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Departments of Medicine and of Cellular and Molecular Physiology, General Clinical Research Center, Pennsylvania State University College of Medicine, Hershey, Pennsylvania 17033; and Department of Exercise Science, University of Georgia, Athens, Georgia 30602
Muscle sympathetic nerve activity (MSNA)
increases with head-down neck flexion (HDNF). The present study had
three aims: 1) to examine
sympathetic and vascular responses to two different magnitudes of HDNF;
2) to examine these same responses
during prolonged HDNF; and 3) to
determine the influence of nonspecific pressure receptors in the head
on MSNA. The first experiment tested responses to two static head
positions in the vertical axis [HDNF and intermediate HDNF
(I-HDNF; ~50% of HDNF)]. MSNA increased above baseline during
both I-HDNF and HDNF (from 219 ± 36 to 301 ± 47 and from 238 ± 42 to 356 ± 59 units/min, respectively;
P < 0.01). Calf blood flow (CBF)
decreased and calf vascular resistance increased during both I-HDNF and
HDNF (P < 0.01). Both the increase in MSNA and the decrease in CBF were linearly related to the magnitude of the downward head rotations (P < 0.01). The second experiment tested responses during prolonged HDNF.
MSNA increased (from 223 ± 63 to 315 ± 79 units/min;
P < 0.01) and CBF decreased (from 3.2 ± 0.4 to 2.6 ± 0.04 ml · 100 ml
1 · min1;
P < 0.01) at the onset of HDNF.
These responses were maintained throughout the 30-min period. Mean
arterial blood pressure gradually increased during the 30 min of HDNF
(from 94 ± 4 to 105 ± 3 mmHg; P < 0.01). In a third experiment,
head-down neck extension was performed with subjects in the supine
position. Unlike HDNF, head-down neck extension did not affect MSNA.
The results from these studies demonstrate that MSNA:
1) increases in magnitude as the
degree of HDNF increases; 2) remains
elevated above baseline during prolonged HDNF; and
3) responses during HDNF are not
associated with nonspecific receptors in the head
activated by increases in cerebral pressure.
autonomic nervous system; cardiovascular control; orthostasis; vascular resistance; vestibulosympathetic reflex; otolith organs; semicircular canals
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