The cardiac action potential has 5 phases: 0 rapid Na⁺ upstroke (~300 V/s), 1 early repolarization (K⁺ Ito), 2 plateau (Ca²⁺ influx balances K⁺), 3 rapid repolarization (K⁺ dominates), 4 resting (~−85 mV). The long plateau (~200 ms) prevents tetanic contraction — the heart must relax before re-excitation.