Cardiac vagal control before, during and after exercise.

There is much evidence showing that the rapid rise in heart rate at the onset of exercise is due to a withdrawal of cardiac vagal tone. This short review discusses the main afferent mechanisms involved in this effect. In addition to signals from central motor command it is shown that muscle mechanoreceptors of group III afferent fibres also play a significant role. Recent studies in man demonstrating this by stretching muscles such as the triceps surae or by direct compression of muscles are briefly reviewed. The evidence also supports the idea that these small fibre mechanoreceptors inhibit the baroreceptor-heart rate reflex. Several studies suggest that skeletal muscle metaboreceptors (mainly group IV) are more important for increasing cardiac sympathetic and vasoconstrictor nerve activity. At the conclusion of exercise the cessation of mechanoreceptor stimulation is an important factor in determining the rapid return of heart rate to resting level.