SUBGROUPS OF ROSTRAL VENTROLATERAL MEDULLARY AND CAUDAL MEDULLARY RAPHE NEURONS BASED ON PATTERNS OF RELATIONSHIP TO SYMPATHETIC-NERVE DISCHARGE AND AXONAL PROJECTIONS

This study was designed to answer three questions concerning rostral v entrolateral medullary (RVLM) and caudal medullary raphe (CMR) neurons with activity correlated to sympathetic nerve discharge (SND). 1) Wha t are the proportions of RVLM and CMR neurons that have activity corre lated to both the cardiac-related and IO-Hz rhythms in SND, to only th e 10-Hz rhythm, and to only the cardiac-related rhythm? 2) Which of th ese cell types project to the spinal cord? 3) Do the outputs of the ca rdiac-related and 10-Hz rhythm generators converge at the level of bul bospinal neurons or their antecedent interneurons? To address these is sues we recorded from 44 RVLM and 48 CMR neurons with sympathetic nerv e-related activity in urethan-anesthetized cats with intact carotid si nus nerves, but sectioned aortic depressor and vagus nerves. Spike-tri ggered averaging, arterial pulse-triggered analysis, and coherence ana lysis revealed that the naturally occurring discharges of 24 of these RVLM neurons and 41 of these CMR neurons were correlated to both the 1 0-Hz and cardiac-related rhythms in inferior cardiac postganglionic SN D. The discharges of the other neurons were correlated to only the IO- Hz rhythm(15 RVLM and 6 CMR neurons) or to only the cardiac-related rh ythm (5 RVLM neurons and 1 CMR neuron) in SND. The time-controlled col lision test Verified that 16 of 18 RVLM and 31 of 34 CMR neurons with activity correlated to both rhythms were antidromically activated by s timulation of the white matter of the first thoracic (T-1) segment of the spinal cord. In contrast, only I of 10 RVLM neurons and 0 of 4 CMR neurons with activity correlated to only the 10-Hz rhythm could be an tidromically activated by stimulation at T1. Also 0 of 3 RVLM neurons with activity correlated to only the cardiac-related rhythm in SND wer e antidromically activated by spinal stimulation. These data show for the first time that bulbospinal sympathetic pathways emanating from th e RVLM and CMR are comprised almost exclusively of neurons whose disch arges are correlated to both the cardiac-related and 10-Hz rhythms in SND. Moreover, the data support the hypothesis that the outputs of the cardiac-related and 10-Hz rhythm generators converge on RVLM and CMR bulbospinal neurons rather than on their antecedent interneurons. Fina lly, the data demonstrate that a substantial proportion of RVLM neuron s and a small group of CMR neurons with activity correlated to SND do not project to the thoracic spinal cord. Their discharges were correla ted to only one of the rhythms in SND. Their axonal trajectories and f unctions are unknown.