<P> The contraction of cardiac muscle (heart muscle) in all animals is initiated by electrical impulses known as action potentials . The rate at which these impulses fire controls the rate of cardiac contraction, that is, the heart rate . The cells that create these rhythmic impulses, setting the pace for blood pumping, are called pacemaker cells, and they directly control the heart rate . They make up the cardiac pacemaker, that is, the natural pacemaker of the heart . In most humans, the concentration of pacemaker cells in the sinoatrial (SA) node is the natural pacemaker, and the resultant rhythm is a sinus rhythm . </P> <P> Sometimes an ectopic pacemaker sets the pace, if the SA node is damaged or if the electrical conduction system of the heart has problems . Cardiac arrhythmias can cause heart block, in which the contractions lose any useful rhythm . In humans, and occasionally in animals, a mechanical device called an artificial pacemaker (or simply "pacemaker") may be used after damage to the body's intrinsic conduction system to produce these impulses synthetically . </P> <P> One percent of the cardiomyocytes in the myocardium possess the ability to generate electrical impulses (or action potentials) spontaneously . A specialized portion of the heart, called the sinoatrial node (SA node), is responsible for atrial propagation of this potential . </P> <P> The sinoatrial node (SA node) is a group of cells positioned on the wall of the right atrium, near the entrance of the superior vena cava . These cells are modified cardiomyocytes . They possess rudimentary contractile filaments, but contract relatively weakly compared to the cardiac contractile cells . </P>

Ability of cardiac muscle cells to contract spontaneously