Cardiac action potential-What are the phases of the cardiac action potential?
In this article i am going to discuss in detail about the Cardiac action potential.
Introduction to cardiac action potential:
When stimulated each cardiac muscle fibre shows an electrical activity known as propagated action potential.
Action potential in the cardiac muscle is different from that of other tissues such as skeletal muscle,smooth muscle and nervous tissue .
This action potential differs significantly in different portions of the heart .
How long is a cardiac action potential?
The action potential of a single cardiac muscle fibre is unusually long and can be divided into five distinct phases. These are,
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Action potential ventromyocyte,Image by-Quasar,[Derivative work-Mnokel(talk),Silvia3(talk)] Source, License-CC BY-SA 3.0 |
1.Phase 0:Rapid depolarization:
The phase 0 is characterized by the depolarization which proceeds rapidly,as in skeletal muscle and nerve.In the mammalian heart , duration of this phase is about 2msec (0.002sec).
In this phase ,the amplitude of depolarization is about +20mV(ranges from +20 to +30 mV).
Ionic basis of rapid depolarization phase:
In this phase voltage-gated Na+ channels opens very rapidly and influx of Na+ ions occurs rapidly ,both these effects are responsible for this rapid depolarization and the overshoot.
The calcium channels also open up at -30 to -40 mV membrane potential,and influx of calcium ions occurs.
2.Phase 1: Initial rapid repolarization:
After the initial rapid depolarization there is another phase namely initial rapid repolarization.
The duration of this phase is very short and that is about 2msec .
In this phase membrane potential reaches from +30 to -10 mV.
Ionic basis of initial rapid depolarization phase :
This phase of the cardiac action potential is due to the closure of Na+ channels and opening of K+ channels.
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Action Potential Heart Contraction,Image by-OpenStax College,Source, License-CC BY 3.0 |
3.Phase 2: Plateau phase,
After the initial rapid depolarization,the cardiac muscle fiber remains in depolarized state for sometime before further repolarization.
It forms the plateau (stable period)in cardiac action potential curve .
In atrial muscle fibers,the duration of the plateau phase is about 200 ms and in the ventricular muscle fibers it lasts for about 300ms.
The membrane potential falls very slowly only to -40 mV during this phase . As the duration of the plateau phase in this action potential is very long .
The contraction time is also longer in cardiac muscle by 5-15times than in skeletal muscle.
Ionic basis of plateau phase :
Plateau phase is characterized by very slow repolarization.
This type of repolarization is due to the ,
➡️Slow influx of Ca2+ ions resulting from opening of sarcolemmal L-type Ca2+ channels.
➡️ Closure of distinct set of K+ channels called the inward rectifying K+ channels.
Plateau phase is also known as the final depolarization phase in this action potential.
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4. Phase 3:Final Repolarization:
After the plateau phase or final depolarization there is another phase ,known as final repolarization phase.
The duration of this repolarization phase in this action potential is about 50ms.
In this phase membrane potential falls to the approximate resting value of -80mV .
Ionic basis of final repolarization phase:
In this slow repolarization phase Ca2+ channels closes and following two types of K+ channels open up ,
➡️Delayed outward rectifying K+ channels,
And ➡️Ca2+ activated channels.
5. Phase 4: Resting potential:
In this phase of resting membrane potential(RMP),the potential is maintained at -90mV.
Ionic basis Of RMP:
At the end of final repolarization phase ,all the sodium ions ,which had entered the cell throughout the process of action potential move out of the cell and potassium ions move into the cell ,by the activation of the sodium-potassium pump.
And through this pump ,excess of calcium ions ,which had entered the muscle fiber also move out .In this way the resting membrane potential is restored.
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