Confluence of barosensory and nonbarosensory inputs at neurons in the ventrolateral medulla in rabbits.

In urethane-anesthetized rabbits, 209 spontaneously active neurons that responded to stimulation of aortic nerve A fibers were found within the ventrolateral medulla (VLM). The neurons, termed barosensory VLM neurons, were inhibited, except for three instances, by stimulation of A fibers. Forty-seven percent of barosensory VLM neurons tested (74 of 159) were activated antidromically by electrical stimulation of the dorsolateral funiculus at the C2 level. Activity of barosensory VLM neurons was enhanced by stimulation of carotid body chemoreceptors or the posterior hypothalamic area, whereas it was diminished by increases in arterial pressure elicited by injection of phenylephrine. Barosensory VLM neurons responded variously to stimulation, with two to three pulses at 40 or 100 Hz, of spinal afferents of cutaneous and muscle origins and the spinal trigeminal complex. Although stimulation of one group of somatosensory fibers could evoke different patterns of neuronal responses consisting of excitatory and inhibitory components, the following responses were most often encountered. Group II cutaneous afferents caused an inhibition. Recruitment of group III afferents brought about a brief excitatory component preceding it. Activation of group IV cutaneous fibers added a long latency excitatory component. Excitation of groups III and IV muscle afferents most often resulted in an inhibition, whereas stimulation of the spinal trigeminal complex elicited various combinations of excitatory and inhibitory components. These results are consistent with the view that neurons in the ventrolateral medulla receive barosensory and nonbarosensory inputs from various peripheral and central sources and participate in the control of sympathetic vasomotor activity and arterial pressure.