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Evidence for differential control of muscle sympathetic single units during mild sympathoexcitation in young healthy humans.

Two subpopulations of muscle sympathetic single units with opposite discharge characteristics have been identified during low-level cardiopulmonary baroreflex loading and unloading in middle-aged adults and heart failure patients. The present study sought to determine if similar subpopulations are present in young healthy adults during cardiopulmonary baroreflex unloading (study 1) and rhythmic handgrip exercise (study 2). Continuous hemodynamic and multi- and single-unit muscle sympathetic nerve activity (MSNA) data were collected at baseline and during non-hypotensive lower body negative pressure (LBNP, n=12) and 40% maximal voluntary contraction rhythmic handgrip exercise (RHG, n=24). Single-unit MSNA responses were classified as anticipated or paradoxical based on whether changes were concordant or discordant with the multi-unit MSNA response, respectively. LBNP and RHG both increased multi-unit MSNA burst frequency (∆5±3 bursts/min, p<0.001; ∆5±8 bursts/min, p=0.005), burst amplitude (∆5±7 %, p=0.04; ∆13±14 %, p<0.001), and total MSNA (∆302±191 AU/min, p=0.001; ∆585±556 AU/min, p<0.001). During LBNP and RHG, 43 and 64 muscle single-units were identified, respectively, which increased spike frequency (∆9±11 spikes/min, p<0.001; ∆10±19 spikes/min, p<0.001) and the probability of multiple spike firing (∆10±12 %, p<0.001; ∆11±26 %, p=0.001). During LBNP and RHG, 36 (84%) and 39 (61%) single units possessed anticipated firing responses (∆12±10 spikes/min, p<0.001; ∆19±19 spikes/min, p<0.001) while 7 (16%) and 25 (39%) exhibited paradoxical reductions (∆-3±1 spikes/min, p=0.003; ∆-4±5 spikes/min, p<0.001). The observation of divergent subpopulations of muscle sympathetic single units in healthy young humans during two mild sympathoexcitatory stressors supports differential control at the fibre level as a fundamental characteristic of human sympathetic regulation.

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