During phase 2 of the cardiac action potential, what ion is responsible for heart muscle contraction?

During phase 2 of the cardiac action potential, what ion is responsible for heart muscle contraction?

• A. Na+
• B. K+
• C. Ca2+
• D. Cl-

Answer: (C)

During the plateau phase, calcium ions entering the cell drive contraction. Voltage-gated L-type calcium channels open and let Ca2+ influx into the cytosol, which then triggers additional Ca2+ release from the sarcoplasmic reticulum (calcium-induced calcium release). The rise in cytosolic Ca2+ binds to troponin C, enabling cross-bridge cycling between actin and myosin and producing cardiac muscle contraction. Sodium would be responsible for the rapid depolarization of the initial phase, and potassium helps terminate the action potential, while chloride is not the main driver of contraction.

During the plateau phase, calcium ions entering the cell drive contraction. Voltage-gated L-type calcium channels open and let Ca2+ influx into the cytosol, which then triggers additional Ca2+ release from the sarcoplasmic reticulum (calcium-induced calcium release). The rise in cytosolic Ca2+ binds to troponin C, enabling cross-bridge cycling between actin and myosin and producing cardiac muscle contraction. Sodium would be responsible for the rapid depolarization of the initial phase, and potassium helps terminate the action potential, while chloride is not the main driver of contraction.