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 (T<sub>2</sub>) 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 T<sub>2</sub> 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.
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