The Skin and Sense of Touch Lab II
Background: (This background may look a bit familiar )
Information from our skin allows us to identify several distinct types of sensations, such as tapping, vibration, pressure,
pain, heat, and cold. What is it that allows us to make these distinctions? First, human skin contains different kinds of
sensory receptors (cells) that respond preferentially to various mechanical, thermal, or chemical stimuli. (The word
"receptor" can mean a receptor cell or a membrane receptor in a cell. Here, it refers to a cell.) Next, these receptors convey
this information to the brain and spinal cord, also known as the central nervous system (CNS), to areas where we
perceive the stimuli. To accomplish this, the nerve endings of the sensory receptors transduce, or convert, mechanical,
thermal, or chemical energy into electrical signals. These electrical signals then travel along neuronal extensions called
axons, to the CNS. Finally, the way we interpret or understand sensations is shaped not only by the properties of receptors
and neurons, but also by previous experiences that are stored in our brains.
What properties of the touch sensory system allow us to discriminate two points pushing on our skin even when they are
only 2 or 3 mm apart (two-point discrimination)? One of the necessary properties is high receptor density - the
receptors must be packed closely enough so that a probe stimulates one or more of them. High receptor density alone,
however, cannot explain why the fingertip can distinguish points so close together while the arm senses two points only
when they are 35 to 40 mm apart. The second property necessary for fine two-point discrimination is that neighboring
receptors must have small receptive fields.
A given central neuron responds to all information from its input area (the skin area that is the gathering field for only that
CNS cell) as if it were coming from one point. This skin area is called the receptive field of the central neuron. On the
arm, each sensory receptor gathers information from a much larger skin area than a receptor on the fingertip, and this
receptor is also connected to a defined central neuron. This central neuron, like the central "finger neuron", interprets all
input as coming from one point, even though the skin area in this case is much larger. In order for a person to feel two
points, two separate central neuronal populations must be activated by stimulation of their respective receptive fields.
When this happens, two points are reported.
Neurologists, doctors who specialize in diseases of the central (brain and spinal cord) and peripheral (nerves to all the
organs and muscles) nervous systems, sometimes test patients for two-point discrimination. They may do this if they
suspect a problem with sensory information entry to the skin, the pathways to the brain, or the interpretation of sensory
information. For example, if a patient has cut a finger badly, a neurologist may test for two-point discrimination at the
time of injury to see if the nerve was cut. After the original injury has healed for a number of weeks, the neurologist will
again test two-point discrimination and compare it with the normal fingers to see if the nerve has regenerated.
This lab will focus on the tactile/somatosensation sense of the skin, which allows us to distinguish different kinds of
stimuli upon the surface of the body. By using our tactile sense, we detect superficial and deep pressure and sensations we
describe as brushing, vibration, flutter, and indentation. As mentioned above, our skin is also sensitive to temperature and
pain, which we sense with different sets of receptors.
Objective:
Using the background information given, you will test the sensitivity of multiple areas of skin with the tip of a paper clip.
Materials:
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Laboratory Handout
Paper Clip
Metric Ruler
Blindfold
Procedure:
1. Work in groups of 2.
2. Un-twist a paper clip so that you have two points.
3. Subject 1 should blindfold their eyes, while subject 2 conducts the first set of trials.
4. Begin tests to see at what distance (in mm) Subject 1 can detect there are two points touching instead of one.
Different areas of the skin should require different distances.
5. Test at all of the places shown below, as well as 3 additional locations chosen by the group.
6. Record every distance tried (in mm), and highlight the final distance where Subject 1 can tell there are two points
in Data Table 1 (shown below)
7. Once the first set of trials is completed, repeat steps #3-5 for Subject 2.
8. Answer the concluding questions in the space provided.
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Data Table 1 – Subject 1:
Distance 1
Distance 1
Distance 3
Distance 4
Distance 5
Distance 6
Distance 1
Distance 1
Distance 3
Distance 4
Distance 5
Distance 6
Back of Hand
Palm of Hand
Inside Arm
Ankle
Cheek
Chin
Back of Neck
Lower Back
Tip of Thumb
Tip of Index Finger
Location #1:
Location #2:
Location #3:
Data Table 2 – Subject 2:
Back of Hand
Palm of Hand
Inside Arm
Ankle
Cheek
Chin
Back of Neck
Lower Back
Tip of Thumb
Tip of Index Finger
Location #1:
Location #2:
Location #3:
Concluding Questions:
1. What part of the body seemed most sensitive? Were both partners the same?
2. Investigate results from around the room. Does it seem that males and females differ in sensitivity?
3. Explain what a ‘threshold stimulus’ is and how it applies to this Sense of Touch investigation.
4. Relate your findings above with our study of the Homunculus in the Nervous System unit. How do your findings
relate to surface area?
5.
How and why did you decide on your three additional touch locations?
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