SA Node Action Potential

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 (I_f)
    • repolarization activates HCN channels
    • non-selective monovalent ion channel
      • evokes Na⁺ mediated depolarization
    • I_f 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 → ↑ I_f → ↑ 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 → ↓ I_f → ↓ rate of phase 4 depolarization → ↓ heart rate
        • also ↑ K⁺ conductance → ↑ outflow K⁺ current via K⁺-ACh receptors
        • an example of a negative chronotropic effect