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NEUROHUMORAL CONTROL OF CIRCULATION AND HYPERTENSION

Human cerebrovascular and autonomic rhythms during vestibular activation

¹U.S. Army Institute of Surgical Research, Fort Sam Houston, Texas 78234; ²Department of Biological Sciences, Michigan Technological University, Houghton, Michigan 49931; and ³Department of Physics, University of Turku, Turku, Finland 20014

Submitted 4 August 2003 ; accepted in final form 5 January 2004

Otolith activation increases muscle sympathetic nerve activity (MSNA), and MSNA activation may alter associations among autonomic oscillators, including those modulating cerebral hemodynamics. The purpose of this study was to determine the influence of vestibulosympathetic activation on cerebral and autonomic rhythms. We recorded the ECG, finger arterial pressure, end-tidal CO₂, respiration, cerebral blood flow velocity, and MSNA in eight subjects. Subjects breathed at 0.25 Hz for 5 min in the prone and head-down positions. We analyzed data in time and frequency domains and performed cross-spectral analyses to determine coherence and transfer function magnitude. Head-down rotation increased MSNA from 7 ± 1.3 to 12 ± 1.5 bursts/min (P = 0.001) but did not affect R-R intervals, arterial pressures, mean cerebral blood flow velocities (V_mean), or their power spectra. Vestibular activation with head-down rotation had no effect on mean arterial pressure and V_mean transfer function magnitude. The two new findings from this study are 1) head-down rotation independently activates the sympathetic nervous system with no effect on parasympathetic activity or V_mean; and 2) frequency-dependent associations between arterial pressures and V_mean are independent of vestibular activation. These findings support the concept that vestibular-autonomic interactions independently and redundantly serve to maintain steady-state hemodynamics.

power spectral analysis; cross-spectral analysis; transcranial Doppler; sympathetic microneurography; vestibulosympathetic reflex

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