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.
In cardiac myocyte: its stable.
In SA node: it shows slow diastolic depolarization → makes the SA node self-excitable.