How the body differentiates between a scorch and a scratch

May 20th, 2009 - 1:27 pm ICT by ANI  

Washington, May 20 (ANI): American scientists have shed new light on how the body figures out whether it has been stuck by a pin or burnt by a match.

Researchers from the California Institute of Technology (Caltech) and the University of California, San Francisco (UCSF) have found that this sensory discrimination begins in the skin at the very earliest stages of neuronal information processing, with different populations of sensory neurons-called nociceptors-responding to different kinds of painful stimuli.

“Conventional wisdom was that the nociceptive neurons in the skin can’t tell the difference between heat and mechanical pain, like a pin prick. The idea was that the skin is a dumb sensor of anything unpleasant, and that higher brain areas disentangle one pain modality from another, to tell you if you’ve been scorched or scratched,” says David Anderson, Seymour Benzer Professor of Biology, a Howard Hughes Medical Institute (HHMI) Investigator, and one of the paper’s lead authors.

However, that was not sufficient to understand the control of pain-avoidance behavior, the researcher added.

“We were asking the cells what the cells can sense, not asking the animal what the cells can sense,” he said.

For their study, Anderson and his colleague Allan Basbaum, chair of the Department of Anatomy at UCSF, created a genetically engineered mouse in which specific populations of pain-sensing neurons could be selectively destroyed.

The researchers were then able to see if the mouse continued to respond to different types of stimuli by pulling its paw away, when exposed to a relatively gentle heat source or poked with a nylon fishing line.

When they killed off a certain population of nociceptor neurons, the mice stopped responding to being poked, but still responded to heat.

When the researchers injected a toxin to destroy a different population of neurons, the mice stopped responding to heat, but their sense of poke remained intact.

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

The researcher further said that neither of the two classes of sensory neurons seemed to be required for responding to a painful cold stimulus, like dry ice.

He said that research into pinpointing that population of cells was ongoing.

“This tells us that the discernment of different types of painful stimuli doesn’t happen only in the brain–it starts in the skin, which is therefore much smarter than we thought. That’s a pretty heretical point of view,” said Anderson.

The study has been published in the early online edition of the Proceedings of the National Academy of Sciences (PNAS). (ANI)

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