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Ask Dr. K: Pain signals tell brain where and how much it hurts

DEAR DOCTOR K: Let’s say I stub my toe. How does my brain know where it hurts and how bad?

DEAR READER: Pain serves as the body’s warning system. It alerts you to an injury or when something, such as an infection, has gone wrong inside your body. Pain can also help in healing. It tells you to avoid touching a wound or using a joint that’s damaged. But in order to respond appropriately, your brain must identify the location and severity of your pain.

Pain that stems from injury to body tissue is known as nociceptive pain. Examples include burns, sprains, broken bones and inflammation from an infection or arthritis. Pain from such injury is triggered by damage to little structures called nociceptors that are part of the nerve endings in the injured area. A pain-sensitive area of the body such as your fingertip or tongue has thousands of nociceptors in a tiny fraction of a square inch.

If you suffer a papercut, for example, the nociceptors in the nerve endings in the skin of your fingers are stimulated by the injury. They send a pain signal that starts at the point of the injury and travels up the nerve to the spinal cord. Then the pain signal travels up another nerve in the spinal cord to various parts of the brain. (I’ve put an illustration of the pain pathway on my website, AskDoctorK.com.)

Pain signals are carried by two types of nerve fibers. The A-delta fibers carry the first, sharp pain you feel. The C fibers carry the dull, throbbing pain that follows.

Nerve fibers throughout the body connect to the spinal cord. (If the pain is in your head, nerve fibers connect directly to your brain.) In the spinal cord, incoming messages can be amplified, dampened or blocked altogether.

Pain signals reach different parts of the brain. Some signals reach the part of the brain that controls spatial awareness. It tells us where in the body a pain signal is coming from. Other signals travel to the part of the brain that generates and controls emotion. Pain signals also reach the hypothalamus, a part of the brain that controls sleep and temperature regulation.

The pain we feel can be modified by chemical signals in the spinal cord and in the brain. We know about some of them, but there are probably others that we haven’t discovered yet. Indeed, little more than 100 years ago we didn’t really know anything about how we felt pain. Human beings have been on Earth for 40,000 years, yet all we knew until 100 years ago was that sometimes we hurt.

Scientific research has taught us many things about how we feel pain, and some of that understanding has translated into new treatments. Still, we have a great deal more to learn, particularly about chronic pain — and how to better relieve the suffering of so many people who live with it.

Dr. Komaroff is a physician and professor at Harvard Medical School. To send questions, go to AskDoctorK.com, or write: Ask Doctor K, 10 Shattuck St., Second Floor, Boston, MA 02115.

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