Hairy or furry skin is hard-wired for petting and stroking sensations, creating intense pleasure when touched this way, a study finds.
The research, published in the latest issue of the journal Nature, helps to explain why pets love to be petted, and possibly why human body pleasure zones tend to be where multiple hair follicles exist.
The ultimate euphoria center, though, is in the brain.
"Scientists care about what feels good to animals, not just about things that feel bad, like painful stimuli," co-author David Anderson told Discovery News. "Both are important for understanding how the brain interprets the world around us and guides our behavior."
Anderson directs the David Anderson Research Group at the California Institute of Technology. He and his colleagues focused their analysis on mice, which often serve as models for other mammals.
The scientists used high tech imaging to monitor how neurons were activated when the mice were touched in various innocuous ways. A custom-designed brush and other methods were used to poke, pinch and stroke the mice.
These experiments revealed a previously undiscovered population of sensory neurons that ""innervate hair follicles," Anderson said. These neurons appear to be solely dedicated to massage-like stroking sensations. They were not activated during the other forms of touch.
"The neurons that detect stroking are probably wired into higher brain circuits that produce a reward or pleasure," he continued.
The researchers suspect similar sensory neurons with comparable properties exist in humans and most furry mammals. Since the sensation is connected to hair follicles, animals with many of them – such as cats and dogs – likely feel waves of pleasure when being petted. They certainly look like they do, as many pet owners could attest.
In terms of research directly on humans, a prior study found that hairy arm skin responds to gentle stroking more so than areas of skin with fewer hair follicles.
"We don't know if the striking sensation would be lost if hair/fur fell out," Anderson said. Since the neurons are connected to follicles, it is possible that hair and fur are not necessary. A bald person, for example, can enjoy a head massage just as much as someone who has a full head of hair.
It is not clear why furry and hairy mammals possess the specialized neurons, but the scientists suspect that they evolved to promote the social and physical benefits tied to personal grooming and the grooming of others.
In humans, they might also help to explain the purpose and location of pubic hair. This hair could additionally promote warmth and protection, as well as facilitate the release of pheromones to attract mates.
Ardem Patapoutian of the Scripps Research Institute is a leading expert in sensory neuron function. He said that he agrees with the new paper's conclusions.
"Also," Patapoutian added, "I do believe that humans might also have similar specialized neurons."
He and Dr. Anderson, however, say that further studies are needed before that theory is fully confirmed.