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Special Senses
Chapter 22
CHAPTER SUMMARY
This chapter begins with a brief introduction to the special senses. The characteristics of the receptors and neural
pathways for the senses of smell and taste are described. The important anatomical characteristics of the sense of vision,
including accessory structures, anatomy of the eyeball, and neural pathway, are all portrayed in detail. The major
structural characteristics of the sense of hearing, including the external ear, middle ear, internal ear, and neural pathway,
are all explained. The major anatomical characteristics of the sense of equilibrium, including saccule, utricle, semicircular
ducts, and neural pathway, are all concisely described. The development of the eyes and ears is portrayed. The effects of
aging on the special sense organs is explained. A glossary of key medical terms associated with special senses is provided.
This chapter concludes with a thorough study outline, an excellent self-quiz, critical thinking questions, and answers to
questions that accompany chapter figures.
STUDENT OBJECTIVES
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8.
Describe the olfactory receptors and the neural pathway for olfaction.
Describe the gustatory receptors and the neural pathway for gustation.
List and describe the accessory structures of the eye and the structural components of the eyeball and their functions.
Describe how the retina processes visual information, and describe the neural pathway between the retina and the
brain.
Describe the anatomy of the structures in the three principal regions of the ear.
List the principal events involved in hearing.
Identify the receptor organs for equilibrium and explain how they function.
Describe the auditory and equilibrium pathways.
LECTURE OUTLINE
A. Special Senses
1. Olfaction: Sense of Smell (p. 672)
i. The olfactory epithelium is located in the superior portion of the nasal cavity and consists of three
major types of cells:
a. olfactory receptor cells (i.e., chemoreceptors) are bipolar neurons which:
- have olfactory hairs (cilia) projecting from a knob-shaped dendrite; these cilia are the
sites where odorant molecules are detected
- have an axon that projects to the olfactory bulb
- live for only about one month
b. supporting cells are columnar epithelial cells that provide physical support, nourishment, and
electrical insulation for the olfactory receptor cells; they also help detoxify chemicals that
contact the olfactory epithelium
c. basal stem cells are stem cells that produce new olfactory receptor cells to replace those that
die
ii. The connective tissue that supports the olfactory epithelium contains olfactory (Bowman’s) glands
which secrete mucus that moistens the surface of the olfactory epithelium and dissolves odorant gases.
iii. Both the supporting cells and the olfactory glands are innervated by branches of the facial (VII) nerve.
iv. Olfactory pathway:
a. axons of the olfactory receptor cells converge to form the olfactory (I) nerves which pass
through foramina in the cribriform plate of the ethmoid bone
b. these nerves extend to the paired masses of gray matter of the brain called olfactory bulbs
where the nerve axons synapse with olfactory bulb neurons
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c.
axons of the olfactory bulb neurons extend posteriorly and form the olfactory tracts which
deliver nerve impulses to :the lateral olfactory area (which is probably the site of the primary
olfactory area)where conscious awareness of smell begins, and thereafter to the frontal lobes
where the sensation of smell is perceived
2. Gustation: Sense of Taste (p. 674)
i. There are five priamry tastes that can be distinguished: sour, sweet, bitter, salty, and umami (which is
described as “meaty” or savory”); all other flavors are various combinations of the primary tastes plus
accompanying olfactory and tactile sensations.
ii. Taste buds are located on the tongue, soft palate, larynx, and pharynx.
iii. Each taste bud is an oval-shaped structure that contains three types of epithelial cells:
a. supporting cells form a capsule
b. the capsule encloses about 50 gustatory receptor cells (i.e., chemoreceptors), each of which
has a single hairlike gustatory hair (microvillus) that projects through on opening in the taste
bud called a taste pore; these gustatory hairs are the sites where taste stimuli are detected
c. basal cells produce supporting cells which in turn develop into gustatory receptor cells that
have a life span of about 10 days
iv. Taste buds are located in elevations on the tongue called papillae:
a. circumvallate papillae are disc-shaped and are located on the back of the tongue; they all
contain taste buds
b. fungiform papillae are mushroom-shaped and are scattered over the entire surface of the
tongue; most contain taste buds
c. foliate papillae are located on the lateral margins of the tongue; most of their taste buds
degenerate in early childhood
d. filiform papillae are pointed threadlike structures and are distributed over the entire surface of
the tongue; they rarely contain taste buds
e. tastants dissolved in saliva contact gustatory hairs to trigger nerve impulses.
v. Gustatory pathway:
a. Nerve impulses are transmitted from taste buds to the medulla oblongata via three pairs of
cranial nerves:
- facial (VII) nerves innervate the anterior two-thirds of the tongue
- glossopharyngeal (IX) nerves innervate the posterior one-third of the tongue
- vagus (X) nerves innervate the pharynx and epiglottis
b. From the medulla, some taste fibers project to the limbic system and the
hypothalamus while others project to the thalamus; from the thalamus, fibers
extend to the primary gustatory area in the parietal lobe where the sensation of
taste is perceived
3. Vision (p. 676)
i. Accessory Structures of the Eye:
a. upper and lower eyelids or palpebrae
- the upper eyelid is moved by the levator palpebrae superioris
- the space between the eyelids is the palpebral fissure whose angles are the lateral
commissure and the medial commissure; in the medial commissure is the lacrimal
caruncle containing sebaceous and sudoriferous glands
- each eyelid consists of skin and underlying structures including the following:
- fibers of the orbicularis oculi muscle
- tarsal plate
- tarsal or Meibomian glands
- palpebral conjunctiva that lines the inner aspects of the eyelids and is continuous
with the bulbar conjunctiva that covers the anterior surface of the eyeball (lateral to
the cornea)
b. eyelashes and eyebrows
- protect the eyeballs against foreign objects, perspiration, and direct rays of the sun
- sebaceous ciliary glands are located at the bases of the eyelash hair follicles and
secrete a lubricating fluid into the follicles
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c.
ii.
lacrimal apparatus is a group of structures:
- lacrimal glands secrete lacrimal fluid or tears which are delivered by excretory
lacrimal ducts onto the conjunctiva of the upper eyelid
- after flowing medially over the anterior surface of the eyeball, tears enter two lacrimal
puncta and then enter the two lacrimal canals which lead into the lacrimal sac and
then into the nasolacrimal duct that carries the lacrimal fluid into the nasal cavity
- lacrimal fluid, which includes a bacteriocidal enzyme called lysozyme, serves to
protect, clean, lubricate, and moisten the eyeball
d. six extrinsic eye muscles that move each eye (superior rectus, inferior rectus, medial
rectus, lateral rectus, superior oblique, and inferior oblique)
- coordinated movement of the two eyes involves circuits in the brain stem and
cerebellum
Anatomy of the Eyeball (see Table 22.1 for summary):
a. The adult eyeball is about 2.5 cm in diameter and is protected by the orbit, into which it fits.
b. The wall of the eyeball consists of three layers:
- fibrous tunic
- vascular tunic
- retina
c. Fibrous tunic:
- outer layer of the wall of the eyeball
- consists of:
- anterior cornea which is a nonvascular, transparent, fibrous coat that covers the
colored iris; it helps focus light onto the retina
- posterior sclera which is a white coat of dense connective tissue which gives shape
to the eyeball, makes it more rigid, and protects its internal parts
- at the junction of the cornea and sclera is an opening called the scleral venous sinus (or
canal of Schlemm)
d. Vascular tunic or uvea:
- middle layer of the wall of the eyeball
- it consists of three portions:
1. posterior, brown-black, vascular portion called choroid that lines most of the
internal surface of the sclera
2. anterior portion called the ciliary body that extends from the ora serrata to a
point just behind the sclerocorneal junction; the ciliary body consists of:
- ciliary processes that secrete aqueous humor; they also attach to zonular
fibers (suspensory ligaments) which connect to the lens
- ciliary muscle that encircles and alters the shape of the lens, in order to
adapt the lens for near or far vision
3. colored, circular iris located between the cornea and the lens and is attached at its
outer margin to the ciliary processes; it consists of circular muscles (constrictor
pupillae) and radial muscles (dilator pupillae) that, under autonomic control,
constrict or dilate the pupil, a hole at the center of the iris, to regulate the amount
of light that enters the eyeball’s vitreous chamber
e. Retina:
- inner layer of the wall of the eyeball
- lines the posterior three-quarters of the eyeball
- near its center is the optic disc where the optic nerve exits the eyeball; bundled with the
optic nerve are the central retinal artery and vein
- consists of two portions:
1. outer pigment layer that helps the choroid absorb stray light rays
2. inner neural layer that is a multilayered outgrowth of the brain
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f.
g.
- the neural layer has three sublayers of retinal neurons (separated by two synaptic layers
where synaptic connections are made):
1. photoreceptor layer whose cells contain photopigments:
- rods that provide vision of shades of gray in dim light
- cones that provide color vision in bright light
2. bipolar cell layer
3. ganglion cell layer
- the neural portion also has horizontal cells and amacrine cells which modify signals
transmitted from photoreceptors to bipolar cells to ganglion cells
- at the center of the retina is the macula lutea which has at its center a small depression
called the central fovea which lack rods; the fovea is the area of highest visual acuity
(or resolution)
- the principal blood vessels of the retina are the central retinal artery and the central
retinal vein, both of which travel through the optic disc
- the axons of the ganglion cells converge toward the optic disc where they unite to form
the optic nerve; since the optic disc lacks photoreceptors, it is the location of the blind
spot
Lens:
- the nonvascular, transparent lens is located posterior to the iris and pupil
- it contains proteins called crystallins arranged like the layers of an onion
- it is enclosed by a clear connective tissue capsule encircled by zonular fibers, which
are attached to the ciliary processes
- the lens fine-tunes focusing of light rays onto the retina for clear vision
Interior of the eyeball:
- the lens divides the interior into two cavities:
1. anterior cavity located anterior to the lens
2. vitreous chamber (posterior cavity) located posterior to the lens
- the anterior cavity consists of an anterior chamber, located anterior to the iris, and a
posterior chamber, located posterior to the iris; it is filled with a watery aqueous
humor (secreted by the ciliary processes) which produces an intraocular pressure that
maintains the shape of the eyeball and prevents it from collapsing
- the vitreous chamber (or posterior cavity) is filled with a jellylike vitreous body which
contributes to intraocular pressure and presses the retina flush against the choroid;
unlike the aqueous humor, the vitreous body is not continuously replaced
- the hyaloid canal is a narrow channel that travels through the vitreous body from the
optic disc to the posterior aspect of the lens
iii. Visual Pathway
a. There is extensive retinal processing of visual input which arises from:
- extensive convergence (and some divergence) of visual information
- activities of horizontal cells and amacrine cells
b. The ganglion cells initiate nerve impulses that are carried by their axons into the optic (II)
nerve.
c. The optic nerves pass into the optic chiasm where some axons cross to the opposite side while
others remain uncrossed.
d. These axons then continue in the optic tract to the lateral geniculate nucleus of the thalamus;
here they synapse with neurons whose axons form the optic radiations that project to the
primary visual areas of the cerebral cortex.
4. Hearing and Equilibrium: (p. 684)
i. The ear consists of three major regions (see Table 22.2 for summary):
a. external (outer) ear
b. middle ear
c. internal (inner) ear where the mechanoreceptors for hearing and equilibrium are located
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ii.
External (outer) ear:
a. The external ear collects sound waves and passes them inward.
b. It consists of:
- auricle (pinna) whose rim is the helix and inferior portion is the lobule
- external auditory canal that is about 2.5 cm long and lies in the temporal bone; it
leads to the eardrum
- eardrum or tympanic membrane which is a thin wall of fibrous connective tissue
(covered by epithelium on both sides) that separates the external auditory canal from
the middle ear
c. Near its opening, the external auditory canal contains several hairs and ceruminous glands
which secrete cerumen (earwax) that help prevent dust and foreign objects from entering the
ear.
iii. Middle ear:
a. The middle ear is a small air-filled cavity (tympanic cavity) in the temporal bone that is lined
by epithelium.
b.
Extending across the middle ear are three linked auditory ossicles:
- malleus which is attached to the inner surface of the eardrum
- incus
- stapes whose base or footplate rests against the oval window of the inner ear; below
the oval window is the round window that is covered by the secondary tympanic
membrane
c. Attached to the ossicles are two skeletal muscles which prevent damage to the inner ear that
may result from loud sounds:
- tensor tympani muscle
- stapedius muscle
d. Its anterior wall contains an opening that leads directly into the auditory (pharyngotympanic
or Eustachian) tube that travels to the nasopharynx; the auditory tube’s function is to equalize
the air pressure on both sides of the tympanic membrane so that it may vibrate freely.
iv. Internal (inner) ear or labyrinth:
a. The internal ear consists of two main divisions:
- outer bony labyrinth
- inner membranous labyrinth
b. The bony labyrinth is a series of cavities in the temporal bone that is divided into three areas:
1. semicircular canals
2. vestibule
3. cochlea
c. The bony labyrinth is lined with periosteum and is filled with perilymph which surrounds the
membranous labyrinth.
d. The membranous labyrinth is a series of interconnected sacs and tubes that are lined with
epithelium and are filled with endolymph.
e. The vestibule of the bony labyrinth contains two interconnected sacs of the membranous
labyrinth called the utricle and saccule.
f. Posterior and superior to the vestibule are the three bony semicircular canals (called anterior,
posterior, and lateral semicircular canals) which are oriented at approximately right angles to
each other; one end of each canal enlarges into a swelling called the ampulla.
g. Within the semicircular canals are the membranous semicircular canals that communicate
with the utricle.
h. The vestibular branch of the vestibulocochlear (VIII) nerve consists of ampullary, utricular,
and saccular nerves which innervate the corresponding structures; cell bodies of the sensory
neurons are located in the vestibular ganglia.
i. Anterior to the vestibule is the bony, spiral-shaped cochlea that makes almost 3 turns around a
central bony core called the modiolus.
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j.
The cochlea contains three channels that spiral alongside each other:
1. scala vestibuli which ends at the oval window and contains perilymph
2. scala tympani which ends at the round window and also contains perilymph
- these two channels communicate via the helicotrema at the apex of the
cochlea
3. intermediate cochlear duct (scala media) which is separated from the scala vestibuli
by the vestibular membrane and from the scala tympani by the basilar membrane
- resting on the basilar membrane is the spiral organ or organ of Corti) which
contains inner and outer hair cells (i.e., mechanoreceptors) with hairlike
processes called stereocilia (specialized microvilli that form a hair bundle)
that detect auditory stimulation; the bases of the hair cells synapse with firstorder sensory neurons and with motor neurons from the cochlear branch of
the vestibulocochlear (VIII) nerve (the cell bodies of the sensory neurons are
located in the spiral ganglion)
- projecting over and in contact with the hair cells is the gelatinous tectorial
membrane
v. Mechanism of hearing: (p. 690)
a. The auricle directs sound waves into the external auditory canal.
b. Sound waves strike the tympanic membrane and cause it to vibrate at the same frequency as the
incoming sound waves.
c. The eardrum’s vibrations are transmitted to the malleus, to the incus, and then to the stapes.
d. The stapes vibrates against the membrane that covers the oval window.
e. The latter vibrations initiate fluid pressure waves in the perilymph of the scala vestibuli; these
pressure waves are transmitted to the scala tympani and eventually to the round window.
f. These pressure waves cause the vestibular membrane to vibrate which in turn initiates pressure
waves in the endolymph of the cochlear duct.
g. The latter pressure waves cause the basilar membrane to vibrate and as a result, the hair cells
move against the tectorial membrane; bending of the stereocilia of the hair cells results in nerve
impulses being generated in the first-order neurons in cochlear nerve fibers
vi. The auditory pathway:
a. First-order sensory neurons in the cochlear branch of each vestibulocochlear (VIII) nerve
transmits auditory impulses to the cochlear nuclei in the medulla oblongata (where many fibers
cross to the opposite side) and then via the superior olivary nuceli in the pons onwards to the
inferior colliculus in the midbrain to the medial geniculate body of the thalamus to the primary
auditory area of the cerebral cortex
vii. Mechanism of equilibrium: (p. 692)
a. There are two types of equilibrium (balance):
- static equilibrium which refers to maintenance of the position of the body (mainly the
head) relative to the force of gravity
- dynamic equilibrium which refers to maintenance of body position (mainly the head)
in response to sudden movements such as rotation, acceleration, and deceleration
b. The receptor organs for equilibrium are collectively called the vestibular apparatus, which
includes:
- saccule
- utricle
- the above two are called the otolithic organs
- semicircular ducts
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c.
Each utricle and saccule contains a small thickened region called a macula; the maculae
detect the position of the head in space (for static equilibrium) and detect linear acceleration
and deceleration (for dynamic equilibrium)
- the two maculae are perpendicular to each other
- they contain two types of cells:
- hair cells have hair bundles which consist of numerous stereocilia (which are
actually microvilli) plus one kinocilium (a standard cilium)
- supporting cells which probably secrete the gelatinous otolithic membrane that
rests on the hair cells
- a layer of otoliths (calcium carbonate crystals) extends over the entire
surface of this membrane
- gravity, linear acceleration, or linear deceleration pull the otoliths and otolithic
membrane resulting in bending of the hair bundles; this leads to the generation of nerve
impulses that are carried away by the first-order neurons in the vestibular branch of the
vestibulocochlear (VIII) nerve
d. The three semicircular ducts are oriented at right angles to each other and function in dynamic
equilibrium:
- the two vertical ones are the anterior and posterior semicircular ducts
- the horizontal one is the lateral semicircular duct
- this orientation permits detection of rotational acceleration or deceleration
- the ampulla of each duct contains a small elevation called a crista:
- each crista contains hair cells and supporting cells covered by a gelatinous cupula;
rotational acceleration or deceleration of the head causes the endolymph to flow over
the hair bundles and bend them, resulting in nerve impulses being carried away by the
vestibular branch of the vestibulocochlear (VIII) nerve
viii. Equilibrium pathways:
a. Most of the vestibular branch fibers terminate in the vestibular nuclei in the medulla and pons;
some fibers enter the cerebellum via the inferior cerebellar peduncle.
b. Various pathways between the vestibular nuclei, cerebellum, and cerebrum enable the
cerebellum to play a key role in maintaining static and dynamic equilibrium; the cerebellum
monitors sensory information from the vestibular apparatus and makes corrective adjustments
to the motor output to skeletal muscles originating in the cerebral cortex.
B. Development of the Eyes and Ears (p. 696)
1. Development of the eyes:
i. The eyes begin to develop when ectoderm of the prosencephalon bulges out to form the optic grooves
which enlarge to form the optic vesicles; the overlying ectoderm thickens to form the lens placode.
ii. The distal portions of the optic vesicles invaginate to form the optic cups which remain attached to the
prosencephalon by the optic stalks.
iii. The lens placodes develop into lens vesicles which eventually develop into the lenses.
iv. The inner wall of the optic cup forms the neural layer while the outer layer forms the pigmented layer
of the retina; axons of the neural layer form the optic (II) nerve.
v. The ciliary body, iris, ciliary muscle and zonular fibers develop from the anterior portion of the optic
cup and its neighboring mesenchyme; cavities in the latter form the anterior and posterior chambers.
vi. The eyelids form from surface ectoderm and mesenchyme.
2. Development of the ears:
i. The internal ear develops when the otic placodes (thickenings of the surface ectoderm) form the otic
pits which in turn form the otic vesicles; the latter form the membranous labyrinth while the
neighboring mesenchyme forms the bony labyrinth.
ii. The middle ear develops from the first pharyngeal (branchial) pouch; the auditory ossicles develop
from the first and second pharyngeal (branchial) pouches.
iii. The external ear develops from the first pharyngeal cleft.
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C. Aging and the Special Senses (p. 698)
i. Deterioration of the senses of smell and taste usually are not noticeable
before age 50 due to the loss of olfactory and gustatory receptor cells coupled with slower rates of their
replacement.
ii. Age-related changes in the eyes include presbyopia, cataracts, variable types of discoloration of the
sclera, slower adjustments of the iris to changes in light intensity, age-related macular disease,
glaucoma, dry eyes, reduction in visual acuity, reduction of color and depth perception, and an
increase in “vitreal floaters.”
iii. Noticeable hearing loss, especially for higher pitched sounds, is not uncommon by age 60 due to
damaged or lost hair cells in the organ of Corti or degeneration of the neural pathway for hearing; the
incidence of tinnitus (ringing in the ears) and vestibular imbalance also increases with aging.
D. Key Medical Terms Associated with Special Senses (p.699)
1. Students should familiarize themselves with the glossary of key medical terms.