<P> The pacemaker cells are connected to neighboring contractile cells via gap junctions, which enable them to locally depolarize adjacent cells . Gap junctions allow the passage of positive cations from the depolarization of the pacemaker cell to adjacent contractile cells . This starts the depolarization and eventual action potential in contractile cells . Having cardiomyocytes connected via gap junctions allow all contractile cells of the heart to act in a coordinated fashion and contract as a unit . All the while being in sync with the pacemaker cells; this is the property that allows the pacemaker cells to control contraction in all other cardiomyocytes . </P> <P> Cells in the SA node spontaneously depolarize, ultimately resulting in contraction, approximately 100 times per minute . This native rate is constantly modified by the activity of sympathetic and parasympathetic nerve fibers via the autonomic nervous system, so that the average resting cardiac rate in adult humans is about 70 beats per minute . Because the sinoatrial node is responsible for the rest of the heart's electrical activity, it is sometimes called the primary pacemaker . </P> <P> If the SA node does not function properly and is unable to control the heart rate, a group of cells further down the heart will become the ectopic pacemaker of the heart . These cells form the atrioventricular node (or AV node), which is an area between the left atrium and the right ventricle within the atrial septum, will take over the pacemaker responsibility . </P> <P> The cells of the AV node normally discharge at about 40 - 60 beats per minute, and are called the secondary pacemaker . </P>

When does the backup pacemaker in the av node initiate an impulse to pace the heart
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