Raphe magnus inhibition of feline T1-T4 spinoreticular tract cell responses to visceral and somatic inputs

Background activity of spinoreticular tract neurons in the T1-T4 segments was on average inhibited 80% by electrical stimulation of nucleus raphe magnus. Nucleus raphe magnus stimulation inhibited responses of spinoreticular tract neurons to somatic input produced by touching the skin and hair (innocuous stimulus) or pinching the skin and muscle (noxious stimulus). Inhibition of responses to noxious and innocuous somatic inputs was not significantly different. Inhibition produced during nucleus raphe magnus stimulation was less effective when the activity of spinoreticular tract cells increased. This relationship was consistent for both background activity and responses to somatic noxious or innocuous input. Nucleus raphe magnus stimulation inhibited responses of spinoreticular tract neurons to visceral input produced by electrical stimulation of cardiopulmonary sympathetic afferent fibers. Responses to C-fiber sympathetic afferent fibers were more effectively inhibited than were responses to A-delta sympathetic afferent fibers. In conclusion, stimulation of the nucleus raphe magnus inhibits T1-T4 spinoreticular tract neuronal responses to visceral and somatic inputs. Since spinoreticular neurons project to the medullary reticular formation, activation of the nucleus raphe magnus could modulate affective-motivational behavior and cardiovascular adjustments that often occur during angina pectoris.