Imaging of brainstem sites involved in cardiovascular control

The brainstem is critically important in the beat-to-beat control of arterial pressure. Our current understanding of the baroreflex circuitry – a classic negative feedback loop – comes primarily from studies in anesthetized animals. Primary afferent axons from the baroreceptors project to the caudal region of the nucleus tractus solitarius (NTS), where they synapse onto second-order neurons, which in turn send excitatory projections onto inhibitory neurons within the region of the caudal ventrolateral medulla (CVLM). These CVLM neurons synapse directly onto excitatory neurons within the RVLM and serve to inhibit the spontaneous activity of RVLM premotor sympathetic neurons. Nucleus ambiguus and the dorsal motor nucleus of the vagus also receive excitatory projections from NTS, activating vagal cardiac efferents and slowing the heart. Recent work in awake human subjects, in which functional magnetic resonance imaging (fMRI) has been coupled with microelectrode recordings of muscle sympathetic nerve activity (MSNA), has confirmed the operation of these medullary nuclei in humans.