We've been comparing the action potentials in cardiac pacemaker cells and cardiac contractile cells, and now we want to look at the molecular physiology of these contractile cells in more detail. Now, remember when I say molecular physiology, we're going to be looking at what ions are crossing the membrane when in a way that gives us a graph that looks like this. Alright. So let's just remember here these contractile cells, what made them a little bit unique is that they have this slow repolarization. Remember, they depolarize, and then the graph kind of plateaued, and they stayed depolarized for a little while before they repolarized again. Now the other thing to remember, like all cardiac cells, we're dealing with 3 ions here. We have our sodium ions. We have our calcium or Ca²⁺ ions, I'm going to write, and we have the potassium ions. Now, remember these contractile cells? These are our pumpers. So our goal here, they're going to squeeze, force the blood out of the heart, and then they're going to relax again so that the blood can fill the heart. Now, an important concept for understanding how that works is going to be absolute refractory period. Now, absolute refractory period is the period when cells cannot respond to action potentials because they are not repolarized. Now, this is actually a kind of simple concept. Right? An action potential is the depolarization and then repolarization of the cell. Well, if a cell is depolarized, when another action potential comes in and stimulates the cell, it can't do anything. It's already depolarized. So in the middle of that action potential, the cell is basically unresponsive until the cell is able to depolarize again. Well, when we look at the graph of these cardiac contractile cells, remember they had that plateau phase. Remember, they stay depolarized for a little while. The fact that they stay depolarized prolongs or lengthens that absolute refractory period, and that forces relaxation of the heart muscle. Now why it forces relaxation, we'll take a look at in a second, but we can understand why that's important. Right? These cells need to pump the blood, they need to squeeze, and then they need to relax so that blood can come in and fill the heart again. If they just squeezed, and then just stayed squeezing, well, that would be really bad. Right? Your heart wouldn't be beating. It would just contract and stay contracting. So to understand how this all works, we're going to break down these graphs. We're actually going to break down 3 graphs here. We're going to spend the most time on this first one though. And this one, we've, looked at the shape of this graph previously. So we see on the y-axis, we have millivolts. This time, it's going from negative 80 millivolts to positive 40 millivolts. Now again, I'm not going to be calling out specific values as we go through here. There is a chance you need to know that depending on your professor. If you do, they're on this graph for reference. On the x-axis, we're going to have time. Alright. So the first thing you're going to notice here, we're going to highlight it in pink, label it number 1. We have a rapid depolarization of the cell. Alright. So let's look at what's going on there. Well, this is basically like your standard ol' action potential in your skeletal muscle or in your neuron. We're going to say that sodium channels open. And when the sodium channel opens, that causes the cell to depolarize the cell depolarizes. We can look at our little image here. We have our different channels drawn. You'll see that we actually have 2 of these potassium channels drawn in green. We'll talk about why there are 2 of those in just a second. But we're going to start looking at this pink channel, this first sodium channel. So you can see here the sodium channel opens, the ions flow into the cell, and that causes that very rapid depolarization. Alright. Next. Next, this is the part where it gets interesting. We see highlighted there in that sort of orange or gold color there labeled number 2. Well, now we hit that plateau phase where it just stays depolarized for a little while. So let's look at what's going on there. We're going to say here that calcium, or Ca²⁺ channels, and potassium channels open, but importantly those potassium channels open slowly. So if we look over at our image here, our well, we can see now those sodium channels are closed, and now the calcium channels open. Calcium's coming into the cell, and remember calcium is a positively charged ion, so it's bringing a positive charge into the cell. But potassium, well, these channels are open as well. Now this is why we have 2 channels drawn here. You'll see these are opening slowly, so not all of these potassium channels are open yet, just some of them. The potassium is flowing out, And again, these are positive ions going in opposite directions, so they're kind of canceling each other out. If calcium were just flowing into the cell, that would cause the cell to get more and more positive, would cause it like to become even more depolarized, you could say. But here, the potassium's canceling it out, going in the other direction. So we have the calcium Ca²⁺ into the cell potassium out. That means that the cell plateaus, and it just sort of stays sort of at this kind of level depolarized state. Alright. You'll note here again that I've sort of put in the green box this potassium channels, that potassium leaving the cell. That's just to remind you when we're looking at these cardiac cells, we're looking at which phase are we gonna try and slow down, and we slow it down by having potassium ions going in the opposite direction of another ion at the same time. So again, to highlight that, I've put that potassium in the green box there in that step. Alright. Well, we had that plateau. Now we've stretched out this action potential. Well, now it's time to repolarize, and we're going to highlight that in green and label it number 3 on the graph here. And again, this is going to happen basically like your standard old action potential. Well, we're going to have those calcium channels closed, CA²⁺ channels closed. So now all the other channels are closed, but now the potassium or K⁺ channels open fully. So now we have all these potassium channels open. The ions are all flowing out of the cell very rapidly, And as they flow out of the cell rapidly, the cell repolarizes. Again, basically like a regular old action potential. Alright. Let's look back at that plateau phase though. Right? We were talking about the absolute refractory period and said a new action potential can't come in as long as this cell is depolarized. So we can look at the, at the absolute refractory period here. It starts when the cell depolarizes, and now this cell cannot be stimulated again until it's repolarized. Now, why does that matter? To look at this, we're going to look at the tension in a cardiac cell. That's going to be the second graph here. So again, on the y-axis, we have tension, and on the x-axis, we have time. Alright. When it depolarizes, that stimulates this heart cell to contract. So tension is going to increase, but the cell has enough time to contract. Tension increases, and even then it's going to start relaxing. As we follow the action potential, we can see when the new action, but when the next action potential could come in is right here. It has to be sort of repolarized at this end of this absolute refractory period. So that is the soonest that this heart cell could be stimulated again. Now, if we compare that to the muscle tension, we can just sort of draw this down. Well, by that time, the muscle cells already relaxed or at least almost completely relaxed. So that stretching out, that plateau phase, increases the absolute refractory period and forces that muscle to fully squeeze and relax before a new action potential can cause it to contract again. Now to understand why that's important, we can compare what happens to skeletal muscle. So our final graph here is again gonna have tension on the y axis, but this time for skeletal muscle. And here we're gonna indicate action potentials with little arrows. So every one of these arrows represents an action potential. Remember, skeletal muscle has a refractory period of, like, 2 or 3 milliseconds. That's how long that action potential lasts. It's really, really short. So that means that they can come in really, really rapidly. The muscle does not have time to relax, so it just keeps squeezing and squeezing and squeezing. So we can say here that the skeletal muscle has short refractory periods. Those short refractory periods lead to rapid action potentials or at least the possibility of rapid action potentials. And when action potentials come in that rapidly, that leads to tetany or a sustained muscle contraction. Now that's important in skeletal muscle. If you're carrying something heavier, heck, holding a baby. Right? You don't want your muscles to just relax. You wanna have a sustained muscle contraction. In your heart, that would be bad. So we wanna stretch out that refractory period and force the muscle to relax so that you can have a heartbeat. Alright. We're going to compare these action potentials in cardiac pacemaker cells and contractile cells one more time. But before we get there, we have some practice and examples for you to look at. Give them a try.