What is pain, anyway?

May 28, 2010  |  General

My next book, due out in mid-July, is about healing. Important to understanding healing is understanding the symptom of pain. What follows is an excerpt from my book that talks about this universal symptom – what it is and why it is.

Pain, in all its varieties and subtleties, is among the most complex of human symptoms. It has been described in uncounted ways by writers and portrayed by actors, but we read or view these characterizations through the lens of the pains we ourselves have had. Even though we all have felt pain, and in that sense have shared the experience with all other humans, it is also unique to us. Pain is both universal and profoundly personal. It’s a complicated subject.

Pain is not limited to humans, of course. All mammals certainly feel pain. Some aspects of the pain response reach far down below mammals in the animal kingdom to quite primitive creatures. How these creatures perceive it, if that is even the right word, is mysterious, but this observation tells us pain has been with us for many eons. That fact alone should tell us it must serve some important purpose.

All of us know that pain comes in many forms. There is the sharp pain from stepping on a tack. There is the vague, dull aching of a twisted knee, the cramping pain in the lower abdomen that comes with the flu, the pounding inside the skull of a migraine headache, the gnawing pain of a toothache. There is the restless pain that persists in spite of what positions you take, as well as the pain that only relents when you lie completely still. All of us could think of many more examples.

Pain is reported to the brain via a dense network of nerve fibers. Think of this network as an intricate grid of electrical wires, because that is what nerve signals are made of — electricity. These wires are of several kinds, but there are two principal ones. They differ in how well insulated they are. Instead of the plastic insulation that protects electrical wires, the body uses a substance called myelin to insulate the neural wiring. Some wires are more tightly wrapped with myelin than are others. Some nerve fibers have no myelin at all. The more wrapping, the faster the electrical signal travels, so myelinated fibers transmit signals faster than those without myelin.

The nervous system uses a series of switching stations to pass a signal from, for example, the end of your finger to your brain. The first of these are in the spinal cord. When you prick your finger, an electrical signal goes from a nerve fiber there, up your arm, and on to a relay station in the spinal cord in your neck. From there, it continues on up your spinal cord to your brain. What happens to it when it reaches your there is fascinating — and complicated.

Pain is a subjective feeling, meaning no one besides yourself can know precisely how you are feeling it. This means no two people will experience pain in the same way; the exact same finger prick may be perceived quite differently by two different people. An injured person can even be initially unaware of his injury because he does not feel it at first. Probably you have experienced the situation in which, distracted by something else, you did not feel a stubbed toe or a bug bite to the same extent you would have if your mind were not focused on something else.

This variability in how pain is perceived, of the discomfort it causes us, is because the simple electrical signal running up your finger from that needle prick gets modulated by a maze of other nerve cells in the spinal cord and in the brain. Some of these influences dampen down the signal, others ramp it up. The result is when it finally gets to your upper brain, where your consciousness lies, all sorts of things have affected the signal, things that are unique to you and your brain.

You have several kinds of nerve fibers in your finger. The ones that transmit the fastest signals, the heavily myelinated ones, mostly are concerned with light touch and position sense, which is knowing where your finger is in space. This makes sense, because these bits of information are things the brain needs to learn as rapidly as possible. If you want to demonstrate this for yourself, close your eyes, open your mouth, and rapidly stick your finger in your mouth. You can do this without poking yourself in the eye because your brain knows, every millisecond, just where your finger is in space in relation to your mouth. These nerves are also involved in the pain response, particularly in blocking some of its input in the spinal cord. When they do not work, the perceived pain from a pricked finger is worse.

The nerve fibers in your finger that transmit pain signals, the ones with less or no myelin insulation, can sense three kinds of things: mechanical forces like hard pressure, hot and cold, and chemical substances. If you pay attention when you whack your finger with a hammer, hard as that may be to do, you can distinguish between them in action. You first feel a very sharp, very localized pain. This is a signal from the insulated fibers, which gets to your brain first. An instant later you begin to feel a more diffuse, deeper pain that is less well localized to the precise spot. This is input from the slower fibers with no myelin.

Another way we experience the difference between fast and slow fibers is when we bark our shins on a piece of furniture when walking in the dark. We first feel our leg hit the furniture — those are the insulated touch and position sense fibers at work. After a perceptible lag, we feel like yowling in pain — those are the uninsulated pain fibers catching up with their messages.

I’ll put up another post about what things we can use to treat pain and how they work.


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