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Chapter 8
Special Senses
Introduction:
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We are usually told that we have 5
senses that keep us in touch with what
is going on in the external
world:
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Touch, Taste, Smell, Sight, and Hearing
However, TOUCH is a mixture of the
general senses of temperature, pressure,
and receptors of the skin, muscles, and
joints.
Introduction:
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However, we still have 5 SPECIAL
SENSES. These include:
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Smell
Taste
Sight
Hearing
EQUILIBRIUM (Not Touch)
Introduction:
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These special senses contain SPECIAL
SENSE RECEPTORS.
Special sense receptors are either large,
complex sensory organs (eyes and ears) or
localized cluster of receptors (taste buds).
This chapter focuses on the functional
anatomy of each of the special sense organs
individually, but
keep in mind that sensory
inputs are overlapping.
The Eye and Vision:
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Vision is the special sense that has been
studied most.
Of all the sensory receptors in the body,
70% are in the eyes.
Vision is also the sense that requires
the most “learning.”
Anatomy of the Eye
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The adult eye is a sphere that measures
about 1 inch in diameter.
Only 1/6 of the eye’s surface can be seen
while the rest of it is enclosed and protected
by fat.
External and Accessory Structures:
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Extrinsic Eye Muscles
Eyelids
Conjunctiva
Lacrimal Apparatus
EXTRINSIC EYE MUSCLES
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(External Eye Muscles)
External eye muscles that are attached
to the outer surface of each eye
These muscles produce eye movements
and make it possible for the eyes to
follow a moving object
EYELIDS
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They meet at the medial and lateral
corners of the eye
They protect the eyes anteriorly.
Eyelashes - project from the border of
each eyelid
CONJUNCTIVA
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It is a delicate membrane that lines the
eyelids and covers part of the outer surface
of the eyeball
It secretes mucus, which helps to lubricate
the eyeball and keep it moist.
Inflammation of the conjunctiva is known as
conjunctivitis or pinkeye - infection caused
by bacteria or viruses and is highly
contagious.
LACRIMAL APPARATUS
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It consists of the lacrimal gland and many
ducts that drain the lacrimal secretions into
the nasal cavity
Lacrimal glands - they continually release a
salt solution (TEARS) onto the anterior
surface of the eyeball through several small
ducts
Lacrimal secretion also contains antibodies
and lysozyme - an enzyme that destroys
bacteria. It cleanses and protects the eye
surface as it moistens and lubricates it.
LACRIMAL APPARATUS
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If lacrimal secretion increases, tears spill over
the eyelids and fill the nasal cavities. This
causes congestion and the “sniffles.”
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This can occur if the eyes are irritated by foreign
objects or chemicals. The enhanced tearing acts
to wash away or dilute the irritating substance.
This can also occur when we are emotionally
upset. Some scientists say that the importance of
“emotional tears” is that it reduces stress.
Internal Structures
The Eyeball
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Eyeball - known as the eye itself - a hollow sphere
The wall of the eyeball is composed of 3 layers
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(1) Sclera - outermost layer that protects the eye
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(2) Choroid - middle layer
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This is the “white” of the eye
Consists of the cornea - central part of the sclera where light enters
the eye
Contains a dark pigment. This pigment prevents light from scattering
inside the eye.
Iris - contains a rounded opening called the pupil - where light passes
(3) Retina - innermost layer
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The retina contains millions of receptor cells called photoreceptors
A photoreceptor responds to light.
There are 2 types of photoreceptors: rods and cones
Rods and Cones:
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Rods - allow us to see in gray tones in
dim light, and provide for our peripheral
vision
Cones - allow us to see the details of
our world in color under bright light
Rods and Cones:
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There are 3 varieties of cones - Each type is most
sensitive to particular wavelengths of visible light
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One responds to blue light
One responds to green light
One responds to a range including both green and red light
**NOT THE 3 PRIMARY COLORS: YELLOW, RED, AND
BLUE
A lack of all 3 cone types results in total COLOR
BLINDNESS. But, a lack in just one cone is partial
color blindness. Color blindness occurs almost
exclusively in males.
Lens
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Focuses the light entering the eye on
the retina
Cataracts - a condition that results
from the hardening of the lens and
causes vision to become hazy.
Treatment = lens implant
Vision problems:
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Myopia - also known as NEARSIGHTEDNESS
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Hyperopia - also known as FARSIGHTEDNESS
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It occurs when the light rays from distant objects fail to
reach the retina and instead are focused in front of it
Distant objects appear blurry - able to see close up
It occurs when the light rays from distant objects are
focused BEHIND the retina.
Nearby objects appear blurry - able to see distances
Astigmatism - caused by unequal curvatures in
different parts of the cornea or lens
The Ear: Hearing and Balance
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Our hearing apparatus allows us to hear
an extraordinary range of sound, and it
also allows us to maintain our balance.
Anatomy of the Ear
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The ear is divided into three major areas:
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outer or external
middle
inner or internal
The outer and middle ear structures are
involved with HEARING ONLY.
The inner ear functions in BOTH
EQUILIBRIUM and HEARING.
Outer (External) Ear:
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Composed of the pinna and the external auditory
canal.
Pinna - also known as the auricle - the shell-shaped
structure surrounding the auditory canal opening.
External Auditory Canal - a short, narrow
chamber located in the temporal bone of the skull
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Composed of ceruminous glands - secretes a waxy yellow
substance called earwax or cerumen
Sound waves that enter the external auditory canal
eventually hit the tympanic membrane or the eardrum
and cause it to vibrate
Middle Ear:
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Also known as the tympanic cavity - a small, airfilled cavity within the temporal bone
Two openings in the middle ear:
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Oval Window
Round Window
Auditory Tube - links the middle ear with the
throat.
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Normally this tube is flat and closed, but swallowing or
yawning can open it briefly to equalize the pressure in the
middle ear cavity (forces air into auditory tube)
If the pressure is unequal, it causes hearing difficulty (voices
sound far away).
Middle Ear
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The tympanic cavity contains 3 small bones which are
known as the OSSICLES:
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Hammer - also known as the malleus
Anvil - also known as the incus
Stirrup - also know as the stapes
When the ear drum moves, the hammer moves with
it and transfers the vibration to the anvil. The anvil
passes it on to the stirrup, which presses on the oval
window of the inner ear. The movement of the oval
window sets the fluids of the inner ear into motion
which excites the hearing receptors.
Inner (Internal) Ear:
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3 subdivisions of the inner ear:
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Cochlea - resembles a snail shell and
houses the hearing receptors
Vestibule - located between the cochlea
and the semicircular canals
Semicircular Canals - appears as a maze
of circles
Mechanism of Hearing:
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Within the cochlea is the ORGAN OF
CORTI. It contains the hearing
receptors or HAIR CELLS.
The hair cells are stimulated by sound
vibrations
Hearing Deficits or Problems:
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DEAFNESS - hearing loss of any degree - from a
slight loss to a total inability to hear sound
CONDUCTION DEAFNESS - occurs when
something interferes with the conduction of sound
vibrations to the fluids of the inner ear
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Causes = buildup of earwax, a ruptured eardrum, or fusion
of the ossicles
SENSORINEURAL DEAFNESS - occurs when there
is degeneration or damage to the receptor cells in the
organ of Corti
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Causes = listening to excessively loud sounds
Chemical Senses: Taste and
Smell
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Chemical substances MUST be dissolved
in water to excite the receptors for
smell and taste.
Olfactory Receptors and the
Sense of Smell:
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OLFACTORY RECEPTORS - receptors
for the sense of smell and are located in
the superior part of the nasal cavity
Sniffing causes more air to flow across
the olfactory receptors which intensifies
the sense of smell
Taste Buds and the
Sense of Taste:
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TASTE BUDS - receptors for the sense of
taste and are scattered in the oral cavity
(most are located on the tongue)
There are 4 basic taste sensations:
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Sweet receptors - responds to sugars
Sour receptors - responds to acidic solutions
Bitter receptors - responds to alkaloids
Salty receptors - responds to metal ions
Location of the taste sensations - DRAW
LOCATIONS!!
Developmental Aspects of the
Special Senses
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Special sense organs are formed early in
embryonic development.
The eyes begin to develop by the fourth week
Maternal infections during the first five or six
weeks of pregnancy may cause visual
abnormalities as well as deafness in the
developing child.
An infant is farsighted and lacks color vision
and depth perception at birth.
Developmental Aspects of the
Special Senses
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The eye continues to grow and mature until
the eighth or ninth year of life but the lens
grows throughout life.
The newborn infant can hear sounds, but
initial responses are reflexive.
By the toddler stage, the child is listening
critically, begins to imitate sounds, and
language development begins.
Taste and smell are sharp at birth and
decrease in sensitivity after the age of 40.