Marieb-Essentials-of-Anatomy-Physiology-12th.pdf

8 Special Senses WHAT The special senses respond to stimuli involved in vision, hearing, balance, HOW smell, and taste....

8 Special Senses WHAT The special senses respond to stimuli involved in vision, hearing, balance, HOW smell, and taste. A variety of receptors, housed in special sense organs such as the eye, ear, and nose, help detect stimuli in your surroundings. WHY Without your special senses, you could not smell or taste your favorite food, appreciate colors, or hear your INSTRUCTORS favorite song. New Building Vocabulary Coaching Activities for this chapter are assignable in P eople are responsive creatures. Hold freshly baked bread before us, and our mouths water. Sudden thunder makes us jump. These stimuli (the bread and thunder) and many others continually greet us and are interpreted by proprioceptors of muscles and joints. The other four “traditional” senses—smell, taste, sight, and hearing—are called special senses. Receptors for a fifth special sense, equilibrium, are housed in the ear, along with the organ of hearing. In con- our nervous system. trast to the small and widely distributed general We are told that we have five senses that keep receptors, the special sense receptors are either us in touch with what is going on in the external large, complex sensory organs (eyes and ears) or world: touch, taste, smell, sight, and hearing. localized clusters of receptors (taste buds and Actually, touch is a mixture of the general senses olfactory epithelium). that we considered in Chapter 7—the temperature, In this chapter we focus on the functional pressure, and pain receptors of the skin and the anatomy of each special sense organ, but keep in 278  Chapter 8: Special Senses 279 mind that sensory inputs overlap. What we experi- The adult eye is a sphere that measures about 1 ence—our “feel” of the world—is a blending of inch (2.5 cm) in diameter. Only the anterior one- stimulus effects. sixth of the eye’s surface is normally seen. The rest ➔ of it is enclosed and protected by a cushion of CONCEPTLINK fat and the walls of the bony orbit. The acces- Recall the three basic functions of the nervous system sory structures of the eye include the extrinsic (Figure 7.1, p. 226). Each of the special senses gathers eye muscles, eyelids, conjunctiva, and lacrimal unique sensory information that, once integrated, will apparatus. influence motor output. For example, if you saw a ball Anteriorly the eyes are protected by the eye- moving toward your head, this sensory input might lids, which meet at the medial and lateral corners result in a motor output that would move your body of the eye, the medial commissure (canthus) out of the path of the ball. Additionally, recall that each and lateral commissure (canthus), respectively type of sensory information is processed in a special- (Figure 8.1). The space between the eyelids in an ized area of the cerebrum (Figure 7.13c, p. 242). ➔ open eye is called the palpebral fissure. Projecting from the border of each eyelid are the eyelashes. Modified sebaceous glands associated with the PART I: THE EYE AND VISION eyelid edges are the tarsal glands. These glands Of all the senses, vision has been studied most. produce an oily secretion that lubricates the eye (Figure 8.2a, p. 280). Ciliary glands, which are 8 Nearly 70 percent of all sensory receptors in the body are in the eyes. The optic tracts that carry modified sweat glands, lie between the eyelashes information from the eyes to the brain are massive (cilium = eyelash), and their ducts open at the bundles, containing over a million nerve fibers. We eyelash follicles. On the medial aspect of each eye rely heavily on our sight and often have to “see it is the lacrimal caruncle (see Figure 8.1), a raised to believe it” (Figure 8.1). area containing sebaceous and sweat glands that produce an oily, whitish secretion that also lubri- Anatomy of the Eye cates the eye. A delicate membrane, the conjunctiva (kon- External and Accessory Structures junk–ti¿vah), lines the eyelids and covers part of ➔ Learning Objective the outer surface of the eyeball (Figure 8.2a). It □ When provided with a model or diagram, identify ends at the edge of the transparent cornea by fus- the accessory eye structures, and list the functions ing with the corneal epithelium. The conjunctiva of each. secretes mucus, which helps to lubricate the eye- ball and keep it moist. Site where Eyebrow conjunctiva merges with Eyelid cornea Eyelashes Palpebral Pupil fissure Lacrimal caruncle Lateral Medial commissure commissure (canthus) (canthus) Iris Sclera Figure 8.1 Surface (covered by anatomy of the eye and Eyelid conjunctiva) accessory structures. 280 Essentials of Human Anatomy and Physiology The lacrimal apparatus (Figure 8.2b) con- sists of the lacrimal gland and a number of ducts Lacrimal that drain lacrimal secretions into the nasal cavity. Excretory duct gland of lacrimal gland The lacrimal glands are located above the lateral end of each eye. They continually release a dilute Conjunctiva salt solution (tears) onto the anterior surface of the eyeball through several small ducts. The tears Anterior flush across the eyeball into the lacrimal canalic- aspect uli medially, then into the lacrimal sac, and finally into the nasolacrimal duct, which empties Eyelid into the inferior meatus of the nasal cavity (see Eyelashes Figure 8.2b). Tears also contain mucus, antibodies, and lysozyme (li¿so-zı̄m), an enzyme that destroys Tarsal glands bacteria. Thus, they cleanse and protect the eye surface as they moisten and lubricate it. When lac- (a) Eyelid rimal secretion increases substantially, tears spill over the eyelids and fill the nasal cavities, causing congestion and the “sniffles.” This happens when the eyes are irritated by foreign objects or chemi- cals and when we are emotionally upset. In the case of irritation, the enhanced tearing acts to Lacrimal Lacrimal sac wash away or dilute the irritating substance. The gland importance of “emotional tears” is poorly under- stood, but some suspect that crying is important in Excretory ducts of lacrimal gland reducing stress. Anyone who has had a good cry would probably agree, but this has been difficult to prove scientifically. Lacrimal canaliculus Homeostatic Imbalance 8.2 Nasolacrimal duct Because the nasal cavity mucosa is continuous Inferior meatus with that of the lacrimal duct system, a cold or of nasal cavity nasal inflammation often causes the lacrimal mucosa to become inflamed and swell. This Nostril impairs the drainage of tears from the eye surface, causing “watery” eyes. ______________________ ✚ Six extrinsic eye muscles (external eye (b) muscles) are attached to the outer surface of each Figure 8.2 Accessory structures of the eye. eye. These muscles produce gross eye movements (a) Sagittal section of the accessory structures and make it possible for the eyes to follow a mov- associated with the anterior part of the eye. (b) Anterior ing object. Figure 8.3 gives the names, locations, view of the lacrimal apparatus. actions, and cranial nerve serving each of the extrinsic muscles. Homeostatic Imbalance 8.1 Did You Get It? 1. What is the role of the eyelids? Inflammation of the conjunctiva, called conjuncti- 2. Which four accessory glands or structures help vitis, results in reddened, irritated eyes. Pinkeye, lubricate the eye? its infectious form caused by bacteria or viruses, is 3. What is the role of lysozyme in tears? highly contagious. __________________________ ✚ 4. What is the visual role of the external eye muscles? For answers, see Appendix A.  Chapter 8: Special Senses 281 Trochlea Superior oblique muscle Superior oblique tendon Axis at Superior center of rectus muscle eye Conjunctiva Inferior rectus muscle Lateral rectus Medial muscle rectus muscle Lateral rectus muscle Optic Inferior Inferior rectus oblique Common 8 nerve muscle muscle tendinous ring (a) (b) Controlling Name Action cranial nerve Lateral rectus Moves eye laterally VI (abducens) Medial rectus Moves eye medially III (oculomotor) Superior rectus Elevates eye and turns it medially III (oculomotor) Inferior rectus Depresses eye and turns it medially III (oculomotor) Inferior oblique Elevates eye and turns it laterally III (oculomotor) Superior oblique Depresses eye and turns it laterally IV (trochlear) (c) Figure 8.3 Extrinsic muscles of the eye. (a) Lateral view of the right eye.(b) Superior view of the right eye. The four rectus muscles originate from the common tendinous ring, a ringlike tendon at the back of the eye socket. (c) Summary of cranial nerve supply and actions of the extrinsic eye muscles. Internal Structures: The Eyeball The eye itself, called the eyeball, is a hollow sphere (Figure 8.4, p. 282). Its wall is composed ➔ Learning Objectives of three tunics, or layers, and its interior is filled □ Name the layers of the wall of the eye, and with fluids called humors that help to maintain its indicate the major function of each. shape. The lens, the main focusing apparatus of □ Explain how the functions of rods and cones differ. the eye, is supported upright within the eye cavity, □ Define blind spot, cataract, and glaucoma. dividing it into two chambers. □ Trace the pathway of light through the eye to the retina. Layers Forming the Wall of the Eyeball □ Discuss the importance of an ophthalmoscopic Now that we have covered the general anatomy of examination. the eyeball, we are ready to get specific. 282 Essentials of Human Anatomy and Physiology Q: Which layer of the eye would be the first to be affected by deficient tear production? Sclera Ciliary body Choroid Ciliary zonule Retina Cornea Fovea centralis Iris Pupil Optic nerve Aqueous humor (in anterior segment) Lens Scleral venous sinus Central artery (canal of Schlemm) and vein of Vitreous humor the retina (in posterior segment) Optic disc (blind spot) (a) Ciliary body Vitreous humor in posterior segment Iris Retina Margin of pupil Choroid Sclera Aqueous humor Fovea centralis (in anterior segment) Optic disc Lens Optic nerve Cornea Ciliary zonule (b) Figure 8.4 Internal anatomy of the eye (sagittal section). (a) Diagrammatic view. (b) Photograph. tears. A: its cornea), which normally is continuously washed by The outermost fibrous layer (the sclera and especially  Chapter 8: Special Senses 283 Fibrous Layer The outermost layer, called the The transparent inner neural layer of the ret- fibrous layer, consists of the protective sclera ina contains millions of receptor cells, the rods (sklĕ¿rah) and the transparent cornea (kor¿ne-ah). and cones, which are called photoreceptors The sclera (thick white connective tissue) is seen because they respond to light (Figure 8.5, p. 284). anteriorly as the “white of the eye.” The central Electrical signals pass from the photoreceptors via anterior portion of the fibrous layer is crystal clear. a two-neuron chain—bipolar cells and then gan- This “window” is the cornea through which light glion cells—before leaving the retina via the enters the eye. The cornea is well supplied with optic nerve and being transmitted to, and inter- nerve endings. Most are pain fibers, and when the preted by, the optic cortex. The result is vision. cornea is touched, blinking and increased tear The photoreceptor cells are distributed over production occur. Even so, the cornea is the most the entire retina, except where the optic nerve exposed part of the eye, and it is very vulnerable leaves the eyeball; this site is called the optic disc. to damage. Luckily, its ability to repair itself is Since there are no photoreceptors at the optic extraordinary. Furthermore, the cornea is the only disc, it results in a blind spot in our vision. When tissue in the body that is transplanted from one light from an object is focused on the optic disc, person to another without the worry of rejection. the object disappears from our view and we can- Because the cornea has no blood vessels, it is not see it. beyond the reach of the immune system. The rods and cones are not evenly distributed in the retina. The rods are densest at the periph- 8 Vascular Layer The middle, or vascular layer, ery, or edge, of the retina and decrease in number of the eyeball, has three distinguishable regions. as the center of the retina is approached. The rods Most posterior is the choroid (ko¿roid), a blood- allow us to see in gray tones in dim light, and they rich nutritive tunic that contains a dark pigment. provide our peripheral vision. The pigment prevents light from scattering inside the eye. Moving anteriorly, the choroid is modified Homeostatic Imbalance 8.3 to form two smooth muscle structures, the ciliary (sil¿e-er-e) body, which is attached to the lens by Anything that interferes with rod function hinders a suspensory ligament called the ciliary zonule, our ability to see at night. This condition, called and the iris. The pigmented iris has a rounded night blindness, dangerously impairs our ability opening, the pupil, through which light passes. to drive safely at night. Its most common cause is Circularly and radially arranged smooth muscle prolonged vitamin A deficiency, which eventually fibers form the iris, which acts like the diaphragm causes the neural retina to deteriorate. Vitamin A of a camera. That is, it regulates the amount of is one of the building blocks of the pigments the light entering the eye so that we can see as clearly photoreceptor cells need to respond to light (see as possible in the available light. In close vision “A Closer Look” on p. 285). Vitamin A supple- and bright light, the circular muscles contract, and ments will restore function if taken before degen- the pupil constricts, or gets smaller. In distant erative changes in the neural retina occur. _____ ✚ vision and dim light, the radial fibers contract to enlarge (dilate) the pupil, which allows more light Cones are discriminatory receptors that allow to enter the eye. Cranial nerve III (oculomotor) us to see the details of our world in color under controls the muscles of the iris. bright light conditions. They are densest in the cen- ter of the retina and decrease in number toward Sensory Layer The innermost sensory layer of the retinal edge. Lateral to each blind spot is the the eye is the delicate two-layered retina (ret¿ı̆- fovea centralis (fo¿ve-ah sen-tră¿lis), a tiny pit that nah), which extends anteriorly only to the ciliary contains only cones (see Figure 8.4). Consequently, body. The outer pigmented layer of the retina is this is the area of greatest visual acuity, or point composed of pigmented cells that, like those of of sharpest vision, and anything we wish to view the choroid, absorb light and prevent light from critically is focused on the fovea centralis. scattering inside the eye. They also act as phago- There are three varieties of cones. Each type is cytes to remove dead or damaged receptor cells most sensitive to particular wavelengths of visible and store vitamin A needed for vision. light (Figure 8.6). One type responds most  Visible light Pigmented layer of retina 420 nm 530 nm (blue cones) (green cones) Light absorption by cone populations Rod 560 nm (red cones) Cone Bipolar cells Ganglion Pathway cells of light (a) 400 450 500 550 600 650 700 Wavelength (nanometers) Figure 8.6 Sensitivities of the three cone types to different wavelengths of visible light. Pigmented vigorously to blue light, another to green light. Neural layer layer of retina of retina The third cone variety responds to a range includ- ing both green and red wavelengths of light. However, this is the only cone population to Central respond to red light at all, so these are called the artery “red cones.” Impulses received at the same time and vein from more than one type of cone by the visual of retina cortex are interpreted as intermediate colors, simi- Optic disc lar to what occurs when two colors of paint are mixed. For example, simultaneous impulses from blue and red color receptors are seen as purple or violet tones. When all three cone types are stimu- lated, we see white. If someone shines red light into one of your eyes and green into the other, you will see yellow, indicating that the “mixing” Sclera and interpretation of colors occur in the brain, not Optic nerve Choroid in the retina. (b) Homeostatic Imbalance 8.4 Figure 8.5 The three major types of neurons composing the retina. (a) Notice that light must pass Lack of all three cone types results in total color through the thickness of the retina to excite the rods and blindness, whereas lack of one cone type leads cones. Electrical signals flow in the opposite direction: to partial color blindness. Most common is the from the rods and cones to the bipolar cells and finally to lack of red or green receptors, which leads to two the ganglion cells. The ganglion cells generate the nerve varieties of red-green color blindness. Red and impulses that leave the eye via the optic nerve. (b) Schematic green are seen as the same color—either red or view of the posterior part of the eyeball illustrating how the axons of the ganglion cells form the optic nerve.  A Visual Pigments—The Actual CLOSER LOOK Photoreceptors T he names of the tiny pho- not blinded and unable to see in Light Li ght ht toreceptor cells of the ret- bright sunlight. Lig ina reflect their shapes. As A good deal is known about the shown to the left, rods are structure and function of rhodopsin, slender, elongated neurons, the purple pigment found in rods (see Process of whereas the fatter cones figure below). It is formed from the bipolar cell Synaptic taper to pointed tips. In each union of a protein (opsin) and a endings type of photoreceptor, there is modified vitamin A product (retinal). Rod Inner a region called an outer seg- When combined in rhodopsin, retinal cell fi ers Rod cell ment, attached to the cell has a kinked shape that allows it to body body Cone body. The outer segment cor- bind to opsin. But when light strikes cell responds to a light-trapping rhodopsin, retinal straightens out and body Nuclei dendrite, in which the discs releases the protein. Once straight- Outer containing the visual pigments ened out, the retinal continues its fi er Mitochondria are stacked like a row of conversion until it is once again vita- pennies. min A. As these changes occur, the The behavior of the visual purple color of rhodopsin changes to pigments is dramatic. When the yellow of retinal and finally Inner segment light strikes them, they lose becomes colorless as the change to their color, or are “bleached”; vitamin A occurs. Thus the term shortly afterward, they regen- “bleaching of the pigment” accurately erate their pigment. Absorp- describes the color changes that occur tion of light and pigment when light hits the pigment. Rhodop- bleaching cause electrical sin is regenerated as vitamin A is changes in the photoreceptor again converted to the kinked form of cells that ultimately cause retinal and recombined with opsin in nerve impulses to be transmit- an ATP-requiring process. The cone ted to the brain for visual pigments, although similar to rhodop- interpretation. Pigment regen- sin, differ in the specific kinds of pro- eration ensures that you are teins they contain. Pigmented layer Discs containing visual pigments Retinal (visual yellow) Outer Light absorption segment Pigment cell causes Releases nucleus Melanin granules Opsin Rhodopsin (visual purple) Bleaching of the pigment 285 green, depending on the cone type present. Many the vitreous body (see Figure 8.4). Vitreous color-blind people are unaware of their condition humor helps prevent the eyeball from collapsing because they have learned to rely on other cues— inward by reinforcing it internally. Aqueous humor such as differences in intensities of the same is similar to blood plasma and is continually color—to distinguish green from red, for example, secreted by a special area of the choroid. Like the on traffic signals. Because the genes regulating vitreous humor, it helps maintain intraocular color vision are on the X (female) sex chromo- (in–trah-ok¿u-lar) pressure, the pressure inside the some, color blindness is a sex-linked condition. It eye. It also provides nutrients for the avascular occurs almost exclusively in males. ___________ ✚ lens and cornea. Aqueous humor is reabsorbed into the venous blood through the scleral venous Lens sinus, or canal of Schlemm (shlĕm), which is Light entering the eye is focused on the retina by located at the junction of the sclera and cornea. the lens, a flexible biconvex crystal-like structure. Recall the lens is held upright in the eye by the Homeostatic Imbalance 8.6 ciliary zonule and attached to the ciliary body (see Figure 8.4). If drainage of aqueous humor is blocked, fluid backs up like a clogged sink. Pressure within the Homeostatic Imbalance 8.5 eye may increase to dangerous levels and com- press the delicate retina and optic nerve. The In youth, the lens is transparent and has the consis- resulting condition, glaucoma (glaw-ko¿mah; tency of firm jelly, but as we age it becomes increas- “vision going gray”), can lead to blindness unless ingly hard and cloudy. Cataracts, the loss of lens detected early. Glaucoma is a common cause of transparency, cause vision to become hazy and dis- blindness in the elderly. Unfortunately, many forms torted and can eventually cause blindness. Other of glaucoma progress slowly and have almost no risk factors for forming cataracts include diabetes symptoms at first. Thus, sight deteriorates slowly mellitus, frequent exposure to intense sunlight, and and painlessly until the damage is done. Signs of heavy smoking. Current treatment of cataracts is advanced glaucoma include seeing halos around either special cataract glasses or surgical removal of lights, headaches, and blurred vision. A simple the lens and replacement with a lens implant. instrument called a tonometer (to-nom¿e-ter) is used to measure the intraocular pressure, which should be tested yearly in people over 40. Glaucoma is commonly treated with eyedrops that increase the rate of aqueous humor drainage. Laser or surgical enlargement of the drainage channel is another option. ___________________ ✚ – The ophthalmoscope (of-thal¿mo-skop) is an instrument that illuminates the interior of the eye- ball, allowing the retina, optic disc, and internal blood vessels at the fundus, or posterior wall of the eye, to be viewed and examined (Figure 8.7). Such an examination can detect certain pathologi- cal conditions, such as diabetes, arteriosclerosis, The cataract in this photo appears as a milky structure that and degeneration of the optic nerve and retina. seems to fill the pupil. __________________________________________ ✚ Did You Get It? 5. What is the meaning of the term blind spot in relation The lens divides the eye into two segments, or to the eye? chambers. The anterior (aqueous) segment, ante- 6. What function does the choroid of the vascular layer rior to the lens, contains a clear watery fluid called have in common with the pigmented layer of the retina? aqueous humor. The posterior (vitreous) seg- 7. How do the rods and cones differ from each other? ment, posterior to the lens, is filled with a gel-like substance called vitreous (vit¿re-us) humor, or For answers, see Appendix A.  Chapter 8: Special Senses 287 Q: Fovea Macula Blood Optic disc Retina As you look at this figure, are your lenses relatively thick centralis vessels or relatively thin? Retina Light from distant source Focal point (a) Light from near source Focal point Lateral Medial Retina 8 Figure 8.7 the posterior wall (fundus) of the retina as seen with an ophthalmoscope. Notice the optic disc, from which the blood vessels radiate. Physiology of Vision Pathway of Light through the Eye (b) and Light refraction Figure 8.8 relative convexity of the lens during ➔ Learning Objectives focusing for distant and close vision. (a) Light rays from a distant object are nearly parallel as they reach □ Describe image formation on the retina. the eye and can be focused without requiring changes □ Define the following terms: accommodation, in lens convexity. (b) Diverging light rays from close astigmatism, emmetropia, hyperopia, myopia, objects require that the lens bulge more to focus the and refraction. image sharply on the retina. When light passes from one substance to another substance that has a different density, its speed changes and its rays are bent, or refracted. Light close object tends to scatter and diverge, or spread rays are bent in the eye as they encounter the cor- out, and the lens must bulge more to make close nea, aqueous humor, lens, and vitreous humor. vision possible (Figure 8.8b). To achieve this, the The refractive, or bending, power of the cor- ciliary body contracts, allowing the lens to become nea and humors is constant. However, that of the more convex. This ability of the eye to focus spe- lens can be changed by changing its shape—that cifically for close objects (those less than 20 feet is, by making it more or less convex, so that light away) is called accommodation. The image can be properly focused on the retina. The greater formed on the retina as a result of the light-bending the lens convexity, or bulge, the more it bends the activity of the lens is a real image—that is, it is light. The flatter the lens, the less it bends the reversed from left to right, upside down (inverted), light. and smaller than the object (Figure 8.9, p. 288). The resting eye is “set” for distant vision. In gen- The normal eye is able to accommodate eral, light from a distant source (over 20 feet away) properly. However, vision problems occur when a approaches the eye as parallel rays (Figure 8.8a), and the lens does not need to change shape to A: would be bulged and thus relatively thick. focus properly on the retina. However, light from a You would be using your close vision, so your lenses 288 Essentials of Human Anatomy and Physiology Fixation point Figure 8.9 Real image (reversed left to right, and upside down) formed on the retina. Notice that the farther away the object, the smaller its image on Right eye Left eye the retina. lens is too strong (overconverging) or too weak Optic (underconverging) or when there are structural nerve problems of the eyeball (as described in “A Closer Optic Optic Look” on near- and farsightedness on p. 289–290. chiasma tract Visual Fields and Visual Pathways to the Brain ➔ Learning Objective □ Trace the visual pathway to the visual cortex. Axons carrying impulses from the retina are bun- dled together at the posterior aspect of the eyeball and leave the back of the eye as the optic nerve. Optic At the optic chiasma (ki-as¿mah; chiasm = cross) radiation the fibers from the medial side of each eye cross Thalamus Occipital lobe over to the opposite side of the brain. The fiber (visual cortex) tracts that result are the optic tracts. Each optic Figure 8.10 Visual fields of the eyes and visual tract contains fibers from the lateral side of the pathway to the brain (inferior view). Notice that eye on the same side and the medial side of the the visual fields overlap considerably (area of binocular opposite eye. The optic tract fibers synapse with vision). Notice also the retinal sites at which a real image neurons in the thalamus, whose axons form the would be focused when both eyes are fixed on a close, optic radiation, which runs to the occipital lobe pointlike object. of the brain. There they synapse with the cortical cells, and visual interpretation, or seeing, occurs. (Figure 8.10 shows the visual pathway from the Homeostatic Imbalance 8.7 eye to the brain). Hemianopia (hem–e-ah-no¿pe-ah) is the loss of Each side of the brain receives visual input the same side of the visual field of both eyes, from both eyes—from the lateral field of the eye which results from damage to the visual cortex on on its own side and from the medial field of the one side only (as occurs in some strokes). Thus, other eye. Also notice that each eye “sees” a the person would not be able to see things past slightly different view but that their visual fields the middle of the visual field on either the right or overlap quite a bit. As a result of these two phe- left side, depending on the site of the stroke. Such nomena, humans have binocular vision. Binocular individuals should be carefully attended and vision, literally “two-eyed vision,” provides for warned of objects in the nonfunctional side of the depth perception, also called “three-dimensional” visual field. Their food and personal objects should vision, as our visual cortex fuses the two slightly always be placed on their functional side, or they different images delivered by the two eyes into might miss them. ____________________________ ✚ one “picture.”  A Bringing Things into Focus CLOSER LOOK T he eye that focuses images cor- rectly on the retina is said to have emmetropia (em–ĕ-tro¿pe-ah), liter- blurry to myopic people. Nearby objects are in focus, however, because the lens “accommodates” (bulges) to Farsightedness, or hyperopia (hi–per-o¿pe-ah; “far vision”), occurs when the parallel light rays from dis- ally, “harmonious vision.” Part (a) focus the image properly on the ret- tant objects are focused behind the of the figure shows an emmetro- ina. Myopia results from an eyeball retina—at least in the resting eye, in pic eye. that is too long, a lens that is too which the lens is flat and the ciliary Nearsightedness, or myopia strong, or a cornea that is too curved. muscle is relaxed; see part (c) in the (mi–o¿pe-ah; “short vision”), occurs Correction requires concave corrective figure. Hyperopia usually results from when the parallel light rays from dis- lenses that diverge the light rays an eyeball that is too short or from a tant objects fail to reach the retina before they enter the eye, so that they “lazy” lens. People with hyperopia and instead are focused in front of it; converge farther back on the retinal see distant objects clearly because see part (b) in the figure. Another way surface. In other words, nearsighted their ciliary muscles contract continu- to think of this is the focal point of a people see near objects clearly and ously to increase the light-bending myopic eye falls short of the retina. need corrective lenses to focus on dis- power of the lens, which moves the Therefore, distant objects appear tant objects. focal point forward onto the retina. Focal Correction plane None required Concave lens (a) Emmetropic eye (b) Myopic eye (nearsighted) Convex lens (c) Hyperopic eye (farsighted) ➤ 289 A Closer look Bringing Things into Focus (continued) However, the diverging rays from Correction of hyperopia requires con- astigmatism (ah-stig¿mah-tizm). In nearby objects are focused so far vex corrective lenses that converge this condition, blurry images occur behind the retina that even at full the light rays before they enter the because points of light are focused “bulge,” the lens cannot focus the eye. Thus, farsighted people can see not as points on the retina but as image on the retina. Therefore, faraway objects clearly and require lines (astigma = not a point). Special nearby objects appear blurry, and corrective lenses to focus on nearby cylindrically ground lenses or hyperopic individuals are subject to objects. contacts are used to correct eyestrain as their endlessly contract- Unequal curvatures in different astigmatism. ing ciliary muscles tire from overwork. parts of the cornea or lens cause Eye Reflexes often result in what is commonly called eyestrain. When you read for an extended time, look up ➔ learning objective from time to time and stare into the distance. This □ Discuss the importance of the convergence and pupillary reflexes. temporarily relaxes all the eye muscles. Both the internal and external (extrinsic) eye Did You Get It? muscles are necessary for proper eye function. 8. What are the refractory media of the eye? The autonomic nervous system controls the inter- 9. What name is given to the ability of the eye to focus nal muscles. As mentioned earlier, these muscles on close objects? 10. What is the difference between the optic tract and include those of the ciliary body, which alters lens the optic nerve? curvature, and the radial and circular muscles of 11. In what way does the photopupillary reflex protect the iris, which control pupil size. The external mus- the eyes? cles are the rectus and oblique muscles attached 12. How is astigmatism different from myopia and to the eyeball exterior (see Figure 8.3), which con- hyperopia? trol eye movements and make it possible to fol- For answers, see Appendix A low moving objects. They are also responsible for convergence, which is the reflexive movement of the eyes medially when we view close objects. PART II: THE EAR: HEARING When convergence occurs, both eyes are aimed AND BALANCE toward the near object being viewed. The extrinsic At first glance, the machinery for hearing and muscles are controlled by somatic fibers of cranial balance appears crude. Fluids must be stirred to nerves III, IV, and VI (see Figure 8.3). stimulate the receptors of the ear: sound vibra- When the eyes are suddenly exposed to bright tions move fluid to stimulate hearing receptors, light, the pupils immediately constrict; this is the whereas gross movements of the head disturb flu- photopupillary reflex. This protective reflex pre- ids surrounding the balance organs. Receptors that vents excessively bright light from damaging the respond to such physical forces are called mecha- delicate photoreceptors. The pupils also constrict noreceptors (mek–ah-no-re-sep¿terz). reflexively when we view close objects; this Our hearing apparatus allows us to hear an accommodation pupillary reflex provides more extraordinary range of sound, and our highly sen- acute vision. sitive equilibrium receptors keep our nervous sys- Reading requires almost continuous work by tem continually up to date on the position and both sets of muscles. The muscles of the ciliary movements of the head. Without this information, body bring about the lens bulge, and the circular it would be difficult if not impossible to maintain (or constrictor) muscles of the iris produce the our balance or to know which way is up. Although accommodation pupillary reflex. In addition, the these two sense organs are housed together in the extrinsic muscles must converge the eyes as well ear, their receptors respond to different stimuli and as move them to follow the printed lines. This is are activated independently of one another. why long periods of reading tire the eyes and 290  Chapter 8: Special Senses 291 Anatomy of the Ear structure surrounding the auditory canal open- ing. In many animals, the auricle collects and ➔ Learning Objective directs sound waves into the auditory canal, but in □ Identify the structures of the external, middle, and humans this function is largely lost. internal ear, and list the functions of each. The external acoustic meatus (or auditory canal) is a short, narrow chamber (about 1 inch Anatomically, the ear is divided into three major long by ¼ inch wide) carved into the temporal areas: the external, or outer, ear; the middle ear; bone of the skull. In its skin-lined walls are the and the internal, or inner, ear (Figure 8.11). The ceruminous (sĕ-roo¿mı̆-nus) glands, which external and middle ear structures are involved secrete waxy yellow cerumen, or earwax, which with hearing only. The internal ear functions in provides a sticky trap for foreign bodies and both equilibrium and hearing. repels insects. External (Outer) Ear Sound waves entering the auditory canal even- tually hit the tympanic (tim-pan¿ik; tympanum = The external ear, or outer ear, is composed drum) membrane, or eardrum, and cause it to of the auricle and the external acoustic meatus. vibrate. The canal ends at the eardrum, which The auricle (aw¿ri-kul), or pinna (pin¿nah), is separates the external from the middle ear. what most people call the “ear”—the shell-shaped 8 External (outer) ear Middle ear Internal (inner) ear Vestibulocochlear nerve Auricle (pinna) Semicircular canals Oval window Cochlea Vestibule Round window Pharyngotympanic (auditory) tube Tympanic membrane (eardrum) Hammer Anvil Stirrup (malleus) (incus) (stapes) External acoustic meatus Auditory ossicles (auditory canal) Figure 8.11 Anatomy of the ear. Note that the inner ear structures (vestibule, cochlea, semicircular canals) represent a cast of the cavity formed by the bony labyrinth. 292 Essentials of Human Anatomy and Physiology Middle Ear (sta¿pˉez). Like dominoes falling, when the eardrum The middle ear cavity, or tympanic cavity, is a moves, it moves the hammer and transfers the small, air-filled, mucosa-lined cavity within the tem- vibration to the anvil. The anvil, in turn, passes the poral bone. It is flanked laterally by the eardrum vibration on to the stirrup, which presses on the and medially by a bony wall with two openings, oval window of the inner ear. The movement at the the oval window and the inferior, membrane- oval window sets the fluids of the inner ear into covered round window. The pharyngotympanic motion, eventually exciting the hearing receptors. (think throat-eardrum: pharynx-tympanic) tube, or auditory tube, runs obliquely downward to Internal (Inner) Ear link the middle ear cavity with the throat, and the The internal ear is a maze of bony chambers mucosae lining the two regions are continuous. called the bony labyrinth, or osseous labyrinth Normally, the pharyngotympanic tube is flattened (lab¿ı̆-rinth; “maze”), located deep within the and closed, but swallowing or yawning can open temporal bone behind the eye socket. The three it briefly to equalize the pressure in the middle ear subdivisions of the bony labyrinth are the spiraling, cavity with the external, or atmospheric, pressure. pea-sized cochlea (kok¿le-ah, “snail”), the vestibule This is an important function because the eardrum (ves¿ti-bˉul), and the semicircular canals. The ves- does not vibrate freely unless the pressure on both tibule is situated between the semicircular canals of its surfaces is the same. When the pressures are and the cochlea. The views of the bony labyrinth unequal, the eardrum bulges inward or outward, typically seen in textbooks, including this one, are causing hearing difficulty (voices may sound far somewhat misleading because we are really talking away) and sometimes earaches. The ear-popping about a cavity. Figure 8.11 can be compared to a sensation of the pressures equalizing is familiar to cast of the bony labyrinth—that is, a labyrinth that anyone who has flown in an airplane. was filled with plaster of paris and then had the bony walls removed after the plaster hardened. The shape of the plaster then reveals the shape of Homeostatic Imbalance 8.8 the cavity that worms through the temporal bone. Inflammation of the middle ear, otitis media The bony labyrinth is filled with a plasmalike (o-ti¿tis me¿de-ah), is a fairly common result of a fluid called perilymph (per¿ı̆-limf). Suspended in sore throat, especially in children, whose pharyn- the perilymph is a membranous labyrinth, a sys- gotympanic tubes run more horizontally. In otitis tem of membrane sacs that more or less follows media, the eardrum bulges and often becomes the shape of the bony labyrinth. The membranous inflamed. When large amounts of fluid or pus labyrinth itself contains a thicker fluid called endo- accumulate in the cavity, an emergency myringot- lymph (en¿do-limf). omy (lancing of the eardrum) may be required to relieve the pressure. A tiny tube is implanted in Did You Get It? the eardrum that allows pus to drain into the 13. Which region(s) of the ear (external, middle, or internal) serve hearing only? external ear canal. The tube usually falls out by 14. Which structures of the ear transmit sound vibrations itself within the year. ________________________ ✚ from the eardrum to the oval window? The more horizontal course of the pharyngo- For answers, see Appendix A. tympanic tube in infants also explains why it is never a good idea to “prop” a bottle or feed them when they are lying flat (a condition that favors Equilibrium the entry of the food into that tube). ➔ Learning Objectives The tympanic cavity is spanned by the three □ Distinguish between static and dynamic smallest bones in the body, the ossicles (os¿sı̆- equilibrium. kulz), which transmit the vibratory motion of the □ Describe how the equilibrium organs help maintain eardrum to the fluids of the inner ear (see balance. Figure 8.11). These bones, named for their shape, are the hammer, or malleus (mă¿le-us); the anvil, The equilibrium sense is not easy to describe or incus (in¿kus); and the stirrup, or stapes because it does not “see,” “hear,” or “feel.” What  Chapter 8: Special Senses 293 it does is respond (frequently without our aware- ness) to various head movements. The equilibrium receptors of the inner ear, collectively called the vestibular apparatus, can be divided into two Membranes in vestibule branches—one branch is responsible for moni- toring static equilibrium, and the other monitors dynamic equilibrium. Static Equilibrium Within the membrane sacs of the vestibule are receptors called maculae (mak¿u-le; “spots”) that Otoliths are essential to our sense of static equilibrium (Figure 8.12). The maculae report on changes in Otolithic the position of the head in space with respect to membrane the pull of gravity when the body is not moving Hair tuft (static = at rest). Because they provide information on which way is up or down, they help us keep Hair cell our head erect. The maculae are extremely impor- Supporting cell tant to divers swimming in the dark depths (where 8 most other orienting cues are absent), enabling Nerve fibers of them to tell which way is up (to the surface). Each vestibular division (a) macula is a patch of receptor (hair) cells with their of cranial nerve VIII “hairs” embedded in the otolithic membrane, a gelatinous mass studded with otoliths (o¿tŏ-liths), Force of Otolithic Otoliths gravity tiny stones made of calcium salts. As the head membrane moves, the otoliths roll in response to changes in Hair cell the pull of gravity. This movement creates a pull on the gel, which in turn bends the hairs of the hair cells. Bending activates the hair cells, which send impulses along the vestibular nerve (a division of cranial nerve VIII) to the cerebellum of the brain, informing it of the position of the head in space. Dynamic Equilibrium The dynamic equilibrium receptors, found in the semicircular canals, respond to angular or rotational movements of the head rather than to straight-line movements. When you twirl on the dance floor or suffer through a spinning carnival ride, these receptors are working overtime. The semicircular canals (each about ½ inch, or 1.3 cm, around) are oriented in the three planes of space. Head upright Head tilted Thus, regardless of which plane you move in, (b) there will be receptors to detect the movement. Within the ampullae, swollen regions at the Figure 8.12 Structure and function of maculae base of each membranous semicircular canal (static equilibrium receptors). (a) Diagrammatic view of part of a macula. (b) When the head is tipped, (Figure 8.13a, p. 294), are multiple receptor the otoliths in the gelatinous otolithic membrane move regions, each called a crista ampullaris in the direction of gravitational pull, stimulating the (kris¿tah am–pu-lar¿is), or simply crista, which con- maculae. This creates a pull on the hair cells, causing sists of a tuft of hair cells covered with a gelatinous them to bend. cap called the cupula (ku¿pu-lah) (Figure 8.13b). 294 Essentials of Human Anatomy and Physiology Semicircular Ampulla canals Endolymph Ampulla Vestibular nerve Vestibule Cupula of crista ampullaris Flow of endolymph (a) (b) Cupula Figure 8.13 Structure and function of the crista Nerve ampullaris (dynamic equilibrium receptor region). fibers (a) Arranged in the three spatial planes, the semicircular ducts Direction of body in the semicircular canals each have a swelling called an movement ampulla at their base. (b) Each ampulla contains a crista ampullaris, a receptor that is a cluster of hair cells that project (c) into a gelatinous cap called the cupula. (c) When head position changes during rotation or in an angular direction, inertia causes the endolymph in the semicircular ducts to lag behind; and as the cupula moves, it drags across the endolymph, which bends the hair cells in the opposite direction. The bending results in increased impulse transmission in the sensory neurons. This mechanism adjusts quickly if the angular motion (or rotation) continues at a constant speed. When your head moves in an arclike or angular Although the receptors of the semicircular direction, the endolymph in the canal lags behind canals and vestibule are responsible for dynamic the movement (Figure 8.13c). Then, as the cupula and static equilibrium, respectively, they usually drags against the stationary endolymph, the cupula act together. Besides these equilibrium senses, bends—like a diver’s fins in water—with the sight and the proprioceptors of the muscles and body’s motion. This stimulates the hair cells, and tendons are also important in providing the cere- impulses are transmitted up the vestibular nerve to bellum with information used to control balance. the cerebellum. Bending the cupula in the oppo- site direction reduces impulse generation. When Did You Get It? you are moving at a constant rate, the receptors 15. What sense do the vestibule and semicircular canals gradually stop sending impulses, and you no lon- serve? ger have the sensation of motion until your speed 16. Describe the different receptors for static and dynamic equilibrium and their locations. or direction of movement changes. 17. What are otoliths, and what is their role in equilibrium? For answers, see Appendix A.  FOCUS Physical Therapy Assistant ON C AREERS Patients trying to regain loose electric cords that the patient mobility rely on physical could trip on. Finally, she leaves therapy assistants. instructions with the patient to exer- cise on his or her own. A s the population ages, a growing number of people find them- selves needing in-home medical care Anatomy is an important part of physical therapy work, Burgess says. “Working with various deviations of as they recover from injuries or surgi- movement, you need to know what cal procedures. Many of these bones and muscles are involved so patients rely on physical therapy assis- that you know which bones and mus- tants like Leslie Burgess. cles to strengthen and show patients Burgess works for Amedisys Home how to regain their mobility.” Health Care, and 90 to 95 percent of In some cases, part of her job is to Physical therapy assistants work in her patients are senior citizens. Once help her patients and their families hospitals, nursing homes, and clin- a doctor prescribes physical therapy, recognize that they will not be ics—anywhere physical therapists are a licensed physical therapist visits the exactly the way they were before, found. They usually work directly with patient and writes a treatment plan. particularly if they have suffered a patients, putting them through exer- Based on the nature of the problem, stroke or other severe injury. The fact cises under the supervision of a physi- this regimen may incorporate that patients may also be coping with cal therapist. Not all patients are strength, movement, and/or balance hearing or vision loss complicates geriatric—some are recovering from training, with the goal of improving their therapy. serious injuries or have conditions mobility, reducing pain, and/or help- “As we start to age, we begin to such as cerebral palsy. ing the patient function with a dis- lose our independence,” she says. “So Many states require that physical ability. The therapist also sets goals: what can we do to change our life- therapy assistants complete an associ- for example, the patient will be able style so that we can still be as inde- ate’s degree and pass a board exam, to walk 300 feet with a cane after 6 pendent as possible?” in addition to completing continuing weeks. Burgess’s job is to help the education. patients carry out these treatment For more information, contact: plans, visiting the patient two or three times a week, for 6 to 8 weeks The fact that American Physical Therapy Association or more, depending on the patient’s progress. In some cases, she will use patients may also 1111 N. Fairfax St. Alexandria, VA 22314-1488 electrical stimulation or ultrasound to stimulate nerves or muscles. If the be coping with (800) 999-2782 http://www.apta.org patient has a new piece of equip- ment, such as a cane or a walker, hearing or vision For additional information on this career and others, click the Focus on she helps him or her learn to use it. She reviews any prescribed medica- loss complicates Careers link at. tion to make sure the patient is tak- ing it and discusses safety concerns their therapy. with the patient and family, such as 295 296 Essentials of Human Anatomy and Physiology Temporal Perilymph in scala vestibuli bone Vestibular Spiral Vestibular Hair (receptor) Tectorial membrane organ of membrane cells of spiral membrane Corti organ of Corti Afferent fibers of the cochlear nerve Temporal bone Cochlear Fibers of duct (contains Perilymph in Basilar Supporting the cochlear endolymph) scala tympani membrane cells nerve (a) (b) Figure 8.14 Anatomy of the Corti in the cochlear duct. The (b) Detailed structure of the spiral cochlea. (a) A cross-sectional view cavities of the bony labyrinth contain organ of Corti. The receptor cells of one turn of the cochlea, showing perilymph. The cochlear duct (hair cells) rest on the basilar the position of the spiral organ of contains endolymph. membrane. Hearing basilar membrane in the spiral organ of Corti, are stimulated by the vibrating movement of the ➔ Learning Objectives basilar membrane against the gel-like tectorial □ Explain the function of the spiral organ of Corti in (tek-to¿re-al) membrane that lies over them. hearing. The “hairs” of the receptor cells are embedded □ Define sensorineural deafness and conductive in the stationary tectorial membrane such that deafness, and list possible causes of each. when the basilar membrane vibrates against it, □ Explain how a person is able to localize the source the “hairs” bend (see Figure 8.14b). The length of of a sound. the fibers spanning the basilar membrane “tunes” Within the cochlear duct, the endolymph-con- specific regions to vibrate at specific frequen- taining membranous labyrinth of the cochlea cies. In general, high-pitched sounds disturb the is the spiral organ of Corti (kor¿te), which shorter, stiffer fibers of the basilar membrane and contains the hearing receptors, or hair cells stimulate receptor cells close to the oval window, (Figure 8.14a). The chambers (scalae) above and whereas low-pitched sounds affect longer, more below the cochlear duct contain perilymph. Sound floppy fibers and activate specific hair cells fur- waves that reach the cochlea through vibrations ther along the cochlea (Figure 8.16, p. 298). of the eardrum, ossicles, and oval window set Once stimulated, the hair cells transmit the cochlear fluids into motion (Figure 8.15). As impulses along the cochlear nerve (a division of the sound waves are transmitted by the ossicles cranial nerve VIII—the vestibulocochlear nerve) to from the eardrum to the oval window, their force the auditory cortex in the temporal lobe, where (amplitude) is increased by the lever activity of interpretation of the sound, or hearing, occurs. the ossicles. In this way, nearly the total force Because sound usually reaches the two ears at dif- exerted on the much larger eardrum reaches the ferent times, we could say that we hear “in stereo.” tiny oval window, which in turn sets the fluids Functionally, this helps us to determine where of the inner ear into motion, and these pressure sounds are coming from in our environment. waves set up vibrations in the basilar mem- When the same sounds, or tones, keep reach- brane. The receptor cells, positioned on the ing the ears, the auditory receptors tend to adapt,  Chapter 8: Special Senses 297 EXTERNAL EAR MIDDLE EAR INTERNAL EAR Auditory Ear- Hammer, Oval Fluids in cochlear canals Pinna canal drum anvil, stirrup window Upper and middle lower Pressure Spiral organ Time One Amplitude fi of Corti vibration in middle ear stimulated Figure 8.15 Route of sound waves through the ear. To excite the hair cells in the spiral organ of Corti in the inner ear, sound wave vibrations must pass through air, membranes, bone, and fluid. 8 or stop responding, to those sounds, and we are Sensorineural deafness occurs when there no longer aware of them. This is why the drone of is degeneration or damage to the receptor cells in a continuously running motor does not demand the spiral organ of Corti, to the cochlear nerve, or our attention after the first few seconds. However, to neurons of the auditory cortex. This often hearing is the last sense to leave our awareness results from extended listening to excessively loud when we fall asleep or receive anesthesia and is sounds. Thus, whereas conduction deafness results the first to return as we awaken. from mechanical factors, sensorineural deafness is a problem with nervous system structures. A person who has a hearing loss due to con- Hearing and Equilibrium duction deafness will still be able to hear by bone Deficits conduction, even though his or her ability to hear air-conducted sounds (the normal conduction Homeostatic Imbalance 8.9 route) is decreased or lost. In contrast, individuals with sensorineural deafness cannot hear better by Children with ear problems or hearing deficits either conduction route. Hearing aids, which use often pull on their ears or fail to respond when skull bones to conduct sound vibrations to the spoken to. Under such conditions, tuning fork or inner ear, are generally very successful in helping audiometry testing is done to try to diagnose the people with conduction deafness to hear. They are problem. Deafness is defined as hearing loss of not helpful for sensorineural deafness. any degree—from a slight loss to a total inability to Equilibrium problems are usually obvious. hear sound. Generally speaking, there are two Nausea, dizziness, and problems in maintaining kinds of deafness, conduction and sensorineural. balance are common symptoms, particularly when Temporary or permanent conduction deafness impulses from the vestibular apparatus “disagree” results when something interferes with the con- with what we see (visual input). There also may duction of sound vibrations to the fluids of the be strange (jerky or rolling) eye movements. inner ear. Something as simple as a buildup of ear- A serious pathology of the inner ear is wax may be the cause. Other causes of conduction Ménière’s (mān–e-airz¿) syndrome. The exact deafness include fusion of the ossicles (a problem cause of this condition is not fully known, but sus- called otosclerosis [o–to-sklĕ-ro¿sis]), a ruptured pected causes are arteriosclerosis, degeneration of eardrum, and otitis media (inflammation of the cranial nerve VIII, and increased pressure of the middle ear). inner ear fluids. In Ménière’s syndrome, progressive 298 Essentials of Human Anatomy and Physiology Stapes Fibers of Figure 8.16 Activation of the cochlear hair cells. Scala sensory (a) The cochlea is drawn as though it were uncoiled to vestibuli neurons make the events of sound transmission occurring Oval Perilymph there easier to follow. Sound waves of low frequency, window below the level of hearing, travel entirely around the cochlear duct without exciting hair cells. But sounds of higher frequency penetrate through the cochlear duct and basilar membrane to reach the scala tympani. This causes the basilar membrane to vibrate maximally in certain areas in response to certain frequencies of sound, stimulating particular hair cells and sensory neurons. The differential stimulation of hair cells is Round Scala Basilar Cochlear perceived in the brain as sound of a certain pitch. window tympani membrane duct (b) The length and stiffness of the fibers spanning the (a) basilar membrane tune specific regions to vibrate at specific frequencies. The higher notes—20,000 Hertz (Hz)—are detected by shorter, stiffer hair cells along Fibers of basilar membrane the base of the basilar membrane. Apex Base (short, (long, stiff fibers) floppy fibers) 20,000 2,000 200 20 (High notes) (Low notes) Frequency (Hz) (b) deafness occurs. Affected individuals become nauseated and often have howling or ringing PART III: CHEMICAL SENSES: sounds in their ears and vertigo (a sensation of SMELL AND TASTE spinning) that is so severe that they cannot stand ➔ Learning Objectives up without extreme discomfort. Anti–motion sick- □ Describe the location, structure, and function of the ness drugs are often prescribed to decrease the olfactory and taste receptors. discomfort. _________________________________ ✚ □ Name the five basic taste sensations, and list factors that modify the sense of taste. Did You Get It? 18. From the air outside the body, through what The receptors for taste and olfaction are classified substances do sound waves travel to excite the as chemoreceptors (ke–mo-re-sep¿terz) because receptor cells of the cochlea? they respond to chemicals in solution. Five types 19. Which nerve transmits impulses from the spiral organ of taste receptors have been identified, but the of Corti to the brain? olfactory receptors (for smell) are believed to be 20. Do high-pitched sounds peak close to or far from the oval window? sensitive to a much wider range of chemicals. The 21. How do sensorineural deafness and conduction receptors for smell and taste complement each deafness differ from each other? other and respond to many of the same stimuli. For answers, see Appendix A. Olfactory Receptors and the Sense of Smell Even though our sense of smell is far less acute than that of many other animals, the human nose is still no slouch in picking up small differences  Chapter 8: Special Senses 299 Q: How does sniffing help to identify scents? Olfactory bulb Cribriform plate of ethmoid bone Olfactory tract Olfactory filaments of the olfactory nerve Supporting cell Olfactory Olfactory receptor mucosa cell Olfactory hairs Mucus layer (cilia) (a) Route of inhaled air containing odor molecules 8