Cardiac Action Potential Phases

The cardiac action potential has 5 phases (0-4) with distinct ion movements that create the electrical signal for heart contraction.

Detailed Explanation

The cardiac ventricular action potential consists of 5 phases:

Phase 0 - Rapid Depolarization:

Voltage-gated Na+ channels open

Rapid Na+ influx → rapid upstroke

"Fast response" in atrial/ventricular myocytes

Phase 1 - Early Repolarization:

Na+ channels inactivate

Transient K+ efflux (Ito current)

Brief, small repolarization

Phase 2 - Plateau:

L-type Ca2+ channels open → Ca2+ influx

Balanced by K+ efflux

Long plateau distinguishes cardiac from skeletal muscle

Ca2+ triggers calcium-induced calcium release (CICR)

Phase 3 - Repolarization:

Ca2+ channels close

K+ efflux predominates (IKr, IKs)

Return to resting potential

Phase 4 - Resting Potential:

Stable at ~-90mV in working myocytes

K+ leak channels maintain potential

Na+/K+ ATPase restores ion gradients

Clinical Correlation

Class I antiarrhythmics block Na+ channels (Phase 0), Class III block K+ channels (Phase 3), and Class IV block Ca2+ channels (Phase 2). Long QT syndrome involves prolonged Phase 3 due to defective K+ channels.

Memory Trick

"0-Sodium opens, 1-K+ out briefly, 2-Calcium plateau, 3-K+ repolarizes, 4-Rest at -90"