PHYSIOLOGY

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KAAF UNIVERSITY COLLEGE
Physician Assistance studies.
The special senses
Dr Joseph Somuah Akuamoah
Senior lecturer

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The Special Senses
Contents
• Pain
• Eye vision
• Ear: hearing and equilibrium
• Chemical senses:- taste and smell

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1. PAIN
• Pain is primarily a protective mechanism meant to
bring to conscious awareness the fact that tissue
damage is occurring or about to occur.
• Furthermore, storage of painful experiences in
memory helps us avoid potentially harmful events in
the future.
• There are 3 categories of pain receptors or
nociceptors
 Mechanical nociceptors:- cutting, crushing, pinching
 Thermal nociceptors:- temperature extremes e.g heat
 Polymodal nociceptors:- all kinds of damaging stimuli

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2. EYE: VISION
• Eyes capture the patterns of illumination in the
environment as an “optical picture” on a layer of light
sensitive cells, the retina (camera captures an image
on a film)
• The image on the retina is transmitted through a
series of steps until it is finally perceived as a visual
likeness of the original image.

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EYE: Protective mechanisms..1
• The eyeball, except for the anterior (front) part is
sheltered by the bony socket in which it is positioned
• The eyelids act like shutters to protect the anterior
portion of the eye from insults.
• The eyelids close reflexly to cover the eye under
threatening circumstances such as dazzling light or
rapidly approaching objects or instances when the
surface of the eye is touched
• Frequent spontaneous blinking of eyelids helps
disperse the lubricating, cleansing, bactericidal tears.

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EYE: Protective mechanisms (2)
• Tears are produced continuously by the lacrimal
glands in the upper lateral corner under the eyelid.
• This fluid flows across the anterior surface of the eye
and drains into tiny canals in the corner of each eye
eventually emptying into the back of the nasal
passageway.
• The eyes are also equipped with protective
eyelashes, which trap fine, airborne debris such as
dust before it can fall into the eye.

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EYE: fluid filled sphere
• Each eye is a spherical, fluid filled structure enclosed
by three layers. From outermost to innermost these
are:-
1.The scleral cornea
2. The choroid/ciliary body/iris and
3. The retina.
• Most of the eyeball is covered by a tough outer layer
of connective tissue, the sclera, which forms the
visible white part of the eye.
• Anteriorly, the outer layer consists of the transparent
cornea, through which light rays pass into the interior
of the eye. Cornea and lens has no blood supply.

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EYE: fluid filled sphere…2
• The middle layer underneath the sclera is the highly
pigmented choroid, which contains many blood
vessels that nourish the retina.
• The choroid layer becomes specialized anteriorly to
form the ciliary body and iris.
• The innermost coat under the choroid is the retina,
which consists of an outer pigmented layer and an
inner nervous-tissue layer.
• The inner nervous tissue layer contains the rods and
cones, the photoreceptors that convert light energy
into nerve impulses.

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EYE: fluid filled sphere…3
• The interior of the eye consists of two fluid filled
cavities separated by an elliptical lens.
• The larger, posterior cavity between the lens and
retina is a semifluid jellylike substance, the vitreous
humor. Maintains the spherical shape of the eyeball.
• The anterior cavity between the cornea and lens
contains a clear, watery fluid, the aqueous humor.
• The fluid, produced by the ciliary body at 5ml/day,
carries nutrients for the cornea and lens and drains
into a canal at the edge of the cornea into the blood.
• Glaucoma (blockage of drainage canal) leads to
accumulation of aqueous fluid which causes
increased pressure on the eye.

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EYE: amount of light entering the eye
• Not all light passing through the cornea reaches the
light sensitive photoreceptors, because of the
presence of the iris.
• The pigment in the iris is responsible for eye color.
• The varied flecks, lines and others makes the iris
unique for each individual, hence the basis for
identification (instead of finger printing or DNA)
• The center of the iris through which light enters the
eye is the pupil. Contraction of the iris muscles
controls amount of light entering the eye

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EYE: diseases
• With advancing age the cells of the lens die and
become stiff. With loss of elasticity, the lens can no
longer assume the spherical shape required to
accommodate for near vision…Presbyopia, affect
most people by age 45 to 50 requiring them to use
corrective lenses for near vision (reading)
• The elastic fibers in the lens are transparent. These
fibers occasionally become opaque so that light rays
cannot pass through...Cataract
• Myopia, near sightedness (short sighted or lens is
too strong) or far sightedness…hyperopia are other
vision disorders (long sighted or lens is too weak).

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3. EAR: HEARING AND EQUILIBRIUM
• The ear consists of 3 parts; the external, middle
and inner ear.
• The external and middle ear transmit airborne sound
waves to the fluid -filled inner ear, amplifying the
sound energy in the process.
• The inner ear houses two different sensory systems:
the cochlea, which contains the receptors for
conversion of sound waves into nerve impulses,
making hearing possible, and the vestibular
apparatus, which is necessary for the sense of
equilibrium.

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EXTERNAL EAR
• The external ear consists of the pinna (ear), external
auditory meatus (ear canal) and tympanic
membrane (eardrum).
• The pinna (cartilaginous) collects sound waves and
channels them down the external auditory meatus.
Many species like dogs can cock their ears in the
direction of the sound to collect more sound waves
but human ears are immobile.
• The entrance to the ear canal is guarded by fine
hairs. The skin of the canal contains modified sweat
glands that produce cerumen (ear wax). The hairs
and earwax prevent particles from reaching the inner
ear to accumulate or injure the tympanic membrane.

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Tympanic membrane
• The tympanic membrane vibrates when struck by
sound waves.
• The inside of the eardrum is exposed to atmospheric
pressure via the eustachian tube, which connects
the middle ear to the pharynx (back of the throat).
• During rapid external pressure changes (e.g during
airflight) the ear drum bulges painfully as pressure
outside the ear changes whilst the pressure in the
middle ear remains unchanged.
• Opening the eustachian tube by yawning allows
the pressure on both sides of the tympanic
membrane to equalize, relieving the pressure
distortion as the eardrum pops back into place.

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Fluid movements in the inner ear
• The middle ear transfers the tympanic membrane
vibrations to the fluid of the inner
ear..mechanoreceptors..
• This transfer is facilitated by the sensitive 3 small
bones or ossicles (malleus, incus and stapes).
Malleus is attached to the tympanic membrane and
stapes attached to the oval window.
• The ossicular system amplifies the pressure of the
airborne sound waves.
• Tiny muscles in the middle ear contract reflexly in
response to loud sounds (over 70 dB), causing the
tympanic membrane to tighten and limit movements
of the ossicular chain.

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VESTIBULAR APPARATUS
• The inner ear has another specialized component,
the vestibular apparatus, which provides
information essential for the sense of equilibrium and
for coordinating head movements with eye and
postural movements.
• The vestibular apparatus detects changes in position
and motion of the head.
• All components of the vestibular apparatus contain
endolymph and are surrounded by perilymph.
• Fluid imbalances within the inner ear lead to
Meniere’s disease. It is characterized by severe
vertigo accompanied by pronounced ringing in the
ears (tinnitus) and some loss of hearing.

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DEAFNESS
• Loss of hearing (deafness) may be temporary or
permanent, partial or complete.
• Deafness can classified into two types – conductive
deafness and sensorineural deafness.
• Conductive deafness:- sound waves are not
adequately conducted through the external and
middle portions of the ear to set the fluids in the inner
ear in motion. Possible causes are blockage by ear
wax, rupture of ear drum, middle ear infections.
• In Sensorineural deafness, sound waves are
transmitted to the inner ear, but are not translated
into nerve endings for the brain to interpret. Defect
can lie with the auditory nerves or organ of corti.

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CHEMICAL SENSE: TASTE AND SMELL
• Unlike the eyes’ photoreceptors and the ears’
mechanoreceptors the receptors for taste and
smell are chemoreceptors.
• The sensations of taste and smell in association with
food intake influence the flow of digestive juices and
affects appetite.
• Furthermore, stimulation of taste or smell receptors
induces pleasurable or objectionable sensations and
signals the presence of something to seek (good-
tasting food) or to avoid (a potentially toxic or bad
tasting substance.
• Thus the chemical senses provides a quality-control
check point for substances available for ingestion.

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CHEMICAL SENSE: TASTE AND SMELL
• In lower animals, smell plays a major role in finding
direction, seeking prey or avoiding predators and in
sexual attraction to a male.
• The sense of smell is less sensitive in humans and
much less important in influencing our behavior
(although millions of dollars are spent on perfumes
and deodorants to make us smell better and be more
socially attractive)

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Mechanism of gustation (taste)
• The chemoreceptors for taste sensation are
packaged in taste buds, about 10,000 of which are
present in the oral cavity and throat, with the majority
on the upper surface of the tongue.
• A taste bud consists of about 50 taste receptor cells,
packaged like slices of an orange.
• Each taste bud has a small opening, the taste pore
through which fluids in the mouth touches the surface
of its receptor cells, taste receptor cells.
• We can discriminate among thousands of different
taste sensations, yet all tastes are varying
combinations of five primary tastes: salty, sour,
sweet, bitter and umami (meaty or savory taste).

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Mechanism of Olfactory (smell)
• The olfactory mucosa, a 3cm2 patch of mucosa in
the ceiling of the nasal cavity contains 3 cell types:-
olfactory receptor cells, supporting cells, basal cells.
• The axons of the olfactory receptor cells collectively
form the olfactory nerve.
• The supporting cells secrete mucus, which coats
the nasal passages.
• The basal cells are precursors for new olfactory
receptor cells which are replaced every 2 months.

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Questions
1. Explain why your sense of smell is reduced when you have a
cold, even though the cold virus doesn’t directly adversely
affect the olfactory receptor cells.
2. Kojo Mensah complained to his doctor of bouts of dizziness.
The doctor asked whether by dizziness he meant a syncope
or vertigo?
a) What is syncope
b) What is vertigo?
c) Why is this distinction important in differential diagnosis of his
condition
d) What are some possible causes of each of these symptoms?
3. Patients with certain nerve disorders are unable to feel pain.
Why is this disadvantageous?