What causes pain and how do pain medicines work?

September 6, 2013  |  General

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.

We have two main approaches for treating pain: we can do things that reduce the pain signals coming from the spot that hurts, or we can use medications that confuse the brain into thinking the pain is either not there or is not so bad.

There are several simple things we can do to reduce the pain signals coming up the nerve fibers. A simple one has been known to parents for eons — simple rubbing of a painful spot. Stimulating one set of nerve fibers, particularly the fast, insulated ones, affects how our brain processes sensations. Every parent knows how to do this, although you probably did not know why it works. When your child comes running to you after falling down and bonking her head, what do you do? Generally you rub it, and it really does feel better. This is not just from parental love. Stimulating the touch fibers in the same place where the pain is coming from causes them to intervene and dampen back the pain signal coming from the other fibers. The same thing happens when we rub any body part after we hit it on something.

Cooling the area with an ice pack is another way to reduce the pain signals coming up the nerve network. Yet another is to put a medicine that interferes with how the nerves work right on the painful spot. Examples of this approach include ear drops that can numb the ear drum for a child with an infection or numbing sprays and ointments for a child with sunburn. A dentist injecting a painkiller around a sore tooth is using a more powerful version of these same methods.

The other way to treat pain is to use medications that act directly on the nervous system to alter how the brain reacts to the signals coming up from the painful place. They convince the brain to downplay or even ignore the information. This is how both acetaminophen (Tylenol and many other brands) and ibuprofen (Motrin and many other brands) work. Ibuprofen also relieves pain in another way that acetaminophen does not; ibuprofen can work directly at the site, such as the inflamed finger or ear, to block the production of some of those substances that cause the inflammation. We also have an injectable medication related to ibuprofen, only more potent, called ketorolac (brand-named Toradol).

More severe pain, such as from a broken arm, calls for medications more powerful than Tylenol or Motrin. Members of the opiate family, also called narcotics, are the standard. There are many members of this family, which vary in how they are given, their appropriate dose, and some of their side-effects, but they all work in the same way: they go to the brain and the spinal cord and alter a person’s perception of the pain. They also can alter mood and a person’s level of awareness to things around them.

Even though we give narcotic medications for severe pain, a fascinating thing about them is that they are not really foreign to the body at all. We have similar substances that occur naturally in our body, and presumably these natural narcotics, called endorphins and enkephalins, are performing some useful function inside us, most likely involving pain control. So when we give a child with more severe pain, such as a broken leg, a medication of this type we are really just reinforcing a normal pathway. The presence of these natural substances could explain why some persons, an Indian Yogi for example, can walk across a bed of hot coals without pain because he has learned how to alter his brain’s perception of what is painful.

Pain, uncomfortable as it is, does serve some useful purpose, and in that sense helps a child heal. Pain alerts us that something is wrong and tells us we should try to do something about it. If we cannot feel the pain, worse injury often results. A good example of this is what happens when a person lacks sensation in an arm or a leg. Because he cannot feel there, painful things, such as an ill-fitting shoe, can go unnoticed and lead to injury.

But pain can also interfere with healing. Mild or moderate pain does not seem to affect healing much, but more severe pain, if it persists, can interfere with it. This stems from the effects of what we call stress hormones, substances like adrenaline, which the body releases at times of stress. They are called “fight or flight” hormones because they probably helped our ancient ancestors deal with things like a wild animal attack. Although they can help in times of acute danger, prolonged high levels of these hormones, such as occurs with continuing severe pain, do inhibit proper healing. Researchers have studied this phenomenon in children who have had major surgery, and it is clear that using pain-killers does not just make the children feel better — it also makes them heal better.

Doctors once thought that children do not feel pain to the same degree adults do, and children were often under-medicated with pain drugs for things for which no adult would tolerate not receiving adequate medications. Now we know better. If your child is, for example, in the emergency department with a broken arm, make sure the doctors take care of her pain as well as fix the broken arm. Just because a child can’t tell us about the pain doesn’t mean it’s not there.


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