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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

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Impulse from pacemaker tissue stimulates the cardiac muscle cell

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Opening of fast Na channels

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Rapid influx of large number of Na ions

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Rapid rise in membrane potential

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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

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Net loading of positive ions

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Keeps the membrane depolarized for some time

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Creates a long plateau in action potential

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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

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Net exit of positive charges

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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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