The stroking impulse could have evolved out of the need to keep fur and hair free from parasites
A gentle stroke on the head or arm could feel pleasant because these areas of the skin contain specialised nerve cells that send "pleasure pulses" to the brain when stimulated, scientists have discovered.
The existence of special pleasure detectors in hairy skin has long been debated but now researchers have finally found strong evidence of their existence – in laboratory mice at least.
The discovery could explain why many mammals, from cats and dogs to humans, take pleasure in caressing and grooming one another, the scientists suggest. It could have evolved out of the need to keep fur and hair free of parasites.
The study, carried out by David Anderson and colleagues at the California Institute of Technology in Pasadena, has shown that the hairy skin on the back legs of mice contains sensory nerve endings that respond to pleasurable rubbing, but not to pain.
The study, published in Nature, showed that certain types of neurons or nerve cells in the hind legs of mice were activated when stroked. Unlike pain-detecting neurons, these nerve cells are not covered by a protective fatty sheath, which causes the nerve impulses to the brain to be transmitted at slower speed than usual.
"Despite numerous physiological studies, molecularly defined sensory neurons that detect pleasant stroking of hairy skin in vivo have not been reported," the scientists say in their study.
The new kind of nerve cells, called C-fibres, only appear to exist in hairy skin. However, it has not yet been shown that only certain C-fibres in humans are dedicated to pleasure sensing.