Cardiac pacemaker controls the rate of contraction for the entire heart muscle because its cells have the quickest rate of spontaneous depolarization, thus they initiate action potentials the quickest. The action potential generated by the SA node passes down the electrical conduction system of the heart, and depolarizes the other potential pacemaker cells (AV node) to initiate action potentials before these other cells have had a chance to generate their own spontaneous action potential, thus they contract and propagate electrical impulses to the pace set by the cells of the SA node. This is the normal conduction of electrical activity in the
Cardiac cycle Cardiac cycle The cardiac cycle is the performance of the human heart from the beginning of one heartbeat to the beginning of the next. It consists of two periods: one during which the heart muscle relaxes and refills with blood, called diastole (die-ASS-toe-lee), followed by a period of robust contraction and pumping of blood, dubbed systole (SIS-toe-lee). After emptying, the heart immediately relaxes and expands to receive another influx of blood "returning from" the lungs and other systems of the body, before again contracting to "pump blood to" the lungs and those systems. A normally performing heart must be fully