How scratching can stop an itch

Nerve cells play a key role in itching

However, the effect only seems to occur during itchiness itself - scratching at other times makes no difference.

While it is widely-known scratching relieves an itch, the physiological mechanisms for how this works are little understood.

The University of Minnesota study appears in Nature Neuroscience.

Previous research has suggested that a specific part of the spinal cord - the spinothalamic tract - plays a key role.

Nerve cells in this area have been shown to be more active when itchy substances are applied to the skin.

Blocks activity

The latest work, in primates, found that scratching the skin blocks activity of nerve cells in the spinothalamic tract during itchiness - preventing the spinal cord from transmitting signals from the scratched area of skin to the brain.

Researcher Dr Glenn Giesler hopes the work could lead to ways to relieve chronic itch effectively for the first time. However, he said more information was still needed about the chemistry underpinning the effect.

Professor Gil Yosipovitch, an expert on itching from Wake Forest University in North Carolina, said the finding was "potentially significant".

He said: "Although there is a long way to go, methods that can induce a pleasurable scratch sensation without damaging the skin, via mechanical stimuli or drugs that can inhibit these neurons, could be developed to treat chronic itch."

However, Professor Yosipovitch stressed that scratching and itching were complex phenomena involving factors such as emotions as well as physiology.

"The main open question is what happens in patients who suffer from chronic itch where scratching may actually aggravate itch perception."

Professor Patrick Haggard, of University College London, said: "We all know that scratching helps alleviate itch, but this elegant study helps to show how this mechanism works.

"It's an interesting illustration of a very general principle of the brain controlling its own inputs, in this case by making movements that triggers an interaction between scratchy touch and itch."

Dr Paul Bays, based at UCL's Institute of Cognitive Neuroscience, agreed that the study provided an important part of a physiological explanation for how the sensation of itch is reduced.

"However, it is still unclear why scratching should have this effect, or why it is only effective for itches and not for painful sensations - which are transmitted to the brain through the same pathway."