Your body is covered in hairy skin.* Below the surface of this skin are wispy sensory nerves known as a C-fiber tactile afferents, or CTs. These nerves are designed to respond to gentle contact - even the slightest of indentations can turn them on, starting a cascade of electrical signals that ends with a feeling of touch. For a long time, the most notable fact about these nerves was their lack of speed: because CTs had no myelin insulation, they were about 50 times slower at transmitting sensory signals to the brain than myelinated A-fiber nerves.
And so a simple model of the touch system emerged: we had a fast pathway, modulated by A-fibers, which gave us quick and precise information about the surface of the body. Such a system had an obvious function, allowing us to touch the world, manipulate objects and monitor the body in space.
But if we have this fast sensory system, then why are the vast majority of nerves in hairy skin slow CT fibers? It’s like a customer with a broadband keeping around a dial-up modem, just in case.
In recent years, however, it’s become clear that CT fibers are not merely an archaic back-up or useless redundancy. Rather, they are endowed with their own unique purpose, which is just as essential as the speedy transmission of A-fibers. In a new Perspective published in Neuron, the neuroscientists Francis McGlone, Johan Wessberg and Hakan Olausson lay out the argument. They suggest that a particular kind of Cfiber nerve is largely responsible for the emotional quality of touch, passing along crucial information about the “affective and rewarding properties” of the most tender contact. When we talk about the power of touch – say, the healing properties of a hug, or a gentle caress – we are talking about the powers of these slow nerves.
There are multiple strands of evidence. The first is neurological patients with selective damage to A-fibers, leaving them with a touch pathway composed exclusively of C-fibers. These people are mostly numb. However, this numbness comes with a strange loophole – if their skin is brushed gently at a low velocity (between 1 and 10 centimeters per second), their interior bodies can be filled with pleasurable sensations. The feeling is vague – some patients couldn’t even identify the body quadrant that was being stroked - but everyone felt it.
The second piece of evidence is the inverse situation: patients with a rare genetic mutation that wipes out their C-fiber pathway, so that only A-fibers remain. While these patients have primarily been studied for their inability to feel pain – they are often oblivious to severe wounds, such as that from a broken bone – it turns out that they’re also less likely to experience pleasure from a soft touch.
These differences in the function of A and C fibers are echoed in the brain. While skin stroking in normal subjects triggers activation in the somatosensory cortex – the part of the brain that tells us where the sensation is coming from – patients with only C-fibers show a selective activation in the posterior insular cortex and other limbic areas. According to McGlone et. al, this suggests that a class of touch sensitive C-fibers have “excitatory projections mainly to emotion-related” systems in the brain. They are designed to fill us with feeling, not to tell us where in the flesh these feelings are coming from.
This all makes sense, if you think about. We are creatures of touch, naked apes that still enjoy getting groomed. We soothe children with soft strokes and kiss the limbs of lovers; the skin is a social organ. While neuroscience tends to focus on vision and hearing as conduits for social information, McGlone et. al. point that the epidermis is also “the site of events and processes crucial to the way we think about, feel about, and interact with one another.”
These touches are most important during development. As Harry Harlow first observed, the absence of comforting contact is deeply stressful for young monkeys, leaving them with a wound from which they never recover. More recent studies have found that separating infant monkeys from their mother with a transparent screen – they could still hear, smell and see her – led to chronic activation of stress pathways in the brain. The stress was only diminished if the young monkeys were allowed to form “peer touch relationships,” suggesting that physical contact is required for normal brain development. Michael Meaney, meanwhile, has shown that rat pups born to mothers that engaged in lots of licking and grooming were much better at coping with stressful situations, such as the open-field test. They solved mazes more quickly, were less aggressive with their peers and lived longer lives. Meaney argues that these differences are driven by differences in the brain, as rat pups exposed to a surfeit of tender contact have fewer receptors for stress hormone and more receptors for the chemicals that attenuate the stress response.
And then there’s the tragic evidence from early 20th century orphanages and foundling hospitals. In these childcare institutions, there was an intense focus on cleanliness and efficiency. As the psychologist Robert Karen notes, this meant that babies were “typically prop-fed, the bottle propped up for them so that they wouldn’t have to be held during feeding. This was considered ideally antiseptic, and it was labor-saving as well.”
Unfortunately, such routines proved deadly. Although these hospitals supplied infants with adequate nutrition and warmth, they struggled to keep them alive. A 1915 review of ten infant foundling hospitals in the Eastern United States, for instance, concluded that up to 75 percent of the children died before their second birthday. (The best hospital in the study had a 31.7 percent mortality rate.) In fact, it wasn’t until the early 1930s, when pediatricians like Harry Bakwin began insisting that nurses touch the babies that mortality rates declined. The soft touches, carried along by those CT nerves, were a kind of sustenance.
Of course, the newfound recognition of C-fibers doesn’t mean the mystery of emotional touch has been solved. The pleasure of contact isn’t just a bottom-up phenomenon, triggered by some peripheral nerves in the flesh. Rather, it’s entangled with all sorts of higher order variables, from the context of touch to the “relationship of the touchee with the toucher.” If anything, the fact that we’re only now beginning to outline the mechanics of the caress is a reminder that the nervous system is full of unknowns, threaded with wires we don’t understand. Somehow, in the milliseconds after the skin is stroked, we turn that mechanical twitch into a powerful feeling, which eases our anxiety and reminds us why it’s good to be alive.
*The only non-hairy parts of the skin - so-called glabrous skin - are found on the soles of the feet and the palms of the hands.
McGlone, Francis, Johan Wessberg, and Håkan Olausson. "Discriminative and Affective Touch: Sensing and Feeling." Neuron 82.4 (2014): 737-755.