What defines phase four of the cardiac cycle?

Phase four of the cardiac cycle is defined as the resting potential of cardiac cells, particularly the cardiac myocytes. During this phase, the cells are at rest after repolarization and are ready for the next stimulation. At this point, ion channels are closed, and the intracellular environment of cells is restored to its resting state, typically characterized by a negative membrane potential.

This phase is critical because it ensures that the heart is in a stable state, ready for the next action potential that will lead to another contraction, thereby maintaining a consistent and rhythmic heartbeat. Understanding this phase is essential for grasping how the heart cycles through electrical activity and the mechanics of contraction.

In contrast, other phases pertain to different activities during the cardiac cycle, such as contraction or repolarization, making the definition of phase four distinctly tied to the concept of resting potential.