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Identifying a central command network of sympathetic control in the awake human

      Using concurrent microneurography and fMRI we have been able to identify the medullary nuclei responsible for the spontaneous generation of muscle sympathetic nerve activity (Macefield and Henderson, 2010) and have since extended upon this work by examining the suprabulbar areas involved in the control of both muscle sympathetic nerve activity (MSNA) and skin sympathetic nerve activity (SSNA). In the present study we aim to determine the whole brain regions common and unique to MSNA and SSNA and establish a central network of autonomic control. A tungsten microelectrode was inserted percutaneously into a fascicle of the common peroneal nerve, at the level of the fibular head, to record multiunit sympathetic nerve activity. MSNA was recorded in 10 subjects; spontaneous SSNA was recorded in 13 subjects. Gradient echo, echo planar fMRI was performed using a 3 T scanner (Phillips Achieva). Two hundred scans were collected continuously in a 4 s-ON, 4 s-OFF protocol (46 axial slices, TR=8 s, TE=4 s, flip angle=90 deg, raw voxel size=1.5 mm3). Total sympathetic burst amplitudes were measured from the RMS-processed mean voltage amplitude during the final 3 s of the 4 s-OFF period. A group analysis of Blood Oxygen Level Dependent (BOLD) changes in signal intensity (SPM5: random effects, minimum cluster size 10 voxels, uncorrected p<0.05) was measured during the subsequent 4 s-ON period. A comparative analysis was then performed between individuals in the SSNA group and the MSNA group (SPM5: fixed effects, minimum cluster size 10 voxels, uncorrected, p<0.05). Preliminary results have shown signal intensity increases in the left amygdala and the medial prefrontal cortex when SSNA was contrasted against MSNA. Conversely, when MSNA was compared against SSNA signal intensity increases occurred in the right insula and right cerebellar cortex. These initial analyses illustrate discrete regions that are specifically involved in the generation of sympathetic outflow to either muscle or skin.
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