The effects of exercise and training on human cardiovascular reflex control.

During physical activity, there is a graded withdrawal of vagal cardiac tone and a graded increase in sympathetic cardiac and vasomotor tone, initiated through both central command from the somatic motor cortex and muscle chemoreceptive and mechanoreceptive inputs. In parallel, there is an upward resetting of the operating point of the arterial baroreflex, with preserved reflex sensitivity. In contrast to the traditional interpretation that blood flow through exercising muscle is independent of vasomotor neural influences because of the dominance of local dilator metabolites, recent evidence suggests that both constrictor and dilator sympathetic neural influences may be involved in determining absolute levels of perfusion. Post-exercise, there is a period of relative hypotension that is associated with decreased peripheral resistance. Some, but not all, evidence indicates a causal role for reduced sympathetic drive. Chronic exercise training appears to reduce resting sympathetic activity, with parallel changes in the gain of a variety of cardiovascular autonomic reflexes initiated from cardiovascular sites. These changes may be attributable at least partly to masking of arterial baroreflexes by the impact of elevated blood volume on low-pressure baroreceptors. The reductions in sympathetic drive that follow training are more pronounced in patients with essential hypertension than in normotensive individuals and are likely to underlie the anti-hypertensive effect of exercise.