Properties of ventrolateral medullary neurons that respond to muscular contraction

Previous results from this laboratory have suggested that neurons in the ventrolateral medulla (VLM) modulate the pressor response to muscular contraction. The purpose of the present study was to determine 1) if VLM neurons with a discharge pattern related to sympathetic discharge and/or the cardiac cycle are stimulated during muscular contraction, 2) if the neurons activated by muscular contraction project to the intermediolateral columns of the spinal cord and 3) the location of glutamate immunoreactive neurons in the medulla. Single-unit responses of ventrolateral medullary neurons to hindlimb muscular contraction evoked by ventral root (L7 and S1) stimulation were recorded in one group of anesthetized cats. Computer analyses were performed to determine if the resting discharge of VLM neurons correlated temporally with sympathetic nerve discharge and/or the cardiac cycle. The discharge rate of 21 of 27 neurons which had a discharge related to sympathetic nerve activity increased during muscular contraction. Neurons in some of the experiments were tested for axonal projections to the intermediolateral nucleus (T2 or T5) of the spinal cord with antidromic activation techniques. The discharge pattern of 78% of the VLM neurons which were activated antidromically was related to the cardiac cycle or sympathetic nerve discharge. Most (92%) reticulospinal VLM neurons with cardiovascular related discharge were excited by muscular contraction. In a second set of experiments, glutamate immunoreactivity was demonstrated in neurons within an area overlapping the location of VLM neurons which were excited by muscular contraction. These findings suggest that reticulospinal neurons in the ventrolateral medulla which have a discharge pattern related to cardiovascular activity contribute to the pressor reflex evoked by muscular contraction. These neurons may utilize glutamate as a neurotransmitter.