Why we itch has long been a mystery. As soon as we feel an itch, we scratch without thinking. But people with constant itching can irritate their skin and make the inflammation worse, so scientists are looking for effective ways to stop chronic itch.
Many scientists link scratching with keeping clean: it rids us of poisonous plants, insects and the diseases they carry. A study by scientists at the University of Minnesota in Minneapolis, published in Nature Neuroscience, suggests that nerve cells in the spinal cord trigger the urge to itch, and that the same cells switch it off after scratching.
The researcher, Dr Glenn Giesler, said more than 50 conditions cause severe itching, such as AIDS and Hodgkin's disease, as well as side effects of drugs used for advanced cancer, although some medicines can ease certain kinds of itch.
In monkeys, the researchers found nerve cells near the bottom of the spinal cord that connect to the brain and carry the sensation of itch. Anaesthetised monkeys were treated with itch-causing chemicals, and the activity in their spinal cord was measured.
The scientists used a device that imitates scratching with three fingers. It reduced the nerve signals for itch, but when used without the itch (without the chemicals), it increased the activity instead.
The same cells that respond to itch also respond to pain. "We want to know how this happens so we can stop the feeling of itch," the researcher said, explaining that future drugs might relieve itch either by blocking the nerve message to the brain or by imitating scratching.
Dr Gil Yosipovitch of Wake Forest University commented: "This is a very important study. Millions of people around the world suffer from itch."
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