The organs of special senses include the gustatory, olfactory, visual, auditory, and vestibular systems. The gustatory apparatus, consisting of taste buds,

is discussed in Chapter 13, and the olfactory epithelium is treated in Chapter 12. This chapter details sensory endings: the microscopic morphology of the eye, involved with visual sensations, and the ear, involved with auditory and vestibular sensations.

SENSORY ENDINGS

Sensory endings are located at the terminus of dendrites. These specialized receptors (see Table 19-1) are members of the general somatic or general visceral afferent pathways and are:

•specialized to respond to stimuli, such as pressure, touch, temperature, and pain, on the external surface of the body (exteroceptors); additionally, there are special senses of sight and hearing as well as smell and taste

•incorporated into muscles and tendon to perceive the localization of the body in three-dimensional space (proprioceptors), and

•distributed within organs to monitor the activity of these organs as components of the general visceral afferent pathways.

EYE

The eye is a sensory organ whose lens focuses rays of light originating in the external environment onto photosensitive cells of the retina (see Graphic 19-1). The intensity, location, and wavelengths of the transmitted light are partially processed by the retina, and the assembled information is transmitted, via the optic nerve, for further processing and interpretation by the visual cortex of the brain as three-dimensional color images of the external milieu.

•Because the eyes are set apart and because their visual fields overlap, such three-dimensional imaging becomes possible.

•Each eyeball (bulb, globe), protected by the eyelids, is movable by means of a group of extrinsic skeletal muscles that insert into its fibrous outer tunic, thus assisting in suspending it in its bony orbit and directing the pupil to the most advantageous position for perceiving the image viewed.

Three coats constitute the wall of the bulb: the outer fibrous tunic, the middle vascular tunic (uvea), and the inner retinal tunic.

•The fibrous tunic (corneoscleral layer) is composed of the opaque, white sclera that covers the posterior aspect of the bulb and the transparent cornea that covers the anterior one-sixth of the eyeball.

The junction between the sclera and the cornea is known as the limbus.

•The vascular tunic consists of several regions: the anteriorly positioned iris and ciliary body and the posteriorly located, highly vascular and pigmented choroid.

Melanocytes located in the epithelium and stroma of the iris block light from passing through the iris, except at the pupil.

Additionally, eye color is related to the abundance of melanin produced by these melanocytes:

A large amount of melanin imparts dark eyes, whereas less melanin renders the eyes light in color.

Intrinsic smooth muscles, represented by the sphincter pupillae and dilatator pupillae muscles, adjust the aperture of the iris.

The ciliary smooth muscles alter tension of the suspensory (zonular) fibers anchored in the lens and thus alter the shape of the lens (accommodation) for near and far vision.

•The innermost retinal tunic is composed of 10 layers responsible for photoreception and impulse generation (see Table 19-2). The two deepest layers, the retinal pigment epithelium and the layer of rods and cones, bear the major responsibility for photoreception. Retinal pigment epithelium functions in esterifying vitamin A and transporting it to the rods and cones, phagocytosing the shed tips of rods and cones, and synthesizing melanin, which absorbs light after rods and cones have been stimulated.

Rods are sensitive to low light intensity and possess many flattened discs containing rhodopsin (an integral membrane protein, opsin, bound to retinal, the aldehyde form of vitamin A) in their outer segment.

When light is absorbed by rhodopsin, it dissociates into retinal and opsin (bleaching),permitting diffusion of bound Ca2+ into the outer segment.

Excess levels of Ca2+ hyperpolarize the cell by closing Na+ channels, thus preventing the entry of Na+ into the cell.

The electrical potential thus generated is relayed to other rods via gap junctions and then along the pathway to the optic nerve.

Dissociated retinal and opsin reassemble, and the Ca2+ ions are recaptured, establishing a normal resting potential.

Cones, sensitive to light of high intensity, producing greater visual acuity, are much more numerous

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than rods, and they produce iodopsin, the photopigment responsible for distinguishing color. Three different moieties of opsin are sensitive to either red, green, or blue light.

The mechanism of transducing photoenergy into electrical energy for transmission to the brain via the optic nerve is similar to that described in the rods.

Rods and cones are either stimulated (on) or inhibited (off) by light, that is, they indicate the location of a light pixel and, in the case of cones, its color.

Dendrites of 10 different types of bipolar cells receive information from the rods and cones, and then this information is conveyed by the axons of the bipolar cells into specific strata of the inner plexiform layer of the retina.

The further transmission of the impulses is monitored and modulated by one or more of the 27 types of amacrine cells, whose axons can span several millimeters or just a few micrometers of the retinal expanse.

The outer layer of the retina contains 12 types of ganglion cells whose interactions with bipolar cells and amacrine cells result in the

transmission of 12 different moving images (a continuous moving stream that resembles but is not created frame by frame) of the same scene via the optic nerve to the visual cortex of the brain for further analysis, assembly, and interpretation.

These moving images are very different from each other, in that some consist of highlights, others consist of line drawings of outlines, and still others generate shadows.

It is the function of the visual cortex to assemble these movies into the world that we recognize.

It must be stressed that this is a simplified description of some of the current concepts of vision that will certainly be modified as more information is gained from research in this field.

An additional type of ganglion cells, which make up less than 3% of the ganglion cell population, appear to function in the establishment of the circadian rhythm.

These ganglion cells possess the light-sensitive pigment melanopsin that responds to blue light even in individuals who are blind.

The axons of these ganglion cells project to the suprachiasmatic nucleus, the region of the brain responsible for the regulation of circadian rhythm.

It appears, then, that the suprachiasmatic nucleus receives information from these specialized ganglion cells of the retina as to when there is daylight.

The optic disc, the region where the optic nerve exits the eyeball, contains neither cones nor rods; consequently, it represents and is called a blind spot. Just lateral to the blind spot is the fovea centralis, a depression in the retina. The fovea contains mostly cones that are packed so tightly that not all layers of the retina are present. Visual acuity is the greatest in the fovea centralis.

•Accessory structures of the eye include the conjunctiva, eyelids, and lacrimal gland.

The conjunctiva is a transparent mucous membrane that lines the eyelids and reflects onto the eyeball.

The eyelids contain modified sebaceous glands, the meibomian glands, which are responsible for altering the surface tension of the watery tears, thus slowing evaporation.

The lacrimal glands secrete tears, a complex fluid composed of water, proteins, salts, peptides, and other organic molecules which keep the conjunctiva and cornea moist.

Tears also contain lysozyme, an antibacterial enzyme.

The additional components of the interior of the eyeball are the aqueous humor, a fluid; the vitreous body, a gel; and the lens, all of which serve as parts of the refractive media.

The aqueous humor, located in the posterior and anterior chambers of the eye, and the vitreous body, located behind the lens of the eye, are also important in providing nutrients to the avascular lens and cornea.

EAR

The ear functions in the reception of sound as well as in the perception of the orientation of the head and therefore the body in relation to the directional forces of gravity (see Graphic 19-2). To perform both functions of hearing and equilibrium (balance), the ear is subdivided into the external, middle, and inner ears.

•The external ear is composed of a cartilaginous, skin-covered auricle (pinna) and the external auditory meatus, with its cartilaginous outer and bony inner aspects, whose internal extent is separated from the middle ear by the thin tympanic membrane.

•The tympanic cavity of the middle ear houses the three auditory ossicles, connected in series to each other: the outermost malleus (hammer), the middle

incus (anvil), and the innermost stapes (stirrup). This cavity is connected to the nasopharynx via the cartilaginous auditory (eustachian) tube, which permits equalization of atmospheric pressures on either side of the tympanic membrane. Sound waves are funneled by the auricle to the tympanic membrane, whose vibrations are amplified and transmitted by the movements of the three bony ossicles to the oval window of the inner ear’s cochlea.

•The bony labyrinth of the inner ear, subdivided into the semicircular canals, vestibule, and cochlea, is filled with perilymph. Loosely contained within it and all of its subdivisions is the endolymph-containing membranous labyrinth.

Movements of the fluid environment within this system are perceived by apical “hairs” of specialized sensory cells contained within the membranous labyrinth and ultimately transduced to electrical impulses for transmission to the brain.

The saccule and utricle, specialized structures of the membranous labyrinth in the vestibule, contain type I and type II hair cells (neuroepithelial cells containing many stereocilia and a single kinocilium), whose free ends are embedded in the otolithic membrane containing calcium carbonate crystals known as otoliths (otoconia).

Static equilibrium and linear acceleration are determined by movements (or the lack of movements) in the stereocilia or kinocilia of these hair cells.

Threshold bending of the stereocilia or kinocilia will depolarize the hair cells, which, in turn, relay (via neurotransmission) information to the processes of primary vestibular neurons located in Scarpa’s ganglion.

Semicircular ducts, specializations of the membranous labyrinth in the semicircular canals,

contain neuroepithelial hair cells located in the cristae ampullares (sensory regions) of the ampullae.

Free ends of these hair cells have stereocilia that are embedded in a viscous glycoprotein known as the cupula.

Movement of the endolymph bathing the cupula depolarize the hair cells, which, in turn, alter the activity in the synaptic endings associated with the base of the hair cells.

This process is sensitive to rotational acceleration in any of the three directions of orientation of the semicircular canals.

Thus, these structures are responsible for the vestibular sensations of balance and orientation.

TABLE 19-3 • Cells of the Spiral Organ of Corti

The endolymphatic sac (terminal end of the endolymphatic duct) contains phagocytic cells in its lumen and may function in resorption of endolymph.

The cochlear duct (scala media) contains the spiral organ of Corti, which is bordered by the scala vestibuli and the scala tympani (both scalae contain perilymph and communicate at the helicotrema).

The vestibular membrane located between the scala vestibuli and the cochlear duct functions to maintain the high ion gradient between the perilymph and endolymph.

The spiral organ of Corti (see Table 19-3), sitting on the basilar membrane, contains, among other supporting cells, neuroepithelial inner and outer hair cells whose free ends are embedded in the gel-like tectorial membrane.

-Sound transmission/conduction via the tympanic membrane and ossicles to the oval window sets waves translated to the oval window and sets the perilymph of the scala tympani in motion, which displaces the basilar membrane, thus moving the hair cells but not the tectorial membrane.

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-Bending of the hair cells causes them to release neurotransmitter substance, exciting the bipolar cells of the spiral ganglion, resulting in transmission of the impulse to higher centers of the brain.

-Although the basilar membrane vibrates at many frequencies, certain regions vibrate optimally at specific frequencies.

-For example, low-frequency sound waves are detected farther away from the oval window.

-It should be noted that loud sounds, such as those at rock concerts, create a great deal of energy within the hearing mechanism, such that it may take 2 or 3 days for the energy to be completely dissipated and the buzzing to stop.

Blue Eye Color

Until approximately 6,000 to 10,000 years ago, every human being had brown eyes; then, a small mutation in the switch that turned off OCA2 gene resulted in the inability of that individual to manufacture P protein in the iris. Protein P is involved in melanin formation; thus, the person with this particular mutation was able to synthesize melanin normally except in the iris, and instead of having brown eyes, that person’s eyes were blue. Thus, it is believed that all blue-eyed individuals are descendants of that one person born in that era.

Myopia and Hyperopia

As an individual ages, the longitudinal axis of the eye changes, as may the curvature of the cornea, and the lens, instead of focusing the image on the retina, focuses it either in front of the retina (myopic vision) or behind the retina (hyperopic vision). The condition may be corrected with lenses (eyeglasses or contact lenses) or by refractive surgery, assisting the lens in focusing on the retina.

Glaucoma

Glaucoma is a condition of high intraocular pressure caused by an obstruction that prevents the aqueous humor from exiting the anterior chamber of the eye. If left untreated, the pressure damages the optic nerve to such an extent that blindness may result.

Cataract

Cataract, a common condition of aging, is caused by excessive UV radiation and by pigments and other substances accumulating in the lens, making it opaque and thus impairing vision. This condition may be corrected by excising the lens and replacing it with a plastic lens.

This photomicrograph is from the lens of an older patient who presented with age-related cataract. Observe the presence of cortical extracellular clefts and globules. (Reprinted with permission from Mills SE, Carter, D, et al., eds. Sternberg’s Diagnostic Surgical Pathology, 5th ed., Philadelphia: Lippincott Williams & Wilkins, 2010, p. 981.)

Detached Retina

Detached retina may result from a trauma in which the neural and pigmented layers of the retina become separated, resulting to ischemic damage to the neurons. This condition may cause partial blindness, but it may be corrected by surgical intervention.

Retinoblastoma

Retinoblastoma is a malignancy of the very young child, usually detected at about 2 years of age, although at the time of diagnosis the child may be 5 or 6 years old. Approximately a third of the cases have familial components, but at least 60% occur without a familial incidence. The tumor appears white with regions of calcification

and yellow foci of necrosis. The tumor may spread by individual cells invading the optic nerve as well as the choroid. The patient has to lose the eyeball to prevent metastasis.

This photomicrograph is from the eyeball of a child with retinoblastoma. Observe the relatively normal retina on the left-hand side of the image. The arrows indicate regions of necrosis in a field of perivascular tumor cells. (Reprinted with permission from Mills SE, Carter, D, et al., eds. Sternberg’s Diagnostic Surgical Pathology, 5th ed., Philadelphia: Lippincott Williams & Wilkins, 2010, p. 982.)

Conductive Hearing Loss

Conductive hearing loss may arise from a middle ear infection (otitis media), an obstruction, or otosclerosis of the middle ear.

Nerve Deafness

Nerve deafness results from a lesion in the cochlear portion of the vestibulocochlear nerve (cranial nerve VIII). This condition may be the result of disease, prolonged exposure to loud sounds, and/or drugs.

Observe that the footplate of the stapes is fixed to the densely sclerotic bone forming the perimeter of the oval window. (Reprinted with permission from Mills SE, Carter D, et al., eds. Sternberg’s Diagnostic Surgical Pathology, 5th ed., Philadelphia: Lippincott Williams & Wilkins, 2010, p. 944.)

Ménière’s Disease

Ménière’s disease is an inner ear disorder characterized by symptoms such as hearing loss due to excess fluid accumulation in the endolymphatic duct, vertigo, tinnitus, nausea, and vomiting. In severe cases, surgical treatment may be required.

Acoustic Neuroma

The condition known as acoustic neuroma is a benign tumor whose cells of origin are the Schwann cells of the vestibulocochlear nerve (cranial nerve VIII). It is manifested by loss of hearing, loss of balance, vertigo, and tinnitus. If the tumor is not treated early enough, it may involve other cranial nerves in its vicinity. Recent studies suggest the possibility that long-term exposures to the electromagnetic radiation emitted by cell phones may be a causative factor in the development of acoustic neuroma in susceptible individuals.

Eye • 1-19 GRAPHIC

Stapes

Incus

Malleus

External auditory meatus Tympanum

Outer Hair

Cell

Inner

Hair Cell

Outer hair cell

Cells of Hensen

Outer spiral sulcus

Outer phalangeal cells

Outer pillar cell

Cochlear nerve

Inner pillar cell

Acoustic

nerve

Semicircular canals

Cochlea

Auditory tube

Scala vestibuli

Tectorial membrane

Vestibular membrane

Spiral ganglion

Organ of Corti

Basilar membrane

Scala tympani

Organ of Corti

Tectorial membrane

Inner spiral cells

Inner hair cell

Inner phalangeal cells

Cochlear nerve fibers

Ear• 2-19 GRAPHIC

Body Ciliary and Iris, Sclera, Cornea, ye, • E1-19 PLATE

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FIGURE 1. Cornea. Monkey. Paraffin section. ×132.

The cornea is a multilayered, transparent structure. Its anterior surface is covered by a stratified squamous nonkeratinized epithelium (Ep) on the right-hand side of the image, deep to which is a thin, acellular Bowman’s membrane. The bulk of the cornea, the stroma (St), is composed of regularly arranged collagen fibers (CF) and intervening fibroblasts, whose nuclei (N) are readily evident. The posterior surface of the cornea is lined by a simple squamous-to-cuboidal epithelium (Ep) on the left-hand side of the image. A thin, acellular Descemet’s membrane lies between the simple epithelium and the stroma. Inset. Cornea. Monkey. Paraffin section. ×270. A higher magnification of the anterior surface displays the stratified squamous epithelium (Ep) as well as the acellular Bowman’s membrane (BM). Note the regularly arranged bundles of collagen fibers (CF) and intervening fibroblasts (F), whose nucleus is labeled by the lead line.

FIGURE 3. Iris. Monkey. Paraffin section. ×132.

The iris (I) is a pigmented diaphragm that delineates the pupil (P) of the eye. It separates the anterior chamber (AC) from the posterior chamber (PC). The iris is composed of three layers: an outer discontinuous layer of melanocytes and fibroblasts; the intermediate fibrous layer (FL), housing pigment cells (Pc) and fibroblasts; and the posterior double-layered pigmented epithelium (PEp). The sphincter (sM) and dilatator muscles are composed of smooth muscle and smooth muscle–like myoepithelial cells, respectively. The pupillary region of the iris contacts the capsule (Ca) of the lens (L) in living individuals.

FIGURE 2. Sclera. Monkey. Paraffin section. ×132.

The sclera is similar to and continuous with the cornea, but it is not transparent. Note that the epithelium (Ep) of the conjunctiva covers the anterior surface of the sclera. Deep to the epithelium is the loose episcleral tissue (ET), whose small blood vessels (BV) are evident. The stroma (St) is composed of thick collagen fiber (CF) bundles, between which numerous fibroblasts (F) can be seen. The deepest layer of the sclera is the suprachoroid lamina (SL), whose melanocytes (M) containing dark melanin pigment characterize this layer.

FIGURE 4. Ciliary body. Monkey. Paraffin section. ×132.

The ciliary body is composed of ciliary processes (CP), projecting into the posterior chamber (PC), from which suspensory ligaments (zonular fibers) extend to the lens. The bulk of the ciliary body is composed of smooth muscle (SM) disposed more or less in three layers, not evident in this photomicrograph. Numerous pigment cells (Pc) are present in this region. Note that the epithelium of the ciliary body is composed of two layers: an outer pigmented (OP) and an inner nonpigmented (IN) epithelium. The narrow vascular layer (VL) intervenes between the epithelium and ciliary muscles. The base, or root, of the iris is anchored to the ciliary body.

Microscopy Electron Scanning and Light Retina,• 2-19 PLATE

466 S P E C I A L S E N S E S

FIGURE 1. Tunics of the eye. Monkey. Paraffin section. ×14.

This survey photomicrograph is of an anterolateral section of the globe of the eye, as evidenced by the presence of the lacrimal gland (LG). Note that the three layers of the globe of the eye are extremely thin in relation to its diameter. The sclera (S) is the outermost layer. The pigment choroid (Ch) and multilayered retina (Re) are easily distinguishable even at this low magnification. The posterior compartment (PCo) lies behind the lens and houses the vitreous body. A region similar to the boxed area is presented at a higher magnification in Figure 2.

FIGURE 3. Rods and cones. Monkey. Scanning electron microscopy. ×6,300.

This scanning electron micrograph of the monkey retina displays regions of several cones (C) that display their thicker morphology and wider nuclear zone and of a few rods (R) whose diameter is narrower, with a thinner nuclear zone. The inner segments of the lamina of rods and cones (2), external limiting membrane (3), and outer nuclear layer (4) are clearly recognizable. The microvilli (Mv) noted in the vicinity of the external limiting membrane belong to Müller’s cells, which were removed during specimen preparation. Observe the longitudinal ridges (arrows) along the surface of the inner segments. (From Borwein B, Borwein D, Medeiros J, McGowan J. The ultrastructure of monkey foveal photoreceptors, with special reference to the structure, shape, size, and spacing of the foveal cones. Am J Anat 1980;159:125–146.)

FIGURE 2. Retina. Pars optica. Monkey. Paraffin section. ×270.

The pars optica of the retina is composed of 10 distinct layers. The pigment epithelium (1), the outermost layer, is closely apposed to the vascular and pigmented choroid (Ch). Various regions of the rods (R) and cones (C) characterize the next four layers. These are the lamina of rods and cones (2), external limiting membrane (3), outer nuclear layer (4), and outer plexiform layer (5). The inner nuclear layer (6) houses the cell bodies of various associative glial (Müller’s) and neural cells. The inner plexiform layer

(7) is a region of synapse formation, whereas the ganglion cell layer (8) houses the cell bodies of multipolar neurons and associated neuroglia. The centrally directed (toward the central nervous system) fibers of these ganglion cells form the optic nerve fiber layer (9), whereas the inner limiting membrane (10) is composed of the expanded processes of Müller’s cells along the inner surface of the eye. A region similar to the boxed area is presented in Figure 3, a scanning electron micrograph of the rods and cones.

Cone

Rod

Pigment layer

Photosensitive region

Metabolic region

Section of retina

Glands Lacrimal and Eyelid, Lens, ovea, • F3-19 PLATE

Ca

Ep

Glands Lacrimal and Eyelid, Lens, ovea, • F3-19 PLATE

Ear Inner• 4-19 PLATE

ochlea • C5-19 PLATE

Corti of Organ piral • S6-19 PLATE