Na+ ion will move

• A. Outside to inside
• B. Inside to outside
• C. Neither direction
• D. Depends on membrane voltage

Answer: (A)

The movement of Na+ across the cell membrane is governed by the electrochemical gradient. There is a high concentration of Na+ outside the cell and a negative resting interior; when Na+ channels are open, both the chemical gradient (outside to inside) and the electrical gradient (negative inside) favor Na+ influx. This inward flow of positively charged ions is what drives the rapid depolarization during the upstroke of the cardiac action potential. The equilibrium potential for sodium is positive, so Na+ would only reverse direction (move outward) if the membrane potential approached that positive value. In typical resting or depolarizing states, Na+ moves from outside to inside, making that option the best choice.