Steps in the Cardiac Cycle: 1)The sinoatrial (SA) node (a.k.a. the pacemaker) located in the R. atrium stimulates muscle cells to contract and relax rhythmically. 2)It generates an electrical signal that spreads over the atria so they contract simultaneously. 3)The signal reaches the atrioventricular (AV) node, which transmits the signal through a bundle of fibres called the bundle of His. 4)The signal is relayed to Purkinje fibres; these fibres initiate the simultaneous contraction of the ventricles.