Is muscle sympathetic nerve activity associated with cerebral blood velocity? A partial coherence analysis.

Despite some evidence, the role of sympathetic nerve activity in the regulation of cerebral blood flow remains controversial. In humans, muscle sympathetic nervous activity (MSNA) is the only direct measure of sympathetic nerve activity that can be recorded with sufficient temporal resolution, to allow association with dynamic regulation of cerebral blood velocity (CBv). This study tested the hypothesis that MSNA is associated with the regulation of CBv at rest and during different physiological maneuvers. Nine healthy subjects underwent two sympathoexcitatory maneuvers: i) isometric handgrip exercise (HGR), and ii) cold pressor test (CPT). Mean arterial pressure (MAP, oscillometric method), CBv (transcranial Doppler ultrasound), and MSNA (microneurography) were measured continuously during experimental protocols. Ordinary and partial coherences of the MAP, CBv and MSNA time series were estimated by transfer function analysis in the low-frequency range (LF: 0.07-0.20 Hz), using MAP and MSNA as inputs and CBv as the output variable. When the influence of MSNA was taken into account, the partial coherences between MAP and CBv were considerably reduced at baseline (P<0.01), HGR (P=0.02), and CPT (P<0.01). Similarly, when the influence of MAP was taken into account, the coherence between MSNA and CBv was considerably reduced at baseline (P<0.01), HGR (P=0.02), and CPT (P=0.01), leading to the conclusion, that MSNA was associated to dynamic regulation of CBv. Partial coherence analysis is a promising method for assessing the influence of the sympathetic nervous system on cerebral hemodynamics.