Which phase of an action potential in cardiac muscle maintains the lengthened refractory period?

The phase of an action potential in cardiac muscle that maintains the lengthened refractory period is the plateau phase. During this phase, which follows depolarization, there is a prolonged influx of calcium ions through voltage-gated calcium channels. This influx sustains the depolarized state for an extended period, preventing early repolarization.

The significance of the plateau phase is that it prolongs the refractory period, which is crucial in preventing the heart muscle from being re-stimulated too soon. This extended refractory period allows the heart to pump effectively by ensuring that the muscle cells have adequate time to contract and relax before another contraction is initiated. This mechanism helps to maintain a coordinated rhythm and reduces the risk of arrhythmias.

In cardiac muscle action potentials, the other phases do not contribute to this lengthened refractory period in the same way. Depolarization is the initial spike in action potential but does not sustain the refractory period. Repolarization is the return to resting state, which signifies the end of the refractory period. Relaxation refers to the period when the muscle is no longer contracting, following the completion of the action potential. Therefore, the plateau phase is the key component that ensures the heart functions effectively without risking premature contractions.