<P> There are important physiological differences between nodal cells and ventricular cells; the specific differences in ion channels and mechanisms of polarization give rise to unique properties of SA node cells, most important, the spontaneous depolarizations necessary for the SA node's pacemaker activity . </P> <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>

Which of the following is the correct sequence of cardiac electrical​ activity