The experience of pain begins just beneath the skin in specialized nerve endings known as nociceptors. (“Noci-” is from a Latin word meaning “hurt.”) Nociceptors respond to three kinds of painful stimuli: thermal, chemical, and mechanical, or at least so it is universally assumed. Remarkably, scientists have not yet found the nociceptor that responds to mechanical pain. It is extraordinary surely that when you whack your thumb with a hammer or prick yourself with a needle, we don’t know what actually happens beneath your outer surface. All that can be said is that signals from all types of pain are conveyed on to the spinal cord and brain by two different types of fibers—fast-conducting A delta fibers (they’re coated in myelin, so slicker, as it were) and slower-acting C fibers. The swift A delta fibers give you the sharp ouch of a hammer blow; the slower C fibers give you the throbbing pain that follows. Nociceptors only respond to disagreeable (or potentially disagreeable) sensations. Normal touch signals—the feel of your feet against the ground, your hand on a doorknob, your cheek on a satin pillow—are conveyed by different receptors on a separate set of A-beta nerves.
Nerve signals are not particularly swift. Light travels at 300 million meters per second, while nerve signals move at a decidedly more stately 120 meters a second—about 2.5 million times slower. Still, 120 meters a second is nearly 270 miles an hour, quite fast enough over the space of a human frame to be effectively instantaneous in most circumstances. Even so, as an aid to responding quickly, we have reflexes, which means that the central nervous system can intercept a signal and act on it before passing it on to the brain. That’s why if you touch something very undesirable, your hand recoils before your brain knows what’s going on. The spinal cord, in short, is not just a length of impassive cabling carrying messages between the body and the brain but an active and literally decisive part of your sensory apparatus.
Several of your nociceptors are polymodal, which means they are triggered by different stimuli. That’s why spicy foods taste hot, for instance. They chemically activate the same nociceptors in your mouth that respond thermally to real heat. Your tongue can’t tell the difference. Even your brain is a little confused. It realizes, at a rational level, that your tongue isn’t literally on fire, but it sure feels that way. What is oddest of all is that the nociceptors somehow allow you to perceive a stimulus as pleasurable if it’s a vindaloo and yelp inducing if it’s a hot match head, even though both activate the same nerves.
The person who first identified nociceptors—who can indeed fairly be called patriarch of the central nervous system altogether—was Charles Scott Sherrington (1857–1952), one of the greatest and most inexplicably forgotten British scientists of the modern era. Sherrington’s life seems to have been lifted straight out of a nineteenth-century boys’ adventure story. A gifted athlete, he played soccer for Ipswich Town while still in school and had a distinguished rowing career at Cambridge. He was above all a brilliant student, winning many honors while impressing all who met him with his modest manner and keen intellect.
After graduating in 1885, he studied bacteriology under the great German Robert Koch, then embarked on a dazzlingly varied and productive career in which he did seminal work on tetanus, industrial fatigue, diphtheria, cholera, bacteriology, and hematology. He proposed the law of reciprocal innervation for muscles, which states that when one muscle contracts, a companion muscle must relax—essentially explaining how muscles work.
While studying the brain, he developed the concept of the synapse, coining the term “synapse” in the process. This in turn led to the idea of proprioception—another Sherrington coinage—which is the body’s ability to know its own orientation in space. (Even with your eyes closed, you know whether you are lying down or whether your arms are outstretched and so on.) And this, in further turn, led to the discovery in 1906 of nociceptors, the nerve endings that alert you to pain. Sherrington’s landmark book on the subject, The Integrative Action of the Nervous System, has been compared to Newton’s Principia and Harvey’s De motu cordis (On the Motion of the Heart) in terms of its revolutionary importance to its field.
But Sherrington’s admirable qualities don’t stop there. He was, by all accounts, a pretty wonderful person: devoted husband, gracious host, delightful company, beloved teacher. Among his students were Wilder Penfield, the authority on memory whom we met in chapter 4; Howard Florey, who won a Nobel Prize for his role in developing penicillin; and Harvey Cushing, who went on to become one of America’s leading neurosurgeons.
In 1925, Sherrington astonished even his closest friends by producing a volume of poetry, which was widely praised. Seven years later, he won a Nobel Prize for his work on reflexes. He was a distinguished president of the Royal Society, a benefactor of museums and libraries, and a devoted bibliophile with a world-class collection of books. At the age of eighty-three in 1940 he wrote a bestselling work, Man on His Nature, which went through several editions and was voted one of the hundred best books of modern Britain at the Festival of Britain in 1951. In it, he invented the expression “the enchanted loom” as a metaphor for the mind. And now, unaccountably, he is almost completely forgotten outside his field and not hugely remembered even there.
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The nervous system is divided in various ways depending on whether you are looking at its structure or its function. Anatomically, it has two divisions. The central nervous system is the brain and spinal cord. The nerves radiating out from this central hub—the ones that reach out to the other parts of your body—are the peripheral nervous system. The nervous system is additionally divided by function into the somatic nervous system, which is the part that controls voluntary actions (like scratching your head), and the autonomic nervous system, which controls all those things like heartbeats that you don’t have to think about because they are automatic. The autonomic nervous system is further divided into sympathetic and parasympathetic systems. The sympathetic is the part that responds when the body needs sudden actions—what is generally referred to as the fight-or-flight response. The parasympathetic is sometimes referred to as the “rest and digest” or “feed and breed” system and looks after a miscellany of other, generally less urgent matters like digestion and waste disposal, the production of saliva and tears, and sexual arousal (which may be intense but not urgent in the fight-or-flight sense).
An oddity of human nerves is that those in the peripheral nervous system can heal and regrow when damaged, whereas the more vital ones in the brain and spinal cord cannot. If you cut your finger, the nerves can grow back, but damage your spinal cord and you are out of luck. Spinal cord injuries are dismayingly common. More than one million people in the United States are paralyzed from them. More than half of spinal cord injuries in America result from car accidents or gunshot wounds, so, as you might expect, men are four times more likely to get a spinal cord injury than women. They are especially susceptible between the ages of sixteen and thirty—just when they are old enough to have guns and cars and foolish enough to misuse them.