Describe the electrical conduction system of the heart and how it initiates the heart beat

The sinoatrial node (SAN) is the pacemaker of the heart and initiates a wave of excitation (impulse) at a regular interval. This wave of excitation sweeps across the walls of the atria and in doing so causes the depolarization of the muscle cells, initiating their contraction. The wave of depolarization then arrives at another area of specialised conduction tissue known as the atrioventricular node. Here, the impulse is delayed for a short time before being conducted down the bundle of his and spreading up across the walls of the ventricles via the Purkinje fibres. As this happens the ventricular muscle cells depolarise and they also contract forcing blood up and out of the ventricles into the great vessels of the heart.