We see that the muscle actually begins to contract before our action potential has finished. So basically, when we're somewhere in between our plateau phase, that's when our muscle contraction basically almost reaches the highest portion. So about when we reach this point here does our muscle contraction actually begin to take place. And this is important in our heart, because the heart must actually create a single, forceful and steady muscle contraction. So that means we need to have this extended period to actually depolarize the adjacent muscle cells inside our heart. So in cardiac muscle cells, the action potential also causes the release of calcium ions into the cytoplasm of the cardiac myoside, the cardiac muscle cell.