The dorsal horn relay neurones convey the sensory input of pain from the periphery. Their activity is inhibited by local interneurones and descending noradrenergic and serotonergic systems. Both local and descending routes can be activated by endogenous enkephalins either in the brain stem or the spinal cord. Enkephalins act on opiate receptors at both sites.
Narcotic analgesics are similar to enkephalins and act primarily on the mu-subtype of opiate receptor in the periaqueductal grey matter, raphe nucleus and spinally to inhibit ascending nocioceptive input. Stimulation of receptors at other sites accounts for common side effects:
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