The ventricular muscle cells depolarize at the start of this phase. During this phase, Na entry through the funny channels causes the pacemaker potential of SA nodal cells to gradually become less negative until it reaches threshold. The ventricular muscle cells repolarize right before this phase. The cytosolic concentration of calcium in the contractile cells of the ventricle is high during this phase, and it drops at the very end of this phase. A. ventricular ejection B. isovolumetric contraction C. ventricular filling D. isovolumetric relaxation