Myocardial Action Potential

Overview

• The myocardial action potential consists of five distinct phases (0-4), each representing different ionic movements across the cardiac cell membrane.

Phase 0: Rapid Depolarization

• Key Event: Rapid depolarization of cardiac cells.
• Mechanism:
  • Opening of fast sodium (Na+) channels.
  • Massive influx of sodium ions into the cytoplasm causes the rapid uprise.

Phase 1: Initial Repolarization

• Key Event: Rapid repolarization.
• Mechanism:
  • Opening of potassium (K+) channels allowing efflux of K+ ions.
  • Fast sodium channels close quickly.
• Characteristics:
  • This is the shortest phase in the action potential cycle.

Phase 2: Plateau Phase

• Key Event: Plateau in the action potential.
• Mechanism:
  • Voltage-sensitive calcium (Ca2+) channels open.
  • Influx of calcium ions balances the repolarizing effects of potassium efflux.
• Characteristics:
  • Maintains a balance to create a plateau in the action potential.

Phase 3: Rapid Repolarization

• Key Event: Rapid repolarization of cardiac cells.
• Mechanism:
  • Calcium channels close.
  • Unopposed potassium efflux current leads to repolarization.

Phase 4: Resting Membrane Potential

• Key Event: Preparation for the next action potential cycle.
• Mechanism:
  • Sodium re-enters cells through slow, spontaneous permeation.
  • Prepares cardiac cells for the next cycle beginning with phase 1.