Respiratory influences on sympathetic vasomotor outflow in humans

We have attempted to synthesize findings dealing with four types of respiratory system influences on sympathetic outflow in the human. First, a powerful lung volume-dependent modulation of muscle sympathetic nerve activity (MSNA) occurs within each respiratory cycle showing late-inspiratory inhibition and late-expiratory excitation. Secondly, in the intact human, neither reductions in spontaneous respiratory motor output nor voluntary near-maximum increases in central respiratory motor output and inspiratory effort, per sec, influence MSNA modulation within a breath, MSNA total activity or limb vascular conductance. Thirdly, carotid chemoreceptor stimuli markedly increase total MSNA; but most of the MSNA response to chemoreceptor activation appears to be mediated independently of increased central respiratory motor output. Fourthly, repeated fatiguing contractions of the diaphragm or expiratory muscles in the human show a metaboreflex mediated time-dependent increase in MSNA and reduced vascular conductance and blood flow in the resting limb. Recent evidence suggests that these respiratory influences contribute significantly to sympathetic vasomotor outflow and to the distribution of systemic vascular conductances and blood flow in the exercising human.