Module 1 The Nervous System

1.1.4 page 5

Touch

You are about to be introduced. The room is so hot. Your hand is so sweaty that it feels sticky. Wow, this person has a firm handshake. Your brain tells you that the finger you sprained in basketball practice is hurting from the pressure of the handshake. You wince in pain and pull away quickly. To understand more about touch, read pages 427 to 429 of the textbook.

The mechanoreceptors for touch are all over the body, but they aren’t evenly distributed. Consider how sensitive your fingertips and lips are compared to the back of your hand. They can detect light touch, pressure, pain, and high and low temperatures. From the previous activity, when you put your hand into the ice water and held it there, it wasn’t long before you felt the freezing sensation. How would it feel if it was your elbow in the freezing water? Do you think you could keep your elbow in the cold water longer than your hand?

Some Human Sensory Receptors in the Skin

Inquiry into Biology u5_S12.27_p.429, from McGraw Hill Reproduced by permission.

In the graphic above, the figure marked "free nerve endings" is a free dendrite ending as found in thermoreceptors (hot and cold receptors) as well as some pain receptors. The remaining figures have enclosed dendrites in capsules. These sensory receptors are mechanoreceptors that sense touch, pressure, vibration (on and off pressure), and stretch stimuli. Krause’s end bulb shown by the third figure on the left and Meissner’s corpuscle shown on the top right are sensitive to light touch but are located in different parts of the body.

From the diagram, can you hypothesize which type of receptor is more concentrated in your hand than in your elbow, which would explain why your hand was more sensitive to the cold water?

Try This

Bend a paper clip into a U-shape with the two ends about 2 mm apart.  Close your eyes, and gently push down on the palm of your hand with the paper clip. Then gently press the paper clip on your shoulder, and your elbow. In which location could you distinguish the two prongs as being separate?

What do your results suggest about the number of mechanoreceptors for touch in the palm of your hand, relative to the number in your shoulder? You might want to look at Figure 12.27 on page 429 to review the various types of touch receptors.

Self-Check

Complete the self-check activity below.