Reticulospinal vasomotor neurons in the RVL mediate the somatosympathetic reflex

Sympathoexcitatory neurons in the C1 area of the rostral ventrolateral medulla (RVL) that project to the spinal intermediolateral nucleus play an important role in the maintenance of basal sympathetic tone and in the reflex adjustments of sympathetic activity to internal and external stimuli. We sought to determine whether the sympathetic response to somatic afferent activation, the somatosympathetic reflex, also arises from the excitation of reticulospinal vasomotor neurons in the RVL. In urethan-anesthetized, paralyzed, ventilated rats, single sciatic nerve stimuli evoked an early (latency: 26 ms) and a late (latency: 117 ms) excitation of antidromically identified, RVL-spinal, sympathoexcitatory neurons that paralleled and preceded the biphasic increases in splanchnic nerve activity (peak latencies: 87 and 176 ms). The differences between the respective latencies of the RVL unit responses and those recorded in the splanchnic nerve were comparable to the conduction time in the sympathoexcitatory pathway from the RVL to the splanchnic nerve. Both the threshold intensity and the response amplitude vs. stimulus intensity relationship for the sympathetic response were comparable to those for the A-δ-fiber component of the dorsal root response to sciatic stimulation. We conclude that the rat somatosympathetic reflex consists of an early excitatory component due to the early activation of RVL-spinal sympathoexcitatory neurons with rapidly conducting axons and a later peak that may arise from the late activation of these same neurons as well as the early activation of RVL vasomotor neurons with more slowly conducting spinal axons.