Scientists finally find where to scratch.

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 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.