Touch, Our Most Complex Sense, Is a Landscape of Cellular Sensors

“David Ginty is the emperor of touch,” said Alexander Chesler, a sensory neuroscientist at the National Institutes of Health.

“You look at his publication list and you go, ‘Oh my God,’” said David Hughes, a neuroanatomist at the University of Glasgow. “He’s so massively productive, and all his papers are published in the very highest-impact journals.”

Beyond the technical breakthroughs and the discoveries fit for biology textbooks, it’s the images that stick in his colleagues’ minds. They’re otherworldly, like deep-sea creatures — not at all what you might imagine neurons could look like. These strangely shaped cells are the reason why the experience of touch is so rich and multifaceted — why a buzzing cell phone feels different from a warm breeze or a lover’s caress, from raindrops or a mother’s kiss. To realize that your body is covered in them — that they are a part of you — takes your breath away.

“Each one of these neurons tells a story,” Ginty said. “Each one has a structure that is unique and responds to different things. It’s all about form underlying function. That’s where the beauty is.”

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Of all the senses, the somatosensory system is the most complex, and therefore touch, some researchers argue, is the most difficult to study. Vision and hearing, for instance, are confined to the retina and the cochlea — parts the size of a postage stamp and a pea, respectively. Touch, however, is diffuse: The neurons that relay touch signals reside in clusters outside the spinal cord, from which they extend a vast web of axon fibers, like jellyfish tentacles, into the skin and internal organs. Each axon forms an ending just beneath the skin’s surface; the different types of endings are mechanisms for picking up and interpreting the variety of touch sensations.

While our eyes and ears each process information related to light or sound, touch concerns a smorgasbord of stimuli, including pokes, pulls, puffs, caresses and vibrations, as well as a range of temperatures and chemicals, such as capsaicin in chili peppers or menthol in mint. From these inputs arise perceptions of pressure, pain, itchiness, softness and hardness, warmth and cold, and the awareness of the body in space.

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Ginty will keep counting them. Today he’s asking the same fundamental questions he set out to answer more than a decade ago: Where do the various touch neurons go, what are their end structures, and how do they capture the richness of the physical realm? “We’ve gotten a pretty good handle on who’s who in the skin and what their response properties are,” Ginty said. But what about the heart, lungs, larynx, esophagus, stomach, intestines and kidneys? What are the neurons that make muscles ache and fatigue, or trigger migraines, or cause milk to flow in a mother’s breast when her baby suckles?

Ginty also wants to know how all these neurons connect to the brain to generate perceptions. How does pressure and vibration across millions of nerve endings become a hug? How do we feel wetness, slipperiness or elasticity? “Think about squeezing a balloon,” he said. “Presumably no one sensory neuron type is going to encode squeeziness.”

His work has transformed our understanding not only of individual touch sensors, but also of their connectivity. Until recently, the canonical view was that touch signals, like a telephone conversation, travel along fixed lines all the way to the somatosensory cortex, the part of the outer brain associated with sense information. “So any higher-order feature of the tactile world was seen as an emergent property of the cortex,” Ginty said. But his research and that of others has caused a paradigm shift. It’s now clear that a great deal of information carried by touch neurons converges in the spinal cord and brainstem before reaching the cognitive parts of the brain, suggesting that the touch signals are processed earlier in the neurobiological pathway than once believed.

If you ask Ginty what all this knowledge is good for, he’ll list the predictable applications: better pain drugs, improved treatments for sensory processing disorders  such as autism, more lifelike prosthetics. But what really motivates him is something less tangible: awe. His work, he’ll tell you, has given him a deeper appreciation of this sense we so often take for granted — how nuanced and multidimensional it is, and how much it can still surprise him.

Not long ago, he said, he attended a performance of the Boston Symphony Orchestra. “I put my fingers on the chair, and I closed my eyes and just felt the music.”

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