contraction of heart muscle cells requires depolarization and repolarization of their cell membranes. movement of ions across cell membranes causes these events. the cardiac conduction system (and av node part of it) coordinates myocyte mechanical activity. a wave of excitation spreads out from the sinoatrial node through the atria along specialized conduction channels. this activates the av node. the atrioventricular node delays impulses by approximately 0.09s. this delay in the cardiac pulse is extremely important: it ensures that the atria have ejected their blood into the ventricles first before the ventricles contract.