Pain can be a warning that your body is injured or infected. The doubled-over pain of a ruptured appendix will send you straight to the emergency room; without this warning, you would die, says Carol A. Warfield, M.D., director of the Pain Management Center at Beth Israel Hospital, Boston. This type of acute pain usually lasts until your injury or infection heals. But not all pain serves a purpose. Pain that lasts long beyond the normal recovery period, such as lower back pain or migraine headache, is called chronic pain, says Peter J. Vicente, Ph.D., of the American Pain Society. Disabling chronic pain -- the type of pain that keeps you from working and enjoying life -- needs to be treated, Dr. Vicente recommends.

In terms of numbers of pain receptors, two parts of your anatomy may tie for most sensitive. One is the mouth, says J. Theodore Jastak, D.D.S., Ph.D., chairman of oral and maxillofacial surgery at the University of British Columbia. It’s a very, very sensitive area to pain reception. Think of spicy food that burns your tongue, cold ice cream that sends painful shivers through your teeth, or a throbbing molar, and you get the idea. Peter R. Wilson, M.D., a consultant in anesthesiology and pain at the Mayo Clinic, says there’s evidence that the genitalia are as sensitive to pain as the mouth. I guess that doesn’t require a lot of imagination, he adds.

It’s your brain, not your earlobes, that is least sensitive to pain. Your brain can process pain messages, but it has no pain receptors of its own, says Seymour Diamond, M.D., executive director of the National Headache Foundation, based in Chicago.

There are otherwise normal people who are born incapable of feeling pain. And while you might think living without pain sounds like a good deal, it’s really not. Pain is really protective, Dr. Warfield says. If you don’t have that, you die, because you can’t take care of yourself. There have been several reports of people who can’t feel pain, Dr. Warfield says, and most died in their 20s. Infections, broken bones and other injuries all went unnoticed by these unlucky few.

Pain is an emotional experience, says Dr. Wilson. Doctors can get physical responses -- including increased blood pressure and heart rate -- by trying to stimulate coma patients pain receptors. But without the brain working to register and comprehend the pain, it’s unlikely they feel anything. They probably are not suffering, he adds.

according to Dr. Diamond, who also runs a headache clinic in Chicago. Everybody gets a simple headache now and then, he explains. The National Headache Foundation lists 21 common types of headache that affect more than 45 million Americans annually.

About 70 percent of migraine sufferers are women. Migraine headaches cause a severe throbbing pain on one side of the head, often accompanied by nausea, vomiting or dizziness. About 28 million Americans suffer from migraines. Women seem to suffer migraines more often because fluctuating hormone levels cause contractions in the blood vessels in the scalp.

It depends on the person, not the person’s gender, explains Dr. Warfield. Some people have a higher level of natural pain blockers, such as endorphins and serotonin, that act the same way that narcotics do by stopping pain messages from traveling to the brain, she says. What may feel like agony to you might be perceived as mild discomfort by your next-door neighbor, all thanks to these self-narcotics.

The phenomenon of pain is a perception, says Dr. Vicente, a psychologist who also serves as the executive director of the Pain Rehabilitation Center at Good Samaritan Hospital, Cincinnati. Your brain can, in effect, amplify the pain experience or turn it down to a whisper. This is how yogis can stick needles into themselves or walk on a bed of hot coals. With sustained concentration, they’re able to override the discomfort, Dr. Vicente explains.

But there was a time when doctors believed babies were unable to feel pain and even operated on them without anesthesia. That view has changed, says Dr. Vicente. There is no question that neonates [newborns] can feel pain and respond to it.

A heart attack, for example, can manifest itself as pain in the left arm. Or tooth pain can be felt as an earache, says Dr. Jastak. This is known as referred pain.