Bite Size Med: Cardiac Action Potential Part 1 - Ventricular Action Potential

Introduction

• Focus on ventricular action potential in cardiac muscles.
• Different from nerves and skeletal muscles.
• Two kinds: atria/ventricles (plateau present) and pacemaker cells (sinoatrial node).

Basics of Membrane Potentials

• Resting Membrane Potential: Usually negative.
• Action Potential: Brief changes due to ion movement (sodium, potassium, calcium).
• Neuron Action Potential Recap:
  • Resting potential: ~ -70mV
  • Depolarization: Sodium enters cell, membrane potential becomes more positive.
  • Repolarization: Potassium exits, membrane potential returns to resting.
  • Hyperpolarization: Membrane potential more negative than resting state.

Cardiac Muscle Action Potentials

• Resting Membrane Potential: ~ -90mV (more negative than neurons).
• Phases:
  • Phase 0 (Upstroke/Depolarization): Sodium entry, potential becomes positive.
  • Phase 1 (Initial Repolarization/Notch): Potassium exit begins.
  • Phase 2 (Plateau): Calcium entry balances potassium exit, sustained depolarization.
  • Phase 3 (Repolarization): Calcium channels close, potassium exit dominates.
  • Phase 4 (Rest): Return to resting potential.

Ion Channels and Phases

• Sodium Channels: Fast, responsible for rapid depolarization (Phase 0).
• Calcium Channels: Slow L-type, significant in plateau phase (Phase 2).
• Potassium Channels: Various channels, exit leads to repolarization.

Refractory Periods

• Absolute Refractory Period: No new action potential possible, sodium channels inactivated.
• Relative Refractory Period: Possible with stronger stimulus.

Excitation-Contraction Coupling

• Calcium Influx: During plateau, triggers sarcoplasmic reticulum to release more calcium.
• Muscle Contraction: Calcium causes thin filaments to slide over thick filaments.
• Gap Junctions: Allow simultaneous contraction of atrial/ventricular cells.

Conclusion

• Ventricular action potential explains contraction mechanism.
• Upcoming part 2 will cover sinoatrial node's action potentials.
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