Reflex control of the circulation during exercise.

Current theory is that circulatory control in exercise is governed by central command which sets basic patterns of effector activity that is modulated by arterial baroreflexes and chemo- and mechanoreflexes from active muscle. Because central command acts on vagal activity rather than sympathetic nerve activity (SNA), and because muscle chemoreflexes are not normally active during mild to moderate dynamic exercise, current theory cannot explain why SNA to virtually all organs, including active muscle, increases even during mild exercise. Are arterial baroreflexes involved? Baroreflex sensitivity is maintained during exercise, and most importantly, the reflex is reset to higher blood pressure (BP). A new hypothesis is that central command works by resetting the baroreflex to a higher BP and withdraws vagal activity to raise heart rate, cardiac output and BP at the onset of exercise. The key to the hypothesis is that the rise in cardiac output at exercise onset must be fast enough to raise BP to its new reset level immediately, otherwise a BP error occurs that must be corrected by baroreflex and SNA.