领域：垂类场景 · 食品健康

For every spike in voltage there was a small but predictable increase in pleasure
With so much variety, it is telling when something remains constant. Try an experiment: lick your fingers as though you were about to turn a page. Instinctively, you’ve licked the spot where fingers grip light objects, and at its centre are the concentric ridges and grooves that define your fingerprint. If you move your finger over an object in most directions, the object will run roughly perpendicular to these ridges, allowing friction to tug on each ridge as though toppling a wall. This central, bulbous part of your fingertip also contains the finest, densest set of ridges. You can see this if you follow your finger a short distance toward your palm, where the ridges become progressively wider. It is no coincidence that the ridges are finest, most centred on the part of your finger that first makes contact with an object. It is also where the nerve endings that sense touch are most dense. If you’re the caressing sort, recall how you have touched a lover, your fingertips scanning as they glide slowly over skin. Perhaps your palm lay flat, presenting the largest possible surface for contact.

The ridges of our fingers and hands are densely innervated by sensory neurons, nerve cells that translate pressure into changes in voltage. These sensory neurons come in a variety of forms suited for their tasks, named after neuroscientists like Merkel, Ruffini, Meissner and Pacini. Nerve endings can be capped with structures called disks, capsules or corpuscles – each defined by a distinctive weight or stiffness. These tips make the neurons more or less sensitive to pressure. The nerve endings that sense touch can be buried deep in the skin or can be so near the surface you could find them within the ridge of a fingerprint.

When the pressure and depth of touch are just right, the surface of the sensing neuron is deformed, stretched until the tension opens channels that let electrically charged salt ions flow in and out of the cell. The voltage change caused by the flow of ions zips along a cable-like projection to the spinal cord, where it gets passed on to other nerve cells and eventually to the brain. We can judge how smooth or pliant something is because voltages conveying the complex patterns of pressure arrive quickly enough for our brains to perceive subtle variation in timing. Without this ability, touch would feel like a surveillance tape played at half-speed: blurred and coarse. Like other species, we gain this speed by insulating our cables. Nerve cells are highly specialised, and require companion cells to help them with the daily details of cellular living. Some of these companions have developed means of enveloping the cable-like projections of neurons, becoming flat and wrapping themselves around the exterior of the cable again and again, like a king-sized sheet swaddling an infant. Or like rubber coating wire.

Insulated neurons are responsible for fine touch, but there is a second class of receptors that remain bare. These bare nerve endings are slower, and respond to coarser kinds of stimuli. Science has long known that these unmyelinated neurons respond to temperature, pain, tickle and itch. But we have only recently learned that they also respond to the pleasurable sensation of caress. Researchers in Sweden recorded data from neurons in the skin of human subjects as they exposed them to soft slow touch. For every spike in voltage there was a small but predictable increase in pleasure. While these naked neurons are missing in the hairless skin of our fingers and palms, they are found on the rest of the body, on the places you might touch with affection or consolation. And naked fibres are particularly abundant in the places we like to juxtapose – our lips, nipples, genitals and anus. The clitoris and the glans are enmeshed in the unmyelinated ends of sensory neurons. Inexplicably, we have often assumed these naked fibres were there for the sensation of pain, as though we had never known the joy of sexual touch.

n the naked stream, touch can be warm or rapturous or full of hurt

Each touch receptor propagates voltages upward toward the spinal cord and brain, voltages that float like bottles bearing notes along a waterway defined by the spindly extensions of sensory neurons. Each current conveys its own kind of message, and the myriad currents coalesce into two north-bound streams.

Of these streams, the routes of discriminative touch are particularly well mapped. In the 1930s, Canadian neurosurgeon Wilder Penfield electrically stimulated the brains of epileptics, probing the cortex for the origin of seizures. Patients had to be awake for this procedure so that he could ask them what experiences were evoked by the faint electrical current. Electricity alone was enough to elicit the feeling of being touched on an arm, or, when delivered to a nearby region of cortex, the shoulder.

Penfield found that the brain contained precise maps of the body; he charted duplicate maps of both touch and movement, side by side, along adjacent folds of the cortex. The resulting ‘homunculus’ is an iconic image in neuroscience – a strange representation of the body whose distortions, like early maps of the world, reflect how we value the body’s surface. Those areas where touch is most sensitive are inflated. And three-dimensional reconstructions of these maps reveal a grotesque caricature of our evolutionary past. Our fingers, faces, palms, lips, tongues and genitals are all out-sized. The map of our brain’s control of movement is similarly distorted – our hands and mouths in particular are both exquisitely sensitive and extraordinarily precise. Play the piano or fellate a pianist and you will invoke our specialisations of sensation and motion to equal degrees.

Perhaps the most remarkable attribute of discriminative touch is that it reveals just how malleable our brains can be. The brains of patients born with syndactyly, in which two or more fingers are fused, represent that set of fingers as a single unit. Free the fingers and their cortical maps soon follow, new borders arising from their independence. Professional string musicians use the left hand for the precise fingering of an arpeggio or aria. With each note played glissando or staccato, with each shimmering or soulful vibrato, the left-handed cortices slowly swell.

If use inflates neural representations, disuse causes them to shrink, allowing neighbouring neurons to squat on the vacant real estate. Neurons that register facial touch lie adjacent to representations of our arms; amputees who lose an arm find that the brain’s face grows to take over the now idle regions of the map. Genital touch and the control of pelvic muscles lie side by side along a central nook of cortex, just below the cortical territories of feet. In one of the more provocative examples of neural plasticity, the neuroscientist V S Ramachandran at the University of California, San Diego, cites two amputees who, after losing a foot, seem to have gained genital sensitivity. One patient reported that his orgasm spanned from his genitals to his phantom foot.

A student of Ramachandran has gone on to suggest that such brain reorganisation contributed to the millennial prevalence of footbinding in medieval China. The brutal process, illegal since 1912, involved the bending and binding of a young girl’s foot, accomplished over years, until it was folded over like a billfold or, more generously, a lotus blossom. While the hobbling of women must have been a primary motive, Paul McGeoch, a clinician in San Diego, suggests that these women would also have experienced the atrophy of foot cortices and the encroachment of genital maps. English language scholarship from the 1960s cites texts that extol the virtues of footbinding. Some claim that it promoted vaginal tone, or that the foot became unusually sensitive to erotic touch. This literature seems somehow complicit with the practice and its misogyny – and yet it is consistent with our understanding of cortical plasticity.

The shifting landscape of discriminative touch reveals just how deeply we are shaped by our experiences. Our brains are sculpted by the accretion and erosion of their innumerable connections; the dendrites and spines of our neurons are altered by the information that flows through them. A friend and professional musician has worked his way across Europe, transcribing rare sheets of music written specifically for the viola, and sleeping in bathhouses along the way. At home, he keeps a map with pins in each country whose citizens he has sampled sexually. There are many pins. I imagine what his cortices must look like. Does he touch new skin with his left fingers? Do his lips tremble as he plays a passionate concerto? The ways in which we are changed by our paths through the world suggest an exquisite variety and specificity of experience.


翻译考点：
考点1：“the dendrites and spines of our neurons ”推荐译为“神经元的树突和棘突”
考点2：“the places we like to juxtapose" 中的 "juxtapose"不可译为“并置”，推荐译为“让（身体部位）并列、贴近、接触”
考点3："left-handed cortices"不可译为“左手皮层”在神经科学上是不准确的，可能会误导专业读者。应译为“（大脑中）负责左手的皮层区域”
考点4："provocative" 推荐译为“引人深思的、启发性的”，不可译为“令人激动的”
考点5："more generously" 推荐译为“说得好听一点”或“用一种更美化的方式说”，不可译为“更宽泛地说”
考点6：bathhouses推荐译为“澡堂”，不可译为“浴室”