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Comment: We've all had an itch we need to scratch, but what exactly is it?

As new research claims to hold the key to suppressing the irritating sensation, Steve Connor explains what causes an itch

An itch is not a tickle, although some people may think it is funny. Neither, according to the latest research, is an itch a form of pain, although try telling that to people tormented by chronic, long-term scratching.

The scientific evidence for one of life’s odder sensory experiences is that the sensation of itch depends on a dedicated set of nerve cells leading from the spinal cord to the brain, complete with their own chemical messenger, a neurotransmitter known as Nppb.

Take away Nppb and the itch disappears. Put it back again in the spinal cord, and the irritating sensation returns. It is easy to see a potential for a drug that blocks Nppb in chronic scratchers.

The worst type of itch that most of us will suffer from is the temporary sort that comes after a mosquito bite. This is a result of the immune system releasing a substance called histamine into the skin to counteract all the nasty ingredients found in a mosquito’s saliva.

Most itches – even the seven-year variety – eventually wane and disappear, especially with a bit of an effort to avoid scratching them too much. But there are some itches that just won’t go away – such as the one that led a distressed Massachusetts woman to scratch right through her skull case to expose the brain.

An itch is loosely and simply defined as an unpleasant sensation that provokes a desire to scratch. It is thought to have evolved as a defence against parasites trying to burrow through the skin, but the sensation seems to have a psychological as well as a physical component – an itch can be brought on merely at the thought of something itchy.

Certainly the fear of chronic itching inspired one of the tortures described in Dante’s Inferno: “the burning rage of fierce itching that nothing could relieve”.

One of the worst things about an itch, of course, is that once you develop the habit, it is difficult to stop. Scientists call this the “itch-scratch cycle”. So what may have started out as a pleasantly relieving hobby becomes an addictively painful occupation.