Pathophysiology of pain.

OBJECTIVE: To review the pain pathways in the central and peripheral nervous system and the actions of drugs used to treat pain.
DESIGN: An overview of pain pathways is presented, beginning in the periphery and progressing centrally, and the ascending and descending pathways are described in detail.
RESULTS: The nociceptive pathway, consisting of the classic three-neuron chain, is now understood to be a dual system at each level, and the sensation of pain is thought to arrive in the central nervous system with the discriminative component of pain ("first pain") carried separately from the affective-motivational component of pain ("second pain"). In addition to spinal control mechanisms of nociceptive transmission, descending pathways that originate in three major areas--the cortex, thalamus, and brain stem--can modify functions at the spinal level. At every level of the nervous system, a close relationship prevails between somatic pain pathways and visceral pathways. This relationship likely accounts for the transmission of visceral pain and also for autonomic responses to somatic pain and somatic responses to visceral pain.
CONCLUSION: By understanding the pathways of pain and the transmitters involved, prevention and treatment of pain will be improved.