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Ventricular Action Potential; also Atria, Bundle of His, Purkinje Fibers
  • Resting membrane potential
    • inward rectifier K+ channels set resting membrane potential of cardiac myocyte at -90 mV; they are open at rest, close with depolarization, and open with repolarization.
  • Phase 0, upstroke 
    • cardiac action potential opens activation gates on voltage-gated Na+ channels and increases Na+ conductance
    • inward Na+ current depolarizes cell membrane of cardiac myocyte
    • cardiac myocyte cell membrane depolarization closes inward rectifier K+ channels
  • Phase 1, initial repolarization
    • cardiac myocyte cell membrane depolarization quickly closes inactivation gates on voltage-gated Na+ channels and decreases Na+ conductance
    • slow delayed rectifier K+ channels begin to open and K+ conductance increases
    • outward K+ current initiates cardiac myocyte cell membrane repolarization   
  • Phase 2, plateau 
    • plateau is a long period of relatively stable, depolarized cardiac myocyte cell membrane potential
    • L-type voltage-gated Ca2+ channels open and Ca2+ conductance increases
    • inward Ca2+ current balances outward K+ current
    • inward Ca2+ current induces release of Ca2+ from sarcoplasmic reticulum for excitation-contraction coupling (calcium-induced calcium release)
  • Phase 3, repolarization
    • slow delayed rectifier K+ channels open completely and K+ conductance further increases
    • L-type voltage-gated Ca2+ channels close and Ca2+ conductance decreases
    • outward K+ current exceeds inward Ca2+ current, repolarizing cardiac myocyte cell membrane
    • cardiac myocyte cell membrane repolarization opens rapid delayed rectified K+ channels and reopens inward rectifier K+ channels
  • Phase 4, resting membrane potential
    • slow delayed rectifier K+ channels and rapid delayed rectifier K+ channels close as cardiac myocyte cell membrane repolarizes to resting membrane potential
    • inward rectifier K+ channels reset and maintain membrane potential of cardiac myocyte at -90 mV
 
 

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