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.