Download and Sensory Physiology

Lab 4.1
LAB 4 - Sensory Physiology
OBJECTIVES - after completing this lab you should be able to:
1) describe the effect of receptive field (primary and secondary) size and lateral
inhibition on acuity in a sensory pathway
2) describe the effect of density of receptors on sensitivity
3) define sensory adaptation and explain how it occurs
4) explain how hearing acuity is tested
5) interpret results of tests for deafness, visual acuity, and astigmatism
6) explain what the near point of accommodation measures and how it is correlated
with age
7) explain what causes the blind spot, color blindness, presbyopia, astigmatism,
myopia and hyperopia
8) describe the reflex pathway for the pupillary light and consensual reflexes
BACKGROUND:
The nervous system cannot operate without information about the internal
and external environment. This comes through the receptors of the general (touch,
proprioception, etc.) and special (sight, hearing, etc.) senses.
Not only does each receptor type monitor a different kind of stimulus (change in
the environment), but over time a receptor's sensitivity may vary due to adaptation,
disease or drugs. For this reason, it is a good idea to be familiar with the normal
distribution and response of the various receptors, and to be able to perform diagnostic
tests for sensory function.
A. Two-Point Discrimination Test
Cutaneous receptors have areas within which they can detect stimuli. These regions
are called receptive fields. There is an inverse relationship between the density of
receptors and the size of primary receptive fields: the more receptors in an area of the
skin the smaller their individual receptive fields. The two point discrimination test will tell
you about the size of touch receptor receptive fields in different areas of the skin. From
that information you will infer the density of the receptors.
Equipment: dividers
metric ruler
1. With the divider points very close together, touch your lab partner's skin on the
area of the body indicated. Make sure both points touch the skin at the same
time. Don't let the subject watch! If he can feel two distinct points with the
dividers as close together as they will come, use 2 pins instead.
2. Increase the distance between the divider points until the subject feels two distinct
points.
Sarah Strong 5/2000
BIOL 2102/Strong/Spring 2008
Lab 4.2
3. Record this distance in millimeters in data chart A.
4. Repeat for each area of the body.
Data Chart A - Two Point Discrimination Test
area of body:
face
back of hand
palm of hand
fingertips
back of neck
back of calf
distance when two points felt (in mm)
B. Adaptation of Touch Receptors
Equipment: 4 coins of the same size and weight.
1. Have the subject close his/her eyes.
2. Warm a coin in your hand and place it on the anterior surface of his/her forearm
and time the duration of sensation (from when you placed the coin on his/her
arm until he/she stops feeling it. Record the time in seconds in data chart B.
3. Move the coin to a second location and repeat the test. Record data.
4. Now, stack 3 more coins (same size) on top of the first one. Record the duration
of sensation.
5. To test a different set of receptors, use the tip of a pencil to slowly bend an
individual hair on the subject’s arm and have him/her tell you what happens
to the sensation.
Data Chart B - Adaptation of Touch Receptors
location
1st location, 1 coin
2nd location, 1 coin
2nd location, 4 coins
duration of sensation (in sec.)
C. Adaptation of Temperature Receptors
Equipment: 3 beakers of water: warm (45 C), cool (not ice cold), and room
temperature
1. Immerse the left index finger in warm water for 2 minutes. Then dip the right index
finger into the same container of warm water, noting the difference in sensation
in data chart C.
Sarah Strong 5/2000
BIOL 2102/Strong/Spring 2008
Lab 4.3
2. Remove both fingers and wait 2 minutes before continuing.
3. Immerse the left index finger in warm water and the right one in cool water at the
same time and leave them there for 2 minutes. Then immerse both
simultaneously in a beaker of room temperature water and record your
observations in data chart C.
Data Chart C - Adaptation of Temperature Receptors
Step #1: When the right index finger was immersed in warm water, the
sensation was (warmer or cooler?) than the temperature felt from
the left finger at the same time.
Step #3: When immersed in room temperature water, the left index finger
felt (warm or cool?) and the right index finger felt (warm or cool?)
D. Hearing Tests
The receptors for hearing are located in the organ of Corti in the cochlea.
Not only are they sensitive to varying intensities of sound, but because of their
arrangement and the structure of the basilar membrane, they are also sensitive to
different pitches of sound. Normally, sound waves enter the ear through the external
auditory canal, causing vibration (in turn) of the tympanic membrane, the auditory
ossicles, the oval window and the basilar membrane. When sound waves cannot
reach the cochlea because of damage to the tympanic membrane or the ossicles,
they can still be transmitted, less efficiently, through the temporal bone. Damage
to the eardrum or ossicles causes conduction deafness, which can usually be
partially corrected by surgery or the use of hearing aids. Damage to the cochlea
or cochlear nerve causes nerve or sensorineural deafness.
1. Test for (simulated) hearing impairment (should be performed in a quiet area)
Equipment: kitchen timer
cotton balls
meter stick
a. Seat subject in a quiet area.
b. Plug one of his ears with cotton.
c. With the subject facing forward, with eyes closed, hold the timer next to one of his
ears and move the timer steadily and slowly away from the ear.
d. The subject should indicate when he no longer hears the ticking.
e. Measure this distance and record in data chart D.
f. Repeat this test once for the other ear and record.
Sarah Strong 5/2000
BIOL 2102/Strong/Spring 2008
Lab 4.4
Data Chart D: Hearing Acuity
distance (in inches) right ear
distance (in inches) left ear
2. Rinne Test
In this test, a tuning fork is used to differentiate conduction and
sensorineural deafness. The fork of a vibrating tuning fork is held close to the ear.
If the person cannot hear it, he is deaf. The stem of the vibrating tuning fork is then
placed on the mastoid process. If the person still cannot hear it, he has nerve
deafness. If he can hear it, he has conduction deafness. A normal person, of course,
can hear the sound in both situations. The cotton is used to simulate conduction
deafness.
Equipment: tuning fork
cotton
a. Set the tuning fork in motion by striking it gently against the edge of a book (do not
strike it on a hard surface)
b. Hold the forks near the ear. If the subject cannot hear anything, what is wrong?
c. Now carefully place the stem against the mastoid process behind one ear, with the
fork pointing slightly down and back, away from the ear. (Be careful not to touch
the fork or you will cause it to stop vibrating.) If the subject cannot hear anything,
what is wrong?
d. Repeat the test with the ear plugged. What was the difference?
e. Repeat both tests on the other side.
3. Weber Test
This test also uses a tuning fork to differentiate between conduction and
sensorineural deafness. The stem of a vibrating tuning fork is placed in the center
of the forehead. (How are the vibrations transmitted to the inner ear?) If the subject
has conduction deafness in one ear, the sound will be perceived to be louder in that
ear because the receptors have become more sensitive. If the subject has
sensorineural deafness in one ear, the sound will be perceived to be louder in the
other ear.
Equipment: tuning fork
cotton
a. Set the tuning fork in motion by striking it gently against the edge of a book.
b. Place the stem in the center of the forehead. Does the sound seem louder in one
ear?
c. Plug one ear with cotton to simulate conduction deafness.
d. Repeat test.
Sarah Strong 5/2000
BIOL 2102/Strong/Spring 2008
Lab 4.5
E. Visual Tests
The receptors for vision are located in the retina. But there are other
structures which contribute to vision: the cornea, iris, lens, and optic nerve,
among others. Abnormalities in any of these structures may cause diminished vision.
1. Visual Acuity
Visual acuity refers to the sharpness of vision and is most frequently tested
using the Snellen eye chart which is printed with lines of letters of varying sizes on
different lines. Each size of letter can be read by a normal eye (one with no
abnormalities of the lens or cornea) at a certain number of feet. The top line is
designated 20/200; a normal eye can distinguish the letter "E" at 200 feet. Line 8 is
designated 20/20; a normal eye can distinguish all of the letters in line 8 from 20 feet
away. If you can only distinguish the letter in line 1 while standing 20 feet from the
chart, you have "20/200" vision in that eye. It means you can see at 20 feet away what
a "normal" eye can see from 200 feet away. In other words, you have very poor visual
acuity, and you probably wear corrective lenses. The first or top number is the reference
point—how many feet away the chart is located, usually 20 ft. The second or bottom
number indicates the distance at which a person with normal eyesight could read the
same line you correctly read. It compares your visual acuity with the reference point.
All numbers refer to the farthest point from the chart that a given letter can be clearly
seen.
Equipment: Snellen eye chart
3" x 5" card
a. Stand behind the 20 foot line (marked on the floor with masking tape) and remove
your glasses. (If you wear contacts, leave them in.)
b. Have your lab partner stand close to the eye chart.
c. Cover one eye with the 3x5 card and read aloud the line with the smallest letters
you can see. Your lab partner will check your accuracy.
d. Record your visual acuity (20/?) for that eye.
e. Repeat test with other eye, then with both eyes together and record results below.
f. If you wear glasses, put them back on and repeat for each eye and then both eyes
together.
Data Chart E-1: Visual Acuity
without glasses
left eye alone
right eye alone
both eyes together
Sarah Strong 5/2000
with glasses
BIOL 2102/Strong/Spring 2008
Lab 4.6
2. Near Point of Accommodation
As you age, the elasticity of your lens decreases, so that it becomes more
difficult to accommodate for near vision. The older you are, the higher your near point
(the farther it is from your eye). This is a gradual process that occurs in everyone,
but may be more severe in one person than another.
Equipment: card with letter
meter stick
a. Close one eye and move the card toward the other eye until the image becomes
blurred (when you no longer see a clear, sharp image).
b. Measure and record this distance below.
c. Repeat for the other eye and record below.
Data Chart E-2: Near Point of Accommodation
near point (cm) right eye
near point (cm) left eye
3. Pupillary light reflex and consensual response
The pupillary light reflex occurs when a bright light (the stimulus) is shined into
one or both eyes--the response is pupillary constriction to restrict the amount of light
that enters the eye. The pupillary light reflex, by definition, only occurs in eyes that have
been exposed to bright light. The lack of the pupillary light reflex is a sign of severe
brain trauma or disease.
The consensual response is a different but associated reflex that can occur
simultaneously with the pupillary light reflex if only one eye is stimulated. If bright light
is shined into one eye, the consensual response occurs in the OTHER eye only.
a. move to an area where you are not in extremely bright light
b. have the subject hold an index card or hand between his eyes so that a light shined
into one eye cannot reach the other eye
c. you will need two observers, one for each eye--the response in both eyes has to be
observed simultaneously
d. hold a penlight pointed at the subject’s ear, and then move it rapidly to shine into
his/her eye for a few seconds (2 or 3)
e. watch carefully for changes in pupil diameter—one person should monitor each eye
Sarah Strong 5/2000
BIOL 2102/Strong/Spring 2008