Lesson 2: Cardiac Action Potential: Myocyte

Cardiac Action Potential Overview

• Unique to cardiac myocytes: plateau phase prolongs depolarization, enabling effective heart contraction.

Phases of Myocyte Action Potential

• Phase 0 - Depolarization: Na+ influx. Threshold potential at -70 mV activates fast voltage-gated Na+ channels.

• Phase 1 - Initial Repolarization: K+ efflux and Cl- influx. Inactivation of Na+ channels.

• Phase 2 - Plateau: Ca2+ influx counters K+ efflux, maintaining depolarization; prolongs absolute refractory period.

• Phase 3 - Final Repolarization: K+ efflux dominates; repolarization restores transmembrane potential to -90 mV.

• Phase 4 - Resting Phase: Maintenance of resting potential at -90 mV via K+ leak channels and Na/K-ATPase function (removes Na+, replaces lost K+).

Ionic Movements Summary

• Phase 0: Na+ in.

• Phase 1: Cl- in & K+ out.

• Phase 2: Ca2+ in & K+ out.

• Phase 3: K+ out.

• Phase 4: K+ out & Na/K-ATPase function.