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