Some nerve fibers seem to love a good rubdown. These tendrils, which spread across skin like upside-down tree roots, detect smooth, steady stroking and send a feel-good message to the brain, researchers report in the Jan. 31 Nature.
Although the researchers found these neurons in mice, similar cells in people may trigger massage bliss. The results are the latest to emphasize the strong and often underappreciated connection between emotions and the sensation of touch, says study coauthor David Anderson, a Howard Hughes Medical Institute investigator at Caltech. “It may seem frivolous to be studying massage neurons in mice, but it raises a profound issue — why do certain stimuli feel a certain way?” he says.
It’s no surprise that many people find a caress pleasant. Earlier studies in people suggested that a particular breed of nerve fibers detects a caress and carries that signal to the brain. But scientists hadn’t been able to directly link this type of neuron to good feelings, either in people or in animals. “The beauty of this paper is that it goes one step further and adds behavioral elements,” says cognitive neuroscientist Francis McGlone of Liverpool John Moores University in England.
Directly linking these neurons with pleasure clarifies the importance of touch, McGlone says. “Skin is a social organ,” he says. A growing number of studies show that the sensation of touch, particularly early in life, profoundly sculpts the brain. Young animals deprived of touch grow up with severe behavioral abnormalities. Babies fare better when they are held and touched frequently. And touch sensation can be altered in certain disorders. People with autism, for instance, often dislike caresses.
The new study relied on mice genetically engineered so that a select population of nerve cells would glow when they sensed a caress. These neurons, which Anderson and colleagues described in 2007, possessed the attributes of massage sensors, but they stubbornly refused to respond to touch in experiments in lab dishes. But by touching the genetically engineered animals’ skin, the researchers were able to study these cells in live mice.
A brush stroke with pressure between a light tickle and a forceful push, similar to what a mouse might feel from a firm lick, made the nerve cells light up. A harsher poke, with a more focused point of pressure, didn’t elicit a reaction from the cells. These neurons, which all carry a protein called MRGPRB4, seem tuned to detect a steady stroke, Anderson says.
Next, the researchers tested whether this stroke felt good to mice. For these experiments, the scientists used a different kind of genetically engineered mouse, one with caress-sensitive neurons that a drug could activate. When the researchers dispensed the drug in a particular room, the mice soon learned to prefer that room over others, associating it with the presumably enjoyable sensation of being stroked. “That tells us that activating these neurons is pleasant to the animal,” Anderson says.It’s not yet clear whether the nerve fibers in the mice have exact analogs in humans, cautions clinical neurophysiologist Håkan Olausson of the University of Gothenburg in Sweden. But this new view of caress detection, which he calls “fantastic,” offers a deeper understanding of touch.