(Roughly) Daily

… and, as Ariel Bleicher explains in her report on the work of neurobiologst David Ginty, so much more…

Like many proud parents, David Ginty (opens a new tab) has decorated his office with pictures of his genetic creations. There’s the prickly one sporting a spiked collar and the wannabe cowboy twirling a lasso. There’s the dramatic one, always reacting to the slightest provocation; the observant one that notices every detail; the golden child Ginty loves to boast about. “They’re like a family,” he said. “Each one has its own quirks and individual characteristics.”

They’re not really a family and, anyway, they’re not his children. They have evolved over millions of years to give humans and other mammals an interface with the physical world around us. But Ginty, who heads the neurobiology department at Harvard Medical School, has been studying this quirky cast of characters — the sensory neurons of touch — for more than two decades, and has gotten to know them better than anyone else ever has. He has learned their electrical language and what forces excite them, and charted their intricate paths into the skin and up to the brain. And, through feats of genetic engineering and chemical labeling, he has produced the colorful portraits on his walls.

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

… 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.”

Scientists and philosophers have been enamored with touch for centuries. Aristotle believed that humans’ tactile abilities eclipsed those of all other species, which partially accounted for our superior intelligence. However, we now know that creatures as diverse as sea lions, spiders and star-nosed moles can feel features of the physical world that are imperceptible to us. Yet Aristotle wasn’t wrong to view touch as exceptional.

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.

But how?…

[Bleicher recounts the history of exploration of touch, and unpacks Ginty’s pioneering work…]

… Over the past five years, Ginty and other scientists have analyzed the genetics of thousands of individual touch neurons. Sorting these cells according to the genes they express, Ginty’s team has so far come up with 18 distinct types, maybe more — it’s hard to tell, given the limited resolution of their sorting tools. That total includes the six or seven gentle-touch neurons on which Ginty’s research has primarily focused, as well as six neurons for stronger mechanical stimuli (some of which also respond to temperature and chemical irritants), one neuron for painful heat, one for cold, three or more for sensing body position, and a few whose functions are unknown.

As more touch neurons are analyzed, the count will likely increase. And each genetically distinct type can be further subdivided based on its axon endings. Genetically identical neurons that form Meissner corpuscles for picking up vibrations in glabrous skin, for example, also form lanceolate endings for detecting hair movement in hairy skin. In a 2023 study, Ginty’s team showed that these same touch neurons also innervate the colon, where their axons branch and curl around motor neurons in the gut, enabling us to sense bowel distention. “So you might say there are actually 50 or 60 different touch neuron types,” he said, if you count both genetic and physical variations. “We don’t know how many there are.”

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.”…

Cataloging the neurons beneath everyday sensations: “Touch, Our Most Complex Sense, Is a Landscape of Cellular Sensors,” from @quantamagazine.bsky.social.

* John Keats

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As we celebrate the senses, we might recall that on this date in 1927 a sensory expert in a different domain, Mae West, was sentenced to jail for obscenity.

Her first starring role on Broadway was in a 1926 play entitled Sex, which she wrote, produced, and directed. Although conservative critics panned the show, ticket sales were strong. The production did not go over well with city officials, who had received complaints from some religious groups, and the theater was raided and West arrested along with the cast. She was taken to the Jefferson Market Court House (now Jefferson Market Library), where she was prosecuted on morals charges, and on April 19, 1927, was sentenced to 10 days for “corrupting the morals of youth.” Though West could have paid a fine and been let off, she chose the jail sentence for the publicity it would garner. While incarcerated on Welfare Island (now known as Roosevelt Island), she dined with the warden and his wife; she told reporters that she had worn her silk panties while serving time, in lieu of the “burlap” the other girls had to wear. West got great mileage from this jail stint. She served eight days with two days off for “good behavior”.

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