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Special Senses
Chapter 8
The Special Senses
 Touch
 Taste
 Smell
 Sight
 Hearing
 Equilibrium (Balance)
Special Sense Receptors
 Receptors can be large complex sensory
organs such as the eye and ear
 Or they can be localized clusters of
receptors such as the taste buds and
olfactory system
The Eye and Vision
Anatomy of the Eye
 70% of your sensory receptors are in the eye.
 It is the sense that requires the most learning
 External and Accessory Structures:
 The eye is a sphere about 1 inch in diameter
 Only the anterior 1/6th of the eye is visible the
rest is cushioned with fat and bony structures
 Eyelid: Anterior protection of the eyes which
meet at the lateral and medial canthus
(corner) of the eye
 Eyelashes: projections on the border of the
eyelid
Anatomy of the Eye
 External & Accessory Structures:
 Meibomian glands: modified sebaceous glands on the
eyelid edges. They produce an oily secretion which
lubricates the eye.
 Ciliary glands: modified sweat glands which lie
between the eyelashes
 Conjunctiva: delicate membrane lining the eyelid and
covering part of the outer surface of the eyeball.
Secretes mucus which helps to lubricate the eye and
keep it moist.
 Conjunctivitis: inflammation of the conjunctiva. Eyes
become red, irritated and when infected becomes
pinkeye – caused by a bacteria or virus and is highly
contagious.
Anatomy of the Eye
 Lacrimal Apparatus: Consists of lacrimal gland and
ducts which drain the lacrimal sections of the nasal
cavity.
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Lacrimal Glands: Located above the lateral end of each
eye. They release a diluted salt solution (tears) onto the
anterior surface of the eye through ducts
Tears: contain antibodies, lysozomes, and enzymes
which destroy bacteria. They cleanse and protect the
eye surface as well as moisturize and lubricate it.
Lacrimal canals: Tiny canals through which the tears
drain into. When the lacrimal mucosa is inflamed due to
infection they swell and do not allow proper drainage of
the tears from the eye surface causing watery eyes.
Anatomy of the Eye
 Lacrimal Apparatus:
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Lacrimal sac: another stop for the tears
before they drain into the …
Nasolacrimal duct: empties tears into the
nasal cavity. When tears spill over the
eyelids they fill the nasal cavity and cause
congestion. This can occur because of
irritation to the eyes or when we are
emotionally upset.
Anatomy of the Eye
 Extrinsic/External Eye Muscles:
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Attach to the outer surface of the eye
The muscles provide gross movement of
the eye and make it possible to follow
moving objects.
Anatomy of the Eye
 Internal Structures: The Eyeball
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Eyeball: Hollow sphere that’s wall contain tunics and its
interior is filled with fluids called humors which maintain its
shape.
Tunics:

Sclera:
 Outermost tunic
 Made up of thick, white, connective tissue
 It is sometimes referred to as the fibrous tunic and can be seen
anteriorly as the white of the eye.

Cornea:
 Central anterior portion of the sclera
 Is a transparent window through which light enters the eye
 It has many nerve endings which when it is touched blinking and
increased tears begin
 It has a great ability to heal itself and is the only tissue inn the
body that can be transplanted without worry of rejection.
Anatomy of the Eye
 Internal Structures: The Eyeball
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Choroid:
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Middle tunic of the eye
Is blood rich and contains a dark pigment
which prevents light from scattering inside
the eye
Anteriorly it forms the ciliary body
Cilialry Body:
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Anterior formation of the choroid
Attaches to the lens and the iris
Anatomy of the Eye
 Internal Structures: The Eyeball
 Lens: Main focusing apparatus of the eye
 Cataracts:
 As we age the lens becomes increasingly hard and
opaque causing vision to become hazy and eventually
causing blindness
 Special glasses or removal of the lens and putting a new
one in are the most common treatments.

Aqueous Humor:
 Is Anterior to the lens and contains a clear water-like fluid
similar to blood plasma
 It is continually secreted by a special area of the choroids
 It helps with internal pressure of the eye and provides
nutrients for the lens and cornea which have NO blood
supply
Anatomy of the Eye
 Interior Structures: The Eyeball
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Vitreous Humor:
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Canal of Schlemm:
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Gel-like fluid which is contained within the posterior segment.
Helps prevent the eyeball from collapsing inward by
reinforcing the internal aspect
Reabsorbs the aqueous humor and is located between the
sclera and cornea
Glaucoma
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Result of blocked drainage of the aqueous humor which
compresses the retinal and optic nerve
Pain and possible blindness will occur
Common in the elderly and tends to occur without any obvious
signs
Signs that develop are: halos around lights, headaches, and
blurred vision
Treated with eyedrops which increases the rate of drainage of
aqueous humor
Anatomy of the Eye
 Interior Structures: The Eyeball
 Iris:
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Pigment color of the eye which contains a
rounded opening allowing light to enter
Pupil:
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Rounded opening of the iris where the light
enters.
Made of smooth muscle fibers which regulate
the amount of light entering the eye so that you
can see clearly in the available light
Close vision/Bright light = pupils contract
Distant vision/dim light = pupils dilate
Anatomy of the Eye
 Interior Structures: The Eyeball
 Retina:
 Inner most tunic of the eye
 Extends anteriorly to the ciliary body
 Contains receptor cells called rods and cones
 Rods & Cones:
 Photoreceptors which respond to light
 Electrical signals go to the photoreceptors then to
the retina through the optic nerve and then are
transmitted to the optic cortex to create what we see
(VISION)
 Rods are most dense and on the periphery and
decrease in number as you get closer to the center
of the retina. They allow us to see gray tones in dim
light and provide peripheral vision.
Anatomy of the Eye
 Interior Structures: The Eyeball
 Rods & Cones con’t:
 Night Blindness:
 Result of damage or injury to the rods
 Can be caused by a prolonged deficiency of vitamin A
which will eventually result in total deterioration of the
retina
 Supplements can be taken to restore the damage before
degenerative changes occur.

Cones:
 Discriminatory receptors which allow us to see details in
color under bright light
 They are found near the center of the retina and decrease
in number as you get closer to the outer edge.
Anatomy of the Eye
 Internal Structures: The Eyeball
 Optic Nerve:
 Contains ganglionic cells and is the point where
senses leave the eye.
 Optic Disk/Blind Spot:
 When light from an object focuses on the optic disk
then the object disappears from sight and we can no
longer see it.
 Fovea Centralis:
 Lies lateral to the blind spot and is a tiny pit that
contains only cones
 It is the area of greatest visual acuity (sharpest vision)
and when we are viewing things critically this is the
area we use.
Anatomy of the Eye
 Three Types of Cones:
 Responds to Blue Light
 Responds to Green Light
 Responds to a range of light including green and
red wavelengths of light.
 These are most commonly referred to as the
cones because they are the only ones that can
respond to a red light
When all three cones are stimulated we see white.
If you were to shine a red light into one eye and a
green light into the other eye you will see yellow.
This allows us to see things in multiple colors.
Anatomy of the Eye
 Color Blindness:
 Lack of all three types of cones
 Lack of one type of cone will lead to partial color
blindness
 Most common is the lack of red or green receptors
which leads to two types of color blindness:
 Red and green are seen as the same color = you see
only red or only green
 Color blindness is a sex linked condition and occurs
in males because genes that regulate color are linked
to the X chromosome.
 Ophthalmoscope:
 Instrument used to illuminate the interior of the eye
and allows the retina, optic disk and internal blood
vessels to be viewed and examined.
Pathway of Light through the Eye and
Light Refraction
 When light passes from one substance to another
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substance with a different density the rays are
bent/refracted.
Light enters the cornea, then through the aqueous
humor, then through the lens and into the vitreous
humor = a bend in the rays
The lens can change the bend in the light by
changing its shape so that incoming light can
properly focus in the retina
The more concave the lens is the more bend there
will be in the light
The less concave the lens is the less bend there is in
the rays
Pathway of Light through the Eye and
Light Refraction
 Light from a distance greater than 20 ft away does not require
as much bend in the rays meaning the lens can be flatter or
less concave
 Light from a closer object scatters and the lens must bulge,
causing it to become more concave, in order to make close
vision possible.
 When the ciliary body contracts the lens becomes more convex
(rounded) = Accommodation: the ability to focus on close
objects
 Real Image
 The result of light bending activities of the lens from the
retina
 The real image is reverse from left to right and upside down
and smaller than the actual object
 Vision problems occur when the lens is too strong or too
weak to make the adjustment to the real image.
Pathway of Light through the Eye and
Light Refraction
 Emmetropia:
 Lens which focuses images correctly on the retina
 Myopia:
 Nearsightedness
 Occurs when parallel rays from distant objects fail
to reach the retina and are focused in front of it.
 The distant objects appear blurry and nearby
objects are in focus because the lens
accommodates
 Can result from an eye that is too long or a lens is
too strong or a cornea that is too curved.
 Correction requires concave lenses
Pathway of Light through the Eye and
Light Refraction
 Hyperopia:
Farsightedness
 Occurs when the parallel light rays from distant objects are
focused behind the retina
 Results from an eye that is too short or a lazy lens
 People see distant objects clearly but cannot focus on
objects that are nearby
 Correction requires a convex lens to converge the light
rays before they enter the eye
 Astigmatism:
 Unequal curvatures in different parts of the cornea or lens.
 Blurry images occur because points of light are focused as
lines on the retina
 Special cylindrical lenses or contacts are used to correct
the problem
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Visual Fields and Visual Pathways to
the Brain
 Axons which carry impulses from the retina are
bundled together at the posterior aspect of the eye
and arise from the back of the eye at the optic nerve.
 Optic Chiasma:
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Fibers from the medial side of each eye cross over to
the opposite side.
 Optic Tracts:
 Fiber tracts that result from the optic chiasma.
 Each tract contains fibers from the lateral side of the
eye on the same side and the medial side of the
opposite eye.
 They synapse with neurons in the thalamus
Visual Fields and Visual Pathways to
the Brain
 Optic Radiation:
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Synapse of neurons in the thalamus and
axons.
Controls the occipital lobe of the brain
They then synapse with cortical cells and
visual interpretation (vision) occurs.
 Binocular vision:
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Two Eye vision which provides for depth
perception and requires input from both
eyes.
Visual Fields and Visual Pathways to
the Brain
 Hemianopia:
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Loss of the same side of the visual field of
both eyes.
Result of damage to the visual cortex on
one side only.
The person is unable to see things past the
middle of his/her visual field on either the
right or left side.
Eye Reflexes
 Both internal and external eye muscles are
necessary for proper function of the eye
 Internal muscles are controlled by the autonomic
nervous system
 External muscles control eye movement and
make it possible to follow a moving object.
 Convergence:
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Reflexive movement of the eye medially when we
view close objects.
Uses the extrinsic eye muscles
Eye Reflexes
 Photopupillary reflex:
 When eyes are exposed to bright light the pupils
will immediately constrict
 This is a protective reflex that prevents excessively
bright light from damaging the delicate
photoreceptors within the eye.
 Accommodation papillary reflex:
 Pupil constriction when viewing close objects.
 Provides more acute vision
 Reading requires the use of both reflexes.
The EAR
Hearing and Balance
Hearing
 Fluid must be “stirred” to stimulate the
receptors located within the ear.
 The sound vibrations move the fluid to
stimulate the receptors
 Gross movement of the head disturbs the
fluids surrounding the balance organs
 Mechanoreceptors then respond to these
physical forces allowing us to hear while
moving.
Anatomy of the Ear
 Outer/External Ear:
 Composed of the pinna and external
auditory canal and is responsible for
hearing only.
 Pinna:
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Also called the auricle
Is the “ear” the structure that surrounds the
auditory canal opening.
External Auditory canal:
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Short narrow chamber carved into the
temporal bone of the skull
Anatomy of the Ear
 Outer/External Ear:
 Ceruminous glands:
 Skin lined walls of the external auditory canal
which secretes a waxy, yellow substance
 Cerumen:
 Earwax
 Substance secreted by the ceruminous glands
 Tympanic membrane/Eardrum:
 Eardrum
 Soundwaves enter and cause it to vibrate
 The eardrum separates the outer ear from the
middle ear.
Anatomy of the Ear
 Middle Ear:
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Tympanic Cavity:
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Small air filled cavity within the temporal bone
It is surrounded by the eardrum laterally and
the bony wall medially which has two
openings.
Oval Window: one of the openings
Round Window: the other opening which is
inferior and covered by a membrane.
Anatomy of the Ear
 Middle Ear:
 Auditory Tube:
 Runs obliquely downward to link the middle ear cavity
with the throat and mucosa lining the two are
continuous
 Normally it is flattened and closed but swallowing or
yawning can open it briefly to equalize pressure in the
middle ear
 The eardrum does not vibrate freely unless pressure
is equal on both sides
 If pressure is unequal the eardrum bulges and causes
hearing difficulty
 Otitis Media:
 Inflammation of the middle ear.
 Common result of a sore throat – the eardrum bulges
and becomes inflamed.
Anatomy of the Ear
 Middle Ear:
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Myringotomy:
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Cutting of the eardrum to release pressure
and inserting a tube into it to allow pus
formed in the middle ear to drain into the
outer ear.
Ossicles:
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Three of the smallest bones in the body which
are part of the tympanic cavity.
They transmit vibrations of the eardrum to the
fluids of the inner ear.
Anatomy of the Ear
 Ossicles con’t:
 Hammer/Malleus:
 Named for its hammer like appearance
 When eardrum vibrates the hammer moves the
vibration to the anvil.
 Anvil/Incus:
 Moves the vibration into the stirrup
 Stirrup/Stapes:
 Passes the vibration to the oval window of the
inner ear.
 The movement of the window sets the fluid of
thinner ear in motion.
Anatomy of the Ear
 Inner Ear: Only part of the ear responsible for
hearing and balance.
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Osseus/Labrynth:
 Maze of bony chambers in the ear
 Located deep within the temporal bone and just
behind the eye socket.
 Cochlea:
 Subdivision of the bony labyrinth
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Vestibular:
 Located between the cochlea and the semicircular
canal
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Semicircular canal:
 Canals within the bony labyrinth
Anatomy of the Ear
 Inner Ear:
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Perilymph:
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Membranous Labyrinth:
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Plasma like fluid in the bony labyrinth
System of membrane sacs within the
perilymph which follow the shape of the bony
labyrinth
Endolymph:
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Thick fluid contained within the membranous
labyrinth.
Mechanism of Hearing
 Organ of Corti:
 Contained within the cochlea
 The organ of corti contains tiny hearing
receptors called hair cells.
 Basilar Membrane:
 Contains receptor cells which are
stimulated when the hair cells are bent or
tweaked by the movement of the …
 Tectorial Membrane:
 Gel like fluid which lies over the hair cells.
Mechanism of Hearing
 Cochlear Nerve:
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Division of the vestibulocochlear nerve
where interpretation of sound occurs.
Since sounds reach each ear at different
times we hear in “stereo” and are able to
determine where sound is coming from
Hearing is the last sense to leave our
awareness and the first to return.
Mechanism of Equilibrium
 Equilibrium is a mechanism of response
which often we do not even realize its
occurring.
 Vestibular apparatus:
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The equilibrium receptors of the inner ear.
They are divided into two arms:
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Static Equilibrium
Dynamic Equilibrium
Mechanism of Equilibrium
 Static Equilibrium:
 Maculae:
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Receptors within the membrane sacs of the
vestibulae
They report on the position of the head with
the pull of gravity when the head is not
moving
They help us to stand up straight.
Otolithic Membrane:
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Patch of receptor hair cells with hairs
embedded into it.
Mechanism of Equilibrium
 Static Equilibrium con’t:
 Otoliths:
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Tiny stones made up of calcium salts and
when our head moves they roll in response to
the changes in gravity
This creates a pull on the gell which slides
over the hair cells causing them to bend
Once the hair cells are activated by this
impulses are sent along the vestibular nerve.
Vestibular Nerve:
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Informs the brain of the position of the head in
space.
Mechanism of Equilibrium
 Dynamic Equilibrium:
 Receptors are found in the semicircular canals and
respond to angulatory or rotary movements of the
head. These receptors work in all three planes so
that any way we move they will detect it.
 Crista Ampularis: receptor region consisting of tufts
of hair cells covered with a gelatin like cap called
the …
 Capula: When your head moves in an angular
direction the endolymph in the canal moves in the
opposite direction pushing the capula into a
direction that is opposite of your body movement.
This stimulates the hair cells and transmits
impulses up the vestibular nerve.
Mechanism of Equilibrium
 Dynamic Equilibrium con’t:
 Vestibular Nerve: transmits impulses to the
cerebellum.
 When angular motion stops the flow is in
the opposite direction reversing the
capula’s movement and causes the
reversed motion you feel when stopping
suddenly.
 The static and dynamic equilibrium work
together along with sight and sound to
keep us balanced.
Hearing & Equilibrium Deficits
 Deafness:
 Hearing loss of any degree – from slight to total
 Conduction Deafness:
 Results when something interferes with the conduction
of sound vibrations to the fluids of the inner ear.
 Buildup of earwax can be the cause of the fusion of the
ossicles
 A ruptured eardrum or otitis media may also contribute.
 The person will be able to hear by bone conduction
 In most cases this can be fixed by the use of a hearing
aid.
Hearing & Equilibrium Deficits
 Sensorineural Deafness:
 Occurs when there is degeneration or damage to the
receptor cells in the organ of corti, the cochlear nerve
or neurons of the auditory cortex
 These people cannot hear by any conduction route.
 Meniere’s Syndrome:
 Cause is not fully known
 Suspected causes are arteriosclerosis, degeneration of
the cranial nerve 8 and increased pressure of the inner
ear fluids.
 Progressive deafness occurs and individuals
experience nausea or vertigo which is so bad that they
are unable to stand up without extreme discomfort.
 Anti-motion sickness medications may help with this
discomfort.