Theintrinsic conduction systemsets the basic heart rate. However, the fibers of theautonomic nervous systemmodify the beat and introduce a subtle variability from one beat to the next.

Figure 1.The heart innervation. 1 Vagus nerve (parasympathetic). 2. The sympathetic cardiac nerve.

The sympathetic and parasympathetic neurons exert antagonistic effects on the heart.

交感神经系统(“加速器”)increases both the rate and the force of the heartbeat. Parasympathetic activation (the “brakes”) slows the heart.