Download physiology 1 lab: general cutaneous sensations

PHYSIOLOGY LAB: GENERAL CUTANEOUS SENSATIONS
A sensation is defined as a state of awareness, of the internal, or external, environment. For a
sensation to occur, four criteria must be met. First, there must be a stimulus. This is a change in
the environment, to which we will become aware. Next, there must be a receptor. A receptor is a
cell, or an organ, which is sensitive to the stimulus. There also must be an afferent (sensory)
nerve pathway, to carry signals to the central nervous system. Finally, there must be sensory
cortex, where the signals will be analyzed and interpreted.
The skin is responsive to a number of stimuli. These include light touch, deep pressure, vibration,
and changes in temperature. Any stimulus must be of threshold magnitude to be detected. All
sensory systems have a limit to their sensitivity; any stimulus below a minimum magnitude cannot
elicit a response.
In this activity, you will test sensory neurons in the skin and how they respond to different stimuli.
EXERCISE 1: LOCATION & DENSITY OF CUTANEOUS RECEPTORS
PART A: TOUCH RECEPTOR DENSITY
The density of touch receptors varies with location on the body. The fingertips, toes, and lips have
the greatest density. These areas of the body have the highest tactile resolution: the ability to
discriminate between one and two points of stimulation.
You will use a two-point discrimination test to compare tactile resolution (and receptor density)
between various parts of your body. The minimum distance between points, which can be
determined as two points of contact, is called the two-point threshold.
Begin the test by taping the toothpicks as close as possible on one side of an index card (to a
single point of contact). Next, tape two more 5 mm away on the same side but, other end of the
card. Continue to set up the rest of the toothpicks on the same card 10mm, 15mm, and 20mm.
Now, start with the 0mm toothpicks and gently touch the skin of the test subject. The subject
should have his/her eyes closed. Repeat the process with increasing distances until the subject
reports two points of contact. Record the distance between points, in millimeters:
Area being tested
Two-point threshold (mm)
Back of hand
_________________
Palm of hand
_________________
Index Finger
_________________
Back of neck
_________________
Sole of foot
_________________
Back of calf
_________________
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PART B: TACTILE LOCALIZATION
This tests the ability to determine where tactile stimulation has occurred. Sensory neurons may
connect individually to the CNS or converge with others when synapsing with the CNS. This can
greatly affect the ability of the cerebral cortex to locate the exact position of the stimulated neuron.
Using a felt-tip marker, touch the skin of the test subject. His/her eyes should be closed. The
subject then will try to touch the exact spot with a marker (of a different color); measure the
distance between the two marks.
Area tested
Distance (mm)
Palm of hand
___________
Back of hand
___________
Forearm (anterior)
___________
Forearm (posterior)
___________
Sole of foot
___________
Back of Calf
___________
EXERCISE 2: ADAPATION OF SENSORY RECEPTORS
In many cases, when a stimulus is applied for a prolonged period, the rate of receptor discharge
slows and conscious awareness of the stimulus declines or is lost until some type of stimulus
change occurs. This phenomenon is referred to as adaptation (or fatigue) of sensory receptors.
The adaptation appears to happen because the rate of change within the nerve's membrane is
inadequate to keep up with continuous stimulation. There are many examples of adaptation in
everyday life. For example, our clothing is in constant contact with our skin, which should
constantly stimulate touch receptors in the skin, however, people are rarely aware of the feeling of
their clothes on their skin unless some change occurs.
PART A: ADAPTION OF TOUCH RECEPTORS
Sensory adaptation enhances sensitivity to changes in the environment. It also allows us to
become less sensitive to nonessential stimuli. Being constantly aware of the same pattern of
tactile stimulation (e.g. pressure of clothes on the skin) would be very distracting. A new pattern
(e.g. a scorpion crawling up your leg) probably has far more significance.
Have the test subject close his/her eyes. Place a coin on the back of the person’s hand. Note how
long it takes for the sensation to cease. Record the time interval:
Repeat the test on the forearm, and record the time: ______
Once the sensation has ceased, stack two more coins on top of the first one.
Record how long the sensation persists: _______
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PART B: ADAPTATION OF HAIR RECEPTORS
Rest your forearm on the desk in front of you. With your eyes closed, have your partner gently
displace one hair on your forearm and very carefully maintain the displacement. Tell your partner
as soon as soon as you feel the initial sensation of the displaced hair. Your partner will then
carefully watch the clock until you announce that you can no longer feel the sensation. Repeat this
test two more times, calculate the average duration of your sensation, and record your data in the
table below:
Duration of
Sensation (sec.)
Hair Tested
#1
#2
#3
Average
PART C: ADAPTATION OF TEMPERATURE RECEPTORS
Many sensory nerve fibers respond well to the sudden onset of a stimulus, but then respond
less, or stop responding altogether, when the stimulus remains constant. This decrease in the level
of response despite continued stimulation is called sensory adaptation. One result of sensory
adaptation is that our perceived sensation of cold is greater while skin temperature is falling, as
compared to when skin temperature is constantly cool. Similarly, we feel warmer when the
temperature is rising as compared to constantly warm conditions. As an everyday example of
thermal adaptation, the water in a swimming pool feels much colder when we first enter it, as
compared to after we've been in it a while. Likewise, hot water in a tub feels hotter at first than
after we've been in it a while. Thus our sensations of temperature are more influenced by relative
changes in temperature (in one direction or the other, depending on thermoreceptor type), rather
than by the absolute value of temperature.
1. Use the hot water bath that is set at 45o C. Using a beaker, obtain a beaker full of the hot
water.
2. Have the test subject immerse the index finger in the beaker for 2 minutes. Compare the
sensation at the beginning of immersion with that at the time interval:
a. Beginning ________________________ after two minutes ______________________
3. With the subject’s finger still in the beaker, now immerse the other index finger in the
beaker. Note which finger feels hotter in the water now.
4. Keep both fingers in the beaker until both hands feel the same temperature.
5. Have partner fill another beaker with tap water. Remove one of the fingers from the warm
water beaker, and place it the Tap Water beaker.
Note how your finger feels once immersed in the tap water.
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Name:______________________________________ Period:_________ Date:__________
Lab Questions: (complete sentences with well-thought out answers only!)
1. What is the relationship between the two-point threshold and the nerve density?
2. What is the relationship between the two-point threshold and the receptive field?
3. What is the relationship between tactile localization and neuron convergence?
4. How is referred pain related to the concept of convergence?
5. With the adaptation exercise, were the same receptors being stimulated with 3
coins vs. only 1? Explain.
6. Why did the tap water feel cold at the end of the temperature activity?
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