During the plateau phase of the cardiac action potential, what occurs?

During the plateau phase of the cardiac action potential, what occurs?

• A. Rapid repolarization.
• B. Prolongation of the action potential to prevent tetany and allow filling.
• C. Initiation of contraction.
• D. Initiation of relaxation.

Answer: (B)

The plateau phase is about prolonging the cardiac action potential to support a strong, sustained contraction and give the heart time to fill. During this phase, Ca2+ enters the cardiac cell through L-type calcium channels while potassium ions continue to leave. The inward Ca2+ current balances the outward K+ current, keeping the membrane potential near a depolarized level for a relatively long time. This extended depolarization is crucial because it sustains calcium influx, which triggers calcium release from the sarcoplasmic reticulum and sustains cross-bridge cycling for the contraction. Because the action potential stays prolonged, the muscle remains in a refractory state longer, preventing rapid, repeated contractions (tetany) and ensuring the ventricle has a chance to fill after ejecting blood. The plateau eventually ends when Ca2+ channels inactivate and K+ efflux dominates, allowing repolarization to restore the resting membrane potential. So, the plateau’s primary role is to prolong the action potential to prevent tetany and to provide adequate time for ventricular contraction and filling. Rapid repolarization doesn’t occur during this phase, contraction is already underway due to calcium triggering, and relaxation begins after repolarization, not during the plateau.

The plateau phase is about prolonging the cardiac action potential to support a strong, sustained contraction and give the heart time to fill. During this phase, Ca2+ enters the cardiac cell through L-type calcium channels while potassium ions continue to leave. The inward Ca2+ current balances the outward K+ current, keeping the membrane potential near a depolarized level for a relatively long time. This extended depolarization is crucial because it sustains calcium influx, which triggers calcium release from the sarcoplasmic reticulum and sustains cross-bridge cycling for the contraction.

Because the action potential stays prolonged, the muscle remains in a refractory state longer, preventing rapid, repeated contractions (tetany) and ensuring the ventricle has a chance to fill after ejecting blood. The plateau eventually ends when Ca2+ channels inactivate and K+ efflux dominates, allowing repolarization to restore the resting membrane potential.

So, the plateau’s primary role is to prolong the action potential to prevent tetany and to provide adequate time for ventricular contraction and filling. Rapid repolarization doesn’t occur during this phase, contraction is already underway due to calcium triggering, and relaxation begins after repolarization, not during the plateau.