A gentle rub of the head has long been used by parents to soothe children.

Researchers have found that some nerves in the skin send 'feel good' signals to the brain when activated by just the right amount of stroking.

The 'C fibre' nerves usually transmit messages of pain but record pleasure when the skin is stroked at about four centimetres per second, according to the study carried out by scientists in Britain, Germany and the United States and is published in the journal, Nature Neuroscience.

Researchers demonstrated the effect of C-fibres on volunteers using a 'robotic tactile stimulator' – a mechanical arm fitted with soft brush. Sensually caressed by the robot, the volunteers produced C-fibre signals that could be recorded.

Professor Francis McGlone, one of the scientists from the Unilever company's Port Sunlight laboratories in Wirral, Cheshire, said: "If you get a piece of grit in your eye, have a toothache, or bite your tongue, it hurts so much because there are more C fibres there. The research we have been doing is building evidence for another role of C fibres in the skin that are not pain receptors, but are pleasure receptors."

He said the findings appear to explain "the pleasant… aspects of touch we are all familiar with, such as when grooming or being cuddled".

The nerves are found in skin covered by hair but are absent in the palms of the hands. "We believe this could be Mother Nature's way of ensuring that mixed messages are not sent to the brain when it is in use as a functional tool," said Professor McGlone.

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