Kathleen Koman in Harvard Magazine:
Nociceptive or somaticpain — a normal response to noxious stimuli — is essential for life. It tells you to pull your hand away from a flame or withdraw your mouth from a cup of hot coffee. If you break an ankle, the pain keeps you from walking around on it, so the bone can heal. Nociceptors are sensory receptors, or nerve endings, that react to mechanical, thermal, and chemical stimuli that may damage tissues. They relay nerve impulses — electrical messages from the site of injury in peripheral tissues such as skin, muscles, and joints — to the dorsal horn, an area in the spinal cord that acts as a switchboard. There, different chemicals determine whether these electrical messages reach your brain, where you actually perceive pain.
Nociceptive pain is very clear, says professor of anesthesiology Carol A. Warfield, chief of anesthesia, critical care, and pain medicine at Beth Israel Deaconess Medical Center in Boston. When you cut your finger, she says, you know darn well that it’s your finger that hurts; in fact, you could close your eyes and easily identify the location of the cut. Usually you feel a sharp pain, then throbbing, and finally, after a short time, the pain goes away. Pain that occurs suddenly and has a real, definable source is considered acute pain. Rapid in onset and relatively short in duration, it generally follows a traumatic event such as a bone fracture or a surgical procedure, but can occur in other situations, such as when a hemophiliac suffers internal bleeding. Doctors often treat acute pain with strong drugs, knowing that it will fade as the healing process takes over.
Sometimes, however, the pain message system goes awry, says Warfield, and people perceive pain for much longer than it’s useful.