Mastering Cardiac Action Potential
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Here's the first picture of the four phases. Four phases are needed in order to create a full firing, a full action potential. Four, that phase four that you see there, is pretty much the recovery, the repolarization before a firing actually occurs. And then you've got your zero, then the spike of one, plateau two, and then the repolarization phase of three.
So let's talk about phase zero. Phase zero is when sodium fast gates open, and sodium rushes into the cell. We know that sodium is on the outside of the cell, the majority of it is on the outside of the cell. So when it gets triggered, when the cell gets triggered by, usually, another cell that's stimulating it, then sodium gates open and sodium rushes into the cell.
This brings the negativity in the cell, the cell's typically negative, and it decreases the negativity. And once it decreases the negativity enough, it will create a firing. So the more sodium that rushes in, the closer and closer it gets to that threshold that creates an electrical response that creates an effect.
So this means that we're getting closer to more positive. It's sitting around minus 90, negative, and that sodium rush in increases it to 70 or higher to create an electrical response. Negative 70 I should say. So that's what's happening, when it gets to that threshold, it fires, and that's why you're seeing a spike.
It fires, as you're seeing with zero, this is the first part of the electrical response. Then you see that spike. This is when sodium is really rushing into the cell, and then that spike occurs, and the fast gates of sodium shut.
Now we're at the spike, and we're kinda turning into the plateau, so how does it keep that plateau? What happens in the cells is that we actually have an increase in chloride. Chloride is increased inside the cell, which kinda keeps more of the plateau, it keeps the negativity, or the electrical response, where it's supposed to, and also we have an increase of calcium.
Calcium is a crucial, crucial ion in this case, because what calcium does is it actually triggers actinomycin to contract. It physically allows for the myocardium to contract, it allows for that squeeze. That's what calcium's doing in the plateau.
And so it creates that plateau, because now the body is pretty much just trying to level off the action potential so that way we get that plateau. Our body needs this in order to create squeeze, so that's why we have that plateau. Our body's really smart in increasing chloride, increasing calcium to create that physical squeeze, and prolong the action potential.
And so this is the last part here, is that the third phase is repolarization. What happens here is that gates are opened up from the inside of the cell, and potassium is rushed out of the cell. This will bring the electricity even lower in the negative, and this brings the cell back to its normal state, back to its state before action potential occurred.
So more and more potassium keeps running out until we finally get to the four. Phase four, and this is where the sodium potassium pumps come into play. It'll start to level off where the cell's supposed to be. And once it's leveled off, then the cell is ready to be fired again, and create more action potential.
So when it gets stimulated again, the whole thing takes over again, and then we'll have sodium rush in again, it'll spike, at that spike we'll see chloride come in, calcium come in, create the plateau. And then after the plateau, potassium channels will open and potassium will be shifted outside the cell, and this will repolarize the action potential. And then we'll come back to the four and we'll do it all over again. That is cardiac action potential.
So if you guys are using antiarrythmics, this is specifically what an antirrhythmic looks at. For example, we're gonna talk about amiodarone later this week. Amiodarone blocks potassium, and so what it does is that it actually prolongs phase three, it prolongs the plateau. What this does is it extends the refractory period, which means that it can't refire until after the refractory period.
That's what amiodarone does, is it extends refractory periods, or blocks potassium, which extends phase three, or prolongs phase three. This is the kind of thing that antiarrythmics will do, is it'll target action potential with its effect.
So if you have any questions please post them or email me. Again, like I said, this might be a little bit difficult for you to understand, so please understand ions and mastering electrolytes. There is a course on the membership that you guys can check out, would be really helpful.