Sequence of activation of ventrolateral and dorsal medullary sympathetic neurons.

A unique approach to the problem of evaluating potential regions of origin of the cardiac-related component in basal sympathetic nerve discharge (SND) is described in the present investigation. Specifically a comparison was made of the firing times of cat ventrolateral and dorsal medullary neurons with sympathetic nerve-related activity. These two medullary regions are known to play a role in the maintenance of blood pressure. Spike-triggered averaging and post-R wave interval analysis were used to identify single medullary neurons with spontaneous discharges temporally related to those in the renal or inferior cardiac sympathetic nerve. The results obtained using a ventricular pacing test previously developed in our laboratory allowed us to classify some of these neurons as elements of networks that control SND (i.e., medullary sympathetic neurons). The test is based on the assumption that brain stem sympathetic neurons should exhibit discharges that remain locked in time to the peak of the cardiac-related component of SND during changes in heart rate that shift the phase relations between baroreceptor and sympathetic nerve activity. Electrical stimulation of sites from which the discharges of such units were recorded elicited increases in SND, thus suggesting that these neurons subserved a sympathoexcitatory function. On the average, ventrolateral medullary neurons fired 48 ms later than dorsal medullary neurons during the cardiac-related component in SND. This observation does not support the previous suggestion [R. A. L. Dampney and E. A. Moon. Am. J. Physiol. 239 (Heart Circ. Physiol. 8): H349-H358, 1980] that basal sympathetic tone originates exclusively in circuits of ventrolateral medullary neurons. Possibilities concerning the interconnections of dorsal and ventrolateral medullary neurons involved in maintaining blood pressure are discussed.