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๐Ÿ“š > Physiology > Cardiovascular System > Electrophysiology of Heart_>

Action Potential in Cardiac Muscle

Action Potential

Upstroke or Initial Depolarization (Phase 0)

Resting membrane potential in cardiac muscle is about -90 mV


Impulse from pacemaker tissue stimulates the cardiac muscle cell


Opening of fast Na channels


Rapid influx of large number of Na ions


Rapid rise in membrane potential


Reaches to about +15 mV

Initial Repolarization (Phase 1)

1. Fast Na channels close โ†’ Prevents rapid influx of Na

2. Also, a small quantity of K leaks out of the cell through K channels

โ†“ a small repolarization

Plateau (Phase 2)

1. Fast K channels close โ†’ K efflux stops

2. L-type of Ca channels open โ†’ Ca influx


Net loading of positive ions


Keeps the membrane depolarized for some time


Creates a long plateau in action potential


Provides longer contraction time to cardiac muscle

Final Repolarization (Phase 3)

1. Ca channels close โ†’ influx of Ca stops

2. Slow K channels open โ†’ efflux of K starts


Net exit of positive charges


Potential reach the resting negative potential.

Resting Membrane Potential (Phase 4)
  • is seen till the next action potential.

Restoration of Ionic Balance

  • During action potential...

  • Na and Ca move in &

  • K moves out of the muscle cell.

  • This is corrected by following transport proteins:

Na-K ATPase Pump
  • Moves Na out

  • Moves K in

Na-Ca Exchanger
  • Moves Ca out

Ca Pump
  • Moves Ca out


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โ–ถ๏ธ Action Potential in Cardiac Muscle vs SA Node: A Comparison
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