JOIN USScientists have discovered that humans and animals love to be caressed because a specific class of sensory cells in the skin respond to gentle touch.
Both animals and humans seem to enjoy being stroked, but until now the neuronal circuitry underlying the sensation had been a mystery.
The skin is a human being's largest sensory organ, helping to distinguish between a pleasant contact, like a caress, and a negative sensation, like a pinch or a burn, the 'Daily Mail' reported.
Earlier studies have shown that these sensations are carried to the brain by different types of sensory neurons that have nerve endings in the skin.
Only a few of those neuron types have been identified, however, and most of those detect painful stimuli.
Researchers at the California Institute of Technology identified in mice a specific class of skin sensory neurons that reacts to an apparently pleasurable stimulus.
"We've known a lot about the neurons that detect things that make us hurt or feel pain, but we've known much less about the identity of the neurons that make us feel good when they are stimulated," said lead researcher David Anderson.
"Generally it's a lot easier to study things that are painful because animals have evolved to become much more sensitive to things that hurt or are fearful than to things that feel good," said Anderson.
The team was investigating a type of sensory cell discovered in 2007, the purpose of which had not yet been ascertained.
Researchers bred genetically modified mice with fluorescent molecular markers attached to the neurons which lit up when the particular cells were activated.
"We took advantage of the fact that these sensory neurons are bipolar in the sense that they send one branch into the skin that detects stimuli, and another branch into the spinal cord to relay the message detected in the skin to the brain," Anderson said.
Through a painstaking process of applying stimuli to one tiny area of the animal's body at a time, they found that these particular neurons lit up only when stroked.
Anderson said this was the first time anyone has used this type of conditioned place-preference experiment to show that activating certain neurons in the skin can actually make an animal feel good. P