Baroreceptor mechanisms controlling sympathetic nervous rhythms of central origin.

A study was made in the anesthetized cat of the slow wave of sympathetic nervous discharge (SND) locked in a 1:1 relation to the cardiac cycle (3 cycle/s periodicity). SND was recorded from the preganglionic splanchnic and postganglionic renal nerves. The data contradict the generally accepted view that the slow wave occurs as the direct result of a waxing and waning of baroreceptor nervous discharge. Although baroreceptor denervation unlocked the phase relations between SND and the cardiac cycle, the slow wave persisted and its duration was not changed. Furthermore, the slow wave was aborted by stimuli delivered to the baroreceptor nerves or paramedian reticular nucleus during a time span which accounted for less than 1 percent of the cardiac cycle. It is concluded that the 3 cycle/s periodicity of SND is representative of a vasomotor rhythm of central origin which is entrained to the cardiac cycle by the baroreceptor reflexes. The sympathoinhibitory effect leading to the entrainment of the slow wave is mediated in the brainstem. ta spinal component of baroreceptor-induced sympathoinhibition also was demonstrated.