What occurs during the plateau phase of cardiac muscle cells?

What occurs during the plateau phase of cardiac muscle cells?

• A. Potassium flows out and calcium enters, prolonging depolarization.
• B. Sodium inflow shortens depolarization.
• C. Chloride outflow stabilizes the membrane potential.
• D. Potassium inflow and calcium outflow cause hyperpolarization.

Answer: (A)

The plateau phase is defined by a sustained depolarization kept alive mainly by calcium entering the cell. Voltage-gated calcium channels open and let Ca2+ flood in, while potassium continues to exit to some extent. The inward calcium current counteracts the outward potassium current, so the membrane potential stays elevated rather than snapping back to rest. This prolonged depolarization underlies the long action potential and refractory period of cardiac muscle, allowing coordinated contraction. So, calcium entering prolongs depolarization, with potassium still flowing out as part of the ongoing currents. The other options don’t match this balance: or they imply the wrong ion movements or the wrong effect on the membrane potential.

The plateau phase is defined by a sustained depolarization kept alive mainly by calcium entering the cell. Voltage-gated calcium channels open and let Ca2+ flood in, while potassium continues to exit to some extent. The inward calcium current counteracts the outward potassium current, so the membrane potential stays elevated rather than snapping back to rest. This prolonged depolarization underlies the long action potential and refractory period of cardiac muscle, allowing coordinated contraction.

So, calcium entering prolongs depolarization, with potassium still flowing out as part of the ongoing currents. The other options don’t match this balance: or they imply the wrong ion movements or the wrong effect on the membrane potential.