What phase is marked by the depolarization of ventricular muscle?

The phase marked by the depolarization of the ventricular muscle is known as Phase Zero. During this phase, there is a rapid influx of sodium ions into the cardiac cells through voltage-gated sodium channels, leading to a significant change in the membrane potential. This depolarization causes the ventricles to contract, which is critical for the effective pumping of blood out of the heart.

Phase One, Phase Two, and Phase Three are associated with different aspects of the cardiac action potential. Phase One involves a brief repolarization as potassium channels open, Phase Two is characterized by a plateau phase where calcium influx balances potassium efflux, and Phase Three marks the repolarization phase as more potassium channels open leading to a return to resting membrane potential. Understanding these phases is crucial for recognizing how the heart's electrical activity corresponds to its mechanical function.