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1Departments of Medicine and Cellular and Molecular Physiology, Pennsylvania State University College of Medicine, Hershey, Pennsylvania; 2Department of Biological Sciences, Michigan Technological University, Houghton, Michigan; and 3United States Army Institute of Surgical Research, Fort Sam Houston, Texas
Submitted 14 June 2004 ; accepted in final form 7 October 2004
Autonomic responses may underlie associations among anxiety, vestibular dysfunction, and unexplained syncope. Mental stress (MS), an anxiety-inducing stimulus, causes forearm vasodilation, whereas the vestibulosympathetic reflex (VSR) causes forearm vasoconstriction. The purpose of this study was to examine the combined effects of mental and vestibular stimulation on neurovascular control in the forearm. Heart rate, arterial pressure (Finapres), and forearm blood flow (Doppler) were measured in 10 healthy volunteers in the prone position during 1) head-down rotation (HDR), 2) MS (mental arithmetic), and 3) HDR + MS. Forearm vascular resistance (FVR) increased during HDR (from 232 ± 40 to 319 ± 53 units) and decreased during MS (from 260 ± 57 to 154 ± 22 units). During HDR + MS, FVR did not change [change (
) = –31 ± 50 units] and was not significantly different from the algebraic sum of each trial performed alone ( = –20 ± 42 units). Arm muscle sympathetic nerve activity (MSNA; microneurography) was measured in seven additional subjects. MSNA increased during HDR (from 13 ± 2 to 17 ± 2 bursts/min) and HDR + MS (from 11 ± 2 to 16 ± 2 bursts/min). Increases in MSNA during HDR + MS ( = 5 ± 2 bursts/min) were not different from the algebraic sum of each trial performed alone ( = 6 ± 2 bursts/min). We conclude that an additive neurovascular interaction exists between MS and the VSR in the forearm. Activation of the VSR prevented forearm vasodilation during MS, suggesting that activation of the VSR may help protect against stress-induced syncope.
otolith stimulation; vestibulosympathetic reflex; arterial pressure control; mental arithmetic
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