Phases of the Cardiac Action Potential

The cardiac action potential is a remarkable dance of ions across the cell membrane, and it's composed of five distinct phases:

1. Phase 0 – Rapid Depolarization

   • What happens: Voltage-gated sodium (Na⁺) channels open, and Na⁺ floods into the cell.

   • Think of it as: The opening act where the spotlight suddenly shines, kicking off the performance.

   • Result: The cell’s interior becomes more positive, leading to a rapid upward spike in membrane potential.

2. Phase 1 – Initial Repolarization

   • What happens: Na⁺ channels close, and potassium (K⁺) channels briefly open, allowing K⁺ to exit.

   • Think of it as: A brief pause, adjusting the tempo before the main act.

   • Result: A slight drop in membrane potential.

3. Phase 2 – Plateau Phase

   • What happens: Calcium (Ca⊃2;⁺) channels open, Ca⊃2;⁺ enters the cell while K⁺ continues to leave.

   • Think of it as: A captivating solo that holds the audience—prolonging the excitement.

   • Result: The membrane potential plateaus, prolonging depolarization.

4. Phase 3 – Repolarization

   • What happens: Ca⊃2;⁺ channels close, more K⁺ channels open, increasing K⁺ outflow.

   • Think of it as: The finale where the energy winds down.

   • Result: The cell returns toward its resting negative membrane potential.

5. Phase 4 – Resting Membrane Potential

   • What happens: The cell is at rest, maintained by the Na⁺/K⁺ ATPase pump.

   • Think of it as: Backstage prep for the next performance.

   • Result: The cell is ready for the next action potential.
     
     

     Clinical Connections

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     • Antiarrhythmic Drugs: Many work by altering ion channel function—Class I drugs block Na⁺ channels, Class IV block Ca⊃2;⁺ channels.

     • Electrolyte Imbalances: High potassium levels (hyperkalemia) can affect repolarization, leading to dangerous arrhythmias.

     • Ischemia Impact: Reduced oxygen affects ATP production, impairing Na⁺/K⁺ pumps and disrupting resting membrane potential.