Neural control of cardiac function.

The principal functions of the heart are regulated by the sympathetic and parasympathetic divisions of the autonomic nervous system. In general, the sympathetic nerves to the heart are facilitatory, whereas the parasympathetic (vagus) nerves are inhibitory. The kinetics of the two autonomic divisions differ substantially. The vagal effects develop very rapidly, often within one heartbeat, and they decay quickly as well. Hence, the vagus nerves can exert beat-by-beat control of cardiac function. Conversely, the onset and decay of the sympathetic effects are much more gradual; only small changes are effected within the time of one cardiac cycle. When both autonomic systems act concomitantly, the effects are not additive algebraically, but complex interactions prevail. Such interactions may be mediated either pre-junctionally or post-junctionally with respect to the neuro-effector junction.