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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 → Prevents rapid influx of Na

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

↓ a small repolarization


Plateau (Phase 2)

1. Fast K channels close → K efflux stops

2. L-type of Ca channels open → 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 → influx of Ca stops

2. Slow K channels open → 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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