Download Special Senses Lab

Special Senses
In this exercise you will conduct a brief review of
eye and ear anatomy. You will also work with a
partner to perform multiple special senses tests.
The lab consists of three special senses;
1) Vision
2) Hearing
3) Equilibrium
Vision
• The axons of the ganglion cells of the
retina converge as the optic nerve.
• There are no photoreceptors at the
optic disc which is an area called the
blind spot.
• At the optic chiasm, some of the axons
of the optic nerves cross over to the
opposite side and extend as optic
tracts.
• The optic tract synapse with neurons
in the thalamus and end in the visual
cortex in the occipital lobe.
Vision
• Each eye receives
light from both left
and right hemifields.
• The yellow represents
the left hemifield and
the blue represents
the right hemifield.
Experimental Procedures for Vision
Blind Spot Test
• This is a test for the presence of the blind spot or optic
disc.
• To test the right eye, close the left eye and stare at the
plus sign with the right eye. You should be aware of the
dot in the periphery of your vision.
Vision
• When light rays pass from one medium to another,
their speed of transmission changes and the rays are
bent, or refracted.
• Therefore, the light rays are refracted when they
encounter the cornea, lens, and vitreous humor of
the eye.
• The lens refractive index, or bending power, can be
varied based on the lens shape. The greater the lens
convexity, or bulge, the greater the light will refract.
Vision
Ciliary muscles
Suspensory ligaments
Vision
• The lens becomes more convex when focusing
light from a near object onto the retina.
• This occurs when the ciliary muscle contract,
decreasing the tension on the suspensory
ligament attached to the lens and allowing the
lens to “plump up”. Making these adjustments is
referred to as, accommodation.
Accommodation
Eye Movement
• The intrinsic muscles of the
eyes include the ciliary
muscles, and muscles of the
iris.
• The extrinsic muscles control
eye movement and make it
possible to keep moving
objects focused on the fovea
centralis. They are also
responsible for
convergence, or medial eye
movement which is essential
for near vision.
Experimental Procedures for Vision
Test For Near Point Accommodations
• The closest distance at which one can see an object in
sharp focus is called the near point. Move the page up to
your eye until the first letter “T” becomes blurred;
measure the distance from your eye to the page.
Eye Reflexes Test
• Focusing for close vision requires that the eye make
three adjustments. Look at a distant object, such as a
wall across the room. Have your partner note the size of
your pupils and the position of your eyeballs. Do the
same with an object up close.
Vision
• Visual acuity is the sharpness of vision.
• Astigmatism is a condition resulting is
unequal curvature of either the cornea or lens.
This condition prevents light rays from being
focused with equal sharpness on the retina.
Experimental Procedures for Vision
Visual Acuity Test
• Stand 20 feet away from the Snellen
eye chart, cover one eye and attempt
to read the line with the smallest
letters that you can see.
Test for Astigmatism
• Using the Astigmatism chart, cover
one eye and focus on the circle in the
center. If all the radiating lines
appear equally dark and distinct,
there’s no astigmatism.
Color Vision
• Color vision depends on impulses formed by three
types of cones (receptors for red, green, and blue) in
the retina.
• Color blindness is more common in men than in
women because the genes for the red and green
color receptors are located on the X chromosome, of
which males have only one and females have two.
• The most common type of color blindness is redgreen color blindness. A complete color blind person
would see everything as shades of gray.
Experimental Procedures for Vision
Color Blindness Test
• Have your lab partner hold up the color
blindness test plates about 30 inches from your
eyes.
• Look at the color blindness test plate (Ishihara
plates) for 3 seconds and state which number
you see on the plate.
Hearing
• Hearing begins as sound waves pass through the
external auditory canal to the tympanic
membrane through the middle ear into the inner
ear, where the vibrations reach the spiral organ
of Corti.
Hearing
• Once the
mechanoreceptors are
stimulated they
depolarize and begin the
chain of neural impulses
by way of the cochlear
nerve to the auditory
centers of the temporal
lobe cortex.
Hearing
• Tone or pitch is determined by the frequency of the
waves. Different frequencies stimulate receptors in
different areas of the cochlea and brain.
• Loudness is determined by the frequency of the nerve
impulses that reach the brain which depends on the
amplitude of the vibrations that produce these impulses.
Hearing
• Conduction deafness is a result from blockage of
sound waves from reaching the inner ear. This can
be corrected by surgery or hearing aids and is
detected with a Rinne test.
• Nerve deafness is caused by damage to the sounds
receptors or neurons that send the impulses to the
brain. This usually is a result of loud noise and is not
correctable.
• A Weber test detects if there is a difference between
the left and right ear simultaneously.
Experimental Procedures for Hearing
Rinne Test
• Produce vibrations in a tuning fork by holding it by
the handle and striking it against the palm of your
hand. Do not strike it against a hard object!
• Place the handle of the tuning fork against the
mastoid process of the temporal bone. When the
sound is no longer audible, position the tines just
outside of the external auditory meatus.
• Compare air conduction (AC) to bone conduction
(BC).
Rinne Test
• If AC > BC then hearing
is normal
• If BC > AC then
conduction deafness
• If AC > BC but both are
reduced then nerve
deafness
Experimental Procedures for Hearing
Weber Test
• Produce vibrations in the tuning fork and place the handle of
the tuning fork against the top of the subject’s head.
• If conduction deafness is present in one ear, the sound will be
heard more strongly in the ear with hearing loss due to bone
conduction of the skull.
Interpreting Results
Equilibrium
• The equilibrium apparatus of the inner ear is in the vestibule and
semicircular canals. The three semicircular ducts are involved in
the mechanism of dynamic equilibrium.
• When your head position changes in an angular direction, the
endolymph in the canal lags behind, pushing the cupula in the
opposite direction of the movement, bends the hair cells and
creates an action potential.
Experimental Procedure for Equilibrium
Barany Test
• Evaluates the semicircular canals and detects
nystagmus, which is the trailing of the eyes
slowly in one direction, followed by their rapid
movement in the opposite direction.
• If the semicircular canals are operating normally
one will demonstrate nystagmus after rotation;
abnormal otherwise.
Experimental Procedures for Equilibrium
Romberg Test
• Used for the clinical assessment of disequilibrium
or ataxia from sensory and motor disorders
• Equilibrium is maintained through proprioception,
vestibular, and visual sensory input.
• Should maintain position for 20 seconds with
minimal swaying