Action Potential

Cardiac Action Potential

Ventricular cardiomyocytes have a unique action potential lasting 200-400 ms (compared to 1-2 ms in skeletal muscle).

Five Phases

0. Phase 0 - Rapid depolarization

   • Fast Na⁺ channels open

   • Membrane potential: -90 mV → +20 mV

1. Phase 1 - Early repolarization

   • Na⁺ channels close

   • Transient K⁺ efflux

2. Phase 2 - Plateau

   • Ca²⁺ influx = K⁺ efflux

   • Unique to cardiac muscle!

   • Prevents tetanic contraction

3. Phase 3 - Rapid repolarization

   • Ca²⁺ channels close

   • K⁺ efflux continues

   • Returns to -90 mV

4. Phase 4 - Resting potential

   • Na⁺/K⁺-ATPase restores gradients

Key Concept: The long plateau phase ensures complete ventricular emptying and prevents summation of contractions.