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Special Senses
I Senses of the body
A. General sensory receptors are found all over the
body.
1. Proprioceptors of muscles and joints.
2. Pressure, pain, touch, and temperature
receptors of the skin.
3. Special senses include the following with
special receptors/organs:
Hearing
Taste
Vision
Smell
Equilibrium
II The eye and vision (70% of all sensory receptors are
in the eye)
A. Structures of the eye
1. Eyelashes – Prevent debris from entering eye
2. Eyelids – Cover and protect the eye
3. Conjunctiva – Lines the eyelid and part of
the surface of the eyeball. Secretes mucus for
lubrication.
4. Lacrimal apparatus – Glands secrete the
dilute salt solution with enzymes and
antibodies, wash across the eye to the canals
(in the corner of the eye) then the sac and
finally the nasolacrimal duct.
5. Extrinsic eye muscles – Six muscles
attached to the outer eye surface,
responsible for gross motor movement of the
eyeball.
6. Meibomian glands – specialized sebaceous
glands to lubricate eye.
7. Ciliary glands – modified sweat glands
between eyelashes for lubrication.
8. Eye ball – hollow sphere made from 3 tunics (coats) and is filled
with a liquid (humors). Divided into two chambers by the lens.
a. Tunics
i. Sclera – outermost layer, thick and white with a transparent
center called the cornea (where light enters the eye).
ii. Choroid – blood rich nutritive tunic with a dark pigment
(prevents light from scattering inside the eye). Forms the
ciliary body and the iris (smooth muscles). Iris has a rounded
opening – pupil – which regulates the amount of light
entering the eye.
1. Ciliary body(contains the ciliary muscle – attaches to
the lens to modify shape,
2. Iris – contracts and relaxes to control pupil size.
Sheep eye
iii. Retina – Sensory tunic;
location of cones (detects
color) and rods (low levels of
light, grays and peripheral
vision). These receptor cells
lead to sensory neurons that
converge to form the optic
nerve.
1. Blind spot – on the
retina where the optic
nerve is found (no sensory
receptors so no vision at
this point)
2. Fovea centralis – area
lateral to each blind spot
where the highest
concentration of sensory
receptors is located (area
of highest visual
acuity:reading, watching
TV, driving)
-center of the macula)
b. Humors
i. Aqueous humor – in the anterior
chamber of the eye. Helps maintain
intraocular pressure and provides
nutrients for lens and cornea (they lack
blood supply). Secreted by the ciliary
body and contantly replenished.
ii. Vitreous humor – in the posterior
chamber of the eye. Gel like liquid used
to prevent collapse of the eye. –Is
stagnant and not replenished
If the drainage angle of the
eye is blocked, excess fluid
cannot flow out of the eye
(right).
Normally drains through
Schlemm’s canal.
B. Physiology of vision
1. Light waves travel towards the eye and enter through
the pupil.
2. The movement of the light waves through the structures
of the eye, bend the light rays (hopefully to focus directly
on the retina). This is called refraction.
3. Depending on the distance of the object you are looking
at, the lens must change shape (with contraction of
ciliary body) to focus light waves on the retina.The
ability to change and focus on objects at different
distances is called accommodation. The image is also
inverted as the light waves travel through the lens.
eye animation
Notice that the farther the
image, the smaller the
image on the retina.
4.
5.
6.
Once the receptors have been stimulated, thereby
changing the shape of pigment proteins in the cell, this
change causes electrical changes creating the nerve
impulse (Action potential!!).
The optic nerves from both eyes cross at the optic
chiasma, which results in the left side of the brain
receiving visual information from the right eye and vice
versa.
Once the impulse has reached the occipital lobe, the
visual information will be interpreted and understood by
known visual information stored in that area of the brain.
Ex. recognizing someone you have met before.
7. Eye muscle fatique – Internal and External muscles
a. External muscles- responsible for convergence –
reflexive movement of the eyes medially (toward the
center) when you view something close. Strabismus
“crossed “ eyes is caused by unequal pulls by these
muscles. (This is not the same thing as “lazy eye”, it
is associated with strabismus and can result in
decreased vision in the affected eye.) The muscles
must be retrained. (If the eyes cannot be retrained a
patient may choose to have surgery to realign the
eyes)
b. Internal muscles – ciliary body constrict focusing lens
and iris contracts reducing the size of the pupil.
c. Over time these muscles can become fatigued such
as when you read for long periods of time. Take a
break and look off in the distance to relax those
muscles.
Cataracts- clouding of the lens of the eye
Glaucoma- increased intraocular pressure
III
Hearing and Equilibrium
A. Mechanoreceptors are used which respond to physical
forces (movement of liquid) to convey information on
hearing and equilibrium. There are two different systems in
the ear, one for hearing and one for equilibrium.
B. Hearing
1. Anatomy of the ear
a. External ear
i. Pinna or Auricle – external auditory
meatus, gathers sound waves and
directs them down the canal.
ii. External canal – (about 1 inch), conduct
waves through the temporal bone.
iii. Ceruminous glands – secrete wax,
contains antibodies and filters debris,
also a deterrent for mosquitoes
iv. Tympanic membrane – vibrates when
sound waves strike it and transmit
vibrations to middle ear.
b. Middle ear (about the size of a dime)
i. Auditory tube – links middle ear
with throat and helps to equalize
pressure in middle ear. Typically is
closed.
ii. Oval window – connects the stirrup
(stapes) to the inner ear on
cochlea.
iii. Ossicles
1. Hammer / malleus –
transmits vibrations of the
eardrum to the next ossicle.
2. Anvil / incus – second ossicle,
transmits vibrations.
3. Stirrup / stapes – transmits
vibrations to cochlea.
c. Inner ear (bony
chamber/labyrinth) –just
behind eye orb.
i. Cochlea (contains the
organ of corti) – location
of hair cells (hearing
receptors). Receives
vibrations from oval
window. Made up of
vestibule and semicircular
canals. Contains a fluid
that transmits vibrations to
auditory nerves
2. Pathway and mechanism of
hearing – the journey of
the vibration
3. Hearing Deficit Causes
a. Conduction deafness – There is an
obstruction to the conduction of
vibrations on the way to the organ of
corti. Causes include ear infection,
build up of wax in canal, fusion of
ossicles, and ruptured eardrum.
Hearing aids can help by sending the
vibrations via the bone.
b. Sensorineural deafness –
Degeneration or damage to the
receptor cells. Causes include over
exposure to very loud sounds. No
treatment.
C. Equilibrium
1. Monitored by the vestibular apparatus (includes
the vestibule and semicircular canals of inner
ear). Help control balance with the cerebellum!
Where are you in space or deep in the water?
2. Two different functions:
a. Static equilibrium – receptors in vestibule
monitor the position of the head with respect
to gravity when you are not moving. (i.e. –
sitting in your chair and you turn your head to
the side). When your head moves, the gel
moves and triggers the hair cells sending the
message to the brain via the 8th cranial nerve
(vestibular).
b. Dynamic equilibrium – Receptors are in
semicircular canals and respond to the
circular movements of the head (ex.
twirling on the dance floor, the movements of
your body while on a boat in rough seas). The
endolymph(fluid) within the semicircular
canals moves opposite the direction that
your body is moving, this motion shifts the
cupula (gel like material) thereby stimulating
the receptor hairs. Once stimulated the
information about the movement of the
head and body is transmitted to the brain via
the vestibular nerve.
IV
Chemical Senses – These receptors respond to specific
chemicals in solution
A. Olfactory (smell)
1. Olfactory receptors are located in the roof of each
nasal cavity, there are 1000’s that each detect a
different chemical odor. These receptors are at
the top of the nasal cavity, which is why “sniffing”
intensified the smell, because more air is forced
to the top of the cavity, stimulating the receptors.
2. The receptor cells have hairs on them (similar to
equilibrium and hearing). They are bathed in a
thick layer of mucus. As the olfactory chemicals
diffuse through the mucus, they stimulate the
receptors which send the message via the
olfactory filaments to the olfactory nerve (CN I).
3. This pathway goes directly through the limbic
system (emotional center), therefore there tends
to be strong emotional ties to smells. These
receptors, like others, can adapt to stimuli that
are constant, which is why women or men don’t
smell their own cologne or perfume after a period
of time.
V Sense of Taste
A. Receptors are called taste buds and
there are over 10,000 located on the
tongue. Very few are located on your soft
palate and the inside of your cheek.
1. There are 4 basic taste sensations:
a. Sweet – respond to sugar,
saccharine, and some amino
acids.
b. Sour – respond to H+ ions
(acids)
c. Bitter – respond to alkaloids
(basic)
d. Salty – respond to metal ions
2. The regions of the tongue don’t
truly hold true, because
the
receptors are pretty well evenly
distributed over the surface of the
tongue.
B. Sense of taste and certain cravings
help to satisfy homeostatic
imbalances. When you crave
carbohydrates or orange juice it
can be your body’s way of
communicating to your
consciousness to fulfill these
deficiencies.