The cardiac cycle regulation is myogenic (self stimulated). The sympathetic part of the nervous system accelerates the heart beat, this system meets the demands of the body, the parasympathetic system stops and slows things down, this part of the nervous system think that the body is fine and not in any urgent need of anything.
Electrical activity within the heart happens as follows;
As the SA nodes contract, they send electrical impulses the SA nodes are located in the upper wall of the right atrium these are responsible for electrical stimulation, these impulses travel to the right and left atrium. These impulses cause the the upper two chambers in the heart to contract. The electrical impulses then move to the AV (atrio- ventricular) node. Once the impulses reach here, they are held for a short period, the reason for this is so that the right and left atrium can continue to empty it's blood into the ventricles.
Image from: saintlukes health system
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