Physiology_Levy

Physiology_Levy. Bilateral lesions of the auditory cortex have some effect on the ability to distinguish the frequency or intensity of different sounds, and they reduce the abilities to localize sound and to understand speech. Unilateral lesions, however, have little effect, especially if the nondominant (for language) hemisphere is involved (see ). Evidently, frequency discrimination depends on activity at lower levels of the auditory pathway, possibly the inferior colliculus. Twosimpletestsareoftenusedclinicallytodistinguishthemostimportanttypesofdeafness:conduction loss andsensorineural loss. Conductionhearinglossoccursindisordersoftheexternalear(e.g.,earcanalblockedbycerumen)ormiddleear(e.g.,ruptureoftheeardrum).Sensorineuralhearinglossreflectsdisordersoftheinnerear,thecochlearnerve,orcentralconnections.


Physiology_Levy


Physiology_Levy

Physiology_Levy. The Vestibular System The vestibular system detects angular and linear accelerations of the head. Signals from the vestibular system allow the body to make adjustments in posture that maintain balance and trigger head and eye movements to stabilize the visual image on the retina. The following description of the vestibular system emphasizes the sensory aspects of vestibular function, and it introduces the central vestibular pathways. The role of the vestibular apparatus in motor control is discussed in The Vestibular Apparatus Structure of the Vestibular Labyrinth The vestibular apparatus, like the cochlea, consists of a component of the membranous labyrinth located within CHAPTER 8 The Special Senses the bony labyrinth. The vestibular apparatus on each side is composed of three semicircular canals and two otolith organs ( Fig. 8.23 Fig. 8.17A).


Physiology_Levy

Physiology_Levy. CHAPTER 8 The Special Senses the bony labyrinth. The vestibular apparatus on each side is composed of three semicircular canals and two otolith organs ( Fig. 8.23 Fig. 8.17A). These structures contain endolymph and are surrounded by perilymph. The semicircular canals are named the horizontal, anterior, and posterior canals. The otolith organs are the utricle and the saccule (together indicated as “Vestibule” in Fig. 8.17A ). Each semicircular canal has a swelling called an ampulla at the point where it joins the utricle. The saccule connects with the cochlea, through which endolymph (produced by the stria vascularis of the cochlea) can reach the vestibular apparatus. The three semicircular canals on one side are matched with corresponding coplanar semicircular canals on the other side. The horizontal canals on each side of the head correspond, as do the anterior canal on one side and the posterior canal on the other side (see


Physiology_Levy

Physiology_Levy. Fig. 8.23B ). This arrangement allows the sensory epithelia, in corresponding pairs of canals on the two sides, to cooperate in sensing acceleration of the head about three nearly orthogonal axes in space. The horizontal canals are not truly horizontal; rather, they lie in the horizontal plane if the head is tilted down 30 degrees in relation to the horizon. The ampulla of each of the semicircular canals contains a sensory epithelium called a crista ampullaris, or ampullary crest ( Fig. 8.24 ). An ampullary crest consists of a ridge, transverse to the long axis of the canal, that is covered by epithelium containing vestibular hair cells. These hair cells are innervated by primary afferent fibers of the vestibular nerve, which is a subdivision of the vestibulocochlear nerve (CN VIII).


Physiology_Levy

Physiology_Levy. Like cochlear hair cells, each vestibular hair cell contains a set of stereocilia on its apical surface. However, unlike cochlear hair cells, vestibular hair cells also contain a large single kinocilium. The cilia on ampullary hair cells are embedded in a gelatinous structure called the cupula. The cupula and the crista occlude the lumen of the ampulla completely. Movement of endolymph, produced by angular acceleration of the head about an axis perpendicular to the plane of the canal, deflects the cupula and consequently bends the cilia on the hair cells. The cupula has the same specific gravity as endolymph, and thus it is unaffected by linear acceleratory forces, such as gravity. The sensory epithelia of the otolith organs are called the macula utriculi and the macula sacculi (


Physiology_Levy

Physiology_Levy. The sensory epithelia of the otolith organs are called the macula utriculi and the macula sacculi ( Fig. 8.25 ). The utricle is oriented nearly horizontally; the saccule is oriented vertically. Their hair cells are embedded in the epithelium that overlies each macula. As in the ampullary crests, the stereocilia and kinocilia of the macula project into a gelatinous mass. However, the gelatinous mass in the macula contains numerous otoliths (“ear stones”) composed of calcium carbonate crystals. Together, the gelatinous mass and its otoliths are known as an otolithic membrane. The otoliths increase the specific gravity of the otolithic membrane to about twice that of the endolymph. Hence, the otolithic membrane tends to move when subjected to acceleration, whether linear (such as that produced by gravity) or angular, particularly when the center of rotation is outside the head.


Physiology_Levy

Physiology_Levy. • Fig. 8.23 A, Lateral view of the right semicircular canals of a rhesus monkey that were dissected after being filled with plastic. Note the ampullae associated with each canal. Scale is in millimeters. B,Overheadviewofthebaseoftheskullshowingtheorientationofstructuresoftheinnerear.Coplanar pairsofsemicircularcanalsincludethehorizontalcanals,aswellastheanteriorandcontralateralposterior canals.(A,CourtesyofDr.JohnSimpson,New YorkUniversity SchoolofMedicine.B,Redrawnfrom Haines DE [ed]. Fundamental Neuroscience for Basic and Clinical Applications. 3rd ed. Philadelphia: ChurchillLivingstone;2006.) •Fig. 8.24 Drawing of an Ampullary Crest Inside an Ampulla.Thestereocilia andthekinocilium of eachhaircellextendintothecupula,whichextendsacrosstheentirecross-sectionoftheampulla.Head movement(acceleration)totherightwouldresultinendolymphpressuretotheleftanddeflectionofthe cupulatotheleft. Innervation of Sensory Epithelia of the Vestibular Apparatus


Physiology_Levy

Physiology_Levy. Innervation of Sensory Epithelia of the Vestibular Apparatus The cell bodies of the primary afferent fibers of the vestibular nerve are located in the Scarpa ganglion. The neurons are bipolar, and their cell bodies, as well as axons, are myelinated. Peripherally, the vestibular nerve gives off separate • Fig. 8.25 Structure of one of the otolith organs, the saccule. Note the orderly variation in kinocilium orientation, as well as their mirror symmetry with regard to the striola. (Redrawn from Lindeman HH. Adv Otorhinolaryngol 1973;20:405.) CHAPTER 8 The Special Senses 153 branches to each of the vestibular epithelia; centrally, it accompanies the cochlear and facial nerves as they enter the internal auditory meatus of the skull.


Physiology_Levy

Physiology_Levy. CHAPTER 8 The Special Senses 153 branches to each of the vestibular epithelia; centrally, it accompanies the cochlear and facial nerves as they enter the internal auditory meatus of the skull. Like cochlear hair cells, vestibular hair cells are functionally polarized, and the transduction mechanism is presumed to be similar. When the stereocilia are bent toward the longest cilium (in this case, the kinocilium), conductance of the apical membrane increases for cations and, because of the high K+ concentration of the endolymph, K+ enters, and the vestibular hair cell is depolarized (


Physiology_Levy

Physiology_Levy. Fig. 8.26 ). Conversely, when the cilia are bent away from the kinocilium, the hair cell is hyperpolarized. The hair cell releases an excitatory neurotransmitter (either glutamate or aspartate) tonically, so that the afferent fiber on which it synapses has a resting discharge. When the hair cell is depolarized, more neurotransmitter is released, and the discharge rate of the afferent fiber increases. Conversely, when the hair cell is hyperpolarized, less neurotransmitter is released, and the firing rate of the afferent fiber slows. Angular accelerations of the head produce minute movement of the endolymph in relation to the head (


Physiology_Levy

Physiology_Levy. Angular accelerations of the head produce minute movement of the endolymph in relation to the head ( Fig. 8.27 ). This happens because the inertia of the endolymph causes it to resist the initial acceleration of the membranous labyrinth. This lag pushes on the cupula, causes the cilia to bend, and consequently changes the discharge rates of the vestibular afferent fibers. All the cilia in a given ampullary crest are oriented in the same direction. In the horizontal canal, the cilia are oriented toward the utricle, and in the other ampullae, they are oriented away from the utricle. • Fig. 8.26 Functional Polarization of Vestibular Hair Cells. When the stereocilia are bent toward the kinocilium, the hair cell is depolarized, and the afferent fiber is excited. When the stereocilia are bent away from the kinocilium, the hair cell is hyperpolarized, and the afferent discharge slows or stops. (Redrawn from Kandel ER, Schwartz JH. Principles of Neural Science. New York: Elsevier; 1981.)


Physiology_Levy

Physiology_Levy. Turning motion of head AmpullaAxis of Axis of hair cells •Fig. 8.27 Effect of Leftward Head Movement on the Activity of Vestibular Afferent Fibers Supplying Hair Cells in the Horizontal Semicircular Canals. The perspective in the figure is from above the head looking down. In this case, head rotation (acceleration) to the left causes endolymph pressure to the right and the result is increased output from the left canal and decreased output from the right canal. Small arrows intheampullaindicatethefunctionalpolarityofthehaircells.Thelarge curved arrow atthetopindicatesmovementofthehead;smaller curved arrows indicaterelativemovementoftheendolymph. The way in which angular acceleration of the head affects the discharge of vestibular afferent fibers is exemplified by the activity that originates from the horizontal canals. Fig.


Physiology_Levy

Physiology_Levy. 8.27 shows the horizontal canals and utricle, as seen from above as the head is rotated (accelerated) to the left. As acceleration to the left begins, the inertia of the endolymph in the horizontal canals increases pressure toward the right side. This causes the cilia to bend on hair cells of the ampulla of the left horizontal canal toward the utricle and bends the cilia of the right canal away from the utricle. These actions increase the firing rate in the afferent fibers on the left and decrease the firing rate of the afferent fibers on the right. Once the head is moving at a constant velocity of rotation (i.e., no acceleration), there would be no force on either cupula, and therefore the hair cells of both canals would be firing as they do at rest. However, when the indicated rotation is stopped, the inertia of the endolymph creates a force on both cupulas, but in the direction opposite to that caused by the original acceleration. This results in an increase in the discharge rate


Physiology_Levy

Physiology_Levy. is stopped, the inertia of the endolymph creates a force on both cupulas, but in the direction opposite to that caused by the original acceleration. This results in an increase in the discharge rate of afferent fibers on the right side and a decrease in the discharge rate on the left. This postrotatory effect is of functional and clinical significance.


Physiology_Levy

Physiology_Levy. Unlike the hair cells in the ampullary crests, not all the hair cells in the otolith organs are oriented in the same •Fig. 8.28 Functional Polarization of Hair Cells in the Otolith Organs. A, Thesaccule.B, Theutricle.Thestriolaineachcaseisindicatedbythedotted line. (RedrawnfromSpoendlinHH.In:WolfsonRJ[ed].The Vestibular System and Its Diseases. Philadelphia:UniversityofPennsylvaniaPress;1966.) Irritationofthevestibularlabyrinth,asinMeniere’s disease, canresultinrhythmicconjugatedeviationsoftheeyes,followedbyquickreturnsaccades.Thisconditionisknownasnystagmus (see Chapter9).Theseeye movementsareaccompaniedbyasenseofvertigo andoftennausea. Thebraininterpretsadifferenceintheinputfromthetwosidesofthevestibularsystemasheadmotion.Irritation(ordestruction)ofonelabyrinthproducesanasymmetryofinputthatresultsinabnormaleyemovementandassociatedpsychologicaleffects. direction. Instead, they are oriented in relation to a ridge, called the striola, along the otolith organ (see


Physiology_Levy

Physiology_Levy. direction. Instead, they are oriented in relation to a ridge, called the striola, along the otolith organ (see Fig. 8.25 ). In the utricle, the hair cells on either side of the striola are polarized toward the striola, whereas in the saccule they are polarized away from the striola. Because the striola in each otolith organ is curved, there are hair cells with all orientations in the plane of the organ (


Physiology_Levy

Physiology_Levy. Fig. 8.28 ). In any particular orientation of the head, the cilia of the hair cells are bent to varying extents according to their orientation in relation to the gravitational vector. This results in a particular pattern of input from the otolith organs to the CNS. When the head is tilted to a new position, the orientation of the otolithic membranes in relation to the gravitational vector changes, and so the cilia of the hair cells are bent in a new way. This change in the bending of the cilia of the hair cells changes the pattern of input from the otolith organs to the CNS and creates the sensation of movement, as well as possibly triggering various reflexes. Similarly, a linear acceleration caused by other forces, such as might occur in a fall or the angular acceleration when a car turns around a curve (angular accelerations have linear centripetal and instantaneous tangential components), also affects output from the otolith organs.


Physiology_Levy

Physiology_Levy. The vestibular afferent fibers project to the brainstem through the vestibular nerve. As previously mentioned, the cell bodies of these afferent fibers are located in the Scarpa ganglion. These primary afferent fibers terminate in the vestibular nuclei (see Fig. 4.6 D to E), which are located in the rostral medulla and caudal pons and in specific regions of the cerebellum, most prominently in the nodulus. The vestibular nuclei give rise to various projections, including projections through the medial longitudinal fasciculus (see


Physiology_Levy

Physiology_Levy. The vestibular nuclei give rise to various projections, including projections through the medial longitudinal fasciculus (see Fig. 4.6C to E ) to the oculomotor nuclei. Therefore, it is not surprising that the vestibular nuclei exert powerful control over eye movements (the vestibule-ocular reflex). Other projections give rise to the lateral and medial vestibulospinal tracts, which, respectively, provide for the activation of trunk and neck muscles and thereby contribute to equilibrium and to head movements (vestibulocolic reflex). There are vestibular pathways to the cerebellum, the reticular formation, and the contra-lateral vestibular complex, as well as to the thalamus. The pathway to the thalamus, via a projection to the cerebral cortex, mediates conscious sensation of vestibular activity. Vestibular reflexes and clinical tests of vestibular function are described in The Chemical Senses


Physiology_Levy

Physiology_Levy. The Chemical Senses The senses of gustation (taste) and olfaction (smell) help detect chemical stimuli that are present either in food and drink or in the air. In the evolution of humans, these chemical senses apparently did not have the survival value of some of the other senses, but they contribute considerably to quality of life and food selection, and they are important stimulants of digestion. In other animals, the chemical senses have greater survival value, and their activation evokes a number of social behaviors, including mating, territoriality, and feeding. The stimuli that we commonly know as tastes are actually mixtures of five elementary taste qualities: salty, sweet, sour, bitter, and umami. Taste stimuli that are particularly effective in eliciting these sensations are, respectively, sodium chloride, sucrose, hydrochloric acid, quinine, and mono-sodium glutamate. Umami has been described as having a proteinaceous, meaty character.


Physiology_Levy

Physiology_Levy. bThe existence of a sixth, taste, fat (free fatty acids), is currently being debated. CHAPTER 8 The Special Senses The sensation of taste depends on the activation of chemoreceptors located in taste buds. A taste bud consists of a group of 50 to 150 receptor cells, as well as supporting cells and basal cells (Fig. 8.29A ). The chemoreceptor cells synapse at their bases with primary afferent nerve fibers, and their apices have microvilli that extend toward a taste pore. Chemoreceptor cells live only about 10 days. They are continuously replaced by new chemoreceptor cells that differentiate from basal cells located near the base of the taste bud.


Physiology_Levy

Physiology_Levy. Chemoreceptor molecules, each specialized for one type of taste stimulus, sit on the microvilli of chemoreceptor cells and detect molecules that diffuse into the taste pore from the overlying mucus of the tongue, part of which originates from glands adjacent to the taste buds. Some stimuli can pass directly into the cell to depolarize it (Na+ for salty and H+ for sour) or open cation channels to generate a receptor potential (also salty and sour), whereas others (sucrose, quinine, and glutamate for sweet, bitter, and umami) activate a second messenger that can either open cation channels or directly activate intracellular Ca++ stores (see Fig. 8.29B ). In each case, depolarization of the receptor results in the release of an excitatory neurotransmitter and, consequently, action potentials in the primary afferent nerve fiber that are transmitted to the CNS.


Physiology_Levy

Physiology_Levy. Coding of taste, however, is not based entirely on the selectivity of the chemoreceptors for the different primary qualities because each cell responds to a range of stimuli, although most intensely to one. Because most natural tastes have chemicals that effect responses from a number of chemoreceptors, recognition of taste quality appears to depend on the patterned input from a population of chemoreceptors, each responding differentially to the components of the stimulus. The intensity of the stimulus is reflected in the total amount of activity evoked. Distribution and Innervation of Taste Buds Taste buds are located on different types of taste papillae found on the tongue, palate, pharynx, and larynx. Types of taste papillae include fungiform and foliate papillae on the anterior and lateral aspects, respectively, of the tongue and circumvallate papillae on the base of the tongue (see Fig. 8.29C).


Physiology_Levy

Physiology_Levy. Fig. 8.29C). The circumvallate papillae may contain several hundred taste buds. The tongue in humans may have several thousand taste buds. The sensitivity of different regions of the tongue for different taste qualities varies slightly because taste buds responding to each type of taste are widely distributed. The taste buds are innervated by three cranial nerves. The chorda tympani branch of the facial nerve (CN VII) innervates taste buds on the anterior two thirds of the tongue, and the glossopharyngeal nerve (CN IX) innervates taste buds on the posterior third of the tongue (see Fig. 8.29C ). The vagus nerve (CN X) innervates a few taste buds in the larynx and upper esophagus. The cell bodies of taste fibers in cranial nerves VII, IX, and X are located in the geniculate, petrosal, and nodose ganglia, respectively. The central processes of the afferent fibers enter the medulla, join the solitary tract, and synapse in the nucleus of the solitary tract (see Fig. 4.6


Physiology_Levy

Physiology_Levy. Fig. 4.6 D to E). In some animals, including several rodent species, the second-order taste neurons of the solitary nucleus project rostrally to the ipsilateral parabrachial nucleus. The parabrachial nucleus then projects to the small-celled (parvocellular) part of the ventroposterior medial (VPMpc) nucleus of the thalamus. In monkeys, the solitary nucleus projects directly to the VPMpc nucleus. The VPMpc nucleus is connected to two different gustatory areas of the cerebral cortex: one in the face area of the S1 cortex and the other in the insula. An unusual feature of the central gustatory pathway is that it is predominantly an uncrossed pathway (unlike the central somatosensory pathways, which are predominantly crossed).


Physiology_Levy

Physiology_Levy. The sense of smell is much better developed in some animals (macrosmatic animals) than in humans. The ability of dogs to track other animals on the basis of odor is legendary, as is the use of pheromones by insects to attract mates. However, olfaction contributes to humans’ emotional life, and odors can effectively conjure up memories. It also helps Release of • Fig. 8.29 A, A taste bud is shown with the taste pore at the top and its innervation below. B, basal cell; R, ciliated taste receptor cells; S, supporting cells. B, Taste receptor cell showing second messenger, ligand-gated, and direct depolarization resulting in depolarization of the cell.


Physiology_Levy

Physiology_Levy. • Fig. 8.29, cont’d C, Distribution of the taste buds on the tongue and their innervation. (Redrawn from Squire LR, et al [eds]. Fundamental Neuroscience. San Diego, CA: Academic Press; 2002.) people avoid consuming spoiled food and detect dangerous situations. For example, an unpleasant odorant is added to odorless, colorless natural gas so that people can easily detect a leak. Odor has more primary qualities than taste does. As many as 1000 different odor receptors are coded in the human genome, and although only approximately 350 types are functional, they represent the largest population of G protein–coupled receptors in the genome. The olfactory mucosa also contains somatosensory receptors of the trigeminal nerve. When performing clinical tests of olfaction, clinicians must avoid activating these somatosensory receptors with thermal or noxious stimuli, such as the ammonia used in “smelling salts.”


Physiology_Levy

Physiology_Levy. The olfactory chemoreceptor cells are located in the olfactory mucosa, a specialized part of the nasopharynx. Olfactory chemoreceptors are bipolar nerve cells ( Fig. 8.30 ). The cThe conscious perception of flavor, particularly of foods, is the result of both olfactory and gustatory input based on directly inhaled odor, taste from the food as it is macerated in the mouth, and retronasal odor from the volatile molecules that are released by maceration and pass up into the nasal cavity from the pharynx.


Physiology_Levy

Physiology_Levy. CHAPTER 8 The Special Senses 157 nonmotile cilia on the apical surface of these cells contain chemoreceptors that detect odorant chemicals dissolved in the overlying mucus layer. From its opposite side, the cell gives off an unmyelinated axon that joins other olfactory nerve filaments and penetrates the base of the skull through openings in the cribriform plate of the ethmoid bone. These olfactory nerves synapse in the olfactory bulb, a portion of the cerebral hemisphere of the brain located at the base of the cranial cavity, just below the frontal lobe ( Fig. 8.31 Humans have about 10 million olfactory chemoreceptors. Like taste cells, olfactory chemoreceptors have a short life span (about 60 days), and they are also continuously replaced. However, olfactory receptor cells are true neurons and, as such, are the only neurons that are continuously regenerated throughout life.


Physiology_Levy

Physiology_Levy. The olfactory mucosa is exposed to odorant molecules by ventilatory air currents or from the oral cavity during feeding. Sniffing increases the influx of odorants. The odorants are temporarily bound in mucus to an olfactory binding protein that is secreted by a gland in the nasal cavity. Olfactory coding resembles taste coding in that most natural odors are complex and consist of many molecules that excite a wide variety of olfactory chemoreceptors. Coding for a particular perceived odor depends on the responses of many olfactory chemoreceptors, and the strength of the odorant is represented by the overall amount of afferent neural activity. The initial synapse of the olfactory pathway is located in the olfactory bulb, which is a specialized portion of the cerebral cortex and located on the underside of the frontal lobe. It contains mitral cells, interneurons (granule cells; periglomerular cells), and distinct synaptic clusters (glomeruli; see


Physiology_Levy

Physiology_Levy. Fig. 8.31 ) in which theses interact with olfactory afferent fibers. As the olfactory afferent fibers reach the olfactory bulb from the olfactory mucosa, they branch as they approach an olfactory glomerulus to synapse on the dendrites of mitral cells. Each glomerulus is the target of thousands of olfactory afferent fibers, but all the afferent fibers to a single glomerulus convey input from the one type of olfactory receptor. This is all the more remarkable because olfactory receptor cells are being regenerated continuously and new axons must therefore navigate their way to a correct glomerulus.


Physiology_Levy

Physiology_Levy. The granule and periglomerular cells are inhibitory inter-neurons. They form dendrodendritic reciprocal synapses with the dendrites of mitral cells. Activity in a mitral cell depolarizes these inhibitory cells, and they in turn inhibit the original and adjacent glomeruli. Because each glomerulus is specialized by being the target of afferent fibers for a unique combination of odor qualities, this appears to be a way of enhancing stimulus contrast, much the way horizontal cells do in the retina. In addition, it provides a mechanism for adaptation to continuous stimulation.


Physiology_Levy

Physiology_Levy. • Fig. 8.30 Olfactory chemoreceptors and supporting cells. (Redrawn from de Lorenzo AJD. In: Zotterman Y [ed]. Olfaction and Taste. Elmsford, NY: Pergamon; 1963.) • Fig. 8.31 Drawing of a Sagittal Section Through an Olfactory Bulb, Showing Terminations of the Olfactory Chemoreceptor Cells in the Olfactory Glomeruli and the Intrinsic Neurons of the Olfactory Bulb. The axons of the mitral cells are shown exiting in the olfactory tract to the right. (Modified from House EL, Pansky B. A Functional Approach to Neuroanatomy. 2nd ed. New York: McGraw-Hill; 1967.) CHAPTER 8 The Special Senses


Physiology_Levy

Physiology_Levy. CHAPTER 8 The Special Senses The axons of mitral cells leave the olfactory bulb and enter the olfactory tracts. From here, the olfactory connections become highly complex. Within the olfactory tracts is a nucleus, called the anterior olfactory nucleus, that receives input from the olfactory bulb and projects to the contralateral olfactory bulb through the anterior commissure. As each olfactory tract approaches the base of the brain, it splits into the lateral and medial olfactory striae. Axons of the lateral olfactory stria synapse in the primary olfactory cortex, which includes the prepiriform cortex (and, in many animals, the piriform lobe). The medial olfactory stria includes projections to the amygdala, as well as to the basal forebrain. These structures are portions of, or directly connected to, the limbic system (see


Physiology_Levy

Physiology_Levy. Of note is that the olfactory pathway is the only sensory system that does not have an obligatory synaptic relay in the thalamus before signals reach the cortex. However, olfactory information does reach the mediodorsal nucleus of the thalamus, and it is then transmitted to the prefrontal and orbitofrontal cortex. The functional roles of olfaction, in addition to the conscious perception of odor, include providing much of the subtleties of taste by enhancing the 1. Light enters the eye through the cornea and lens and is focused on the retina, which lines the back of the eye. The cornea is the most powerful refractive surface, but the lens has a variable power that allows images of near objects to be focused on the retina. The iris regulates depth of field and the amount of illumination that enters the eye. 2.


Physiology_Levy

Physiology_Levy. 2. The outer segments of the photoreceptor cells transduce light. Photoreceptors synapse on retinal bipolar cells, which in turn synapse on other interneurons and on ganglion cells. The ganglion cells project to the brain through the optic nerve. The optic disc, where the optic nerve leaves the retina, contains no photoreceptors and is therefore a blind spot. The portion of the retina with the highest degree of spatial resolution is the fovea and the surrounding macula. 3. Rod photoreceptors have high sensitivity, do not discriminate among colors, and function best under low light levels. Cone photoreceptors have lower sensitivity but higher spatial resolution. Color vision relies on the three types of cones that have different spectral sensitivities. 4.


Physiology_Levy

Physiology_Levy. 4. Bipolar cells and many ganglion cells have concentric receptive fields with an on-center/off-surround or off-center/on-surround organization. Horizontal cells mediate this center-surround antagonism. Photoreceptor, bipolar, and horizontal cells respond to stimulation by modulating their membrane potential and their release of neurotransmitters,


Physiology_Levy

Physiology_Levy. Olfactionisnotgenerallyexaminedinaroutineneurologicalexamination.However,smellcanbetestedbyhavingthepatientinhaleandidentifyanodorant.Onenostrilshouldbeexaminedatatimewhiletheothernostrilisoccluded.Strongodorants,suchasammonia,shouldbeavoidedbecausetheyalsoactivatetrigeminalnervefibers.Smellsensationcanbelost(anosmia) afterabasalskullfractureorafterdamagetooneorbotholfactorybulbsortractsbyatumor(suchasanolfactory groove meningioma).Concussionscancauseanosmiabecausethesuddenmovementofthebraininsidetheskullcanshearthesmallunmyelinatedolfactorynervefibers.Anauraofadisagreeableodor,oftenthesmellofburningrubber,occursduringuncinate seizures, whichareepilepticseizuresthatoriginateinthemedialtemporallobe. narrow range of gustatory receptors with the wide repertoire of olfactory receptors. In addition, via its intimate connections with limbic and, by extension, hypothalamic structures, it provides input to subconscious mechanisms related to emotions, memory, and sexual behavior.


Physiology_Levy

Physiology_Levy. but ganglion cells respond by generating action potentials. 5. The axons of ganglion cells in the temporal retina project to the brain ipsilaterally; those in the nasal retina cross in the optic chiasm. Because the lens inverts the image that falls on the retina, each side of the visual field is projected to the contralateral side of the brain for both eyes. In the lateral geniculate nucleus (LGN) of the thalamus, the input from each eye terminates in separate layers, and the M ganglion cells (sensitive to movement) and P ganglion cells (sensitive to detail and color) project to separate layers as well. 6.


Physiology_Levy

Physiology_Levy. 6. The LGN projects to primary visual (striate) cortex via the visual radiation and terminates largely in layer 4, where there is an orderly retinotopic map. Within the map, information from each eye maps to alternating adjacent points to create ocular dominance columns that extend vertically in the cortex. Striate cortical neurons outside of layer 4 respond best to bar or edge stimuli oriented in a particular way. Cells that “prefer” a particular stimulus orientation are grouped in orientation columns. 7. The extrastriate visual areas have different functions. Some in the inferotemporal cortex are influenced chiefly by P cells, and they function in form detection, color vision, and face discrimination. M cells influence regions of the middle temporal and parietal cortex, which function in motion detection and the control of eye movements. 8.


Physiology_Levy

Physiology_Levy. 8. A pure tone is characterized in terms of its amplitude, frequency, and phase. Natural sounds are combinations of pure tones. Sound pressure is measured in decibels (dB), in relation to a reference level. 9. The pinna and auditory canal convey airborne sound waves to the tympanic membrane. The three small bones (ossicles) of the middle ear transmit the vibrations of the tympanic membrane to the oval window of the fluid-filled inner ear. Hearing is most sensitive at about 3000 Hz because of the dimensions of the auditory canal and the mechanics of the ossicles. 10. The cochlea of the inner ear has three main compartments: the scala vestibuli, the scala tympani, and the intervening scala media (cochlear duct). The cochlear duct is bounded on one side by the basilar membrane, on which lies the organ of Corti, the sound transduction mechanism. 11.


Physiology_Levy

Physiology_Levy. 11. When the basilar membrane oscillates in response to pressure waves introduced into the scala vestibuli at the oval window, the stereocilia of the hair cells of the organ of Corti are subjected to shear forces, which open mechanoreceptor K+ channels. This results in a membrane conductance change that modulates the release of neurotransmitters on to cochlear nerve fibers. 12. High-frequency sounds best activate the hair cells near the base of the cochlea, and low-frequency sounds activate cells near the apex. Such a tonotopic organization is also present in central auditory structures, including the cochlear nuclei, superior olivary complex, inferior colliculus, medial geniculate nucleus, and primary auditory cortex. 13. Auditory processing at many sites in the central auditory pathway contributes to sound localization, frequency and intensity analysis, and speech recognition. 14.


Physiology_Levy

Physiology_Levy. 13. Auditory processing at many sites in the central auditory pathway contributes to sound localization, frequency and intensity analysis, and speech recognition. 14. The vestibular apparatus is part of the inner ear. It includes three semicircular canals (horizontal, anterior, and posterior) and two otolith organs (utricle and saccule) on each side. These transduce, respectively, angular and linear accelerations of the head. The three semicircular canals are mutually orthogonal, so they can resolve angular acceleration of the head about any axis of rotation. 15. In each semicircular canal, there are sensory hair cells whose cilia extend into a cupula, which blocks the cross-section of the endolymph-filled canal. Angular head acceleration displaces the endolymph and the cupula, bending the cilia. If the stereocilia bend toward the kinocilium, the hair cell is depolarized, which causes an increase in the firing rate in the afferent fiber. 16.


Physiology_Levy

Physiology_Levy. 16. In the otolith organs, the cilia project into an otolithic membrane. Acceleration of the head, as with linear movement, or change in position in relation to gravity displaces the otolithic membrane (because of the mass of the otoliths) and changes the firing patterns of the hair cells, depending on their orientation. 17. Central vestibular pathways include afferent connections to the vestibular nuclei and the cerebellum. Activation of the vestibular afferent fibers is detected by the brain as head acceleration or position change and is relayed via the vestibular nuclei to pathways that mediate compensatory eye movements, neck movements, and adjustments to posture. 18.


Physiology_Levy

Physiology_Levy. 18. Taste buds contain chemoreceptor cells arranged around a taste pore. Taste buds are located on several kinds of papillae on the tongue and in the pharynx and larynx. Five types of taste-receptor cells detect the five elementary qualities of taste: salty, sweet, sour, bitter, and umami. Complex flavors are signaled by the patterned activity of multiple classes of taste receptor and by central correlation with accompanying olfactory input. 19. Afferent taste fibers synapse in the nucleus of the solitary tract. The thalamic relay is via a part of the ventroposterior medial nucleus to the taste-receiving areas located in the S1 cortex and the insula. 20. Odors are detected by olfactory chemoreceptor cells, which are continuously regenerated in the olfactory mucosa. These cells are true neurons that are endowed with a wide array of G protein–coupled receptors that enable the detection of hundreds of odor molecules. 21.


Physiology_Levy

Physiology_Levy. 21. Individual olfactory axons project to olfactory glomeruli, specific for each stimulus type, in the olfactory bulb. They synapse on the dendrites of mitral cells, which have reciprocal synapse with inhibitory interneurons. This synaptic organization in the glomerulus underlies stimulus adaptation and contrast enhancement. Squire L, Berg D. Fundamental Neuroscience. 4th ed. New York: Academic Press; 2012. Upon completion of this chapter, the student should be able to answer the following questions: 1. What is a motor neuron, and how are α and γ motor neurons different? 2. What is a motor unit? How does the “size principle” apply to the orderly recruitment of motor units? 3. What is a reflex, and why are reflexes useful for clinical and scientific understanding? 4. What information about the state of the muscle is sensed by the muscle spindles, and what afferent fibers convey this information to the central nervous system (CNS)? 5.


Physiology_Levy

Physiology_Levy. 4. What information about the state of the muscle is sensed by the muscle spindles, and what afferent fibers convey this information to the central nervous system (CNS)? 5. How do γ motor neurons modulate the responses of the muscle spindle? 6. What are the pathways and functions of the basic spinal reflexes? 7. What is a central pattern generator, and what types of movements can it be used for? 8. What distinguishes the pathways of the medial and lateral descending pathways in motor control? 9. What is decerebrate rigidity, and what are its implications for the control of muscle tone? 10. What distinguishes the cortical motor areas from each other? 11. What motor parameters are coded for in the activity of neurons in motor cortex? 12. How does the organization of the mossy and olivocerebellar (climbing) fiber afferent systems to the cerebellum differ in their origins, topography, and synaptic connections. 13.


Physiology_Levy

Physiology_Levy. 12. How does the organization of the mossy and olivocerebellar (climbing) fiber afferent systems to the cerebellum differ in their origins, topography, and synaptic connections. 13. What is the geometric relationship between the major cellular elements of the cerebellar cortex? 14. What are simple and complex spikes in Purkinje cells? 15. What are the direct and indirect pathways in the basal ganglia, and how does their activity influence movement? 16. How is the balance of activity between the direct and indirect pathways altered in Parkinson’s disease and Huntington’s disease? 17. How do the vestibuloocular and optokinetic reflexes act to stabilize gaze? How do they complement each other? 18. What are the roles of saccades and smooth pursuit movements in visual tracking? 19. What is nystagmus, and what types of sensory stimulation can drive nystagmus in a normal individual? 20.


Physiology_Levy

Physiology_Levy. 18. What are the roles of saccades and smooth pursuit movements in visual tracking? 19. What is nystagmus, and what types of sensory stimulation can drive nystagmus in a normal individual? 20. What is the somatotopic organization of the different CNS regions involved in motor control. ovements are the major way in which humans interact with the world. Most activities— including running, reaching, eating, talking, writing, and reading—ultimately involve motor acts. Thus motor control is a major task of the central nervous system (CNS), and from an evolutionary perspective, it is probably the reason that nervous systems first arose. Not surprisingly, a large amount of the CNS is devoted to motor control, which can be defined as the generation of signals to coordinate contraction of the musculature of the body and head, either to maintain a posture or to make a movement (transition between two postures).


Physiology_Levy

Physiology_Levy. Because large amounts of the nervous system are involved in motor control, damage or diseases of the nervous system often result in motor abnormalities. Conversely, particular motor symptoms help determine the location of the damaged or malfunctioning region; thus assessment of motor function is an important clinical tool.


Physiology_Levy

Physiology_Levy. In this chapter, each major CNS area involved in motor control is described, starting with the spinal cord and continuing with the brainstem, cerebral cortex, cerebellum, and basal ganglia. Eye movements are discussed at the end of the chapter because of their importance and the specialized circuits involved in their generation. Each CNS area is described separately; however, CNS regions do not function in isolation, and most movements result from the coordinated action of multiple brain regions. For example, even spinal reflexes, which are mediated by local circuits in the spinal cord, can be modified by descending motor commands, and virtually all voluntary movements, which arise from cerebral activity, are ultimately generated by activation of the spinal cord circuitry (or analogous brainstem nuclei for muscles in the head and face). Principles of Spinal Cord Organization


Physiology_Levy

Physiology_Levy. The spinal cord has a cylindrical shape in which the white matter is located superficially and the gray matter is found deep to the white matter shell. The gray matter forms a continuous column that runs the length of the cord. However, the nerve roots that enter and exit the spinal cord bundle into discrete nerves, which form the basis for naming the •Fig. 9.1 Musculotopic Organization of Motor Neurons in the Ventral Horn of the Spinal Cord. A, Schematicviewofthecervicothoracicspinalcordandassociatedcross-sections,showingtheloca-tionsofmotorneuronsthatinnervateaflexor(blue dots) andanextensor(red dots). B, Spinalcordcross-section,withlocationsofdifferentmusclesrepresentedbyadrawingofthearm.(RedrawnfromPurvesD,etal[eds].Neuroscience. 3rded.Sunderland,MA:Sinauer;2004.)B Distal muscles Proximal muscles specific levels (“segments”) of the spinal cord (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal). When viewed in cross-section, the gray matter column typically has an “H”


Physiology_Levy

Physiology_Levy. muscles specific levels (“segments”) of the spinal cord (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal). When viewed in cross-section, the gray matter column typically has an “H” or butterfly shape. The “butterfly wings” are divided into dorsal and ventral horns that are separated by an intermediate zone (


Physiology_Levy

Physiology_Levy. Fig. 9.1 ) (at some spinal cord levels, a small lateral horn is also present; see ). The ventral horn is where motor neurons reside, and thus it has primarily a motor function. Correspondingly, it is the main target of descending motor pathways from the brain. In contrast, the dorsal horn is the major recipient of incoming sensory information and the main source of ascending sensory pathways (e.g., the spinothalamic tract; see Chapter 7). The description of the motor function of the spinal cord therefore begins with the properties of motor neurons and their organization in the ventral horn. Similar organizational principles hold for cranial nerve nuclei that are involved in controlling the musculature of the head and face (e.g., the facial and motor trigeminal nuclei).


Physiology_Levy

Physiology_Levy. A motor neuron is a neuron that projects to muscle cells. Because motor neurons represent the only route for CNS activity to control muscle activity, motor neurons have been termed the final common pathway. There are both somatic motor neurons and autonomic motor neurons. However, in this chapter, the term motor neuron refers only to somatic motor neurons; autonomic motor neurons are discussed in . Somatic motor neurons innervate the skeletal (striated) muscles of the body. Two main classes are distinguished on the basis of their axonal diameters: α and γ motor neurons. The α motor neurons are large, multipolar neurons that range in size up to 70 µm in diameter (see


Physiology_Levy

Physiology_Levy. The α motor neurons are large, multipolar neurons that range in size up to 70 µm in diameter (see Fig. 4.10A ). Their axons leave the spinal cord through the ventral roots and the brainstem via several cranial nerves and are distributed to the appropriate skeletal muscles via peripheral nerves. The α motor neuron axon also projects to other neurons by giving off collateral axons before leaving the CNS. The main axon terminates by synapsing onto the extrafusal muscle fibers. These synapses are called neuromuscular junctions or end plates. Extrafusal fibers are large muscle fibers that make up the bulk of a skeletal muscle and essentially generate its contractile force (a muscle also contains intrafusal fibers whose functions are detailed later in this chapter; also see


Physiology_Levy

Physiology_Levy. A key functional aspect of the motor neuron projection pattern out of the CNS is that each neuron’s axon innervates only one muscle, but it branches to innervate multiple fibers within that muscle. Moreover, each extrafusal muscle fiber in mammals is supplied by only one α motor neuron. Thus a motor unit can be defined as an α motor neuron and all of the skeletal muscle fibers that its axon supplies. The motor unit can be regarded as the basic unit of movement because when an α motor neuron discharges under normal circumstances, all of the muscle fibers of the motor unit contract. That is, the safety factor (see ) of the neuromuscular junction is greater than 1, and so each action potential in the motor neuron axon triggers an action potential in every muscle fiber of the motor unit.


Physiology_Levy

Physiology_Levy. Of importance is that the average size of the motor unit (i.e., the number of muscle fibers innervated by an axon) varies between muscles, depending on how fine a control of the muscle is required. For finely controlled muscles, such as the eye muscles, an α motor neuron may supply only a few muscle fibers. However, in a proximal limb muscle, such as the quadriceps femoris, a single α motor neuron may innervate thousands of muscle fibers.


Physiology_Levy

Physiology_Levy. The muscle fibers that belong to a given motor unit are called a muscle unit. All the muscle fibers in a muscle unit are of the same histochemical type (i.e., they are all either slow-twitch [type I] or fast-twitch [type IIA or IIB] fibers). For an in-depth description of muscle fiber types, see . Of importance in this chapter is that a number of physiological properties are correlated with this histochemical classification scheme. In particular, slow-twitch fibers, which contract and relax slowly as implied by their name, also generate low-force levels but essentially never fatigue. In contrast, the fast-twitch fiber contract and relax rapidly, generate higher levels of force, and fatigue at varying rates. The first motor units to be activated in many cases, either by voluntary effort or during reflex action or just


Physiology_Levy

Physiology_Levy. The first motor units to be activated in many cases, either by voluntary effort or during reflex action or just CHAPTER 9 Organization of Motor Function to maintain posture, are those with the smallest motor axons. These motor units contain slow-twitch fibers and thus generate the smallest contractile force, allowing the initial contraction to be finely graded. These units tend to be active much of the time, if not continuously, and so their lack of fatigability makes good functional sense. As more motor units are recruited for a motor act, motor neurons with progressively larger axons become involved, and these axons synapse onto the fast-twitch fibers. Thus they generate progressively larger amounts of tension. The most powerful such motor units are typically recruited only for tasks requiring large amounts of force (e.g., sprinting, jumping, and lifting a heavy weight), tasks that people can perform for only for short periods of time.


Physiology_Levy

Physiology_Levy. The orderly recruitment of motor units helps the CNS generate a large range of forces and also maintain relatively precise control at the different force levels. This recruitment pattern is called the size principle because the motor units are recruited in order of motor neuron axon size. The size principle depends on the fact that small motor neurons are activated more easily than are large motor neurons. Recall that if an excitatory synapse is active, it opens channels in the postsynaptic membrane and causes an excitatory postsynaptic current. The same size excitatory postsynaptic current generates a larger potential change at the initial segment of an axon of a small motor neuron than it does at a larger motor neuron, simply as a consequence of Ohm’s law (V = IR) and the fact that smaller motor neurons have higher membrane resistance than larger motor neurons do. Thus because excitatory postsynaptic potentials in the CNS are small and need to summate to reach threshold for


Physiology_Levy

Physiology_Levy. smaller motor neurons have higher membrane resistance than larger motor neurons do. Thus because excitatory postsynaptic potentials in the CNS are small and need to summate to reach threshold for triggering spikes, as the level of synaptic bombardment rises from zero, the resulting depolarization will reach spiking threshold in smaller motor neurons first. As the size principle is usually obeyed, this assumption generally appears to hold; however, there can be exceptions, and in these cases, the descending motor pathways presumably must provide differing levels of synaptic drive to the different-sized motor neurons.


Physiology_Levy

Physiology_Levy. The γ motor neurons are smaller than α motor neurons; they have a soma diameter of about 35 µm. The γ motor neurons that project to a particular muscle are located in the same regions of the ventral horn as the α motor neurons that supply that muscle. γ Motor neurons do not supply extrafusal muscle fibers; instead, they synapse on specialized striated muscle fibers called intrafusal muscle fibers, which traverse receptors called muscle spindles that are embedded in skeletal muscles. The function of γ motor neurons is to regulate the sensitivity of these receptors (discussed later). Topographic Organization of Motor Neurons in the Ventral Horn The spatial distribution of motor neurons in the spinal cord is highly organized. (This is also true in the cranial nerve nuclei.) A given skeletal muscle is supplied by a group


Physiology_Levy


Physiology_Levy

Physiology_Levy. of α motor neurons, called a motor nucleus, located in the ventral horn. Each such motor nucleus takes the form of a rostrocaudally running column that can span several spinal cord levels (see Fig. 9.1A ). Motor neurons that supply the axial musculature collectively form a column of cells that extends the length of the spinal cord. In the cervical and lumbosacral enlargements, these cells are located in the most medial part of the ventral horn; at other levels, they essentially form the entire ventral horn. The motor neurons innervating the limb muscles are in the cervical and lumbosacral enlargements, where they form columns that are lateral to those for the axial muscles. Motor neurons to muscles of the distal part of the limb are located most laterally, whereas those that innervate more proximal muscles are located more medially (


Physiology_Levy

Physiology_Levy. Fig. 9.1B). Also, motor neurons to flexors are dorsal to those that innervate extensors. Note that the α and γ motor neurons to a given muscle are found intermixed within the same motor neuron column. The interneurons that connect with the motor neurons in the enlargements are also similarly topographically organized. In general, interneurons that supply the limb muscles are located mainly in the lateral parts of the deep dorsal horn and the intermediate region between the dorsal and ventral horns. Those that supply the axial muscles, however, are located in the medial part of the ventral horn. All of these interneurons receive synaptic connections from primary afferent fibers and from the axons of pathways that descend from the brain, and thus they are part of both spinal reflex arcs and descending motor control pathways.


Physiology_Levy

Physiology_Levy. An important aspect of interneuronal systems is that the laterally placed interneurons project ipsilaterally to motor neurons that supply the distal or the proximal limb muscles, whereas the medial interneurons project bilaterally. This arrangement of the lateral interneurons allows the limbs to be controlled independently. In contrast, the bilateral arrangement of the medial interneurons allows bilateral control of motor neurons to the axial muscles to provide postural support to the trunk and neck.


Physiology_Levy

Physiology_Levy. Although motor neurons are the final common pathway from the CNS to muscles and thus shape how neuronal activity is transformed into muscular contraction, each motor neuron directly acts on only a single muscle. Normal movements (or postures), however, are rarely, if ever, caused by the isolated contraction of an individual muscle. Rather, they reflect the coordinated activity of large groups of muscles. For example, elbow flexion involves an initial burst of activity in flexor muscles, such as the biceps, and in relaxation of extensor muscles, such as the triceps. This activity is then succeeded by a burst of activity of the triceps and then a second burst of activity in the biceps to stop the flexion movement at the desired position. Furthermore, other muscles are also activated during the elbow flexion to maintain overall balance and posture.


Physiology_Levy

Physiology_Levy. As the elbow flexion example shows, different roles are played by each muscle during a movement. The muscle that initiates, and is the prime cause of, the movement is called the agonist. Muscles that act similarly to the agonist are called synergists, whereas muscles whose activity opposes the action of the agonist are antagonists. In addition, muscles can act as fixators to immobilize a joint and in postural roles. Moreover, the relationship two muscles have to each other may depend on the specific movement being performed. For example, during elbow flexion, the triceps acts an antagonist to the biceps. In contrast, during supination of the forearm without rotation occurring about the elbow, the biceps (which also acts to supinate the forearm) is again an agonist, but the role of the triceps is that of an elbow fixator.


Physiology_Levy

Physiology_Levy. Thus motor control requires flexibly linking (and unlinking) the activity of groups of motor neurons that connect to different muscles. The circuits of the spinal cord are a major mechanism used by the CNS for this aspect of motor control. Indeed, descending pathways from the brain target primarily the interneurons of the spinal cord, although there are some descending axons that synapse directly onto motor neurons.


Physiology_Levy

Physiology_Levy. Spinal cord circuitry has several levels of organization. The most basic is the segmental level: that is, a circuit that is largely confined to a single or several neighboring segments and that is repeated again at many levels. The basic spinal reflexes covered below (i.e., the myotatic, inverse myotatic, and flexion reflexes) are mediated by such circuits. Superimposed on this segmental organization is the propriospinal system, which is a series of neurons whose axons run up and down the spinal cord to interconnect the different levels of the cord. This system allows the coordination of activity at different spinal levels, which is important for behavior involving both the forelimbs and the hind limbs, such as •Fig. 9.2 Muscle Proprioceptors. Skeletalmusclescontainsensoryreceptorsembeddedwithinthemuscle(spindles)andwithintheirtendons(Golgitendonorgans).A,


Physiology_Levy

Physiology_Levy. both the forelimbs and the hind limbs, such as •Fig. 9.2 Muscle Proprioceptors. Skeletalmusclescontainsensoryreceptorsembeddedwithinthemuscle(spindles)andwithintheirtendons(Golgitendonorgans).A, Schematicviewofamuscle,showingthearrangementofaspindleinparallelwithextrafusalmusclefibersandatendonorganinserieswithmusclefibers.B, Structureandinnervation(motorandsensory)ofamusclespindle.C, Structureandinnervationofatendonorgan.A B C Ib afferent neuron Muscle fibers Tendon 250 µm Bag fibers Chain fibersGolgi tendon organ


Physiology_Levy

Physiology_Levy. The neural innervation of an intrafusal fiber differs significantly from that of an extrafusal fiber, which is innervated by a single motor neuron. Intrafusal fibers are multiply innervated and receive both sensory and motor innervation. The sensory innervation typically includes a single group Ia afferent fiber and a variable number of group II afferent fibers (see


Physiology_Levy

Physiology_Levy. Fig. 9.2B). Group Ia fibers belong to the class of sensory nerve fibers with the largest diameters and conduct at 80 to 120 m/sec; group II fibers are intermediate in size and conduct at 35 to 75 m/sec. A group Ia afferent fiber forms a spiral-shaped termination, referred to as a primary ending, on each of the intrafusal muscle fibers in the spindle. Thus primary endings are found on both types of nuclear bag fibers and on nuclear chain fibers. The group II afferent fiber forms a secondary type ending on nuclear chain and bag2 fibers, but not on bag1 fibers. The primary and secondary endings have mechanosensitive channels that are sensitive to the level of tension on the intrafusal muscle fiber. The motor supply to a muscle spindle consists of two types of γ motor axons (see Fig. 9.2B ). Dynamic γ motor axons end on nuclear bag1 fibers, and static γ motor axons end on nuclear chain and bag2 fibers.


Physiology_Levy

Physiology_Levy. Fig. 9.2B ). Dynamic γ motor axons end on nuclear bag1 fibers, and static γ motor axons end on nuclear chain and bag2 fibers. Muscle spindles respond to changes in muscle length because they lie in parallel with the extrafusal fibers and therefore are also stretched or shortened along with the extrafusal fibers. Because intrafusal fibers, like all muscle fibers, display spring-like properties, a change in their length changes the tension that they are under, and this change is sensed by mechanoreceptors of the group Ia and group II spindle afferent fibers. The nonselective cation channel Piezo2 has been identified as the principal transduction channel that allows spindle sensory afferent fibers to sense changes in mechanical stress that occur when a muscle changes length.


Physiology_Levy

Physiology_Levy. Fig. 9.3 shows the changes in activity of the afferent fibers of a muscle spindle when the muscle is stretched. It is clear that group Ia and group II fibers respond differently to stretch. Group Ia fibers are sensitive both to the amount of muscle stretch and to its rate, whereas group II fibers respond chiefly to the amount of stretch. Thus when a muscle is stretched to a new longer length, group II firing increases in proportion to the amount of stretch (see Fig. 9.3 , left), and when the muscle is allowed to shorten, its firing rate decreases proportionately (see Fig. 9.3 , right). Group Ia fibers show this same static-type response, and thus under steady-state conditions (i.e., constant muscle length), their firing rate reflects the amount of muscle stretch, similar to that of group II fibers.


Physiology_Levy

Physiology_Levy. While muscle length is changing, however, group Ia firing also reflects the rate of stretch or shortening that the muscle is undergoing. Its activity overshoots during muscle stretch and undershoots (and possibly ceases) during muscle shortening. These are called dynamic responses. This dynamic sensitivity also means that the activity of group Ia fibers is much more sensitive to transient and oscillatory stretches, such as shown in the middle diagrams of


Physiology_Levy

Physiology_Levy. Fig. 9.3 . In particular, the tap profile is what occurs when a reflex hammer is used to hit the muscle tendon and thereby cause a brief stretching of the attached muscle. The change in muscle length is too brief for significant changes in group II firing to occur, but because the magnitude of the rate of change (slopes of the tap profile) is so high with this stimulus, large dynamic responses are elicited in the group Ia fibers. Thus the functionality of reflex arcs involving group Ia afferent fibers is what is being assessed when a reflex hammer is used to tap on tendons. γ Motor Neurons Adjust the Sensitivity of the Spindle Up to this point, we have described only how muscle spindles behave when there are no changes in γ motor neuron activity. The efferent innervation of muscle spindles is extremely important, however, because it determines the sensitivity of muscle spindles to stretch. For example, in Fig. 9.4A, the activity of a muscle spindle afferent fiber is shown during


Physiology_Levy

Physiology_Levy. Fig. 9.4A, the activity of a muscle spindle afferent fiber is shown during CHAPTER 9 Organization of Motor Function •Fig. 9.3 Responses of a Primary Ending (Group Ia) and a Secondary Ending (Group II) to Changes in Muscle Length. Notethedifferenceindynamicandstaticresponsivenessoftheseendings.Thewaveformsatthetoprepresentthechangesinmusclelength.ThemiddleandbottomrowsshowthedischargesofagroupIafiberandagroupIIfiber,respectively,duringthevariouschangesinmusclelength.


Physiology_Levy

Physiology_Levy. •Fig. 9.4 The Activity of γ Motor Neurons Can Counteract the Effects of Unloading on the Discharge of a Muscle Spindle Afferent Fiber. A, Theactivityofamusclespindleafferentfiberduringsteadystretch.B, Stimulationofα motorneuronat0mseccausescontractionoftheextrafusalfibers,whichleadstomuscleshorteningandincreasedmuscletensionbutunloadingofthetensionacrossthemusclespindle,whichinturninducestheafferentfibertostopfiring.Uponrelaxation,themusclereturnstoitsoriginallength,andtensionisrestoredontheintrafusalfibers,causingthereturnofactivityinthegroupIaafferentfiber.C, Coactivationofα andγ motorneuronscausesshorteningofbothextrafusalandintrafusalfibers.Thusthereisnounloadingofthespindle,andtheafferentfibermaintainsitsspontaneousactivity.(RedrawnfromKufflerSW,NichollsJG.From Neuron to Brain. Sunderland,MA:Sinauer;1976.) a steady stretch. If only the extrafusal muscle fibers were may stop discharging and become insensitive to further to contract (this can be done experimentally by selective


Physiology_Levy

Physiology_Levy. a steady stretch. If only the extrafusal muscle fibers were may stop discharging and become insensitive to further to contract (this can be done experimentally by selective decreases in muscle length. However, the unloading of the stimulation of α motor neurons; see


Physiology_Levy

Physiology_Levy. Fig. 9.4B ), the muscle spindle can be prevented if α and γ motor neurons are spindle would be unloaded by the resultant shortening of stimulated simultaneously. Such combined stimulation the muscle. If this happens, the muscle spindle afferent fiber causes the intrafusal muscle fibers of the spindle to shorten along with the extrafusal muscle fibers, maintaining the baseline tension on the equatorial portion of the intrafusal fibers (see Fig. 9.4C •Fig. 9.5 Effects of Static and Dynamic γMotor Neurons on the Responses of a Primary Ending to Muscle Stretch. A, Thetimecourseofthestretch.B, ThedischargeofgroupIafibersintheabsenceofγmotorneuronactivity.C, Stimulationofastaticγmotoraxon.D, Stimulationofadynamicγmotoraxon.(RedrawnfromCroweA,MatthewsPBC.J Physiol. 1964;174:109.)


Physiology_Levy

Physiology_Levy. Note that only the two polar regions of the intrafusal muscle contract; the equatorial region, where the nuclei are located, does not contract because it has little contractile protein. Nevertheless, when the polar regions contract, the equatorial region elongates and regains its sensitivity. Conversely, when a muscle relaxes (α motor neuron activity drops) and thus elongates (if its ends are being pulled), a concurrent decrease in γ motor neuron activity allows the intrafusal fibers to relax (and thus elongate) as well and thereby prevent the tension on the central portion of the intrafusal fiber from reaching a level at which firing of the afferent fibers is saturated. Thus the γ motor neuron system allows the muscle spindle to operate over a wide range of muscle lengths while retaining high sensitivity to small changes in length.


Physiology_Levy

Physiology_Levy. For voluntary movements, descending motor commands from the brain in fact typically activate α and γ motor neurons simultaneously, presumably to maintain spindle sensitivity as just described. This has two important functions. By maintaining the muscle spindle’s sensitivity as the muscle changes length, the spindle remains capable of sensing, and signaling to the CNS, any disturbances to the ongoing movement that cause an unexpected stretch of the muscle, and this in turn allows the CNS to initiate both reflex (see next section) and voluntary corrections. Second, if the spindle were to become unloaded during the movement, this would oppose the intended movement by decreasing the excitatory drive, via the group Ia reflex arc (see next section), to the α motor neurons driving the agonist muscles. As mentioned earlier, there are two types of γ motor neurons: dynamic and static (see Fig. 9.2).


Physiology_Levy

Physiology_Levy. As mentioned earlier, there are two types of γ motor neurons: dynamic and static (see Fig. 9.2). This allows the CNS to have very precise control over the sensitivity of the muscle spindle. Dynamic γ motor axons end on nuclear bag1 fibers, and static γ motor axons synapse on nuclear chain and bag2 fibers. Thus when a dynamic γ motor neuron is activated, the response of the group Ia afferent fiber is enhanced, but the activity of the group II afferent fibers is unchanged; when a static γ motor neuron discharges, the responsiveness of the group II afferent fibers and the static responsiveness of the group Ia afferent fibers are increased. The effects of stimulating the static and dynamic fibers on a group Ia afferent fiber’s response to stretch are illustrated in


Physiology_Levy

Physiology_Levy. Fig. 9.5 . Descending pathways can preferentially influence dynamic or static γ motor neurons and thereby alter the nature of reflex activity in the spinal cord and also, presumably, the functioning of the muscle spindle during voluntary movements. The Phasic (or Ia) Stretch Reflex The reflex arc responsible for the phasic stretch reflex is depicted in


Physiology_Levy

Physiology_Levy. Fig. 9.6 ; the rectus femoris muscle serves as an example. A rapid stretch of the rectus femoris muscle strongly activates the group Ia fibers of the muscle spindles, which then convey this signal into the spinal cord. In the spinal cord, each group Ia afferent fiber branches many times to form excitatory synapses directly (monosynaptically) on virtually all α motor neurons that supply the same (also known as the homonymous) muscle and with many α motor neurons that innervate synergists, such as the vastus intermedius muscle in this case, which also acts to extend the leg at the knee. If the excitation is powerful enough, the motor neurons discharge and cause a contraction of the muscle. Note that the group Ia fibers do not contact the γ motor neurons, possibly to avoid a positive-feedback loop situation. This selective targeting of α motor neurons is exceptional in that most other reflex and descending pathways target both α and γ motor neurons.


Physiology_Levy

Physiology_Levy. Other branches of group Ia fibers end on a variety of interneurons; however, one type, the reciprocal Ia inhibitory interneuron (black cell in Fig. 9.6 ), is particularly important with regard to the stretch reflex. These interneurons are identifiable because they are the only inhibitory interneurons •Fig. 9.6 Reflex Arc of the Stretch Reflex. Thepathwaybacktotherectusfemorisinthisarccontainsasinglesynapsewithinthecentralnervoussystem;hence,itisamonosynapticreflex.Theinterneuron,showninblack, isagroupIainhibitoryinterneuron.DRG,dorsalrootganglion. that receive input from both the group Ia afferent fibers and Renshaw cells (see


Physiology_Levy

Physiology_Levy. that receive input from both the group Ia afferent fibers and Renshaw cells (see Fig. 9.12 ). They end on α motor neurons that innervate the antagonist muscles—in this case, the hamstring muscles, including the semitendinosus muscle—which act to flex the knee. Other branches of the group Ia afferent fibers synapse with yet other neurons that originate ascending pathways that provide various parts of the brain (particularly the cerebellum and cerebral cortex) with information about the state of the muscle. The organization of the stretch reflex arc guarantees that one set of α motor neurons is activated and the opposing set is inhibited. This arrangement is known as reciprocal innervation. Although many reflexes involve such reciprocal innervation, this type of innervation is not the only possible organization of a motor control system; descending motor pathways can override such patterns.


Physiology_Levy

Physiology_Levy. The stretch reflex is quite powerful, in large part because of its monosynaptic nature. The power of this reflex also derives from the essentially maximal convergence and divergence that exist in this pathway, which is not apparent from the circuit diagrams, such as Fig. 9.6 , that are typically used to illustrate reflex pathways. That is, each group Ia fiber contacts virtually all homonymous α motor neurons, and each such α motor neuron receives input from every spindle in that muscle. Although its monosynaptic nature makes the group Ia reflex rapid and powerful, it also means that there is relatively little opportunity for direct control of CHAPTER 9 Organization of Motor Function activity flow through its reflex arc. The CNS overcomes this problem by controlling muscle spindle sensitivity via the γ motor neuron system as described previously. The Tonic Stretch Reflex


Physiology_Levy

Physiology_Levy. The Tonic Stretch Reflex The tonic stretch reflex can be elicited by passive bending of a joint. This reflex circuit includes both group Ia and group II afferent fibers from muscle spindles. Group II fibers make monosynaptic excitatory connections with α motor neurons, but they also excite them through disynaptic and polysynaptic pathways. Normally, there is ongoing activity in the group Ia and group II afferent fibers that helps maintain a baseline rate of firing of α motor neurons; therefore, the tonic stretch reflex contributes to muscle tone. Its activity also contributes to the ability to maintain a posture. For example, if the knee of a soldier standing at attention begins to flex because of fatigue, the quadriceps muscle is stretched, a tonic stretch reflex is elicited, and the quadriceps contracts more, thereby opposing the flexion and restoring the posture.


Physiology_Levy

Physiology_Levy. The foregoing discussion suggests that stretch reflexes can act like a negative-feedback system to control muscle length. By following the stretch reflex arc, it is possible to see that changes in its activity act to oppose changes in muscle length from a particular equilibrium point. For example, if the muscle’s length is increased, there will be an increase in firing by group Ia and group II fibers, which excites homonymous α motor neurons and leads to contraction of the muscle and reversal of the stretch. Similarly, passive shortening of the muscle unloads the spindles and leads to a decrease in the excitatory drive to the motor neurons and thus relaxation of the muscle. So how are humans able to rotate their joints? It is partly because the γ motor neurons are coactivated during a movement and thereby shift the equilibrium point of the spindle and partly because the gain or strength of the reflex is low enough that other input to the motor neuron can override the stretch reflex.


Physiology_Levy

Physiology_Levy. The inverse myotatic reflex acts to oppose changes in the level of force in the muscle. Just as the stretch reflex can be thought of as a feedback system to regulate muscle length, the inverse myotatic, or group Ib, reflex can be thought of as a feedback system to help maintain force levels in a muscle. With the upper part of the leg as an example, the group Ib reflex arc is depicted in Fig. 9.7. The arc starts with the Golgi tendon organ receptor, which senses the tension in the muscle. Golgi tendon organs are located at the junction of the tendon and the muscle fibers and thus lie in series with the muscle fibers, in contrast to the parallel arrangement of the muscle spindles (see Fig. 9.2 ). Golgi tendon organs have a diameter of about 100 µm and a length of about 1 mm. A Golgi tendon organ is innervated by the terminals of group Ib afferent fibers. These terminals wrap about bundles of collagen fibers in the tendon of a muscle (or in tendinous inscriptions within the muscle).


Physiology_Levy


Physiology_Levy

Physiology_Levy. *Ingeneral,gain ofasystemisdefinedasitsoutputforagiveninput.Inthiscase,theinputtothesystemistheimposedstretch,andtheoutputismovementcausedbythestretchreflex–evokedcontraction. Because of their in-series relationship to the muscle, Golgi tendon organs can be activated either by muscle stretch or by muscle contraction. In both cases, however, the actual stimulus sensed by the Golgi tendon organ is the force that develops in the tendon to which it is linked. Thus the response to stretch is the result of the spring-like nature of the muscle (i.e., by Hooke’s law, the force on a spring is proportional to how much it is stretched). To distinguish between the responsiveness of the muscle spindles and Golgi tendon organs, the firing patterns of group Ia and group Ib fibers can be compared when a muscle is stretched and then held at a longer length (Fig.


Physiology_Levy

Physiology_Levy. 9.8 ). The firing rate of the group Ia fibers maintains its increase until the stretch is reversed. In contrast, the group Ib fiber shows an initial large increase in firing, reflecting the increased tension on the muscle caused by the stretch, but then shows a gradual return toward its initial firing rate as the tension on the muscle is lowered because of cross-bridge recycling and the resultant lengthening of the •Fig. 9.7 Reflex Arc of the Inverse Myotatic Reflex. Theinterneuronsincludebothexcitatory(white) andinhibitory(black) interneurons.Thisisanexampleofadisynapticreflex.


Physiology_Levy

Physiology_Levy. •Fig. 9.8 Changes in Group Ia and Group Ib Firing Rates When Muscle Is Stretched to a New Length. Afteratransientburst,thefiringrateofthegroupIafiberremainsconstantatanewhigherlevelthatisproportionaltotheincreaseinlength(compareblue lines in upper and lower graphs).Incontrast,thegroupIbfibershowsaninitialrapidincreaseinfiringfollowedbyaslowdecreasebacktowarditsoriginallevel(lower graph, red line) andhasafiringprofilethatmatchesthetensionlevelinthemusclecausedbythestretch(upper graph, red line). sarcomeres. Therefore, Golgi tendon organs signal force, whereas spindles signal muscle length. Further evidence of this distinction is that group Ib firing is correlated with force level during isometric contraction even though muscle length and therefore group Ia activity are unchanged.


Physiology_Levy

Physiology_Levy. CHAPTER 9 Organization of Motor Function •Fig. 9.9 The Reflex Arc of the Flexion Reflex. Black interneuronsareinhibitory,andwhite onesareexcitatory.FRA,flexionreflexafferentfiber.++––FRASemitendinosusNociceptorFlexionExtensionRectusfemoris


Physiology_Levy

Physiology_Levy. The group Ib afferent fibers branch as they enter the spinal cord and end on interneurons. There are no mono-synaptic connections to α motor neurons. Rather, the group Ib afferent fibers synapse onto two classes of interneurons: interneurons that inhibit α motor neurons that supply the homonymous muscle (in this case the rectus femoris muscle) and excitatory interneurons that activate α motor neurons to the antagonist (the semitendinosus muscle). Because there are two synapses in series in the CNS, this is a disynaptic reflex arc. Because of these connections, group Ib fiber activity should have the opposite action of the group Ia stretch reflex during passive stretch of the muscle, which explains the group Ib reflex’s other name, the inverse myotatic reflex.


Physiology_Levy

Physiology_Levy. Functionally, however, the two reflex arcs can act synergistically, as the following example shows. Recall that the Golgi tendon organs monitor force levels across the tendon that they supply. If during maintained posture (such as standing at attention) knee extensors (such as the rectus femoris muscle) begin to fatigue, the force pulling on the patellar tendon declines. The decline in force reduces the activity of Golgi tendon organs in this tendon. Because the group Ib reflex normally inhibits the α motor neurons to the rectus femoris muscle, reduced activity of the Golgi tendon organs enhances the excitability of (i.e., disinhibits) the α motor neurons and thereby helps reverse the decrease in force caused by the fatigue. Simultaneously, bending of the knee stretches the knee extensors and activates the afferent fibers from the muscle spindles, which then excite the same α motor neurons. Thus coordinated action of afferent fibers from both the muscle spindle and Golgi tendon organ


Physiology_Levy

Physiology_Levy. and activates the afferent fibers from the muscle spindles, which then excite the same α motor neurons. Thus coordinated action of afferent fibers from both the muscle spindle and Golgi tendon organ help oppose the decrease in contraction of the rectus femoris muscle due to fatigue and thereby work together to maintain the standing posture.


Physiology_Levy

Physiology_Levy. The flexion reflex starts with activation of one or more of a variety of sensory receptors, including nociceptors, whose signals can be carried to the spinal cord via a variety of afferent fibers, including group II and group III fibers, collectively called the flexion reflex afferent (FRA) fibers. In flexion reflexes, afferent volleys (1) cause excitatory inter-neurons to activate the α motor neurons that supply the flexor muscles in the ipsilateral limb and (2) cause inhibitory interneurons to inhibit the α motor neurons that supply the antagonistic extensor muscles ( Fig. 9.9). This pattern of activity causes one or more joints in the stimulated limb to flex. In addition, commissural interneurons evoke the opposite pattern of activity in the contralateral side of the spinal cord (see


Physiology_Levy

Physiology_Levy. Fig. 9.9 ), which results in extension of the opposite limb, the crossed extension reflex. For lower limbs in humans (or for both forelimbs and hind limbs in quadrupeds), the crossed extension part of the reflex helps in maintaining balance by enabling the contralateral limb to be able to support the additional load that is transferred to it when the flexed limb is lifted.


Physiology_Levy

Physiology_Levy. Because flexion typically brings the affected limb in closer to the body and away from a painful stimulus, flexion reflexes are a type of withdrawal reflex. In Fig. 9.9 , the neural circuit of the flexion reflex is shown for neurons that affect only the knee joint. Actually, however, considerable divergence of the primary afferent and interneuronal pathways occurs in the flexion reflex. In fact, all the major joints of a limb (e.g., hip, knee, and ankle) may be involved in a strong flexor withdrawal reflex. Details of the flexor withdrawal reflex vary, depending on the nature and location of the stimulus.


Physiology_Levy

Physiology_Levy. The interneurons subserving flexion reflexes also appear to be part of the central pattern generator (CPG) for generating locomotion and thus are an example of how reflex circuits are used for multiple purposes. A CPG is a set of neurons and circuits capable of generating the rhythmic activity that underlies motor acts, even in the absence of sensory input. For example, activation of the FRA inter-neurons leads to a pattern of flexor excitation and extensor inhibition on one side and the converse pattern on the opposite side, and if the FRA interneurons on each side of the spinal cord alternated in being active, a stepping pattern would emerge. That is, walking motion could result from alternately activating the FRA interneurons on each side. Note that such a rhythmic activity pattern in the FRA circuits need not be dependent on activity from the FRA fibers themselves (e.g., they could be activated by descending pathways from the brain).


Physiology_Levy

Physiology_Levy. To show that these circuits are actually involved in generating the locomotion rhythm, spinal cord preparations were made that showed spontaneous locomotion (i.e., if the brainstem is transected and weight is supported, the spinal cord circuits can generate activity that causes the limbs to perform a normal locomotion sequence). In one such preparation, the electromyographic signals from the flexors and extensors of a limb were recorded, and the FRA fibers then stimulated to demonstrate the effect on locomotion rhythm ( Fig. 9.10 ). Before any stimulus, a spontaneous alternating pattern of flexor and extensor electromyographic (EMG) activity exists. If the FRA fibers were not involved in the locomotion circuit, or at least were not a critical part of the circuits responsible for generating the rhythm (see Fig.


Physiology_Levy

Physiology_Levy. Fig. 9.10B ), the stimulus would be expected to produce only a transient response (i.e., a single burst in the flexor EMG record and brief inhibition of activity in the extensor EMG record) but to have no long-term effect on ongoing EMG pattern. Such a transient response is observed (see


Physiology_Levy

Physiology_Levy. Fig. 9.10A ; EMG records just after the stimulus). However, the stimulus also causes a permanent, approximately 180degree phase shift in locomotor rhythm, as can shown from a comparison of the times of contractions before and after the stimulus. The dashed vertical lines indicate the times at which a flexor EMG response would be expected if the stimulus had produced no phase shift from the EMG activity pattern. Before the stimulus, each vertical line is aligned with the onset of a flexor EMG burst, whereas after the stimulus, each vertical line occurs at the end of the flexor burst. Therefore, the stimulus affected the locomotor CPG itself and the FRA interneurons are a critical part of this CPG (see Fig. 9.10C


Physiology_Levy

Physiology_Levy. Fig. 9.10C A second important point illustrated by this experiment is that the locomotion CPG (and CPGs in general) can be influenced by strong afferent fiber activity. The afferent fiber’s influence ensures that the pattern generator adapts to changes in the terrain as locomotion proceeds. Such changes may occur rapidly during running, and locomotion must then be adjusted to ensure proper coordination. Determining Spinal Cord Organization Through the Use of Reflexes Convergence and divergence are important aspects of reflex pathways and of neuronal circuits in general. Several examples of these phenomena have been described in the previous discussion of the reflexes. Reflexes can be used to identify and characterize these phenomena in the spinal cord. For example, convergent input can be demonstrated through the phenomenon of spatial facilitation, which is illustrated in Fig. 9.11


Physiology_Levy

Physiology_Levy. Fig. 9.11 In this example, a monosynaptic reflex is elicited by electrical stimulation of the group Ia fibers in each of two nerves (see Fig. 9.11A ). The reflex response is characterized by a recording of the discharges of α motor axons from the appropriate ventral root (as a compound action potential). When nerve A is stimulated, a small compound action potential is recorded as reflex A. Similarly, when nerve B is stimulated, reflex B is recorded. Fig. 9.11B depicts the motor neurons contained within the motor nucleus.


Physiology_Levy

Physiology_Levy. Fig. 9.11B depicts the motor neurons contained within the motor nucleus. CHAPTER 9 Organization of Motor Function •Fig. 9.10 Phase Reset of Locomotion Rhythm by Flexion Reflex Afferent (FRA) Fiber Stimulation Helps Identify Neuronal Components of the Underlying Central Pattern Generator (CPG). A, EMGrecordsfromkneeflexorandextensormuscles.Notetherhythmicalternatingpatternbeforeapplicationofthestimulus.Thesolid vertical lines beloweachtraceindicatethetimesatwhichflexorcontractionisinitiated.Thedashed vertical lines indicatethetimesatwhichflexorcontractionwouldhavebeeninitiatedifthestimuluscausednolastingeffectontherhythmicpattern.B andC, TwopossiblemodelsfortheCPGunderlyingthelocomotorrhythmdepictedinA. B, TheFRAinterneurons(INs)intheCPGarenotshown.C, TheFRAinterneuronsareshown.ThedatashowninA supportthemodelshowninC. MN,motoneuron.(DatafromHultbornH,etal.Ann N Y Acad Sci. 1998;860:70.)


Physiology_Levy

Physiology_Levy. The α motor neurons in the discharge zones are activated above threshold when each nerve branch is stimulated separately. Thus a distinct pair of α motor neurons spike when each nerve is stimulated alone. In addition, each of these motor neuron pairs is surrounded by a subliminal fringe of eight additional motor neurons that are excited but not sufficiently to trigger spikes. When the two nerves are stimulated at the same time, a much larger reflex discharge is recorded (compare RA and RB with RA + RB recordings at the right of Fig. 9.11B ). As the figure demonstrates, this reflex represents the discharge of seven α motor neurons: the four that spiked after the singular stimulation of each nerve (two per nerve) and three additional α motor neurons (located in the facilitation zone) that are made to discharge only when the two nerves are stimulated simultaneously because they lie in the subliminal fringe for both nerves.


Physiology_Levy

Physiology_Levy. A similar effect could be elicited by repetitive stimulation of one of the nerves, provided that the stimuli occur close enough together that some of the excitatory effect of the first volley still persists after the second volley arrives. This effect is called temporal summation. Both spatial summation and temporal summation depend on the properties of the excitatory postsynaptic potentials evoked in α motor neurons by the group Ia afferent fibers (see Fig. 6.8). Convergence can also lead to inhibitory interactions between stimuli, a phenomenon called occlusion. If a volley in one of the two nerves in Fig. 9.11 reaches the motor nucleus at a time when the motor neurons are highly excitable, the reflex discharge is relatively large (see


Physiology_Levy

Physiology_Levy. Fig. 9.11 reaches the motor nucleus at a time when the motor neurons are highly excitable, the reflex discharge is relatively large (see Fig. 9.11C ). A similar volley in the other nerve might also produce a large reflex response. However, when the two nerves are excited simultaneously, the reflex can be less than the sum of the two independently evoked reflexes if the cells reaching threshold to activation of either of the two nerves alone overlap significantly. In this case, each afferent nerve activates 7 α motor neurons, but the volleys in the two nerves together cause only 12 α motor neurons to discharge because two motor neurons lie in the individual discharge zones of both afferent nerves.


Physiology_Levy

Physiology_Levy. The phenomena of spatial and temporal summation and occlusion can also be used to demonstrate interactions between spinal cord neurons and the various reflex circuits. To start, a monosynaptic reflex discharge can be evoked by stimulation of the group Ia afferent fibers in a muscle nerve. This is a test of the reflex excitability of a population of α motor neurons. The discharges of either extensor or flexor α motor neurons can be recorded if the proper muscle nerve to be stimulated is chosen. Other kinds of afferent fibers •Fig. 9.11 Spatial Facilitation. A, Arrangementforusingelectricallyevokedafferentvolleysandrecordingsfrommotoraxonsinaventralroottostudyreflexes.B, Experimentinwhichcombinedstimulationofafferentfibersintwomusclenervesresultedinspatialsummation(RAandRB).Thedischargezones(pink areas) encloseα motorneuronsthatareactivatedabovethresholdwheneachnervebranchisstimulatedseparately.InC, thecombinedvolleyscausedocclusion.(RedrawnfromEyzaguirreC,FidoneSJ.Physiology of the


Physiology_Levy

Physiology_Levy. areas) encloseα motorneuronsthatareactivatedabovethresholdwheneachnervebranchisstimulatedseparately.InC, thecombinedvolleyscausedocclusion.(RedrawnfromEyzaguirreC,FidoneSJ.Physiology of the Nervous System. 2nded.Chicago:Mosby–YearBook;1975.) are then stimulated along with the homonymous group Ia afferent fibers from the muscle to demonstrate whether the response to the group Ia stimulation changes. For example, stimulation of group Ia afferent fibers in the nerve to the antagonist muscles produces inhibition of the response to the homonymous group Ia stimulation (which is mediated by the reciprocal group Ia inhibitory interneuron described previously).


Physiology_Levy

Physiology_Levy. As another example, if the small afferent fibers of a cutaneous nerve are stimulated to evoke a flexion reflex, the responses to group Ia stimulation of the α motor neurons that innervate the extensor muscles are inhibited (and those of α motor neurons that innervate flexor muscles are potentiated). As a final example, stimulation of a ventral root causes inhibition of group Ia responses and inhibits the reciprocal group Ia inhibition. Because the ventral root contains only motor neuron axons, this result implies the presence of axon collaterals that excite inhibitory interneurons that feed back onto the same motor neuron population (


Physiology_Levy

Physiology_Levy. Fig. 9.12 ). These interneurons are named Renshaw cells. Because ventral root stimulation also inhibits the group Ia inhibition of antagonist motor neurons, but no other classes of interneurons, the reciprocal group Ia interneurons are uniquely inhibited by ventral root stimulation (and activated by group Ia stimulation). Experiments like these described have been used to provide a detailed knowledge of the circuitry of the spinal cord. •Fig. 9.12 Renshaw Cell (RC) Connections With Motor Neurons and Group Ia Inhibitory Interneurons. ThecircuitsshownmediategroupIareciprocalinhibitionofantagonistmuscles(inthiscase,anextensor)andinhibitionofthisreciprocalinhibitionbyRenshawcells.NotethatequivalentnumbersofRenshawcellsandgroupIainhibitoryinterneuronsareassociatedwithextensormotorneuronsandgroupIainputfromspindlesinextensormuscles,buttheyarenotshownforsimplicity.Orange cells areinhibitory,andblue andgreen cells areexcitatory. Classification of Descending Motor Pathways


Physiology_Levy

Physiology_Levy. Classification of Descending Motor Pathways Descending motor pathways were traditionally subdivided into pyramidal and extrapyramidal pathways. This terminology reflects a clinical dichotomy between pyramidal tract disease and extrapyramidal disease. In pyramidal tract disease, the corticospinal (pyramidal) tract is interrupted. The signs of this disease were originally attributed to the loss of function of the pyramidal tract (so named because the corticospinal tract passes through the medullary pyramid). However, in many cases of pyramidal tract disease, the functions of other pathways are also altered, and most signs of pyramidal tract disease (see the later section “ Caused by Lesions of Descending Motor Pathways ”) are apparently not caused solely by loss of the corticospinal tract but also reflect damage to additional motor pathways. The


Physiology_Levy

Physiology_Levy. Caused by Lesions of Descending Motor Pathways ”) are apparently not caused solely by loss of the corticospinal tract but also reflect damage to additional motor pathways. The CHAPTER 9 Organization of Motor Function term extrapyramidal is even more problematic. Thus this classification system is not used in this book. Another way of classifying the motor pathways is based on their sites of termination in the spinal cord and the consequent differences in their roles in the control of movement and posture. The lateral pathways terminate in the lateral portions of the spinal cord’s gray matter ( Fig. 9.13 ). The lateral pathways can excite motor neurons directly, although interneurons are their main target. They influence reflex arcs that control fine movement of the distal ends of limbs, as well as those that activate supporting musculature in the proximal ends of limbs. The medial pathways end in the medial ventral horn on the medial group of interneurons (see


Physiology_Levy

Physiology_Levy. Fig. 9.13 ). These interneurons connect bilaterally with motor neurons that control the axial musculature and thereby contribute to balance and posture. They also contribute to the control of proximal limb muscles. In this book, the terms lateral and medial are used to classify the descending motor pathways. However, even this terminology is not perfect, partly because although motor neuron cell bodies form localized columns, motor neuron dendritic trees are rather large and typically span most of the ventral horn. Thus any motor neuron can potentially receive input from so-called medial or lateral system pathways. The Lateral System


Physiology_Levy

Physiology_Levy. The Lateral System The corticospinal and corticobulbar tracts originate from a wide region of the cerebral cortex. This region includes the primary motor, premotor, supplementary, and cingulate motor areas of the frontal lobe and the somatosensory cortex of the parietal lobe. The cells of origin of these tracts include both large and small pyramidal cells of layer V of the cortex, including the giant pyramidal cells of Betz. Although Betz cells are a defining feature of the primary motor cortex, they represent a small minority (<5%) of the cells that contribute to these tracts, in part because they are found only in the primary motor cortex, and even there they represent a minority of the cells contributing to the tracts. These tracts leave the cortex and enter the internal capsule, then traverse the midbrain in the cerebral peduncle, pass through the basilar pons, and emerge to form the pyramids on the ventral surface of the medulla (see


Physiology_Levy

Physiology_Levy. Fig. 9.13A). The corticobulbar axons leave the tract as it descends in the brainstem and terminate in the motor nuclei of the various cranial nerves. The corticospinal fibers continue caudally, and in the most caudal region of the medulla, about 90% of them cross to the opposite side. They then descend in the contralateral lateral funiculus as the lateral corticospinal tract. The lateral corticospinal axons terminate at all spinal cord levels, primarily on interneurons, but also on motor neurons. The remaining uncrossed axons continue caudally in the ventral funiculus on the same side as the ventral corticospinal tract, which belongs to the medial system. Many of these fibers ultimately decussate (cross) at the spinal cord level at which they terminate.


Physiology_Levy

Physiology_Levy. •Fig. 9.13 Descending Motor Pathways. Majorpathwaysconnectingthecorticalandbrainstemmotorareastothespinalcordareshown.A, Lateralsystempathways,corticospinal(red) andrubrospinal(blue) pathways.NotethattheventralcorticospinalpathwayispartofthemedialsystembutisshowninA forsimplicity.B, Medialsystempathways,medullary(blue) andpontine(green) reticulospinalandlateralvestibulospinal(red) pathways.C-B,corticobulbar;C-P,corticopontine;C-S,corticospinal;LCST,lateralcorticospinaltract;VCST,ventralcorticospinaltract.


Physiology_Levy

Physiology_Levy. The lateral corticospinal tract is a relatively minor tract in lower mammals but is quantitatively and functionally very important in primates, particularly in humans, in which it contains more than 1 million axons. This number still represents a relatively small proportion of the outflow from the cortex because there are approximately 20 million axons in the cerebral peduncles. Nevertheless, the corticospinal pathway is critical for the fine independent control of finger movement, inasmuch as isolated lesions of the corticospinal tract typically lead to a permanent loss of this ability, even though other movement abilities are often recovered with such lesions. Indeed, in primates, corticospinal synapses directly onto motor neurons are particularly prevalent for the motor neurons controlling finger muscles and are probably the basis of the ability to make independent, finely controlled finger movements.


Physiology_Levy

Physiology_Levy. The corticobulbar tract, which projects to the cranial nerve motor nuclei, has subdivisions that are comparable with the lateral and ventral corticospinal tracts. For example, part of the corticobulbar tract ends contralaterally in the portion of the facial nucleus that supplies muscles of the lower part of the face and in the hypoglossal nucleus. This component of the corticobulbar tract is organized like the lateral corticospinal tract. The remainder of the corticobulbar tract ends bilaterally.


Physiology_Levy

Physiology_Levy. The rubrospinal tract originates in the magnocellular portion of the red nucleus, which is located in the midbrain tegmentum. These fibers decussate in the midbrain, descend through the pons and medulla, and then take up a position just ventral to the lateral corticospinal tract in the spinal cord. They preferentially affect motor neurons controlling distal musculature, as do the corticospinal fibers. Red nucleus neurons receive input from the cerebellum and from the motor cortex; thus making this an area of integration of activity from these two motor systems. The Medial System