Direct pathway from neurons in the ventrolateral medulla relaying cardiovascular afferent information to the supraoptic nucleus in the cat.

In chloralose anesthetized cats experiments were done to electrophysiologically identify neurons in the ventrolateral medulla (VLM) which relay cardiovascular afferent information directly to the supraoptic nucleus (SON). Action potentials elicited antidromically by electrical stimulation of the SON were recorded from 69 histologically verified single units in the VLM. Single units responded with latencies corresponding to conduction velocities of 7.8 +/- 0.6 m/s. Of these units 26 were excited orthodromically by stimulation of the buffer nerves; 12 responded to stimulation of only the carotid sinus nerve, 7 responded to stimulation of only the aortic depressor nerve, and 7 responded to both buffer nerves. The axons of VLM units that responded to buffer nerves conducted at a significantly slower velocity than those of non-responsive units (5.7 +/- 0.4 and 9.1 +/- 0.8 m/s, respectively). These data provide electrophysiological evidence of two different populations of VLM neurons which project directly to the SON, and suggest that the direct pathway from the VLM to the SON is involved in the release of vasopressin by SON neurons during activation of baroreceptor and chemoreceptor afferent fibers.