Location and conduction velocity of raphespinal neurones in nucleus raphe magnus and raphe pallidus in the cat.

Extracellular recordings were made from raphespinal neurones identified by antidromic stimulation of their axons in the upper lumbar cord. Conduction velocities ranged from 1.25 to 67 m/sec. Neurones with axonal conduction velocities of 1.25-5 m/sec were found mainly ventrally, in nucleus raphe pallidus; many of these are presumably tryptaminergic cells with unmyelinated axons. In nucleus raphe magnus most raphespinal neurones had axonal velocities between 7 and 67 m/sec; these axons must be myelinated. This indicates that most raphespinal neurones in nucleus raphe magnus are non-tryptaminergic, and it is suggested that analgesia mediated by raphespinal neurones may not be solely due to the action of 5-hydroxytryptamine descending systems.