It is well established that neurons in ventrolateral medulla play a key role in determining the vasomotor tone. The purpose of present study is to identify sympathetic related, medullospinal tract neurons in ventrolateral medulla and to show that these mediate somato-sympathetic reflex. Medullospinal tract cells were identified by antidromic stimulation to intermediolateral nucleus (IML) of the second thoracic (T2) spinal cord in anesthetized cats. Peripheral nerves were stimulated for orthodromic activation of these cells and peripheral receptive fields were determined. Post R wave histogram of unit and spike triggered averaging of sympathetic nerve discharge (SND) were used to define sympathetic related cell. A total of 113 neurons was recorded in ventrolateral medulla that had the axonal projections to T2 spinal cord. Thirty four of these medullospinal cells showed spontaneous discharges and the others not. Between these two groups, rostro-caudal coordinate of the distribution from obex [4.7±0.2 (mean S.E.) mm, 4.1 0.1 mm], depth from dorsal surface (5.5±0.2mm, 4.9±0.1mm and conduction velocity (9.9±1.7m/sec, 16.7±1.9 m/sec) were significantly different (p<0.05). In spontaneously discharging group, characteristics of rostral and caudal groups were significantly different and we demonstrated that cells in rostral group mediate somatosympathetic reflex. From these results, we conclude that a certain portion of spontaneously discharging medullospinal tract cells in rostral ventrolateral medulla comprise the efferent outputs of somatosympathetic reflex to sympathetic preganglion neurons.
