Friday, December 11, 2009

Why pain sometimes lingers : Nature News

A once-mysterious neural pathway may have a crucial role in making injured areas overly sensitive to touch, a study in mice suggests.

When a person has any kind of injury — a broken shin, for example, or a sunburn — the pain system becomes hypersensitized, firing up in response to normally painless sensations induced by, for instance, walking or a gentle massage. Normally, this tenderness protects the vulnerable tissue as it heals. But occasionally the pain can overstay its usefulness, becoming chronic in conditions such as arthritis.

Now, neuroscientists Robert Edwards and Allan Basbaum from the University of California, San Francisco, and their colleagues have found that a small subset of nerve fibres, the function of which remained a puzzle since their discovery decades ago1, could be routing innocuous touch sensations to the pain pathway when there's an injury.

"Surprise would be an understatement," says Basbaum, referring to the findings. "No one knew anything about what these fibres were doing."

The team's findings are published by Nature2.

The researchers found that the fibres, called unmyelinated low-threshold mechanoreceptors (C-LTMRs), are easily stimulated, unlike classic pain fibres, which respond only when the sensation is intense. But C-LTMRs aren't usually used to detect light touch — this falls to another another major group of sensory neurons — so their role was unclear. The small population of cells have remained enigmatic because they have been difficult to target specifically.

The authors cleared that hurdle when they discovered

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