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 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 node is the natural pacemaker, and the resultant
rhythm is a
sinus rhythm.