Action Potential & Automaticity – The Premier EKG Resource for Medical Professionals – EKG MD

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1. Action Potential & Automaticity
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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

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