The Science of Pain

U.S. scientists say they've determined how the human body differentiates various kinds of pain, overturning what has been conventional wisdom.

Scientists from the California Institute of Technology and the University of California-San Francisco say they've shown how different sensory neurons -- called nociceptors -- respond to different kinds of pain stimuli.

"Conventional wisdom was that nociceptive neurons in the skin can't tell the difference between heat and mechanical pain, like a pin prick," Caltech Professor David Anderson said. "The idea was that the skin is a dumb sensor of anything unpleasant, and higher brain areas disentangle one pain modality from another ... "

But that theory, Anderson said, didn't explain control of pain-avoidance behavior.

Anderson and UCSF Professor Allan Basbaum created a genetically engineered mouse in which pain-sensing neurons can be selectively destroyed. They discovered when a certain population of nociceptor neurons was killed, the mice stopped responding to being poked, but still responded to heat. Conversely, when the researchers destroyed a different population of neurons, the mice stopped responding to heat, but their sense of poke remained.

"This tells us the fibers that mediate the response to being poked are neither necessary nor sufficient for a behavioral response to heat," Anderson said, "And vice versa for the fibers that mediate the response to heat."

The findings appear in the early online edition of the Proceedings of the National Academy of Sciences.