During phase 2 of the cardiac action potential, which process is driven by calcium influx into the cell?

• A. Calcium-induced calcium release from the sarcoplasmic reticulum leading to contraction via actin-myosin assembly
• B. Direct depolarization by sodium ions
• C. Potassium efflux causing repolarization
• D. Chloride influx causing depolarization

Answer: (A)

During the plateau phase, calcium entering the cell through L-type calcium channels maintains the depolarized state. This incoming calcium triggers the sarcoplasmic reticulum to release more calcium into the cytosol in a process called calcium-induced calcium release. The rise in cytosolic calcium binds to troponin C, exposing the actin-myosin binding sites and driving cross-bridge cycling for contraction. So the key event is calcium-induced calcium release from the sarcoplasmic reticulum leading to contraction. Sodium-driven depolarization happens earlier, potassium efflux leads to repolarization, and chloride flux is not the main driver of this phase.