What causes chronic pain – or – how does pain become chronic?

Pain, as terrible as it can be when it outlasts its stay, is actually a vital protective function of our nervous system. The body detects pain through a subgroup of primary sensory neurons, called nociceptors, that innervate the entire body and which normally respond only to high-threshold stimuli such as extreme heat or strong mechanical stimulation. This nociceptive response to potentially tissue damaging stimuli is critical for reflexes and coordinated responses to the stimulus which generally result in a protective reaction (such as withdrawing your hand from a hot stove). Hence, the activation of peripheral nociceptors by pain-inducing stimulation serves a crucial teaching function insofar as it is the signal that protects us from further damage. This fact is best exemplified by studies of rare cases where individuals have a genetic mutation that makes them insensitive to pain. For instance, a family was recently discovered in Pakistan wherein mutations in a voltage-gated sodium channel (called Nav1.7) involved in generating pain signals in nociceptors led to a total lack of pain sensation. Members of this family were working as street entertainers, performing incredible feats such as placing daggers through their arms. Horrifically, one of these young men died after jumping off a roof during one such performance. As tragic as this story is, it serves as an excellent example of how we depend on pain signals to keep us safe from potentially life-threatening injuries.