During the plateau phase, which ions move and in which directions?

During the plateau phase, which ions move and in which directions?

• A. Potassium flows out and calcium enters, prolonging depolarization.
• B. Sodium flows in and potassium flows out
• C. Magnesium flows in and calcium flows out
• D. Chloride flows in and sodium flows out

Answer: (A)

During the plateau phase, calcium ions enter the cardiac muscle cell while potassium ions continue to leave. The inward calcium current via L-type calcium channels counteracts the outward potassium current, keeping the membrane potential near a steady level around 0 mV for a brief period. This prolonged depolarization allows enough time for calcium to trigger excitation–contraction coupling, enabling the heart to contract before repolarization occurs. Sodium influx is the main driver of the initial rapid depolarization (phase 0), and sodium channels are largely inactivated by the plateau, so they don’t drive the plateau current. Magnesium and chloride movements are not the primary forces shaping the plateau in ventricular action potentials, which is why those options don’t fit.

During the plateau phase, calcium ions enter the cardiac muscle cell while potassium ions continue to leave. The inward calcium current via L-type calcium channels counteracts the outward potassium current, keeping the membrane potential near a steady level around 0 mV for a brief period. This prolonged depolarization allows enough time for calcium to trigger excitation–contraction coupling, enabling the heart to contract before repolarization occurs.

Sodium influx is the main driver of the initial rapid depolarization (phase 0), and sodium channels are largely inactivated by the plateau, so they don’t drive the plateau current. Magnesium and chloride movements are not the primary forces shaping the plateau in ventricular action potentials, which is why those options don’t fit.