Download Special Senses 2016

SPECIAL SENSES
Overview
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
Introductory Notes for Vision
The innermost tunic of the eye is the retina. The retina has a neural tissue layer composed of two types
of photoreceptors, rods and cones. Photoreceptors convert light energy into a nerve impulse. These nerve
impulses will be transmitted to the optic cortex in the brain resulting in visual perception. Theses photoreceptors
are distributed over the entire retina except for where the axons of the ganglion cells exit through the optic
nerve. This area is called the optic disc and results in a blind spot.
At the optic chiasm, fibers from the medial retina from each eye cross over to the opposite side and the
lateral fibers of each eye remain on the same side. The fibers then extend as optic tracts and synapse with
neurons in the lateral geniculate body of the thalamus and terminate in the visual cortex in the occipital lobe.
Each eye receives light from both left and right hemifields. In Figure 1.1 below the yellow represents the
left hemifield and the blue represents the right hemifield. Notice that each eye receives light from both
hemifields.
Figure 1.1
1 When light rays pass from one medium to another, their speed of transmission changes and the rays are
bent, or refracted. Light travels faster in air than through other mediums such as liquid therefore; light rays slow
down and are refracted when they pass through the cornea, lens, and vitreous humor of the eye to reach the
retina. The cornea and the vitreous humor refractive index remains constant however, 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.
The lens does not need to change its convexity for it to focus light from a distant object, greater than
twenty feet away. On the other hand, the lens becomes more convex when focusing light from a near object
onto the retina. This occurs when the ciliary muscles 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.
Both intrinsic and extrinsic muscles are needed for proper eye functioning. The intrinsic muscles of the
eyes are smooth muscle and include the ciliary muscles (which alters the lens curvature in focusing), and
muscles of the iris (which controls pupil size). The extrinsic muscles are skeletal muscle and attach to the
outside of the eye. These 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.
Visual acuity, or sharpness of vision is tested with a Snellen eye chart, which consists of letters of
various sizes printed on white paper. This test is based on the fact that letters of a certain size can be seen
clearly by normal vision at a specific distance.
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. The astigmatism chart is designed to
test for defects in the refracting surface of the lens and/ or cornea.
The sensation of color vision depends on the degree to which impulses are 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 red- green color blindness, which
is caused by a deficit in cones stimulated by either red or green light. People with this deficit have difficulty
distinguishing between red and green. A completely color blind individual would see everything as shades of
gray.
Introductory Notes for Hearing
The mechanism of hearing begins as sound waves pass through the external auditory canal to the
tympanic membrane, through the middle ear, and into the inner ear, where the vibrations reach the spiral organ
of Corti. This is where the mechanoreceptors for hearing are located and stimulated by sounds of various
frequencies and amplitude. Once they are stimulated they depolarize and begin a chain of nerve impulses by
way of the cochlear nerve to the auditory centers of the temporal lobe cortex.
Tone or pitch is determined by the particular receptors that are stimulated based on the frequency of the
vibrations and the part of the brain that receives it. High frequency waves result in high-pitch sounds detected
close to the oval window. Low frequency waves result in low-pitch sounds detected near the apex of the
cochlea. The amplitude of the sound waves, which will increase the frequency of the nerve impulses that reach
the brain, determines loudness.
There are two types of hearing loss. Conduction deafness results from the blockage of sound waves
reaching the inner ear. This can be corrected by surgery or hearing aids and is detected with a Rinne test. Nerve
deafness (a.k.a sensorineural) 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 difference
between the left and right ear simultaneously. 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.
2 Introductory Notes for Equilibrium
The equilibrium apparatus of the inner ear is in the vestibule and semicircular canals. The vestibule
contains the saccule and utricle, and the semicircular canals contain the semicircular ducts. The three
semicircular ducts are involved in the mechanism of dynamic equilibrium and are oriented in a horizontal,
frontal, and sagittal plane. The ampulla is located at the base of each duct and houses the receptor region called
crista ampullaris, which consists of hair cells covered with a gelatinous cap, or cupula. 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.
Maculae in the saccule and utricle contain the hair cells, receptors involved in the mechanism of static
equilibrium. The maculae respond to gravitational pull providing information on which way is up or down, and
to linear changes in speed. The hair cells in each macula are embedded in the otolithic membrane which
contains calcium carbonate (otoliths). When the head moves, the otoliths move in response to gravitational pull.
Tests such as the Barany Test detect 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. Romberg Test is used for the
clinical assessment of disequilibrium or ataxia from sensory and motor disorders. Equilibrium is maintained
through proprioception, vestibular, and visual sensory input.
Experimental Procedures for Vision
Blind Spot Test
This is a test for the presence of the blind spot or optic disc. The optic disc is the point at the back of the
eyeball where the optic nerve exits. To perform this test, hold Figure 1.2 about 18 inches from your
face. 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. While continuing to stare at the plus sign, slowly move the page
toward your face. At a certain distance, the dot will disappear from your peripheral vision as it comes into focus
on the optic disc. To test the left eye, close the right eye and stare at the dot with the left eye while slowly
moving the page toward your face. The plus sign will disappear from your peripheral vision when it falls on the
optic disc. Answer the questions on the data sheet.
Figure 1.2
3 Test For Near Point Accommodations
The closest distance at which one can see an object in sharp focus is called the near point. Close one eye and
look at the letter “T” at the beginning of this paragraph. Move the page closer to your eye until the letter
becomes blurred; then move it away until you get a sharp image. Have your partner measure the distance from
your eye to the page using the provided ruler. This distance is your near point of accommodation. Repeat this
procedure with the other eye. Record your near point for each eye. Typical near points are given below. Why
does the near point increase with age?
Age
20 years
30
40
50
60
Near point
3.5” (9 cm)
4.5” (11.5 cm)
6.5” (16.5 cm)
20.5” (52 cm)
to 33” (84 cm)
Eye Reflexes Test
Focusing for close vision requires that the eye make three adjustments. Two of those adjustments can be
easily observed. Look at a distant object, such as a wall across the room (not a light or window). Have your
partner note the size of your pupils and the position of your eyeballs. Then, focus on a pencil held about 10
inches from your face. Again, have your partner note the size of your pupils and the position of your eyeballs.
Answer the questions on the data sheet.
Visual Acuity Test
Test your visual acuity with the Snellen eye chart. Stand 20 feet away from the eye chart, cover one eye and
attempt to read the line with the smallest letters that you can see. Do not squint. Have your lab partner note the
visual acuity value associated with that line on the chart. Repeat with the other eye. If you wear glasses, perform
this test with and without your glasses. If you wear contacts, keep them in but note it in your documentation.
Normal vision is 20/20. If your vision tested 20/50, you can see at 20 feet what a person with normal vision can
see at 50 feet. Record your results in the data sheet.
Test for Astigmatism
Using the Astigmatism chart, stand 20 feet away from the 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. If some of the lines
are blurred or appear less dark than others, then some degree of astigmatism is present. If this is the case, record
the number (I-XII) of the line that appears light or blurred in you data sheet. Repeat the process with the other
eye.
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. Refer to
the answer key which accompanies the test plates. If you are unable to read the numbers, this may indicate that
you are color blind. Answer the questions in your data sheet.
4 Experimental Procedures for Hearing
Rinne Test
Produce vibrations in a tuning fork by holding it by the handle and striking the prongs (tines) against the
palm of your hand. Do not strike it against a hard object! Do not touch the tines; hold it by the handle only.
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 1 inch from the external acoustic meatus. Ask your subject where the sound
was loudest. If the sound is louder at the meatus, then hearing is not impaired. If it is louder at the mastoid
process, then the subject has a degree of conduction deafness in that ear. Repeat with the other ear and record
your results on the data sheet.
Next, produce vibrations in a tuning fork and hold it outside of the external auditory meatus. When the
sound is no longer audible, place the handle of the tuning fork against the mastoid process. If sound is again
audible, some conduction deafness if present. If nothing is heard at the meatus or the mastoid process then nerve
deafness is present.
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 it is equally loud in both ears, you have equal hearing or equal loss in both ears. Next, plug one ear to
simulate conduction deafness and repeat the test. The sound should appear louder in the plugged ear. Answer
the questions in your data sheet.
Barany Test (Induction of Nystagmus and Vertigo)
This evaluates the semicircular canals and should be conducted as a group effort to protect test subjects
from possible injury. Before beginning this experiment, please adhere to the following. Select a subject who is
not easily inclined to dizziness during rotational movements. Be sure to stop rotation immediately if the subject
feels nauseated and have other group members ready to catch, hold, or support the subject and the end of the
rotation as the subject will experience vertigo and loss of balance.
Instruct the subject to sit on a rotating chair and to hold onto the seat or chair arms, and have the feet off
the ground. The subject’s head should be tilted forward approximately 30 degrees (almost touching the chest).
The horizontal (lateral) semicircular canal will be stimulated when the head is in this position. The subject’s
eyes are to remain open during the test.
Four classmates should position themselves so that the subject is surrounded on all sides. The classmate
behind the subject will rotate the chair to the subject’s right for 10 revolutions in 10 seconds, then abruptly stop
the rotation. Immediately note the direction of the subject’s nystagmus. Ask the subject to describe the feelings
of movement, indicating speed and direction of sensation. Record the information on the data sheet.
Role of Vision in Maintaining Equilibrium
The Romberg test determines the integrity of the dorsal white column of the spinal cord, which is
responsible for transmitting impulses from the proprioceptors involved in posture to the brain. Have the subject
stand with their back to the whiteboard. On the whiteboard, draw a series of parallel vertical lines, about an inch
apart from each other, on both sides of the subject. The subject should stand straight with their eyes open
looking straight ahead. Have one lab partner stand in front and observe the subject’s degree of side-to-side
movement and another to act as a spotter incase they lose their balance. Repeat the test and observations with
the subject’s eyes closed. Then have the subject raise their left foot approximately 30 cm off the floor and hold
there for 1 minute with eyes open. Record the observations on the data sheet. Rest for 1 minute and then repeat
the experiment with the same foot raised but with eyes closed. Record the observations on the data sheet.
5 NAME: ___________________________
Lab Section:________________________
Data Collection and Follow Up Questions
Visual Pathways of the Brain
After examining visual pathways of the brain in figure 1.1, determine what effects lesions in the following areas
would have on vision.
1. In the right optic nerve: _________________________________________________________
2. Through the optic chiasma: ______________________________________________________
3. In the left optic tract: ___________________________________________________________
4. In the right visual cortex: ________________________________________________________
Blind Spot Test
1. Explain why the optic disc is a blind spot. _________________________________________________
_____________________________________________________________________________________
2. Why are you not aware of this blind spot except when performing this test. ___________________________
_________________________________________________________________________________________
Test For Near Point Accommodations
1. Near point for right eye: ______________ inches. Is it close to average for your age? _________________
2. Near point for left eye: _______________ inches. Is it close to average for your age? _________________
3. Why does the near point increase with age? ____________________________________________________
_________________________________________________________________________________________
Eye Reflexes
1. List the three adjustments that took place when looking at the pencil held 10 inches away. Explain why each
is necessary.____________________________________________________________________________
_________________________________________________________________________________________
_________________________________________________________________________________________
_________________________________________________________________________________________
6 Visual Acuity Test
1. Visual acuity, right eye without glasses: _20/_______
2. Visual acuity, right eye with glasses: _20/_______
3. Visual acuity, left eye without glasses: _20/_______
4. Visual acuity, left eye with glasses: _20/_______
Test for Astigmatism
1. Is there astigmatism present in the right eye? _______ If yes, what line number(s) was blurring? _________
2. Is there astigmatism present in the left eye? _______ If yes, what line number(s) was blurring? __________
3. What does the presence of astigmatism indicate? _______________________________________________
________________________________________________________________________________________
Color Blindness Test
1. Is there any indication that you have some degree of color blindness? _______
2. If so, what type? _________________________________________________________________
Rinne Test
1. State and interpret your results for the right ear.
_____________________________________________________________________________________
_____________________________________________________________________________________
2. State and interpret your results for the left ear.
_____________________________________________________________________________________
_____________________________________________________________________________________
3. If you could hear the sound from the mastoid process, but not the external auditory canal, what specifically
would that indicate about your hearing?______________________________________________________
_______________________________________________________________________________________
Weber Test
1. Is the sound equal in both ears, or is it louder in one ear? _______________________________________
_______________________________________________________________________________________
2. What did you notice when you plugged one ear using this test and why did that occur? _______________
_______________________________________________________________________________________
7 Barany Test
1. Describe the subject’s feelings of movement, indicating speed and direction sensation. _______________
_______________________________________________________________________________________
2. When the subject is rotated to the right, which direction does the cupula bend? ______________________
3. Describe the nystagmus observed. Were the subject’s eyes moving more quickly to one side than the other?
_______________________________________________________________________________________
Romberg Test & Role of Vision in Maintaining Equilibrium
1. Did you observe any swaying movement when the eyes were open?
_______________________________________________________________________________________
_______________________________________________________________________________________
2. Describe the degree of swaying movement observed when the eyes were closed. Compare observations to
when eyes were open.
________________________________________________________________________________________
3. Does vision play a role in equilibrium? Explain your answer based on your observations.
_______________________________________________________________________________________
_______________________________________________________________________________________
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