Neurophysiology of pain and pain modulation.

An endogenous central nervous system pain-modulating network, with links in the mid brain, medulla, and spinal cord, has recently been discovered. This system produces analgesia by interfering with afferent transmission of neural messages produced by intense stimuli. Although other neurotransmitters are involved, the analgesia produced by this system depends on the release of endogenous opioid substances, generically referred to as endorphins. The system is set in motion by clinically significant pain--such as that resulting from bony fractures or postoperative pain. The analgesia network monitors the pain and controls it at the level of the spinal cord. Complex psychologic factors play an important role in the variability of perceived pain, partly because of their ability to trigger this pain-suppressing system. For example, this system contributes to the analgesic potency of placebo administration and is also activated by stress. Knowledge of this analgesia system has greatly expanded our understanding of the mechanisms underlying pain management. Opiates, like morphine and meperidine, produce analgesia by mimicking the action of endorphins in the pain. Tricyclic drugs may produce analgesia by enhancing the nonendorphin links of the same system. Future research on this system will provide new insights and, consequently, new approaches to the management of pain.