The pacemaker controls heart rhythm.
The sinoatrial (SA) node (pacemaker) generates electrical impulses, which spread rapidly through intercalated disks between cells , causing both atria to contract in unison. The impulses pass to the atrioventricular (AV) node, where they are delayed for 0.1 second to ensure that the atria empty completely during atrial systole. Muscle fibers called bundle branches and Purkinje fibers conduct the signals to the apex. Signals spread throughout the ventricles, which contract in ventricular systole.