Pattern of sympathetic discharges and their relation to baroreceptor and respiratory activities

Abstract

(1) The patterns of sympathetic background discharges and sympathetic reflex responses were recorded and compared in pre- and postganglionic fibers in the white rami at T 1−2, T 10−11, L 1−2, cervical sympathetic nerve, splanchnic, cardiac and renal nerves in chloralose-urethane anesthetized cats. (2) Background discharges recorded from white rami at various levels of sympathetic outflow were similar. Background discharges recorded from postganglionic fibers appeared to be much different from those of the white rami. In the postganglionic fibers there was clear arterial-pulse synchronous activity. After severance of all baroreceptor afferents, rhythmic activity was still present but it was not pulse synchronous. (3) Sympathetic reflexes evoked by stimulation of the sciatic nerve showed spinal (early) and supraspinal (late) reflex responses in white rami of T 10–11 and L 1–2, but at the T 1–2 level and in cardiac and cervical sympathetic nerves these two reflexes could not be separated because of the small difference in their latencies. Recordings from postganglionic fibers of renal nerves generally showed supraspinal reflexes only; the spinal reflexes were small or could not be distinguished from background discharges except in a few instances. (4) Background discharges as well as the sympathetic reflexes recorded from pre- and postganglionic sympathetic fibers were strongly altered by respiratory neuron activity. They were augmented during the phases of the respiratory cycle when phrenic discharges were occuring and they were depressed during absence of phrenic discharges. (5) Activation of carotid sinus baroreceptors by distention completely inhibited both background and supraspinal reflex responses recorded from pre- and postganglionic fibers, but the spinal reflex was not affected. The degree of inhibitory effect was found to be the same in all recordings, i.e., from the white rami of various cord levels as well as in postganglionic renal and cardiac nerves. (6) The decrease in systemic blood pressure caused by repetitive stimulation of somatic myelinated fibers at low frequency (2–5/sec) was further augmented by baroreceptor activation. The two effects summated. The pressor effects and the increased sympathetic activity evoked by high frequency stimulation of myelinated (A-) fibers, or excitation of both A- and C-fibers together, were counteracted by baroreceptor activation; augmented sympathetic discharges were completely abolished for a few seconds and a decrease in blood pressure occurred. (7) It was concluded that the modulation of sympathetic activities produced by baroreceptor afferents, by the respiratory center activity and by somatic nerve excitation through supraspinal centers are similar in all sympathetic efferents.