We've previously introduced how the action potentials in the cardiac pacemaker cells and the cardiac contractile cells, those pacers and the pumpers of the heart, how they're different from each other. And we also said that they're going to be different from the action potentials that you're already familiar with, such as in skeletal muscle. Well, now we want to take a closer look at this molecular physiology in these pacemaker cells. Now by molecular physiology, I just mean we're going to take a look at what ions are crossing the membrane when in order to give us a graph that looks like that. Alright. So let's dive in. Let's remember that these pacemaker cells, we sort of called out what's unique about these is that they have this period of slow depolarization, that sort of slow ramp up that I mentioned. Now these cells also, like all cardiac cells, are going to be using 3 ions. We have sodium ions, we have calcium or Ca2+ ions, and we have potassium ions. Now, what makes pacemaker cells so important is they set the heart rate. They start the action potentials that then spread out through the entire heart and cause the heart to beat, and they do that to a rhythm. Now, we call that ability autorhythmicity, and autorhythmicity we're going to break down this word here. We see auto means self, and rhythmicity, what we see in their rhythm, these set their own rhythm. They set the rhythm by themselves. This is the ability of these certain heart cells to create their own action potentials. Remember, in the heart, the action potentials start in the pacemaker cells. They do not need a signal from the nervous system to start an action potential.

Alright. The way they set the rate of your heartbeat, the way they spread out those action potentials is going to be through something called the pacemaker potential, and this part we've actually already called out. This is that slow depolarization that starts without any outside signal. Alright, so let's really break this down. We're going to break down this graph here. Before we really get into it, let's orient ourselves to the graph. You can see on the y-axis, we have millivolts, and we have it labeled from negative 60 to 0 millivolts. Now, I'm not going to be calling out specific voltage values as we go through this. There is a chance you need to know that for your class. It just depends on your class and your professor. If you do, they're on this graph for a reference. On the x-axis, we see time. And time, we don't have a value there because it's going to change depending on your heart rate, But we can assume that from beginning to end in a resting heart rate, this is going to take about 8 tenths of a second or 800 milliseconds. Alright. The first thing that we want to call out there is that slow ramp up, and we're highlighting it in pink, that slow depolarization, and we have it labeled there in 1. What's going on? Alright. We're going to say that special voltage-gated channels open, and these special voltage-gated channels allow sodium in and potassium out of the cell. And we see here an image. We see the 3 different channels we'll be talking about, and we see on the left this pink channel. We see these pink sodium ions coming in, and at the same time, through the same channel, we see these green potassium ions going out. Now this channel is unlike other channels you have learned about previously. What we're talking about here is a channel that's only used create these pacemaker potentials. We have sodium and potassium going through the same channel at the same time. Because these are both positively charged ions, as they go past each other, they bring in their charge with them and they basically cancel each other out. But you'll note that there's more sodium going in than there is potassium going out because there's just a little bit more sodium coming in than potassium going out that leads to that very slow depolarization, that slow ramp up that we see, and we call that the pacemaker potential.

Now one other thing I just want to call out here. I've put potassium out in a green box there. That's because remember in the cardiac cells, we're always looking which step are we trying to slow down and the step that we're trying to slow down, we do that by having potassium go in the opposite direction of another ion. This is the step we're trying to slow down. So I'm calling out that potassium going in the opposite direction, canceling something out there by putting it in that green box. Alright. Well, now if you look at the graph, we've had that slow ramp up, and now this graph changes directions pretty quickly there. Right? We can see I'm highlighting it in a sort of orange or gold color there. It sort of has a relatively rapid depolarization that comes up next. So we're going to save for number 2, at threshold voltage-gated calcium or Ca2+ channels open. So we can look it over at our image there, we can see that those special, sodium-potassium channels, the pink channels now closed. And now this calcium channels open, and we can see this calcium now coming into the cell. So we're going to say calcium enters the cell. Well, calcium is a positively charged ion. So as it comes into the cell, that's going to lead to depolarization. Alright. Now importantly, when you've been talking about other action potentials when they depolarize, they do that with sodium. These cells are different. These cells depolarize using these calcium channels and calcium ions. Now that we've depolarized, though, well, that's our action potential. Now this action potential can spread from cell to cell in the heart, and this is what's going to start the heart contracting. So our pacemaker cells at this point, they've done their job, but now they need to repolarize. So we can see that on our graph there, and we're going to highlight that part in green and we're going to label it number 3. And this is going to happen basically like any other action potential. Well, the calcium channels are going to close, so all the other channels are closed now, and voltage-gated potassium channels open. And we can see that in our image here. Now, this final green channel is open. This is our potassium channel, our potassium channel. Our potassium is now flowing out of the cell. Potassium is a positively charged ion. So we're going to say here the potassium exits the cell, and as it leaves the cell, it brings its positive charge with it that causes repolarization, and the cells now repolarize. Now in other action potentials, the cell would just sort of stay repolarized. It would stay in this polarized state, and we would call that its resting potential. It would just wait there until it was stimulated again. Not here. Alright. If you look here, you'll see it sort of touches that negative 60 millivolts line. It repolarizes and then it just starts going back up again. So we're going to actually say step number 4 is go back to 1. We're going to say here there is no resting potential. As soon as this is repolarized, well, that pacemaker potential starts again. So the pacemaker potential starts again. Those sodium, potassium channels are open. They're cal you have that slow ramp up. At a threshold, the calcium channel is open. You have depolarization that sets an action potential off through the heart. You repolarize. Go back again. Slow ramp up, depolarization, action potential, repolarization again and again and again and again. That's your heartbeat. Alright. The last thing that we just want to note here is that the intrinsic rate of depolarization is about 100 times per minute. If these cells are just left to do things on their own, the heart's pacemaker will set a heart rate of about 100 beats per minute. You'll note your heart's not normally beating at 100 beats per minute. If you're exercising, it's going to be above that. If you're at rest, it's likely below that. Remember, we have extrinsic factors, the autonomic nervous system, which is going to sort of turn up and turn down this heart rate. It's going to do that by affecting that pacemaker potential. How quickly that depolarization happens is going to spread out those action potentials or have them be closer together. Importantly, though, the actual rate is kept in these pacemaker cells, and the action potential starts in these pacemaker cells. Alright. With that, we're going to take a deeper dive and look at these cardiac contractile cells, the pumpers, next. But before that, we have examples and practice problems to follow. I'll see you there.