Bidirectional cardiovascular connections between ventrolateral medulla and nucleus of the solitary tract

Single-unit recording experiments were done in chloralose-anesthetized, paralyzed and artificially ventilated cats to identify neurons in ventrolateral medulla (VLM) that send efferent axons directly to the region of the nucleus of the solitary tract (NTS) and receive cardiovascular afferent inputs from the carotid sinus (CSN) and aortic depressor (ADN) nerves and the NTS. Units in VLM were identified by antidromic excitation to stimulation of functionally and histologically verified sites in the NTS complex. Antidromic potentials were recorded from 34 units in VLM. Units responded with a mean antidromic latency of 4.37 +/- 0.32 ms corresponding to a mean conduction velocity of 0.93 +/- 0.07 m/s. Of these 34 units, 18 were excited orthodromically by stimulation of the CSN and/or ADN. Furthermore, 10 of the 18 units responding to stimulation of the buffer nerves were also orthodromically excited by stimulation of NTS. An additional 76 units were identified in VLM that only responded orthodromically to stimulation of NTS with a mean latency of 9.75 +/- 2.93 ms, of which 33 also responded orthodromically to stimulation of the buffer nerves. These data provide electrophysiological evidence of a bidirectional connection between neurons in VLM that receive and integrate peripheral cardiovascular afferent inputs and send efferent axons directly back to the region of NTS. These results suggest that neurons in the VLM may be part of a medullary feedback reflex loop through which afferent information from cardiovascular receptors exerts an influence on NTS neurons involved in the control of the circulation.