Pain Pathways
Tissue injury can lead to cellular changes involving release of chemicals (eg, histamine) that start or quicken neuronal impulses that are interpreted as pain. Many neuronal pathways transmit pain sensation. For example, pain from peripheral injury reaches the CNS via primary afferent neurons, whose cell bodies form the DRG.
Disorders such as phantom limb pain may involve abnormal
DRG structure or function. Primary afferents end mainly in the dorsal horn of
the spinal cord. Secondary neurons cross
the spinal cord and ascend in pathways to the thalamus, the cerebral cortex, and other sites. A
descending system of opioid (endorphins, enkephalins), 5HT (eg, from raphe
nuclei), and noradrenergic (eg, from locus ceruleus) pathways can lessen
afferent signals. Drugs that act at pathways mediating pain sensation or
perception are local (eg, lidocaine) and general (eg, halothane) agents,
opioids (eg, morphine), and nonopioids (eg, aspirin
and acetaminophen).
