<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>

Where is the primary pacemaker of the heart located