<Table> <Tr> <Td> </Td> <Td> This article needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (December 2009) (Learn how and when to remove this template message) </Td> </Tr> </Table> <Tr> <Td> </Td> <Td> This article needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (December 2009) (Learn how and when to remove this template message) </Td> </Tr> <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>

What part of the cardiac conduction system is pacing the heart