Nucleus paragigantocellularis lateralis and lumbar sympathetic discharge in the rat.

Two classes of medullospinal units with inhibitory baroreceptor inputs [nucleus paragigantocellularis lateralis (PGCL) units] were recorded in PGCL (conduction velocity 3.1 and 0.67 m/s, respectively). Single-pulse stimulation of PGCL produced two peaks of activation of the lumbar sympathetic chain [sympathetic nerve discharge (SND)] with latencies of 98 and 206 ms. After substracting the delay due to impulse propagation between cord and sympathetic chain (64 ms), a conduction velocity of 2.8 and 0.67 m/s could be estimated for the medullospinal portion of the two sympathoexcitatory pathways stimulated in the PGCL. R wave-triggered activity histograms of PGCL unit discharge and lumbar SND exhibited a similar pattern with one major trough, occurring 108 ms later at the level of the sympathetic chain. It is concluded that PGCL contains two classes of medullospinal neurons with sympathoexcitatory function, the fast-conducting type being predominantly responsible for the cardiac-related rhythm of lumbar SND. Finally, averaging techniques revealed a large respiratory rhythm of lumbar SND in spontaneously breathing rats, but little or no such rhythm could be detected at the level of most PGCL sympathoexcitatory cells.