The Sinoatrial Node generates a small electrical pulse that causes your heart muscles to contract. The Atrioventricular Node then paces the pulses to create a heartbeat (it also acts as a back up electric generator if the Sinoatrial fails). Finally, the Purkinje Fibers contract to create the force necessary to eject blood out of the heart (it can also act as an emergency electric generator if the other two fail, but it is significantly weaker).