Cardiac sympathetic premotor neurons

Abstract

To locate premotor neurons controlling the cardiac sympathetic supply and to determine their relation to brain stem vasomotor pathways, the rostral ventrolateral medulla (RVLM) was mapped in seven chloralose-anesthetized cats, with the use of microinjections of sodium glutamate (5-10 nl, 0.1 M) to excite neuronal cell bodies. Cardiac sympathetic responses were recorded from the ipsilateral inferior cardiac nerve, while recordings were made simultaneously from postganglionic vasoconstrictor fibers to skeletal muscle (ipsilateral peroneal nerve). Baroreceptors were denervated to eliminate the reflex effects of blood pressure changes. Most of the 115 injected RVLM sites excited both sympathetic nerves. Inferior cardiac nerve activity increased by up to 395% (mean 105 +/- 86%, SD), and muscle vasoconstrictor activity increased by up to 487% (110 +/- 107%). Their relative response varied with injection site, however. For 16 of the most rostromedial injections, the inferior cardiac nerve-to-muscle vasoconstrictor response ratio exceeded that expected by two- to sevenfold; for 9 very caudolateral injections that ratio was strongly reversed, favoring muscle vasoconstrictors by two to fivefold. Intervening sites gave more equal responses. Overall, the response ratio varied systematically with injection site. These findings demonstrate that neurons with preferential or selective actions on the cardiac sympathetic outflow are present in the RVLM and are organized topographically. The simplest interpretation is that a population of selective cardiac sympathetic premotor neurons occupies a territory substantially overlapping, but centered rostromedially to, the population controlling vasoconstriction in muscle.