From Harvard Magazine:
Those who suffer the devastating effects of chronic pain may fantasize about a life that is completely pain-free. In fact, such a life is far from idyllic. People who are born with congenital insensitivity to pain, a rare genetic disorder, chew their tongues and lips to pieces, burn their flesh, and fracture their bones without realizing the harm they are doing to their bodies. Lacking a warning system to protect themselves from dangers in the environment, they tend to die young, often in their twenties. 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.
More here.