LECTURES:
1. Action Potential & Automaticity
2. Complete ECG Course
3. The EKG Guy Videos
Action Potential = depolarization (activation) + repolarization (recovery)
- Occurs in all cardiomyocytes, but varies in cell type
- Flow of electrically charged ions => electrical current + electrical conductors (ie, surrounding fluids & tissues) => electrodes => ECG
- Note: Electrical potential of conducting system is too small to be detected; therefore, only atrial & ventricular activity recorded
Sinoatrial node:
- Automaticity = ability of specialized cardiac cells to achieve spontaneous depolarization & function as “pacemakers” to form new cardiac-activating impulses
- Pacemaker potential (4) = Na+ influx at resting state
- …Na+ influx (4) => AP near -40 mV + Ca2+ influx + depolarization (0) => K+ efflux => repolarization (3)…
- AP propagates via gap junctions
Contractile myocyte:
- True resting potential (4) & thus must be stimulated
- …K+ efflux (resting state, -90 mV) => cell stimulated => rapid Na+ influx => depolarization (0) => K+ efflux => early repolarization (1) => Ca2+ influx => plateau phase (2) => Ca2+ channels close + K+ efflux => repolarization (3)…
- Total AP duration: atria ~200 ms; ventricles ~300 ms
- Absolute refractory period: no stimulus can trigger new action potential
- Relative refractory period: strong stimulus can trigger new action potential