Epineurium From Wikipedia, the free encyclopedianavigation search Epineurium Transverse section of human tibial nerve. (Epineurium labeled at upper right. )Nerve structure Details Identifiers Latin Epineurium Me SH D010525TA A14.2.00.016FMA 12234Anatomical terminology [ edit on Wikidata]The epineurium is the outermost layer of dense irregular connective tissue surrounding a peripheral nerve. [1] It usually surrounds multiple nerve fascicles as well as blood vessels which supply the nerve. Smaller branches of these blood vessels penetrate into the perineurium. In addition to blood vessels which supply the nerve, lymphocytes and fibroblasts are also present and contribute to the production of collagen fibers that form the backbone of the epineurium. In addition to providing structural support, lymphocytes and fibroblasts also play a vital role in maintenance and repair of the surrounding tissues. [2]When the spinal nerve leaves the vertebral canal via an intervertebral foramen, two layers of the spinal meninges, the arachnoid and the dura invaginate the nerve to form a dural sleeve of connective tissue, which is the epineurium. The outer portion of this sleeve comprises the external epineurium which permits longitudinal nerve excursion and absorption of longitudinal stress. The layer of the epineurium that extends within the nerve to define the fascicles is termed the internal epineurium. Together, these two layers form the epineurium, which varies in thickness along the course of a nerve. [3] The epineurium is usually most abundant around joints, as its function is to protect the nerves from stretching and subsequent injury. A common surgical procedure to repair a torn nerve via the epineurium is termed an epineurial repair. [4]See also [ edit]Connective tissue in the peripheral nervous system Endoneurium References [ edit]^ Mc Cracken, Thomas (1999). New Atlas of Human Anatomy. China: Metro Books. pp. 96–97. ISBN 1-5866-3097-0.^ Kulkarni, GS (2009). Textbook of Orthopaedics and Trauma. India: Jaypee Brothers Medical Publication. pp. 895–898. ISBN 9350908522.^ Payne, Samuel H. (2001). "Nerve Repair and Grafting in the Upper Extremity". Journal of the Southern Orthopaedic Association. Medscape. 10 (2): 1–7. Retrieved 22 March 2015.^ Saladin, Kenneth (2014). Anatomy & Physiology: The Unity of Form and Function. USA: Mc Graw-Hill Science. pp. 484–485. ISBN 0073403717. External links [ edit]Anatomy photo: nervous/pns/nerve1/nerve1 - Comparative Organology at University of California, Davis - "PNS, nerve (LM, Low)"Anatomy photo: nervous/pns/nerve2/nerve1 - Comparative Organology at University of California, Davis - "PNS, nerve (LM, Medium)"Diagram at Howard [ hide]v t e Nervous tissue CNSTissue Types Grey matter White matter Projection fibers Association fiber Commissural fiber Lemniscus Funiculus Fasciculus Nerve tract Decussation Commissure Neuropil Meninges Cell Types Neuronal Pyramidal Purkinje Granule Glialinsulating: Myelination: Oligodendrocyteother Astrocyte Radial glial cell Ependymal cells Tanycyte Microglia PNSGeneral Dorsal Root Ganglion Ramus Ventral Root Ramus Ramus communicans Gray White Autonomic ganglion ( Preganglionic nerve fibers Postganglionic nerve fibers)Connective tissues Epineurium Perineurium Endoneurium Nerve fascicle Neuroglia Myelination: Schwann cell Neurilemma Myelin incisure Node of Ranvier Internodal segment Satellite glial cell Neurons / nerve fibers Parts Soma Axon hillock Axon Telodendron Axon terminals Axoplasm Axolemma Neurofibril/neurofilament Dendrite Nissl body Dendritic spine Apical dendrite / Basal dendrite Types Bipolar Unipolar Pseudounipolar Multipolar Interneuron Renshaw Afferent nerve fiber / Sensory neuron GSA GVA SSA SVA fibers Ia or Aα Ib or Golgi or Aα II or Aβ and Aγ III or Aδ or fast pain IV or C or slow pain Efferent nerve fiber / Motor neuron GSE GVE SVE Upper motor neuron Lower motor neuronα motorneuron β motorneuronγ motorneuron Termination Synapse Electrical synapse / Gap junction Chemical synapse Synaptic vesicle Active zone Postsynaptic density Autapse Ribbon synapse Neuromuscular junction Sensory receptors Meissner's corpuscle Merkel nerve ending Pacinian corpuscle Ruffini ending Muscle spindle Free nerve ending Nociceptor Olfactory receptor neuron Photoreceptor cell Hair cell Taste bud Categories: Neurohistology 
Anatomy Chapter 8 185 terms xolynleyxo23Anatomy Chapter 8Learn Flashcards Write Spell Test Match Gravity Advertisement Upgrade to remove ads Like this study set? Create a free account to save it. Create a free account Maybe later Sort Skin or Cutaneous membrane composed of different tissues, is physically distinct from surrounding organs, and performs functions specific to its structure.integumentary system skin is most prominent organ but is also comprised of orther organs called skin derivatives (or accessory organs)skin derivatives include hair, nails, and variety of microscopically small glnadsliteral moving of integumentary coveringepidermis is the thinner, superficial layer of the skin consisting of stratified squamous epitheliumliteral meaning of epidermis epi = on dermis = skindermis the thicker layer, lies deep to the epidermis and consists mostly of dense irregular and areolar connective tissuesstrata "layers" of finer tissue. located in both the epidermis and dermis. this is what makes skin a stratified organliteral meaning of strata "layers"hypodermis or subcutaneous tissue not part of the skin. it is what underlys the skin. composed of areolar and adipose connective tissuethin skin covers the entire body except the palms, the anterior surfaces of the fingers, the soles of the feet, and the plantar (inferior) surfaces of the toes. has a FOUR layer epidermishirsute skin thin skin is sometimes called this because occasionally it contains hairthick skin has a relatively deep, FIVE layer epidermis. Covers the anterior surface of the hands and fingers and the plantar surface of the feet and toesglabrous skin thick skin lacks hair so it is sometimes called this (since it lacks hair, it lacks sebaceous (oil) glands)is the epidermis or dermis thicker? DERMISthin skin and thick skin only apply to... epidermisfunctions of integumentary system protection, temperature regulation, emergency blood reserve, excretion, sensation, vitamin D synthesis, identification, communication of emotions, sexual differentiation, and diagnosing and treating health problemsexcretion process of expelling wasteepidermal surface features are classified into three groups surface pattern lines, flexure lines, and friction ridgessurface pattern lines extremely small, triangular or polygonal-shaped line patterns seen only on thin skin. increase surface area, allowing it to stretch more evenly during movementflexure lines deeper than surface pattern lines and exist in regions where skin is flexed (bent) repeatedly. examples are "life lines" on palms and wrinkles and grooves on the face. pattern of these usually corresponds to orientation of parallel bundles of collagen fibers in underlying dermisepidermal ridges or friction ridges slightly elevated, parallel lines separated by shallow grooves on the surface of thick skin. they provide a rough surface that allows the fingers and palms to grip objects more effectively, and they help prevent feet from slipping. these correspond to rows of tiny projections, called dermal papillae, located at the superficial margin of the dermisepidermal grooves shallow grooves between epidermal ridges or friction ridges which correspond to indentations of the epidermis, called epidermal pegs, into the dermis.fingerprints example of friction ridges. these are telltale impressions of the ridges themselves.three basic patterns of fingerprints whorl, loop, and archepidermis the most superficial region of the skin, and lies superficial tot he dermis.epidermal pegs downward projections of the epidermisdermal papillae the upward projections of the dermisadvantages of epidermal pegs and dermal papillae structure increase amount of surface area contact between epidermis and dermis, firmly anchors the epidermis to the dermis and it increases the surface area over which nutrients can diffuse from dermal blood vessels to epidermal cellsis epidermis vascular or avascular avascular (lacks vessles)strata the different layers in the epidermis. there are four in thin skin and five in thick skin. the strat consists of one or more rows of similar-looking cells.order of skin layers in thin skin stratum corneum, stratum granulosum stratum spinosum, and stratum basaleorder of skin layers in thick skin stratum corneum, stratum lucidum, stratum granulosum stratum spinosum, and stratum basalekeratinocytes most cells in the epidermis are these. they are named this because they secrete keratinkeratin a structural, filamentous proteins, and is abundant in the horns of certain animals (mainly cows)keratinization the transformation of living keratinocytes, in the deeper strata, to dead flat cells full of keratin protein, at the apical surface. this process requires about four weeksstratum basale single row of cells (most of these are keratinocytes) that form the base (basal layer) of the epidermis.functions of keratinocytes in stratum basale synthesize part of the basement membrane (basal lamina) by secreting proteoglycans, they synthesize keratin polypeptides, and they are the principal source of epidermal growthbasal cells keratinocytes comprising the stratum basale are called this and is attached to neighboring ______ cells by desmosomesbasal lamina the superficial portion of the basement membrane that separates the epidermis from the dermis. made of proteoglycans from which keratinocytes secrete.hemidesmosomes after a basal cell keratinocyte divides (undergoes mitosis and cytokinesis), the deeper of the two daughter cells remains attached to the basement membrane by thismelanocytes spider-shaped cells that synthesize melanin. between 5% and 25% of the stratum basale are these cellsmelanin a chemical that darkens the skin and helps protect underlying tissues from the ultraviolet rays of the sundendrites the arm-like projections of a melanocyte that extend between adjacent basal cells and outward between keratinocytes in the stratum spinosum.merkel cells cells that are also scattered throughout the stratum basale. they are thought to be modified keratinocytes. they are in close contact with a nerve ending located in the upper region of the dermis.merkel disc made of a single merkel cell and the nerve ending beneath it. it functions as a receptor of light touchstratum spinosum contains eight to ten rows or keratinocytes, and is superficial to the stratum basale.spiny (prickle) cells keratinocytes in the stratum spinosum are sometimes called this because when the cells in the stratum spinosum lose water and crenate (shrivel) and the desmosomes continue to hold the cells together, the stretched plasma membranes appear to have spines.stratum germinativum the stratum basale and stratum spinosum together are sometimes called this because some of the cells in the stratum spinosum are still capable of dividing, which means they can contribute to the outward growth of the epidermiskeratin tonofilaments the resulting protein strands that arise int he stratum spinosum where more keratin polypeptides form within the keratinocytes and combing with existing intermediate filaments of the cytoskeleton. these are thicker and denser than the intermediate filaments found within the stratum basalelangerhans cells belong to a class of white blood cells called macrophages. they are located in the stratum spinosum. they are motile and can perform phagocytosis to remove foreign particles from the epidermisfree nerve endings sensory receptors which are root-like branches of neurons, that extend between the basal cells and into the stratum spinosum. important for temperature detection and generation of pain signals when the epidermis is damagedstratum granulosum a thin region, containing only three to five rows of cells, and located immediately superficial to the stratum spinosum.autophagia where lysosomes in the cytoplasm are digesting other organelles. this occurs in the stratum granulosum where keratinocytes are flatter than the spiny cells in the stratum spinosum, and most are dying and undergoing this.apoptosis programmed cell death. some say that the reason keratinocytes die in the stratum granulosum is due to lack of nutrients or this..keratin fibrils thick strands formed when keratinocytes are pushed outward into the stratum granulosum, and their keratin tonofilaments intertwine to form these thick strandskeratohyalin granules of this attach to the keratin fibrils. these are part protein. they absorb dye and cause the keratinocytes to appear dark and "grainy"lamellar granules lipid-rich vesicles which stack like small saucers. they fuse with the plasma membrane and release their lipids into the extracellular spaces between the keratinocytes. keratinocytes consist of these in the stratum granulosumstratum lucidum a thin, clear region seen only in the thick skin of the palm and sole. contains 3 to 5 rows of keratinocytes, all of which are dead.eleidin enzymes transform the keratohyalin granules into this chemical, causing the cell's cytoplasm to appear clear, not grainy.stratum corneum is the most superficial stratum of the epidermis. may consist of a bunch of layer of keratinocytes (thick skin) or only a few layers of keratinocytes (thin skin)squame cells cells in the stratum corneum that are dead, flattened keratinocytes. have a rough leathery texture because the cells are keratinized (or cornified)desquamation or exfoliation process when the desmosomes holding adjacent squame cells together break, allowing the outermost cells to peel off.dandruff if the epidermis becomes too dry, large clumps of squame cells may detach together, causing the skin surface to look flaky.keratinization or cornifcation five step process 1. synthesis of keratin and formation of intermediate filaments in the stratum basale 2. formation of keratin tonofilaments in the stratum spinosum 3. binding of tonofilaments with kertohyalin to form keratin fibrils in the stratum granulosum 4. conversion of keratohyalin to eleidin (in the upper stratum granulosum or stratum lucidum) 5. aggregation of keratin fibrils into bundles in the stratum corneuminsensible perspiration process of water loss that we are unaware of. occurs because the epidermis is not waterproof, so in a dry environment, water constantly diffuses out of the skin - water lost by diffusion through the epidermissensible perspiration process of water loss that we are aware of or can feel. like sweatingdehydrocholesterol UVB radiation converts this compound, found inside keratinocytes of the stratum germinativum, into cholecalciferol. Keratinocytes synthesize this from cholesterolcholecalciferol also known as vitamin D3. its added to milk to promote good health. this is an inactive (unusable) form of vitamin D. this diffuses into the blood and travels to the livercalcidiol the liver converts the vitamin D3 into this, which travels through the blood to reach the kidneyscalcitriol the kidneys convert the calcidiol into the active (usuable) form of vitamin D called this. shows importance of this vitamin to calcium homeostasisdermis or stratum corium deep to the epidermis and forms the bulk of the skin (by weight and by volume). consits of dense irregular connective tissue, with collagen fibers accounting for about 70% of the total weight.dermis is divided into two regions 1. papillary region (thin) reticular region (thick)papillary region located deep to the basement membrane of the epidermis and is named for its tiny projections (the dermal papillae). contains areolar connective tissue.dermal papillae upward projections that interlock with the epidermal pegs. consist of areolar connective tissue that contains fibroblasts, mast cells, very thin collagenous and elastin fiber, tiny loops of capillaries, and one or more sensory receptors.functions of papillary region 1. secures the epidermis to the dermis, 2. provides nutrients to the cells of the epidermistwo parts of the basement membrane 1. reticular lamina 2. basal laminawhat secretes the reticular lamina fibroblasts within the dermal papillae secrete compounds that form this bottom halfwhat secretes the basal lamina basal keratinocytes in the stratum basale secrete this superficial portionreticular region the scaffolding of the skin. fibrous region of the dermis that is located between the papillary region and the hypodermis, and it is the thickest part of the dermis. consists of dense irregular connective tissuetension lines or lines of cleavage collagen fibers in the reticular region are organized into bundles that align in parallel rows. these bundles usually align in the same direction that the skin stretches, which is why they are known as thiswrinkles skin creases that become more prevalent as we get older, when the epidermis and the dermis become thinnerreticular plexus "braided" network. its the largest vessels in the skin that are sandwiched between the reticular dermis and the hypodermis.papillary plexus arterioles at the junction between the reticular and papillary regions of the dermis form this fin network. small loops of capillaries extend superficially from this into the dermal papillae, where nutrients diffuse across the basement membrane into the epidermis.vasodilation rings of smooth muscle that surround the dermal arterioles relax, causing the vessels to widen. dermal blood flow increases, so more heat radiates to external environmentvasoconstriction ring-like muscles surrounding the dermal arterioles contract, causing the vessels to narrow. dermal blood flow decreases, and less heat radiates from the blood to the external environment.hypodermis or subcutaneous tissue not part of the skin. helps attach the skin to the body. consists of areolar and adipose connective tissue. cushions the skin against hard, deep structures such as bonessuperficial fascia another name for the hypodermis. called this because the hypoderis is superficial to underlying musclescutaneous receptors nerve endings in the epidermis, dermis, and hypodermis. also us to experience different cutaneous sensations (awareness of stimuli in the skin). this receptor initiates an impulse that passes along a nerve to the brain, where the impulse is interpreted.three types of cutaneous receptors - structurally 1. encapsulated nerve endings 2. free nerve endings 3. epidermal nerve endingsencapsulated nerve endings are surrounded by a well-defined "shell" of connective tissue. type of cutaneous receptor that is classified structurallyfree nerve endings consists of a nerve endings not surrounded by a capsule of connective tissue. type of cutaneous receptor that is classified structurallyepidermal nerve endings attach to a specific type of cell in the epidermis, or a structure that is derived from epidermal cells. type of cutaneous receptor that is classified structurallythree types of cutaneous receptors - how they respond to certain stimulus 1. mechanoreceptor 2. nociceptor 3. thermoreceptormechanoreceptor responds to mechanical forces, such as compression and stretch. most of these are encapsulated nerve endings. there are two different typestwo different mechanoreceptors tactile receptor and deep pressure receptortactile receptor touch receptor. its a mechanoreceptor that generates an impulse when something makes light contact with the skindeep pressure receptor a mechanoreceptor that generates an impulse when something presses hard enough against the skin to compress the hypodermisnociceptor a free nerve ending that generates an impulse interpreted as pain. stimulated most often by tissue damage, resulting from a wound, extreme hot or cold temperature, or stretching. certain chemicals whether foreign or those released from surrounding tissues, can stimulate these.thermoreceptor generates an impulse in response to changes in temperature. most of these are free nerve endingsfree nerve endings (FNEs) the only cutaneous receptor found in both the epidermis and the dermis, and they are found in both thin and thick skin. these extend throughout the dermis and extend through the basement membrane into the stratum basale and stratum spinosum. most of these are nociceptor, but sometimes are mechanoreceptors or thermoreceptors.merkel discs or tactile discs epidermal nerve endings that attach to merkel cells int he stratum basale of both thin and thick skin. the Merkel cell causes this to initiate an impulse, which you feel as a light touch. these branch from a single nerve fiber in the dermis and extend to the base of epidermal peg. Meissner corpuscles or tactile corpuscles are mechanoreceptors sensitive to light touch, and are found mainly in the dermal papillae of thick skin. most abundant in fingertips. these are a fluid-filled, encapsulated receptor, consisting of several layers of flat epithelial cells intertwined with a single nerve ending. this corpuscle has an outer covering made of fine elastic fibers and fibroblasts. Hair receptors or hair root plexuses found only in think skin containing hair follicles, the tubes from which individual hairs grow. the hair follicle is derived from epidermal cells so these are epidermal nerve endings. these consists of numerous nerve endings. this doesn't reach the hair, but when something touches the hair hard enough to bend its follicle, this initiates an impulse allowing you to feel your hair movepacinian corpuscles or lamellated corpuscles largest cutaneous receptors and respond to deep pressure and vibration. some are found deep in the dermis of both thick and thin skin, but most are found in hypodermis, where they are examples of subcutaneous receptors. these are encapsulated mechanoreceptorslamellae the capsule of the pacinian corpuscles surrounding the nerve ending consists of flat fibroblasts arranged in concentric layer called thisruffini's endings are located mainly in the lower dermis of both thick and thin skin, and in the hypodermis, where they respond mostly to skin stretch and vibration. looks like cylinder with a connective tissue shell surrounding one or two nerve endings.pigment an organic molecule that gives color to a cell, tissue, or organmelanin the only endogenous skin pigment; the only pigment made by the skin itself. melanocytes in the stratum basale secrete this. this is a complex polymer derived from the amino acid tyrosine.melanocytes make two kinds of melanin eumelanin and pheomelanineumelanin darkest and most common form, is brown-to-black in color - darker skinned people usually produce more of thispheomelanin red-to-amber in color - light skinned people typically produce more of thismelanosomes vesicles where melanin molecules (granules) are packaged. this is in a melanocyte. keratinocytes use phagocytosis to engulf the dendritic tips containing these, causing these to release their granules into the cytoplasm.two pigments not produced in the skin heme and caroteneheme a constituent of hemoglobin, the oxygen-carrying molecule in red blood cells. dramatic evidence of this is during blushing.carotene a yellow-orange, lipid-soluble precursor to vitamin A. It can diffuse from the blood into keratinocytes of the epidermis and adipocytes of the hypodermis. this pigment is only noticeable in light-skinned people3 different skin glands eccrine, apocrine and sebaceous glandseccrine sweat glands most important skin derivatives. they regulate body temperature and excretion. more prevalent in palms and soles. stimulated by the nervous systemsweat pore in eccrine glands. the sweat-forming portion of the gland is a tightly coiled tube housed in the reticular region of the dermis. the gland's secretions are released into a single tubular duct that opens directly onto the skin's surface through this...the duct of an eccrine gland has two parts a spiraled part in the epidermis and a straight part in the dermissuoriferous gland eccrine sweat glands are a type of this because it produces sweatevaporative cooling the release of heat that takes place when water evaporates off the skin surface. the watery protion of eccrine sweat is an important source of this.eccrine sweat glands are simple, coiled tubular (one duct and one coiled tube-like gland). they are also sudoriferous (produce sweat), exocrine (have a duct and release secretions to the outside of the body), and merocrine (release secretions by exocytosis) glands.myoepithelial cells whenever the nervous system stimulates eccrine sweat glands, the nervous system stimulates specialized contractile cell which surround the secretory cells at the base of the gland. These cells contract and squeeze the sweat into the duct leading to the hair follicle. Apocrine sweat glands unique sweat glands, like eccrine glands, have a sweat-forming portion that is a highly coiled tube. mainly attached to hair follicles. located in thin skin of the axillary, anal, areola (nipple), and pubic regions. these secrete sweat into the hair follicle to which they are attached. contain lipids and proteins, which make it more viscous (thicker). considered a merocrine gland. stimulated by hormones. they have myoepithial cells surrounding their secretory cells that squeeze sweat into the secretory ducts.two kinds of apocrine glands ceruminous glands and mammary glandsceruminous glands located within the skin of the external ear canals. waxy secretion called cerumencerumen waxy secretion, also called earwax, that waterproofs the ear canal and may help repel tiny insectsmammary glands located within the breasts. produce milk, but this occurs only during lactation period following childbirth.sebacous glands also called oil glands. secrete sebum. attached to "leaning" side of hair follicles. not located on palms or soles. lack the water-resistant oily covering present on other parts of your body. stimulated by hormones. temperature can also affect these. HOLOCRINE GLANDSsebum oily fluid secreted by sebacous glands. lubricates hair within hair follicles and helps make the hair and epidermis water resistant. viscous secretion consisting mostly of tiglycerides and free fatty acids.functions of fatty acids in sebum 1. decrease the sebum's p H, which can inhibit the growth of certain types of bacteria and fungi on the skin's surface 2. after entering a hair follicle, sebum coats the hair inside making it more flexible and able to repel water 3. when sebum reaches the skin's surface, it coats the epidermal cells and helps keep them flexible and water resistantpilosebaceous unit the whole unit of the sebaceous gland and the hair follicle which secretes through the hair folliclefree sebaceous glands sebaceous glands that do not attach to hair follicles, but secrete sebum directly onto skin's surface. found in skin of eyelids, certain regions on the external genitalia, and in the areola of the nipples Hair or pili grow from tube-like structures called hair follicles and consist of densely packed keratinized cells.two types of hairs vellus and terminalvellus hairs often called "peach fuzz" - covers most of the body and are extremely thin. most of these lack pigmentterminal hairs eight times thicker than vellus hairs. usually contain pigment. these develp from vellus hairs at puberty, in the axillary and pubic regions.functions of hair 1. scalp hairs shield the scalp, the skin that covers the skull, from the damaging effects of UV radiation. scalp hairs also help to insulate the scalp from heat and cold 2. eyebrows reduce glare around the eyes, and eyelashes prevent foreign particles from reaching the surface of the eye 3. hairs help keep dust and other foreign objects from entering the eyes, nasal cavities and ear canals 4. hairs in the axillary regions reduce friction while the arms are swinging and hair in the pubic region reduces skin friction during sexual intercoursehair follicle is a tube-like organ that produces a hair. its a pouch-like extension of the epidermis that plunges deep into the superficial region of the hypodermis in some cases. deepest part of this is in the reticular region of the dermis.hair bulb the base of the hair follicle that fits like a cup over the hair papilla. contains mostly keratinocytes but also has some melanocytes and Langerhans cellshair papilla consists of areolar connective tissue and blood vessels. provides nutrients to the hair bulbfour layers of the hair follicle dermal root sheath, glassy membrane, the outer root sheath and the inner root sheath.dermal (fibrous) root sheath consists of collagen fibers and fibroblasts, and serves to anchor the hair follicle to the surrounding dermal tissueglassy membrane the basement membrane separating the epidermis from the dermis extends into the hair follicle called this. has a clear appearance. separates epidermal tissue (outer root sheath) from dermal tissue (dermal root sheath)outer root sheath lies between the glassy membrane and the inner root sheath. contains mostly keratinocytes, and is continuous with the stratum basaleinner root sheath contains mostly keratinocytes and is continuous with the stratum spinosum. makes contact with the hair from the hair bulb to about the level of the sebaceous gland, but at the sebaceous gland, this disintegrates.arrector pili muscle attached to the dermal root sheath of most hair follicles. extends just below sebaceous gland to the papillary region of the dermis. located on "leaning" side of hair follicle. contracts in response to cold temperature, making goose bumpspiloerection process when the arrector pili contracts in response to cold temperatures and yanks the follicle and its hair into a vertical position. this pushes the skin into goose bumpsgoose bumps or cutis anserina (physiological name) when the skin around the hair is pushed into a little peak See more  A&P BIO 141: TISSUE TYPES AND STRUCTURES IN THE EPIDERMIS AND THE DERMIS A&P BIO 141: TISSUE TYPES AND STRUCTURES IN THE EPIDERMIS AND THE DERMIS41 cards | Created by amyrlin026 | 9 years ago Flashcards Memorize Quiz Match Gravity The Integumentary System Overview For BIO 141, Anatomy & Physiology. Notes From Textbook And Class. When Studying From These Cards, Pay Close Attention To bold, Colored, And/or Larger Font Sized Areas Within The Text. These Are Important Terms And Generally Focus On The Specific Information That Will Be Asked In Class. There Are Essay Questions... View Flashcards Preview Flashcards Front Back What is the Integumentary System? The skin and its derivatives (sweat and oil glands, hairs and nails) make up a complex set of organs that serves several functions, mostly protective. Together, these organs form the integumentary system. Skin is composed of two layers: Superficial Epidermis Deeper Dermis What is the subcutaneous tissue just deep to the skin know as? What is another name for this tissue and why is it called this? The Hypodermis, also called superficial fascia because it is superficial to the tough connective tissue wrapping (fascia) of the skeletal muscles (consists mostly of adipose tissue). Describe the functions of the hypodermis layer. Besides storing fat, they hypodermis anchors the skin to the underlying structures (mostly to muscles), but loosely enough that the skin can slide relatively freely over those structures. Because of its fatty composition, they hypodermis also acts as a shock absorber and an insulator that reduces head loss from the body. Keep in mind that the hypodermis is NOT skin (aka superficial fascia)What is the epidermis? A keratinized stratified squamous epithelium consisting of four distinct cell types and four or five distinct layers. What are the cells of the epidermis? The cells populating the epidermis include: karatinocytes melanocytes epidermal dendric cells tactile cells (Most epidermal cells are keratinocytes)What is the chief role of karatinocytes? ("Keratin cells") The chief role of karatinocytes is to produce keratin. Define keratin. Fibrous protein that helps give the epidermis its protective properties. What are melanocytes? The spider-shaped epithelial cells that synthesize the pigment melanin. They are found in the deepest layer of the epidermis. What happens when melanin is created? What role does it play in our skin? As melanin is made, it is accumulated in membranebound granules called melanosomes that are moved along actin filaments by motor proteins to the ends of the melanocyte's processes (the "spider arms). From there they are taken up by nearby keratinocytes. The melanin granules accumulate on the superficial, or "sunny," side of the keratinocyte nucleus, forming a pigment shield that protects the nucleus from damaging effects of ultraviolet (UV) radiation in sunlight. Skin color and racial differences - All races have melanocytes that produce melanin. - Racial variations are due to the type of melanin produced. (dark brown, tan, yellow, red)List the different colors of skin and their medical terms. (Give example (s) of cause (s) for each different pigment) Redness - Erythema (Burn, blush, heat/temperature, infection, inflamation, trauma) Pale - Palor (Fright, loss of blood) Blue - Cyanotic (Prolonged loss of blood) Yellow - Jaundice (Liver disease, (sometimes seen in infants)) Bronze - Adrenal (Gland disorder) Orange - Carotene (Eating foods with lots of carotene in them, like carrots) Black and Blue - Ecchymosis (Flat bleeding under the skin due to trauma) Hematoma (Bleeding under the skin (collection of raised blood))What are Langerhans cells? What do they do? The star-shaped epidermal dendritic (Langerhans) cells arise from bone marrow and migrate to the epidermis. They ingest foreign substances and are key activators of our immune system. Their slender processes extend among the surrounding keratinocytes, forming a more or less continuous network. -Phagocytes, Immune cells. What are Merkel cells? What do they do? Occasional tactile (Merkel) cells are present at the epidermal-dermal junction. Shaped like a spiky hemisphere, each tactile cell is intimately associated with a disclike sensory nerve ending. The combination, called a tactile or Merkel disc, functions as a sensory receptor for touch. What are the layers of the epidermis? Stratum Basale (Basal Layer) Stratum Spinosum (Prickly Layer) Stratum Granulosum (Granular Layer) Stratum Lucidum (Clear Layer) Stratum Corneum (Horny Layer)Variations in epidermal thickness determine if skin is thick or thin. Explain. In thick skin, which covers the palms, fingertips, and soles of the feet, the epidermis consists of five layers (strata). (Thickened is NOT the same as thick!) In thin skin, which covers the rest of the body, the stratum lucidum appears to be absent and the other strata are thinner. Composed of four strata. Stratum Lucidum: -Second layer only present in thick skin -Vague, clear layer, composed of flat, dead, keratinocytes (whose plasma membranes are not evident) -These cells contain keratohyaline granules (a gum-like substance that adhears to the keratin in the cells)Describe the stratum basale. Deepest epidermal layer; one row of actively miotic stem cells; some newly formed cells become part of the more superficial layers. See occasional melanocytes and epidermal dendritic cells. -Deepest layer -Row of live mitotic kertinocytes Melanocytes, Merkel cells. Describe the stratum spinosum. Several layers of keratinocytes unified by desmosomes. cells contain thick bundles of intermediate filaments made of pre-keratin. -Live cells -Prickle cells -Histological artifacts (cells die and dry up, and the desmosomes remain connected. Cells shrink and die, lose water/dry up, look prickly. )Describe the stratum granulosum. Three to five layers of flattened cells, organelles deteriorating; cytoplasm full of lamellated granules (release lipids) and keratohyaline granules. -Third layer -Contains live cells -Contains keratohyaline granules (grainy) -Glycolipids Describe the stratum corneum. Most superficial layer; 20-30 layers of dead cells represented only by flat membranous sacs filled with keratin. Glycolipids in extracellular space. -Stratified squamous epith. -Composed of 20-30 layers of dead keratinocytes (cells that produce the protein keratin - overlap like shingles on a roof - connected by desimosomes) -Most superficial layer (replace entire epidermis every month, turns into dust) -waterproof our body (glycolipids) (Possible essay question: DESCRIBE THE LAYERS AND CELLS OF THE EPIDERMIS) Take time now and write a brief essay for practice. 1. KNOW THE TISSUE TYPES AND STRUCTURES IN THE EPIDERMIS AND THE DERMIS 2. KNOW THE LAYERS AND CELLS ( INCLUDING FUNCTIONS) IN THE EPIDERMIS AND DERMIS 3. KNOW THE FACTORS THAT INFLUENCE SKIN COLORWhat is the dermis? The dermis, the second major skin region, is strong, flexible connective tissue. Its cells are typical of those found in any connective tissue proper: fibroblasts, macrophages, and occasional mast cells and white blood cells. Two layers: Papillary layer Reticular layer Explain the papillary layer. The superficial papillary layer is areolar connective tissue in which fine interlacing collagen and elastic fibers form a loosely woven mat that is heavily invested with small blood vessels. The looseness of this connective tissue allows phagocytes and other defensive cells to wander freely as they patrol the area for bateria that may have breached the skin. -20% of the dermis -Areolar connective tissue (CT) -Contains finger-like projections. Dermal papillae -The epidermal ridges form friction ridges or fingerprints (sweat glands leave a sweat imprint)Explain the reticular layer. The deeper reticular layer, accounting for about 80% of the thickness of the dermis, is course, irregularly arranged, dense fibrous connective tissue. The network of blood vessels that nourishes this layer, the cutaneous plexus, lies between this layer and the hypodermis. Its extracellular matrix contains pockets of adipose cells here and there, and thick bundles of interlacing collagen fibers, The collagen fibers run in various planes, but most run parallel to the skin surface. Seperations, or less dense regions, between these bundles form cleavage, or tension, lines in the skin. -80% of the dermis -Dense irregular CT -sweat glands, oil glands, blood vessels, and nerves What are tension lines? Fine lines visible on the skin. -Areas where the collagen is sparse. (less dense) (places where there is no collagen) -Foundation of surgical incisions (lines on skin)What are flexure lines? Any place where a joint flexes. -Areas over-lining joints that flex where the reticular dermis is tightly bound down to the underlying surface. Describe sweat glands. Sweat glands, also called sudoriferous glands, are distributed over the entire skin surface except the nipples and parts of the external genitalia. Regardless of type, the secretory cells that contract when stimulated by the nervous system. Their contraction forces the sweat into and through the gland's duct system to the skin surface. Two types: Eccrine Apocrine What are eccrine sweat glands? Eccrine sweat glands, also called merocrine sweat glands, are far more numberous and are particularly abundant on the palms, soles of the feet, and forehead. Each is a simple, coiled, tubular gland. (These sweat pores are different from the so-called pores of a person's complexion, which are actually the external outlets of hair follicles. )What are apocrine sweat glands? Approximately 2000 of them, are largely confined to the axillary and anogenital areas, In spite of their name, they are merocrine glands, which release their product by exocytosis like the eccrine sweat glands. They begin functioning at puberty under the influence of androgens and have little role to play in thermoregulation. Their precise function is not yet known, but they are activated by sympathetic nerve fibers during pain and stress. What are ceruminous glands? Are modified apocrine glands found in the lining of the external ear canal. Their secretion mixes with sebum produced by nearby sebaceous glands to form a sticky, bitter substance called cerumen, or earwax, that is thought to deter insects and block entry of foreign material. What are mammary glands? Another variety of specialized sweat glands, secret milk. What are the three types of skin cancer? Basal Cell Carcinoma Squamous Cell Carcinoma Melanoma Explain basal cell carcinoma. It is the least malignant and most common skin cancer. It accounts for nearly 80% of skin cancers. Stratum basale cells proliferate, invading the dermis and hypodermis. The cancer lesions occur most often on sun-exposed areas of the face and appear as shiny, dome-shaped nodules that later develop a central ulcer with a pearly, beaded edge. Basal cell carcinoma is relatively slow glowing, and metastasis seldom occurs before it is noticed. Full cute by surgical excision is the rule in 99% of cases. Explain squamous cell carcinoma. Is the second most common skin cancer, arises from the keratinocytes of the stratum spinosum. The lesion appears as a scaly reddened papule (small, rounded elevation) that arrises most often on the head (scalp, ears, and lower lip), and hands. It tends to grow rapidly and metastasize if not removed. If it is caught early and removed surgically or by radiation therapy, the chance of complete cure is good. Explain melanoma. Cancer of melanocytes. It is the most dangerous skin cancer because it is highly metastic and resistand to chemotherapy. It accounts for only 2-3% of skin cancers, but its incidence is increasing rapidly (by 3-8% per year in the United States). Melanoma can begin wherever there is pigment. Most such cancers appear spontaneously, and about one-third develop from preexisting moles. It usually appears as a spreading brown to black patch that metastasizes rapidly to surrounding lymph nodes. The key to surviving melanoma is early detection. The chance of survival is poor if the lesion is over 4 mm thick. The usual therapy for melanoma is wide surgical excision accompanied by immunotherapy. What is the ABCD rule? (American Cancer Society) Used in recognising melanoma: A. Asymmetry: The two sides of the pigmented spot or mole do not match. B. Border irregularity: The borders of the lesion exhibit indentations. C. Color: The pigmented spot contains several colors (blacks, browns, tans, and sometimes blues and reds). D. Diameter: The spot is larger than 6 mm in diameter. What is a burn? Burns are a devastating threat to the body primarily because of their effects on the skin. A burn is tissue damage inflicted by intense heat, electricity, radiation, or certain chemicals, all of which denature cell proteins and cause cell death in the affected areas. What is the rule of nine? In adults, the volume of fluid lost can be estimated by computing the percentage of body surface burned (extend of the burns) using the rule of nines. This method divides the body into 11 areas, each accounting for 9% of total body area, plus an additional area surrounding the genitals accounting for 1% of body surface area. (burns on head of hands or soles of feed are more severe.) Burn patients also need thousands of extra food calories daily to replace lost proteins and allow tissue repair. Describe first-degree burns. Only the epidermis is damaged. Symptoms include localized redness, swelling, and pain. First-degree burns tend to heal in two to three days without special attention. sunburn is usually a first-degree burn. No scarring. Describe second-degree burns. Injure the epidermis and the upper region of the dermis. Symptoms mimic those of first-degree burns, but blisters also appear. The burned area is red and painful, but skin regeneration occurs with little or no scarring within three to four weeks if care is taken to prevent infection. First- and second-degree burns are referred to as partial-thickness burns. Erythema, pain, blistering, due to accumulation between skin layers. Describe third-degree burns. Are full-thickness burns, involving the entire thickness of the skin. The burned area appears gray-white, cherry red, or blackened, and initially there is little or no edema. Since the nerve endings in the area have been destroyed, the burned area is not painful. Although skin regeneration might eventually occur by proliferation of epithelial cells at the edges of the burn or from stem cells in hair follicles, it is usually impossible to wait that long because of fluid loss and infection. For this reason, skin grafting is usually necessary. Dermis damaged, extensive fluid loss, high risk of infection. (Possible essay questions: COMPARE AND CONTRAST THE THREE TYPES OF SKIN CANCER COMPARE AND CONTRAST THE THREE TYPES OF BURNS) Take time now and write a brief essay for practice. 4. KNOW THE STRUCTURE, NAME, PARTS AND FUNCTIONS OF ALL GLANDS 5. KNOW THE FUNCTIONS OF THE SKIN 6. KNOW THE DIFFERENT TYPES OF CANCER AND ASSOCIATED DETAILS 7. KNOW THE DIFFERENT TYPES OF BURNS AND THE ASSOCIATED DETAILS 8. DESCRIBE THE RULE OF NINES  A & P Chapter 6 82 terms Robert_trel A & P Chapter 6Learn Flashcards Write Spell Test Match Gravity Advertisement Upgrade to remove ads Like this study set? Create a free account to save it. Create a free account Maybe later Sort Appendages the hair, the nails, and the skin glands Integument skin Membranes anatomicaaly simple but functionally important sheetlike structures. Connective tissue membranes composed exclusively of various types of connective tissue; no epithelial cells are present in this type of membrane Cutaneous membrane skin; primary organ of the integumentary system Serous membranes found only on surfaces within closed cavities Parietal portion serous membrane, which lines the walls of a body cavity much like wallpaper covers the walls of a room Visceral portion the other type of serous membrance which covers the surface of organs found in body cavities. Pleural Membranes that line the thoracic cavity or pertaing to the thoracic cavity Peritoneum Membrane surrounding the organs in the abdomen Pleurisy very painful pathological condition characterized by inflammation of the serous membranes (pleura) that line the chest cavity and cover the lungs Peritonitis inflammation of the serous membranes in the abdominal cavity Mucous membranes epithelial membranes that line body surfaces opening directly to the exterior Mucus a slimy material that keeps things moist and soft Mucocutaneous junction used to describe the transitional area that serves oa a point of "fusion" where skin and mucous membranes meet Synovial membranes lining the spaces between bones and joints that move are classified as connective tissue membranes. do not contain epithelial cells Synovial fluid the slick. thick, colorless fluid that the synovial membranes secrete Bursae small cushionlike sacs found between moving body parts Epidermis the outermost layer of skin. It is a relatively thin sheet of stratified squamous epithelium Dermis the deeper of the 2 layers. It is thicker than the epidermis and is made up of largely connective tissue. "true skin"Subcutaneous tisse/ Hypodermis thick layer of loose connective tissue called fat. Support the layers of skin Stratum gerinativum cells of the innermost layer. they continually reproduce the new cells then move towrd the surface Pigment layer responsible for the production of a pigment called melanin Melanin gives color to the skin Melanocytes pigment producing cells Cyanosis reduced blood oxygen levels or blood flow causing the skin to turn a bluish gray color Freckle small flat macules that occur most often in light-skinned indiviuals Dermal-epidermal junction the junction that lays between the thin epidermal layer of skin above and the dermal layer below Dermal papillae parallel rows of peg-like projections Strawberry hemangioma a collection of dilated vessels that may initially appear as a bruise at birth an then grow rapidly during the first year into a bright red nodule Port-wine stain pigmented vascular birthmarks Stork Bite dilation of dermal capillaries at the nape of the neck of a baby in development Follicles required for hair growth Lanugo hair of a newborn infant. extremely fine and soft Hair Follicle when cells in epidermal layer of skin grow down into dermis, forming a small tube Alopecia hair loss of any kind Arrector pili tine, smooth (involuntary) muscle Lamellar (Pacini) corpuscle skin receptors; detects pressure deep in the dermis Tactile (Meissner) corpuscle more superficial; detects light touch Cuticle fold of skin that hides the nail root Lunula crescent-shaped white area of the nail Onycholysis separation of a nail from its normal attachment to the nail bed Sebaceous gland a cutaneous gland that secretes sebum (usually into a hair follicle) for lubricating hair and skin Eccrine eccrine sweat glands are the more numerous, important, and widespread sweat glands in the body. Perspiration Sweat Apocrine found primarily in the skin of the armpit (axilla) and in the pigmented skin areas around the genitals. Larger than eccrine glands and instead of watery sweat, they secrete a thicker, milky secretion Pores outlets of small ducts from the errine sweat glands Sebum lubricates the hair and skin Skin grafts required to provided some degree of protection to areas of the body that are no longer covered by skin because of burns or to replace skin destroyed by disease or trauma Burns one of the most serious and frequent problems that affect the skin First-degree burn superficial burn; no blisters, superficial damage to the epidermis Second-degree burn destroys the epidermis and part of the dermis; characterized by pain, blisters, swelling, and discoloration. Thrid-degree burn damage to the epidermis, corium, and subcutaneous layers Fourth-degree burn destroy all layers of skin and involve tendons, muscles, and sometimes bones. Rule of Nines method used to calculate the amount of fluid lost as the result of a burn; divides the body into 11 areas, each accounting for 9% of the total body area Excoriation Skin sore or abrasion produced by scratching or scraping. Areola Circular pigmented area of skin surrounding the nipple Basement membrane The structure that attaches most epithelia to underlying tissue; consists of carbohydrates and proteins secreted by the epithelia and the underlying connective tissue. Blister a painful red swelling on the skin that contains liquid, caused usually by continuous rubbing, especially on your foot, or by burning Hair papilla at bottom of bulb; blood-rich connective tissue, hair growth begins here Integumentary System organ system that includes har, skin, and nails and protects the body from pathogens and maintains homeostasis Keratin hard protein material found in the epidermis, hair, and nails Parietal peritoneum Serous membrane that lines the abdominal cavity Parietal pleura the outer layer of the pleura that lines the walls of the thoracic cavity, covers the diaphragm, and forms the sac containing each lung Stratum corneum outermost layer of the epidermis, which consists of flattened, keratinized cells Subcutaneous tissue fatty layer found below the dermis that gives smoothness and contour to the body, contains fat for use as energy, and also acts as a protective cushion for the outer skin; also called adipose or subcutis tissue Sudoriferous (sweat) gland tiny, coiled tubular structures that emerge through pores on skin Visceral pleura inner layer of pleura that surrounds each lung Visceral peritoneum the inner layer of the peritoneum that surrounds the organs of the abdominal cavity Blackhead accumulated sebum dries, oxidizes and darkens Decubitus ulcer A pressure-induced ulceration of the skin occurring in persons confined to bed for long periods of time Dehydration a state in which the body has lost more water than has been taken in Dermatosis abnormal condition of skin Melanoma malignant tumor of melanocytes that commonly begins in a darkly pigmented mole and can metastasize widely Plaque a solid mass greater than 1 cm in diameter, limited to the surface of the skin Squamous cell carcinoma Type of skin cancer more serious than basal cell carcinoma; often characterized by scaly red papules or nodules. Striae Stretch marks caused by the tearing of the dermis Subcutaneous injection Injection into the tissue between the skin and muscle; a medication delivery route. Third-degree burn a burn involving all layers of the skin; characterized by the destruction of the epidermis and dermis, with damage or destruction of subcutaneous tissue Wart flesh colored, raised area caused by a viral infection Partial-thickness burn a burn involving the epidermis and dermis that usually involves blisters; commonly called a second-degree burn Full-thickness burn a burn in which all the layers of the skin are damaged. There are usually areas that are charred black or areas that are dry and white. Also called a third-degree burn.  What is Epimysium? Epimysium is a type of connective tissue that completely surrounds and encases muscles in humans and many animals. It acts as a sort of buffer that protects muscles from rubbing into bones or other muscular fibers, and promotes smooth movement in the limbs and most joints. Nearly every muscle in the body has this sort of protective casing. Beyond serving as an insulator, it also plays an important role in the formation of tendons, which act as thick cord-like structures that facilitate tension and movement. Deep tissue massages known as myofascial massages often target the epimysia of many of the body’s biggest muscle groups, both as a way of releasing pressure and tension and relieving pain. The tissue’s proximity to major muscles makes it a good conductor of targeted energy. Where to Find It This tissue layer essentially acts as the outermost shell or casing for most muscles. As such, it’s more or less ubiquitous in the body. Humans and animals depend on the musculoskeletal system for everything from basic weight stability to movement and reflexes. Muscles are more than just strong strips of flesh, though, and tend to have their own complicated anatomy that helps them do their job well. At a very elemental level, muscles are made of individual fibers, each covered by a protective layer known as the endomysioum, that are bundled together to form a muscle fascicle. A layer called the “ perimysium ” holds the fascicles and a number of blood vessels and nerve tissues in an even bigger bundle, which is enclosed at the top by epimysium fibers. What It’s Made Of Epimysia differ from perimysia and endomysium primarily in location. From a composition standpoint, all are very similar. All three layers are dense, irregular connective tissue that consists primarily of tightly packed fibroblasts and collagen fibers without any particular directional orientation. Together they are part of the fascia — a grid of dense, irregular, connective tissue that lines the muscles and subcutaneous layer. Main Function There are many different biological purposes for this outermost layer, but protection from strain and friction is one of the most important. The smoothness of the tissues promotes seamless gliding, and can also protect joints and other high-impact areas from rips or tears. Muscles tend to be strong, but they can be damaged, particularly in the event of accidents or overuse. A tough fibrous coating can provide a bit of cushion against this. The epimysium of any muscle also plays a role in tendon formation and anchoring, along with the perimysium and endomysium. These components flow together into the insertion of the muscle where they combine to form a tendon cord. This tendon pulls on the insertion of the muscle while the origin remains stationary, thereby causing movement. For example, tendons in the upper arm cross over to the forearm over the inside of the elbow. When muscles of the upper arm contract, they pull on their tendons, which pull on the forearm and cause various different types of motion. Role in Stress Release People often carry a lot of tension in their muscles, both from everyday stress and strains caused by posture, heavy lifting, or many hours spent standing. Those who suffer from muscular disorders also frequently have a lot of pain in their major muscle bands. Myofascial massage is a type of massage that focuses on the fascia, including the epimysium around muscles. This type of massage is often classified as “therapeutic,” which means that it’s designed more for a medial purpose than a general feeling of relaxation as might be sought in a spa. Massage therapists who perform this sort of procedure often work in physical therapists’ offices or in medical rehabilitation clinics, though there are some exceptions. Myofascial massage is often painful at first, as the fascia and muscles are stretched and rolled beyond what a person may be used to. With time and regular treatments, however, patients usually report a reduction in pain, joint stress, and muscle fatigue. Ad  Myelin: Molecular Architecture of CNS and PNS Myelin Sheath Encyclopedia of Neuroscience2009, Pages 1169–1180Myelin: Molecular Architecture of CNS and PNS Myelin Sheath S. S. Scherer, E. J. Arroyo Show morehttps://doi.org/10.1016/B978-008045046-9.01019-6 Get rights and content Schwann cells and oligodendrocytes make the myelin sheaths of the peripheral nervous system (PNS) and central nervous system (CNS), respectively. Their myelin sheaths are structurally similar, consisting of multiple layers of specialized cell membrane that spiral around axons. Both kinds of myelin sheaths mainly consist of compact myelin, which is mostly composed of lipids as well as a few unique proteins. PNS myelin sheaths also have prominent regions of noncompact myelin, in which tight junctions, gap junctions, and adherens junctions join apposed layers of the myelin sheath. CNS myelin sheaths do not have adherens junctions, and they have a different arrangement of tight junctions and gap junctions. Despite their structural and molecular differences, the nodal regions of the PNS and CNS are similarly organized. The nodal axolemma contains high concentrations of voltage-dependent Na + and K + channels that are linked to the spectrin cytoskeleton by ankyrin-G. The paranodal membrane contains septate-like junctions, which are composed of both glial and axonal components. The juxtaparanodal and internodal regions lack distinct morphological specializations, but the apposed axonal and glial membranes have distinct molecular components that are probably organized by trans molecular interactions. Author Keywords Adherens junctions; Cell adhesion molecules; Connexins; Gap junctions; Incisures; Myelin;Neuropathy; Nodes; Oligodendrocytes; Potassium channels; Schwann cells; Sodium channels; Tight junctions The Function of Myelin Sheaths A myelin sheath is a multilamellar spiral of specialized cell membrane that surrounds a segment of an axon that is typically larger than 1 μm in diameter ( Figure 1 ). Myelinated axons are completely covered by myelin sheaths (internodes) except at nodes of Ranvier, the small gaps (less than 1 μm in length) directly exposed to the extracellular milieu. By reducing the capacitance and clustering ion channels at nodes, myelin sheaths reduce current flow across the internodal axonal membrane, thereby facilitating saltatory conduction, one of the fundamental adaptations of vertebrates. Figure 1. Ultrastructure of a myelinated fiber in the PNS. (a) This electron micrograph shows an axon, its myelin sheath (m), basal lamina (arrowheads), as well as the inner (i) and outer (o) mesaxons (from a 4-day-old mouse). (b) This electron micrograph shows that the compact myelin sheath is composed of alternating major dense (arrows) and intraperiod lines. Note the ‘double nature’ of the intraperiod line. From SS Scherer (1997) Molecular genetics of demyelination: New wrinkles on an old membrane. Neuron 18: 13–16, with permission of Elsevier. Figure options The Circumferential Organization of Peripheral Nervous System Myelin Sheaths Ramon y Cajal (1928) understood that a myelinated peripheral nervous system (PNS) axon “consists of an external solid membrane – sheath of Schwann – of a tubular cell – Schwann cell – of a fatty sheath, and of an axon.” He understood that the sheath of Schwann was not part of the Schwann cell, and that it continued across nodes of Ranvier. With the advent of electron microscopy (EM), the sheath of Schwann was shown to be composed of a basal lamina and associated collagen fibers. This basal lamina contains laminin 2 (composed of a trimer of α 2, β 1, and γ 1 laminin chains; Figure 2 ), type IV collagens, entactin/nidogen, fibronectin, agrin, and glypican. Of these, α 2 and γ 1 laminin are essential because eliminating either one disrupts the formation of the basal lamina and results in developmental defects in myelination. Figure 2. Extracellular matrix receptors of myelinating Schwann cells. The dystroglycan receptor complex of myelinating Schwann cells is linked to three members of the dystrophin family – utrophin, DRP2, and Dp116. Of these, only utrophin has an actin binding domain, whereas DPR2 binds periaxin, which is shown binding to itself via its PDZ domains. Three sarcoglycans – β, δ, and γ – associate with dystroglycan. α 6 β 4 integrin also binds to laminin-2, which is a trimer of α, β, and γ subunits. The figure was modified from the one kindly provided by Drs. Diane Sherman and Peter Brophy. Figure options By analogy to epithelial cells, the basal/abaxonal cell membrane apposes the basal lamina ( Figure 1, Figure 2 ; Figure 3 ). The abaxonal membrane contains at least two receptors for laminin 2 – the integrin α 6 β 4 and dystroglycan ( Figure 2 ; Figure 3 ). In epithelia such as the skin, α 6 β 4 heterodimers are found in hemidesmosomes, which join epithelial cells to their basal lamina via intermediate filaments. Schwann cells, however, do not have hemidesmosomes; α 6 β 4 integrin may be linked to the actin cytoskeleton and not to intermediate filaments. Mice lacking either the α 6 or the β 4 integrin subunit die soon after birth because of a profound skin defect, but myelination appears normal in co-cultures established from mice lacking β 4 integrin. Dystroglycan is composed of α and β subunits, which are products of the same gene. As depicted in Figure 2, dystroglycan binds to laminin 2 and to agrin. The intracellular domain of β dystroglycan forms a complex with periaxin, dystroglycan-related protein 2 (DRP2), a short dystrophin isoform (Dp116), utrophin, and β, δ, and ϵ sarcoglycan. In contrast to the continuous localization of α 6 β 4 integrin and even dystroglycan, DRP2 and periaxin are specifically localized to regions lacking cytoplasm ( Figure 4 ), indicating that DRP2 and/or periaxin may bind to a component of compact myelin. Figure 3. Circumferential organization of myelinating glial cells. This schematic drawing depicts the circumferential organization of a myelinated axon from the CNS and the PNS, including representative molecules. Adapted from Scherer SS and Arroyo EJ (2002) Recent progress on the molecular organization of myelinated axons. Journal of the Peripheral Nervous System 7: 1–12, with permission of The Peripheral Nerve Society. Figure options Figure 4. Cytoplasmic domains in myelinating Schwann cells. (a) This is a confocal image of three teased fibers from a rat sciatic nerve, double labeled with a rabbit antiserum against DRP2 (green) and a mouse monoclonal antibody against caveolin-1 (red). The positions of a node (apposed arrowheads) and two nuclei (n) are indicated. Reproduced from Scherer SS and Arroyo EJ (2002) Recent progress on the molecular organization of myelinated axons. Journal of the Peripheral Nervous System 7: 1–12, with permission of The Peripheral Nerve Society. Scale bar = 10 μm. (b) This is a drawing made by Ramon y Cajal (1928) of teased fibers from an adult cat, stained with reduced silver. He subdivided the Schwann cell cytoplasm into a ‘perinuclear mass,’ ‘longitudinal stripes,’ and ‘transverse trabeculae’; as shown in a, the Schwann cell membrane in these regions contains caveolin-1 but not DRP2. Used with permission of Oxford University Press. (c) These are transmission (a and b) and scanning (c) EM images of the outer surface of a myelin sheath. a, Cross-cut myelinated fiber in thin section. Arrowheads indicate two caveolae. b, Longitudinally cut myelinated fiber in thin section. The plane of section passes through three thin cytoplasmic regions (arrows) devoid of large organelles. c, Outer surface of freeze-fracture sheath. The P-face of the Schwann cell membrane has been exposed. Conspicuous convexities of the fracture face studded with caveolar stomata correspond to organelle-rich cytoplasmic regions such as is represented in a. The fingerlike elevations correspond to organelle-poor cytoplasmic regions shown in b. The flat portions in between correspond to semicompact regions. Lines 1 and 2 demarcate planes corresponding approximately to those shown in thin section in a and b, respectively. om, outer mesaxonal furrow. The triangle indicates the cross-fractured mesaxonal cytoplasm, and the arrow indicates the mesaxonal tight junctional strands. Reproduced from Schnapp BJ and Mugnaini E (1978) Membrane architecture of myelinated fibers as seen by freeze-fracture. In: Waxman SG (ed.) Physiology and Pathobiology of Axons, pp. 83–123. New York: Raven Press, with permission of Blackwell Publishing. Figure options The importance of the dystroglycan complex in myelinating Schwann cells remains to be fully elucidated, but dystrophin as well as α, β, δ, and γ sarcoglycan are all essential in skeletal muscle cells since the absence of any one of these components results in myopathy. In mice, eliminating dystroglycan or especially the β 1 integrin gene in Schwann cells causes developmental defects in myelination. Recessive mutations in the mouse Large gene, one of the genes involved in glycosylation of β dystroglycan, also results in abnormal myelination. In humans, Dp116 may be required for proper myelination since a splice site mutation in the Dystrophin gene that abolishes dystrophin expression in peripheral nerve has been reported to cause a demyelinating neuropathy. The best evidence for an essential role of the dystroglycan complex is that recessive Periaxin mutations in humans cause an inherited demyelinating neuropathy known as Charcot–Marie–Tooth disease 4F (CMT4F). In addition to a demyelinating neuropathy, periaxin null mice also show a loss of the cytoplasmic channels/Cajal bands, the cytoplasmic channels on the outside of the myelin sheath ( Figure 4 ). Cajal bands contain microtubules that provide the highways for trafficking cargo to and from the perinuclear region, where the rough endoplasmic reticulum and Golgi apparatus are located. The cell membranes of Cajal bands are enriched in caveole (and caveolin-1), as well as the K + channels Kir2.1, Kir2.3, and a Ca 2+ -activated K + channel. In periaxin null mice, the cytoplasm uniformly surrounds the compact myelin, resulting in abnormally short myelin internodes. Compact Myelin Figure 5 (a) shows a schematic PNS myelinated fiber. It depicts two internodes, one of which has been unrolled to reveal its trapezoidal shape. The myelin sheath has two domains – compact and noncompact myelin – each of which contains a nonoverlapping set of proteins ( Figure 5 (b). Compact myelin forms the bulk of the myelin sheath and is a spiral of closely packed membranes, with a periodicity of approximately 14 nm in conventional transmission EM ( Figure 1 ; Figure 6) and slightly greater (18 nm) in fresh nerves by X-ray diffraction. The apposed membranes form the intraperiod line; the thin space that separates the membranes is contiguous with the extracellular space. The major dense line is contiguous with the cytoplasm. Figure 5. Schematic PNS myelinated axon; the proteins of CNS and PNS myelin sheaths. (a) One myelinating Schwann cell has been unrolled, revealing the regions that form noncompact myelin – the incisures and paranodes. Adherens junctions are depicted as two continuous (purple) lines; these form a circumferential belt and are also found in incisures. Gap junctions are depicted as (orange) ovals; these are found between the rows of adherens junctions. The nodal, paranodal, and juxtaparanodal regions of the axonal membrane are colored blue, red, and green, respectively. (b) CNS and PNS myelin sheaths contain distinct sets of proteins. In the PNS, compact myelin contains P 0, PMP22, and MBP; in the CNS, it contains PLP, MOBP, and MBP. In the PNS, the noncompact myelin contains E-cadherin, MAG, and Cx32; in the CNS, it contains Cx32 and claudin-11. Note that P 0 and MAG have extracellular Ig-like domains (semicircles), and that claudins, PLP, PMP22, and Cx32 all have four transmembrane domains. Adapted from Scherer SS and Arroyo EJ (2002) Recent progress on the molecular organization of myelinated axons. Journal of the Peripheral Nervous System 7: 1–12, with permission of The Peripheral Nerve Society. Figure options Figure 6. The localization of PNS and CNS myelin proteins in compact myelin. (Left) Electron micrograph of compact (PNS) myelin, which consists of alternating layers known as the intraperiod line (actually a double line) and the major dense line. (Right) Schematic depiction of how apposed cell membranes create the intraperiod and major dense lines. The disposition of P 0 (tetramers), PMP22 (dimers), PLP, and MBP monomers as well as the glycolipids galactocerebroside and sulfatide are shown. The approximate thickness of the lipid bilayers (based on X-ray diffraction), as well as the intracellular and extracellular spaces, is indicated for PNS myelin. Adapted from Scherer SS and Arroyo EJ (2002) Recent progress on the molecular organization of myelinated axons. Journal of the Peripheral Nervous System 7: 1–12, with permission of The Peripheral Nerve Society. Figure options Compact myelin is largely composed of lipids, mainly cholesterol and sphingolipids, including galactocerebroside and sulfatide. The targeted disruption of the genes required for the synthesis of sulfatide ( cst null mice) or galactocerebroside and sulfatide ( cgt null mice) provided the unprecedented opportunity to perturb the lipid components of myelin. Surprisingly, except for the splitting of some central nervous system (CNS) myelin sheaths, compact myelin has a normal ultrastructure. Despite this normal appearance, axonal conduction velocity is dramatically slowed, which is likely related to abnormal paranodes. The main myelin proteins of compact myelin are myelin protein zero (P 0 ), peripheral myelin protein 22 k Da (PMP22), and myelin basic protein (MBP). P 0 is by far the most abundant and is the main adhesive molecule in compact myelin. It is an immunoglobulin (Ig)-like cell adhesion molecule (CAM); its extracellular domain forms tetramers in the plane of the membrane. As shown in Figure 5 ; Figure 6, these tetramers interact with each other both in cis (in the plane of the membrane) and in trans (from apposed membranes). The Schwann cells in Mpz null mice still form a multilamellar spiral of membrane around axons, but the myelin is not compact. Mice that are heterozygous for a null Mpz allele also develop a late-onset demyelinating neuropathy, indicating that a reduction in P 0 causes instability of compact myelin. Thus, one of the ways that mutations in the human MPZ gene could cause demyelination (the disease is called CMT1B) is by haplotype insufficiency resulting in a dose-related decrease in the amount of P 0 protein. PMP22 is a hydrophobic, intrinsic membrane protein of unknown function, and it is much less abundant than P 0. Nevertheless, the amount of PMP22 in compact myelin is critical. Humans with three (instead of two) copies of the PMP22 gene have CMT1A, the most common inherited demyelinating neuropathy, probably because there is slightly more PMP22 in compact myelin. Conversely, humans with one copy of the PMP22 gene have a different kind of inherited demyelinating neuropathy – hereditary neuropathy with liability to pressure palsies – probably owing to a modest decrease of PMP22 in compact myelin. The complete absence of PMP22, which has been created in Pmp22 null mice (but has not been described in humans), leads to a severe demyelinating neuropathy, further demonstrating an essential role of PMP22 in compact myelin. MBP is a cytoplasmic protein, and it comprises the bulk of the major dense line of compact myelin ( Figure 1 ; Figure 6 ), as shown by its absence in shiverer mice, which completely lack MBP because of a recessive mutation in the Mbp gene. PNS myelin is not affected in shiverer mice at least in part due to the basic cytoplasmic domain of P 0, which may effectively substitute for MBP, as demonstrated by the phenotype of mice that lack both MBP and P 0. These data demonstrate that both P 0 and PMP22 play essential roles in myelinating Schwann cells, and that a modest reduction in P 0 or PMP22 or a modest increase in PMP22 results in demyelination. Because perturbations in the stoichiometry of any one component appear to alter the integrity of entire structure, compact myelin has been likened to a liquid crystal. How lipids, PMP22, and the other minor intrinsic membrane proteins ‘fit’ into the lattice of P 0 tetramers remains to be determined. Noncompact Myelin As depicted in Figure 5 ; Figure 7, noncompact myelin is found in the paranodes, the lateral borders of the trapezoid, and in Schmidt–Lanterman incisures (the funnel-shaped interruptions in the compact myelin). Noncompact myelin contains adherens junctions, tight junctions, and gap junctions; these join adjacent layers of the myelin sheath. Some of these junctions are also found in the inner and/or outer mesaxons, where the innermost or outermost layers of the myelin membrane contact the adjacent layer. Figure 7. Tight junctions, adherens junctions, and gap junctions in PNS myelin sheaths. These are teased fibers that have been immunostained for claudin-19, E-cadherin, connexin32 (Cx32), and neurofilament heavy (NF-H), as indicated; DAPI is a nuclear counterstain. Claudin-19 and E-cadherin labeling are found in incisures (arrowheads) and mesaxons (arrows). Claudin-19, E-cadherin, and Cx32 labeling are found at paranodes (nodes are marked by apposing arrowheads). Scale bars = 10 μm. Figure options Adherens junctions (or ‘desmosome-like’ junctions in the older literature) typically span many layers and are most prominent in the outer layers of the myelin sheath ( Figure 5 ; Figure 7 ). The adherens junctions of myelinating Schwann cells are composed of E-cadherin, perhaps the most prevalent member of the cadherin family. E-cadherin is a Ca 2+ -dependent CAM that forms ‘strand dimers’ that bind homophilically with those on the apposing membrane (in trans ). The cytoplasmic domain of E-cadherin binds p120 catenin and β -catenin; the latter links E-cadherin to the actin cytoskeleton. The role of adherens junctions is uncertain because deleting the E-cadherin gene in myelinating Schwann cells does not have an obvious effect on myelin sheaths. On the other hand, expressing a dominant negative p120 in myelinating Schwann cells has been reported to result in a loss of incisures. Tight junctions limit the diffusion of ions and small molecules between apposed cell membranes, the paracellular pathway ( Figure 5 ; Figure 7 ). A family of molecules, the claudins, are the essential functional component of tight junctions. In myelinating Schwann cells, claudin-19 forms the tight junctions between the adjacent paranodal loops as well as at the inner and outer mesaxons; it is unsettled whether claudin-5 forms the tight junctions in some incisures. Claudins have a PDZ binding domain, and claudin-19 (and claudin-5) may bind to PATJ, MUPP1, ZO-1, ZO-2, and Par-3, all of which are PDZ proteins localized to paranodes and/or incisures. Except for the lack of tight junctions, the PNS myelin sheaths of claudin-19 null mice appear normal. The mice have subtle motor findings, but a subset of myelinated axons have slowed conduction, indicating that the tight junctions between the layers of the myelin sheath contribute to the normal propagation of action potentials. Curiously, no neurologic phenotype has been noted in humans with homozygous recessive mutations in claudin-19. Small gap junction plaques had been noted between the rows of tight junctions in the PNS myelin sheath by freeze-fracture EM for decades. Their obscurity ended when mutations in GJB1 were discovered to cause CMT1X, an inherited demyelinating neuropathy. GJB1 encodes connexin32 (Cx32), which belongs to a family of approximately 20 connexin genes that encode gap junction proteins. Gap junctions formed by Cx32 are found in the paranodes and incisures of myelinating Schwann cells ( Figure 5 ; Figure 7) and appear to provide a direct pathway for the diffusion of ions and small molecules across the layers of the myelin sheath. Mutations in GJB1 may disrupt the diffusion of small molecules across the myelin sheath. Cx29 (the human ortholog is Cx31.3) is also localized to paranodes and incisures, but its functional role is unclear because it does not appear to form functional gap junctions on its own and does not appear to interact with Cx32. The CNS Myelin Sheath Unlike Schwann cells, which generate only one myelin sheath, each oligodendrocyte typically makes multiple myelin sheaths; the number varies from tract to tract and appears to relate to the caliber of the axons. Oligodendrocytes make fewer sheaths in tracts containing large myelinated fibers, which is the result of axon–oligodendrocyte interactions rather than an intrinsic trait of the oligodendrocytes themselves, as shown by transplantation studies. Oligodendrocytes also differ from Schwann cells in that their myelin sheaths lack a basal lamina, and their incisures lack distinguishing molecular markers. Another key difference is that oligodendrocytes are coupled to astrocytes (but not to themselves) by gap junctions. As depicted in Figure 8, astrocyte/oligodendrocyte (A/O) gap junctions are probably formed by two discrete pairs of connexins – Cx43/Cx47 and Cx30/Cx32. Astrocytes express Cx43 and Cx30; oligodendrocytes express Cx47 and Cx32. Cx47 is mainly found on oligodendrocyte somata, whereas Cx32 is also found on the outer aspects of myelin sheaths. There is compelling genetic evidence that A/O coupling is essential because recessive mutations in GJA12/Cx47 cause a devastating CNS dysmyelinating disease in humans known as Pelizaeus–Merzbacher-like disease, and mice that lack both Cx32 and Cx47 develop severe CNS demyelination. The functional role of these A/O junctions is incompletely understood, but buffering the K + that is released during neural activity has been implicated. In addition, Cx32 is localized to the paranodal region of CNS myelin sheaths, although not as prominently as in PNS myelin sheaths. Figure 8. Astrocyte/oligodendrocyte coupling. This figure shows that astrocytes (A) are coupled to astrocytes and oligodendrocytes (O) but not to neurons (N). Note that oligodendrocytes are not coupled to themselves and that A/A junctions are homotypic (Cx30/Cx30 or Cx43/Cx43), whereas A/O junctions are heterotypic (Cx30/Cx32 or Cx43/Cx47). Adapted from Rash JE, Yasumura T, Dudek FE, and Nagy JI (2001) Cell-specific expression of connexins and evidence of restricted gap junctional coupling between glial cells and between neurons. Journal of Neuroscience 21: 1983–2000, with permission of The Society for Neuroscience. Figure options CNS myelin sheaths are polarized. In addition to Cx32 and Cx47, their outer cell membrane contains myelin–oligodendrocyte glycoprotein (MOG), and their inner cell membrane contains myelin-associated glycoprotein (MAG) and Nogo ( Figure 3 ). The molecular components of compact myelin in the CNS myelin sheaths partially overlap with those of the PNS ( Figures 5 and 6 ). Both contain high amounts of lipids, especially cholesterol and sphingolipids, including galactocerebroside and sulfatide. P 0 and PMP22 are not present in CNS compact myelin; instead, proteolipid protein (PLP) and its alternatively spliced isoform, DM20, are the main proteins. Conversely, PLP is not present in PNS myelin, although DM20 is present in noncompact PNS myelin. Both PLP and DM20 are highly hydrophobic, intrinsic membrane proteins. Furthermore, either deletion or duplication of the human PLP gene results in CNS dysmyelination (Pelizaeus–Merzbacher disease), demonstrating again the importance of gene dosage for myelin proteins. Remarkably, CNS myelin appears nearly normal in Plp null mice – a finding that underscores our lack of knowledge about the function of PLP and DM20. As shown in Figure 5, the major dense line contains MBP and myelin–oligodendrocyte basic protein (MOBP). As demonstrated by the severe dysmyelinating in shiverer mice, MBP plays an essential role in CNS myelin, whereas CNS myelin appears normal in Mobp null mice. CNS myelin has a distinctive structural feature that is not seen in PNS myelin – the so-called ‘radial component.’ This is a series of radially arranged, intralamellar tight junctions that span the thickness of the compact myelin, usually in a single sector ( Figure 9 ). These tight junctions are composed of claudin-11, which also forms tight junctions between the paranodal loops. Like claudin-19 null mice, the neurological findings in claudin-11 null mice include mild conduction slowing. In contrast to mice lacking either PLP or claudin-11 alone, compact myelin in mice lacking both PLP and claudin-11 is decompacted, indicating that both of these proteins stabilize its structure. Figure 9. Claudin-11 is localized to the radial component of compact myelin. (a) Freeze-fracture EM of a mouse brain. A series of interlamellar strands (arrows; enlarged in the inset) are found between each layer of the compact myelin sheath. (b and c) Immunoelectron microscopy localizes claudin-11 (arrows) to the interlamellar strands of freeze-fractured mouse brain (b) and optic nerve (c). The asterisks indicate axons. Reproduced from Morita K, Sasaki H, Fujimoto K, Furuse M, and Tsukita S (1999) Claudin-11/OSP-based tight junctions of myelin sheaths in brain and Sertoli cells in testis. Journal of Cell Biology 145: 579–588, with permission of The Rockefeller University Press. Figure options Axonal Specializations at Nodes of Ranvier Despite the differences between PNS and CNS myelin sheaths, the organization of the nodal region is quite similar. Transmission EM of the nodal axolemma reveals an electron-dense undercoating and scanning EM reveals a high density of large intramembranous particles (1200 μm 2 ). These large particles are mainly voltage-dependent Na + (Na V) channels, which comprise a family of ten genes; however, Na V 1.6 appears to be the main one because it is expressed at all mature nodes. The gene encoding Na V 1.6 is mutated in mice with a recessively inherited motor endplate disease ( med ), which die of respiratory failure approximately 20 days after birth. Na V 1.6 is probably not the only nodal Na V channel because nerve conduction velocity of sensory axons is only minimally slowed in med mice; Na V 1.2 and Na V 1.8 are the best substantiated. The actual Na V channel is composed of a single α subunit – a large, glycosylated, polytopic protein composed of four homologous domains. Mature channels are composed of one α and two β subunits ( Figure 10 ); the latter are members of the Ig CAM superfamily. Four genes encode different β subunits; β 1 and β 3 are more closely related to each other than to β 2 or β 4. Both β 1 and β 2 have been localized to nodes; whether β 3 or β 4 are localized to nodes remains to be determined. In addition, it is unknown whether different combinations of β subunits have different interactions with, and thus different effects on, Na v 1.2, Na v 1.6, and Na v 1.8. Figure 10. The molecular organization of the nodal region. Schematic drawing of possible cis and trans interactions between the molecular components of nodes, paranodes, and juxtaparanodes. The pathways for diffusion of molecules through the paranodal tight junctions and the septate-like junctions are shown by the small and large red dotted lines, respectively. Adapted from Scherer SS and Arroyo EJ (2002) Recent progress on the molecular organization of myelinated axons. Journal of the Peripheral Nervous System 7, 1–12, with permission of The Peripheral Nerve Society. Figure options A variety of functions have been proposed for β subunits. In transfected cells, β subunits increase the delivery of α subunits to the cell membrane and can alter their electrophysiological characteristics. As CAMs, β subunits have multiple potential interactions with various nodal components: β 1 and β 2 can also interact homophilically and heterophilically with the extracellular matrix molecules tenascin-C and tenascin-R; β 1 and β 3 (but not β 2) can interact in cis with neurofascin and Nr-CAM; and β 1 (but not β 2) can interact with contactin and phosphacan, a secreted isoform of receptor protein tyrosine phosphatase β (RPTP β ). Furthermore, the cytoplasmic domains of β 1 and β 2 can recruit ankyrin-G. It remains to be determined which of these potential interactions play important roles at nodes of Ranvier, but the findings that β 2 is required for normal density of nodal Na v channels in vivo and that β 4 interacts with Na V 1.6 to produce the ‘resurgent’ current that enables action potentials to be generated at high rates are provocative. Two members of the L1 family of CAMs, neurofascin186 and Nr-CAM, are found in the nodal axolemma. Both bind to the ankyrin-G isoforms that are found at nodes (ankyrin-G 480 and 270 k Da). The binding of ankyrin-G 480/270 k Da to neurofascin can be modulated since phosphorylation of a highly conserved tyrosine in neurofascin abolishes binding. As depicted in Figure 10, these ankyrin-G isoforms also interact with the α subunits of Na V channels and with the K + channels KCNQ2 and KCNQ3. Like Na V channels, these K + channels are localized to nodes and axon initial segments; KCNQ2 appears to be localized to all nodes and initial segments, whereas KCNQ3 is localized to a subset. KCNQ2 and/or KCNQ3 likely produce the slow K + current (K s) that has been recorded from patches of nodal membrane. The importance of K s is shown by a dominant mutation in the human KCNQ2 gene; the mutant KCNQ2 reduces the activity of both KCNQ2 and KCNQ3, and it produces repetitive firing of motor axons (known as neuromyotonia or neuromyokymia). Both KCNQ2 and KCNQ3 have an amino acid sequence in their C-terminus that is strikingly similar to the sequence (found in the intracellular loop between the second and third domains) of Na V channels that binds to ankyrin-G; this sequence is necessary and sufficient for binding to ankyrin-G 480/270. Ankyrins are a family of three genes, each with extensive alternative splicing. The ankyrin-G 480/270 isoforms found at nodes and initial segments are distinguished by ANK repeats, a spectrin-binding domain, and a serine/threonine-rich domain. Ankyrins interact with the spectrin cytoskeleton, a lattice composed of α and β heterodimers that are localized just beneath the cell membrane. There are two α and four β spectrin genes, leading to many potential combinations. One particular splice variant of spectrin β IV ( β IVΣ1) is specifically localized to initial segments and nodes, and it presumably binds ankyrin-G 480/270. Evidence for a role for spectrin β IV in this regard comes from the analysis of quivering mice, in which recessive mutations in the spectrin 4 gene cause altered ion channel distributions in myelinated axons. These data indicate that ankyrin-G 480/270 is a multivalent protein and links CAMs (neurofascin186, Nr-CAM, and β subunits of Na V channels) and ion channels (Na V, KCNQ2, and KCNQ3) to the spectrin cytoskeleton at nodes. In keeping with this idea, inactivation of the ankyrin-G gene in the cerebellum results in the mislocalization of neurofascin186, Nr-CAM, Na V 1.6, KCNQ2, KCNQ3, and spectrin β IVΣ1 in the initial segments of Purkinje cells. Since initial segments and nodes share many molecular characteristics, the nodal membranes of Purkinje cells should be similarly affected. Glial Specializations at Nodes of Ranvier The lateral borders of the Schwann cell cytoplasm are tipped with microvilli ( Figure 5 ; Figure 10 ). These contain F-actin and the ERM proteins erzin, radixin, and moesin, which probably link intrinsic membrane proteins such as syndecan-3 and -4 to the actin cytoskeleton. Electron microscopists have long known that the tips of these microvilli contact the nodal axolemma, indicating that there are molecular interactions between the two cell types. These interactions likely begin in developing nerves, as clusters of Na V channels are found at the lateral edges of developing myelin sheaths; the clusters (heminodes) of two adjacent internodes fuse to form a node. Furthermore, clusters of neurofascin and Nr-CAM appear before those of ankyrin and Na V channels. An intrinsic membrane protein called gliomedin has been found based on its ability to bind to both neurofascin and Nr-CAM. Gliomedin is expressed by Schwann cells, localized to their microvilli, appears before ankyrin and Na V channels, and is both necessary and sufficient to cluster neurofascin186, Nr-CAM, ankyrin-G, and Na V channels. Thus, Schwann cells appear to induce the formation of nodes via their focal expression of gliomedin. Dystroglycan also appears to play a role at nodes because deleting dystroglycan in Schwann cells results in disorganized microvilli, reduced density of Na V channels, and slowed conduction. In the CNS, the processes of ‘perinodal astrocytes’ (astrocytes and glial progenitors) have been postulated to have a similar function as Schwann cell microvilli. These processes express tenascin-R, which can bind to Na v channels and alter their electrophysiological properties; this may be the reason why myelinated optic axons in mice lacking tenascin-R have slowed conduction. Although CNS nodes contain RPTP β, which can interact with tenascin-R and both the α and the β subunits of Na v channels, the distribution of nodal Na v channels as well as conduction velocity of CNS myelinated axons are normal in RPTP β -deficient mice. Both tenascin-R and RPTP β also interact with the high-molecular-weight form of contactin, which (in contrast to the PNS) is found at CNS nodes. Contactin regulates the surface expression of Na v channels, and mice lacking contactin have reduced nodal Na v channels clusters. Specializations at Paranodes At the paranode, the lateral edge of the myelin sheath contacts the axon; a loop of cell membrane slightly indenting the axon as it spirals around it, forming the axoglial junctions ( Figure 11 ). For most myelin sheaths, the entire lateral edge remains in contact with the paranodal axolemma; some of the paranodal loops of the largest PNS myelin sheaths, however, do not touch the axonal membrane. A unique specialization known as septate-like junctions (or transverse bands) is found at axoglial junctions; freeze-fracture EM reveals that these are composed of rows of large particles on the glial membrane that are in register with a double row of smaller particles on the axolemma. These junctions not only resemble invertebrate septate junctions but also contain similar molecules that are expressed by the axon or the myelinating Schwann cell: Contactin (a GPI-linked protein) and contactin-associated protein (Caspr, an intrinsic membrane glycoprotein) are localized to the paranodal axolemma, and neurofascin-155 (an intrinsic membrane protein) is localized to the paranodal loops. There is some uncertainty regarding whether neurofascin-155 binds to contactin or to contactin–Caspr heterodimers, but eliminating any of these three proteins in mice demonstrates that all of them are essential for the formation of septate-like junctions. Gene knockout studies in mice also show that galactocerebroside and sulfatide, as well as the tetraspanin protein CD9, are also required for septate-like junctions; all are expressed by myelinating Schwann cells and may be required to deliver and/or stabilize neurofascin-155 to septate-like junctions. In the absence of septate-like junctions, the paranodal loops of myelinating Schwann cells are slightly separated from the axolemma, whereas those of oligodendrocytes are often detached. Figure 11. The ultrastructure of the paranode. (a) A transmission electron micrograph showing a CNS paranode in longitudinal section. The terminal loops of the myelin sheath indent the axonal membrane, to which they are connected to septate-like junctions. (b) A transmission electron micrograph showing lanthanum outlining the space between the septate-like junctions of CNS axons. The arrows indicate the locations of the spaces between adjacent glial loops; these spaces are larger than the spaces between adjacent rows of septate-like junctions. (c) A longitudinal freeze-fracture EM through a CNS paranode. The P face of the paranodal axolemma (Al P) shows scalloping (arrows) produced by the pockets of paranodal cytoplasm that are visible at the sides (P). The fracture plane then passes across the axon (Ax) so that the B face (Al E) of the axolemma on the other side comes into view. (a) Reproduced from Hirano A and Dembitzer HM (1967) A structural analysis of the myelin sheath in the central nervous system. Journal of Cell Biology 34, 555, with permission of Rockefeller University Press. (b) Reproduced from Hirano A and Dembitzer HM (1982) Further studies on the transverse bands. Journal of Neurocytology 11, 861–866, with permission of Elsevier. (c) Reproduced from Peters A, Palay SL, and Webster H de F (1991) The Fine Structure of the Nervous System, 3rd edn. New York: Oxford University Press, with permission from Oxford University Press. Figure options The cytoplasmic domain of Caspr contains a binding site for protein 4.1, which in turn can bind to the spectrin cytoskeleton. Of the family of 4.1 proteins, only 4.1B has been found in axons, and it is found throughout the internode but not at nodes ( Figure 10 ). The cytoplasmic domain of Caspr also contains a SH3 binding site, which could allow Caspr to participate in signaling pathways within the axon. Septate junctions function similarly to vertebrate tight junctions, forming intercellular junctions that prevent the diffusion of small molecules and ions. Because septate-like axoglial junctions do not prevent the diffusion of lanthanum ( Figure 11) or microperoxidase into the periaxonal space, they are not ‘tight’ in the conventional sense. In addition to limiting the diffusion of large molecules, paranodal septate-like junctions have also been shown to sequester Kv1.1 and Kv1.2 K + channels. In the absence of septate-like junctions, erstwhile juxtaparanodal Kv1.1/Kv1.2 channels occupy the paranodal region, thereby apposing nodal Na V channels, leading to current shunting and the slowing of action potential propagation. Specializations at Juxtaparanodes By freeze-fracture EM, the axolemma in the region extending 10–15 μm from the paranode contains clusters of five or six particles. The distribution of these juxtaparanodal particles corresponds to the distribution of delayed rectifying K + channels, likely composed of Shaker -type Kv1.1 and Kv1.2, which mix in varying proportions to form tetramers, the functional channels. The size of the particles (10 nm in diameter) compares well to the expected size of K + channels. Each Kv1.1 and 1.2 subunit is associated with a Kv β 2 subunit. Although they appear to be concealed under the myelin sheath, juxtaparanodal Kv1.1/Kv1.2 channels are thought to have an important physiological function, dampening the excitability of myelinated fibers. The finding that Kcnac1 / Kv1.1 null mice have abnormal impulse generators near the neuromuscular junctions supports this idea. Similarly, mutations in the human KCNAC1 gene cause a form of familial episodic ataxia that is associated with ectopic impulse generators in the distal aspects of motor axons, causing neuromyotonia/neuromyokymia. Caspr2 and TAG-1 (transiently expressed glycoprotein-1, an Ig CAM) are also found in the juxtaparanodal axolemma in both the CNS and the PNS. Caspr2 has a similar domain structure to Caspr, especially in the extracellular region; its intracellular C-terminus contains a band 4.1 binding domain, a PDZ binding domain, but not an SH3 domain ( Figure 10 ). Caspr2 co-localizes and even co-immunoprecipitates with Kv1.1 and Kv1.2, perhaps because they all bind to a common PDZ protein. Although PSD95 is localized to juxtaparanodes, Caspr2, Kv1.1, Kv1.2, and Kv β 2 are not mislocalized in mice that lack PSD95. Rather, cis interactions of Caspr2 with homodimers of TAG-1 may localize the complex of Caspr2/Kv1.1/Kv1.2/Kv β 2 to juxtaparanodes because TAG-1 can form homodimers with TAG-1 on the adaxonal membrane of the myelin sheath ( Figure 10 ). In keeping with this model, Kv1.1 and Kv1.2 are mislocalized in mice that lack either TAG-1 or Caspr2 but not in mice that lack Kv1.1, Kv1.2, or Kv β 2. The adaxonal Schwann cell membrane in the juxtaparanodal region contains hexamers (12 nm in diameter) by freeze-fracture EM. These particles have the appearance of hemichannels and are composed of Cx29. The importance of these putative hemichannels is unknown because the myelinated axons in cx29 null mice appear normal, and how such hemichannels might function is a matter of speculation. Because TAG-1 and Cx29 are unlikely to interact directly, there must be other proteins that mediate this interaction ( Figure 10 ). The Internodal Region The apical/adaxonal surface of the myelin sheath apposes the axon and is highly enriched in MAG in both the PNS and the CNS ( Figure 3 ). MAG can bind to certain complex gangliosides, including GD1a and GT1b, but it remains to be shown that these are preferentially distributed on the internodal axonal membrane. In the CNS, the adaxonal membrane is also enriched in the Nogo, which may bind to the Nogo receptor on the internodal axonal membrane. The internodal axonal membrane lacks the conspicuous structural specializations of the nodal region. Nevertheless, at least one important molecule, Na,K-ATPase, is distributed rather uniformly throughout the internode of myelinated axons, including the juxtaparanodes. Na,K-ATPase is composed of an α and a β subunit; the α subunit exchanges intracellular Na + for extracellular K +. In the PNS, myelinated axons express α 1, α 3, or a combination of the two; in the CNS, myelinated axons express α 3. How α subunits are tethered to the internodal axonal cytoskeleton is unknown; in other cell types, Na,K-ATPase interacts with ankyrins. In the PNS, a tripartite strand of glial proteins – consisting of a central strand of NF155 flanked on either side by strands of Cx29 – is localized on the adaxonal Schwann cell membrane at the inner mesaxon and at the innermost aspect of incisures ( Figure 12 ). A tripartite strand of axonal proteins – consisting of a central strand of Caspr staining, flanked on either side by strands of Kv1.1/1.2/ β 2 and Caspr2 immunoreactivity – apposes the inner mesaxon (‘juxta-mesaxonal’) and the innermost aspect of incisures (‘juxta-incisural’). This precise apposition suggests that the same trans -molecular interactions between myelinating Schwann cells and axons that are found at paranodes and juxtaparanodes extend into the internodal region. In contrast, there is no mesaxon-like staining of Cx29 or NF155 in CNS myelin sheaths, and myelinated CNS axons only have internodal staining of Caspr (and not Kv1.1/1.2/ β 2 and Caspr2) at the inner mesaxon. Figure 12. Internodal specialization of myelinating Schwann cells. This is a schematic summary of the putative identities and distributions of intramembranous particles on the axonal and adaxonal Schwann cell membranes in the internodal region, as visualized by freeze-fracture electron microscopy. The location of the tight junction (TJ) at the inner mesaxon is indicated. Adapted from Stolinski C, Breathnach AS, Thomas PK, Gabriel G, and King RH (1985) Distribution of particle aggregates in the internodal axolemma and adaxonal Schwann cell membrane of rodent peripheral nerve. Journal of Neurological Sciences 67: 213–222, with permission of Elsevier. Figure options See also Demyelinating Diseases; Demyelination and Demyelinating Antibodies; Neuropathy: Peripheral; Neurotransmitter and Hormone Receptors on Oligodendrocytes and Schwann Cells; Oligodendrocyte and Schwann Cell Identification Methods; Oligodendrocyte Morphology; Schwann Cell Development; Schwann Cell Morphology; Schwann Cells and Axon Relationship; Schwann Cells and Plasticity of the Neuromuscular Junction; Transplantation of Myelin Forming Cells Further Reading Bennett and Baines, 2001 V. Bennett, A. J. Baines Spectrin and ankyrin-based pathways: Metazoan inventions for integrating cells into tissues Physiological Reviews, 81 (2001), pp. 1353–1392 [SD-008]Dyck and Thomas, 2005 P. J. Dyck, P. K. Thomas Peripheral Neuropathy (4th edn. )Saunders, Philadelphia (2005) [SD-008]Hille, 2001 B. Hille Ion Channels of Excitable Membranes Sinauer, Sunderland, MA (2001) [SD-008]Kamasawa et al., 2005 N. Kamasawa, A. Sik, M. Morita, et al. Connexin-47 and connexin-32 in gap junctions of oligodendrocyte somata, myelin sheaths, paranodal loops and Schmidt–Lanterman incisures: Implications for ionic homeostasis and potassium siphoning Neuroscience, 136 (2005), pp. 65–86 [SD-008]Lazzarini, 2004 R. A. Lazzarini Myelin Biology and Disorders Elsevier, San Diego (2004) [SD-008]Peters et al., 1991 A. Peters, S. L. Palay, H.de F. Webster The Fine Structure of the Nervous System (3rd edn. )Oxford University Press, New York (1991) [SD-008]Poliak and Peles, 2003 S. Poliak, E. Peles The local differentiation of myelinated axons at nodes of Ranvier Nature Reviews Neuroscience, 4 (2003), pp. 968–980 [SD-008]Previtali et al., 2001 S. C. Previtali, M. L. Feltri, J. J. Archelos, A. Quattrini, L. Wrabetz, H. P. Hartung Role of integrins in the peripheral nervous system Progress in Neurobiology, 64 (2001), pp. 35–49 [SD-008]Ramón and Cajal, 1928 y. Ramón, S. Cajal R. M. May (Ed. ), Degeneration and Regeneration of the Nervous System Oxford University Press, Oxford (1928) [SD-008]Salzer, 2003 J. L. Salzer Polarized domains of myelinated axons Neuron, 40 (2003), pp. 297–318 [SD-008]Scherer and Arroyo, 2002 S. S. Scherer, E. J. Arroyo Recent progress on the molecular organization of myelinated axons Journal of the Peripheral Nervous System, 7 (2002), pp. 1–12 [SD-008]Schnapp and Mugnaini, 1978 B. J. Schnapp, E. Mugnaini Membrane architecture of myelinated fibers as seen by freeze-fracture S. G. Waxman (Ed. ), Physiology and Pathobiology of Axons, Raven Press, New York (1978), pp. 83–123 [SD-008]Waxman et al., 1995 S. G. Waxman, J. D. Kocsis, P. K. Stys The Axon Oxford University Press, Oxford (1995) [SD-008]Willison and Yuki, 2002 H. J. Willison, N. Yuki Peripheral neuropathies and anti-glycolipid antibodies Brain, 135 (2002), pp. 2591–2625 [SD-008]Relevant Websitesncbi.nlm.nih.govhttp://www.ncbi.nlm.nih.gov – National Center for Biotechnology Information [SD-008]neuro.wustl.eduhttp://www.neuro.wustl.edu – Neuromuscular Disease Center of Washington University in St. Louis [SD-008]molgen.ua.ac.behttp://www.molgen.ua.ac.be – University of Antwerp, Molecular Genetics Department [SD-008]Copyright © 2009 Elsevier Ltd. All rights reserved.  CHAPTER 4 TENDONS, SKELETAL MUSCLES, AND LIGAMENTS IN HEALTH AND DISEASE Healing Mechanisms Tendon Skeletal Muscle Ligaments ANATOMY CONNECTIVE TISSUE Connective tissue has two components: cells and intra-cellular matrix. (20) The cells in unspecialized connective tissue consist of fibroblasts, macrophages, plasma cells, mast cells, fat cells, and pigment cells. Fibroblasts, which are concerned with the production of the intracellular matrix, are the most numerous. They are very active during wound repair and multiply to produce large numbers of cells embedded in granulation tissue, a mixture of fibrous tissue and capillaries. Mesenchymal cells also can give rise to new generations of fibroblasts, or fibroblasts themselves act as stem cells. A specialized contractile fibroblast, the myofibroblast, is responsible for contraction of granulation tissue in wound healing. (12)The intracellular matrix has two components: amorphous ground substance and fibrous elements. (13) The ground substance is a viscous gel containing a high proportion of water, which is bound to carbohydrate molecules and carbohydrate-protein complexes and some tropocollagen. Therefore, the carbohydrate can take the form of glycosaminoglycans (mucopolysaccharides) or glycoproteins. The fibrous elements in the intracellular matrix consist of three fiber types: collagen, reticulin, and elastin. White collagen fibers are the most numerous. They are collected into bundles, which are oriented in a straight or sinuous path. Each fiber is composed of bundles of fibrils, each fibril 20 nm to 100 nm in diameter. A fibril can be broken up into its fibrillar units of tropocollagen, which in turn can be further subdivided into three polypeptide chains that are rich in the amino acids glycine, hydroxyproline, and hydroxylysine. The biosynthesis of collagen and the involvement of fibroblasts have been studied extensively. (13)Reticulin fibers are produced by fibroblasts in most connective tissue and form the supporting framework for collagen fiber bundles. Elastin fibers rejoin freely and stretch easily with an almost perfect recoil. They are composed mainly of the protein elastin, with some glycoprotein, traces of collagen, ground substance, and lipid. (16)Connective tissues vary considerably in appearance, consistency, and composition depending on function. The variances are related to the type of cell that is predominant; the concentration, arrangement, and types of fibers; and the nature of the ground substance. Ordinary connective tissue is classified as irregular or regular. The latter includes highly fibrous tissues, such as fascia, aponeuroses, ligaments, and tendons, in which the fibers are oriented regularly in relation to one another. The fibroblasts that secrete the fibers may eventually become trapped within the fibrous structure. Old and mature fibroblasts are often termed fibrocytes. BURSAE True or anatomical bursae are connective tissue sacs lined by secretory endothelium, the synovial membrane, and containing a viscous fluid, synovia. They are interposed between moving parts and at points of unusual pressure. Thus, they are situated between tendons, ligaments and muscles, and many bony prominences, or occasionally between two tendons. Bursae are usually flattened sacs of synovial membrane supported by dense irregular connective tissue. The apposed walls are separated only by a film of synovial fluid and are attached to structures that move in relation to one another, that is, periosteum, tendon, muscle, ligament. Bursae are therefore described as being subtendinous, submuscular, subfascial, and interligamentous. The majority of bursae are close to joints, and some communicate with the joint. There are up to 11 bursae in the front legs of most dogs and 10 bursae in the hind leg. The anatomical sites are described elsewhere. (See Chapter 70.) Acquired or false bursae can develop in animals, usually between skin and bony prominences, as a result of chronic trauma. They begin as areas of hemorrhage and necrosis, which, as a result of the repeated trauma, cannot be absorbed by normal body mechanisms of repair. These areas instead become surrounded by a dense zone of fibrous tissue. The acquired bursae resemble closely the anatomical bursae; however, they are lined not by a synovial membrane but by a flattened layer of granulation tissue. Foci of hyaline cartilage can be found in the wall, which is composed mainly of dense collagenous tissue. The typical sites for acquired bursal formation are discussed elsewhere. (See Chapter 70. )TENDONS AND TENDON SHEATHS (SYNOVIAL SHEATHS) Most skeletal muscles are attached to bone or cartilage by a tendon, aponeurosis, or by direct attachment. Tendon attachment is by far the most important and is present wherever the point of insertion is distant or the muscle must exert its forces of contraction across a joint. A tendon is collagenous tissue that is highly resistant to extension, but it is relatively flexible and can therefore be angulated around bones or joints. A tendon is usually round, oval, or elongate in cross section and consists of fascicles of collagen fibers, which are embedded in a ground substance and generally run parallel to the long axis of the tendon. Connective tissue is associated with a tendon in the form of a sheath on its surface, called epitenon. This areolar tissue is continuous with the endotenon, a loose connective tissue that permeates the substance of the tendon, separating the fascicles and providing a route for vessels and nerves. The epitenon, on the surface of the tendon, is continuous not only with the endotenon, but also with loose areolar tissue, the paratenon. This loose arrangement allows minimal resistance to movement of the tendon through tissues and must be preserved if tendon gliding and function are to persist. The epitenon and paratenon contain elastic as well as irregularly arranged collagen fibers. In areas in which there is a marked change in direction of a tendon or in which a tendon passes under a ligamentous band or through a fascial sling, a tendon sheath or synovial sheath surrounds it. The tendon invaginates into the sheath to form a closed double-walled cylinder. The inner layer, the visceral layer, is closely attached to the tendon by areolar tissue. The outer layer, the parietal layer, is attached to adjacent connective tissue or periosteum. Because of this method of formation, the inner and outer layers are usually connected by an elongated mesentery, the mesotendon. The latter is important in passage of blood vessels and nerves into the tendon tissue within a sheath. Tendon sheaths, like most anatomical bursae, tend to be completely flattened structures, not saclike as they are usually described. The inner and outer surfaces actually slide over each other, lubricated by a few drops of synovia. This is clearly a device to reduce friction, but its most fundamental aspect is to produce a discontinuity between structures that must move in relation to each other. (20)Tendon sheaths, like bursae, are lined by a secretory endothelium on both the parietal and visceral surfaces. The wall of the parietal layer external to the endothelium is composed of a strong fibrous sheath. Some tendons have a more complex arrangement than bursae or sheaths, the presence of osseous or cartilaginous sesamoid bones. In general, these bones protect tendons at points at which they are bent around a bone surface. The apposed surfaces of the sesamoid and underlying bone are cartilage covered, and the whole is enveloped in a bursa or sheath. The tensile strength of tendon is similar to that of bone and is much in excess of ordinary demand. A tendon of cross-sectional area of 1 cm will support 600 kg to 1000 kg. Tendons have slightly extensile and elastic properties. Tendons have a sparse but adequate blood supply from two main sources. Small arterioles enter the tendon from adjacent muscle and run longitudinally in the interfascicular areolar tissue, accompanied by veins and Iymph vessels. These longitudinally situated vessels anastomose freely with small vessels from adjacent para-tendon tissue or synovial sheaths by means of the meso-tendon. There does not appear to be passage of vessels from bone to attached tendon. The nerve supply of tendons appears to be almost entirely afferent, and specialized afferent receptors, neurotendinous endings, exist in tendons, especially near the muscle-tendon junction. The insertion of tendons and ligaments on bone has been studied. A clear progression from tendon to fibrocartilage to mineralized fibrocartilage to bone is apparent. (4)APONEUROSES AND FASCIA Aponeuroses are flat sheets of densely arranged collagen fibers and generally consist of several layers. These broad sheets of dense connective tissue are associated with the attachment of muscle. In some muscles, the whole attachment is aponeurotic (e.g., m. obliquus externus abdominis); in other muscles an attachment can be partly tendinous and partly aponeurotic. In most cases, the aponeurosis eventually is attached to bone, providing a wide and additional attachment of muscle to a bone or bones. The term fascia is used to describe connective tissue that is collected into masses large enough to be visible grossly. Fascia occurs on the surface of muscles as investing fascia. Here it is in the form of loose areolar tissue that aids movement between structures and also allows passage of vessels and nerves. Superficial fascia occurs beneath the skin, where it facilitates movement between skin and underlying structures, allows passage of vessels and nerves, and acts as a thermal insulation. In animals, skin muscles such as platysma, in addition to varying amounts of fat, occur extensively in this fascia. The superficial fascia is bound to underlying fibrous structures, the deep fascia. This fascia is normally more compact than superficial fascia and may be indistinguishable from aponeurotic tissue. The deep fascia can almost encircle the limbs, and many muscles arise from its inner layer. In other areas of the body, deep fascia exists as intermuscular septa, and in the case of flexor tendons, localized transverse thickenings of deep fascia form retinacular or annular ligaments that encircle tendons, hold them close to the bone, and thus prevent bowing of the tendon when its muscle contracts (Fig. 4-1). In the dog, very important annular ligaments encircle the superficial and deep flexor tendons. The proximal one, lg. metacarpeum transversum superficiale or the palmar annular ligament, lies at the metacarpophalangeal joint and runs between the borders of the sesamoid bones. The middle one, the proximal digital annular ligament, is situated on the middle of the first phalanx, and the distal one, the distal digital annular ligament, is situated immediately distal to the proximal interphalangeal joint. These annular ligaments are extremely important in repair of severed flexor tendons. They must be identified and the tendon must be retained within the encircling ligament. FIG. 4-1 Muscles on the medial side of the left forearm and forepaw. (Evans HE, Christensen GC: Miller's Anatomy of the Dog, p 360. Philadelphia, WB Saunders, 1979)In the lower limb, deep fascia can be a very important component of the mechanism of venous return by enclosing veins, together with muscles, in closed compartments. When muscles contract, pressure is exerted on veins, in which blood can flow only one way as a result of the action of venous valves. LIGAMENTS Ligaments are composed of regular connective tissue in which the fibers form a thick bundle. This regular connective tissue is predominantly collagenous, but some elastic components are present. SKELETAL MUSCLES The Motor Unit and Innervation The motor unit is the functional unit of the neuromuscular system and consists of (1) a motor neuron in the ventral horn of the spinal cord; (2) the axon of that neuron leaving the cord and terminating on the motor end-plate; and (3) the muscle fibers innervated by the motor neuron. (6,20)Motor nerve fibers are admixed with sensory nerve fibers in individual peripheral nerves such that even in principally "motor" nerves about 50% of the nerve fibers are motor fibers. A single neuromuscular junction lies along the midpoint of the muscle fibers. The muscle fibers within a motor unit are supplied by a single motor neuron and its axon and constitute a field of randomly distributed cells sharing certain physiologic, biochemical, and histochemical features. The number of muscle fibers in a motor unit is thought to be related to the refinement of the movements of the muscle (e.g., extrinsic muscle of the eye has 10 muscle fibers per neuron; leg muscle up to 2000 fibers per neuron). The actual anatomical area of the muscle fibers in a single motor unit is usually round or oval, and there is considerable overlap between the area of distribution of muscle fibers of one unit and the area of another unit. These units are autonomous even though they overlap, and each unit can be stimulated with increasing voluntary effort, thus explaining the gradation of power of a muscle. Each skeletal muscle receives one or more nerves of supply. In general, the nerve enters the deep surface of the muscle near its origin, where the muscle is relatively immobile. Blood vessels accompany the nerve to enter the muscle at the neurovascular hilum. Each nerve contains both motor and sensory fibers that supply motor end-plates, vascular smooth muscle (autonomic efferents), and various afferent sensory endings in muscle fibers, neurotendinous sensory endings, and endings in fascia. When the nerve enters the muscle, it breaks up into a plexus that runs into the epimysium and perimysium before entering into the endomysium between fibers in a fasciculus. Each somatic motor axon terminates in a number of branches, each of which joins a muscle fiber at the motor endplate. THE MUSCLE FIBER The units of skeletal muscle are the muscle fibers or muscle cells, each having many hundreds of nuclei that are slender and oval. They are oriented parallel to the length of the fiber and lie peripherally in the zone immediately within the cell membrane or sarcolemma. Satellite cell nuclei lie between the basement membrane and the sarcolemma. They make up about 4% to 10% of all nuclei in the cells and may play an important role in muscle fiber regeneration. The cytoplasm of the fiber or cell is divided into longitudinal threads of myofibrils, each about 1 um in diameter. In longitudinal section, the myofibrils are seen to be traversed by striations, apparently continuous across the fiber and therefore involving many myofibrils at the same level. These cross striations are very complex and include isotropic or I bands, anisotropic or A bands, Z bands, and H bands, (6,20) further details of which will not be presented here. The number of myofibrils in each fiber or cell varies considerably depending on the size of the fiber, but on the average, each fiber contains hundreds. Myofibrils are composed of thousands of regularly arranged filaments of contractile proteins, the myosin filaments and the actin filaments (Fig. 4-2); they also contain myoglobins, glycogen, mitochondria, Iysosomes, and fat globules. The physiology and biochemistry of normal muscular contraction have been reviewed. (8,17)FIG. 4-2 Diagram showing successive levels of organization within a skeletal muscle, from whole muscle, through fasciculi, fibers, myofibrils, myofilaments, down to molecular dimensions. (Williams PL, Warwick R: Gray's Anatomy, 36th ed. Philadelphia, WB Saunders, 1980. Copyright 1980, Churchill Livingstone)Muscle fibers vary in diameter from 10 um to 100 um depending on the function of the muscle, degree of exercise, and sex (males tend to have larger fibers than females). Each fiber is elongated, and most extend from one end of the muscle to the other, achieving a length of 30 cm in long muscles. Some fibers traverse only part of the length of a muscle and end in connective tissue within the muscle body. In humans and experimental animals, two types of muscle fibers have been defined based on their physiochemical characteristics and speed of contraction. The two fiber types are intermingled within a muscle, and the predominance of one over the other depends on the function of the muscle. Type I fibers, or slow-twitch fibers, are known as red fibers and are capable of a slow twitch and sustained or weight-bearing action. Type II fibers, or fast-twitch fibers, are paler (white fibers) and are capable of sudden action. Muscle fibers are arranged in bundles (fasciculi or myonemes) of various sizes and patterns. Connective tissue, or endomysium, fills the space between fibers in a fasciculus. Each fasciculus or bundle is surrounded by a strong connective tissue sheath, the perimysium. The strong connective tissue sheath, the epimysium, surrounds the muscle body and is continuous internally with the other perimysial septa and externally with the connective tissue of surrounding structures (Fig. 4-3). MUSCLE FORM Muscles vary widely in size, shape, and complexity (Fig. 4-4). (13) They may be grouped according to orientation of their fasciculi or bundles, which can be parallel, oblique, or spiral relative to the final direction of pull at their attachments. Where the fasciculi are approximately parallel to the line of pull, muscle bodies vary from flat, short, and quadrilateral to long, straplike muscles in which the individual muscle fibers often pass from one end to the other. Some straplike muscles have transverse tendinous intersections at intervals along the muscle belly (m. rectus abdominus). In fusiform-shaped muscles, there are similar, approximately parallel fasciculi. FIG. 4-3 Schematic drawing of connective tissue sheaths. (De Girolami U, Smith, TW: Pathology of Skeletal Muscle Disease, Am J Pathol Teaching Monograph, 1982) pp 235-271Where fasciculi are oblique to the line of pull, muscle shapes are triangular or pennate. The latter can be unipennate, bipennate, or multipennate. Because the line of force of muscle contraction is oblique to the line of pull of the tendons of insertion of the muscle, range of movement can be restricted, but considerable force of contraction can be developed. Spiral muscles with fasciculi that are spiral relative to the final direction of pull very often tend to "despiral" when they contract. MUSCLE ACTION As stated previously, the rapid, repetitive synchronous stimulation of all, or even the majority of, motor units does not occur naturally, and physiological actions and movements are the result of a synchronous contraction of only a portion of available motor units. Therefore, at any one time during normal activity, the motor units in a muscle are in various physiological states, some relaxing, some undergoing stimulation, some contracting, and some quiescent. At the next point in time, the individual motor unit could well show a different activity. The proportion of units undergoing a particular activity, such as contraction, varies depending on the functional demand. Kinesiology reveals that the action of muscles is exceedingly complex. In general, there is a "prime mover" in the initiation and maintenance of a particular movement, and an "antagonist" that can appose such a movement. In general, at the beginning of a movement, there is obviously activity of the prime mover, and there is a variable transient burst of activity in the antagonist. However, the antagonist then tends to remain quiescent until the final brief deceleration. However, the activity of prime movers is not unrestrained but is balanced against the various passive, inertial, and very important gravitational effects. One situation in which the events described above do not occur is seen in activities in which a part of the body is fixed in position. In this circumstance, obviously prime movers and antagonists can contract together. In some situations, a joint can be fixed by the interplay of prime movers and the force of gravity. FIG. 4-4 The morphologic "types" of muscle based upon their general form and fasicular architecture. (Williams PL Warwick R: Gray's Anatomy, 36th ed. Philadelphia, WB Saunders, 1980. Copyright 0 1980, Churchill Livingstone)An important aspect of kinesiology is the prevention of unwanted movements by the action of synergic muscles, which can be seen to be partially antagonistic to the prime mover. For example, when a flexor muscle of the digits contracts, it can flex not only the digit (a desired activity), but also the carpus (usually an undesirable activity). Therefore, synergic muscles are activated to maintain the position of the carpus, or even to extend it. Thus the majority of complex movements are the result of a finely integrated interplay of forces between prime movers, antagonists, synergics and fixators, and other forces such as inertia and gravity. The patterns of movement are continuously monitored by afferent nerve endings and continuously synchronized by analysis by and activity of the central nervous system. Even more complex kinesiologic studies of joint and muscle mechanics have been described. (9-11)BLOOD SUPPLY Neighboring arteries provide muscular branches that usually enter the muscle together with the nerve at the neurovascular hilum. Additional arteries are usually present, and these enter the muscle at its ends. They branch into smaller arteries and arterioles, which spread into the perimysium, and these in turn give off capillaries that are situated in the endomysium. Although capillaries mainly parallel the muscle fibers, they form frequent transverse anastomoses, thus presenting a three-dimensional lattice. Numerous arteriovenous anastomoses are present in the epimysium and perimysium, and these circuits allow blood flow without passage into the endomysial capillary beds, passage that may be impeded during muscle contraction. (7)HEALING MECHANISMSTENDON PHASES OF HEALING Following surgical coaptation of a divided tendon, fibroblasts from surrounding connective tissue migrate into the wound, including the space between the tendon ends. The healing process follows the general pattern common to virtually all tissue, and the fibroblasts synthesize and discharge into the wound monomeric collagen and mucopolysaccharides needed for synthesis of mature scar. Rapid polymerization of monomeric subunits converts the initial fine reticular network immersed in a viscous ground substance into discernible fibrils and finally into a dense connective tissue scar. At this early stage, the concept of a common wound throughout the healing area is an important one (Fig. 4-5). (14) The scar is uniform in consistency and physical properties in and around the severed tendon and throughout adjacent wounded tissue such as fascia, muscle, and even dermis. In long tendons, a gliding function will be restored by selective differentiation of various parts of the scar during secondary remodeling of scar tissue. It is obvious that there are two aspects of tendon healing that distinguish healing of this tissue from healing of other tissues. The first is the tremendous tensile strength that must develop between the tendon ends. The second aspect is the development of a type of separation in the single scar such that the healed tendon can move or glide within the surrounding scar tissue. Injury is followed by inflammation, a vascular and cellular response capable of defending the body against foreign substances, of disposing of devitalized tissue, and of initiating the mechanism responsible for repair. Although wound healing has been divided into processes, usually four, in fact the changes seen after surgery and in the healing process are the processes of inflammation. It is convenient to discuss a substrate phase, a repair phase, and a maturation phase. FIG. 4-5 The sequence of events following severing of a tendon. There is an initial hematoma between the tendon ends that is continuous with the general wound hematoma in injured surrounding tissue. (Rockwood CA, Green DP (eds): Fractures. Philadelphia, JB Lippincott, 1975) p 97-118SUBSTRATE PHASE (INFLAMMATORY AND DEBRIDEMENT PROCESSES). 1. )Immediately following injury, there is vasoconstriction of small vessels in the area and gradual vascular occlusion at the point of injury. This process tends to limit bleeding; however, the effect lasts for 5 to 10 minutes and is followed by active vasodilation.2. )At the injured site, blood flows into the gap created by the cutting instrument, fills the space, and clots, uniting the edges of the wound. Fibrinogen molecules from the blood quickly link up into interconnected strands of fibrin.3. )Almost immediately after injury, vascular, cellular, and fluid changes occur in the injured area. Small vessels dilate, leukocytes begin to adhere to endothelium, particularly of venules, and definite adhesion of erythrocytes and platelets occurs. Leakage of fluid from the venules begins through obvious "gaps" in the vessel walls. Leukocytes begin to move through the vessel wall by a process of diapedesis. This process carries nutrients, antibodies, and cells into the injured or healing area.4.) Beginning about 6 hours after injury, natural debridement commences. White blood cells migrate into the wound and remove and break down cellular debris, bacteria, and other foreign material. The first of these cells are the neutrophils, which can ingest organisms by phagocytosis. In a clean wound such as the one made by a surgeon, neutrophils have few bacteria to ingest. Under these circumstances, the cells degenerate and die. At this point, the outer membranes of the neutrophils rupture, and enzyme containing granules of various sizes pour into the wound. As the enzymes are released from the granules, they attack the extracellular debris at the site of injury. Within the first 12 hours after injury, monocytes begin to migrate into the wound. On entering the wound, monocytes become macrophages, phagocytic cells that remove most of the debris from the injured area by ingesting and then partially digesting this debris. Thus, inflammatory exudate, composed of escaping fluid from vessels, migrating leukocytes, and dead tissue, accumulates in the injured area, whether or not infection is present. The surgeon wants a minimal amount of this exudate in order that debridement can continue to remove all dead tissue, and proliferation can then proceed unhindered. The factors that determine whether the exudate is minimal or enough to constitute an abscess are the extent of injury to normal tissue, the extent of the cellular reaction, and the extent to which polymorphonuclear cells accumulate and die. Within limits, the inflammatory response shows a typical "dose-response" curve in relation to the severity of the trauma; therefore, it is desirable to minimize tissue injury. In addition, the state of the local tissue determines whether a healthy inflammatory reaction can occur and whether local tissues can effectively drain away accumulating fluid. The initiating factors in the inflammatory response are not clearly defined. It does appear that intracellular materials are released when cells are injured. These substances, such as histamine, serotonin, proteolytic enzymes, and biologically active peptides known as kinins, are responsible for the vascular dilatation, the increased permeability of venules, and the chemotactic effect on leukocytes. REPAIR PHASE Repair processes commence almost immediately after injury and proceed as fast as necrotic tissue, blood clots, and other barriers are removed from the injured area. In uncomplicated simple wounds, debris is usually removed by the third to fifth day, at which time fibroblast proliferation and capillary infiltration can commence in the wound area. Many early workers postulated that wound fibroblasts arise from white blood cells. Current experiments have shown that the fibroblasts in a wound originate in undifferentiated mesenchymal cells in nearby connective tissue. The fibroblasts secrete the ground substances and collagen that form scar tissue, which eventually serves to replace the wound defect. The migrating fibroblasts appear to use the strands of fibrin as a scaffold, and the fibrin disappears coincidentally with collagen deposition. The removal of fibrin, as collagen is deposited, is due to the capillaries. The endothelial cells of new capillaries contain a plasminogen activator, such that fibrinolysis occurs and the fibrin network is broken down and removed as it is replaced by collagen. Initially, fibroblasts manufacture and secrete the protein-polysaccharides and various glycoproteins of the ground substance in the healing tissue. About the fourth or fifth day, collagen synthesis commences and continues at a very rapid rate. Thereafter a balance is reached until eventually collagen synthesis ceases. The fibroblastic phase of repair lasts 2 to 4 weeks, depending on the nature of the wound. At the end of this time, many capillaries regress and the number of synthesizing fibroblasts diminishes. The rate of total collagen synthesis decreases and eventually balances the rate of collagen destruction during the maturation phase of the wound healing. New capillaries originate as budlike structures on nearby vessels, penetrate the wound, and flow into loops that ramify through the wound. The new tissue formed by the fibroblasts and the budlike capillaries constitute what is commonly referred to as granulation tissue. In the early stages of wound repair this network of capillaries provides large quantities of oxygen for the cells that are actively synthesizing protein in the wound. Before the new capillary network forms, there is a marked variation in the amount of oxygen within the wound, with the center of the wound being the most deficient. This gradient may be partially responsible for the branching of new vessels into the region. Once the continuity of the connective tissue has been reestablished, many of the new capillaries regress. MATURATION PHASE The final process in wound healing is maturation of the scar. In a healing wound there is normally an overproduction of collagen fibers, leading to a hypertrophied scar. As fibrogenesis proceeds, purposefully oriented fibers appear to become thicker, presumably because they accrue more collagen particles. Nonpurposefully oriented fibers seem to disappear at this time. This maturation and control appear to be the result of a delicate balance between collagen production and collagen destruction. DEVELOPMENT OF STRENGTH IN A WOUND In many descriptions of wound healing, the first stage is termed the "lag phase." It has been clearly shown that from a cellular and biochemical aspect, a lag phase does not exist; however, the term can be usefully preserved for the development of strength in a wound. In terms of overall healing, the term lag phase should be replaced by the term "production" or "substrate" phase. During the lag phase of 4 to 6 days, wound strength does not increase appreciably. However, even in the first 24 hours after injury, a properly coapted wound has some strength. This is due to the formation of fibrin clot in the wound, and shortly afterwards, to some strength in the regenerating capillaries and in the newly formed ground substance, derived from serum glycoprotein that leaks into the wound from the circulation. After 4 to 6 days, strength increases significantly to reach an early maximum strength at 14 to 16 days. This phase of wound healing is associated with the rapid period of fibroplasia and production of collagen. Hydroxy- proline, a measure of collagen concentration, increases rapidly beginning at day 4, with the highest rate between days 5 and 12, a lesser rate of increase between days 12 and 21, and a markedly lower rate from day 21 to day 60. After the collagen content of a wound has stabilized, strength continues to increase as a result of cross-linking and reorientation of the already formed collagen fibers. The arrangement of collagen fibers in scar tissue is disorganized, and this is corrected to a degree during maturation; however, some disorganization persists. There is an almost imperceptible gain in strength for at least 2 years; however, the strength of the scar never reaches that of normal tissue. The development of strength in the healing tissue between tendon ends follows reorientation of the collagen fibers between the cut ends of the tendon. It is obviously this same process that leads to restoration of the gliding mechanism. In remodeling of the scar tissue, collagen fibers surrounding the healing tendon will be subjected to less linear tension than the fibers between the tendon ends. Therefore, these fibers seem to disappear and be replaced by thinner and fewer fibers. Throughout the history of tendon surgery, many methods have been used to prevent the formation of adhesions between healing tendon and surrounding healing tissue. These methods have centered on the use of artificial barriers between the tendon and surrounding tissue. Barriers have included blood vessels, fetal membranes, scar tissue, and several synthetic membranes including Millipore membrane. However, it has been shown conclusively that tendon healing is dependent upon migration of surrounding cells into the tendon defect, including fibroblasts, white cells, and cells forming capillary buds. In addition, oxygen and nutrients enter from surrounding tissue. Tendons contain longitudinally oriented blood vessels; however, these vessels are not capable of nourishing a healing tendon, and collateral flow is needed. Tendons contain some fibrocytes that appear to be inactive, and fibroblasts involved in healing come from the epitenon and paratenon and surrounding areolar tissue (15)The most important factor in promoting an active gliding mechanism after tendon healing is not the prevention of adhesion formation between the tendon and surrounding tissue, but the reduction of scar tissue to the absolute minimum. This reduction requires minimal trauma, minimal hematoma and abscess formation, and rest to allow optimal healing to occur. It is not valid to consider the concept of "breaking down" adhesions as they form. This produces further inflammation and further scar tissue formation. A healed tendon with satisfactory gliding function is characterized not by the absence of adhesions but by the presence of adhesions that have been the source of fibroblasts and blood vessels and that now appear to have a length that is adequate to allow tendon gliding. There is no sudden breaking down of tissue around a healing tendon to allow gliding; rather, there is an almost imperceptible gradual gain in gliding motion as a result of slowly developing changes in the physical properties of the connective tissue scar. Among these changes, a change in length of bundles of collagenous fibers is probably the most important. How a change in the length of fiber bundles occurs is not known, nor is it known whether longitudinal slippage occurs at a molecular, fibril, fiber, or fiber-bundle level. It seems more likely, for various reasons, that alteration occurs in the physical weave of relatively large subunits rather than in chemical cross-links. This aspect of tendon healing and maturation is discussed in depth by Peacock and Van Winkle. (14)An important but unanswered question is why, during the healing and maturation process, new tissue between the tendon ends regains great physical strength and why new tissue that forms around this can be remodeled into loose tissue resembling and acting as paratenon. One suggestion has been the various linear forces that are applied to this developing scar. Peacock and Van Winkle have offered the suggestion that newly synthesized scar tissue remodels according to the architecture of the tissue that it adjoins. If this is so, it becomes clear why minimal trauma to the outside of tendons is a prerequisite for restoration of a gliding mechanism. Excessive trauma leads to destruction of paratenon tissue around the tendon, such that remodeling scar tissue is in juxtaposition with tendon tissue, not paratenon tissue. Healing of flexor tendons involves special features not found generally in healing of extensor tendons. First, many flexor tendons are surrounded by tendon sheaths and annular ligaments over part of their strength. Secondly, flexor tendons usually require more gliding function than extensor tendons. This aspect is much more important in humans than in animals. In the human hand, the amplitude of motion in the deep flexor tendon in the midpalm region is approximately 5 cm. In the dog the same tendon in the same site acts mainly as a weight-bearing structure with important but limited gliding function. When tendon ends are left apart in loose connective tissue, the tendon stumps become atrophic and rounded. However, the ends are also embedded in the healing matrix filling the healing zone. At first this healing matrix is the gelatinous scar tissue, which eventually becomes fibrous with the production of collagen. Following production of this scar tissue, a remodeling of it occurs so that the portion between the tendon ends becomes longitudinally oriented and separates from the transversely oriented scar tissue. Thus a new tendinous structure appears to have formed in a new fibrous sheath, and it appears grossly that the bundles within the tendon have sprouted new tendon tissue in a manner similar to regeneration of a peripheral nerve. In truth this has not occurred. The new tendonlike tissue between the tendon ends is organized fibrous tissue that surrounds the rounded tendon ends and blends with the connective tissue surrounding the tendon and tendon bundles. As a result of an efficient secondary remodeling process of collagen fibrils between the tendon ends, the longitudinally oriented fibrils accrue more monomeric particles and become thicker and stronger, and fibrils that are not longitudinally oriented disappear. The collagen fibrils not between the tendon ends behave in a reverse fashion, such that transversely oriented fibrils become stronger and thicker. Thus a type of tendon sheath or paratenon is formed. The final product can be highly efficient reformation, not regeneration, of a tendon which, however, has little gliding function. The reformed tendinous tissue does not in all instances rejoin severed ends of a tendon. In many cases, it will join the tendon ends to adjacent structures such as bone or joint capsule. A very efficient supporting structure can be formed that allows weight bearing without collapse of the floot. However, the flexor muscle is no longer connected to the digit or foot by a gliding tendon. Tendons do not appear to contain cells with the potential to synthesize new collagen. Similarly, tendon sheaths do not contain these cells. When a tendon is ruptured within an intact sheath, an uncommon occurrence, the tendon ends become atrophic and rounded and do not rejoin. In the more common injury, both the tendon sheath and tendon are injured; therefore, the healing ends of the tendon are exposed to surrounding tissues that contain cells that can lead to reformation of the tendon, in the same manner as described above for tendon ends in loose connective tissue. Also, the tendon sheath can reform, and minimal gliding function can be present. The more common finding is limited gliding function, that is, many adhesions that can give good supporting function but no flexor function. In human tendon surgery, restoration of gliding function in flexor tendons is achieved with great difficulty. In general, in the region of the flexor sheaths, a decision is first made between primary repair and secondary replacement, using a graft. Secondary repair is rarely done, since restoration of gliding function is rarely achieved by this means. For primary repair to be successful in retaining gliding function, conditions must be ideal. These include a clean laceration, definitive surgical care within a few hours of injury, minimal contamination, superb facilities, and an experienced surgeon. These factors are rarely, if ever, present in animal surgery. In human tendon surgery, it is agreed that the prognosis for restoration of gliding function in flexor tendons located within a tendon sheath can be excellent in a secondary replacement procedure. The accepted rule is to attempt to make the tendon suture line away from the injured surrounding tissue. This is the basis for secondary replacement rather than secondary repair. In the latter, the tendon repair zone is in the same plane or in the surrounding scar. In secondary replacement, the initial wound is allowed to heal. A new tendon is then found to bridge the scar area, and this tendon is placed in fresh tissue away from the scarred tissue. The anastomotic sites between graft and original tendon are away from the scarred area and, incidentally, away from the flexor sheaths. Thus, the situation in humans is exceedingly complex owing to the importance of restoration of gliding function in flexor tendons. In animals the functional priorities are different, and, in general, supporting function is mandatory while gliding function has secondary significance. As described previously, if two tendon ends are left apart in connective tissue, they are rejoined by reformed tendon that has little gliding function. Unfortunately, this "healed" tendon may also have little supporting function because it is now too long. Some contraction of the reformed tendon will, however, occur with scar maturation. Satisfactory supporting function and variable gliding function will be present if the tendon ends are sutured together or held together by an external splint that holds the leg in flexion. The end result will be a scar that actually joins together the tendon ends without tendon lengthening, or a scar that joins the distal tendon stump to adjacent bone or other firm structure. In either case, excellent supporting function can be preserved. When a flexor tendon is divided within a tendon sheath in an animal, the situation should be assessed differently than in a human. It should be accepted that gliding function will be lost and that support is needed. Therefore, the choice of surgical repair is not between primary repair and secondary repair. Primary repair can involve surgical exploration with tendon suture or application of an external device to keep the severed tendon ends as close together as possible. Unfortunately, primary repair that involves suturing is subject to the same restrictions in animals as in humans, including the need for a clean laceration with minimal contamination and an experienced surgeon. These factors are rarely present. Therefore, primary surgical repair of a tendon is almost always contraindicated in animals. Instead, the wound should be treated appropriately, closed or left open, and an external device applied with the limb in flexion. It is likely that successful primary surgical repair in animals can be attributed to the use of the supplementary splints or casts, and not to the skills of the surgeon. Secondary repair of a tendon in animals is needed following failure of primary repair or when no primary care was given to the tendon. It is usually done in 2 to 4 weeks after the wound has healed. As stated previously, this is not recommended in humans because gliding function is almost invariably lost. However, it is simple and direct and can be relied upon to give good supporting function. In general, the severed tendon ends are found embedded in the general scar tissue, which may already be showing some longitudinal orientation of collagen fibrils between the tendon ends. The tendon stumps are mobilized together with the interposed connecting scar tissue, and the tendon ends are brought together so that the original tendon length is restored. In some cases, the actual stump cannot be accurately approximated because of overall shortening of the muscle and tendon. The reformed tendon tissue can then be used for anastomosis. Tendon ends can be severed transversely and accurately apposed for tendon suturing, or they can be left long by using the reformed tendon tissue and overlapped for suturing. It must be remembered that since restoration of gliding function is usually a forlorn hope, neatness at the anastomotic site is secondary to strength. The ultimate goal is restoration of tendon length for support. If this involves restoration of gliding function, so much the better. Restoration of length for support is attained either by joining the distal tendon stump to adjoining firm structures, by new tendon tissue, or by a combination of both. When primary repair by suturing is attempted, the surgeon should not attempt to repair the tendon sheath around the sutured tendon ends but should remove the fibrous sheath in the immediate vicinity of the tendon anastomosis. One of the important considerations in tendon healing is management of the sutured tendon to produce maximum strength and minimum adhesions. Early post-surgical active motion has been advocated in an attempt to prevent formation of peritendinous adhesions and to form a type of bursal lining around the tendon and on the tissue adjacent to the tendon. However, this early active motion is not desirable. The healing tendon is not able to develop a blood supply from surrounding tissue, and dense collagenous adhesions are formed. Optimal healing is achieved by a 3-week rest period following surgery. The tendon ends receive a blood supply, healing takes place, and minimal scar tissue forms around the tendon. This peritendinous scar tissue is then remodeled when motion is resumed after 3 weeks. The remodeling process, as described previously, leads to the formation of paratenon-like tissue that allows gliding. The fundamental principle in postsurgery motion is not to rupture adhesions but to encourage secondary remodeling of scar tissue. After a 3-week period of absolute rest, a sutured tendon is not sufficiently strong that active motion and full weight bearing can be allowed. Increased collagen synthesis has been measured for as long as 35 days after tendon anastomosis. After 3 weeks of immobilization, the tendon should be supported by less vigorous external splintage for at least an additional 3 weeks. Continuous absolute immobilization beyond 3 weeks is probably not desirable. It can be shown that with continued immobilization there is not significant increase in tensile strength at 5 weeks as compared with that at 3 weeks. However, if active motion is allowed after 3 weeks of immobilization, the tensile strength at 5 weeks is three times as great as that at 3 weeks. (13)SKELETAL MUSCLE Wounds of skeletal muscle occur during surgical dissection and by lacerating wounds, spontaneous ruptures, and contusions. Muscle tears and ruptures are particularly common in racing greyhounds. (5,19)Wounds of skeletal muscle can heal both by fibrosis and by regeneration of myofibrils. If the edges of the muscle wound are displaced, healing is by the usual fibrous protein synthesis and formation of scar tissue. This scar tissue has the ability to strangle the myofibrils and prevent regeneration. Between muscle ends the scar tissue can remodel and elongate to such an extent that muscle function is reduced. If the edges of a muscle wound are carefully debrided of dead and devitalized tissue (in a fresh wound) or of scar tissue (in an old wound) and are approximated by sutures, optimal healing can be obtained by fibrous tissue formation and regeneration of myofibrils. Muscle does not heal by cell division but by myofibril regeneration across the defect. Fibers on each side of the defect break up into nucleated cylinders of cytoplasm. Macrophages remove dead material but leave the basement membrane intact. The muscle fiber cylinders now fuse and grow back inside the original basement membrane to form a myotube, until eventually the two growing undamaged ends fuse and fill this gap (1-3) The surviving portion of the muscle begins to form these fine outgrowths in 6 to 7 days, and cross striations can develop in 8 to 14 days. The rate of muscle regeneration by this type of sarcoplasmic outgrowth appears to be about 1.5 mm/day. LIGAMENTS The basic concepts of ligament healing resemble those of tendon healing; however, the process is simpler, since gliding is not involved. FIG.4-6 Surgical approximation of a severed ligament leads to a ligament that more closely approximates the surgical structure and usually gives better function. (Rockwood CA, Green DP (eds): Fractures. Philadelphia, JB Lippincott, 1975) p. 97-118Under suitable conditions, injured ligaments have the ability to reform a structure that very closely approximates the original structure: connective tissue in which the fibers are regularly oriented with respect to one another to form a thick bundle. Healing is by formation of collagen and remodeling of this scar tissue. Fibroblastic activity comes from the ligament itself as well as from surrounding tissues. As in tendon healing, the "one wound" concept prevails, and healing proceeds in the wounded ligament and the wounds in surrounding structures as in a wound of a single structure. If the ligament ends are separated, a gap of irregular fibrous tissue is formed between the ends. If the ends are sutured carefully, an organized scar develops in which collagen fibers are regularly oriented. The tensile strength of this organized scar exceeds that of the irregular fibrous tissue between separated ends of a ligament. In addition, if ligament ends are allow, ed to heal in a separated mode, the resulting healed ligament is not only weaker than a sutured ligament, it is also longer. This leads to instability of the joint supported by the ligament (Fig. 4-6). REFERENCES1. Allbrook DB, Baker W de C, Kirkaldy-Willis WH: Muscle regeneration in experimental animals and in man. J Bone Joint Surg 48B, No. 1:153, 1966 2. Bischoff R: Regeneration of single skeletal muscle fibers in vitro. Anat Rec 182:215, 1975 3. Carlson BM: The regeneration of skeletal muscles: A review. Am J Anat 137: 119, 1973 4. Cooper RR, Misol S: Tendon and ligament inserhon. J Bone Joint Surg 52A, No. 1:1, 1970 5. Davis PE: The diagnosis and treatment of muscle injuries in the racing greyhound. Aust Vet J 43:519, 1967 6. De Girolami U, Smith TW: Pathology of skeletal muscle diseases. Am J Pathol Teaching Monograph, 1982 7. Grant RT, Payling Wright H: Further observations on the blood vessels of skeletal muscle (rat cremas- ter). J Anat 103:553, 1968 8. Huxley HE: Molecular basis of contracture in cross-striated muscles. In Bourne GH (ed): The Structure and Function of Muscle, vol 1, Part 1, pp 301-387. New York, Academic Press, 1972 9. Jackson KM, Joseph J, Wyard SJ: Sequential muscular contraction. J Biomech 10:97, 1977 10. Joseph J: Sequential contraction of muscles producing the same movement at a joint. In Desmedt JE (ed): New Developments in EMG and Clinical Neurophysiology, pp 665-674. Basel, Karger, 1973 11. Mac Conaill MA, Basmajiam JV: Muscles and Move- ment. New York, Krieger, 1977 12. Majno G: The story of the myofibroblasts. Am J Surg Pathol 3, No. 6:535, 1979 13. Mason ML, Allen H: The rate of healing of tendons and experimental study of tensile strength. Ann Surg 113:424, 1941 14. Peacock EE, Van Winkle W: Wound Repair, 2nd ed. Philadelphia, WB Saunders, 1976 15. Potenza AD: Detailed evaluation of healing processes in canine flexor digital tendons. Milit Med 237:34, 1962 16. Serafini-Fracassini A, Smith JW: The Structure and Biochemistry of Cartilage. Edinburgh, Churchill- Livingstone, 1974 17. Stein RB: Nerve and Muscle Membranes, Cells and Systems. New York, Plenum Press, 1980 18. Uitto J, Lichtenstein JR: Defects in the biochemistry of collagen in diseases of connective tissue. J Invest Dermatol 66: 59, 1978 19. Vaughan LC: Gracilis muscle injury in greyhounds. J Sm Anim Pract 10:363, 1969 20. Williams PL, Warwick R (eds): Gray's Anatomy,36th ed. Philadelphia, WB Saunders, 1980  Perineurium From Wikipedia, the free encyclopedianavigation search Perineurium Transverse section of human tibial nerve (perineurium labeled at upper right)Nerve structure Identifiers Me SH D010525TA A14.2.00.015FMA 52585Anatomical terminology [ edit on Wikidata]In the peripheral nervous system, the myelin sheath of each axon in a nerve is wrapped in a delicate protective sheath known as the endoneurium. Within the nerve, axons targeting the same anatomical location are bundled together into groups known as fascicles, each surrounded by another protective sheath known as the perineurium. [1] Several fascicles may be in turn bundled together with a blood supply and fatty tissue within yet another sheath, the epineurium. This grouping structure is analogous to the muscular organization system of epimysium, perimysium and endomysium. The perineurium is composed of connective tissue, which has a distinctly lamellar arrangement consisting of roughly seven to eight concentric layers. The perineurium is cellular, and is composed of perineurial cells, which are epithelioid myofibroblasts. Perineurial cells are sometimes referred to as myoepithelioid due to their epithelioid and myofibroblastoid properties including tight junctions, gap junctions, external laminae and contractility. The perineurium is a smooth, transparent tubular membrane which may be easily separated from the fibers it encloses. In contrast, the epineurium is a tough and mechanically resistant tissue which is not easily penetrated by a needle. See also [ edit]Nerve fascicle Endoneurium Epineurium Connective tissue in the peripheral nervous system References [ edit]^ Mc Cracken, Thomas (1999). New Atlas of Human Anatomy. China: Metro Books. pp. 96–97. ISBN 1-5866-3097-0. External links [ edit]Histology image: 1_03 at the University of Oklahoma Health Sciences Center - "Peripheral nerve"Anatomy photo: nervous/pns/nerve1/nerve2 - Comparative Organology at University of California, Davis - "PNS, nerve (LM, Low)"Histology image: 21401loa – Histology Learning System at Boston University Diagram at Howard Histology at ucla.edu [ hide]v t e Nervous tissue CNSTissue Types Grey matter White matter Projection fibers Association fiber Commissural fiber Lemniscus Funiculus Fasciculus Nerve tract Decussation Commissure Neuropil Meninges Cell Types Neuronal Pyramidal Purkinje Granule Glialinsulating: Myelination: Oligodendrocyteother Astrocyte Radial glial cell Ependymal cells Tanycyte Microglia PNSGeneral Dorsal Root Ganglion Ramus Ventral Root Ramus Ramus communicans Gray White Autonomic ganglion ( Preganglionic nerve fibers Postganglionic nerve fibers)Connective tissues Epineurium Perineurium Endoneurium Nerve fascicle Neuroglia Myelination: Schwann cell Neurilemma Myelin incisure Node of Ranvier Internodal segment Satellite glial cell Neurons / nerve fibers Parts Soma Axon hillock Axon Telodendron Axon terminals Axoplasm Axolemma Neurofibril/neurofilament Dendrite Nissl body Dendritic spine Apical dendrite / Basal dendrite Types Bipolar Unipolar Pseudounipolar Multipolar Interneuron Renshaw Afferent nerve fiber / Sensory neuron GSA GVA SSA SVA fibers Ia or Aα Ib or Golgi or Aα II or Aβ and Aγ III or Aδ or fast pain IV or C or slow pain Efferent nerve fiber / Motor neuron GSE GVE SVE Upper motor neuron Lower motor neuronα motorneuron β motorneuronγ motorneuron Termination Synapse Electrical synapse / Gap junction Chemical synapse Synaptic vesicle Active zone Postsynaptic density Autapse Ribbon synapse Neuromuscular junction Sensory receptors Meissner's corpuscle Merkel nerve ending Pacinian corpuscle Ruffini ending Muscle spindle Free nerve ending Nociceptor Olfactory receptor neuron Photoreceptor cell Hair cell Taste bud Categories: Neurohistology  Structure of normal skin From top to bottom, skin consists of 3 layers: Epidermis Dermis Subcutis A. Epidermis The epidermis is the uppermost or epithelial layer of the skin. It acts as a physical barrier, preventing loss of water from the body, and preventing entry of substances and organisms into the body. Its thickness varies according to body site. The epidermis consists of stratified squamous epithelium. That means it consists of layers of flattened cells. Skin, hair and nails are keratinised, meaning they have a dead and hardened impermeable surface made of a protein called keratin. Mucous membranes are non-keratinised and moist. The epidermis has three main types of cell: Keratinocytes (skin cells)Melanocytes ( pigment -producing cells)Langerhans cells ( immune cells ). Special stains are often required to tell the difference between melanocytes and Langerhans cells. The Merkel cell is a fourth, less visible, epidermal cell. The epidermis forms an undulating appearance, with intermittent regular protrusions of the epidermis layer (rete pegs) into the upper layers of the underlying dermis. In some areas of the body such as the palms and soles, the rete pegs are less pronounced. The pillars of dermis next to the rete pegs form the rete ridges. The small area of epidermis between rete pegs is called the suprapapillary plate. Keratinocytes The keratinocytes become more mature or differentiated and accumulate keratin as they move outwards. They eventually fall or rub off. They form four distinct layers, described in the table below from the most superficial to the deepest. Layer Cell type Stratum corneum ( horny layer)Called corneocytes or squames. Dead, dried-out hard cells without nuclei. Stratum granulosum ( granular layer)Cells contain basophilic granules. Waxy material is secreted into the intercellular spaces. Stratum spinulosum (spinous, spiny or prickle cell layer)Intercellular bridges called desmosomes link the cells together. The cells become increasingly flattened as they move upward. Stratum basale ( basal layer)Columnar (tall) regenerative cells. As the basal cell divides, a daughter cell migrates upwards to replenish the layer above. Immediately below the epidermis is the basement membrane, a specialised structure that lies between the epidermis and dermis. It includes various protein structures linking the basal layer of keratinocytes to the basement membrane ( hemidesmosomes) and the basement membrane to the underlying dermis (anchoring fibrils). The basement membrane has an important role in making sure the epidermis sticks tightly to the underlying dermis. The epidermis gives rise to a number of specialised appendages also called adexal structures or adnexae. Hair and nails are both examples, i.e. they are specialised structures formed by direct extension of the epidermis. The hair follicles are associated with sebaceous (oil) glands and arrector pili smooth muscle. This muscle is responsible for goose bumps appearing on the skin in response to cold. The epidermis also gives rise to eccrine (sweat) glands, a tangle of tubules deep within the dermis that secrete a watery salt solution into a duct that ends on the skin surface. Larger apocrine sweat glands are found in the armpits and groin. Different areas of the body have different proportions of the adnexal and hair follicle structures present. For example: Dense hair on the scalp and none on the palms Intense sweating from armpits, palms and soles compared with elsewhere. Melanocytes Melanocytes are found in the basal layer of the epidermis. These cells produce pigment called melanin, which is responsible for different skin colour. Melanin is packaged into small parcels (or melanosomes), which are then transferred to keratinocytes. Langerhans cells Langerhans cells are immune cells found in the epidermis, and are responsible for helping the body learn and later recognise new ‘ allergens ’ (material foreign to the body). Langerhans cells break the allergen into smaller pieces then migrate from the epidermis into the dermis. They find their way to lymphatics and blood vessels before eventually reaching the lymph nodes. Here they present the allergen to immune cells called lymphocytes. Once the allergen is successfully ‘presented’, the lymphocytes initiate a sequence of events to (1) initiate an immune reaction to destroy the material, and (2) stimulate proliferation of more lymphocytes that recognise and remember the allergen in the future. Merkel cells Merkel cells are cells found in the basal layer of the epidermis. Their exact role and function is not well understood. Special immunohistochemical stains are needed to visualise Merkel cells. B. Dermis The dermis is the fibrous connective tissue or supportive layer of the skin. The major fibres are: Collagen fibres: this type of fibre predominates in the dermis. Collagen fibres have enormous tensile strength and provide the skin with strength and toughness. Collagen bundles are small in the upper or papillary dermis, and form thicker bundles in the deeper or reticular dermis. Elastin: this type of fibre provides the properties of elasticity and pliability to the skin. The collagen and elastin fibres are bound together by ground substance, a mucopolysaccharide gel in which the nutrients and wastes can diffuse to and from other tissue components. The dermis also contains nerves, blood vessels, epidermal adnexal structures (as described above), and cells. The normal cells in the dermis include: Mast cells. These contain granules packed with histamine and other chemicals, released when the cell is disturbed. Vascular smooth muscle cells. These allow blood vessels to contract and dilate, required to control body temperature. Specialised muscle cells. For example, myoepithelial cells are found around sweat glands and contract to expel sweat. Fibroblasts. These are cells that produce and deposit collagen and other elements of the dermis as required for growth or to repair wounds. A resting fibroblast has very little cytoplasm compared with an active cell and appears to have a ‘naked’ nucleus. Immune cells. There are many types of immune cell. The role of tissue macrophages (histiocytes) is to remove and digest foreign or degraded material (this is known as phagocytosis ). There are also small numbers of lymphocytes in the normal dermis. Transient inflammatory cells or leukocytes are white cells that leave the blood vessels to heal wounds, destroy infections or cause disease. They include: Neutrophils (polymorphs). These have segmented nuclei. They are the first white blood cells to enter tissue during acute inflammation. T and B Lymphocytes. These are small inflammatory cells with many subtypes. They arrive later but persist for longer in inflammatory skin conditions. They are important in the regulation of immune response. Plasma cells are specialised lymphocytes that produce antibody. Eosinophils. These have bilobed nuclei and pink cytoplasm on H&E stain. Monocytes. These form macrophages. The skin cells communicate by releasing large numbers of biologically active cytokines and chemotactic factors that regulate their function and movement. These are too small to see on light microscopy. C. Subcutis The subcutis is the fat layer immediately below the dermis and epidermis. It is also called subcutaneous tissue, hypodermis or panniculus. The subcutis mainly consists of fat cells ( adipocytes ), nerves and blood vessels. Fat cells are organised into lobules, which are separated by structures called septae. The septae contain nerves, larger blood vessels, fibrous tissue and fibroblasts. Fibrous septae may form dimples in the skin (so-called cellulite ).  sheath Correct all you're your grammar errors instantly. Try it now.sheath Also found in: Dictionary, Thesaurus, Legal, Encyclopedia, Wikipedia. Related to sheath: cable sheath, carotid sheathsheath [ shēth] a tubular case or envelope.arachnoid sheath the delicate membrane between the pial sheath and the dural sheath of theoptic nerve.carotid sheath a portion of the cervical fascia enclosing the carotid artery, internal jugular vein,vagus nerve, and sympathetic nerves supplying the head.connective tissue sheath of Key and Retzius endoneurium.crural sheath femoral sheath.dural sheath the external investment of the optic nerve.femoral sheath the fascial sheath of the femoral vessels. Henle's sheath endoneurium.lamellar sheath the perineurium.medullary sheath myelin sheath.myelin sheath ( nerve sheath) the sheath surrounding the axon of myelinated nerve cells,consisting of concentric layers of myelin formed in the peripheral nervous system by the plasmamembrane of Schwann cells, and in the central nervous system by the plasma membrane of oligodendrocytes. It is interrupted at intervals along the length of the axon by gaps known as nodes of Ranvier. Myelin is an electrical insulator that serves to speed the conduction of nerveimpulses (see saltatory conduction ).pial sheath the innermost of the three sheaths of the optic nerve.root sheath the epidermic layer of a hair follicle.sheath of Schwann neurilemma.synovial sheath synovial membrane lining the cavity of a bone through which a tendon moves.tendon sheath a lubricated fibrous or synovial layer of tissue in which the tendon is housed andthrough which it moves. Miller-Keane Encyclopedia and Dictionary of Medicine, Nursing, and Allied Health, Seventh Edition. © 2003 by Saunders, an imprint of Elsevier, Inc. All rights reserved.sheath ( shēth ), 1. Any enveloping structure, such as the membranous covering of a muscle, nerve, or bloodvessel. Any sheathlike structure. Synonym (s): vagina (1)2. The prepuce of male animals, especially of the horse.3. A specially designed tubular instrument through which special obturators or cuttinginstruments can be passed, or through which blood clots, tissue fragments, and calculi can be evacuated.4. A tube used as an orthodontic appliance, usually on molars. [A. S. scaeth]Farlex Partner Medical Dictionary © Farlex 2012sheath ( shēth) a tubular case or envelope.arachnoid sheath the continuation of the arachnoidea mater around the optic nerve, formingpart of its internal sheath.carotid sheath a portion of the cervical fascia enclosing the carotid artery, the internal jugularvein, and the vagus nerve.crural sheath femoral s.dentinal sheath the layer of tissue forming the wall of a dentinal tubule.dural sheath the external investment of the optic nerve.femoral sheath the investing fascia of the proximal portion of the femoral vessels.sheath of Henle endoneurium. Hertwig sheath root s. (1).sheath of Key and Retzius endoneurium.lamellar sheath the perineurium. Mauthner's sheath axolemma.medullary sheath , myelin sheath the sheath surrounding the axon of myelinated nerve cells,consisting of concentric layers of myelin formed in the peripheral nervous system by the plasmamembrane of Schwann cells, and in the central nervous system by oligodendrocytes. It is interrupted at intervals along the length of the axon by gaps known as nodes of Ranvier. Myelin is an electrical insulator that serves to speed the conduction of nerve impulses.pial sheath the continuation of the pia mater around the optic nerve, forming part of its internalsheath.root sheath1. an investment of epithelial cells around the unerupted tooth and inside the dental follicle.2. the epithelial portion of a hair follicle.sheath of Schwann neurilemma.synovial sheath synovial membrane lining the cavity of a bone through which a tendon moves.tendon sheath epitendineum. Dorland's Medical Dictionary for Health Consumers. © 2007 by Saunders, an imprint of Elsevier, Inc. All rights reserved.sheath (shēth)n. pl. sheaths (shē th z, shēths)An enveloping tubular structure, such as the tissue that encloses a muscle or nerve fiber. The American Heritage® Medical Dictionary Copyright © 2007, 2004 by Houghton Mifflin Company. Published by Houghton Mifflin Company. All rights reserved.sheath [shēth]Etymology: AS, scaetha tubular structure that surrounds an organ or any other part of the body, such as the sheath of the rectus abdominis muscle or the sheath of Schwann, which covers various nerve fibers. Mosby's Medical Dictionary, 9th edition. © 2009, Elsevier.sheath ( shēth) 1. Any enveloping structure, such as the membranous covering of a muscle, nerve, or bloodvessel; any sheathlike structure. Synonym (s): vagina (1).2. The prepuce of male animals, especially of the horse.3. A specially designed tubular instrument through which special obturators or cuttinginstruments can be passed, or through which blood clots, tissue fragments, or calculi can be evacuated.4. A tube used as an orthodontic appliance, usually on molars. [A. S. scaeth]Medical Dictionary for the Health Professions and Nursing © Farlex 2012sheath (sheth)1. A covering structure of connective tissue, usually of an elongated part, such as themembrane covering a muscle.2. An instrument introduced into a vessel during angiographic procedures when multiplecatheter changes are anticipated. It facilitates ease of change and decreases morbidity at thepuncture site. Patient care The sheath introduced into the femoral artery, the preferred vascular access route forpercutaneous coronary intervention, is a 4 to 6 French (1.35 to 2 mm) in size. The sheathremains in place after completion of the procedure and removal of the catheter untilanticoagulation is reversed or anticoagulants are below peak action. The sheath is connected to high-pressure tubing and a flushing system; manual or automatic flushing keeps the linepatent. A stopcock connected to the system permits drawing of blood samples.axon sheath A myelin sheath or a neurilemma. See: myelin sheathcarotid sheath The portion of cervical or pretracheal fascia enclosing the carotid artery, interior jugular vein, andvagus nerve.crural sheath The fascial covering of femoral vessels.dural sheath A fibrous membrane or external investment of the optic nerve.femoral sheath The fascia covering the femoral vessels.sheath of Henle See: Henle sheathsheath of Hertwig See: Hertwig's root sheathsheath of Key and Retzius Endoneurium.lamellar sheath A connective tissue sheath covering a bundle of nerve fibers. Synonym: nerve sheath; perineuriummedullary sheath An obsolete term for myelin sheath. MYELIN SHEATHmyelin sheath Layers of the cell membrane of Schwann cells (peripheral nervous system) or oligodendrocytes (central nervous system) that wrap nerve fibers,providing electrical insulation and increasing thevelocity of impulse transmission. Synonym: Schwann sheath See: nerve fiber; neuron; illustrationnerve sheath Lamellar sheath.periarterial lymphoid sheath The tissue composed of T lymphocytes that surrounds each arteriole in the spleen. The sheaths areattached to lymphoid follicles containing B cells and make up much of the white pulp. See: spleenpial sheath An extension of the pia that closely invests the surface of the optic nerve.sheath of Schweigger-Seidel The thickened wall of a sheathed artery of the spleen.rectus sheath A strong fibrous sleeve in which the rectus abdominis and pyramidalis muscles contract. Thesheath is formed from the aponeuroses of the abdominal wall muscles as they meet in the lineaalba at the abdominal midline.root sheath1. One of the layers of a hair follicle derived from the epidermis. It includes the outer rootsheath, which is a continuation of the stratum germinativum, and the inner root sheath, whichconsists of three layers of cells that closely invest the root of the hair. See: hair2. The epithelial covering that induces root formation in teeth. Also called Hertwig's rootsheath.synovial sheath Synovial tendon sheath.tendon sheath A dense fibrous sheath that confines a tendon to an osseous groove, converting it into an osteofibrous canal. It is found principally in the wrist and ankle. See: synovial sheath Medical Dictionary, © 2009 Farlex and Partnerssheath1. An enveloping structure or part, usually tubular.2. A condom. Collins Dictionary of Medicine © Robert M. Youngson 2004, 2005sheath ( shēth) 1. Tube used as an orthodontic appliance, usually on molars.2. Any enveloping structure, such as membranous covering of a muscle, nerve, or bloodvessel.3. Specially designed tubular instrument through which special obturators or cuttinginstruments can be passed, or through which blood clots, tissue fragments, and calculi can be evacuated. [A. S. scaeth]Medical Dictionary for the Dental Professions © Farlex 2012sheath1. a tubular case or envelope.2. vernacular for prepuce.adrenal pericapsular sheathcontains a plexus of large nerve trunks with numerous ganglion cells.arachnoid sheaththe delicate membrane between the pial sheath and the dural sheath of the optic nerve.carotid sheatha portion of the cervical fascia enclosing the carotid artery, internal jugular vein, and vagus or vagosympathetic nerve.carpal tendon s'ssheaths to the tendons of the muscles which course over the carpus.crural sheathfemoral sheath.dural sheaththe external investment of the optic nerve.femoral sheaththe fascial sheath of the femoral vessels. Henle's sheaththe endoneurium, especially the delicate continuation around the terminal branches of nerve fibers.lamellar sheaththe perineurium.medullary sheath, myelin sheaththe sheath surrounding the axon of myelinated nerve cells, consisting of concentric layers of myelin formed in the peripheral nervous system by the plasma membrane of Schwanncells, and in the central nervous system by the plasma membrane of oligodendrocytes. It is interrupted at intervals along the length of the axon by gaps known as nodes of Ranvier. Myelin is an electrical insulator that serves to speed the conduction of nerve impulses.pial sheaththe innermost of the three sheaths of the optic nerve.root sheaththe epidermal layer of a hair follicle.sheath of Schwannneurilemma.synovial sheatha synovial membrane sleeve through which a tendon moves; found commonly where a tendon passes over a joint. Saunders Comprehensive Veterinary Dictionary, 3 ed. © 2007 Elsevier, Inc. All rights reserved Want to thank TFD for its existence? Tell a friend about us, add a link to this page, or visit the webmaster's page for free fun content . Link to this page: Facebook Twitter Finally, Farlex brings you all the rules of English grammar, all in one place, explained in simple terms. The Farlex Grammar Bookis available now in paperback and e Book formats.  Histology@ Yale Skin Lab Learning Objectives Name and distinguish the four layers of the epidermis in terms of structure and function. Identify the two layers of the dermis and the hypodermis and explain their functional significance. Describe the basic structure and function of several key epidermal derivatives. Contrast the three modes of exocrine secretion and give examples of cells that exhibit each type. Describe the structure of the mammary glands. Recognize some key pathological examples affecting skin and epidermal derivatives. Keywords This is an experimental portion of the website. Each keyword starts a script that searches for the keyword on DBpedia which is the structured data version of Wikipedia. The search returns a description to the keyword and an associated image if available. If the search does not return a results, a link to a Google search is presented.epidermis stratified squamous epitheliumkeratin dermishypodermis stratum basalestratum spinosum cytokeratinstratum granulosum keratohyalinstratum corneum melanocyte Langerhans cell Merkel cellpapillary dermis reticular dermis Meissner's corpuscle fibroblast Pacinian corpuscle hair follicleroot sheath anagencatagen telogensebaceous gland sebumarrector pili muscle eccrine sweat glandmyoepithelium apocrine sweat glandmammary gland merocrine sweat glandholocrine secretion Pre-Lab Reading Introduction The skin is the largest organ of the body and varies greatly in different regions. It has five main functions: protection, thermoregulation, sensation, metabolic functions (vitamin D, adipose metabolism), and sexual attraction. Skin is composed of several subunits. The three main divisions are:the epidermis, or surface epithelium, which is a self-regenerating stratified squamous epithelium that produces a protective protein layer of keratin.the dermis, an underlying layer of dense collagenous connective tissue that contains hair follicles, sweat glands, blood and lymphatic vessels, sensory receptors and nerves, and connective tissue cells.the hypodermis, another connective tissue layer that is rich in white adipose cells and contains large blood vessels that supply the smaller vessels of the dermis. Epidermis The epidermis is a stratified squamous epithelium that contains discrete layers of proliferating, differentiating, and differentiated cells called keratinocytes. It is divided into four layers that have different structural appearances: Basal Cell Layer - Keratinocytes begin in the deepest layer of the epidermis, the stratum basale, which is a row of columnar cells resting on the basal lamina that separates the epidermis from the dermis. Mitosis occurs exclusively at the basal cell layer and allows for the replacement of cells lost from the surface. Stratum Spinosum - After forming in the basal cell layer, keratinocytes migrate upwards into the stratum spinosum. In this layer, they develop short projections that attach via desmosomes to adjacent cells. The stratum spinosum is also known as the "prickly layer" because of these characteristic spines. The cells in this layer produce cytokeratin, an intermediate filament precursor to keratin. Stratum Granulosum - The third layer is the stratum granulosum. In this layer, the keratinocytes have become squamous cells that contain granules of keratohyaline, a precursor to the extracellular keratin that protects the skin tissue from abrasion. Stratum Corneum - The most superficial layer of the epidermis is the aceullar stratum corneum. It is the most functionally important layer of the skin and consists of flat, keratinized scales that are shed and replaced continuously. This is the layer that includes the final keratin product, which is a combination of cytokeratin and keratohyaline. Recall from the Laboratory of Epithelia that epithelia differ in their degree of keratinization - those exposed to abrasion and desiccation are heavily keratinized, but those that form mucous membranes do not have much keratin. For example, the skin is highly keratinized, but the esophagus, anal canal,and vagina are not. Instead of protection by keratin, these mucous membranes are kept moist by glandular secretions. Mucous membranes lack a stratum granulosum and stratum corneum. The epidermis contains several characteristic cell types: Melanocytes occur at intervals among the basal keratinocytes and produce melanin pigment, which is most abundant in sun-exposed skin and in areas surrounding body openings. Melanin is synthesized from tyrosine and transferred as melanin granules to the surrounding epithelial cells. While the number of melanocytes is the same in light- and dark-skinned people, they are far more active in the latter. Langerhans cells are typically located in the stratum spinosum and are the equivalent of macrophages in the skin tissue. Merkel cells are attached to keratinocytes by desmosomes and are most commonly found in highly sensitive areas like the fingertips - these serve as touch receptors. Dermis and Hypodermis The dermis consists of two layers: Papillary layer - The most superficial layer of the dermis is the papillary layer, which consists of loose connective tissue immediately beneath the epidermal basement membrane. Reticular layer - The reticular layer is composed of dense, irregular collagenous connective tissue. Most blood vessels, nerves, and sensory receptors occur in the papillary layer. This region also contains Meissner's corpuscles, which sense light touch. The hypodermis is the fatty layer beneath the dermis. It is thickest in the abdominal wall and virtually absent in the eyelid, scrotum, penis, and the dorsal side of the hand. This layer contains a significant number of fibroblasts, which synthesize collagen and elastin. The hypodermis contains Pacinian corpuscles, which sense deep touch. Epidermal Derivatives Many structures are derived from epidermal tissue. Keep in mind that just because a structure is derived from the epidermis does not mean that it is located in the epidermis. Hair follicles are encased by an invagination of the epidermis into the dermis known as the external root sheath. They contain specially organized keratin built into long tubular structures. Remember that hair follicles have generous blood and nerve supplies. There are three states of hair follicles: Anagen - Growing follicles synthesize hair. They are long and most numerous in the scalp. Catagen - Resorbing follicles are in a short phase of regression that signals the end of active hair growth. Telogen - Resting follicles contain a fully formed hair. Sebaceous glands are pear-shaped alveolar glands that secrete an oily substance called sebum, which moisturizes and waterproofs hair. They are usually attached to hair follicles near the arrector pili muscle, which allows the hair to "stand up." An extensive capillary plexus characterizes sebaceous glands. Eccrine sweat glands occur throughout most of the skin. They consist of long tubules extending from the epidermis deep into the dermis or hypodermis. The secretory portion of each gland is tightly wound and appears as a collection of cross-sectioned tubules. It is encased by myoepithelial cells, which contain actin filaments. These cells receive input from nerve fibers, which cause them to contract and expulse the sweat from the gland. Keep in mind the organs that do not have eccrine sweat glands: the glans penis, the inner surface of the foreskin, the clitoris, and the labia minora. Apocrine sweat glands are much larger than eccrine glands and produce a thicker secretion. They have straight, narrow ducts that run parallel to hair follicles and frequently open into the pilosebaceous canal. These sweat glands occur in the axilla, the areola of the nipple, the labia majora, and the circumanal region. Mammary Glands Mammary glands are one of the most complex epidermal derivatives. These glands are present in both sexes, but only develop fully in females after parturition. They begin to undergo dramatic structural changes at puberty. The basic structure of the mammary glands involves alveoli that contain two layers of cells: an inner cuboidal epithelium and an outer layer of myoepithelial cells. The alveoli make up tubuloalveolar glands, or lobes, which connect via lactiferous ducts to the base of the nipple. After milk is produced, it is secreted and travels through the ducts into spindle-shaped enlargements beneath the areola known as lactiferous sinuses. Important structural changes occur in the mammary glands over the course of the female's lifetime: In a nonpregnant, sexually mature female, the glandular tissue consists of ducts with small terminal alveoli embedded in an abundant connective tissue stroma that contains many adipose cells. During pregnancy, hormonal stimulation results in the proliferation of the intralobar ducts and terminal alveoli. The epithelial cells become enlarged and vacuolated, as milk fat production increases. After parturition, the gland enters its active secretory phase and produces watery milk containing membrane-bound lipid droplets, as well as milk proteins, lactose, and cellular debris. Suckling causes release of prolactin from the anterior pituitary and oxytocin from the posterior pituitary (processes that you will study in detail during Physiology). Prolactin maintains milk production and oxytocin causes the contraction of myoepithelial cells and ejection of milk. Types of Exocrine Secretion In your study of Histology, you may hear three different terms to describe exocrine cells. These can often be confusing. Merocrine, or eccrine, secretion occurs by exocytosis. This is the mode of secretion of both eccrine and apocrine glands, which can be very confusing. Apocrine secretion occurs when a portion of the plasma membrane containing the secretion buds off from the cell. This is the mode of secretion of the mammary glands and mucous-producing cells, but not the apocrine sweat glands. Holocrine secretion occurs when the entire cell disintegrates in order to release its secretion. Sebaceous glands exhibit holocrine secretion, as the sebum is released with remnants of dead cells. Pre-Lab Quiz Name the four layers of the epidermis and the state of keratin associated with each. Answer: What are the important differences between sebaceous glands, eccrine sweat glands, and apocrine sweat glands? Answer: What is the structure of the mammary gland, and what key differences do you expect to see between active and inactive mammary tissue? Answer: Name the three types of exocrine secretion, their key characteristics, and an example of a cell that demonstrates each one. Answer: Slides Please select whether to view the slides in study mode or quiz mode. In study mode, the images will contain labels and a description. In quiz mode, labels and description will be hidden. Study Mode Quiz Mode Skin Epidermis Stratum Basale and Stratum Spinosum Stratum Basale and Stratum Spinosum EMStratum Granulosum and Stratum Corneum Melanocytes and Langerhans Cells Pacinian Corpuscle Meissner's Corpuscle Hair Hair Follicle Sebaceous Glands Eccrine Sweat Glands Apocrine Sweat Glands Inactive Mature Breast Active Mature Breast Breast Acini Virtual Microscope Slides Layers of the Skin At low magnification, distinguish the epidermis, dermis an hypodermis. In which layers do you find blood vessels? Zoom in to identify the four layers of the epidermis. Which layer appears granulated and why? Which layer appears prickly and why? Hair Follices Focus on a single hair follicle at high magnification. Can you identify the medulla and cortex? Try to find the hair papilla and hair bulb. Mammary Gland At low magnification, differentiate between glandular lobules and connective tissue. Is this sample from an inactive or active breast? Pathology Please select whether to view the slides in study mode or quiz mode. In study mode, the images will contain labels and a description. In quiz mode, labels and description will be hidden. Study Mode Quiz Mode Basal Cell Carcinoma Melanoma Epidermolysis Bullosa Quiz Name the layer on the left. Answer: What cell type synthesizes the brown material? In what cell type does most of the melanin reside? Answer: Identify this tissue. Answer: Identify the structure on the left and its primary function? Answer: What type of glands are these, what is their function, and what is their mode of secretion? Answer: This slide shows a sweat gland. Identify A and B, and explain their different functions. Answer: Is there anything abnormal in the epidermis? If so, what might it indicate. Answer: This image is of skin with a tattoo. Why is the tattoo permanent? Answer: This image is an example of psoriasis that is an inflammatory disorder that results in increased scaling and shedding of the skin. What differences from normal skin do you observe? Answer: It is thought that acne results from alterations in hair follicles, and particularly their secretions. If this were the case, what type of glands would be responsible for acne? Answer:  Nervous System The two major divisions of the nervous system are the central nervous system (CNS), consisting of the brain and spinal cord, and the peripheral nervous system (PNS) consisting of the peripheral nerves, nerve receptor endings, and ganglia distributed throughout the body. Embryologically, the central nervous system is derived from the neural plate which closes to form the neural tube. From the anterior neural tube additional folding and development produces the complex regions of the brain. The neural crest cells migrate to give rise to most of the peripheral nervous system. Cells of the Nervous System - Neurons The major function of the nervous system is provided by neurons. Neurons come in a variety of shapes and sizes. In general, they have the following components: Cell body: the nucleus and cytoplasm containing organelles are contained here. The cytoplasm contains rough endoplasmic reticulum grouped to form the Nissl substance. There are neurofilaments that can occur singly or in clusters. The cell body is receptive to input, but only a minority of impulses arrive directly onto the cell body. Dendrites: these small arborizing processes receive almost all of the input (nerver impulses). They contain some organelles and many neurofilaments. Axon: a long process that conducts impulses (action potentials) away from the cell body. Most neurons have a single axon, though some can have more than one. At the distal end there is a terminal arborization with synapses to other neurons. Axons contain organelles and neurofilaments. Some axons are covered in a sheath of myelin. Neurons can be subdivided into: Motor neurons: these efferent neurons extend distally to send nerve impulses to muscles for voluntary movement. Sensory neurons: these afferent neurons conduct impulses from sensory stimuli to the central nervous system. Interneurons: these are connectors between neurons in the CNS. Cells of the Nervous System - Glial Cells There are a variety of cells which support neuronal function. These include: Astrocytes: these cells in the brain have a small, round nucleus and many very thin processes that radiate from them. The processes extend between neurons. At least one process either extends to and surrounds a capillary or extends to the overlying pia of the brain. The cytoplasm of astrocytes contains intermediate filaments that are composed of glial fibrillary acidic protein (GFAP) that serves to identify these cells. Oligodendroglia: the oligodendroglial cells are small round cells that are most prominent in white matter. They produce the myelin that surrounds the axons of myelinated neurons of the CNS. A single oligodendrocyte can provide myelin for as many as 40 neurons. Microglia: these cells are part of the mononuclear phagocyte system of fixed macrophages in tissues. They are ordinarly not numerous. However, when injury occurs, they become more numerous Schwann cells: these cells provide the myelin that covers neurons in the peripheral nervous system. Larger neurons are covered in a myelin sheath. Myelin is high in lipid and serves the same purpose as insulation around electrical wires. Myelinated axons conduct nerve impulses faster. Structural Components and Regions These cells form the substance of the brain, spinal cord, and peripheral nervous system. The brain is composed of the basic units of the cerebral hemispheres, brainstem, and cerebellum. The cerebral hemispheres can be further categorized by structure and function. Grossly, structures such as the cerebral cortex have many neurons and appear grey, the so-called "grey matter." Regions of the brain that are composed primarily of fiber tracts making connections appear lighter, the so-called "white matter". Histologically, the cortex of the cerebral hemispheres can be categorized as "neocortex" with six layers, considered more developed, and the "paleocortex" with three layers, seen in central structures controlling more primitive reflexive, emotional functions of the limbic system, such as the hippocampus. The hippocampus is also involved in processing short term memories. The cerebellum is involved with controlling and fine-tuning movement. There are three layers comprising the cerebellar cortex. The spinal cord is arranged with outer white matter and inner grey matter with neurons such as the anterior horn cells. From the spinal cord emanate the spinal nerve roots. Meninges The central nervous system is covered by specialized connective tissues that are divided into three layers: Dura mater: the outer covering of the brain just inside the cranial cavity is a very tough layer of connective tissue--the dura. The potential space outside the dura next to the skull is known as the epidural space. The potential space beneath the dura between the dura and the brain is known as the subdural space. Arachnoid: this thin avascular connective tissue layer has irregular channels below it comprising the subarachnoid space, though which the cerebrospinal fluid accumulates. Pia: this thin layer of connective tissue is directly adjacent to the CNS. The pia and arachnoid are so close and so similar in structure that they are often called the pia-arachnoid, better known as the leptomeninges. Ependymal Lining and Choroid Plexuses The cerebral ventricles and spinal canal containing the cerebrospinal fluid are lined by an ependyma, which consists of a single layer of cuboidal to columnar cells that have cilia or microvilli. In the lateral ventricles and the fourth ventricle, the ependyma is specialized to form the choroid plexuses. These plexuses consist of a highly vascularized, arborizing stroma covered by cuboidal cells with fine microvilli. The choroid plexus produces approximately 0.5 to 1.5 liters of cerebrospinal fluid per day. This CSF is an ultrafiltrate of plasma that provides a "shock absorber" function for the brain. The CSF circulates through the ventricles and spinal canal. The CSF is reabsorbed at the arachnoid granulations at the vertex of the brain. If the flow of CSF is blocked, a condition known as hydrocephalus can occur. Blood Brain Barrier (BBB)The BBB is a diffusion barrier that prevents the influx of most compounds from blood to brain. The three cellular elements of the BBB are endothelial cells, astrocytic foot processes, and pericytes (PCs). Pericytes (PCs) encircle endothelial cells and provide structural support and aid in controlling blood flow. The tight junctions (TJs) between cerebral endothelial cells form the selective diffusion barrier. Astrocytic foot processes closely adhere to small vessels and aid in the induction and maintenance of the TJ barrier. However, the astrocytes do not have a direct barrier function. Loss of the astrocytes leads to downregulation of TJ proteins such as occludin and claudin, with resultant diffusion of some, but not all, proteins through the endothelial cells. The BBB is present in all brain regions, except for certain areas that regulate autonomic nervous system functions and endocrine functions of the body such as the pituitary. Fenestrations in blood vessels in these select areas permit diffusion of molecules across the vessel wall. Peripheral Nervous System The PNS is comprised of several structures. There are ganglia that are formed of a group of cell bodies. The Schwann cells are specialized to form satellite cells that invest the cell bodies. The dorsal root ganglia next to the spinal cord are involved with sensory function. The autonomic nervous system has autonomic ganglia that can be further divided into sympathetic and parasympathetic ganglia. The axons of neurons are typically grouped together to form nerve trunks. There are connective tissue layers that surround the peripheral nerves. An endoneurium surrounds an individual axon. Bundles of axons called fascicles are surrounded by a perineurium. The largest nerve trunks composed of multiple fascicles are surrounded by an epineurium. The peripheral nervous system has ganglia that consist of localized collections of nerve cell bodies surrounded by a connective tissue capsule. A ganglion provides a location for synapses. There are dorsal root ganglia adjacent to the spinal cord. There are ganglia associated with the autonomic nervous system known as parasympathetic and sympathetic ganglia. The actual sensory input into the periphral nervous system comes from a variety of specialized nerve endings and receptors. The sense of touch is one example. One of the largest, most impressive receptors is the pressoreceptor, or Pacinian corpuscle.  Galea aponeurotica Galea aponeurotica Dr Maxime St-Amant and Dr Jeremy Jones ◉ et al. The galea aponeurotica (also called the Galeal or epicranial aponeurosis or the aponeurosis epicranialis) is a tough fibrous sheet of connective tissue that extends over the cranium, forming the middle (third) layer of the scalp. Gross anatomy Attachmentsanteriorly: frontalisposteriorlyoccipitalisexternal occipital protruberance and theoccipital bonelaterally: the auricular muscles Related pathologyscalp haematoma References Related articles Head and neck anatomyskeleton of the head and neckcranial vaultscalp ( mnemonic)galea aponeuroticasutures [+]skull landmarks [+]frontal bone [+]temporal bone [+]parietal bone [+]occipital bone [+]skull base ( foramina) [+]facial bones [+]cervical spinehyoid bonelaryngeal cartilages [+]muscles of the head and neck [+]deep spaces of the neck [+]surgical triangles of the neck [+]orbit [+]ear [+]paranasal sinuses [+]nasal cavity [+]oral cavity [+]pharynx [+]larynx [+]viscera of the neck [+]blood supply of the head and neck [+]innervation of the head and neck [+]lymphatic drainage of the head and neck [+]  Dura mater Dura mater From Wikipedia, the free encyclopedianavigation search Dura mater Meninges of the CNSThe dura mater extends into the skull cavity as the falx cerebri and tentorium cerebelli etc. Details Pronunciation UK: / ˈ d j ʊər ə ˈ m eɪ t ər / or US: / ˈ d ʊər ə ˈ m æ t r /Precursor Neural crest Part of Meninges surrounding the brain and spinal cord Identifiers Latin Dura mater Me SH D004388TA A14.1.01.101FMA 9592Anatomical terminology [ edit on Wikidata]Dura mater or dura, is a thick membrane that is the outermost of the three layers of the meninges that surround the brain and spinal cord. It is derived from Neural crest. [1]The other two meningeal layers are the arachnoid mater and the pia mater. The dura surrounds the brain and the spinal cord and is responsible for keeping in the cerebrospinal fluid. Contents [ hide ]1 Structure1.1 Folds and reflections1.2 Blood supply1.3 Drainage1.4 Nerve supply2 Clinical significance3 History4 Additional images5 See also6 References7 External links Structure [ edit]The dura mater has several functions and layers. The dura mater is a membrane that envelops the arachnoid mater. It surrounds and supports the dural sinuses (also called dural venous sinuses, cerebral sinuses, or cranial sinuses) and carries blood from the brain toward the heart. The dura mater has two layers, or lamellae: The superficial layer (also called the periosteal layer), which serves as the skull's inner periosteum, called the endocranium; and a deep layer called the meningeal layer; the actual dura mater. When it covers the spinal cord it is known as the dural sac or thecal sac. Folds and reflections [ edit]Spinal reflections and folds. The dura separates into two layers at dural reflections (also known as dural folds ), places where the inner dural layer is reflected as sheet-like protrusions into the cranial cavity. There are two main dural reflections: The tentorium cerebelli exists between and separates the cerebellum and brainstem from the occipital lobes of the cerebrum. [2]The falx cerebri, which separates the two hemispheres of the brain, is located in the longitudinal cerebral fissure between the hemispheres. [3]Two other dural infoldings are the cerebellar falx and the sellar diaphragm: The cerebellar falx (or falx cerebelli) is a vertical dural infolding that lies inferior to the cerebellar tentorium in the posterior part of the posterior cranial fossa. It partially separates the cerebellar hemispheres. The sellar diaphragm is the smallest dural infolding and is a circular sheet of dura that is suspended between the clinoid processes, forming a partial roof over the hypophysial fossa. The sellar diaphgram covers the pituitary gland in this fossa and has an aperture for passage of the infundibulum (pituitary stalk) and hypophysial veins. Blood supply [ edit]This depends upon the area of the cranial cavity: in the anterior cranial fossa the anterior meningeal artery (branch from the ethmoidal artery) is responsible for blood supply, in the middle cranial fossa the middle meningeal artery and some accessory arteries are responsible for blood supply, the middle meningeal artery is a direct branch from the maxillary artery and enter the cranial cavity through the foramen spinosum and then divides into anterior (which runs usually in vertical direction across the pterion) and posterior (which runs posteriosuperiorly) branches, while the accessory meningeal arteries (which are branches from the maxillary artery) enter the skull through foramen ovale and supply area between the two foramina, and the in posterior cranial fossa the dura mater has numerous blood supply from different possible arteries: A. posterior meningeal artery (from the ascending pharyngeal artery through the jugular foramen) B. meningeal arteries (from the ascending pharyngeal artery through hypoglossal canal) C. meningeal arteries (from occipital artery through jugular or mastoid foramen) D. meningeal arteries (from vertebral artery through foramen magnum)Drainage [ edit]The two layers of dura mater run together throughout most of the skull. Where they separate, the gap between them is called a dural venous sinus. These sinuses drain blood and cerebrospinal fluid from the brain and empty into the internal jugular vein. Arachnoid villi, which are outgrowths of the arachnoid mater (the middle meningeal layer), extend into the venous sinuses to drain CSF. These villi act as one-way valves. Meningeal veins, which course through the dura mater, and bridging veins, which drain the underlying neural tissue and puncture the dura mater, empty into these dural sinuses. A rupture of a bridging vein causes a subdural hematoma. Nerve supply [ edit]The supratentorial dura mater membrane is supplied by small meningeal branches of the trigeminal nerve (V1, V2 and V3). [4] The innervation for the infratentorial dura mater are via upper cervical nerves. Clinical significance [ edit]Many medical conditions involve the dura mater. A subdural hematoma occurs when there is an abnormal collection of blood between the dura and the arachnoid, usually as a result of torn bridging veins secondary to head trauma. An epidural hematoma is a collection of blood between the dura and the inner surface of the skull, and is usually due to arterial bleeding. Intradural procedures, such as removal of a brain tumour, or treatment of trigeminal neuralgia via a microvascular decompression require that an incision is made to the dura mater. To achieve a watertight repair and avoid potential post-operative complications the dura is typically closed with sutures. In the event that there is a dural deficiency, a dural substitute may be used to replace this membrane. Small gaps in the dura can be covered with a surgical sealant film. In 2011, researchers discovered a connective tissue bridge from the rectus capitis posterior major to the cervical dura mater. Various clinical manifestations may be linked to this anatomical relationship such as headaches, trigeminal neuralgia and other symptoms that involved the cervical dura. [5] The rectus capitis posterior minor has a similar attachment. [6]The dura-muscular, dura-ligamentous connections in the upper cervical spine and occipital areas may provide anatomic and physiologic answers to the cause of the cervicogenic headache. This proposal would further explain manipulation's efficacy in the treatment of cervicogenic headache. [7]The American Red Cross and some other agencies accepting blood donations consider dura mater transplants, along with receipt of pituitary-derived growth hormone, a risk factor due to concerns about Creutzfeldt–Jakob disease. [8]Cerebellar tonsillar ectopia, or Chiari, is a condition that was previously thought to be congenital but can be induced by trauma, particularly whiplash trauma. [9] Dural strain may be pulling the cerebellum inferiorly, or skull distortions may be pushing the brain inferiorly. Dural ectasia is the enlargement of the dura and is common in connective tissue disorders, such as Marfan syndrome and Ehlers–Danlos syndrome. These conditions are sometimes found in conjunction with Arnold–Chiari malformation. Spontaneous cerebrospinal fluid leak is the fluid and pressure loss of spinal fluid due to holes in the dura mater. History [ edit]The name dura mater derives from the Latin for tough mother, a loan translation of Arabic أم الدماغ الصفيقة (umm al-dimāgh al-ṣafīcah), literally thick mother of the brain, matrix of the brain, [10] [11] and is also referred to by the term "pachymeninx" (plural "pachymeninges"). [10] The dura has been described as "tough" and "leather-like". [ citation needed]Additional images [ edit]Dura mater (spinal section)Diagrammatic representation of a section across the top of the skull, showing the membranes of the brain, etc. Diagrammatic transverse section of the medulla spinalis and its membranes. Spinal cord. Spinal membranes and nerve roots. Deep dissection. Posterior view. Spinal cord. Spinal membranes and nerve roots. Deep dissection. Posterior view See also [ edit]This article uses anatomical terminology; for an overview, see Anatomical terminology. Meninges Pia mater Arachnoid mater References [ edit]^ Gray's Anatomy. 2016. p. 287. ISBN 9780702052309.^ Shepherd S. 2004. "Head Trauma." Emedicine.com.^ Vinas FC and Pilitsis J. 2004. "Penetrating Head Trauma." Emedicine.com.^ 'Gray's Anatomy for Students' 2005, Drake, Vogl and Mitchell, Elsevier^ Frank Scali; Eric S. Marsili; Matt E. Pontell (2011). "Anatomical Connection Between the Rectus Capitis Posterior Major and the Dura Mater". Spine. 36: E1612–E1614. doi: 10.1097/brs.0b013e31821129df. PMID 21278628.^ Hack, GD (Dec 1, 1995). "Anatomic relation between the rectus capitis posterior minor muscle and the dura mater". Spine. 20: 2484–6. doi: 10.1097/00007632-199512000-00003. PMID 8610241.^ Gary D. Hack; Peter Ratiu; John P. Kerr; Gwendolyn F. Dunn; Mi Young Toh. "Visualization of the Muscle-Dural Bridge in the Visible Human Female Data Set". The Visible Human Project, National Library of Medicine.^ International Red Cross and Red Crescent Movement - redcross.org^ Freeman, MD (2010). "A case-control study of cerebellar tonsillar ectopia (Chiari) and head/neck trauma (whiplash)". Brain Injury. 24 (7-8): 988–94. doi: 10.3109/02699052.2010.490512. PMID 20545453.^ a b medterms.com^ D. Harper - ety External links [ edit]Media related to Dura mater at Wikimedia Commons [ show]v t e Meninges of the brain and spinal cord [ show]v t e The spinal cord Categories: Meninges  Structure Structure The soft tissue envelope of the cranial vault is called the scalp. The scalp extends from the external occipital protuberance and superior nuchal lines to the supraorbital margins. The scalp consists of 5 layers (seen in the image below): the skin, connective tissue, epicranial aponeurosis, loose areolar tissue, and pericranium. The first 3 layers are bound together as a single unit. This single unit can move along the loose areolar tissue over the pericranium, which is adherent to the calvaria. [ 1, 2, 3, 4]Coronal section of scalp that shows layers of the scalp (From Snell RS, Clinical Anatomy for Medical Students, 5th ed)View Media Gallery Skin The skin of the scalp is thick and hair bearing and contains numerous sebaceous glands. As a result, the scalp is a common site for sebaceous cysts. Connective tissue (superficial fascia)The superficial fascia is a fibrofatty layer that connects skin to the underlying aponeurosis of the occipitofrontalis muscle and provides a passageway for nerves and blood vessels. Blood vessels are attached to this fibrous connective tissue. If the vessels are cut, this attachment prevents vasospasm, which could lead to profuse bleeding after injury. Epicranial aponeurosis (galea aponeurotica)The epicranial aponeurosis is a thin, tendinous structure that provides an insertion site for the occipitofrontalis muscle. Posterolaterally, the epicranial aponeurosis attachment extends from the superior nuchal line to the superior temporal line. Laterally, the epicranial aponeurosis continues as the temporal fascia. Anteriorly, the subaponeurotic space extends to the upper eyelids due to the lack of a bony insertion. This loose areolar tissue provides a potential subaponeurotic space that allows fluids and blood to pass from the scalp to the upper eyelids. Loose areolar tissue Areolar tissue loosely connects the epicranial aponeurosis to the pericranium and allows the superficial 3 layers of the scalp to move over the pericranium. Scalp flaps are elevated along a relatively avascular plane in craniofacial and neurosurgical procedures. However, certain emissary veins traverse this layer, which connects the scalp veins to the diploic veins and intracranial venous sinuses. Pericranium The pericranium is the periosteum of the skull bones. Along the suture lines, the pericranium becomes continuous with the endosteum. A subperiosteal hematoma, therefore, forms in the shape of the skull bones. Occipitofrontalis muscle The occipitofrontalis muscle consists of 2 occipital bellies and 2 frontal bellies. The occipital bellies arise from the superior nuchal lines on the occipital bone. The frontal bellies originate from the skin and superficial fascia of the upper eyelids. The occipital and frontal bellies insert into the epicranial aponeurosis. Each occipital belly is innervated by the posterior auricular branch of the facial nerve, and each frontal belly is innervated by the frontal branch of the facial nerve. The frontal bellies can raise the eyebrows. Next: Nerve Supply  Arachnoid mater Arachnoid mater Coronal view of the brain showing the Arachnoid mater and neighboring anatomical structures. Details Part of Meninges Identifiers Me SH D001099Neuro Names 1464TA A14.1.01.201FMA 9591Anatomical terms of neuroanatomy [ edit on Wikidata]The arachnoid mater is one of the three meninges, the protective membranes that cover the brain and spinal cord. The arachnoid mater is a derivative of the Neural crest mesectoderm in the embryo. Contents [ hide ]1 Structure2 Function3 Etymology4 Additional images5 References6 External links Structure [ edit]It is interposed between the two other meninges, the more superficial and much thicker dura mater and the deeper pia mater, from which it is separated by the subarachnoid space. The delicate arachnoid layer is attached to the inside of the dura and surrounds the brain and spinal cord. It does not line the brain down into its sulci (folds), as does the pia mater, with the exception of the longitudinal fissure, which divides the left and right cerebral hemispheres. Cerebrospinal fluid (CSF) flows under the arachnoid in the subarachnoid space. The arachnoid mater makes arachnoid villi, small protrusions through the dura mater into the venous sinuses of the brain, which allow CSF to exit the subarachnoid space and enter the blood stream. The arachnoid mater and dura mater are very close together throughout the cranium and spinal canal all the way to S2, where the two layers fuse into one and end in the filum terminale, which attaches to the coccygeal end of the spinal canal. Sandwiched between the dura and arachnoid maters lie some veins that connect the brain's venous system with the venous system in the dura mater. [1]The arachnoid mater covering the brain is referred to as the "arachnoidea encephali," and the portion covering the spinal cord as the "arachnoidea spinalis." The arachnoid and pia mater are sometimes considered as a single structure, the leptomeninx, or the plural version, leptomeninges. ("Lepto"- from the Greek root meaning "thin"). Similarly, the dura in this situation is called the pachymeninx. There are two subdivisions of arachnoid mater surrounding the subarachnoid space, the dorsal layer and the ventral layer. The dorsal layer covers internal cerebral veins and fixes them to the surrounding tela choroidea. The ventral layer of arachnoid membrane, on the other hand, is a direct anterior extension of this arachnoid envelope that the dorsal layer forms over the pineal region. [2]Function [ edit]See also: Cerebrospinal fluid and Subarachnoid space CSF circulates in the subarachnoid space (between arachnoid and pia mater). Cerebrospinal fluid is produced by the choroid plexus (inside the ventricles of the brain, which are in direct communication with the subarachnoid space so the CSF can flow freely through the nervous system). Cerebrospinal fluid is a transparent, colourless fluid and it is produced at about 500 ml/day. Its electrolyte levels, glucose levels, and p H are very similar to those in plasma, but the presence of blood in cerebrospinal fluid is always abnormal. [3]Etymology [ edit]The arachnoid mater is named after the Greek word " Arachne " ("spider"), the suffix "-oid" ("in the image of"), and the Latin word "mater" ("mother"), because of the fine spider web -like appearance of the delicate fibres of the arachnoid which extend down through the subarachnoid space and attach to the pia mater. Additional images [ edit]Meninges Diagrammatic section of scalp. The arachnoid mater lies under the dura mater, and arteries and veins run on top of it. Spinal dura mater opened, arachnoid mater visible. The medulla spinalis and its membranes. Spinal cord. Spinal membranes and nerve roots. Deep dissection. Posterior view. Meninges and superficial cerebral veins. Deep dissection. Superior view. Meninges and superficial cerebral veins. Deep dissection. Superior view.the arachnoid cells continue inside the brain covering the so call Vircho Robin spaces or perivascular spaces. For that reason some meningiomas can appear as completely inside the brain. References [ edit]^ ¨Microscopic Morphology and Histology of the Human Meninges¨. Mar2005. p22-34.^ "Overview of Adult Traumatic Brain Injuries." Retrieved on 2008-01-16. The Distribution of arachnoid membrane within the velum interpositum". Acta neurochirurgica Sept2012Vol 154 Issue 9, page 1711-1715. Authors: Zhang, Xi-an; Qi, Songtao; Fan, Jun; Huang, Guanglong; Peng, Junxiang; Xu, Jiaming.^ "The intracranial arachnoid mater". Child's Nervous system; Jan2013, Vol29, Issue 1, p17-33, 17p. Authors: Adeeb, Nimer; Deep, Aman; Griessernauer, Christoph; Mortazavi, Martin; Watanabe, Koichi; Loukas, Marios; Tubbs, R.; Cohen, Aaron. Orlando Regional Healthcare, Education and Development. 2004. "Overview of Adult Traumatic Brain Injuries." Retrieved on 2008-01-16. External links [ edit]Anatomy figure: 02:05-09 at Human Anatomy Online, SUNY Downstate Medical Center Slide [ show]v t e Meninges of the brain and spinal cord [ show]v t e The spinal cord  Muscles 33 terms nadinejean04Muscles Learn Flashcards Write Spell Test Match Gravity Advertisement Upgrade to remove ads Sort Perimysium - Dense Irregular Connective tissue covering surround a fascicle, groups of 10 to 100 or more muscle fibers -they give a cut of meat its characteristic cut of grain Endomysium -separates individual muscle fibers -mostly made of reticular fibers T tubules Transverse tubules; finger like invaginations of plasma membrane; extend into interior of fiber and surround myofibrils Sarcolemma Plasma membrane of skeletal muscle fiber Epimysium -Dense Irregular Connective tissue covering surrounding muscle -surrounds the whole muscle Sarcomere -smallest contractile unite within individual muscle fibers -thick and thin filaments overlap each other which gives the appearance of striation -separated by Z discs Sarcoplasmic Reticulum SR; stores calcium within muscle fiber Triad Two terminal cisternae and a T Tuble Myofibrils Rod- like structures within skeletal muscle fiber that contain thin and thick filaments organized into sarcomeres Skeletal Muscle -striated in appearance -attaches to skin, bone or fascia -voluntary control of contraction and relaxation Cardiac Muscle -striated in appearance -involuntary control -autorhythmic because of built in pace maker Smooth muscle -attached to hair follicles in skin -in walls of hallow organs, blood vessels, and GI tract -non striated -involuntaryfunctions of muscle tissue -body movement -stabilizing position -regulating organ volumes -movement of substances w/in the body -heat production Properties of muscle tissue -Excitability -Conductivity -Contractility -Extensibility -Elasticity Excitability respond to chemicals released from nerve cells Conductivity ability to propagate electrical signals over membrane Contractility ability to shorten and generate force Extensibility ability to be stretched with out damaging the tissue Elasticity ability to return to original shape after being stretched Superficial Fascia loose connective tissue and fat underlying the skin Deep Fascia dense irregular connective tissue around the muscle Tendon -attached muscle to the periosteum of the bone -rope like structure made up of epimysium, perimysium, and endomysium Nerve and Blood Supply -each skeletal muscle is supplied by a nerve, artery, and two veins Neuromusclar Junction the synapse b/w a somatic motor neuron and a skeletal muscle fiber Somatic Motor Neuron -the neurons that stimulate skeletal muscle to contract Sacroplasma filled with tiny threads of myofibrils and myoglobin I Band - thin filaments - light color A Band -Thick filaments -dark color H Zone -light bands on either side of M line M Line -middle of sarcomere Z Disc -narrow plate shape regions of dense protein material -separate one sarcomere from the next DOMS Delayed Onset Muscle Sornes -12 to 48 hours after muscle stimulation Myofibrils are built of 3 kinds of protein -contractile proteins (mysoin, actin) -regulatory proteins (troponin, tropomyosin);turn contraction on and off -structural proteins (titin, myomesin, nebulin and dystrophin); provide proper alignment, elasticity, and extendability Advertisement Upgrade to remove ads Features Quizlet Live Quizlet Learn Diagrams Flashcards Mobile Help Sign up Help Center Honor Code Community Guidelines Students Teachers About Company Press Jobs Privacy Terms Follow us Language© 2018 Quizlet Inc.  Nerves structure/Types 28 terms phoenix180 PLUSNerves structure/Typesnerves structure/types; page 15 of the lab review guide Learn Flashcards Write Spell Test Match Gravity Advertisement Upgrade to remove ads Sortnerve a collection of bundles of nerve cell processes (axons and dendrites); appear as cables or ribbons; transmit impulses from the receptors to the CNS and from the CNS to the effectorsepineurium connective tissue that encloses each nerve; insulates the nerve from other nervesperineurium connective tissue surrounding each bundle (fasciculus) of nerve fibers in the PNSfasciculus a bundle of axons (nerve fibers) enclosed by the perineuriumendoneurium delicate layer of connective tissue that encloses the neurolemma, which is the outer layer of the neurolemmocytespinal nerves there are 31 pairs and are considered to be mixed; consist of afferent and efferent fibers and components of the PNScranial nerves there are 12 pairs; includes all three types-afferent, efferent, and mixed nervessensory nerve also can be called an afferent nerve; when the receptor is a sense organ this nerve is called an afferent nerve; bundles of dendrites of afferent (sensory) neurons that carry impulses from receptors toward the CNSmotor nerve also can be called an efferent nerve; when the effector is a muscle, this nerve is called an efferent nerve; carry impulses away from the CNS toward the effectorsmixed nerve composites of bundles of dendrites and axons that transmit impulses to and from the CNS, functioning both in afferent and efferent manner; ex: spinal nervesreceptor reflex arc component; acknowledges the stimuli and converts it into an impulseafferent nerve fiber reflex arc component; also called a sensory nerve fiber; conveys the sensory impulse to the CNSreflex center reflex arc component; also called integration (CNS) center; the brain stem and/or spinal cord interprets the impulseefferent nerve fiber reflex arc component; also called motor nerve fiber; transmits the motor impulse to the effectoreffector reflex arc component; this is the muscle or gland; performs the actual responseextent of spinal cord travel spinal cord anatomy; arises at the medulla oblongata, passes into the vertebral canal, and descends to the level of the disk between L1 and L2 where it endscauda equina spinal cord anatomy; bundles of spinal nerves from the lower region of the spinal cordwhite matter spinal cord anatomy; located on the outside; encloses the gray mattergray matter spinal cord anatomy; enclosed by the white matter; consists of unmyelinated neuron cell bodies and their processes which serve as a reflex and integration center for certain responses such as those initiated by paingray commissure spinal cord anatomy; thin strip of gray matter that surrounds the central canal of the spinal cordcentral canal spinal cord anatomy; located at the center of the gray matter; cavity which extends throughout the cord and communicates with the ventricles of the brain; filled with cerebrospinal fluid (CSF)dorsal horn spinal cord anatomy; part of the gray matter; receives sensory impulses from the afferent fibers of the spinal nervesventral horn spinal cord anatomy; part of the gray matter; transmit motor impulses to the efferent fibers of the spinal nerveslateral horn spinal cord anatomy; part of the gray matter; associated with the regulation of the involuntary activities via a specific set of nerves of the PNS called the autonomic systemspinal nerve spinal cord anatomy; it's considered to be composed of mixed nervesdorsal root spinal cord anatomy; find sensory nerves theredorsal root ganglion located on the dorsal root; contains cell bodies of neurons in afferent spinal nervesventral root spinal cord anatomy; find motor nerves there Advertisement Upgrade to remove ads Features Quizlet Live Quizlet Learn Diagrams Flashcards Mobile Help Sign up Help Center Honor Code Community Guidelines Students Teachers About Company Press Jobs Privacy Terms Follow us Language© 2018 Quizlet Inc.  histology nervous tissue 97 terms Edgar_Penado PLUShistology nervous tissue Learn Flashcards Write Spell Test Match Gravity Advertisement Upgrade to remove ads Sortnervous tissue systems central nervous system peripheral nervous system CNS brain and spinal cord, developed from neural plate (ectoderm)PNS 31 pairs of spinal nerves 12 pairs of cranial nerves peripheral ganglianervous tissue is made of neurons neuroglia cellsneurons functional units of nervous system non dividing cellsprominent features of neurons excitability conductivityexcitability action potential, has power to generate a nerve impulseconductivity nerve impulse rapidly propagates along the nerve3 types of neuron functions sensory motor interneuronsensory neuron carry impulse from receptors to CNS there are 2 types somatic afferent from muscles skin and joints visceral afferent from internal organs blood vessels and heartmotor neurons carry impulse away from cns to muscle or gland somatic efferent impulse to skeletal muscle (Golgi type I neurons) efferent impulse to organsinterneuron integrate between sensory and motor (Golgi type II neurons)structure of neuron cell body dendrites axonsneuron cell body large nucleus nissl bodiesdendrites numerous short branches that carry impulses toward cell body that contain nissl bodiesaxon long single stem that transmits impulses away from cell body has no nissl bodiesaxon hillock pale staining area of cytoplasm attached to cell bodyaxoplasm no nissl bodies but have mitochondria and microtubulesaxolemma cell membrane of the axonmyelin sheath layer that surrounds the axon rich in lipidnodes of ranvier myelin free gapsneurolemma outside layer of schwann cell cytoplasmmyelin in the PNS are formed by schwann cellsmyelin in the CNS are formed by oligodendrocytestypes of neurons unipolar bipolar multipolarunipolar, pseudo-unipolar neurons one process that divides into 2 parts dendrite and axon like the spinal gangliabipolar neurons 2 processes like the retinamultipolar neuron more than 2 processes have 4 subtypes 1. stellate (star shape, autonomic ganglia) 2.pyramidal (cerebral cortex) 3.piriform (pear shaped, purkinje cells) 4. granule cells (small mostly nucleus, (cerebellum)what is the function of unipolar and bi polar neurons sensorywhat is the function of multipolar motorsynapses sites of contact between neurons or neuron and muscle, organ or glandfunction of synapses facilitate transmission of impulses from presynaptic neuron to a postsynaptic neuron through a synaptic cleftsynapse morphological classifications 1. axodendritic 2. axosomatic 3. axoaxonic 4. dendrodendriticnerve impulse conduction types chemical synapse electrical synapsechemical synapse release of neurotransmitters like acetyl choline and norepinephrineelectrical synapse gap junctions like in cardiac and smooth muscle as in the transmission of ionsexplain how electrical synapse work presynaptic terminals contain synaptic vesicles, ions are transported across cell membrane of neuron and muscle or organ... and create a membrane potential, the unequal electric charges on the opposite sides of the cell membrane polarize, the start of a nerve impulse is accompanied by an opening of ion channels and unbalanced electrical charges depolarization--> action potentialneural tube gives rise to brain and spinal cordneural crest gives rise to ganglia satellite cells of ganglia Schwann cells of peripheral nerves melanocytes of the skin adrenal medulla para follicular cells of thyroid glandcentral nervous system is formed of 2 basic components gray matter white mattergray matter contains cell bodies of neurons, neuroglia and dendrites and blood vesselswhite matter consists mainly of myelinated dendrites, neuroglia and blood vesselsspinal cord gray matter H shaped the central is formed of the anterior ventral horn which contains motor neurons the posterior dorsal horn contains sensory neurons the lateral horn contains neurons of sympathetic nervous systemspinal cord white matter myelinated fibers outside of the gray matter arranged in ascending and descending tracks, neuroglia cellscerebral cortex the gray matter is external and arranged into 6 layers containing different types of neuronscerebellar cortex the grooves called sulci and folds called gyri make up the Arbor vitae the gray matter is formed of 3 layers 1. Outer molecular layer (small neuron) 2. in her granular layer (small neuron) 3. Middle layer contains large neuron (Purkinje cells)neuroglia (brain glue) derived from the ectoderm they provide metabolic structural support for the central nervous system4 types of neuroglia oligodendrocytes astrocytes microglia ependymal cellsoligodendrocytes small treelike cells that produce myelin in the central nervous systemastrocytes star-shaped cells whose feet adhere to capillaries and neurons which contain intermediate filaments that support the cell body and processes2 types of astrocytes fibrous astrocytes protoplasmic astrocytesfibrous astrocytes have few straight processes situated in white matterprotoplasmic astrocytes numerous short processes situated in gray matterfunctions of astrocytes - movement of metabolites and waste to and from neurons - regulate ionic concentrations in intracellular compartment -maintain tight junctions of capillaries that form the blood brain barrier - in the central nervous system if damaged they proliferate and create cellular scar tissue also known as gliosismicroglia tiny cells with long processes phagocytic cells of the central nervous system derived from blood monocytesependymal cells lined the brain ventricles and central canal of spinal cord they form a continuous lining layer called ependyma covering the call choroid plexuses that secrete cerebral spinal fluidblood brain barrier continuous tight junctions that exist between endothelial cells of brain blood capillaries Astro Citic feet maintain blood brain barrierconnective tissue membranes of meninges Pia matter arachnoid matter dura mater Pia matter innermost layer with delicate vascular loose connective tissue tightly adherent to the surface of brain and spinal cordarachnoid matter spiderweb like middle layer made up of a membranous roof and irregular trabeculaesubarachnoid space space under arachnoid matter that contains cerebral spinal fluid and blood vesselsleptomeninges Pia and arachnoid matterdura mater thick tough nonstretchable dentist in regular connective tissue also known as packymeninxepidural space space outside dura matersubdural space deep in dura matercerebral spinal fluid secreted by choroid plexuses fills the brain ventricles subarachnoid space and central canal of spinal cordchoroid plexus is located inside brain ventriclescerebrospinal fluid movement from brain ventricles to subarachnoid space drained by arachnoid villi and graduations most to Venous sinuses of the dura mater where it is absorbed into venous blood and then taken into general circulationfunction of the cerebrospinal fluid acts as a cushion and transports nutrients and waste productsperipheral nervous system consists of ganglia nerves and nerve endings it is a collection of neurons2 types of ganglia in the peripheral nervous system sensory ganglia motor\autonomic gangliasensory ganglia contain cell bodies of sensory and a friend neuronsmotor ganglia contain cell bodies of motor neurons to cardiac and smooth muscle and glandscranial nerves associated with cranial ganglia nerves 5,7,8, 9,10spinal ganglia (sensory) surrounded by connective tissue capsule the ganglion cells are pseudounipolar arranged in linear groups separated by connective tissue and nerve fibers derived from neural crest nucleus appears to be like an owl's eye -myelinated nerve fibers and connective tissueautonomic ganglia (motor) connective tissue capsule on the outside multipolar ganglia eccentric nuclei non-myelinated nerve fibers and connective tissue fibers between cellsperipheral nerves soft mushy but strong and resilientepineurium outer dense connective tissue sheath that surrounds the whole nerveperineurium in a dense connective tissue that surrounds the bundles or fascicles of nerve fibersendoneurium loose connective tissue that invests each nerve fiber.myelinated nerve fibers versus in unmyelinated fibers unmyelinated nerve fibers have a slower conduction rate they are smaller and Schwann cells also known as Lima do not produce myelin around themnerve endings termination of nerve cell processes in relation to other nerve cells2 main categories for nerve endings receptors effectorsreceptors can initiate a nerve impulse in response to a stimulus classified by 1. extroceptors 2. enteroceptor 3. proprioceptorextroceptors react to stimuli from external environment for example pain temperature or touchnociceptor receptor for painthermoreceptors receptor for temperaturemechanoreceptors receptors for touchenteroceptor react to a stimuli from within the body the degree of stretch of blood vessels change in oxygen and carbon dioxideproprioceptors react to stimuli from body walls and extremities in relation to sense of position and stretching of the muscle and movement Golgi tendon organ similar to muscle spindle but found in between tendon fibers at musculotendinous functioneffectors nerve endings relating motor impulses leading to contraction for example the motor and plate or secretions from glandsautonomic nervous system part of the nervous system that regulates the activity of smooth muscles and cardiac muscles as well as glands motor part of visceral nervous system to motor neuron chains one inside one outside of the central nervous system2 parts of the autonomic nervous system sympathetic parasympatheticsympathetic flight or fight responseparasympathetic rest and digest responseantegrade degeneration the generation of an axon distal to the site of injury also known as wallerian  Epidermis (skin) Save The epidermis is the outer (epi in Greek meaning "over" or "upon") of the two layers that make up the skin (or cutis ), the inner layer being the dermis. [1] It provides a barrier to infection from environmental pathogens [2] and regulates the amount of water released from the body into the atmosphere through transepidermal water loss (TEWL). [3] The outermost part of the epidermis is composed of stratified layers of flattened cells, [4] that overlies a basal layer ( stratum basale) composed of columnar cells arranged perpendicularly. In addition to rows of cells, the epidermis also contains hair follicles, and two types of sweat glands: sebaceous glands and eccrine glands [5] (See Fig. 1 in the reference). These cells develop from the stem cells in the basal layer of the epidermis. Structure Cellular components The epidermis has no blood supply and is nourished almost exclusively by diffused oxygen from the surrounding air. [6] It is 95% keratinocytes [4] ( proliferating basal and differentiated suprabasal) but also contains melanocytes, Langerhans cells, Merkel cells, [1] and inflammatory cells. Rete ridges (or rete pegs) are epidermal thickenings that extend downward between dermal papillae. [7] Blood capillaries are found beneath the epidermis, and are linked to an arteriole and a venule. Cell junctions In the epidermis, the cells are tightly interconnected to serve as a tight barrier. The junctions between the epidermal cells is adherens junction type. These junctions are formed by transmembrane proteins called cadherins. Inside the cell, the cadherins are linked to actin filaments. In immunofluorescence microscopy, the actin filament network appears as a thick border surrounding the cells [5] (See Fig. 3 in reference). Yet, as noted, the actin filaments are located inside the cell and run parallel to the cell membrane. Because of the proximity of the neighboring cells and tightness of the junctions, the actin immunofluorescence appears as a border between cells. [5]Layers Schematic image showing a section of epidermis with epidermal layers labeled. The epidermis is composed of 4 or 5 layers depending on the region of skin being considered. [8] Those layers in descending order are: [2]cornified layer ( stratum corneum)Confocal image of the stratum corneumclear/translucent layer ( stratum lucidum, only in palms and soles)granular layer ( stratum granulosum)Confocal image of the stratum corneumspinous layer ( stratum spinosum)Confocal image of the stratum spinosum already showing some clulsters of basal cellsbasal/germinal layer ( stratum basale/germinativum ). Confocal image of the stratum basale already showing some papillae The term Malpighian layer (stratum malpighi) is usually defined as both the stratum basale and stratum spinosum. [4]The epidermis is separated from the dermis, its underlying tissue, by a basement membrane. Cellular kinetics Cell division The stratified squamous epithelium is maintained by cell division within the stratum basale. Differentiating cell delaminate from the basement membrane and are displaced outwards through the epidermal layers, undergoing multiple stages of differentiation until, in the stratum corneum, losing their nucleus and fusing to squamous sheets, which are eventually shed from the surface ( desquamation ). Differentiated keratinocytes secrete keratin proteins which contribute to the formation of an extracellular matrix and is an integral part of the skin barrier function. In normal skin, the rate of keratinocyte production equals the rate of loss, [4] taking about two weeks for a cell to journey from the stratum basale to the top of the stratum granulosum, and an additional four weeks to cross the stratum corneum. [2] The entire epidermis is replaced by new cell growth over a period of about 48 days. [11]Calcium concentration Keratinocyte differentiation throughout the epidermis is in part mediated by a calcium gradient, increasing from the stratum basale until the outer stratum granulosum, where it reaches its maximum, and decreasing in the stratum corneum. Calcium concentration in the stratum corneum is very low in part because those relatively dry cells are not able to dissolve the ions. This calcium gradient parallels keratinocyte differentiation and as such is considered a key regulator in the formation of the epidermal layers. [3]Elevation of extracellular calcium concentrations induces an increase in intracellular free calcium concentrations. [12] Part of that intracellular increase comes from calcium released from intracellular stores [13] and another part comes from transmembrane calcium influx, [14] through both calcium-sensitive chloride channels [15] and voltage-independent cation channels permeable to calcium. [16] Moreover, it has been suggested that an extracellular calcium-sensing receptor (Ca SR) also contributes to the rise in intracellular calcium concentration. [17]Development Epidermal organogenesis, the formation of the epidermis, begins in the cells covering the embryo after neurulation, the formation of the central nervous system. In most vertebrates, this original one-layered structure quickly transforms into a two-layered tissue; a temporary outer layer, the periderm, which is disposed once the inner basal layer or stratum germinativum has formed. [18]This inner layer is a germinal epithelium that give rise to all epidermal cells. It divides to form the outer spinous layer ( stratum spinosum ). The cells of these two layers, together called the Malpighian layer (s) after Marcello Malpighi, divide to form the superficial granular layer (Stratum granulosum) of the epidermis. [18]The cells in the stratum granulosum do not divide, but instead form skin cells called keratinocytes from the granules of keratin. These skin cells finally become the cornified layer (stratum corneum), the outermost epidermal layer, where the cells become flattened sacks with their nuclei located at one end of the cell. After birth these outermost cells are replaced by new cells from the stratum granulosum and throughout life they are shed at a rate of 0.001 - 0.003 ounces of skin flakes every hour, or 0.024-0.072 ounces per day. [19]Epidermal development is a product of several growth factors, two of which are: [18]Transforming growth factor Alpha ( TGFα) is an autocrine growth factor by which basal cells stimulate their own division. Keratinocyte growth factor (KGF or FGF7) is a paracrine growth factor produced by the underlying dermalfibroblasts in which the proliferation of basal cells is regulated. Function Barrier The epidermis serves as a barrier to protect the body against microbial pathogens, oxidant stress ( UV light) and chemical compounds and provides mechanical resistance. Most of that function is played by the stratum corneum. [10]Characteristics of the barrier Physical barrier through keratinocytes attached together via cell–cell junctions and associated to cytoskeletal proteins, which gives the epidermis its mechanical strength. [3]Chemical barrier through the presence of highly organized lipids, acids, hydrolytic enzymes and antimicrobial peptides. [3]Immunologically active barrier through humoral and cellular constituents of the immune system. [3]Water content of the stratum corneum drops towards the surface, creating hostile conditions for pathogenic microorganism growth. [10]An acidic p H (around 5.0) and low amounts of water make it hostile to many microorganic pathogens. [10]The presence of non-pathogenic microorganism on the epidermis surface help defend against pathogenic one by limiting food availability and through chemical secretions. [10]Factors that alter the barrier Psychological stress, through an increase in glucocorticoids, compromises the stratum corneum and thus the barrier function. [20]Sudden and large shifts in humidity alter stratum corneum hydration in a way that could allow entry of pathogenic microorganisms. [21]Skin hydration The ability of the skin to hold water is primarily due to the stratum corneum and is critical for maintaining healthy skin. [22] Lipids arranged through a gradient and in an organized manner between the cells of the stratum corneum form a barrier to transepidermal water loss. [23] [24]Skin color The amount and distribution of melanin pigment in the epidermis is the main reason for variation in skin color in Homo sapiens. Melanin is found in the small melanosomes, particles formed in melanocytes from where they are transferred to the surrounding keratinocytes. The size, number, and arrangement of the melanosomes varies between racial groups, but while the number of melanocytes can vary between different body regions, their numbers remain the same in individual body regions in all human beings. In white and Asian skin the melanosomes are packed in "aggregates", but in black skin they are larger and distributed more evenly. The number of melanosomes in the keratinocytes increases with UV radiation exposure, while their distribution remain largely unaffected. [25]Clinical significance Laboratory culture of keratinocytes to form a 3D structure ( artificial skin) recapitulating most of the properties of the epidermis is routinely used as a tool for drug development and testing. Additional images Epidermis and dermis of human skin Cross-section of all skin layers Illustration of epidermal layers Optical coherence tomography of fingertip See also Skin repair References James, William; Berger, Timothy; Elston, Dirk (2005) Andrews' Diseases of the Skin: Clinical Dermatology (10th ed.). Saunders. Page 2-3. ISBN 0-7216-2921-0. Marks, James G; Miller, Jeffery (2006). Lookingbill and Marks' Principles of Dermatology (4th ed.). Elsevier. pp. 1–7. ISBN 1-4160-3185-5. Proksch, E.; Brandner, J.; Jensen, J. M. (2008). "The skin: an indispensable barrier". Experimental Dermatology. 17 (12): 1063–1072. PMID 19043850. doi: 10.1111/j.1600-0625.2008.00786.x. Mc Grath, J. A.; Eady, R. A.; Pope, F. M. (2004). Rook's Textbook of Dermatology (7th ed.). Blackwell Publishing. pp. 3.1–3.6. ISBN 978-0-632-06429-8. Hanukoglu I, Boggula VR, Vaknine H, Sharma S, Kleyman T, Hanukoglu A (January 2017). "Expression of epithelial sodium channel (ENa C) and CFTR in the human epidermis and epidermal appendages". Histochemistry and Cell Biology. 147 (6): 733–748. PMID 28130590. doi: 10.1007/s00418-016-1535-3. Stücker, M; Struk, A; Altmeyer, P; Herde, M; Baumgärtl, H; Lübbers, DW (2002). "The cutaneous uptake of atmospheric oxygen contributes significantly to the oxygen supply of human dermis and epidermis". The Journal of Physiology. 538 (3): 985–994. PMC 2290093. PMID 11826181. doi: 10.1113/jphysiol.2001.013067. The Free Dictionary > rete ridge Citing: The American Heritage Medical Dictionary Copyright 2007, 2004The Ageing Skin - Structurehttp://www.radcliffe-oxford.com/books/samplechapter/7750/01_bensouillah-241a6c80rdz.pdf Elias, P. M. (2007). "The skin barrier as an innate immune element". Seminars in Immunopathology. 29 (1): 3–14. PMID 17621950. doi: 10.1007/s00281-007-0060-9. Iizuka, Hajime (1994). "Epidermal turnover time". Journal of Dermatological Science. 8 (3): 215–217. PMID 7865480. doi: 10.1016/0923-1811 (94)90057-4. Hennings, H; Kruszewski, FH; Yuspa, SH; Tucker, RW (1989). "Intracellular calcium alterations in response to increased external calcium in normal and neoplastic keratinocytes". Carcinogenesis. 10 (4): 777–80. PMID 2702726. doi: 10.1093/carcin/10.4.777. Pillai, S; Bikle, DD (1991). "Role of intracellular-free calcium in the cornified envelope formation of keratinocytes: Differences in the mode of action of extracellular calcium and 1,25 dihydroxyvitamin D3". Journal of cellular physiology. 146 (1): 94–100. PMID 1990023. doi: 10.1002/jcp.1041460113. Reiss, M; Lipsey, LR; Zhou, ZL (1991). "Extracellular calcium-dependent regulation of transmembrane calcium fluxes in murine keratinocytes". Journal of cellular physiology. 147 (2): 281–91. PMID 1645742. doi: 10.1002/jcp.1041470213. Mauro, TM; Pappone, PA; Isseroff, RR (1990). "Extracellular calcium affects the membrane currents of cultured human keratinocytes". Journal of cellular physiology. 143 (1): 13–20. PMID 1690740. doi: 10.1002/jcp.1041430103. Mauro, TM; Isseroff, RR; Lasarow, R; Pappone, PA (1993). "Ion channels are linked to differentiation in keratinocytes". The Journal of membrane biology. 132 (3): 201–9. PMID 7684087. doi: 10.1007/BF00235738. Tu, CL; Oda, Y; Bikle, DD (1999). "Effects of a calcium receptor activator on the cellular response to calcium in human keratinocytes". The Journal of Investigative Dermatology. 113 (3): 340–5. PMID 10469331. doi: 10.1046/j.1523-1747.1999.00698.x. Gilbert, Scott F (2000). "The Epidermis and the Origin of Cutaneous Structures". Developmental Biology. Sinauer Associates. ISBN 0-87893-243-7. Weschler, Charles J. (April 8, 2011). "Squalene and Cholesterol in Dust from Danish Homes and Daycare Centers". Environ. Sci. Technol. 45: 3872–3879. PMID 21476540. doi: 10.1021/es103894r. Retrieved 3 July 2014. Denda, M.; Tsuchiya, T.; Elias, P. M.; Feingold, K. R. (2000). "Stress alters cutaneous permeability barrier homeostasis". Am J Physiol Regul Integr Comp Physiol. 278 (2): R367–372. PMID 10666137.http://onlinelibrary.wiley.com/doi/10.1021/js950219p/abstract Blank, IH (1952). "Factors which influence the water content of the stratum corneum". The Journal of Investigative Dermatology. 18 (6): 433–40. PMID 14938659. doi: 10.1038/jid.1952.52. Downing, DT; Stewart, ME; Wertz, PW; Colton, SW; Abraham, W; Strauss, JS (1987). "Skin lipids: An update". The Journal of Investigative Dermatology. 88 (3 Suppl): 2s–6s. PMID 2950180. doi: 10.1111/1523-1747.ep12468850. Bonté, F; Saunois, A; Pinguet, P; Meybeck, A (1997). "Existence of a lipid gradient in the upper stratum corneum and its possible biological significance". Archives of dermatological research. 289 (2): 78–82. PMID 9049040. doi: 10.1007/s004030050158. Montagna, William; Prota, Giuseppe; Kenney, John A. (1993). Black skin: structure and function. Gulf Professional Publishing. p. 69. ISBN 0-12-505260-X.  . Integumentary System I. Overview Diagram Two Distinct regions of the integumentary system: epidermis & dermis. Only dermis is vascularized Nutrients reach epidermis by diffusing through the tissue from the blood vessels Underneath dermis is hypodermis (superficial fascia), also called subcutaneous tissue which is composed of loose connective tissue (mostly areolar andadipose). II. Epidermis Thick keratinized stratified squamous epithelium consisting of four distinct cell types and five distinct layers. A. Cells: Keratinocytes, Melanocytes, Merkel Cells, and Langerhans Cells Keratinocytes Cells producing keratin (fibrous protein)Keritinocytes produce keratin continuously until reaching the surface of the skin, then these scale-like structures are sloughed off. The hands and feet have accelerated keratin production Keratinocyte production and keratin formation are regulated by a hormone, epidermal growth factor (EGF). Melanocytes Cells synthesizing the pigment melanin These cells are located in deep layers of epidermis and contain branching processes that contact all keratinocytes in basal epidermal layer The melanin formed in melanosomes are transported to branching processes, shed, and engulfed by keratinocytes It must be noted that all races have the same relative number of melanocytes. Langerhans cells Star-shaped cells arising from bone marrow that migrate to epidermis They are macrophages (engulf foreign particles by phagocytosis)Their primary role is immunologic, the first line of defense. Merkel cells Hemisphere-shaped cells located at epidermal-dermal junction and are associated with a sensory nerve fiber (sensory/touch perception)B. Layers of Epidermi s (strata) Diagram Stratum basal (stratum germinativum)Basal layer, separated from dermis by thin basement membrane; single row of cells (mostly columnar keratinocytes) with rapid mitotic division; also contain merkel cells and melanocytes. Stratum spinosum Many layers, less mitosis; the keratincoytes contain tononfilaments (keratin filaments); superficial stratum spinosum receives less nutrients, therefore keratin production increases (and cells less viable begin to die). Stratum granulosum Three to five layers of flattened cells; cells contain tonofilaments and laminated granules which contain a waterproofing glycolipid that is secreted into theintercellular space to slow water loss across the epidermal layer. Stratum lucidum (clear layer)Thin translucent band, few rows of flattened dead keratinocytes. Stratum corneum (horny layer)Very thick (25 + layers) with shingle- like dead cells. III. Dermis Flexible connective tissue layer containing elastic, collagen, and reticular fibers; dermal cellular components include fibroblasts, macrophages, mast cells (produce heparin and histamine), and white blood cells; dermis also contains nerve fibers, blood vessels, and lymphatic vessels; there two major dermal layers: the papillary and reticular layers. Papillary layer - loose connective tissue with superior surface arranged into nipple-like projections (dermal papillae) that indent the epidermis above; may contain capillary loops, free (bare) nerve endings (pain receptors), and touch receptors (Meissner's corpuscles); ventral portion of hands and feet have definite patterns (looped and whorled) called epidermal ridges which increase/enhance gripping ability; sweat pores in ridges cause films of sweat (fingerprints). Reticular layer - (80% of dermis) dense connective tissue containing bundles of interlocking collagen fibers that run in various planes parallel to the skin surface; elastic and reticular fibers also present; separations or less dense regions are referred to as lines of cleavage or tension lines in skin; cleavage lines run longitudinally in limbs and circular around neck andtrunk; third type of skin marking = flexure lines, dermal folds positioned near or at jointsresult from dermis being tightly secured to deeper structures by the hypodermis - skin cannot slide easily to accommodate joint movement (obvious on wrists, palms, soles of feet, fingers, and toes). IV. Skin Color Three pigments contribute to skin color: melanin, carotene, and hemoglobin Racial differences are due to the amount and variations of melanin produced Ultraviolet light stimulates production of melanin and excessive exposure causes solar elastosis (clumping of elastin fibers) and DNA damage Carotene derived from vitamin A accumulates in stratum corneum and adipose tissue and imparts an orange color to the skin. Lastly hemoglobin in red blood cells give a pinkish hue on fair skin. Skin color varies as a result of disease or emotional states: Redness (erythmia) - reddened skin, embarrassment, fever, hypertension, inflammation, or allergy Pallor/blanching - pale skin, emotional distress or anemia, low blood pressure Jaundice - liver disease, bile deposited in tissue Bronzing - bronze coloration (addison's disease) hypofunction of adrenal cortex Black & blue - bruises, escaped blood clots in tissue spaces (clotted blood masses = hematomas)NOTE: know dermatomes and the rules of nine!V. Clinically Related Terms Macule - localized change in color (freckle)Wheal - accumulated fluid in papillary layer (hives)Papule - solid elevated area, epidermal and papillary (insect bite)Nodule - large papules extending into subcutaneous layer (cyst)Vesicle - papule with fluid core (second degree burn)Pustule - papule with pus core (acne)Erosion - ruptured vesicle (ulcer)Xerosis - "dry skin"Hemangiomas - benign tumor in the dermis (capillary and cavernous)Sebaceous hyperplasia - enlargement of the sebaceous gland Pruritis - irritating itching sensation of the skin Seborrheic dermatitis - inflammation around abnormally active sebaceous glands Basal cell carcinoma - malignant cancer originating in the germinative layer Squamous cell carcinoma - malignant cancer originating in the top layer of the skin Malignant melanomas - metastasizing melanocytes  The Nervous System 107 terms ishi The Nervous System Learn Flashcards Write Spell Test Match Gravity Advertisement Upgrade to remove ads Sort Neuroglia cell A type of nerve cell that supports, protects, and nourishes the neuron. There are four kinds: astrocytes, microglia, ependymal cells, and oligodendrocytes.dendrites branching extensions of neuron that receives messages from neighboring neuronsaxon long nerve fiber that conducts away from the cell body of the neuronnodes of ranvier small gaps in the myelin sheath of medullated axonsnucleus a part of the cell containing DNA and RNA and responsible for growth and reproductionendoneurium inner layer of connective tissue on the spinal nerve; composed of delicate connective tissue fibers that surround individual axonsmyelin sheath a layer of myelin encasing (and insulating) the axons of medullated nerve fibersnissl bodies rough ER in neurons; site of proten synthesisschwann cell any cell that covers the nerve fibers in the peripheral nervous system and forms the myelin sheathcentral canal The narrow cavity in the center of the spinal cord that is continuous with the fluid-filled ventricles of the brain.anterior median fissure wide, deep crease along the ventral surface of the spinal cordposterior median sulcus shallow longitudinal groove on the dorsal surface of the spinal cordganglia groups of nerve cellsepineurium outermost layer of connective tissue on the spinal nerve; a dense network of collagen fibersperineurium connective tissue that covers a bundle of nerve fibersfascicles Bundle of muscle fibersendoneurium delicate connective tissue around individual nerve fibers in nervemedulla oblongata Lower or hindmost part of the brainoptic chiasma the crossing of the optic nerves from the two eyes at the base of the brainconus medullaris where the spinal cord tapers to a conical tip, which is at or inferior to the level of the first lumbar vertebrae (L1)cauda equina collection of spinal nerves below the end of the spinal cordcervical plexus a nerve plexus lying beneath the sternocleidomastoid musclebrachial plexus a network of nerves formed by cervical and thoracic spinal nerves and supplying the arm and parts of the shoulderlumbar plexus a plexus of nerves formed by the ventral branches of the first four lumbar nervessacral plexus a nerve plexus formed by the 4th and 5th lumbar and 1st, 2nd, 3rd sacral nervessciatic nerve arises from the sacral plexus and passes about halfway down the thigh where it divides into the common peroneal and tibial nervescerebral aqueduct a canal connecting the third and fourth ventricleschoroid plexus a vascular plexus of the cerebral ventricles that regulate intraventricular pressurecentral sulcus a brain fissure extending upward on the lateral surface of both hemispheresprecentral gyrus the convolution of the frontal lobe that is bounded in back by the central sulcus and that contains the motor areaintercostal nerves aid in inhalation and relaxation, skin of thorax region, abdominal walllateral ventricles paired; one deep within each cerebral hemisphere; large C-shaped chambers that reflect the pattern of cerebral growth; each ventricle communicates with third ventriclethird ventricle narrow; in the diencephalon; recieves communication from lateral ventricles; continuous with fourth ventriclefourth ventricle an irregular ventricle between the third ventricle and the central canal of the spinal cordpostcentral gyrus the convolution of parietal lobe that is bounded in front by the central sulcuslateral sulcus separates the temporal lobe from the parietal lobelongitudinal fissure separates the two cerebral hemispherestransverse fissure separates cerebrum from cerebellumcerebellum the "little brain" attached to the rear of the brainstem; it helps coordinate voluntary movement and balancepons a band of nerve fibers linking the medulla oblongata and the cerebellum with the midbrainoculomotor nerve Cranial Nerve IIItrochlear nerve Cranial Nerve IVtrigeminal nerve Cranial Nerve Vabducens nerve Cranial Nerve VIfacial nerve Cranial Nerve VIIvestibulocochlear nerve Cranial Nerve VIIIglossopharyngeal nerve CN IXvagus nerve Xaccessory nerve CN XIhypoglossal nerve Cranial Nerve XIIpituitary gland is the master gland/produces hormones/regulates and controlscorpus callosum a broad transverse nerve tract connecting the two cerebral hemispheresseptum pellucidum A structure that lies immediately ventral to the corpus callosum, is most prominent anteriorly, and consists of two thin-walled membranes separated by a narrow cleft, forming a small cavity (cavum of septum pellucidum).thalamus large egg-shaped structures of gray matter that form the dorsal subdivision of the diencephalonmassa intermedia The commissure that is located in the middle of the third ventricle, it is a route of communication between the left and right diencephalon, which is largely separated by the third ventriclethird and fourth ventricles What does the cerebral aqueduct connect?midbrain the middle division of brain responsible for hearing and sight; location where pain is registered; includes temporal lobe, occipital lobe, and most of the parietal lobearbor vitae branching pattern of the white matter of the cerebellumhypothalamus a basal part of the diencephalon governing autonomic nervous systempineal gland a small endocrine gland in the brainpia mater the highly vascular innermost of the 3 meningesdura mater the outermost (and toughest) of the 3 meningescerebrum anterior portion of the brain consisting of two hemispheresspinal cord a major part of the central nervous system which conducts sensory and motor nerve impulses to and from the brainsclera whitish fibrous membrane (albuginea) that with the cornea forms the outer covering of the eyeballcornea transparent anterior portion of the outer covering of the eyechoroid layer beneath sclera; helps supply retina with blood; dark, pigmented area that reduces reflection in the eyeciliary body the part of the tunic of the eye between the choroid coat and the irisora serrata retinae junction of the retina and ciliary bodylens the transparent structure behind the pupil that changes shape to help focus images on the retinasuspensory ligaments a fibrous membrane that holds the lens of the eye in placeiris diaphragm muscular diaphragm that controls the size of the pupilpupil contractile aperture in the iris of the eyemacula lutea a yellowish central area of the retina that is rich in cones and that mediates clear detailed visionfovea centralis area consisting of a small depression in the retina containing cones and where vision is most acuteoptic disc the point where the optic nerve enters the retinaoptic nerve the cranial nerve that serves the retinaanterior cavity between the back of the cornea and the front of the lens, contains aqueous humorposterior cavity between the lens and retina and contains vitreous humorrectus group (superior, inferior, medial, lateral) rectus groupobliques (superior, inferior) obliquesconjuctiva the lining of the eyelids and the c Overing of the scleralacrimal gland any of the glands in the eyes that secrete tearspinna the externally visible cartilaginous structure of the external earexternal auditory meatus external passage for sounds collected from the pinna to the tympanumtympantic membrane also known as the eardrum, is located between the outer and middle earossicles three tiny bones in the middle earmalleus hammer; first of the three auditory ossicles of the middle earincus the ossicle between the malleus and the stapesstapes the stirrup-shaped ossicle that transmits sound from the incus to the cochleapharyngotympanic tube connects middle ear and pharynxbony labyrinth cavity in the petrous part of the temporal bone that contains the membranous labyrinthvestibule middle part of the inner ear in front of the semicircular canals and behind the cochlea that contains the utricle and sacculesemicircular canals three loops of the fluid-filled tubes that are attached to the cochlea; They help us with our sense of balancecochlea the snail-shaped tube (in the inner ear coiled around the modiolus) where sound vibrations are converted into nerve impulses by the Organ of Cortimembranous labyrinth the sensory structures of the inner ear including the labyrinthine receptors and the cochleasaccule a small sac or pouch (especially the smaller chamber of the membranous labyrinth)utricle a small pouch into which the semicircular canals openampulla the dilated portion of a canal or duct especially of the semicircular canals of the earcochlear duct Membrane tube in cochlea, filled with fluid that vibrates when sound waves are transmitted by the stapes.fungiform papillae Mushroom-like protuberances often containing taste buds and located on the sides and tip of the tongue.filiform papillae ...circumvallate papillae Large, obvious protuberances always containing taste buds and distinguished easily because they form a "v" near the back of the tongue.papillae small rough elevations on tongue and roof of mouth; contain taste budstaste bud an oval sensory end organ on the surface of the tongueskeletal muscle a muscle that is connected at either or both ends to a bone and so move parts of the skeletonstratified squamous layers of flattened cells; used in protection (covers areas of high abrasion); found in skin and upper digestive tract (mouth, esophagus)  Chapter 5: Integumentary System 76 terms Thuy VChapter 5: Integumentary System Test 1: Chapter 5,6,7Learn Flashcards Write Spell Test Match Gravity Advertisement Upgrade to remove ads Sort Integumentary System forms the external body covering and protects deeper tissues from injury. Synthesize vitamin D and houses cutaneous (pain, pressure) receptors and sweat/oil glands. Layers of the integumentary system Epidemis and dermis Epidermis superficial region; epithelial cells; nonvascularized; 4 cell types and 4 or 5 layers What are the cells of the epidermis keratinocytes, melanocytes, epidermal dendritic cells, and tactile cells Keratinocytes produce fibrous protein keratin; arise in stratum basale; these cells undergo continuous mitosis Composition and function of Keratinocytes Which cells are tightly connected to one another by desmosomes. Arise in the stratum basale and undergo constant mitosis for epidermal growth. They are dead by the time they reach the surface of the skin. Keratin fibrous protein that helps give the epidermis its protective properties. Melanocytes epithelial cells that synthesize the pigment melanin. Found in the stratum basale. Epidermal dendritic (Langerhans) cells macrophages that help activate the immune system; first line of defense against pathogens Tactile (Merkel) cells touch receptors that expand into the epidermis. Associated with sensory nerve endings. What are the 5 layers of the epidermis ? (1) stratum corneum (2) structum lucideum (3) Stratum granulosum (4) stratum spinosum (5) Stratum Basale Thick skin covers the palms, fingertips, and soles of the feet Thin skin What kind of skin covers the rest of the body Stratum Corneum (horny layer) 20 to30 cell layers thick that accounts for up to ¾ of the epidermal thickness. Keratin and thickened plasma membranes of cells protect the skin against abrasion and penetration and glycolipids b/w its cells waterproofs this layer. Barrier against biological, chemical, and physical assaults. Statum Lucidum thin translucent band, consists of rows of clear, flat, dead kertinocytes. Visible only in thick skin. Stratum Granulosum consists of 3-5 layers in which the cells flatten. Keratinization begins. Cells flatten, nuclei and organelles begin to disintegrate and begin to accumulate 2 types of granules: keratohyaline granules and the lamellated granules. Keratohyaline granules What granules help to form keratin in the upper layers Lamellated granules granules that contain water resitant glycolipid that is spews into the extracellular space and is a major factor in slowing water loss across the epidermis. Stratum Spinosum Which stratum is several cell layers thick; Contain intermediate filaments mainly pre-keratin filaments that attaches to the desosomes. Abundant melanin granules and dendritic cells. Stratum Basale deepest epidermal layer, contain single row of stem cells, youngest keratinocytes, 10-25% of cells are the melanocytes. Also called, germinativum: cells that undergo rapid division Dermis Middle region, vascularized, bulk of the integumentary system; fibrous connective tissue. Dermis strong flexible connective tissue. Cells include: fibroblasts, macrophages, mast cells and white blood cells. Supplied with nerve fibers, lood vessels, and lymphatic vessels. What are the 2 layers of the dermis? Papillary and Reticular Papillary layer What layer forms the thin superficial, areolar connective tissue in which collagen and elastic fibers form woven mat with small blood vessels. Dermal papillae superior surface of papillary layer contain capillary loops, non encapuslated free nerve endings (pain receptors) and ecapsulated touch receptors (meissner's corpuscles)Reticular Layer makes up 80%of the thickness of dermis. Dense fibrous connective tissue, elastic fibers provide stretch-recoil properties. Hypodermis deepest region; anchors to muscles; subcutaneous deep into skin in various regions. Shock absorber and insulator Cutaneous Plexus network of blood vessels that nourishes the hypodermis it also contains collagen fibers that forms cleavage of tension lines. Collagen Fibers single most abundant protein in the body Adipose tissue fat storage, oils having unsaturated bonds (c=c; carbon-carbon double bonds) found in the hypodermis layer. What are the 3 pigments that contribute to skin color? Melanin, carotene and hemoglobin Melanin produced in melanocytes; migrates to keratinocytes where it forms "pigment shields" for nuclei (protects DNA from UV light damage); concentrated amounts found in freckles and pigmented moles Carotene yellow to orange, mostly in the palms and soles; Vitamin A precursor (necessary for normal vision); acquired from diet Hemoglobin oxygen carrying pigment of red blood cells. Produce pink or red color skin when concentrated in surface capillaries. What are the appendages of the skin? sweat glands, oil glands, hairs and hair follicles, nails Sweat (Sudoriferous) glands Eccrine and Apocrine sweat glands Eccrine sweat glands (merocrine exocytosis secretion) abundant on palms, soles, and forehead; 99% water, Na Cl, vitamin C, antibodies, and metabolic wastes; ducts connect to pores; body temperature regulation by cooling as the water of sweat evaporation Apocrine sweat glands (merocrine exocytosis glands) approx. 2000 of the; largely confined to axillary and anogenital areas. produce during puberty. Larger than eccrine glands and lie deeper in the dermis. Ducts connect to hair follicles. Ceruminous glands Modified apocrine glands in external ear canal; secrete cerumen (wax); mammary glands Mammary glands specialized sweat glands, secrete milk Sebaceous (oil) glands develop from hair follicles; become active at puberty; create sebum (oily substance)Sebum oily holocrine secretion (secretion by cell rupture) softens and lubricates the hair and skin, prevents hair from becoming brittle and slows water loss from the skin; has bactericidal action Hair Functions guarding the scalp against physical trauma; helps prevent heat loss; protects from sunlight (UV rays)Hair consists of dead keratinized cells; contains hard keratin; more durable than soft keratin of skinmelanin (yellow, rust brown, black) hair pigmentsgray/white hair decreases melanin production, increased air bubbles in shaft. Hair follicle extends from the epidermal surfaced into dermis; two-layered wall: outer connective tissue root sheath, inner epithelial root sheath Hair bulb expanded deep end of the hair Root hair plexus Hair follicle receptor; sensory nerve ending around each hair bulb Arrector Pilli smooth muscle attached to follicle; responsible for goose bumps. Vellus pale, fine body hair of children and adult females Terminal coarse, long hair of eyebrows, scalp, axillary, and pubic regions (and face and neck of males)Alopecia hair thinning in both sexes after age 40True baldness genetically determined; sex-linked trait (male pattern baldness)Structure of a nail scale-like modifications of the epidermis on fingers and toes. Hard keratin composition Functions of the integumentary system 1) protection 2) body temperature regulation 3) cutaneous sensations 4)metabolic function 5) blood reservoir 6) excretion What are the 3 types of barriers used for Protection (1) chemical, (2) physical/mechanical, and (3) biological. Chemical barrier which barrier has low p H secretion (acids) and defensins (antimicrobial proteins) retard bacterial activity. Physical/Mechanical barrier keratin and glycolipids block most water and water-soluable substances; limit penetration of skin by lipid-soluble substances (poison ivy)Biological barrier macrophage and macrophage-like cells; dendritic (taking up the bacteria and activate immune system) cells of the epidermis Cutaneous sensation temperature, touch, and pain; exteroceptors-- respond to stimuli arising outside the body Metabolic function nutrients reach the surface of the skin by diffusion; synthesis of Vitamin D precursor (UV light converts precursor to Vitamin D); what function of the integumentary system is this? Blood Reservoir What integumentary system function holds up to 5% of body's blood volume (not in epidermis)Excretion what integumenatary system function has nitrogenous wastes and salt in sweat (ammonia urea, uric acid)Skin Cancer most skin tumors (undifferentiated cells) are benign; primary risk factor is overexposure to UV radiation. Liposomes enzymes in skin lotions that can fix damage DNA3 Major types of skin cancer 1) Basal Cell Carcinoma 2) Squamous cell carcinoma 3) Melanoma Basal Cell Carcinoma least malignant, most common; stratum basale cells (undifferentiated) proliferate and slowly invade dermis and hypodermis. Cured by surgical removal. Squamous Cell Carcinoma second most commom; involves keratinocytes of stratum spinosum; most common on scalp, ears, lower lip, and hands; grow rapidly and metasize. Treated by radiation therapy and surgically removed. Melanoma Most dangerous; involves melanocytes; highly metastatic and resistant to chemotherapy; treated by wide surgical excision accompanied by immunotherapy ABCD rule of Melanoma Asymmetry, Border, Color, and Diameter Burns heat, electricity, radiation, certain chemicals; causes tissue damage, denatured cell protein and cell death Immediate threat of burns dehydration and electrolyte imbalance leading to renal shutdown and circulatory shock. First Degree epidermal damage ONLY. localized redness, edema (swelling), and pain Second Degree epidermal and upper dermal damage; blisters appear Third Degree entire thickness of skin damaged; gray-white, cherry red, or black; no initial edema or pain (nerve endings destroyed) skin grafting usually necessary  endoneurium endoneurium Also found in: Thesaurus, Medical, Encyclopedia, Wikipedia. Related to endoneurium: perineuriumendoneurium ( ˌɛndəʊˈnjʊərɪəm) n (Anatomy) the delicate connective tissue surrounding nerve fibres within a bundle [New Latin, from endo- + neuro- + -ium]Collins English Dictionary – Complete and Unabridged, 12th Edition 2014 © Harper Collins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014Thesaurus Antonyms Related Words Synonyms Legend: Switch to new thesaurus Noun 1. endoneurium - delicate connective tissue around individual nerve fibers in nerveconnective tissue - tissue of mesodermal origin consisting of e.g. collagen fibroblastsand fatty cells; supports organs and fills spaces between them and forms tendons andligaments Based on Word Net 3.0, Farlex clipart collection. © 2003-2012 Princeton University, Farlex Inc. Want to thank TFD for its existence? Tell a friend about us, add a link to this page, or visit the webmaster's page for free fun content . Link to this page: Facebook Twitter  Anatomy Chapter 4 32 terms jessjpowers Anatomy Chapter 4Learn Flashcards Write Spell Test Match Gravity Advertisement Upgrade to remove ads Sort Body Membrane cover surfaces, line body cavities and form protective (and often lubricating) sheets around organs. Epithelial Membrane *covering and lining membranes (which includes the cutaneous, mucous, and serous membranes)Cutaneous Membrane the skin, made up on keratinizing stratified squamous epithelium Mucous Membrane composed of epithelium that rests on a loose connective tissue membrane, lines all body cavities that open to the exterior and the hollow organs of the respiratory, digestive, urinary, and reproductive tracts Serous Membrane composed of a layer of simple squamous epithelium resting on a thin layer of areolar connective tissue Parietal Layer lines a specific portion of the wall of the ventral body cavity outer layer Visceral Layer folds in on itself, and covers the outside of the organs in their cavity inner layer Serous Fluid a clear fluid that allows organs to move easily across the cavity walls and one another without creating friction, VERY IMPORTANT when mobile organs such as the pumping heart and churning stomach are involved Peritoneum the serosa lining the abdominal cavity and covering its organs Pleura the membrane surrounding the lungs Pericardium the membrane surrounding the heart Synovial Membrane composed of soft areolar connective tissue and contain no epithelial cells at all, they line the fibrous capsules surrounding joints where they provide a smooth surface and secrete lubricating fluid Integumentary System essential to keeping water and other precious molecules in the body, it also keeps other foreign substances out Epidermis The outermost layer, make up of stratified squamous epithelium that is capable of keratinizing or becoming hard and tough. Dermis the second layer, underneath the epidermis, is made up of mostly dense connective tissue Hypodermis a subcutaneous tissue, made up on adipose tissue, anchors the skin to underlying organs, the "shock absorber" that cushions the body from falls and other injury Keratinocytes keratin cells, which produce keratin, the fibrous protein that makes the epidermis a tough protective layer Stratum Basale the deepest cell layer of the epidermis, lies closest to the dermis and is connected to it along the wavy borderline that resembles corrugated cardboard. this layer contains epidermal cells that receive the most nourishment from diffusion from the dermis Stratum Spinosum and Granulosum become flatter and increasingly full of keratin, then die and from the clear stratum lucidum Stratum Corneum the outermost layer, 20-30 cell layers thick, accounts for 3/4 of the epidermal thickness. Melanin a pigment that ranges in color from yellow to brown to black, produced by melanocytes, found in the stratum basale, controls the skin color, the more melanin you have in one place the darker it is Dermis the "hide" of the human, it is a strong envelope that helps to hold the body together. Papillary Layer the upper dermal region it is uneven and has peglike projections from its superior surface called dermal papillae Dermal Papillae indents the epidermis, many of these contain capillary loops which furnish nutrients to the epidermis, also have sensory receptors (they can feel)Reticular Layer the deepest skin layer, contains: blood vessels, sweat and oil glands, and deep pressure receptors called pacinian corpuscles. Phagocytes act to prevent bacteria that have managed to get through the epidermis from penetrating any deeper into the body Collagen and Elastic Fibers found thought the dermis, responsible for the toughness of the dermis, also attract and bind water and thus help to keep the skin hydrated, also give elasticity Exocrine Glands release secretions to the skin surface from ducts, formed by the stratum basale, reside in the dermis Sebaceous Glands oil glands, found all over the skin (except palms and soles of feet), their ducts empty into a hair follicle or onto the skin surface Sebum produced by the sebaceous glands, a mixture of oily substances and fragmented cells, a lubricant that keeps the skin soft and moist and prevents hair form becoming brittle, also kills bacteria Eccrine Glands produce sweat, important in regulating heat, supplied with nerve endings that cause them to secrete sweat when the external temperature is high Apocrine Glands largely confined to the axillary (armpits) and genital areas, larger than eccrine glands and empty into hair follicles, they smell Advertisement Upgrade to remove ads Features Quizlet Live Quizlet Learn Diagrams Flashcards Mobile Help Sign up Help Center Honor Code Community Guidelines Students Teachers About Company Press Jobs Privacy Terms Follow us Language© 2018 Quizlet Inc.  Skin Anatomy Drugs & Diseases > Anatomy Skin Anatomy Updated: Nov 29, 2017Author: Bardia Amirlak, MD; Chief Editor: Gregory Gary Caputy, MD, Ph D, FICS more... Share Email Feedback Sections Overview Epidermis Dermis Dermoepidermal Junction Epidermal Appendages Cutaneous Blood Supply Lymphatics Skin Innervation Surface Anatomy Skin Phototype Anatomy of Aging Skin Show All Media Gallery Tables References Overview The skin covers the entire external surface of the human body and is the principal site of interaction with the surrounding world. It serves as a protective barrier that prevents internal tissues from exposure to trauma, ultraviolet (UV) radiation, temperature extremes, toxins, and bacteria. Other important functions include sensory perception, immunologic surveillance, thermoregulation, and control of insensible fluid loss. The integument consists of 2 mutually dependent layers, the epidermis and dermis, which rest on a fatty subcutaneous layer, the panniculus adiposus. The epidermis is derived primarily from surface ectoderm but is colonized by pigment-containing melanocytes of neural crest origin, antigen-processing Langerhans cells of bone marrow origin, and pressure-sensing Merkel cells of neural crest origin. The dermis is derived primarily from mesoderm and contains collagen, elastic fibers, blood vessels, sensory structures, and fibroblasts. [ 1] See the image below. Anatomy of the skin. View Media Gallery During the fourth week of embryologic development, the single cell thick ectoderm and underlying mesoderm begin to proliferate and differentiate. The specialized structures formed by the skin, including teeth, hair, hair follicles, fingernails, toenails, sebaceous glands, sweat glands, apocrine glands, and mammary glands also begin to appear during this period in development. Teeth, hair, and hair follicles are formed by the epidermis and dermis in concert, while fingernails and toenails are formed by the epidermis alone. Hair follicles, sebaceous glands, sweat glands, apocrine glands, and mammary glands are considered epidermal glands or epidermal appendages, because they develop as downgrowths or diverticula of the epidermis into the dermis. [ 1, 2]The definitive multi-layered skin is present at birth, but skin is a dynamic organ that undergoes continuous changes throughout life as outer layers are shed and replaced by inner layers. Skin also varies in thickness among anatomic location, sex, and age of the individual. This varying thickness primarily represents a difference in dermal thickness, as epidermal thickness is rather constant throughout life and from one anatomic location to another. Skin is thickest on the palms and soles of the feet (1.5 mm thick), while the thinnest skin is found on the eyelids and in the postauricular region (0.05 mm thick). Male skin is characteristically thicker than female skin in all anatomic locations. Children have relatively thin skin, which progressively thickens until the fourth or fifth decade of life when it begins to thin. This thinning is also primarily a dermal change, with loss of elastic fibers, epithelial appendages, and ground substance. [ 3]Epidermis The epidermis contains no blood vessels and is entirely dependent on the underlying dermis for nutrient delivery and waste disposal via diffusion through the dermoepidermal junction. The epidermis is a stratified, squamous epithelium that consists primarily of keratinocytes in progressive stages of differentiation from deeper to more superficial layers. The named layers of the epidermis include the stratum germinativum, stratum spinosum, stratum granulosum, and stratum corneum. Keratinocytes The stratum germinativum, or the basal layer, is immediately superficial to the dermoepidermal junction. This single cell layer of keratinocytes is attached to the basement membrane via hemidesmosomes. As keratinocytes divide and differentiate, they move from this deeper layer to the more superficial layers. Once they reach the stratum corneum, they are fully differentiated keratinocytes devoid of nuclei and are subsequently shed in the process of epidermal turnover. Cells of the stratum corneum are the largest and most abundant of the epidermis. This layer ranges in thickness from 15-100 or more cells depending on anatomic location and is the primary protective barrier from the external environment. Melanocytes Melanocytes, derived from neural crest cells, primarily function to produce a pigment, melanin, which absorbs radiant energy from the sun and protects the skin from the harmful effects of UV radiation. Melanin accumulates in organelles termed melanosomes that are incorporated into dendrites anchoring the melanosome to the surrounding keratinocytes. Ultimately, the melanosomes are transferred via phagocytosis to the adjacent keratinocytes where they remain as granules. Melanocytes are found in the basal layer of the epidermis as well as in hair follicles, the retina, uveal tract, and leptomeninges. These cells are the sites of origin of melanoma. In areas exposed to the sun, the ratio of melanocytes to keratinocytes is approximately 1:4. In areas not exposed to solar radiation, the ratio may be as small as 1:30. Absolute numbers of melanosomes are the same among the sexes and various races. Differing pigmentation among individuals is related to melanosome size rather than cell number. Sun exposure, melanocyte-stimulating hormone (MSH), adrenocorticotropic hormone (ACTH), estrogens, and progesterones stimulate melanin production. With aging, a decline is observed in the number of melanocytes populating the skin of an individual. Since these cells are of neural crest origin, they have no ability to reproduce. Langerhans Cells Langerhans cells originate from the bone marrow and are found in the basal, spinous, and granular layers of the epidermis. They serve as antigen-presenting cells. They are capable of ingesting foreign antigens, processing them into small peptide fragments, binding them with major histocompatibility complexes, and subsequently presenting them to lymphocytes for activation of the immune system. An example of activation of this component of the immune system is contact hypersensitivity. Merkel Cells Merkel cells, also derived from neural crest cells, are found on the volar aspect of digits, in nail beds, on the genitalia, and in other areas of the skin. These cells are specialized in the perception of light touch. [ 1, 3]Dermis The primary function of the dermis is to sustain and support the epidermis. The dermis is a more complex structure and is composed of 2 layers, the more superficial papillary dermis and the deeper reticular dermis. The papillary dermis is thinner, consisting of loose connective tissue containing capillaries, elastic fibers, reticular fibers, and some collagen. The reticular dermis consists of a thicker layer of dense connective tissue containing larger blood vessels, closely interlaced elastic fibers, and coarse bundles of collagen fibers arranged in layers parallel to the surface. The reticular layer also contains fibroblasts, mast cells, nerve endings, lymphatics, and epidermal appendages. Surrounding the components of the dermis is the gel-like ground substance, composed of mucopolysaccharides (primarily hyaluronic acid), chondroitin sulfates, and glycoproteins. The deep surface of the dermis is highly irregular and borders the subcutaneous layer, the panniculus adiposus, which additionally cushions the skin. Fibroblasts The fibroblast is the major cell type of the dermis. These cells produce and secrete procollagen and elastic fibers. Procollagen is terminally cleaved by proteolytic enzymes into collagen that aggregates and becomes cross-linked. These tightly cross-linked collagen fibers provide tensile strength and resistance to shear and other mechanical forces. Collagen makes up 70% of the weight of the dermis, primarily Type I (85% of the total collagen) and Type III (15% of the total collagen). Elastic fibers constitute less than 1% of the weight of the dermis, but they play an enormous functional role by resisting deformational forces and returning the skin to its resting shape. [ 1]Dermoepidermal Junction The dermoepidermal junction is an undulating basement membrane that adheres the epidermis to the dermis. It is composed of 2 layers, the lamina lucida and lamina densa. The lamina lucida is thinner and lies directly beneath the basal layer of epidermal keratinocytes. The thicker lamina densa is in direct contact with the underlying dermis. These structures are the target of immunologic injury in bullous pemphigoid and epidermolysis bullosa. Dermal papillae from the papillary dermis contain a plexus of capillaries and lymphatics oriented perpendicular to the skin surface. These fingerlike projections are surrounded by similar projections of the epidermis. This highly irregular junction greatly increases the surface area over which oxygen, nutrients, and waste products are exchanged between the dermis and the avascular epidermis. [ 1]Epidermal Appendages Epidermal appendages are intradermal epithelial structures lined with epithelial cells with the potential for division and differentiation. These are important as a source of epithelial cells, which accomplish reepithelialization should the overlying epidermis be removed or destroyed in situations such as partial thickness burns, abrasions, or split-thickness skin graft harvesting. Epidermal appendages include the following: Sebaceous glands Sweat glands Apocrine glands Mammary glands Hair follicles They often are found deep within the dermis and in the face may even lie in the subcutaneous fat beneath the dermis. This accounts for the remarkable ability of the face to reepithelialize even the deepest cutaneous wounds. [ 1]Sebaceous Glands Sebaceous glands, or holocrine glands, are found over the entire surface of the body except the palms, soles, and dorsum of the feet. They are largest and most concentrated in the face and scalp where they are the sites of origin of acne. The normal function of sebaceous glands is to produce and secrete sebum, a group of complex oils that include triglycerides and fatty acid breakdown products, wax esters, squalene, cholesterol esters, and cholesterol. Sebum lubricates the skin to protect it against friction and makes the skin more impervious to moisture. Sweat Glands Sweat glands, or eccrine glands, are found over the entire surface of the body except the vermillion border of the lips, the external ear canal, the nail beds, the labia minora, and the glans penis and the inner aspect of the prepuce. They are most concentrated in the palms and soles and the axillae. Each gland consists of a coiled secretory intradermal portion that connects to the epidermis via a relatively straight distal duct. The normal function of the sweat gland is to produce sweat, which cools the body by evaporation. The thermoregulatory center in the hypothalamus controls sweat gland activity through sympathetic nerve fibers that innervate the sweat glands. Sweat excretion is triggered when core body temperature reaches or exceeds a set point. Apocrine and Mammary Glands Apocrine glands are similar in structure, but not identical, to eccrine glands. They are found in the axillae, in the anogenital region, and, as modified glands, in the external ear canal (ceruminous glands), the eyelid (Moll's glands), and the breast (mammary glands). They produce odor and do not function prior to puberty, which means they probably serve a vestigial function. The mammary gland is considered a modified and highly specialized type of apocrine gland. Hair Follicles Hair follicles are complex structures formed by the epidermis and dermis. (See the image below.) They are found over the entire surface of the body except the soles of the feet, palms, glans penis, clitoris, labia minora, mucocutaneous junction, and portions of the fingers and toes. Sebaceous glands often open into the hair follicle rather than directly onto the skin surface, and the entire complex is termed the pilosebaceous unit. [ 1, 4, 5]Anatomy of hair follicle. View Media Gallery Caucasian hair follicles are oriented obliquely to the skin surface, whereas the hair follicles of black persons are oriented almost parallel to the skin surface. Asian persons have vertically oriented follicles that produce straight hairs. These anatomic variations are an important consideration in avoiding alopecia when making incisions in the scalp. The base of the hair follicle, or hair bulb, lies deep within the dermis and, in the face, may actually lie in the subcutaneous fat. This accounts for the remarkable ability of the face to re-epithelialize even the deepest cutaneous wounds. A band of smooth muscle, the arrector pili, connects the deep portion of the follicle to the superficial dermis. Contraction of this muscle, under control of the sympathetic nervous system, causes the follicle to assume a more vertical orientation. Hair growth exhibits a cyclical pattern. The anagen phase is the growth phase, whereas the telogen phase is the resting state. The transition between anagen and telogen is termed the catagen phase. Phases vary in length according to anatomic location, and the length of the anagen phase is proportional to the length of the hair produced. At any one time at an anatomic location, follicles are found in all 3 phases of hair growth. This is extremely important for laser hair removal, because follicles in the anagen phase are susceptible to destruction, whereas resting follicles are more resistant. This explains why multiple treatments of an area may be necessary to ensure adequate hair removal. Cutaneous Blood Supply Cutaneous vessels ultimately arise from underlying named source vessels. Each source vessel supplies a 3-dimensional vascular territory from bone to skin termed an angiosome. Adjacent angiosomes have vascular connections via reduced caliber (choke) vessels or similar caliber (true) anastomotic vessels. The cutaneous vessels originate either directly from the source arteries (septocutaneous or fasciocutaneous perforators) or as terminal branches of muscular vessels (musculocutaneous perforators). During their course to the skin, the cutaneous vessels travel within or adjacent to the connective tissue framework and supply branches to each tissue with which they come into close contact (bone, muscle, fascia, nerve, fat). They emerge from the deep fascia in the vicinity of the intermuscular or intramuscular septa or near tendons and travel toward the skin, where they form extensive subdermal and dermal plexuses. The dermis contains horizontally arranged superficial and deep plexuses, which are interconnected via communicating vessels oriented perpendicular to the skin surface. Cutaneous vessels ultimately anastomose with other cutaneous vessels to form a continuous vascular network within the skin. Clinically, this extensive horizontal network of vessels allows for random skin flap survival. [ 6, 7, 8]Thermoregulation In addition to the skin's natural heat conductivity and loss of heat from the evaporation of sweat, convection from cutaneous vessels is a vital component of thermoregulation. Cutaneous blood flow is 10-20 times that required for essential oxygenation and metabolism, and large amounts of heat can be exchanged through the regulation of cutaneous blood flow. The thermoregulatory center in the hypothalamus controls vasoconstriction and vasodilatation of cutaneous vessels through the sympathetic nervous system. Lymphatics Skin lymphatics parallel the blood supply and function to conserve plasma proteins and scavenge foreign material, antigenic substances, and bacteria. Blind-ended lymphatic capillaries arise within the interstitial spaces of the dermal papillae. These unvalved, superficial dermal vessels drain into valved deep dermal and subdermal plexuses. These then coalesce to form larger lymphatic channels, which course through numerous filtering lymph nodes on their way to join the venous circulation near the subclavian vein – internal jugular vein junction bilaterally. Skin Innervation Sensory perception is critically important in the avoidance of pressure, mechanical or traumatic forces, and extremes of temperature. Numerous specialized structures are present in the skin to detect various stimuli. As previously mentioned, Merkel cells of the epidermis detect light touch. Meissner corpuscles also detect light touch. These are found in the dermal papillae and are most concentrated in the fingertips. Pacini corpuscles are found deep within the dermis or even in the subcutaneous tissue. These structures are specialized to detect pressure. Pain is transmitted through naked nerve endings located in the basal layer of the epidermis. Krause bulbs detect cold, whereas Ruffini corpuscles detect heat. Heat, cold, and proprioception also are located in the superficial dermis. Cutaneous nerves follow the route of blood vessels to the skin. The area supplied by a single spinal nerve, or a single segment of the spinal cord, is termed a dermatome. Adjacent dermatomes may overlap considerably, which is important to note when performing field blocks with local anesthesia. [ 1, 9]Surface Anatomy Lines and creases are evident over major and minor joints. Skin contraction produces wrinkles and creases that lie perpendicular to the underlying muscular vector force. Relaxed skin tension lines (RSTL), however, are formed during relaxation and often follow a different direction than age and contracting wrinkles. (See the image below.) Relaxed skin tension lines are created by the natural tension on the skin from underlying structures. [ 10]Four main facial lines show the direction of relaxed skin tension lines. View Media Gallery Papillary ridges on the tips of the digits of the hands and feet and the surface of palms and soles are often used for personal identification. These are also known as friction ridges, since they assist in the ability to grasp. They are formed during fetal development and are unique to each individual, including identical twins. This distinct pattern does not change with aging. Stratum mucosum composes the outer surface of the ridges with underlying dermal papillae. Sweat pores are usually located at the top of the ridges. [ 11]Skin Phototype The amount of melanin pigment in the skin determines an individual's skin color (skin phototype). Skin pigment can be inherited genetically or can be acquired through various diseases. Hormonal changes during pregnancy can also vary the amount of pigmentation. The Fitzpatrick Scale is used to classify skin complexion and response to UV exposure. (See table 1, below.) This classification is based on a personal history of sunburning and suntanning. [ 12] This classification is used clinically for evaluation of facial skin pigmentation before resurfacing procedures and is important for predicting outcomes and adverse effects. Table 1: The Fitzpatrick Scale (Open Table in a new window)Skin Type Color Features I White or freckled skin Always burns, never tans II White skin Burns easily, tans poorly III Olive skin Mild burn, gradually tans IV Light brown skin Burns minimally, tans easily V Dark brown skin Rarely burns, tans easily VI Black skin Never burns, always tans Anatomy of Aging Skin Age-associated skin changes include thinning, skin laxity, fragility, and wrinkles. Sun-exposed areas demonstrate additional aging changes, including dyspigmentation, premature wrinkling, telangiectasia, and actinic elastosis. Cutaneous aging is characterized by intrinsic and extrinsic processes. Intrinsic, or chronologic, aging is a genetically determined and inevitable process in skin, including photoprotected skin. Intrinsic aging naturally occurs and is exacerbated by extrinsic aging, which is environmentally induced. Aging at the cellular level is thought to be related to cellular senescence, specifically, the shortening of telomeres (the terminal portions of chromosomes) with each cell cycle. Telomere shortening ultimately results in cell-cycle arrest or apoptosis once a critical length is reached. Preventable environmental factors that amplify intrinsic aging include sun exposure and smoking. Long-term UV-A radiation exposure accelerates intrinsic aging via the formation of reactive oxygen species (ROS). ROS lead to inflammatory cytokines and the up-regulation of matrix metalloproteinases, which result in the breakdown of collagen. UV-B radiation can also contribute to this aging process by causing direct deoxyribonucleic acid (DNA) mutations. Histopathologically, photoaging is manifest as flattening of the dermal-epidermal junction, resulting in decreased nutrient transfer between the layers, heliodermatitis or chronic inflammation, elongated and collapsed fibroblasts, disorganized collagen fibrils with overall decrease in collagen levels, and the accumulation of abnormal elastin-containing material termed solar elastosis. [ 13, 14]Sections Skin Anatomy Overview Epidermis Dermis Dermoepidermal Junction Epidermal Appendages Cutaneous Blood Supply Lymphatics Skin Innervation Surface Anatomy Skin Phototype Anatomy of Aging Skin Show All Media Gallery Tables References 