Scientists have discovered a distinct set of "pleasure nerves" in the skin that can alleviate pain when gently stroked.
The discovery could lead to new treatments for conditions ranging from chronic itching to depression.
Professor Francis McGlone of Unilever and Liverpool University said yesterday that the nerves responded to being brushed slowly and appeared to be sensitive to the type of stroking and cuddling provided by a mother to an upset child.
He said tests on human volunteers had found that a painful stimulus applied to the skin could be eased significantly by gently stroking the pleasure nerves in a nearby part of the body.
The nerves are part of the so-called C fibres of the nervous system, which produce the sensation of pain in the skin. But instead of stimulating pain, a subset of the fibres also appears to stimulate pleasure.
"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," Professor McGlone told the British Association for the Advancement of Science at Liverpool University.
"There is another sensory nerve fibre system in human skin that appears to code for the pleasant and affiliative aspects of touch we are all familiar with, such as when grooming or being cuddled."
There was growing evidence that touching the skin and gentle stroking of the body stimulated an evolutionary ancient part of the nervous system that made people feel good when cuddled by a loved one or groomed by themselves.
"Grooming behaviours are not at a functional level for removing dirt. I think we groom primarily to feel good ... One of the hallmarks of clinical depression is that people stop looking after themselves."
He said some parts of the body were richer in the pleasure nerves than others. They were not present at all, for instance, on the palms of the hands or the soles of the feet.
"We've tested everything from forehead, forearm, upper leg and the face and we find heterogeneity in response to touch. They haven't been found in the genitalia, but I think another system is responsible for tactile reward in that area.
"With pain it has been clearly established that without such a sense we would not survive, and now we are beginning to understand that without a sense of pleasure, or reward, behaviours that we take for granted, like the caress between lovers and the nurturing of babies we would also not survive."
Doctors have already realised that premature babies do better when cuddled straight after birth.