Cardiac- and noncardiac-related coherence between sympathetic drives to muscles of different human limbs.

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Abstract
Partial coherence analysis was used to evaluate the extent to which coherence between resting muscle sympathetic activity (MSA) in different pairs of limbs in humans is explained by the common baroreceptor input and by other noncardiac-related factors. Multiunit MSA in two or three nerves, arterial blood pressure, and electrocardiogram were recorded simultaneously. Correlated MSA consisted of a sharp periodic component at the heart rate and a wideband component of relatively low power distributed between 0 and 2-2.5 Hz. Quantitative analysis revealed stronger coupling between MSAs in close limbs than in distant limbs (peak coherence leg-leg, 0. 94 +/- 0.03; arm-leg, 0.76 +/- 0.11). Furthermore, the wideband component, unaffected by partialization with circulatory signals, was significantly stronger between leg-leg (0.67 +/- 0.10) than between arm-leg pairs (0.29 +/- 0.10), i.e., noncardiac-related components explained 71% of leg-leg and 38% of arm-leg coherences at the frequency of the heart. Our results indicate that nonuniform relationship exists between resting sympathetic outflow to muscles in close and distant extremities which is, however, partially masked by the effect of the common rhythmic baroreceptor input.