What is the purpose of the plateau phase in the cardiac action potential?

• A. Triggers immediate repolarization
• B. Initiates tetany
• C. Prolongs refractory period to prevent tetany
• D. Causes hyperpolarization

Answer: (C)

During the plateau phase, the cardiac action potential stays depolarized longer because inward calcium current (mainly through L-type Ca2+ channels) counteracts the outward potassium current. This sustained depolarization serves two key roles: it provides the calcium entry needed to trigger calcium release from the sarcoplasmic reticulum to power contraction, and it keeps many voltage-gated channels in states that maintain a prolonged refractory period. That extended refractory period is essential for coordinated heartbeats because it prevents another action potential from firing too soon, avoiding tetany or uncontrolled rapid contractions. So the plateau’s main purpose is to prolong the refractory period to prevent tetany, while also enabling the calcium-driven contraction. It does not cause immediate repolarization, initiate hyperpolarization, or trigger tetany.