'Real-time' imaging of cortical and subcortical sites of cardiovascular control : concurrent recordings of sympathetic nerve activity and fMRI in awake subjects

We review our approach to functionally identifying cortical and subcortical areas involved in the generation of spontaneous fluctuations in sympathetic outflow to muscle or skin. We record muscle sympathetic nerve activity (MSNA) or skin sympathetic nerve activity (SSNA), via a tungsten microelectrode inserted percutaneously into the common peroneal nerve, at the same time as performing functional magnetic resonance imaging (fMRI) of the brain. By taking advantage of the neurovascular coupling delay associated with BOLD (Blood Oxygen Level Dependent) fMRI, and the delay associated with conduction of a burst of sympathetic impulses to the peripheral recording site, we can identify structures in which BOLD signal intensity covaries with MSNA or SSNA. Using this approach we found MSNA-coupled increases in BOLD signal intensity in the mid-insula and dorsomedial hypothalamus on the left side, and in dorsolateral prefrontal cortex, posterior cingulate cortex, precuneus, ventromedial hypothalamus and rostral ventrolateral medulla on both sides. Conversely, spontaneous bursts of SSNA were positively correlated with BOLD signal intensity in the ventromedial thalamus and posterior insula on the left side, and in the anterior insula, orbitofrontal cortex and frontal cortex on the right side, and in the mid-cingulate cortex and precuneus on both sides. Inverse relationships were observed between MSNA and BOLD signal intensity in the right ventral insula, nucleus tractus solitarius and caudal ventrolateral medulla, and between SSNA and signal intensity in the left orbitofrontal cortex. These results emphasize the contributions of cortical regions of the brain to sympathetic outflow in awake human subjects, and the extensive interactions between cortical and subcortical regions in the ongoing regulation of sympathetic nerve activity to muscle and skin in awake human subjects.