Cardiac Action Potentials: Study with Video Lessons, Practice Problems & Examples

The heart's action potentials arise from two main cell types: pacemaker cells, which initiate the heartbeat through autorhythmicity, and contractile cells, which pump blood. Pacemaker cells exhibit a slow depolarization due to simultaneous sodium influx and potassium efflux, while contractile cells show rapid depolarization followed by a plateau phase due to calcium influx and potassium efflux. This plateau extends the absolute refractory period, preventing tetany and ensuring the heart relaxes between beats, crucial for effective circulation.

Introduction to Action Potential in Cardiac Cells

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.

Cardiac Action Potentials Example 1

This example tells us that in the table below, we want to put a check in the boxes under cardiac pacemaker cells and cardiac contractile cells if they possess the given feature. Then we want to match the action potential graphs with the type of cell it is showing. So, we have this table here that we need to fill out, and we have a column for cardiac pacemaker cells and cardiac contractile cells. And then we have 2 graphs over here, graph A and graph B, and we see the different depolarizations and repolarizations there. Now that's the last thing that we're supposed to fill in this table here, but I actually want to do it first because if I can identify these graphs, I think it's going to help me answer other things in this table. So, let's first look at these graphs. This graph A, what do you think that looks like? Does that look like the depolarization and repolarization of a cardiac contractile cell or a cardiac pacemaker cell? Well, as I look, I see that rapid depolarization. Rapid depolarization followed by sort of a plateau, and then it repolarizes. We said that that shape was characteristic of a cardiac contractile cell. So, I'm just going to write this right here: contractile. Well, and then as I look over graph B, then what does graph B look like to you? Well, I see this slow depolarization, slow and steady depolarization, followed by a more rapid depolarization and almost immediately a repolarization, and then we start that slow depolarization again. Alright. That looks like a pacemaker cell. I'm going to write pacemaker here. Alright. Now I can fill in the last line of this table here, the cardiac pacemaker cells. Well, that's graph, graph B, and cardiac contractile cells, graph A. Alright. Let's see the other things that we need to identify here. So, which of these has a slow repolarization? Well, as I look at these graphs again, I have that depolarization, and then in the contractile cells, it stays depolarized for a little while before it repolarizes. So those contractile cells, that seems to have a slow repolarization. I'm going to give that one a checkmark. What about the pacemaker cells? Well, the pacemaker cells, it depolarizes, and here we are depolarizes, and then almost immediately, it repolarizes again. So that's not slow. I'm not going to put a check there under cardiac pacemaker cells. Well, what about slow depolarization? Well, again, I'll look at the contractile cells. Well, look at this. That line looks almost vertical. Right? Rapid depolarization under the cardiac contractile cells, so not that one. What about cardiac pacemaker cells? Well, here we have that slow and steady ramp up, that very slow depolarization. So the slow depolarization, that's a feature of the cardiac pacemaker cells. Alright. Now which one of these utilizes only sodium and potassium ions? That's a trick question. Using sodium and potassium ions only as part of the action potential, we said that was for skeletal muscle. Both cardiac contractile cells and cardiac pacemaker cells use sodium, potassium, and calcium ions. So I am not putting any check marks on that line there. Alright. Our next one is uses sodium, calcium, and potassium ions. Well, we just said that's both of them. They're going to utilize them slightly differently, and that's how we get those different curves over there. But they are both going to use them, so they get a check mark both of them get a check mark there. Alright. With that, I filled out my table. Now, again, we are going to go into both these cells in more detail and understand what ions are moving when to create the graphs that look like this. We'll do that coming up. I'll see you there.

Which of the following correctly identifies a difference between action potentials in cardiac and skeletal muscle?

• A. Skeletal muscles utilize K$^+$ and Ca$^+$$^2$ during action potentials while cardiac contractile cells use K$^+$ and Na$^+$.
• B. Action potentials in cardiac contractile and cardiac pacemaker cells are longer lasting than action potentials in skeletal muscle.
• C. Cardiac pacemaker cells have a slow repolarization while skeletal muscles have a slow depolarization.
• D. The action potentials in each type of tissue are the same.

Pacemakers: Molecular Physiology

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.

Cardiac Action Potentials Example 2

Our example says that the graph below shows the membrane potential of a typical pacemaker cell. Alright. So we look down here. This is our normal pacemaker cell here. We should be familiar with this graph. On the y-axis, we see millivolts. On the x-axis, we see time, and we see this very typical curve here. We see the pacemaker potential. We see that more rapid depolarization, the repolarization, and then that pacemaker potential starting again. Alright. A says, draw how you think the curve would look if the permeability of the sodium potassium channels is increased. Alright. So in a here, we're going to have to redraw this curve, but that special sodium potassium channel, the permeability of that channel is going to be increased. How do you think that would change the shape of this curve? Remember that special sodium and potassium channel where the ions go through the same channel in opposite directions, that's what's open during this pacemaker potential. We're talking about the pacemaker potential here. Just to make that clear, I'm going to highlight this part of the graph. It was those ions again going in opposite directions. They kinda cancel each other out. That gives us that sort of slow ramp up, that depolarization, that pacemaker potential. So as I redraw this, I'm going to think if the permeability's increased, I'm going to be changing that part of the graph. And specifically, I'll start with it going down a little bit. It hits that negative 60 millivolt stress threshold. Now if the permeability is increased, that means it's going to depolarize more rapidly. So this part of the graph should be steeper. Then it hits our negative 40 millivolt threshold. It's going to depolarize using those calcium channels, repolarize using the sodium channels, and then hit that threshold when it's repolarized and start again. But again, this part is going to be nice and steep because we've increased the permeability of these channels and we'll just keep going there. Alright. So, again, the part that has really changed in this graph is right here. I'll highlight it. Right here, this part should be steeper because we've increased the permeability of those channels.

Now B says draw how you think the curve would look if the permeability of the sodium potassium channels is decreased. Alright. So now on this graph here, how would you draw that, do you think? Well, if the permeability is decreased, that means that the ions are going through that channel even slower. That's going to slow down this pacemaker potential even more. So I would draw it start down and then I'm going to draw this slower coming up and then we'll hit that threshold and then our calcium channels will open and our potassium channels will open, and that rest of it should look fairly normal. Alright. So again, I'll highlight that part that's different. That part that's different is going to be right there. Okay.

So then it says, for both cases, indicate how this change would generally affect heart rate. Remember, it's that pacemaker potential that is spreading out the heartbeats because it has that long, slow depolarization between the full polar depolarization, which starts the action potentials. So to figure out how this affects heart rate, well, we can just sorta look at this graph. Right? In this graph, each depolarization here and here, full depolarization that starts the action potential, they're closer together. That means that the heart rate is going to increase. I'm going to say it's going to increase heart rate. Now with the decreased permeability, we have a much longer depolarization. That means that these full depolarizations, which start the action potential, they're going to be spread out more. How is that going to affect the heart rate? I'm going to say it's going to decrease or slow down heart rate. Alright. With that, we've answered the questions. Remember, for those pacemaker potentials, that's what sets the rate of the heart, and it's that long, slow depolarization through those very special sodium potassium channels, those ions moving in opposite directions, they cancel out the charge and kinda slow everything down. More questions to follow. Give them a try.

Calcium ion channels open in response to changes in membrane potential. What type of opening mechanism do calcium ion channels in cardiac muscle exhibit?

• A. Ligand-gated.
• B. Voltage-gated.
• C. Mechanically-gated.
• D. Time-gated.

Autorhythmicity is a unique feature of cardiac pacemaker cells. What feature of cardiac pacemaker cells allows them to be autorhythmic while other cardiac cells are not?

• A. Pacemaker cells use Ca$^+$$^2$ ions, unlike other action potentials.
• B. Sodium channels in pacemaker cells allow both calcium and sodium to pass through.
• C. The pacemaker potential is stimulated by polarization of the cell.
• D. Unlike other action potentials, the first channel to open is the potassium channel, leading to repolarization.

Contractile Tissue: Molecular Physiology

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.

Cardiac Action Potentials Example 3

Our example tells us that two tension graphs are shown below, one for cardiac contractile tissue and one for skeletal muscle tissue. In both graphs, the yellow line represents tension in the cell, and the red arrows represent the arrival of an action potential. Alright. So it says here, a. Identify which graph shows cardiac contractile tissue and which shows skeletal tissue. Well, first off, we'll look at the graphs here. We have the y-axis for both of them as tension, and the x-axis is time. And we see here on the left-hand graph those arrows representing action potentials. We see a lot of them coming in rapidly, and we see that this tension line sort of goes up and down, up and down, up and down, but it keeps on climbing until it just stays with a lot of tension. The graph on the right, we see action potentials. There are two of them. They're nicely spread out, and in between them, we get tension and relaxation. And then again, we get tension and relaxation. Alright. I've been through all that. Hopefully, this is pretty straightforward which one represents skeletal muscle and which one represents cardiac muscle. Well, the one on the left, I'm going to say, is skeletal. Remember, skeletal muscle can have really rapid action potentials, and as they come in rapidly, they come in faster than that muscle has time to contract and relax. So it contracts a little bit. And then before it can relax, another one comes in, and it contracts more. Another one comes in. It contracts more and more and more until you get up to what we call here this titanic contraction, where it stays contracted and just stays squeezing and holds like that. Now, in contrast, cardiac muscle, well, that's this one over here. So right here, cardiac. These cardiac contractile cells, the action potentials are going to come in nice and spread out. That allows these cells to squeeze, to contract, and fully relax again before they are stimulated again. Now, that's important for a heartbeat. Right? Because you want your heart to squeeze and relax and not just squeeze and hold because that wouldn't pump blood. Alright. So it says here for b, the skeletal muscle shown is exhibiting a titanic contraction. How does the molecular physiology of contractile tissue prevent tetanic contractions in heart muscle? Alright. So again, remember, tetanic contraction, those contractions that squeeze and hold, we said that's important for skeletal muscle because if you're holding something heavy or holding your baby, you don't want those muscles just to release. You want it to just stay contracting. But in the heart, that's bad because we need to pump blood, pump, and release. Well, we said that the way that cardiac muscle stops from having these tetanic contractions, we said that they have a prolonged or I'm going to say prolonged absolute refractory period. And that absolute refractory period is the time between action potentials when a cell cannot depolarize because it's already depolarized. An action potential cannot stimulate a depolarized cell. Remember, in those contractile cells, you have that long plateau phase, which stretches out the time that it's depolarized. So even if a new action potential came in, it couldn't tell the cell to contract. This gives those cells time to contract and relax before they can be stimulated by another action potential. Alright. For that, we've matched up our muscles. We've explained why they work that way. More practice to follow. Give them a try.

Cardiac contractile tissue uses sodium, calcium, and potassium channels for depolarization and repolarization. During which phase of the action potential is the calcium channel open?

• A. Depolarization phase.
• B. Plateau phase.
• C. Repolarization phase.
• D. Resting phase.

How would you expect the absolute refractory period of a contractile cell to change if more potassium channels opened sooner after polarization?

• A. Absolute refractory period would increase.
• B. Absolute refractory period would decrease.
• C. There would be no change; absolute refractory period is determined by the calcium ion channels.
• D. It’s impossible to tell as opening potassium channels will have an unpredictable effect on sodium channels.

Comparing Action Potentials in Pacemaker and Contractile Cells

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.

Cardiac Action Potentials Example 4

Our examples say that the graphs below show action potentials for cardiac contractile and cardiac pacemaker cells. Different sections of the attributable graphs are colored in blue, labeled a, orange, labeled b, and green, labeled c. Now for each section, identify which ions are moving out of the cell and which are moving in. Alright. So we look down here, we have these two graphs of membrane potential. On the left, we see cardiac contractile cells, this graph here, and we're going to use that to fill in this table down here. And on the right, we have this graph of the membrane potential for cardiac pacemaker cells and we're going to use that to fill in this table here. So let's start on these contractile cells. We see we have the resting potential and then we have that rapid depolarization during A. So what ions cross in the membrane in what direction causes that rapid depolarization? Well, that rapid depolarization is caused by sodium or Na⁺ ions flowing into the membrane, flowing into the cell. Now remember that's just the same as skeletal muscle or in neurons: rapid depolarization, sodium going through those sodium channels into the cell.

Next, where we have this plateau phase, where we sort of just stay depolarized for a while. Which ions going in which direction cause that? Well, after the sodium ions, well, then the calcium channels open. Calcium channels come into the cell, Ca²⁺, but we're trying to slow things down and spread things out. So whenever that's happening, we're going to have potassium ions come, I'm sorry, and flow out of the cell at the same time. Those 2 positive ions going in opposite directions kind of cancel each other out, and we get this very long plateau phase.

That brings us to c, our repolarization. What's going to cause that? Well, to repolarize, we're going to have potassium ions move out of the cell, bringing their positive charge with them. That brings us over to our second graph here for the pacemaker cell. Here, we see that very slow characteristic depolarization, followed by this more rapid depolarization, and then repolarization. So for A, what ions cause that very slow and steady depolarization? Remember, that's that pacemaker potential, that slow depolarization, and that's going to be caused by sodium ions coming in, but it's going to be slowed down because at the same time potassium ions are flowing out. Remember here, they're going through the same channel, the special channel that allows both ions through to cause that pacemaker potential.

That brings us to b. B, we have this more rapid depolarization. What's going to cause that? Well, after the sodium channels, then calcium channels open. So Ca²⁺ calcium channels open, calcium flows into the cell, bringing its positive charge with it, that further depolarizes the cell. And then finally, we get to c. To repolarize, we're just always going to use potassium ions. Our potassium ions channel is open. Potassium with its positive charge goes out of the cell and repolarizes the cell.

The way I remember this is that you always sort of have this order. Sodium ions move in, calcium ions move in, and then potassium ions move out. That happens in both cells. Sodium, calcium, potassium in that order. But one of these stages, we're trying to slow down, and we slow that down by having that potassium ion channel open, so those ions can flow in the other direction. Remember, for contractile cells, they flow in the opposite direction of the calcium in step b here. For the pacemaker cells, they flow in the opposite direction through the same channel of these sodium ions, and that's in step a. Alright. Practice this more and more problems. Give them a try. I'll see you there.

The movement of ions in opposite directions at the same time is responsible for which of the following?

• A. Slow depolarization in cardiac pacemaker cells.
• B. Slow repolarization in cardiac pacemaker cells.
• C. Slow depolarization in cardiac contractile cells.
• D. Both A & C are correct.

Which statement below correctly describes how the channels that are active during the pacemaker potential are different from other ion channels used in action potentials?

• A. They are voltage gated.
• B. They are active over a much wider range of voltages than other ion channels.
• C. They allow Ca$^+$$^2$ ions to pass out of the cell.
• D. They allow both Na$^+$ and K$^+$ through the same channel.

Here’s what students ask on this topic:

What are the key differences between cardiac pacemaker cells and cardiac contractile cells?

Cardiac pacemaker cells and cardiac contractile cells have distinct roles and characteristics. Pacemaker cells, located in the nodes of the heart, initiate the heartbeat through a process called autorhythmicity. They exhibit a slow depolarization due to simultaneous sodium influx and potassium efflux, which sets the heart rate. In contrast, cardiac contractile cells, which make up the majority of the heart muscle, are responsible for pumping blood. These cells show a rapid depolarization followed by a plateau phase due to calcium influx and potassium efflux. This plateau extends the absolute refractory period, preventing tetany and ensuring the heart relaxes between beats, which is crucial for effective circulation.

How do action potentials in cardiac muscle differ from those in skeletal muscle?

Action potentials in cardiac muscle differ significantly from those in skeletal muscle. In skeletal muscle, action potentials involve a rapid depolarization due to sodium influx and a rapid repolarization due to potassium efflux, occurring within 2-3 milliseconds. In cardiac muscle, there are two types of cells: pacemaker cells and contractile cells. Pacemaker cells exhibit a slow depolarization due to simultaneous sodium influx and potassium efflux, setting the heart rate. Contractile cells show a rapid depolarization followed by a plateau phase due to calcium influx and potassium efflux, extending the action potential to around 200 milliseconds. This plateau phase prevents tetany and ensures the heart relaxes between beats.

What is the role of calcium ions in cardiac action potentials?

Calcium ions play a crucial role in cardiac action potentials, particularly in the plateau phase of contractile cells and the depolarization phase of pacemaker cells. In contractile cells, calcium influx through voltage-gated calcium channels during the plateau phase helps maintain depolarization, extending the action potential and the absolute refractory period. This prevents tetany and ensures the heart muscle relaxes between beats. In pacemaker cells, calcium influx is responsible for the rapid depolarization phase after the slow ramp-up of the pacemaker potential. This calcium-driven depolarization initiates the action potential that spreads through the heart, triggering contraction.

Why is the absolute refractory period important in cardiac muscle cells?

The absolute refractory period is crucial in cardiac muscle cells because it prevents the heart from undergoing tetany, a sustained contraction that would be detrimental to its function. During this period, the cells cannot respond to new action potentials because they are not fully repolarized. In cardiac contractile cells, the plateau phase, caused by calcium influx and potassium efflux, prolongs the absolute refractory period. This ensures that the heart muscle has enough time to contract and then relax before the next action potential arrives, allowing for effective pumping of blood and proper filling of the heart chambers between beats.

How do pacemaker cells set the heart rate?

Pacemaker cells set the heart rate through a process called autorhythmicity. These cells, located in the nodes of the heart, generate action potentials without external stimuli. The key feature of pacemaker cells is their slow depolarization phase, known as the pacemaker potential, which is due to the simultaneous influx of sodium and efflux of potassium through special channels. This slow ramp-up gradually brings the membrane potential to a threshold, at which point voltage-gated calcium channels open, causing rapid depolarization and initiating an action potential. This action potential spreads through the heart, triggering contraction and setting the rhythm of the heartbeat.