<P> In order to maximize efficiency of contractions and cardiac output, the conduction system of the heart has: </P> <Ul> <Li> Substantial atrial to ventricular delay . This will allow the atria to completely empty their contents into the ventricles; simultaneous contraction would cause inefficient filling and backflow . The atria are electrically isolated from the ventricles, connected only via the AV node which briefly delays the signal . </Li> <Li> Coordinated contraction of ventricular cells . The ventricles must maximize systolic pressure to force blood through the circulation, so all the ventricular cells must work together . <Ul> <Li> Ventricular contraction begins at the apex of the heart, progressing upwards to eject blood into the great arteries . Contraction that squeezes blood towards the exit is more efficient than a simple squeeze from all directions . Although the ventricular stimulus originates from the AV node in the wall separating the atria and ventricles, the Bundle of His conducts the signal to the apex . </Li> <Li> Depolarization propagates through cardiac muscle very rapidly . Cells of the ventricles contract nearly simultaneously . </Li> <Li> The action potentials of cardiac muscle are unusually sustained . This prevents premature relaxation, maintaining initial contraction until the entire myocardium has had time to depolarize and contract . </Li> </Ul> </Li> <Li> Absence of tetany . After contracting, the heart must relax to fill up again . Sustained contraction of the heart without relaxation would be fatal, and this is prevented by a temporary inactivation of certain ion channels . </Li> </Ul> <Li> Substantial atrial to ventricular delay . This will allow the atria to completely empty their contents into the ventricles; simultaneous contraction would cause inefficient filling and backflow . The atria are electrically isolated from the ventricles, connected only via the AV node which briefly delays the signal . </Li> <Li> Coordinated contraction of ventricular cells . The ventricles must maximize systolic pressure to force blood through the circulation, so all the ventricular cells must work together . <Ul> <Li> Ventricular contraction begins at the apex of the heart, progressing upwards to eject blood into the great arteries . Contraction that squeezes blood towards the exit is more efficient than a simple squeeze from all directions . Although the ventricular stimulus originates from the AV node in the wall separating the atria and ventricles, the Bundle of His conducts the signal to the apex . </Li> <Li> Depolarization propagates through cardiac muscle very rapidly . Cells of the ventricles contract nearly simultaneously . </Li> <Li> The action potentials of cardiac muscle are unusually sustained . This prevents premature relaxation, maintaining initial contraction until the entire myocardium has had time to depolarize and contract . </Li> </Ul> </Li>

Where does the repolarization of the atria occur