We've been talking about action potentials in cardiac pacemaker cells and cardiac contractile cells, those pacers and the pumpers of the heart. Well, I realize we've been talking about a number of ions in 2 cell types and those cells work differently, so there's a lot that could potentially get crossed in your mind. Here, we want to go through these side by side one more time, just to make sure that we have everything nice and straight. Alright. So let's remember that we have these pacemaker cells. Sometimes I call these the pacers. These pacers, they're located in the nodes of the heart, and they set the rhythm of the heart through that autorhythmicity. Now we also have the contractile cells. Those are what I sometimes call the pumpers. These pumpers, well, that's the majority of the heart muscle. When you think of the heart contracting, you're thinking of these cells. Now we've graphed these out before. We're gonna do it again. On the top, we'll graph the pacemaker cells in that green box. We'll graph the contractile cells on the bottom in that purple box. For both of them, the y axis is going to be millivolts, the x axis is time. Alright. Let's see if we remember what these graphs look like. Let's look at the pacemaker cell graph first. So we're gonna draw that out there. We see, remember, we had that slow depolarization to start, what we call the pacemaker potential, that slow ramp up, followed by a more rapid depolarization, followed by repolarization. Now, in contrast, this contractile cell is going to look very different. We had that rapid depolarization, and then it kind of plateaus. It just sort of stays depolarized for a little while, and then it repolarizes again. Now when we went through these previously, we sort of assigned 3 basic steps to each one of these cells. We're going to go through those steps again now, side by side. As we do it, for each step, I'm going to try and describe what I think is the simpler version of that step, the simpler cell, first, and the more complex one second. So to see what I mean, we're gonna start with the depolarization in these contractile cells. So you can see there, I've highlighted that in pink. We start with that rapid depolarization. Well, that is going to be due to sodium ions that flow into the cell, and that causes that rapid depolarization. And we can see that in this image here. We have sodium on the outside of the cell flowing through this sodium channel, bringing its positive charge with it, depolarizing the cell. Now remember, this is the same as other action potentials. The same old sodium channel that you have in skeletal muscle, in neurons, sodium flows into the cell causing rapid depolarization. Now that was very different in the pacemaker cells. Here you see that slow depolarization, that pacemaker potential that we've been talking about. Well, that pacemaker potential, it also involves sodium, but we have some more complex stuff going on. Sodium is going to flow into the cell at the same time that potassium flows out. And so we see here on our image, we see this sodium flowing through the channel. And at the same time, potassium is flowing through. Remember, the same channel. This is a special type of channel that only exists in these pacemaker potentials. Alright. Now because these are positive ions going in opposite directions, we're going to say here that the opposite flow of ions slows this depolarization. Now remember for each one of these cells, we're going to have one step that we're trying to slow down and kind of stretch out. And when that happens, it's because we have ions flowing in opposite directions and it's always going to involve potassium flowing out. So to remind you of that here, we've put that potassium flowing out and an opposite flow of ions in those green boxes just to call that out. Alright. Next. So next both steps 2 are going to involve calcium ions, but I think it's the pacemaker cells here that's a little simpler to understand. We see this sort of just depolarization that looks like a sort of standard depolarization there, and that's because the calcium flows into the cell and it causes depolarization. And in our image here, we see the calcium channels now open. This calcium ion is positively charged. It comes into the cell, bringing its positive charge with it. Alright. Now contractile cells, it's going to be more complex. Here, we have that plateau phase, where it sort of just stays depolarized for a little while. Well, again, we're going to have calcium channels open. Calcium channels are going to open calcium flows into the cell, but here we have potassium flowing out. So in our image, we see calcium flowing in, bringing its positive charge with it. We have potassium flowing out, bringing its positive charge with it. The opposite flow of ions slows repolarization, in this case. So it just sort of stays there, depolarized for a little while. All right. To finish this up, well, both these cells are going to do the same thing now. They both need to repolarize, and they're going to do that. We see we're going to first highlight that in green on our graphs here, and then we're going to say that they open up those potassium channels. So on the top here, we see potassium flowing out of the cell, and that's going to say potassium flows out of the cell causing repolarization. On the bottom, we also see potassium flowing out of the cell. And remember, we have 2 ion channels here, just to indicate that in our previous step we also had potassium channels open. But in this step, we're opening up all the channels. And so now all those potassium channels are open, potassium flows out of the cell, and we get repolarization. All right. Now for these contractile cells, we're done. We don't need to worry about anything else. It's gonna stay at this resting potential. It's gonna stay polarized. Now, the pacemaker cells. Remember, the pacemaker cells, well, highlighted in here in pink, it's going to start depolarizing again. So I'm going to add one more step here. Now previously, we just said go back to 1, but here we're going to remind ourselves we have no resting potential, that depolarization, that slow depolarization, that pacemaker potential starts again. Alright. Hopefully, going through this one more time helps you keep everything straight in your mind. I realize it is complex, but if you've made it this far, I think you're doing pretty good. Alright. You got more practice problems to follow, and I will see you there.
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18. The Heart
Cardiac Action Potentials
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