Figure 49.9 The Action Potential of Ventricular Muscle Fibers (A) The three phases of the action potential of ventricular muscle fibers are due to the opening and closing of voltage-gated channels. (B) At the initiation of the action potential, voltage-gated Na⁺ channels open rapidly but briefly. At the same time, but more slowly, K⁺ channels are closing and Ca²⁺ channels are opening. The open Ca²⁺ channels sustain the depolarization. Repolarization occurs when the Ca²⁺ channels close, and the slow opening of the K⁺ channels also contributes to the repolarization.