Part of the inspiration for the new groups has come from a deeper understanding of pain made possible by advances in research techniques. Not long ago neuroscientists debated whether pain was a separate sense at all, supplied with its own nerve cells and brain centers like the senses of hearing or taste or touch. Maybe you hurt, the scientists reasoned, because nerve endings sensitive to touch are pressed very hard. To some extent, that is true: Some nerve fibers in your skin will be stimulated by a painful pinch as well as a gentle touch. But neuroscientists now know that there are many small nerve cells with extremely fine nerve fibers that are excited exclusively by intense, potentially harmful stimulation. Scientists call the nerve cells nociceptors, from the word noxious, meaning physically harmful or destructive.

Some nociceptors sound off to several kinds of painful stimulation -- a hammer blow that hits your thumb instead of a nail; a drop of acid; a flaming match. Other nociceptors are more selective. They are excited by a pinprick but ignore painful heat or chemical stimulation. It's as though nature had sprinkled your skin and your insides with a variety of pain-sensitive cells, not only to report what kind of damage you're experiencing, but to make sure the message gets through on at least one channel.

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