CARDIAC SYMPATHETIC PREMOTOR NEURONS

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-an esthetized cats, with the use of microinjections of sodium glutamate ( 5-10 nl, 0.1 M) to excite neuronal cell bodies. Cardiac sympathetic re sponses were recorded from the ipsilateral inferior cardiac nerve, whi le recordings were made simultaneously from postganglionic vasoconstri ctor fibers to skeletal muscle (ipsilateral peroneal nerve). Barorecep tors 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 inferi or cardiac nerve-to-muscle vasoconstrictor response ratio exceeded tha t expected by two- to sevenfold; for 9 very caudolateral injections th at ratio was strongly reversed, favoring muscle vasoconstrictors by tw o to fivefold. Intervening sites gave more equal responses. Overall, t he response ratio varied systematically with injection site. These fin dings demonstrate that neurons with preferential or selective actions on the cardiac sympathetic outflow are present in the RVLM and are org anized topographically. The simplest interpretation is that a populati on of selective cardiac sympathetic premotor neurons occupies a territ ory substantially overlapping, but centered rostromedially to, the pop ulation controlling vasoconstriction in muscle.