Somatic nerve stimulation evokes qualitatively different somatosympathetic responses in the cervical and splanchnic sympathetic nerves in the rat.

Input from unmyelinated and myelinated nociceptors drives somatosympathetic responses to painful stimuli. Here we report that somatosympathetic responses recorded simultaneously in the cervical and splanchnic sympathetic nerves of the urethane-anaesthetized rat are qualitatively different. High intensity electrical stimulation of the sciatic nerve (SN) evoked characteristic biphasic responses in splanchnic nerve activity (N=6), but only monophasic responses in the cervical nerve (N=4). By colliding sympathoexcitatory responses to SN stimulation with precisely triggered baroinhibition evoked by electrical stimulation of the aortic depressor nerve, we found that cervical responses are analogous to the first phase of the splanchnic response, and that the biphasic splanchnic response is due to the arrival of two distinct afferent volleys at the site of sympathetic integration. Extracellular recordings of responses to SN stimulation in barosensitive neurons in the rostral ventrolateral medulla (RVLM; N=16) support these findings; responses were typically biphasic, although the relative magnitudes of the two phases were highly variable, and in some cases the longer-latency volley was completely absent. Our results suggest that sympathetic responses to somatic stimuli, mediated by the RVLM, are non-uniform and are dependent on the target of the particular sympathetic output. The identification of RVLM sympathetic premotor neurons with both biphasic and monophasic responses indicates that the difference in the splanchnic and cervical nerve responses is due to specific channeling of activity evoked by myelinated and unmyelinated nociceptors to the medulla. The results are discussed with regard to the differential control of sympathetic nerve activity.