Carl Zimmer relays a theory about recurring pain:
As time passes, we store the memory of the pain without vividly reliving it every day. But for millions of people the memory doesn’t fade and the pain doesn’t go away. To ?A. Vania Apkarian, a neuroscientist at Northwestern University, the connection between the living memory and the never-ending pain suggests a glitch in the brain.
Ordinary pain might turn chronic, he hypothesizes, when inflammation caused by conditions like arthritis or nerve damage provokes an abnormal rush of signals from nociceptors. When these aberrant signals reach the pain network in the brain, Apkarian argues, they overwhelm it. The brain doesn’t get a chance to forget the pain. Instead it learns to feel it continuously. Eventually the neural connections become so strong that we no longer need the original stimuli anymore. The network begins to sustain itself, continually relearning its pain. It can also send signals back down into the body, turning previously painless sensations into painful ones.
The Dish 