What occurs during phases 1-4 of the cardiac cycle?

During the cardiac cycle, phases 1-4 involve distinct mechanical and electrical events that are crucial for the heart's function. The correct choice focuses on the repolarization of the heart muscle, which occurs specifically during these phases.

Phases 1 through 4 of the cardiac cycle represent the process of electrical stimulation and contraction of the heart. During these phases, after the depolarization of the cardiac muscle cells (which leads to contraction), there is a recovery phase known as repolarization. This is where the muscle cells restore their resting electrical potential after being depolarized. This repolarization is essential, as it prepares the cardiac muscle cells for the next cycle of depolarization and contraction.

Understanding the significance of repolarization emphasizes the heart's electrical activity and its relation to muscle contraction, ensuring that the heart can effectively pump blood during each cycle. The other options touch on important aspects of the cardiac cycle, but do not directly pertain to the electrical changes that occur specifically during phases 1-4.