What does phase zero of the cardiac cycle refer to?

Phase zero of the cardiac cycle refers specifically to depolarization, which is the initial phase of the action potential in cardiac muscle cells. During this phase, there is a rapid inflow of sodium ions (Na+) into the cells through voltage-gated sodium channels. This influx of positive ions leads to a change in the membrane potential, causing it to become more positive and ultimately triggering contraction in the myocardium.

The significance of this phase cannot be understated, as it initiates the heartbeat and plays a crucial role in maintaining the heart's rhythmic contractions. Without effective depolarization, the heart would not be able to contract efficiently, leading to impaired circulation and potential cardiac failure.

The other choices refer to different aspects of the cardiac cycle, such as repolarization, which is the phase where the heart muscle cells return to their resting state, or resting potential, which describes the stable state of the membrane potential prior to depolarization. Ventricular filling is a later phase of the cardiac cycle, occurring after depolarization when the heart chambers fill with blood. Each of these phases is vital for overall cardiac function but are not designated as phase zero in the context of the cardiac cycle.