WHAT IS PAIN AND WHO SUFFERS FROM IT?

50 million U.S. residents live with chronic pain, experts estimate. Pain forces an estimated 36 million of us to miss work every year and results in roughly 70 million doctor visits. Yet scientists know very little about how pain works. They can't even agree on a definition. They do agree it's a huge problem. Pain costs an estimated $100 billion each year. A Gallup Poll done in 2000 found that 80 percent of Americans believe pain is part of getting old, and 64 percent said they'd see a doctor only if their pain became unbearable.
Here’s what we know: One way to divide pain (and perhaps conquer it) is to distinguish between acute and chronic, explains Sally Lawson, a professor of physiology at the University of Bristol in the UK. Acute pain is what you get when you hit your thumb with a hammer. You can also achieve it with a twisted knee or a burn. Chronic pain is long-term, continuous and far more frustrating. It can result from physical injury, viral infections of the nerve, or arthritic damage to joints and degeneration of bones. Nerves in the body serve to warn us when something is acutely dangerous, such as a hot surface, or when our body needs rest or repair. And yes, sometimes those nerves just seem to annoy. Thousands of pain-sensing nerve endings can be packed into tiny spaces in your fingers, between the vertebrae of your back, and just about everywhere else. In the spinal cord, the electrical pain signals trigger release of chemicals called neurotransmitters, which activate other nerve cells that process and transmit the information to the brain. Important decisions occur in the spinal column: Acute pain like that from touching a hot surface raises a red flag and is routed to the brain immediately. Weaker messages are given less priority, so that you don't overreact to a minor scrape.

The type of pain you feel—acute or chronic—probably depends in part on the type of nociceptive fiber that is sending the initial signal. The larger fibers convey electrical impulses very rapidly to the brain, and are thought to cause sharp prickling pain, while the very fine fibers cause ongoing burning, very unpleasant sometimes called ‘unbearable' pain because of its persistence. Pain signals are processed in the brain's thalamus, a sort of switching station. The thalamus forwards the message to three places: Somatosensory cortex (physical sensation),  Limbic system (emotional feeling), Frontal cortex (limbic system).

The brain can do more than just initiate a quick retreat from the source of pain. It might signal nerve cells to release painkillers, such as endorphins. Chronic pain, for which there is often no obvious source, is often more complex and poorly understood. It's generally defined as pain that lasts more than six months.