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(Photo: Tela Chhe/flickr)

The Mystery of Human Touch

• April 07, 2014 • 10:25 AM

(Photo: Tela Chhe/flickr)

Scientists are learning more about how we process feelings of gentle touch. New discoveries published Sunday could help restore fading sensations—and ease painful hypersensitivity.

There’s a fine line between pleasure and pain—and, in some people, it is cruelly drawn.

The normally gentle pressure of a cotton shirt, or what should be the arousing caress of a lover’s tongue, can be painful for tactile allodynia patients.

Two papers published by Nature (here and here) cast new light on how we experience the flutters and pressures of gentle touch. The discoveries could help lead to respite from the unending pain of tactile allodynia. They could also help to restore fading senses of touch for diabetes sufferers, cancer patients undergoing chemo—and for everybody else as they age.

New discoveries could help to restore fading senses of touch for diabetes sufferers, cancer patients undergoing chemo—and for everybody else as they age.

When our skin rubs up against an object, our brain quickly paints a mental picture of it. We get a clear sense of the shape and texture of whatever it is that we’re turning over in our hand, or that’s stroking unwelcomely against us in a crowded bar.

Merkel cells are critical to these feelings. They’re found at the bottom layer of mammals’ skin, where they act as information conveyor belts, feeding data from skin cells into our central nervous systems.

But it’s long been a mystery how Merkel cells actually work. Do they resemble nerve cells—which actively encode and pass along information as subtle electrical pulses? Or do they mimic some of the cells in our inner ears—which act as tiny amplifiers, enhancing sound information gleaned from the vibrations of tiny hairs?

Scientists subjected mice and their Merkel cells to tests. Mice were bred to produce fluorescing proteins involved in the functioning of Merkel cells for some of the experiments; other mice lacked genes needed to produce those proteins at all. The results led scientists to conclude that the way these cells work isn’t an “either/or” proposition—it’s an “and” scenario.

It turns out that Merkel cells actively encode basic information about pressure that’s being exerted when we touch something. And they also amplify and enhance subtler sensations—such as the shape of a keyboard beneath fingertips.

“[T]he Merkel cell–neurite complex is a compound sensory system with two receptor cell types that mediate different aspects of touch,” the scientists write in one of the papers, both of which were posted online Sunday ahead of print publication. “This provides the first direct evidence that Merkel cells are not simply passive mechanical filters in the skin.”

The findings reveal that the cells are special, but their dual systems are not entirely unique. A similar division of labor exists in our retinas, where rods help us see basic details in low light, while cones paint our surroundings in sharp and colorful detail.

Ellen Lumpkin, an associate professor at Columbia University and a co-author of both papers, says that “understanding the basis of gentle touch has implications for unrelieved pain, particularly tactile allodynia”—which she calls a “common and debilitating” condition.

The discovery also pushes forward science’s understanding of touch, which is sorely lacking when compared with its knowledge of vision, smell, taste, and hearing.

John Upton
John Upton is a science journalist with an ecology background. He has written recently for VICE, Slate, Nautilus, Modern Farmer, Grist, and Audubon magazine. He blogs at Wonk on the Wildlife. Upton's favorite eukaryotes are fungi, but he won't fault you for being human. Follow him on Twitter @johnupton.

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