The Mechanics of Chronic Pain

To properly understand chronic pain, it's important to have a basic grasp of how the body detects pain under normal circumstances. The process by which pain messages get handled by the body, while lightning quick, is actually quite complex, and involves steps by several different structures in your nervous system.

The advantage of this system, as you'll see when you read this section, is that it's quite sophisticated about sorting out the relative severity of pain signals and figuring out appropriate responses. The drawback is that, again as you'll see below, there are places throughout this process where the system can break down and start sending erroneous pain signals. While that seldom happens, when it does the result is often a chronic pain condition.

The main players in the pain process are the peripheral nervous system, the spinal cord, and the brain.

Peripheral Nervous System

Scientists often divide the body's nervous system into two major categories: the central nervous system, consisting of the brain and spinal cord, and the peripheral nervous system, which includes everything else (the word "peripheral" refers to that which is on the boundary or outer edges of something). The peripheral nervous system forms a network of nerve fibers that branch off of the spinal cord, radiating throughout your body so that every region has a communication path back to the brain.

Attached to some of these fibers are special nerve endings that can sense pain or injury to bodily tissues. These nerve endings are called nociceptors. There are millions of nociceptors in your skin, bones, joints, muscles and the protective tissues around your internal organs. The concentration of nociceptors tends to vary with how prone the area is to injury — for instance, there may be as many as 1,300 nociceptors in a patch of skin the size of a quarter, but muscles, protected beneath the skin, have fewer nerve endings, and organ membranes, even more thoroughly protected, fewer still.

Nociceptors also tend to be specialized according to the type of stimulus they can sense, such as sharp blows; pressure, temperature, and chemical changes; or inflammation caused by an injury or disease.

When nociceptors detect a harmful stimulus, they relay their pain messages along a peripheral nerve to your spinal cord and brain. The speed by which the messages travel can vary according to severity and type of pain. Dull, aching pain is relayed on fibers that travel at a slow speed. Sharp, severe pain is transmitted almost instantaneously.

The Spinal Cord

Once the pain messages reach the spinal cord, they come upon specialized nerve cells that act like switches, selectively allowing the messages to pass through to the brain depending on their severity and urgency. Weak pain messages, such as from a small cut or bruise, may be refused entry altogether by the spinal cord switch.

Messages about severe pain, however, especially pain that's linked to an immediate danger, get passed by the "switch" directly to the brain by the fastest possible route. Meanwhile, other nerve cells in the spinal cord also react to this emergency message by triggering other parts of the nervous systems into action, such as the motor nerves. For example: if a person accidentally sticks their finger into a flame, the pain signal would be sent through the switch directly to that person's brain — plus their motor nerves would also signal their muscles to snatch your finger away from the harmful fire.

The spinal cord also has a mediating role in handling pain messages. Other sensations transmitted in by other peripheral nerves can reach the spine and modify, or even diminish, the pain message. That's how, for instance, a massage can help relieve the pain of aching muscles — the pressure signals reach the spine and cause it to lessen the intensity of the muscular pain signals. Nerves in the spinal cord can also release chemicals that strengthen or weaken some incoming pain signals, affecting how fast they then make it to your brain.

The Brain

Finally, the pain messages arrive at the thalamus, a kind of central switchboard deep within the brain. The thalamus forwards the pain messages to two other structures of the brain, the cerebral cortex and the limbic system. The cerebral cortex is the part of your brain in which thought occurs, while the limbic system mediates the brain's emotional responses to events and stimuli.

The cerebral cortex reacts to the pain messages by locating the source of the injury, assessing the damage, and determining a course of action, such as shifting your weight off your foot if you've sprained your ankle. The cerebral cortex also relays messages that direct the body's automatic responses to the pain.

For instance, if a child has skinned her knee, the cerebral cortex signals her autonomic nervous system, the system that controls unconscious bodily processes such as blood flow, to send additional blood and nutrients to the tissues around the abrasion. The cerebral cortex also triggers the release of pain-suppressing chemicals such as endorphins, and sends "stop-pain" messages back to the spinal cord.

Meanwhile, the limbic system produces the emotions that can often accompany pain, such as anxiety, fear or frustration, often affecting the way your cerebral cortex receives pain messages and lessening or intensifying the pain you feel. When a child reacts to a skinned knee by getting upset, crying, and seeking the security of her parent, it’s the limbic system that is processing those emotional responses, resulting in the child experiencing the pain more intensely. And when the parent soothes the child, the limbic system inputs the emotional response of security and reassurance, so that the child actually experiences some lessening of the pain.