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SA Node Action Potential

Topic updated on 03/26/15 5:40pm

SA Node Action Potential; also AV Node
  • Phase 0, upstroke  
    • cardiac action potential opens T-type voltage-gated Ca2+ channels and increases Ca2+conductance
    • inward Ca2+ current depolarizes cell membrane of cardiac myocyte  
      • SA node and AV node cardiac myocytes lack voltage-gated Na+ channels
      • SA node and AV node upstroke is neither as rapid nor as sharp as in other cardiac myocytes 
      • slow conduction through AV node ensures that ventricles have sufficient time to fill with blood by the contraction of atria before excitation and contraction to pump blood into systemic circulation
  • Phase 1, initial repolarization
    • absent in SA node action potential
  • Phase 2, plateau
    • absent in SA node action potential
  • Phase 3, repolarization
    • T-type voltage-gated Ca2+ channels close and Ca2+ conductance decreases
    • voltage-gated K+ channels open and K+ conductance increases
    • outward K+ current exceeds inward Ca2+ current, repolarizing cell membrane of cardiac myocyte 
  • Phase 4, diastolic depolarization
    • automaticity is the ability of the SA node cardiac myocytes to spontaneously generate cardiac action potentials without neural input
    • cardiac myocyte cell membrane repolarization opens pacemaker Na+ channels and increases Na+ conductance
    • inward funny Na+ current slowly depolarizes cell membrane of the cardiac myocyte 
    • cardiac myocyte membrane depolarization eventually opens T-type voltage gated Ca2+ channels, initiating a subsequent SA node action potential
  • Heart rate
    • rate of cardiac myocyte cell membrane depolarization by inward funny Na+ current (phase 4) sets heart rate
    • sympathetic nervous system stimulation increases heart rate
      • norepinephrine → (+) β1 receptors in SA node → ↑ inward funny Na+ current → ↑ rate of phase 4 depolarization → ↑ heart rate
    • parasympathetic nervous system stimulation decreases heart rate
      • acetylcholine (ACh) → (+) muscarinic receptors in SA node → ↓ inward funny Na+ current → ↓ rate of phase 4 depolarization → ↓ heart rate


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