8
CHAPTER SUMMARY
The special senses keep us informed as to what is going on in our external world. Sense receptors are
found in large, complex sensory organs like the eye or in localized clusters of receptors like the taste
buds. This chapter focuses on each of the sensory organs individually, but also seeks to show us that
the way we experience the world is, in fact, a blending of the effects of various stimuli.
The eye is the first of the sense organs to be presented. Anatomy of the eye begins with a
discussion of the eye’s external and accessory structures, followed by presentation of its internal
structures. The retina contains receptor cells, rods, and cones, which respond to light. Rods allow us to
see gray tones and help us see better in dim light, whereas cones enable us to see color. A discussion
of color blindness explains the homeostatic imbalance that results from lack of cones. The refractive
properties of the eye are presented next, followed by a description of the visual pathways to the brain
and an explanation of eye reflexes.
The ear and its role in hearing and balance are discussed next. The anatomy of the ear begins
with the outer ear structures, followed by the middle and inner ear. The mechanism of hearing is
explained in detail, with emphasis on the role of the hair cells found in the organ of Corti. The
mechanisms of equilibrium, the fifth sense, are then presented. Static and dynamic equilibrium are
described and differentiated, followed by a discussion of some of the more common hearing and
equilibrium deficits.
The chemical senses of taste and smell are presented in the final section. Chemoreceptors
involved in both taste and smell respond to chemicals in solution. In the case of smell, olfactory
receptors located in the roof of each nasal cavity are sensitive to a wide range of chemicals, whereas
taste receptors located in the oral cavity are identified as five basic types: sweet, sour, bitter, umami,
and salty. Because taste and smell are linked by their response to chemicals and enzymatic reactions in
solution, students quickly understand the reason why food seems to have no taste when they have a
cold, and why salty food tastes sweet when held in the mouth for 20–30 seconds. This and other
examples of homeostatic imbalance related to the special senses are discussed at the end of the
chapter.
SUGGESTED LECTURE OUTLINE
I.
THE EYE AND VISION (pp. 272–284)
A.
Anatomy of the Eye (pp. 272–280)
1. External and Accessory Structures
a. Extrinsic Eye Muscles
i.
Moving objects
ii.
Convergence
b. Lacrimal Apparatus
i.
s
ii.
Saline solution washes and lubricated eyeball
c. Eyelids and Eyelashes
i.
Ciliated glands—modified sweat glands
ii.
Tarsal glands—oily lubrication
d. Conjunctiva
i.
Lubricating mucous
ii.
Conjunctivitis—pinkeye
2.
Internal Structures: The Eyeball
a.
B.
C.
D.
II.
Tunics of the Eyeball
i.
Sclera—outer
ii.
Choroid—middle
iii.
Retina—innermost
b. Lens
Pathway of Light Through the Eye and Light Refraction (pp. 280–281)
1.
Lens
i.
Refraction
ii.
Convexity increased by ciliary body
iii.
Aqueous humor—anterior to lens
iv.
Vitreous humor—posterior to lens
2.
Errors of refraction (pp. 282–283)
i.
Emmetropia
ii.
Myopia—nearsightedness
iii.
Hyperopia—farsightedness
iv.
Astigmatism
v.
Blind spots
vi.
Ophthalmoscopic examination
vii.
Night blindness (p. 276)
viii.
Color blindness (pp.276 and 278)
Visual Fields and Visual Pathways to the Brain (p. 281)
Eye Reflexes (p. 284)
1.
Photopupillary
2.
Accommodation Pupillary
3.
Convergence
THE EAR: HEARING AND BALANCE (pp. 284–291)
A.
Anatomy of the Ear (pp. 284–286)
1.
Outer (External) Ear
a. Pinna
b. External Auditory Canal
c. Tympanic Membrane
2.
Middle Ear
a. Ossicles
b. Auditory Tube
3.
Inner (Internal) Ear
a. Cochlea
b. Vestibule
c. Semicircular Canals
B.
Mechanisms of Equilibrium (pp. 286–288)
1.
Static Equilibrium
a. Vestibule (maculae)
2.
Dynamic Equilibrium
a. Cristae ampullaris
3.
Vision and proprioception
C.
Mechanisms of Hearing (pp. 288–289)
1.
Organ of Corti hair cells
D.
Hearing and Equilibrium Deficits (pp. 289–291)
1.
Deafness
2.
Conduction deafness
3.
Sensorineural deafness
III.
CHEMICAL SENSES: TASTE AND SMELL (pp. 291–295)
A.
Olfactory Receptors and the Sense of Smell (pp. 291–292)
B.
Taste Buds and the Sense of Taste (pp. 292–293)
1.
Sweet
2.
Salt
3.
Sour
4.
Bitter
5.
Umami
C.
Limbic system
D.
Influences on taste
1.
Smell
2.
Temperature
3.
Texture
IV.
DEVELOPMENTAL ASPECTS OF THE SPECIAL SENSES (pp. 293–295)
A.
Embryonic Development
1. Strabismus—congenital eye disorder
2. Infants have poor visual acuity
i. Lack color vision
ii. Lack depth perception
B. Aging
1. Presbyopia—sensorineural deafness
2. Glaucoma
3. Cataracts
4. Atherosclerosis of eye blood vessels
5. Decrease in olfactory and gustatory receptors