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Chapter 17
The Special Senses
Lecture Outline
Principles of Human Anatomy and Physiology, 11e
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Chapter 17
The Special Senses
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Smell, taste, vision, hearing and equilibrium
Housed in complex sensory organs
Ophthalmology is science of the eye
Otolaryngology is science of the ear
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Smell
• Smell is called olfaction
• We recognize over 10,000 distinct odors
• Smell and memory are linked in our brains
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Taste
• Taste is gustation
• Our taste buds help us distinguish different tastes
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Vision
• More than half of the sensory receptors in our body are
in our eyes
• Your eyeball is 1 inch in diameter
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Hearing
• The ear changes sound vibrations into electrical
signals that the brain interprets as sound
• The ear has 3 main parts: external, middle, and
internal ears
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Equilibrium
• The ear is also important in our equilibrium
• Equilibrium helps us balance and maintain upright
posture.
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Development of the Eyes
• The eyes begin to develop 22 days after fertilization,
small grooves called optic grooves appear.
• Eyelids form 8 weeks after fertilization.
• Eyes remain closed until 26 weeks of development.
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Development of the Ears
• The inner ear forms first, 22 days after fertilization.
• Middle ears come next, followed by the outer ear.
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Olfaction
• Olfaction is the sense of smell.
• It is a chemical sense, because smell comes from the
interaction of molecules with sensory neurons called
olfactory receptors.
• The chemicals that cause smell are called odorants
and can evoke strong emotional responses or
memories
• We can recognize 10,000 different smells!
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Diseases/Disorders
• Women have a keener
sense of smell than men
do, especially around the
time of ovulation.
• Smoking impairs the sense
of smell.
• Hyposmia is the reduced
ability to smell, which
occurs in 75% of people
age 80 and older.
– it can also be caused
by head injury, disease,
steroids, and
antihistamines.
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Gustation
• Gustation is the sense of taste.
• It is also a chemical sense.
– To be detected, molecules must be dissolved.
– Taste stimuli classes include sour, sweet, bitter, salty,
and umami (savory).
– Gustation is closely linked to olfaction, without the sense
of smell, you cannot taste.
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Gustatory Sensation: Taste
• 10,000 taste buds found on
tongue, soft palate & larynx
• They are found on
elevations called papillae.
• The number of taste buds
declines with age.
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Anatomy of Taste Buds
• An oval body consisting
of 50 receptor cells
surrounded by
supporting cells
• A single gustatory hair
projects upward through
the taste pore
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Physiology of Taste
• Mechanism
– Dissolved substance (food) contacts gustatory hairs
– Receptor potential results in neurotransmitter release
– Nerve impulses are formed
– These nerve impulses travel to the medulla oblongata,
the hypothalamus, and the cerebral cortex, where they
are interpreted.
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Taste Diseases/Disorder
• Taste aversion is the link that forms between unpleasant
tastes and refusal to eat certain foods.
• sweet foods evoke pleasure
• Bitter foods evoke disgust
• This was an evolutionary adaptation to avoid ingesting
poisons
• People with cancer may develop taste aversions to
most food due to radiation treatment and resulting
nausea
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VISION
• More than half the sensory receptors in the human
body are located in the eyes.
• A large part of the cerebral cortex is devoted to
processing visual information.
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Anatomy of the Eye
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Eyelids
Eyelashes
Eyebrows
Lacrimal (tearing)
apparatus
• Extrinsic Eye muscles
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Eyelids
• The eyelids have several functions:
– Shade the eyes during sleep
– Protect the eyes from excessive light
– Protection from foreign objects
– Keep the eyes moist
– Eyelids have glands that secrete both oil and sweat
– The upper eyelid is more mobile
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Eyelashes & Eyebrows
Eyeball = 1
inch diameter
• Eyelashes & eyebrows help protect from foreign
objects, perspiration & sunlight
• Sebaceous glands are found at base of eyelashes
• An infection of these glands can cause a sty
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Lacrimal Apparatus
• About 1 ml of tears produced per day. Spread over eye by
blinking. Contains bactericidal enzyme called lysozyme.
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Extraocular Muscles
• There are 6 muscles
that control eye
movement
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Tunics (Layers) of Eyeball
• The eye is
constructed of
three layers
– Fibrous Tunic
– Vascular Tunic
– Nervous Tunic
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Fibrous Tunic -- Description of Cornea
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Outer layer of the eyeball
Transparent
The cornea helps focus light
3 layers
Transplants
– common & successful
– no blood vessels so no
antibodies to cause rejection
• Nourished by tears & aqueous
humor
• Makes up the “whites” of our eyes
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Vascular Tunic – Middle Layer
• Contains the iris: the
colored part of the eyeball.
It is shaped like a flattened
donut.
• The iris regulates light
entering the pupil, which is
the hole in the center of the
iris (the black part).
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Vascular Tunic - Description of lens
• Also contains the
lens, which focuses
the light entering the
eye
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Nervous Tunic -- Retina
• Posterior 3/4 of
eyeball
• Where the optic
nerve enters the eye
• Location of the retina,
which supplies blood
to the eye
View with Ophthalmoscope
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Photoreceptors
• Photoreceptors are specialized cells that convert images
(light rays) into nerve impulses
• Rods
– specialized for black-and-white vision in dim light
– permit us to see shapes and movement.
– 120 million rods
• Cones
– specialized for color vision and sharpness of vision (high
visual acuity) in bright light
– There are blue, green and red cones that work together
to interpret colors.
– 6 million cones
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Cavities of the Interior of Eyeball
• Anterior cavity (anterior to lens)
– filled with aqueous humor
• continually drained
• replaced every 90 minutes
• Posterior (Vitreous) cavity (posterior to lens)
– filled with vitreous body (jellylike)
– formed once during embryonic life
– Clears debris from the inner eye
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Diseases/Disorders
1. Glaucoma
– Increased pressure in the eye
– problem with drainage of aqueous humor
– may produce degeneration of the retina and blindness
2. Presbyopia
-As you age, you lenses lose elasticity and you can no
longer read print up close, bifocal glasses can correct this.
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Refraction Abnormalities
• Myopia is nearsightedness (you can’t see things far away)
• Hyperopia is farsightedness (you can’t see things near)
• Astigmatism is a refraction abnormality due to an irregular
curvature of either the cornea or lens.
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Physiology of Vision
• Vision transduction is the process that takes what the eyes
see and changing it into something that the brain can read.
• The first step is that light is absorbed by photopigments in
the photoreceptors (rods and cones)
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Photoreceptors
• The health of your photoreceptors
depends on Vitamin A and
carotene, found in:
– Carrots
– Spinach
– Broccoli
– Yellow squash
– liver
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Application: Color Blindness & Night Blindness
• Color blindness (sex-linked, more common in men)
– inability to distinguish between certain colors
– absence of certain cone photopigments
– red-green color blind person can not tell red from green
• Night blindness (nyctalopia)
– difficulty seeing in low light
– possibly due to deficiency of vitamin A
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Brain
Pathways
of Vision
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Processing of Image Data in the Brain
• Visual information in optic nerve travels to
– hypothalamus to establish sleep patterns based upon
circadian rhythms of light and darkness
– midbrain for controlling pupil size & coordination of head
and eye movements
– occipital lobe for vision
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HEARING AND EQUILIBRIUM
• The external (outer) ear collects sound waves.
• The middle ear (tympanic cavity) is a small, air-filled cavity
in the temporal bone that contains auditory ossicles (middle
ear bones, the malleus, incus, and stapes), the oval window,
and the round window
• The internal (inner) ear is also called the labyrinth because
of its complicated series of canals
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Anatomy of the Ear Region
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External Ear
• The external (outer) ear collects
sound waves and passes them
inwards
• Structures
1. auricle or pinna
• elastic cartilage covered with skin
2. external auditory canal
• curved 1” tube of cartilage & bone leading into temporal
bone
• ceruminous glands produce cerumen = ear wax
3. tympanic membrane or eardrum
– Thin, semi-transparent partition between the external
canal and the middle ear
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Middle Ear Cavity
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Ear Anatomy
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Middle Ear Cavity
• Air filled cavity in the temporal bone
• 3 ear ossicles (bones) connected by synovial joints
– malleus attached to eardrum, incus & stapes attached by
foot plate to membrane of oval window
• Auditory tube leads to nasopharynx
– helps to equalize pressure on both sides of eardrum (such
as when yawning or swallowing)
– “ear popping”
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Muscles of the Ear
• Stapedius muscle inserts onto stapes
– prevents very large vibrations of stapes from loud noises
• Tensor tympani attaches to malleus
– limits movements of malleus & stiffens eardrum to prevent
damage
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Inner Ear - Bony Labyrinth
• The bony labyrinth is a series of cavities and canals in the
temporal bone.
• It is divided into 3 main areas:
– Semicircular canals
– Vestibule
– cochlea
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Semicircular Canals
• Projecting upward and posteriorly from the vestibule are the
three bony semicircular canals.
• One end of each canal enlarges into a swelling called
the ampulla.
• The portions of the membranous labyrinth that lie
inside the semicircular canals are called the
semicircular ducts (membranous semicircular canals).
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Vestibule
• The vestibule is the oval central portion of the bony
labyrinth.
• It contains receptors for equilibrium
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Cochlea
• The cochlea contains receptors for hearing.
• It is shaped like a snail.
• It makes 3 turns around a small bony core called the
modiolus.
• It is attached to the Organ of Corti, which has tiny hairs that
bend when sound waves reach them.
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Physiology of Hearing - Overview
• Auricle (outer ear) collects sound waves
• Eardrum vibrates
– slow vibration in response to low-pitched sounds
– rapid vibration in response to high-pitched sounds
• Ossicles (ear bones) then begin to vibrate.
• The oval window begins to vibrate.
• Fluid pressure waves move through the cochlea
• Small hairs in the inner ear begin to move, and these
attach to sensory neurons that generate nerve impulses
which travel to the brain (nerve VIII to the midbrain,
thalamus, and cerebral cortex where it is interpreted).
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Deafness
• Nerve deafness
– possibly nerve damage (CN VIII), but usually
damage to hair cells from antibiotics, high
pitched sounds, anticancer drugs, etc.
– person may fail to notice loss until they have
difficulty hearing frequencies of speech
• Conduction deafness
– perforated eardrum
– vibrations are not “conducted” to hair cells
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Physiology of Equilibrium (Balance)
• Static equilibrium
– maintain the position of the body (head) relative to the
force of gravity
• Dynamic equilibrium
– maintain body position (head) during sudden movement
of any type--rotation, deceleration or acceleration
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Otolithic Organs: Saccule and Utricle
• Two organs control static equilibrium:
– Saccule: in the inner ear
– Utricle: in the inner ear, larger
– They both contain hair cells and sensory receptors that
detect how our body is moving
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Membranous Semicircular Ducts
• The three semicircular ducts maintain dynamic equilibrium.
– anterior, posterior & horizontal ducts detect different
movements (combined 3-D sensitivity)
• The cristae (hair cells) in the semicircular ducts are the
primary sense organs of dynamic equilibrium.
• When the hairs bend, nerve impulses are sent to the brain
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Equilibrium Pathways in the CNS
Nerve VIII (the vestibulocochlear nerve) sends signals to
-the medulla
-the pons
-the cerebellum
The cerebellum coordinates these signals and helps
correct balance.
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AGING AND THE SPECIAL SENSES
• Age related changes in the eyes
– Presbyopia (can’t see close)
– Cataracts (film on the lenses)
– Weakening of the muscles that regulate the size of the
pupil (trouble seeing in the dark and bright light)
– Diseases such as age related macular disease, detached
retina, and glaucoma (all cause vision loss)
– Decrease in tear production (dry eyes)
– Sharpness of vision as well as depth and color
perception are reduced.
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AGING AND THE SPECIAL SENSES
• After age 50 some individuals experience loss of olfactory
and gustatory receptors (smell and taste)
• Age related changes in the ears
– Presbycusis – hearing loss due to damaged or loss of
hair cells in the organ of Corti
– Tinnitus (ringing in the ears) becomes more common
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DISORDERS: HOMEOSTATIC IMBALANCES
• Meniere’s syndrome is a malfunction of the inner ear that
may cause deafness and loss of equilibrium.
• Otitis media is an acute infection of the middle ear, primarily
by bacteria. It is characterized by pain, malaise, fever, and
reddening and outward bulging of the eardrum, which may
rupture unless prompt treatment is given. Children are more
susceptible than adults.
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DISORDERS: HOMEOSTATIC IMBALANCES
• A cataract is a loss of transparency of the lens that can lead
to blindness.
• Glaucoma is abnormally high intraocular pressure, due to a
buildup of aqueous humor inside the eyeball, which destroys
neurons of the retina. It is the second most common cause
of blindness (after cataracts), especially in the elderly.
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Special Senses Project
• 1. Choose a sense and sign-up for a presentation date.
• 2. Create something to share for your sense.
• Examples from last year:
• Smell: perfume manufacturing; smell + memories; examples of
the 10,000 smells
• Taste: food sampling; drawings of the tongue; 5 different tastes
• Vision: color-blind tests; color wheels; eyeball anatomy
• Hearing: binural beats; music for developing fetuses; music for
studying; affects of different decibels
• Equilibrium: balance testing, yoga poses for balance
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