Scientists finally find where to scratch
(This story appeared on page 245 of the
Oct. 18, 1997, Science News.)
By Sid Perkins
Science News
One of humanity's age-old quests may be nearing an end. Researchers in
Europe report identifying a new kind of nerve fiber that is probably
responsible for transmitting the sensation of itching.
The characteristics of the nerve fibers fit a previously
proposed model for so-called itch units and may help explain why
the fibers haven't been observed before, the scientists report
in the Oct. 15 JOURNAL OF NEUROSCIENCE.
Neurobiologists have long surmised that itch units are a type of nerve
fiber especially sensitive to histamines, says pain researcher H. Erik
Torebjork of Sweden's University of Uppsala. Histamines are chemicals
released when any of a variety of irritants triggers the body's many
allergic responses, one of which is itching.
Furthermore, scientists had evidence that the itch units are unmyelinated
- that is, they lack an insulating sheath of white, fatty material called
myelin. Histamine-induced itching does not stop when the transmission of
electric impulses along myelinated nerve fibers is blocked. However, such
itching does cease if the skin is treated with capsaicin, the chemical
that makes peppers hot and temporarily disables unmyelinated nerve endings
(SN: 11/14/92, p. 333).
The skin contains thousands of unmyelinated nerve endings. In a study of
53 people, Torebjork and his colleagues used the itching response to
locate 56 unmyelinated nerve fibers branching from one of the major
nerves in the lower leg. The researchers caused itching by applying a
histamine gel and driving it into the skin with a mild electric
current.
By inserting a probe deep into each volunteer's knee, the researchers
monitored the patterns of electric impulses as they traveled along the
major nerve from the skin of the shin and foot to the brain.
Eight of the 56 nerve fibers showed a long-lasting electric response to
the histamine. Consistency between the pattern of electric signals in
the nerve fibers and the itching sensation reported by volunteers provides
strong evidence that the team has found the itch units scientists have
sought for more than a century, says Torebjork.
There are several reasons why previous searches for the elusive itch
units proved fruitless, he adds. The small diameter of the nerve fibers
and their lack of a myelin sheath make them difficult to detect, but the
primary reason may be related to the response of the nerve fibers.
Few studies have used reactions to histamines to search for nerve endings.
Until recently, most scientists thought that every unmyelinated nerve
fiber in human skin is polymodal - capable of responding to several
stimuli, including heat, touch, and chemicals such as histamines.
In the recent study, however, none of the eight nerve fibers that showed a
strong reaction to histamine responded to touch, and only five responded
to heat. Because researchers typically use pressure to search for nerve
endings, these "silent" nerve fibers have gone unnoticed, Torebjork
says.
Although the newly found nerve fibers are small, their ends branch into
tendrils that cover a lot of territory - for a nerve, that is. The average
polymodal nerve fiber in the lower leg covers an area only 2.4 centimeters
in diameter; one of the nerve fibers that the researchers found branches
to cover an area of skin 8.5 cm across.
M.W. Greaves, a dermatologist at St. John's Institute of Dermatology in
London, says the new results are "interesting but of uncertain relevance"
because the itch units described make up only a small proportion of
all the nerve fibers identified by the researchers.
Moreover, Greaves says, the study shows only that the newly discovered
nerve endings are sensitive to histamines, whereas itching can be caused
by other substances as well.
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Copyright 1997 by Science Service.
All rights reserved.
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