Bulbospinal sympathoexcitatory pathways in the rat.

Electrical stimulation of the rostral ventrolateral medulla (RVL) in the rat elicited a compound excitatory response on the renal sympathetic nerve (RSN) activity. Three shoulders (probably peaks) with mean latencies of 49, 59, and 71 ms were observed in the RSN excitatory response. Stimulation of the lateral funiculus at T4-T5 level evoked an excitatory response on the RSN with a mean latency of 42 ms. After subtracting the time needed for the impulse to run between the thoracic spinal cord and the recording site in the RSN, the calculated mean conduction velocities for the bulbospinal component of the sympathoexcitatory pathway activated by RVL stimulation were 7.4 m/s, 3.0 m/s, and 1.7 m/s. These conduction velocities corresponded with the axonal conduction velocities of intracellular recorded neurons in the RVL with pulse modulated activity and antidromically activated from the thoracic spinal cord (conduction velocities spanning from 1.1 m/s to 8.2 m/s). These findings are consistent with the idea that spontaneously active RVL barosensitive neurons with spinal projections are sympathoexcitatory and participate in mediating the sympathoexcitatory responses evoked on sympathetic nerves by RVL stimulation.