We've been talking about the intrinsic cardiac conduction system, how the heart starts its own action potentials, and then those action potentials spread out through that cardiac muscle. Now here, we want to talk about those action potentials in a lot more detail. Action potentials are something that you've learned about before when you talked about skeletal muscle and when you talked about neurons. Well, the action potentials in cardiac muscle are going to be kind of similar, but there are some very key differences, and we're going to go over those now. Now we're going to go into the different types of action potentials that happen in different cardiac cells in more detail coming up, but right now, we just want us to look at an overview and compare them to the action potentials you're familiar with. We're going to compare them to these action potentials in skeletal muscle. First, though, let's just remind ourselves that there are sort of two basic types of cells in the heart. We have the pacemaker cells, and sometimes I just call these the pacers. Our pacers set the rhythm for the heart. So in this intrinsic cardiac conduction system, this is what starts those action potentials. Then we have the contractile cells, and these contractile cells, sometimes I call the pumpers. This is basically the rest of the cells in the heart, and these, are what actually contract and squeeze blood and pass that action potential from cell to cell through those gap junctions. Alright. So let's look at how both of these operate, again, in comparison to skeletal muscle. Now we're going to look at this in a graph, and in all of these graphs on the x-axis, we have time, and on the y-axis, we have millivolts. So you'll remember for skeletal muscle, it starts with a negative charge in the cell, and then the action potential is this flipping of charge, this rapid depolarization followed by a rapid repolarization. So we can look at that and how it works. The depolarization comes from sodium ions that flow into the cell, and they bring their positive charge with them. And the repolarization happens when potassium ions flow out of the cell and, again, bring their positive charge with them. So let's look at this graph for our other too, Types of cells here. And in both of them, let's first just look at the shape of this curve and see if we can identify what's different. I bet you can. Alright. So for our first for our pacemaker cells, for these pacers, you can see we start out with this really slow depolarization. It's this very slow ramp up, and then the end, well, kind of looks like the other graph. It's going to be that really slow depolarization, the ramp up that looks very different. Now in our contractile cells and our pumpers over here, well, we have a really rapid depolarization, and then it just kind of stays depolarized. We say it kind of plateaus. It just sort of stays up there depolarized for a little while before it repolarizes. Now, I played a little trick on you here. One thing that makes these look even more different than they are how they draw them. I have time on the x-axis, but I haven't put values on there. So let's look at the values for our x-axis. Well, in our skeletal muscle, we're looking at this entire thing happening over two or three milliseconds. Incredibly fast. Well, for our other cells, it's going to depend on how fast your heart is beating. But if your heart's beating at about 75 beats per minute, which is a normal heart rate, well, then these pacemaker cells, they're going to take 800 milliseconds. That's 100 of times longer than in the skeletal muscle. The cardiac contractile cells, these pumpers, they're not going to take quite as long, but they're still going to take something like 200 milliseconds for that action potential from the time it starts depolarizing to the time it's finished repolarizing. Still way, way longer than that skeletal muscle. Now if we put it on its own scale, you can really see this. Right? The skeletal muscle, we're going to have this really rapid depolarization followed by this really rapid repolarization. It looks almost instantaneous at this scale. We'll write that down. This is characterized by a really rapid depolarization and a rapid repolarization. Well, if we look at the cardiac pacemaker cells, what made that special was that really slow ramp up, this very slow depolarization. And the way that works, well, it's something that's different than you've ever learned about before. Here we have sodium ions that are coming into the cell. That's like a normal action potential, but also through the same channel, we have potassium ions leaving the cell at the same time. Now, these are both positive ions. So as they're going in opposite directions, they're kind of cancelling each other out. And that's what really slows down that depolarization, and that's what sets the pace of the heart. That really slow ramp up spaces out the action potentials and sets your heart rate. Alright. In the cardiac contractile cells, we can see there's something different happening. Here, it sort of it plateaus, and we slow down the repolarization. And we can look how that works. Right? So we're looking at the slow sort of plateau there at the top, and you can see here we're introducing a whole new ion. Here we have calcium ions that flow into the cell. Well, calcium is a positive ion. So if it was just coming into the cell on its own, it would sort of increase the depolarization. It would make it more and more positive. But at the same time, this time through a different channel, potassium is leaving the cell. So again, these are both positive ions, but they're going in opposite directions. So they kind of cancel each other out, and it slows down the entire process. It keeps it depolarized for a while before it finally repolarizes. Again, we're going to look at those two cells and all the different steps in a lot more detail coming up. Right now, I just want you to be able to identify the shape of those curves and understand how they're different from that skeletal muscle. So to sum that up, remember the skeletal muscle uses sodium and potassium, and that's really it. The cardiac muscle is going to use sodium, calcium, CA+2, and potassium. Now the key difference in the shapes of the curve, it either has a slow depolarization, and that was that really slow ramp up that we see in our cardiac pacemaker cells, our pacers there, or it has a really slow repolarization. And that's that plateau that we see in the cardiac pace I'm sorry. In the cardiac contractile cells, our pumpers here. That slowed down phase is going to be due to multiple ions crossing the membrane at once. So you're always going to have potassium leaving the cell and either calcium or sodium flowing in the other direction, and that slows down and spreads out this action potential. So again, as we go forward, we're going to learn all the different steps. But when you look at these curves, the thing that you want to keep an eye on is which part are we trying to slow down. And when we do that, it's going to be due to multiple ions going in opposite directions. We'll look at that more going forward, and I'll see you there.