Downtown Campus Biology > Biology 182 > Lesson 12 Activities > Step 6

In Class Activity - Discrimination Test

Time: This activity will take 20 - 30 min. to complete.

This activity was created originally by Robert Wakefield, biology instructor at Pima's Community Campus. Thanks, Robert!

You need to go get Kit # 15 to do this lab.


Also known as nerve cells, neurons are the structural units of the nervous system. They are highly specialized cells that conduct messages in the form of nerve impulses from one part of the body to another. In addition to their ability to conduct nerve impulses, neurons have some other special characteristics.

  1. They have extreme longevity. Given good nutrition, neurons can function optimally for a lifetime (over 100 years).
  2. They are amitotic. As neurons assume their roles as communicating links of the nervous system, they lose their ability to divide (to do mitosis). As a result they usually are not replaced if destroyed. There are exceptions to this rule—For example, some regions of the hippocampus contain stem cells that can produce new neurons throughout life. This makes sense because the hippocampus is a brain region involved in memory.
  3. They have an exceptionally high metabolic rate and require continuous and abundant supplies of oxygen and glucose. Neurons cannot survive for more than a few minutes without oxygen.

Receptive Fields (see figure below)

A neuron’s receptive field is the area that the neuron is “responsible for.” Receptive fields can:

  1. have an exclusive relationship with a sensory neuron (example below on the right) or
  2. exhibit “convergence” where more than one receptive field is associated with one sensory neuron (example below on the left)

receptive field diagram

Copyright ©2004 Pearson Education, Inc., publishing as Benjamin Cummings

The less receptor fields associated with a sensory neuron (the least convergence), the more sensitive the area.

A Two-Point Discrimination Test illustrates receptive fields. If many neurons are associated with one sensory neuron, a large receptive field is created. Even though two distinct areas are stimulated, only one signal is sent to the CNS. Thus you only feel one point. If few neurons are associated with one sensory neuron, a small receptive field is created. Each of the two areas stimulated will send a signal to the CNS. Thus you feel two points.

The size of receptive fields varies significantly in different areas of the body. In general, areas that have a heightened ability to “feel” have the smallest receptive fields.

Consider the following body areas:

_______ Fingertip

_______ Back of the hand (Posterior)

_______ Palm of the hand (Anterior)

_______ Posterior Forearm

Which of these areas of the body do you predict will have the smallest receptive fields (“feel” the most)? Which will have the largest (“feel” the least)? Rank the above body areas from 1 to 4 on your lab worksheets; 1 being the smallest receptive field and 4 being the largest receptive field.

Determining the Two-Point Threshold:

You can perform this test on yourself, or have a lab partner or BLC staff member do it to you, especially if you are having trouble. It is your choice. It works slightly better if you do this with a partner. Read the following instructions before you perform the test.

1. Using the points of two toothpicks (both held in one hand between the thumb and 1st finger), test your subject’s ability to differentiate two distinct sensations when the skin is touched simultaneously at two points. Start with the points of the toothpicks touching one another (no gap), and then gradually increase the distance between their points, testing your subject’s skin after each adjustment. Be sure that your subject is looking away (cannot see the toothpicks touching the skin).

2. Continue with the procedure until your subject feels two points of contact. This measurement, the smallest distance at which two points of contact can be felt, is the two-point threshold. Once you have reached the threshold (subject can feel two points) hold the two different points of the toothpicks up to the metric ruler and determine the distance (in millimeters) between them. Record the two-point threshold in the table on the following page.

3. Repeat steps 1 and 2 for each of the body areas listed, and record your results in the table on your lab worksheets.

4. Answer the questions on your lab worksheets and cleanup your lab. You should throw away your used toothpicks in the red sharps container located inside of the fume hood.

That's it!

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