Cardiorespiratory coupling of sympathetic outflow in humans : a comparison of respiratory and cardiac modulation of sympathetic nerve activity to skin and muscle

It is well known that microelectrode recordings of skin sympathetic nerve activity (SSNA) in awake human subjects reveal spontaneous bursts of activity with no overt modulation by changes in blood pressure or respiration, in contrast to the clear cardiac and respiratory modulation of muscle sympathetic nerve activity (MSNA). However, cross-correlation analysis has revealed that, like individual muscle vasoconstrictor neurones, the firing of individual cutaneous vasoconstrictor neurones is temporally coupled to both the cardiac and respiratory rhythms during cold-induced cutaneous vasoconstriction, and the same is true of single sudomotor neurones during heat-induced sweating. Here we used cross-correlation analysis to determine whether SSNA exhibits cardiac and respiratory modulation in thermoneutral conditions, and to compare respiratory and cardiac modulation of SSNA with that of MSNA. Oligounitary recordings of spontaneous SSNA (n=20) and MSNA (n=18) were obtained during quiet, unrestrained breathing. Respiration was recorded by a strain-gauge transducer around the chest and ECG recorded by surface electrodes. Respiratory and cardiac modulation of SSNA and MSNA were quantified by fitting polynomials to the cross-correlation histograms constructed between the sympathetic spikes and respiration or ECG. The amplitude of the respiratory modulation (52.5±3.4%) of SSNA was not significantly different from the amplitude of the cardiac modulation (46.6±3.2%). Both were comparable to the respiratory modulation of MSNA (47.7±4.2%), while cardiac modulation of MSNA was significantly higher (89.8±1.5%). We conclude that SSNA and MSNA share similar levels of respiratory modulation, the primary difference between the two sources of sympathetic outflow being the marked cardiac modulation of MSNA provided by the baroreceptors.