Chapter 17: An Introduction to the Special Senses
Learning Outcomes
 17-1 Describe the sensory organs of smell, identify the structures and their functions, and trace the olfactory
pathway to their destinations in the brain. Describe what happens to our sense of smell as we age.
 17-2 Describe the sensory organs of taste, identify the structures and their functions (including lingual papillae),
and discuss the six different taste sensations. Discuss the relationship between olfactory functioning and taste.
 17-3 Identify the internal and accessory structures of the eye, and explain the functions of each. Identify the
functions of tears. Define visual acuity and differentiate between emmetropia, myopia and hyperopia.
 17-4 Identify differences between structure and function of rods and cones. Explain how cones allow for color
vision and what occurs in color blind individuals. Explain what happens in individuals with a severe Vitamin A
deficiency. Trace the visual pathway to their destinations in the brain.
 17-5 Describe the structures of the external, middle, and internal ear, explain their roles in equilibrium and
hearing, and trace the pathway for hearing to their destinations in the brain. Explain how our hearing changes
with age.
 Be able to answer questions similar to clinical scenario application questions.
Chapter 17: An Introduction to the Special Senses
I.
17-1 Olfaction - Olfactory Organs
A. Location: nasal cavity on either side of nasal septum
B. 2 layers
1. Olfactory epithelium
a. Olfactory receptors
b. Supportive cells
c. Basal cells
2. Lamina propria
a. Areolar tissue
b. Blood vessels
c. Nerves
d. Olfactory glands (AKA Bowman glands): secretions + water = mucus
C. Olfactory receptors
1. General information
a. Distinguish among 2000-4000 chemical stimuli
b. 50 primary smells
c. Smell interpreted by CNS based on pattern of receptor activity
d. High turnover rate (basal cells – stem cells which replace olfactory receptors)
e. Number of receptors decreases with age, older individuals become less sensitive to
odors
2. Structure
a. Receptors are highly modified neurons
b. Neurons of olfactory nerves move through cribiform plate and end in olfactory bulb
c. cilia extend from knobs into mucus and bind with dissolved odorants
D. Olfactory Pathways
1. odorant binds to receptor proteins on Cilia
2. ATP converted to cAMP (cAMP opens gated sodium channels, allowing Na+ into receptor cell)
3. Na+ channels open and Action Potential is generated
4. AP moves to olfactory nerve fibers olfactory bulb olfactory tract  olfactory cortex,
hypothalamus, limbic system
a. Hypothalamus and limbic system receive olfactory information
b. Explains why smells can trigger memories or emotional/behavioral responses
II.
17-2 Gustation
A. Provides information about foods/liquids we intake
B. Taste receptors distributed over tongue surface and portions of pharynx/larynx
C. Taste receptors + specialized epithelial cells = taste buds
D. Superior surface of tongue has projections called lingual papillae
1. filiform papillae: 0 taste buds, provide friction to help tongue move objects around mouth
2. fungiform papillae: ~ 5 taste buds
3. circumvallate: 100s of taste buds
E. Olfactory fully functioning = increased sensitivity to tastes
F. Ability to taste changes with age
1. Taste buds decline throughout life (dramatic decline by age 50)
2. Children = foods taste too spicy; elderly = food tastes too bland
Chapter 17: An Introduction to the Special Senses
III.
G. Gustatory Discrimination
1. Four primary tastes
a. Sweet
b. Salty
c. Sour
d. Bitter
2. Two additional taste sensations
a. Umami: taste due to glutamates in food (i.e. broth, Parmesan cheese, anchovies); MSG
b. Water
i.
water receptors in pharynx
ii.
sensory output processed in hypothalamus
iii.
affects water balance and regulation of blood volume
iv.
example: take a long drink = reductions in ADH secretions
17-3 The Eye
Accessory Structure
Palpebrae (aka eyelids)
Sebaceous glands
Conjunctiva
Cornea
Lacrimal gland
Nasolacrimal duct
Description/Function
Blinking keeps surface of eye lubricated, removes dust/debris, protects surface of eye
Produce lipid rich secretions which prevent palpebrae from sticking together; if becomes
infected = sty
Mucous membrane covering inner surface of palpebrae and anterior surface of eye;
inflammation = conjunctivitis (aka pinkeye)
Transparent outer fibrous layer of eye
Produces tears to bathe conjunctival surfaces. F(x) of tears: deliver nutrients/O2 to
cornea, lubrication, remove debris, prevent bacterial infection (via antibacterial enzyme
lysozyme and antibodies)
drain tears into nasal cavity
A. Two cavities of eye
1. Anterior cavity: filled with aqueous humor (watery); circulates to provide nutrients and
remove waste
2. Posterior cavity: filled with vitreous humor (gelatinous)
3. Aqueous and vitreous humor stabilize shape of eye
B. Three layers of eye
4. Fibrous layer
a. Consists of sclera and cornea
b. Functions
v.
Supports and protects
vi.
Serves as attachment site for extrinsic eye muscles
vii.
Contains structures assisting in focusing
5. Vascular layer
a. Pigmented
b. Includes iris, Ciliary body, choroid + BVs, lymphatic vessels and smooth muscle
c. Functions
viii.
Provide route for BVs and lymphatic vessels
ix.
Regulate light entering eye
x.
Secrete/reabsorb aqueous humor
xi.
Controls shape of lens
Chapter 17: An Introduction to the Special Senses
6. Inner layer (retina)
a. Contains retina and optic nerve
b. Photoreceptors located in this layer which are important for vision
C. Internal Eye Structures
Structure
Sclera
Description/Function
White part of the eye
Iris
Pupil
Pigmented around pupils; Contains muscles that change diameter of pupils
Central opening of iris; allows light in
Ciliary body
Suspensory ligaments
Choroid
Attaches to iris; contain ciliary processes which attach to suspensory ligaments
Attach to lens and ciliary processes; hold lens in position posterior to iris/pupil
Vascular layer separates fibrous and inner layer; contains capillaries which deliver oxygen
and nutrients to retina; contains melanocytes
Contains pigments to absorb light
Retina
D. The Eye and Vision
1. lens: focus image on photoreceptors, accomplished by changing shape (accommodation)
a. Cells replace nuclei/organelles with clear proteins (crystallins)
b. Crystallins allow for clarity + focus
c. Cataracts:
i.
loss of lens transparency due to loss of balance between structural an
biochemical characteristics of lens
ii.
results from injuries, drugs, radiation, aging (most common cause)
iii.
Remedied by surgery (removal of lens and replacement by artificial lens)
2. When you look directly at an object, image falls on fovea providing sharp image
E. Visual Acuity: How well you see
1. Emmetropia
a. Normal vision
b. 20/20: you can see clearly at 20 ft what should be seen clearly at 20 feet
2. Myopia
a. Nearsightedness
b. Image projected to front of retina, results in distant images that are blurry
3. Hyperopia
a. Farsightedness
b. At close range, lens cannot focus image on the retina
4. Visual acuity falls below 20/200 = legally blind
F. Vision and Pigments
1. Pigment = light absorbing molecule
2. White light = combination of wavelengths (colors) of light (ROY G BIV)
3. Pigments of Rods
a. rhodopsin (with protein opsin)
b. retinal = synthesized from vitamin A
c. opsin binds to retinal
d. Rods stimulated but not cones = white
Chapter 17: An Introduction to the Special Senses
4. Pigments of Cones
a. Retinal
b. Other forms of opsin
c. Type of opsin stimulated = absorption of specific wavelength of light = color vision
d. Color Vision: Integration of information from red, green, and blue cones (all three
stimulated = white)
e. Colorblindness
iv.
Inability to distinguish between certain colors
v.
Most common form red-green color blindness; red cones missing so individual
cannot distinguish between red and green
G. Visual Pathway
1. Photoreceptors stimulated
2. Information passes to bipolar cells then ganglion cell
a. Great convergence for rods (general); Little converge for cones (specific)
b. M cells monitor rods and provide information about general form, motion, shadows in
dim light
c. P cells monitor cones and provide information about edges, fine detail and color
3. Axons of all ganglia converge on optic disc and information passed to optic nerve
4. ½ of the fibers go to the R hemisphere, ½ of the fibers go to the L hemisphere
5. Information received by many areas including brain stem and visual cortex
a. Example: Epithalamus and pineal gland receive information to establish day-night cycle
b. Affects metabolism, endocrine function, blood pressure, digestion, sleep-wake cycles
6. Information compiled by the visual association area
IV.
17-5 The Ear
Structure
Description/Function
Protects opening of auditory canal, directs sounds
Auricle
External acoustic meatus aka auditory canal
Tympanic membrane aka eardrum
Auditory tube aka Eustachian tube
Auditory ossicles
Passageway ending at eardrum; filled with hairs
that trap debris, ceruminous glands secrete wax
(cerumen) to slow growth of bacteria
Separates external ear from middle ear;
movement of this membrane causes displacement
of auditory ossicles = important in hearing
Equalizes pressure on either side of tympanic
membrane; if invaded by microorganisms = otis
media (middle ear infection)
Three tiny bones in middle ear; connect tympanic
membrane to receptors in inner ear
Malleus
Incus
Stapes
Cochlea
Spiral shaped bony chamber; cochlear duct
contains hair cells within organ of Corti =
important in hearing
Chapter 17: An Introduction to the Special Senses
A. Equilibrium sensation provided by receptors of vestibular complex
1. Semicircular canals: provides information about rotation
2. Vestibule : saccule and utricle provide information about sense of gravity/acceleration
B. Inner ear structure
1. Bony labyrinth: continuous with temporal bone
2. membranous labyrinth: interconnected network of fluid-filled tubes
3. Receptors of internal ear found within tubes
4. perilymph: fluid closely resembling CSF found between bony and membranous labyrinths
5. endolymph: fluid with ion concentrations different from those in typical body fluids; found
within membranous labyrinth
C. How Equilibrium is Monitored
1. Hair cells in semicircular canals and vestibule respond to movement
2. Active when movement occurs
D. Pathway for equilibrium
1. Movement of endolymph stimulates/inhibits hair cells and/or gravity distorts hair cells
2. Hair cells monitored by sensory neurons in vestibular ganglia
3. AP propagates to vestibulocochlear nerve
4. Information relayed to the cerebellum, cerebral cortex
5. Commands sent to motor nuclei in brain stem and spinal cord
V.
The Ear and Hearing
A. Round window: thin membrane separating perilymph of cochlea and middle ear
B. oval window: opening between stapes and vestibule
C. Auditory ossicles
1. Convert pressure fluctuation in air into greater pressure fluctuations in perilymph of cochlea
2. Pressure fluctuation stimulate hair cells in organ of Corti
3. Frequency: Determined by which part of cochlear duct is stimulated
4. Intensity (volume): determined by # of hair cells stimulated
D. Auditory Pathways
1. Sound waves arrive at tympanic membrane
2. Movement of membrane causes displacement of auditory ossicles
3. Movement of stapes at oval window establishes pressure waves in perilymph
4. Pressure waves distort membrane on way to round window
5. Vibration of membrane causes vibration of hair cells
6. Information about region/intensity of stimulation relayed to CNS
7. Auditory information from vestibulocochlear nerve reaches:
a. Medulla oblongata and information then relayed to midbrain; Coordinates auditory
reflexes
b. thalamus: relays most of information to auditory cortex on opposite side
E. Hearing and Age
1. Young children have greatest hearing range
2. Injuries to ear accumulate with age
a. Tympanic membrane loses flexibility
b. Articulations between ossicles stiffen
c. Round windows ossify
3. Injuries to the above ear structures results in older individuals showing hearing loss