How does body differentiate between scorch and scratch?


Washington : How do you know you’ve been pricked by a pin or burnt by a match?

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Scientists from Caltech and University of California, San Francisco (UCSF) have shown that this sensory discrimination begins in the skin at the very earliest stages of neuronal (nerve cells) information processing.

“Conventional wisdom was that the nociceptive (caused by or in response to pain) neurons in the skin can’t tell the difference between heat and mechanical pain, like a pin prick,” said David Anderson, professor of biology and Howard Hughes Medical Institute (HHMI) investigator and study co-author.

“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,” he said.

This conventional wisdom came from recording the electrical responses of nociceptive neurons, where it was shown that these neurons are capable of sensing pretty much every kind of painful stimulus – from pin pricks to heat to cold.

But this, Anderson notes, was not sufficient to understand the control of pain-avoidance behaviour. “We were asking the cells what the cells can sense, not asking the animal what the cells can sense,” he explained.

And so Anderson and co-principal investigator Allan Basbaum, of UCSF, decided to ask the animal. To do so, they created a genetically engineered mouse in which specific populations of pain-sensing neurons can be selectively destroyed.

They 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.

What the researchers found was that when they killed off a certain population of nociceptor neurons, the mice stopped responding to being poked but still responded to heat, said a UCSF release.

Conversely, 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 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,” says Anderson. “That’s a pretty heretical point of view.”

It’s also a potentially useful point of view, as Anderson points out. “If doctors want to repair or replace damaged nerve fibres in conditions such as diabetic neuropathy,” he explains, “they need to make sure they’re replacing the right kind of nerve fibres.”

Their findings were published this week in the early online edition of the Proceedings of the National Academy of Sciences (PNAS).