Wednesday, June 29, 2011

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Cardiac action potential

The action potential of a cardiac muscle fiber can be broken down into several phases:
0- depolarization,
1- initial rapid repolarization,
2- plateau phase,
3- late rapid repolarization,
4- baseline.

Many persons find it hard to understand why the curve is as such. I'll try to give a simple explanation in phases. The diagram shows the action potential and below it is what happens to the different ions. By convention, influx is shown by downward deflection while efflux by upward deflection. If positive ions get inside the curve will show an increase and it will show a decrease if ions get out.

Phase 0
Unlike in skeletal muscles where there is only the fast sodium channels, in cardiac muscles there are both fast sodium channels and slow calcium-sodium channels. Both open simultaneously. Phase 0 is due to the rapid opening of the voltage gated sodium channels that leads to a massive influx of sodium ions that cause the initial rapid depolarisation. The slower calcium-sodium channels are slower to open.

Phase 1
The fast sodium channels close. The voltage gated potassium channels open. So, there is a sudden efflux of potassium ions to the outside causing the initial rapid repolarisation.

Phase 2
The fast sodium channels are already closed. The potassium channels are fully open but the slow calcium-sodium channels are also fully open. So there are sodium and calcium getting into the cell while potassum is going out of the cell. Positive charges getting in and out at the same time, thus the charge inside the cell remains more or less constant leading to the plateau phase.

Phase 3
The calcium-sodium channels are now closed and only the potassium channels are open. So, only potassium is going out of the cell making the cell more negative. This phase is called as the late rapid repolarisation.

Phase 4
The potassium channels take some time to close. While closing they continue to allow some potassium ions to move out leading to a slower repolarisation until the baseline is reached and the voltage causes the potassium channels to close completely.


  1. Just wanted to say how good this post was, I have come across so many of these graphs, that just roughly talk about the different channels but not the consequence of them, it really helped me understand, thank you :)