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Action Potential in Cardiac Muscle vs SA Node: A Comparison

Resting Membrane Potential
  • In cardiac myocyte: -90 mV

  • In SA node: -60 mV

  • Thus, the resting membrane potential is less negative in SA node.

  • This is because of presence of leaky Na and Ca channels on SA node → Na and Ca keep leaking into the cell through these channels → move the potential in positive direction.

Upstroke (Phase 0)
  • In cardiac myocyte: Upstroke is produced by fast Na channels → is steeper.

  • In SA node: Upstroke is produced by slow Na and Ca channels → is less steep.

Initial Repolarization (Phase 1) & Plateau (Phase 2)
  • are seen in cardiac myocytes.

  • Not seen in SA node.

Repolarization (Phase 3)
  • is more gradual in SA node as compared to that in myocyte.

Phase 4
  • In cardiac myocyte: its stable.

  • In SA node: it shows slow diastolic depolarization → makes the SA node self-excitable.


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