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Feeling a Point (or two)

How do you feel? No, I don't mean are you happy or sad? Touch the back of your hand. Did you feel it? How? When you touched your hand, you pressed on nerves in your skin. These nerves reacted and sent a message to your brain, telling you that something touched your hand. Some parts of your skin have more nerves than others. We are going to examine how these nerves are arranged, and see how that can affect the message that your brain gets.

You will need:

  • a paperclip
  • your skin
  • a friend
  • a ruler

Make a simple drawing of your friend's hand and lower arm. It does not have to be a masterpiece. You can even place the hand on a sheet of paper and trace around it.

Straighten the paperclip, and shape it into a large "V" shape. This will let you use each paperclip as a handle, with a point sticking out for testing. Bend the paperclip V until the two ends are about 1/4 inch apart. Have your friend place an arm on a table or flat surface and close her eyes. Gently touch the two points to the skin on her arm. You don't want it to hurt. Touch the skin just enough to feel. Ask her if it feels like one point or two. If it feels like two points, move the ends a little closer. If it feels like one, move them a little farther apart. Try to find the distance where it shifts from feeling like two points to feeling like one. Measure the distance with the ruler. Put an X on the drawing to show the location, and beside that, write the distance.

Next, try the same thing on the back of her hand. Again, find out how far apart the points need to be to have it just feel like one point and measure it.

Try one more time, this time on a fingertip. Be especially gentle here, as your fingers are more sensitive. How close can you get the points this time before it feels like one?

What did you find? On the arm, the points could be quite far apart and still feel like one. Your arm does not need to be very sensitive, as you don't usually use it to feel precisely. You need to know that there is a mosquito on your arm, but you don't need to be able to tell how many legs it has.

In your hand, your nerves are closer together, and so you can feel the individual points much closer. You use your hand for touching things, so it needs to be more sensitive. In your fingertip the nerves are so close together that unless the points are touching, you can still feel both of them. That is what allows people to read Braille, which is writing made up of patterns of bumps. You would find if very difficult to read Braille with your arm, or even with the back of your hand.

You can continue testing to learn more. Are there parts of your arm that are more sensitive than others? Does this same pattern match for your feet? Why, or why not?