Anatomy and Physiology: The Senses
Without receiving messages from the outside world, the rest of the nervous system is only so good. Our survival depends on being able to know what's happening in the outside world and about our place in that world. Students ask me now and then about extrasensory perception (ESP), of which there is no scientific support (which is why it deserves no further mention in this section), but they incorrectly label “it” the sixth sense. They were raised on the idea of five senses, when there are in fact seven. The traditional five—smell, vision, hearing, taste, and touch—are all concerned with receiving information about the outside world.
The other two senses, balance and proprioception, relate to the body's position in the world; balance is concerned primarily with the position of the head, and proprioception is concerned with the position of the muscles in relation to the body (for example, where your arms are). Despite their differences, all seven senses require some stimulus to open ion channels in neurons. So let's look around, listen for nuances, and make some sense out of the senses.
Excerpted from The Complete Idiot's Guide to Anatomy and Physiology © 2004 by Michael J. Vieira Lazaroff. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.