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CHAPTER
15
The Special Senses
Objectives
The Eye and Vision
2. Outline the causes and consequences of cataracts and glaucoma.
4. Outline the causes and consequences of astigmatism, myopia, hyperopia, and presbyopia.
5. Describe the events that convert light into a neural signal.
The Chemical Senses: Smell and Taste
9. Describe the location, structure, and afferent pathways of smell and taste receptors, and
explain how these receptors are activated.
The Ear: Hearing and Balance
10. Describe the structure and general function of the outer, middle, and internal ears.
11. Describe the sound conduction pathway to the fluids of the internal ear, and follow the
auditory pathway from the spiral organ to the temporal cortex.
Developmental Aspects of the Special Senses
15. List changes that occur in the special sense organs with aging.
Suggested Lecture Outline
I. The Eye and Vision (pp. 545–565; Figs. 15.1–15.19)
A. Vision is our dominant sense; 70% of our body’s sensory receptors are found in the eye
(p. 545).
B. Accessory Structures of the Eye (pp. 545–549; Figs. 15.1–15.3)
1. Eyebrows are short, coarse hairs overlying the supraorbital margins of the eye that
shade the eyes and keep perspiration out.
3. The conjunctiva is a transparent mucous membrane that produces a lubricating mucus
that prevents the eye from drying out.
a. The palpebral conjunctiva lines the eyelids, and the bulbar conjunctiva covers the
anterior surface of the eyeball.
5. The movement of each eyeball is controlled by six extrinsic eye muscles that are
innervated by the abducens and trochlear nerves.
a. Four rectus muscles superior, inferior, lateral, and medial, originate at the back of
the orbit, and run straight to their insertion on the eyeball.
C. Structure of the Eyeball (pp. 549–553; Figs. 15.4–15.9)
1. Three layers form the wall of the eyeball:
a. The fibrous tunic is the outermost coat of the eye and is made of a dense avascular
connective tissue with two regions: the sclera and the cornea.
b. The cornea is avascular, but rich in nerve endings that detect pain.
2. Internal Chambers and Fluids
a. The posterior segment (cavity) is filled with a clear gel called vitreous humor that
3. The lens is an avascular, biconvex, transparent, flexible structure that can change
shape to allow precise focusing of light on the retina.
D. Optics and the Eye (pp. 553–556; Figs. 15.10–15.14)
1. Overview: Light and Optics
a. Electromagnetic radiation includes all energy waves from long waves to short
2. Focusing of Light on the Retina
a. Light is bent three times: as it enters the cornea, upon entering the lens, and upon
leaving the lens.
b. The far point of vision is that distance beyond which no change in lens shape
(accommodation) is required and, in a normal eye, is at a distance of about
6 meters, or 20 feet.
f. Myopia, or nearsightedness, occurs when objects focus in front of the retina and
results in seeing close objects without a problem but distant objects are blurred.
g. Hyperopia, or farsightedness, occurs when objects are focused behind the retina and
results in seeing distant objects clearly but close objects are blurred.
h. Astigmatism results from an uneven curvature of the cornea or lens, which
produces blurred images.
E. Photoreceptors and Phototransduction (pp. 556–563; Figs. 15.15–15.18; Table 15.1)
b. Photoreceptors contain visual pigments that change as they absorb light.
c. Rods are highly sensitive and are best suited to night vision.
d. Cones are less sensitive to light and are best adapted to bright light and color vision.
3. Exposure of the photoreceptors to light causes pigment breakdown, which hyperpola-
rizes the receptors inhibiting the release of neurotransmitter conveying the information.
4. Light adaptation occurs when we move from darkness into bright light; retinal
sensitivity decreases dramatically and the retinal neurons switch from the rod to the
F. Visual Pathways and Processing (pp. 563–565; Fig. 15.19)
1. The Visual Pathway to the Brain
a. The retinal ganglion cells merge in the back of the eyeball to become the optic
nerve, which crosses at the optic chiasma to become the optic tracts.
2. Depth perception is created when the visual fields of each eye, which differ slightly,
overlap.
II. The Chemical Senses: Smell and Taste (pp. 565–570; Figs. 15.20–15.23)
A. The receptors for smell and taste are chemoreceptors, which means that they respond to
chemicals in a solution (p. 565).
B. Olfactory Epithelium and the Sense of Smell (pp. 565–567; Figs. 15.20–15.21)
1. The olfactory epithelium is the organ of smell located in the roof of the nasal cavity.
C. Taste Buds and the Sense of Taste (pp. 568–570; Figs. 15.22–15.23)
1. Taste buds, the sensory receptor organs for taste, are located in the oral cavity, with
the majority located within the papillae of the tongue.
2. Taste sensations can be grouped into one of five basic qualities: sweet, sour, bitter,
salty, and umami.
3. Physiology of Taste
a. For a chemical to be tasted it must be dissolved in saliva, move into the taste
4. Afferent fibers carrying taste information from the tongue are found primarily in the
facial nerve and glossopharyngeal cranial nerves.
5. Taste impulses from the few taste buds found on the epiglottis and the lower pharynx
are conveyed via the vagus nerve.
6. Taste is strongly influenced by smell and stimulation of thermoreceptors, mechanore-
ceptors, and nociceptors.
D. Homeostatic Imbalances of the Chemical Senses (p. 570)
III. The Ear: Hearing and Balance (pp. 570–584; Figs. 15.24–15.36; Table 15.2)
A. Structure of the Ear (pp. 570–575; Figs. 15.24–15.27; Table 15.2)
1. The external ear consists of the auricle (pinna) and the external acoustic meatus, which
is lined with skin bearing hairs, sebaceous glands, and ceruminous glands.
4. The internal ear has two major divisions: the bony labyrinth and the membranous
labyrinth.
a. The vestibule is the central cavity of the bony labyrinth and is filled with
perilymph.
d. The spiral, snail-shaped cochlea extends from the anterior part of the vestibule and
contains the cochlear duct, which houses the spiral organ, the receptor organ for
hearing.
B. Physiology of Hearing (pp. 575–579; Figs. 15.28–15.32)
1. Overview: Properties of Sound
a. Sound is a pressure disturbance produced by a vibrating object and propagated by
the molecules of the medium.
2. Airborne sound entering the external acoustic meatus strikes the tympanic membrane
and sets it vibrating.
3. Vibrations are transmitted along the auditory ossicles to the oval window on the
vestibule, producing a pressure wave within the perilymph.
4. The resonance of the basilar membrane processes sound signals mechanically before
they ever reach the receptors.
C. Equilibrium and Orientation (pp. 580–584; Figs. 15.33–15.36)
1. The equilibrium sense responds to various head movements and depends on input
from the internal ear, vision, and information from stretch receptors of muscles and
tendons.
D. Homeostatic Imbalances of Hearing and Equilibrium (p. 584)
1. Deafness is any hearing loss, no matter how slight.
IV. Developmental Aspects of the Special Senses (pp. 584–586)
A. Taste and Smell (pp. 584–585)
1. Smell and taste are highly developed at birth.
2. Women generally have a more acute sense of taste and smell than men.
B. Vision (p. 585)
1. By the fourth week of development, eyes begin to develop and—even before
photoreceptors develop—CNS connections are made.
C. Hearing and Balance (pp. 585–586)
1. The ear begins to develop in the embryo at 3 weeks.
2. Newborn infants can hear and respond reflexively. By the fourth month of life,
hearing includes recognition.
Cross References
Additional information on topics covered in Chapter 15 can be found in the chapters listed below.
1. Chapter 4: Epithelia; exocrine glands; connective tissues
2. Chapter 5: Sebaceous and sudoriferous glands
3. Chapter 8: Synovial joints
4. Chapter 10: Skeletal muscle naming
Lecture Hints
1. Emphasize that each taste sensation is not localized to a specific area, but that there is
significant overlap of the different sensation areas. Students often assume that a
particular point on the tongue responds to a single type of substance.
4. Emphasize that olfactory sensory neurons are the only renewable neurons in the body and
are therefore the one exception to the rule that neurons do not replicate.
5. There is often confusion in the terminology of the chambers of the eye. Point out that the
anterior segment is divided into anterior and posterior chambers by the iris.
Activities/Demonstrations
1. Audiovisual materials are listed in the Multimedia in the Classroom and Lab section of
this Instructor Guide (p. 387).
4. Obtain a 3-D model of an eye and of an ear to illustrate the various anatomical parts of
each.
5. Dissect a fresh (or preserved, if fresh is not available) beef eye to illustrate the
anatomical structure and nature of the tissues and fluids.
Critical Thinking/Discussion Topics
1. Most people with sinus infections can’t smell or taste well. Why?
2. Wine tasting can be a real art. Why are some people more adept at tasting than others?
What effect does smoking, alcohol, and/or sweets have on wine tasting? Why is it useful
to swirl a glass of wine and then sniff it?
6. How is it possible that the cornea is transparent and the sclera is opaque when they are
both constructed of the same material and continuous with each other?
7. Examine the consequences to the anatomy of the eye and vision if aqueous humor
drainage exceeded production.
Library Research Topics
1. How successful are cochlear implants? What surgical techniques are employed?
2. Some permanently deaf individuals have been helped by means of computers and
electrical probes connected to certain areas of the brain. How is this possible and what is
the current research in this area?
List of Figures and Tables
All of the figures in the main text are available in JPEG format, PPT, and labeled & unlabeled
format on the Instructor Resource DVD. All of the figures and tables will also be available in
Transparency Acetate format. For more information, go to www.pearsonhighered.com/educator.
Figure 15.1 The eye and accessory structures.
Figure 15.2 The lacrimal apparatus.
Figure 15.8 Circulation of aqueous humor.
Figure 15.9 Photograph of a cataract.
Figure 15.10 The electromagnetic spectrum and photoreceptor sensitivities.
Figure 15.11 Refraction.
Figure 15.12 Light is focused by a convex lens.
Figure 15.18 Signal transmission in the retina.
Figure 15.19 Visual pathway to the brain and visual fields, inferior view.
Figure 15.20 Olfactory receptors.
Figure 15.21 Olfactory transduction process.
Figure 15.22 Location and structure of taste buds on the tongue.
Figure 15.23 The gustatory pathway.
Figure 15.24 Structure of the ear.
Figure 15.25 The three auditory ossicles and associated skeletal muscles.
Figure 15.26 Membranous labyrinth of the internal ear.
Figure 15.36 Neural pathways of the balance and orientation system.
Table 15.1 Comparison of Rods and Cones
Table 15.2 Summary of the Internal Ear
Answers to End-of-Chapter Questions
Multiple-Choice and Matching Question answers appear in Appendix H of the main text.
Short Answer Essay Questions
31. Rods are dim-light visual receptors that have limited color acuity, while cones are for
bright-light and high-acuity color vision. (pp. 551, 559)
33. In response to light, retinal changes to the all-trans form; the retinal-opsin combination
34. Each cone responds maximally to one of these colors of light, but there is a great deal of
overlap in their absorption spectra that accounts for the other hues. (p. 554)
36. False. Each olfactory sensory neuron has only one type of receptor protein; however,
37. The five basic tastes are sweet, sour, salty, bitter, and umami. Taste is served by cranial
nerves VII (facial), IX (glossopharyngeal), and X (vagus). (p. 569)
38. With age, the lens enlarges, loses its crystal clarity and becomes discolored, and the
dilator muscles of the iris become less efficient. Atrophy of the spiral organ reduces
hearing acuity, especially for high-pitched sounds. The sense of smell and taste diminish
due to a gradual loss of receptors, thus appetite is diminished. (p. 553)
Critical Thinking and Clinical Application Questions
1. Papilledema, a nipple-like protrusion of the optic disc into the eyeball, is caused by
2. Pathogenic microorganisms spread from the nasopharynx through the pharyngotympanic
tube into the tympanic cavity. They may then spread posteriorly into the mastoid air cells
3. The inflammatory condition Mr. Gaspe is experiencing is conjunctivitis. The foreign
object would likely be moved toward the conjunctival sac near the orifice of the lacrimal
canals. (p. 546)
5. The inability to hear high-pitched sounds is called presbycusis, a type of sensorineural
6. Compression of the optic chiasma may cause blindness because visual impulses will be
blocked from reaching the optic disc. (p. 564)
7. Albinism involves a hereditary inability for melanocytes to synthesize tyrosinase, an
enzyme that is needed for the production of melanin, a light-absorbing pigment that is
8. Jan had tinnitus, a constant ringing or howling in the ear, possibly brought on by the
pressure of the wax in her ear against the tympanic membrane. (p. 585)
Suggested Readings
Barinaga, Marcia. “Smell’s Course Is Predetermined.” Science 294 (5545) (Nov. 2001):
1269–1271.
Berson, David M. “Phototransduction by Retinal Ganglion Cells that Set the Circadian
Clock.” Science 295 (5557) (Feb. 2002): 1070–1073.
Firestein, Stuart. “How the Olfactory System Makes Sense of Scents.” Nature 413 (6852)
(Sept. 2001): 211–218.
Frasnelli, J., B. Schuster, and T. Hummel. “Subjects with Congenital Anosmia Have Larger
Peripheral but Similar Central Trigeminal Responses.” Cerebral Cortex 17 (2) (Feb.
2007): 370–377.
219–225.
Lu, Y., et al. “Retinal Nerve Fiber Layer Structure Abnormalities in Early Alzheimer’s Dis-
ease: Evidence in Optical Coherence Tomography.” Neuroscience Letters 480 (1) (Aug.
2010): 69–72.
Nirenberg, S. M., et al. “Retinal Ganglion Cells Act Largely as Independent Encoders.”
Nature 411 (6838) (June 2001): 698–701.
Oliver, Dominik, et al. “Intracellular Anions as the Voltage Sensor of Prestin, the Outer Hair
Cell Motor Protein.” Science 292 (5525) (June 2001): 2340–2343.
Pichaud, Franck, and Claude Desplan. “A New View of Photoreceptors.” Nature 416 (6877)
(March 2002): 139–140.
