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Special Senses
Chapter 8
Special Senses
 The four special senses are smell, taste,
sight, and hearing
Eye and Vision
 Eye ball is approx 1in diameter.
 Protected by fat and the orbit of the skull
 Anteriorly protected by the eyelids, lined in
eyelashes.
 A membrane, conjunctiva, lines the eyelids and
covers part of the outer surface of the eye.
Can get infected with bacteria or virus,
conjunctivitis aka pink eye
 Both the eyelid and conjunctiva produces
mucus to cover, lubricate, and protect the eye.
Eye and Vision
 Lacrimal apparatus consists of the
lacrimal gland and the various ducts.
 The lacrimal gland produces salty tears
filled with antibodies and enzymes used
to lubricate and disinfect the surface of
the eye.
 This liquid is emptied into the nose
through a duct.
 stop
Eye Muscles
 There are six muscles attached to the
eye, you need to know four and their
movements.
 Lateral Rectus – moves eye laterally
 Medial Rectus – moves eye medially
 Superior Rectus – Elevates eye
 Inferior Rectus – lowers eye
Eye: Internal Structure
 Hollow sphere, made up of three layers
called tunics, filled with liquid called
humors.
 The outside tunic of the eye is called the
Sclera. White connective tissue, makes
up the “whites” of the eyes.
 The central anterior portion is modified to
be crystal clear, called the cornea
Eye: Internal Structure
 The second tunic/layer is called the vascular
tunic. Has three sections.
 Choroid – posterior, lots of blood vessels
 Ciliary Body – smooth muscle that holds the
lens in place
 Iris – most anterior, pigmented, smooth muscle
that contracts and relaxes to allow different
amounts of light in. Reflex action
Eye: Internal Anatomy
 The innermost tunic is the retina.
 It covers the posterior area of the eye up to the
ciliary body.
 Contains rods and cones called photoreceptors
that respond to light.
 Light stimulus starts a neuron chain going
through the optic nerve and out to the brain.
 The area of the retina that the optic nerve
connects to is called our blind spot.
Rods Vs. Cones
 Rods see grey tones in dim light, usually found
on the edge of the retina, used for peripheral
vision
 Cones see color. Found most abundant in the
center of the retina and become less dense
toward the edge.
 Color blindness results from a malfunction or
lack of certain cones genetically. Sex linked.
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Lens
 Light entering the pupil has to be focused on
the retina, this is the job of the lens.
 The lens and ciliary body split the eye into
anterior (aqueous humor) and posterior
(vitreous humor).
 Cataract – clouding of the lens
 Glaucoma – condition where aqueous humor
pressure builds and could cause blindness.
Focusing light
 The eye is set up to focus light that is 20ft
away.
 When an object gets closer or further
away, the ciliary body (smooth muscle)
changes the shape of the lens to refocus
light from a different distance.
I Need Glasses
 Emmetropia – perfect focus
 Myopia – nearsighted, light
focuses before the retina,
concave lens corrects
 Hyperopia – farsighted, light
focuses after the retina.
Convex lens corrects.
 Astigmatism – misshapen
eye, light is not focused at a
point, but scattered.
Taste and Smell
 Called Chemoreceptors – responds to
chemicals.
 Olfactory, smell, receptors occupy a
postage stamp size area at the top of our
nasal cavity.
 Olfactory pathways are tied into the
limbic system of our brains, smells are
very much a part of our memories.
 stop
Taste
 Taste buds are specific receptors of
chemicals in the mouth.
 Most reside on the tongue.
 Dorsal tongue is covered in Papillae
allowing for more surface area.
 Five basic taste sensations. Sweet, sour,
bitter, salty, and umami.
Taste
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Sweet – need for carbs
Sour – vitamin C
Bitter – defense from spoiled foods
Salty – need for minerals
Umami – intake of proteins.
Spicy foods excite pain receptors in the
mouth. Fact – humans are the only
animal to eat spicy foods by choice.
Ear Anatomy
 The Ear is in charge of hearing as well as
balance.
 Ear is divided into three major areas:
external, middle, and inner.
Outer Ear
 Composed of the Pinna
(shell shaped structure)
and the external acoustic
meatus (ear canal)
 Ear Canal is 1” by ¼”
and is carved out of the
temporal bone.
 Lined with glands that
secrete wax for
protection.
 Outer ear ends at the
Eardrum (tympanic
Membrane)
Middle Ear
 AKA tympanic Cavity – air filled cavity in
the temporal bone.
 Sound collected by the outer ear vibrates
the tympanic membrane.
 The ossicles (bones of the ear) vibrate in
turn, sending vibration to the inner ear.
 In order, Malleus (hammer), Incus (anvil,
and stapes (stirrup).
Middle Ear
 Eustachian tube
connects middle ear to
throat.
 Usually closed, opens
during yawns and
swallowing.
 Equalizes pressure on
both sides of the
eardrum.
 Infection can spread
from back of throat to
middle ear.
stop
Inner Ear
 Inner ear is in charge of turning vibration
into electrochemical signal.
 Also in charge of balance.
Inner Ear: Hearing
 The cochlea is the bony labyrinth where vibration turns
to neuron impulse.
 The snail shaped cochlea is filled with perilymph,
plasmalike fluid, and also lined with hair cells.
 As the stepes of the middle ear vibrates, this liquid is
disturbed in the cochlea. This disturbance stimulates
the hair cells which start a nerve impulse to the brain.
 The amount and location of the disturbed hair cells
distinguishes tune and volume. Further down the
cochlea, the deeper the sound.
Inner Ear: Balance
 Above the cochlea is the vestibule and
the semicircular canals. These both help
with balance.
Inner Ear: Balance
 Both the vestibule and semicircular canals
have hair cells that are covered in a gel.
 When the head changes position, the gel is
pulled based on gravity. This pulls the hairs,
starting a neuron pathway to the brain.
 Vestibule controls static equilibrium under
water with your eyes closed.
 Semicircular canals control dynamic
equilibrium. Dizziness and sea sickness.
Defects in Hearing
 Hearing lose can be caused by a number of
problems. Ear wax build up, fusion of the
ossicles, etc.
 Most hearing lose is caused by exposure to
excessively loud noise/music. 120-150
decibels
 Talking is ~60db, city traffic is ~80.
 Hair cells in the cochlea are knocked over and
do not recover. Ringing in the ear, tinnitus.
Tears
 When tears overflow the eyelids these
ducts empty a lot of liquid into the nasal
area causing the sniffles.
 Tears are caused by irritants or
emotional situations.
 Clears off dust or irritants
 Emotional tears are not well understood
but seem to help lower stress.