PRIMARY AFFERENT DEPOLARIZATION OF IDENTIFIED CUTANEOUS FIBERS FOLLOWING STIMULATION IN MEDIAL BRAIN-STEM

Stimulation in the raphe magnus nucleus of the cat or the monkey results in primary afferent depolarization of cutaneous fibers classified either as A-δ or as A-α,β fibers on the basis of conduction velocity. The fibers that are depolarized include mechanoreceptors as well as nociceptors. There is no obvious preferential action on A-δ afferents in general or on nociceptors in particular. The ability of stimuli in the raphe magnus nucleus to evoke primary afferent depolarization is eliminated by lesions that interrupt the dorsolateral fasciculi of the spinal cord bilaterally. Stimulation in the nucleus reticularis gigantocellularis also results in primary afferent depolarization of A-δ and of A-α,β cutaneous afferent fibers. However, the descending pathway is in the ventral half of the cord, since lesions of the dorsolateral fasciculi do not prevent the effects of stimulation in this nucleus. Support for the hypothesis that there are at least two distinct pathways originating in the caudal brain stem that evoke primary afferent depolarization in cutaneous fibers comes from the observation that the time course of the depolarization is slower when stimuli are applied in the raphe magnus nucleus than in the nucleus gigantocellularis. It is concluded that the preferential inhibitory action of stimulation in the raphe magnus nucleus on the central actions of A-δ afferent fibers is probably due to another mechanism than presynaptic inhibition.