What happens to cardiac cells during depolarization?

During depolarization, cardiac cells undergo a process where there is a rapid influx of sodium ions into the cells. This influx leads to a change in the cell membrane potential, making the interior of the cell more positively charged compared to the outside, which is essential for initiating a contraction.

As the positive charge accumulates, it triggers the contraction of the cardiac muscle fibers. The rise in intracellular calcium ions that follows the depolarization also plays a critical role in facilitating this contraction. This sequence is vital for the proper functioning of the heart, as it enables the heart to pump blood effectively.

The other options describe different states or actions of cardiac cells that do not align with the process of depolarization. For example, relaxation and filling with blood occurs during diastole, while entering a resting state pertains to the period after repolarization. Producing electrical impulses relates to the overall function of the cardiac conduction system but does not specifically define what occurs during depolarization itself.