Which statement best describes the process occurring in phase 2 of the cardiac action potential?

Which statement best describes the process occurring in phase 2 of the cardiac action potential?

• A. Calcium release from the sarcoplasmic reticulum triggers contraction
• B. Sodium influx drives rapid depolarization
• C. Potassium efflux causes repolarization
• D. Calcium efflux terminates contraction

Answer: (A)

During the plateau phase the cell experiences a sustained depolarization because calcium enters through L-type calcium channels. This influx triggers calcium-induced calcium release from the sarcoplasmic reticulum, raising cytosolic Ca2+ that activates the contractile machinery. So, calcium release from the SR triggering contraction directly ties the ion handling of the plateau to the mechanical response, making it the best description of what happens in this phase. For context, rapid depolarization is driven by sodium influx (phase 0), and repolarization occurs mainly due to potassium efflux (phase 3). Calcium efflux terminates contraction is not the defining event of the plateau; rather, calcium is eventually removed from the cytosol to relax the muscle.

During the plateau phase the cell experiences a sustained depolarization because calcium enters through L-type calcium channels. This influx triggers calcium-induced calcium release from the sarcoplasmic reticulum, raising cytosolic Ca2+ that activates the contractile machinery. So, calcium release from the SR triggering contraction directly ties the ion handling of the plateau to the mechanical response, making it the best description of what happens in this phase.

For context, rapid depolarization is driven by sodium influx (phase 0), and repolarization occurs mainly due to potassium efflux (phase 3). Calcium efflux terminates contraction is not the defining event of the plateau; rather, calcium is eventually removed from the cytosol to relax the muscle.