Sympathetic and vascular responses to head-down neck flexion in
humans.
Shortt, Timothy L., and Chester A. Ray.
Autonomic and Cardiovascular Control Unit, Department of Exercise
Science, University of Georgia, Athens, GA 30602
APStracts 3:0506H, 1996.
Animal studies have demonstrated increases in sympathetic nerve
outflow with vestibular stimulation. The purpose of the present study
was to determine if vestibulosympathetic reflexes are engaged in
humans. Muscle sympathetic nerve activity (MSNA), heart rate,
arterial pressure, calf blood flow (CBF), and calculated calf
vascular resistance (CVR, mean arterial pressure/CBF) were determined
during 10 min of baseline (laying prone with chin supported) and 10
min of head-down neck flexion (HDNF). MSNA responses were measured in
9 subjects, and calf vascular responses were determined in 7 of these
subjects. Heart rate increased during the first minute of HDNF (71 2
to 76 3 beats[beta]min-1; p < 0.05) and remained slightly elevated
(71 2 to 74 3 beats min-1; p < 0.05) for the duration of HDNF.
Diastolic and mean arterial pressures also increased slightly with
HDNF (80 3 to 823 mmHg, and 963 to 983 mmHg, respectively; p < 0.05).
Systolic arterial pressure did not change significantly during HDNF.
CBF decreased 14% (4.630.78 to 3.970.60 ml[beta]min-1[beta]100ml-1; p
< 0.05), and CVR increased 12% (24.04.3 to 27.44.7 units; p < 0.05)
during HDNF. These changes corresponded with significant increases in
MSNA during HDNF. MSNA, expressed as burst frequency, increased from
142 to 202 bursts/min (p < 0.05) and increased 6323% (p < 0.05) when
expressed as percent change in total activity. All variables returned
to baseline during recovery. Thoracic impedance measured in 5
subjects did not change during HDNF (19.61.2 to 19.71.5 ), suggesting
no major change in central blood volume. The results indicate that
HDNF elicits increases in CVR that are mediated by the augmentation
of MSNA. Arterial pressure responses and thoracic impedance data
suggest that high and low pressure baroreflexes were not the
mechanism for sympathetic activation. The immediate increase in MSNA
with HDNF suggests a role for vestibulosympathetic reflexes.
Received 19 July 1996; accepted in final form 29 October 1996.
APS Manuscript Number H647-6.
Article publication pending Am. J. Physiol. (Heart Circ. Physiology).
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 31 December 1996