Muscle reflex stimulates sympathetic postganglionic efferents
innervating the triceps surae muscles of cats.
Hill, Janeen M., Christine M. Adreani, Marc P. Kaufman.
Division of Cardiovascular Medicine, Departments of Internal
Medicine and Human Physiology, University of California, Davis, CA
95616
APStracts 2:0556H, 1995.
Two neural mechanisms contribute to the cardiovascular responses to
exercise. The first, central command, proposes a parallel activation
of central locomotor and brainstem circuits controlling
cardiovascular function. The second, the muscle reflex, proposes that
contraction-activated group III and IV afferents increase
cardiovascular function. In humans, whole nerve recordings of
sympathetic discharge suggest that central command increases
sympathetic outflow to skin but not to skeletal muscle and that the
muscle reflex increases sympathetic outflow to skeletal muscle but
not to skin. We, therefore, tested the hypothesis that the muscle
reflex, but not central command, increases the discharge of single
sympathetic ostganglionic efferents innervating the triceps surae
muscles of decerebrate, unanesthetized cats.Central command was
evoked by electrical stimulation of the mesencephalic locomotor
region. The reflex was evoked by electrical stimulation of the tibial
nerve, which in turn contracted the triceps surae muscles.
Hexamethonium abolished spontaneous and evoked activity, verifying
that the recordings were from sympathetic postganglionic fibers. The
discharge of 13 efferents was increased by static contraction (from
0.6 +/- 0.2 to 1.0 +/- 0.3 imp/s; P<0.05) but was not increased
by central command (from 0.6 +/- 0.2 to 0.8 +/- 0.2 imp/s;
P>0.05). Nevertheless, the discharge of nine efferents, not
increased by central command before alpha-adrenergic blockade (from
0.5 +/- 0.2 to 0.9 +/- 0.4 imp/s; P>0.05), was increased after
blockade (from 1.3 +/- 0.2 to 3.2 +/- 0.8 imp/s; P<0.05). We
conclude that the muscle reflex stimulates sympathetic postganglionic
efferents innervating the vasculature of skeletal muscle.
Furthermore, baroreceptors appear to buffer the central command
-induced increases in the discharge of these efferents.
Received 19 June 1995; accepted in final form 10 Novemeber 1995.
APS Manuscript Number H551-5.
Article publication pending Am. J. Physiol. (Heart Circ. Physiology).
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 23 December 95