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Biology LibreTexts

Part 4: The Somatosensory System

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  • The somatosensory system is the largest sensing system in your body. This system produces sensory feedback whenever you come in physical contact with your environment. This sensory feedback includes body position (proprioception), sensing movement of your body and limbs (kinesthesis), pain (nociception), temperature, and finally touch. For this last sense you can map out touch receptor density in your skin using a simple technique. Most of the tactile feel we receive is gathered by four types of mechanoreceptors which are found in both layers of skin. The two receptors located near the top of the dermis, are called Merkel receptors and Meissner corpuscles. The other two mechanoreceptors located deep in the dermis and hypodermis layer and are the Ruffini and Pacinian corpuscles.

    Working with a friend you are going to test each other's two-point threshold. A two-point threshold test seeks to find at what distance apart does a person perceive one point as two separate points. To test this, two points start together touching the skin. Incrementally they are pulled further apart and reapplied to the skin until the subject can clearly tell there are two different points.

    You will take measurements from four different body part locations: Fingertip, Cheek, Forearm, and Calf. You will perform this experiment twice (ascending and descending with the drafting compass). Finally, the test subject must always keep their eyes closed!

    Experiment Steps:

    1. Before you start this experiment, be sure that you know how to use the compass. Practice touching your arm with it with the points at different distances (millimeter (mm)) apart. Use the ruler to precisely position the points apart from each other. It is important during the experiment that you touch your partner's skin with both tips at the same time; otherwise, they will easily be able to tell there are two points. Once you have the hang of it you are ready to begin!
    2. Once your partner has closed their eyes, have them place their arm on the table with their palm facing up. Now take the compass, make sure the points are together (0 mm apart), and start with your partner's finger tip.
    3. Touch the compass to their finger tip and ask them if they feel one point or two points. They should say one point because there is no distance between the points (0 mm). Remove the compass and increase the distance to 2 mm. Reapply the calipers, be sure to touch both tips at the same time, and ask again. If they still only feel one point, increase by another 2 mm and reapply.
    4. Once your partner detects two points, move on to the next area of skin and repeat the process. Once you have completed the ascending or increasing distances now it’s time to repeat the process using the descending (decreasing distances) measurements. You can start with a distance of 7 mm and repeat the process by decreasing the distance by 2 mm each time. You can enter the data below. Remember to write down the distance that you feel only one point.

    Somatosensory Chart.PNG


    1. Why can your fingertip detect such small distances between points while your arms and legs cannot?
    2. Would you expect to see a difference in males vs. females for the four recorded areas? What about children vs adults?
    3. Why doesn't your brain have the sensitivity of your fingertips all over your body?