Updated: 4/18/2018

SA Node Action Potential

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Overview

SA Node Action Potential; also AV Node 
  • Phase 0, upstroke 
    •  action potential opens L-type voltage-gated Ca2+ channels
      • secondary to T-type voltage-gated Ca2+ channels further depolarizing cell membrane enough to reach threshold for L-type votage-gated Ca2+ activation → upstroke
      • slope of upstroke is slower than other cardiac myocytes
        • due to lack of large Na+ currents in SA and AV nodal cells
        • due to non-rapid Ca2+ movement via channels
  • Phase 1 and 2 are absent in SA node action potential
  • Phase 3: repolarization
    • voltage-gated K+ channels open → ↑ K+ conductance → outward K+ current → repolarization 
  • Phase 4: diastolic depolarization
    • accounts for automaticity of SA node cells
      • to generate an action potential spontaneously without neural input
    • hyperpolarization-activated cyclic nucleotide-gated (HCN) channel generates "funny" current (If)
      • repolarization activates HCN channels
      • non-selective monovalent ion channel
        • evokes Na+ mediated depolarization
      • If slow depolarization activates T-type voltage-gated calcium channels
    • reduced outward K+ current contributes to depolarization
  • Heart rate
    • set by the rate of depolarization in phase 4
      • ↑ rate of phase 4 depolarization → reach threshold faster ↑ action potential per unit time by SA node cells ↑ heart rate
    • pharmacology correlate 
      • sympathetic nerveous system stimulation → ↑ heart rate
        • norepinephrine acts on β1 receptors in SA node → ↑ If → ↑ rate of phase 4 depolarization → ↑ heart rate  
          • example of a positive chronotropic effect
      • parasympathetic nervous system stimulation → ↓ heart rate
        • acetylcholine acts on M2 receptors in SA node → ↓ If → ↓ rate of phase 4 depolarization → ↓ heart rate
          • also K+ conductance → ↑ outflow K+ current via K+-ACh receptors
          • an example of a negative chronotropic effect

 

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