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CHAPTER
15
The Special Senses
Objectives
The Eye and Vision
1. Describe the structure and function of accessory eye structures, eye layers, the lens, and humors of the eye.
2. Outline the causes and consequences of cataracts and glaucoma.
3. Trace the pathway of light through the eye to the retina, and explain how light is focused for distant and close
vision.
4. Outline the causes and consequences of astigmatism, myopia, hyperopia, and presbyopia.
5. Describe the events that convert light into a neural signal.
6. Compare and contrast the roles of rods and cones in vision.
7. Compare and contrast light and dark adaptation.
8. Trace the visual pathway to the visual cortex, and briefly describe the steps in visual processing.
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.
12. Explain how we are able to differentiate pitch and loudness, and localize the source of sounds.
13. List possible causes and symptoms of otitis media, deafness, and Ménière’s syndrome.
14. Explain how the balance organs of the semicircular canals and the vestibule help
maintain equilibrium.
Developmental Aspects of the Special Senses
15. List changes that occur in the special sense organs with aging.
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Chapter 15 – Eye Anatomy
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.
2. Eyelids (palpebrae), eyelashes, and their associated glands help to protect the eye from physical danger as
well as from drying out.
a. Several glands are associated with the eyelid: tarsal glands & ciliary glands that produce oily secretions.
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.
b. The conjunctival sac is the space between the bulbar and palpebral conjunctiva and forms the space in
which medications are dispensed into the eye.
4. The lacrimal apparatus consists of the lacrimal gland, which secretes a dilute saline solution (tears), and
small ducts that drain excess fluid into the nasolacrimal duct.
a. Tears contain mucus, antibodies, and lysozyme to cleanse, moisten, and protect the eyes.
5. The movement of each eyeball is controlled by six extrinsic eye muscles that are innervated by the oculomotor (III), abducens (VI) & trochlear (IV) 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.
b. Two oblique muscles, superior and inferior, run along the side of the eyeball and insert on the eyeball at
an angle.
c. The action of the oblique muscles offsets the action of the superior and inferior rectus, allowing the eyeball to be directly elevated or depressed.
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.
c. The vascular tunic (uvea) is the middle layer and has three regions:
 Choroid that supplies blood to all areas of the eyeball, produce melanocytes, help absorb light and
prevent scattering & reflecting within the eye
 Ciliary body, consisting of smooth muscle and connective tissue that controls lens shape & anchors
the lens in place.
 Iris - “rainbow”, rings of smooth muscle that control dilation of the pupil (central opening that allow
light to enter eye). Muscles: Sphincter pupillae (circular) vs. dilator pupillae (radial). Which is influenced by sympathetic vs. parasympathetic control? Figs. 15.5 Review pigment cells in chapter 5.
d. The inner layer (retina) is made up of two layers:
 Outer pigmented layer absorbs light & prevents scattering; phagocytosis = photoreceptor renewal &
store vitamin A needed by photoreceptors. Composed of simple cuboidal epithelium; melanin – absorbs stray light rays.
 Inner neural layer contains millions of photoreceptors (rods & cones) that transduce (convert) light
energy; action potentials generated. Horizontal cells & amacrine cells – play a role in visual processing.
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o Rods – dim light & peripheral receptors; more numerous & far more sensitive to light than
cones
o Cones – bright light receptors = high resolution color vision.
e. The optic disc is the blind spot & weak spot of the fundus, occurs at the point where the optic nerve exits
the eyeball, and has no photoreceptor cells. Brain uses filling to deal with absence of input.
f. Lateral to the optic disc is the macula lutea (“yellow spot”), containing a central pit, the fovea centralis,
which contains a very high density of cones, allowing precise color vision due to light passing almost directly to photoreceptors rather than layers. Highest visual acuity (resolution).
2. Internal Chambers and Fluids
a. The posterior segment (cavity) is filled with a clear gel called vitreous humor that transmits light, supports the posterior surface of the lens, holds the retina firmly against the pigmented layer, and contributes to intraocular pressure. Contains phagocytic cells. Maintains shape of eyeball & prevent it from
collapsing.
 Vitreous humor forms in embryo and lasts a lifetime. Not replaced.
 Iris divides anterior & posterior chambers
b. The anterior segment (cavity) is filled with aqueous humor that supplies nutrients and oxygen to the lens
and cornea while carrying away wastes.
i. Aqueous humor forms & drains continuously; ciliary capillaries produce & scleral venous sinus drains it
@ corneoscleral junction; supplies nutrients & oxygen to the lens & cornea (some cell of cornea) while
carrying out waste.
ii. Glaucoma – fluid buildup in aqueous humor (blockage) = increased pressure & compression of retina
& optic nerve = halos & blurred vision.
3. The lens is an avascular, biconvex, transparent, flexible structure that can change shape to allow precise focusing of light on the retina. Crystillins form the body of the lens. Becomes thicker with age which impairs
ability to focus.
a. Cataracts – (waterfall) clouding of the lens = frosted glass & distortion. Age-relates most commonly d/t
ex. Diabetes mellitus, smoking, or intense sunlight.
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 waves and includes the visible light that our eyes see as color.
b. Refraction, or bending, of a light ray occurs when it meets the surface of a different medium at an
oblique angle rather than a right angle.
c. A convex lens behind pupil & iris; crystalin proteins (onion) refractory media; avascular. Helps focus images on retina for clear vision.
i. Held in place by a Ciliary zonule (suspensory ligament) attached to the ciliary body
ii. Refracts (bends) light greatly so that it converges at a focal point, forming an image called a real
image, which projects upside down and reversed from left to right on the retina.
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 increases
the refractory power of the lens due to ciliary muscles; controlled by CN III) is required and, in a normal
eye, is at a distance of about 6 meters, or 20 feet. Define refraction, focal point, real image.
c. During distant vision, the ciliary muscles are completely relaxed, causing a maximal flattening of the lens.
d. Focusing for close vision demands that the eye make three adjustments:
 Accommodation of the lens, ciliary muscles contract causing it to thicken and increase light refraction;
define presbyopia. This change in shape of the lens & cornea occurs 6m - 10cm d/t ANS-PS CN III
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 Constriction of the pupils, sphincter pupillae muscles decrease size of pupil which better directs light
to the lens; PS of CNIII prevents divergence of light rays which would = blurred vision
 Convergence of the eyeballs (binocular), allowing the object to remain focused on the foveae. Controlled by somatic fibers of CN III via medial rectus. Closer the object = Greater degree of convergence.
Ex: nose = cross-eyed.
e. The near point of vision occurs at the point of maximal thickening of the lens, and is 10 cm, or 4 inches,
from the eye. Define Emmetropic.
f. Myopia, or nearsightedness, eye ball is too long resulting in objects focused in front of the retina and results in seeing close objects without a problem but distant objects are blurred. Corrected with concave
lens.
g. Hyperopia, or farsightedness, eye ball is too short resulting in objects focused behind the retina and results in seeing distant objects clearly but close objects are blurred. Corrected with convex lens.
h. Astigmatism results from an uneven curvature of the cornea or lens, results in 2 focal points which produces blurred images.
i. Presbyopia – over 50, the lens is non-accommodating
E. Photoreceptors and Phototransduction (pp. 556–563; Figs. 15.15–15.18; Table 15.1)
1. Photoreception is the process by which light energy produces graded receptor potentials.
a. Photoreceptors are modified neurons that have their photoreceptive ends inserted into the pigmented
layer of the retina. Highly vulnerable to damage and immediately begin to degenerate if the retina detaches; destroyed by intense light.
b. Rods are highly sensitive and are best suited for night, noncolor (graytones) vision. Low acuity due to
many rods converge into one ganglion; more numerous than cones (20:1); mostly in peripheral retina.
c. Cones are less sensitive to light and are best adapted to bright light & color vision. High resolution (acuity)
due to one cone per one ganglion; less numerous; mostly in central retina (fovea).
2. The Chemistry of Visual Pigments
a. Within photoreceptors is a light-absorbing molecule, retinal, that combines with opsin proteins to form
one of four types of visual pigments.
b. Cone opsins absorb light within a given range of wavelengths, giving them their names, blue, green, and
red.
c. Colorblindness – Inherited (X-linked); mostly in males; more common red & green; Diagnosed by using
Ishihara Cards.
d. Retinal: related to vitamin A; pigmental cells of retina
3. Exposure of the photoreceptors to light causes pigment breakdown, which hyperpolarizes 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 cone system.
5. Dark adaptation occurs when we go from a well-lit area into a dark one; the cones stop functioning and the
rhodopsin starts to accumulate in the rods, increasing retinal sensitivity.
F. Visual Pathways and Processing (pp. 563–565; Fig. 15.19)
1. The Visual Pathway to the Brain
a. Impulses from ganglion cells are conveyed to optic (II) nerve  optic chiasm (some cross; some do not)
 optic track  thalamus (lateral geniculate)  optic radiations  cerebral cortex 1o visual areas (occipital lobe) = visual perception.
 Review: Superior colliculus – visual tracking (ex: watching a tennis match)
 Watch Video on visual pathway lesions and the effects
2. Depth perception is created when the visual fields of each eye, which differ slightly, overlap.
3. Visual processing occurs when the action of light on photoreceptors hyperpolarizes them, which causes the
bipolar neurons from both the rods and cones to ultimately send signals to their ganglion cells.
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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.
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.
2. The olfactory sensory neurons are bipolar neurons with a thin apical dendrite that terminates in a knob with
several olfactory cilia.
3. To smell a particular odorant, it must be volatile (gaseous state) and it must be dissolved in the fluid coating
the olfactory epithelium that stimulates the olfactory receptors. Sniffing draws odors more superiorly intensifying the sense of smell.
4. In olfactory transduction, an odorant binds to the olfactory receptor, a G protein, and the secondary messenger of cyclic AMP.
5. Axons of the olfactory sensory neurons synapse in the olfactory bulbs, sending impulses down the olfactory
tracts to the thalamus, hypothalamus, amygdaloid body, and other members of the limbic system. Propagate in limbic system and provoke strong emotional responses (memories).
6. Olfactory neurons are unusual because they are one of the few that undergo noticeable turnover throughout adult life. (life span 30-60 days) Replaced by olfactory stem cells
C. Taste Buds and the Sense of Taste (pp. 568–570; Figs. 15.22–15.23)
1. Taste buds – sensory receptor organs for taste located on tongue, soft palate, pharynx & epiglottis. The majority located within the papillae of the tongue.
3 kinds of additional cells:
1. Basal cells – stem cells product supporting cells
2. Supporting cells which develop into gustatory cells
3. Gustatory cells with gustatory hair thru taste pore –detect tastants; 10 day life span.
4. Tongue Papillae – elevations used to increase SA & roughen surface; 4 types.
a. Vallate (Circumvallate) – inverted V-shaped row posterior tongue; 100-300 taste buds each
b. Fungiform – mushroom shaped; scattered; 5 taste buds each.
c. Foliate – lateral margins; most taste buds degenerate in early childhood.
d. Filiform – entire surface of tongue; tactile receptors NO taste buds.
2. 5 Primary taste sensations: Cravings:
 Sweet – Carbohydrates
 Sour – Vitamin C
 Salty – Minerals
 Bitter – Defense
 Umami (Glutamate - MSG) – Protein
3. Physiology of Taste
a. For a chemical to be tasted it must be dissolved in saliva, move into the taste pore, and contact a gustatory hair, producing graded potentials that release neurotransmitters to sensory dendrites.
b. Each taste sensation appears to have its own special mechanism for transduction: salty taste is due to Na+
influx, sour taste is due to H+, bitter, sweet, and umami tastes are triggered through G-protein-triggered
Ca++ release.
4. Afferent fibers carrying taste information from the tongue are found primarily in the facial nerve (anterior
2/3rd) and glossopharyngeal (posterior 1/3rd) CN.
5. Taste impulses from the few taste buds found on the epiglottis and the lower pharynx are conveyed via the
vagus nerve.
a. Afferent fibers synapse in solitary nucleus (medulla)  thalamus  gustatory cortex (insula)
a. Solitary nucleus (PS) increases salivation & gastric processes
b. Some fibers project to hypothalamus & limbic system (adds appreciation for the food your eating)
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6. Taste is strongly influenced by smell and stimulation of thermoreceptors, mechanoreceptors, and nociceptors. Taste is 80% smell = bland taste with nasal congestion.
D. Homeostatic Imbalances of the Chemical Senses (p. 570)
1. Anosmias are olfactory disorders resulting from head injuries that tear the olfactory nerves, nasal cavity inflammation, or aging.
2. Uncinate fits are olfactory hallucinations. (rotting meat)
3. Taste disorders are less common but may be caused by respiratory tract infections, head injuries, chemicals,
medications, or head and neck radiation - (Zinc supplementation)
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.
a. The pharyngotympanic (auditory) tube aka. Eustachian tube - links the middle ear with the nasopharynx, which allows pressure to be equalized between the middle ear and external ear pressure. (Yawning
on an airplane)
2. The tympanic membrane, or eardrum, is a thin connective tissue membrane that serves as the boundary
between the outer and middle ear and transfers sound energy to the auditory ossicles.
3. The middle ear, or tympanic cavity, is a small, air-filled, mucosa-lined cavity in the petrous portion of the
temporal bone, spanned by the auditory ossicles (malleus -“hammer”, incus -“anvil”, stapes –“stirrup”).
b. Ossicles transmit vibratory motion of the eardrum to the oval window = motion of the liquid in inner ear.
i. Tensor tympani (CN V3) & Stapedius muscles (CN VII) = limit ossicles damage reflexively from prolonged loud noises (not a gunshot) and minimizes damage to hearing receptors. (Stapedius muscle is
the smallest muscle; paralysis = hyperacusia: abnormal sensitivity hearing)
 Otitis Media – middle ear inflammation; mc in children with sore throat; TX – antibiotic; myringotomy (lancing of eardrum) if severe.
4. The internal ear (aka. Labyrinth) - Houses receptors for hearing & equilibrium. Has two major divisions:
bony labyrinth & membranous labyrinth.
a. Outer bony portion enclosing inner membrane – series of cavities in petrous (temporal bone). Lined with
periosteum & filled w/ perilymph.
b. The membranous labyrinth, consisting of the saccule & utricle, is suspended in the perilymph within the
bony labyrinth and is filled with endolymph (rich in K+). Houses receptors for hearing & balance. Contains 3 areas: Review Summary on Table 15.2
i. Semicircular Canals – 3 bony canals @ ~ 90o angles. Anterior & posterior canals are vertical while the
lateral semicircular canal is horizontal.
 Semicircular ducts – connect utricle with vestibule. Function: Maintain equilibrium during rotational (angular) motion via the receptor called crista ampullaris.
ii. Vestibule – oval, central portion. Includes semicircular ducts and 2 sacs called utricle & saccule;
Function: Maintain equilibrium of head position relative to gravity; linear acceleration via macula
iii. Cochlea – anterior to vestibule. Contains:
 Cochlear duct – houses receptor of hearing, the spiral organ (aka: organ of Corti). Stereocilia,
hair cells produce receptor potential via cochlear branch CN VIII
 The cavity of the bony cochlea is divided into three chambers: the scala vestibuli, scala media,
& scala tympani.
 The floor of the cochlear duct is composed of the osseous spiral lamina, and the basilar membrane, which is important to sound reception.
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.
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b. Frequency is the number of waves that pass a given point in a given time, and is measured in hertz (Hz).
The higher the frequency; the higher the pitch.
c. Amplitude, or height, of the wave reveals a sound’s intensity (loudness), and is measured in decibels (dB).
Loudness = subjective interpretation of sound intensity. Decibel deafness: hearing loss due to prolonged
or frequent sound intensity greater than 90 dB; threshold of pain = 120 dB
2. Transmission of Sound: sound  external acoustic meatus  tympanic membrane = vibrating  auditory
ossicles  oval window (vestibule) = pressure wave (in perilymph)  sounds (below hearing) travel to helicotrema without exciting cells) OR sounds (hearing range)  cochlear duct = vibrating basilar membrane 
deflecting stereocilia of hair cells in inner ear  spiral ganglion  cochlear nerve  cochlear nuclei (medulla)  to the superior olivary nucleus  inferior colliculus (“startle reflex)  primary auditory cortex.
3. Primary auditory processing = perception of pitch, loudness, and localization of sound. Association center
involved in sound recognition.
 Vestibular Nystagmus – eye movements that occur during immediately after rotation. Often associated
by vertigo.
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.
2. The sensory receptors for static equilibrium are the maculae.
3. The receptor for dynamic equilibrium is the crista ampullaris, found in the ampulla of the semicircular canals and activated by head movement.
4. Information from the balance receptors goes directly to reflex centers in the brain stem, rather than to the
cerebral cortex.
D. Homeostatic Imbalances of Hearing and Equilibrium (p. 584)
1. Deafness is any hearing loss, no matter how slight. Classified as conduction (d/t earwax, perforated eardrum, otitis media or otosclerosis) or sensorineural (loss of hair cells, decibel deafness, cochlear nerve damage, stroke, or tumors of auditory cortex). Cochlear implants.
2. Tinnitus is a ringing or clicking sound in the ears in the absence of auditory stimuli. 1st symptoms of nerve
damage, inflammation or medication (Aspirin)
3. Ménière’s syndrome is a labyrinth disorder that causes a person to suffer repeated attacks of vertigo, nausea, and vomiting. “Howling” tinnitus is common.
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.
3. Beginning in the fourth decade of life, the ability to taste and smell declines as receptors are replaced more
slowly than in younger people.
B. Vision (p. 585)
1. By the fourth week of development, eyes begin to develop and—even before photoreceptors develop—CNS
connections are made.
2. Vision is the only sense not fully developed at birth. Newborn infants see only in gray tones, exhibit uncoordinated eye movements, and often use only one eye at a time.
3. By 5 months, vision has improved, and by 5 years, vision is well developed.
4. With age, lens loses clarity & the pupil stays partly constricted, decreasing visual acuity in people over 70.
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.
3. With age, ability to hear high-pitched sounds declines, & hearing loss is exacerbated by loud noises.
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Chapter 15 Homeostatic Imbalances
Eye
Ear (Vestibulocochlear)
Smell & Taste
Otitis Media
Anosmia
Presbycusis
*Ageusia
Conjunctiva (Pinkeye)
Retinopathies
*Horner’s Syndrome
(p.543)
*ARMD
Watery eyes
*Blepharitis
Deafness (types)
Diplopia
*Enucleation
Otosclerosis
Strabismus (Internal & external) chap 13
*Exophthalmos
Tinnitis
Retinal detachment
*Ophthalmology
Meniere’s Syndrome
Glaucoma
*Optometrist
*Labryinthitis
Cataract
* Papilledema
*Otalgia
Myopia
*Scotoma (pathway)
* Otitis Externa
Hyperopia
*Trachoma
Anopsia (chap 13)
Chalazion (sty)
Astigmatism
Motion Sickness
Effect of measles & rubella
Italics: not in textbook or in different chapter
* located on p.590 (At the Clinic)
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