Human Anatomy and Physiology
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Special Senses

The Eye and Vision
The Ear: Hearing and Balance
Chemical Senses: Smell and Taste
Developmental Aspects of the Special Senses

The Eye and Vision

  1. External/accessory structures of the eye.

    1. Extrinsic eye muscles aim the eyes for following moving objects and for convergence.

    2. Lacrimal apparatus (Figure 8.1)includes a series of ducts and the lacrimal glands that produce a saline solution, which washes and lubricates the eyeball.

    3. Eyelids protect the eyes. Associated with the eyelashes are the ciliary glands, modified sweat glands, and the meibomian glands, which produce an oily secretion that helps keep the eye lubricated.

    4. The conjunctiva is a mucous membrane that covers the anterior eyeball and lines the eyelids. It produces a lubricating mucus (Figure 8.2).

  2. Three tunics form the eyeball.

    1. The sclera is the outer, tough, protective tunic. Its anterior portion is the cornea, which is transparent to allow light to enter the eye.

    2. The choroid is the middle coat, which provides nutrition to the internal eye structures and prevents light's scattering in the eye. Anterior modifications include two smooth muscle structures, the ciliary body, and the iris (which controls the size of the pupil).

    3. The retina is the innermost (sensory) coat, which contains the photoreceptors. Rods are dim light receptors. Cones are receptors that provide for color vision and high visual acuity. The fovea centralis, on which acute focusing occurs, contains only cones.

  3. The blind spot (optic disk) is the point where the optic nerve leaves the back of the eyeball.

  4. The lens is the major light-bending (refractory) structure of the eye (Figure 8.3). The lens shape is changed by the ciliary muscle for close focus. Anterior to the lens is the aqueous humor; posterior to the lens is the vitreous humor. Both humors reinforce the eye internally. The aqueous humor also provides nutrients to the avascular lens and cornea.

  5. Errors of refraction include myopia, hyperopia, and astigmatism. All are correctable with specially ground lenses.

  6. The pathway of light through the eye is cornea > aqueous humor > (through pupil) > aqueous humor > lens > vitreous humor > retina.

  7. Overlap of the visual fields and inputs from both eyes to each optic cortex provide for depth perception.

  8. The pathway of nerve impulses from the retina of the eye is optic nerve > optic chiasma > optic tract > thalamus > optic radiation > visual cortex in occipital lobe of brain (Figure 8.4).

  9. Eye reflexes include the photopupillary, accommodation pupillary, and convergence.

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The Ear: Hearing and Balance

  1. The ear is divided into three major areas (Figure 8.5):

    1. Outer ear structures are the pinna (auricle), external auditory canal, and tympanic membrane (eardrum). Sound entering the external auditory canal sets the eardrum into vibration. These structures are involved with sound transmission only.

    2. Middle ear structures are the ossicles and auditory tube within the tympanic cavity. The ossicles (malleus, incus, and stapes)transmit the vibratory motion from the eardrum to the oval window. The auditory tube allows pressure to be equalized on both sides of the eardrum. These structures are also involved with sound transmission only.

    3. Inner ear, or bony labyrinth, consists of bony chambers (cochlea, vestibule, and semicircular canals) in the temporal bone. The bony labyrinth contains perilymph and membranous sacs filled with endolymph. Within the membranous sacs of the vestibule and semicircular canals are equilibrium receptors. Hearing receptors are found within the membranes of the cochlea.

  2. Hair cells of the organ of Corti (the receptor for hearing within the cochlea) (Figure 8.6) are stimulated by sound vibrations transmitted through air, membranes, and fluids.

  3. Deafness is any degree of hearing loss. Conduction deafness results when the transmission of sound vibrations through the external and middle ears is hindered. Sensorineural deafness occurs when there is damage to the nervous system structures involved in hearing.

  4. Receptors of the semicircular canals (cristae) are dynamic equilibrium receptors, which respond to angular or rotational body movements. Receptors of the vestibule (maculae) are static equilibrium receptors, which respond to the pull of gravity and report on head position. Visual and proprioceptor input are also necessary for normal balance.

  5. Symptoms of equilibrium apparatus problems include involuntary rolling of the eyes, nausea, vertigo, and an inability to stand erect.

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Chemical Senses: Smell and Taste

  1. Chemical substances must be dissolved in water to excite the receptors for smell and taste.

  2. The olfactory (smell) receptors are located in the superior aspect of each nasal cavity (Figure 8.7). Sniffing helps to bring more air (containing odors) over the olfactory mucosa.

  3. Olfactory pathways are closely linked to the limbic system; odors recall memories and arouse emotional responses.

  4. Gustatory (taste) cells are located in the taste buds, primarily on the tongue (Figure 8.8). The four major taste sensations are sweet, salt, sour, bitter.

  5. Taste and appreciation of foods is influenced by the sense of smell and the temperature and texture of foods.

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Developmental Aspects of the Special Senses

  1. Special sense organs are formed early in embryonic development. Maternal infections during the first five or six weeks of pregnancy may cause visual abnormalities as well as sensorineural deafness in the developing child. An important congenital eye problem is strabismus. The most important congenital ear problem is lack of the external auditory canal.

  2. Vision requires the most learning. The infant has poor visual acuity (is farsighted) and lacks color vision and depth perception at birth. The eye continues to grow and mature until the eighth or ninth year of life,

  3. Problems of aging associated with vision include presbyopia, glaucoma (the most common cause of blindness in the U.S.), cataracts, and arteriosclerosis of the eye's blood vessels.

  4. The newborn infant can hear sounds, but initial responses are reflexive. By the toddler stage, the child is listening critically and beginning to imitate sounds as language development begins.

  5. Sensorineural deafness (presbycusis) is a normal consequence of aging.

  6. Taste and smell are most acute at birth and decrease in sensitivity after the age of 40 as the number of olfactory and gustatory receptors decreases.
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