The repolarization phase — and particularly the restoration of the resting membrane potential via the Na⁺/K⁺-ATPase pump — is energy dependent, consuming ATP. The pump actively transports 3 Na⁺ out and 2 K⁺ in per cycle, restoring the ion gradients depleted during depolarization. Depolarization itself is passive (voltage-gated Na⁺ channels open spontaneously down the concentration gradient).