Cardiac Action Potentials - Video Tutorials & Practice Problems
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1
concept
Introduction to Action Potential in Cardiac Cells
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7m
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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 wanna talk about the action potentials in a lot more detail. Action potentials are something that you've learned about before. When you talk about skeletal muscle and when you talked about neurons, well, the action potentials in cardiac muscle are gonna be kind of similar, but there's gonna be some very key differences and we're gonna go over those. Now, now we're gonna 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 to look at an overview and compare them to the action potentials you're familiar with, we're gonna 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 uh are what actually contract and squeeze blood and pass that action potential from cell to cell through those gap junctions. All right. So let's look at how both of these operate again in comparison to skeletal muscle. Now, we're gonna 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 mills. 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. Uh 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 where there are other tubes of two 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. All right. 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 gonna be that really slow depolarization, the ramp up that looks very different. Now in our contractile cells and our pumpers over here, but 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 rep polarizes. Now, I played a little trick on you here. May 1 thing that makes these look even more different than they are how they draw now. Well, 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 gonna 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 gonna take 800 milliseconds. So that's hundreds of times longer than in the skeletal muscle, the cardiac contractile cells, these pumpers, they're not gonna take quite as long, but they're still gonna take something like 200 milliseconds for that action potential from the time it starts depolarizing the time it's finished repoire polarizing, still way way longer than that skeletal muscle. Now, if we put it on its own scale, you can really see the right the skeletal muscle, we're gonna 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 depolarize 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 canceling 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. All right, 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 rep polarizes. Again, we're gonna look at those two cells and all the different steps and 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 skell to muscle. So to sum that up, remember, the skeletal muscle uses sodium and potassium and that's really it, the cardiac muscle is gonna use sodium calcium C A plus two 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 are 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 are pumpers here that slowed down phase is going to be due to multiple ions crossing the membrane at once. So you're always gonna 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 gonna learn all the different steps. But when you look at these curves, the thing that you wanna keep an eye on is which part are we trying to slow down? And when we do that, it's gonna be due to multiple ions going in opposite directions. Look at that more going forward. I'll see you there.
2
example
Cardiac Action Potentials Example 1
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3m
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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 wanna 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 two graphs over here. Graph A and graph B and we see the different depolarizations and repolarization there. Now that's the last thing that we're supposed to fill in in this table here. But I actually want to do it first because if I identify these graphs, I think it's gonna 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 uh the depolarization and re polarization 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 rep polarized. We said that that shape that was characteristic of a cardiac contractile cell. So I'm just gonna 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, right? That, that looks like a pacemaker cell. I'm gonna write pacemaker here. All right. Now, I can fill in the last line of this table here. The cardiac pacemaker cells. Well, that's graph uh graph B and cardiac contractile cells. Graph A. All right, let's see the other things that we need to identify here. So which of these has a slow re polarization? Well, as I look at these graphs again, I have the depolarization and then in the contractile cells, it stays depolarized for a little while before it rep polarizes. So those contractile cells that seems to have a slow repolarization. I'm gonna give that one a check mark. What about the pacemaker cells? Well, the pacemaker cells it depolarizes and here we are depolarized and then almost immediately it rep polarizes again, that's not slow. I'm not gonna 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 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. All right. Now, which one of these us utilizes only sodium and potassium ions. Ah 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. All right, our next one is us, utilizes sodium calcium and potassium ions. Well, we just said that's both of them. They're gonna u utilize them slightly differently and that's how we get those different curves over there, but they are both gonna use them. So they get a check mark, both of them get a check mark there. All right, with that. I filled out my table. Now again, we are gonna 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.
3
Problem
Problem
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.
4
concept
Pacemakers: Molecular Physiology
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8m
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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 gonna 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 gonna take a look at what ions are crossing the membrane. When in order to give us a graph, it looks like that. All right. 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 gonna be using three ions. We have sodium ions, we have calcium or C A plus two 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, auto rhythmicity and auto rhythmicity. We're gonna 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. All right, the way they set the rate of your heartbeat, the way they spread out those action potentials is gonna 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. All right. 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 with the graph you can see on the y axis we have mill volts and we have it labeled from negative 60 to 0 millivolts. Now, I'm not gonna 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 gonna change depending on your heart rate. But we can assume that from beginning to end in a resting heart rate, this is gonna take about 8/10 of a second or 800 milliseconds. All right. The first thing that we wanna 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 one. What's going on. All right, we're gonna 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 in image, we see the three different channels we'll be talking about and we see on the left this paint 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 to 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, well, another thing I just wanna 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. All right. 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 this sort of orange or gold color there. It sort of has a relatively rapid depolarization that comes up next. So we're gonna say for number two at threshold voltage gated calcium or C A plus two channels open. So we can look it over at our image there. We can see that those special um sodium potassium channels, the pink channel is now closed and now this calcium channels open and we can see this calcium now coming into the cell. So we're gonna 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. All right. 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 rep polarize. So we can see that on our graph there. And we're gonna highlight that part in green and we're gonna label at number three. And this is gonna happen basically like any other action potential while the calcium channels are gonna 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 gonna say here the potassium exits the cell and as it leaves the cell, it brings its positive charge with it that causes re polarization and the cells now rep polarized. Now, in other action potentials, the cell would just sort of stay rep polarized, 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. All right, if you look here, you'll see it sort of touches that ne negative 60 millivolts line. It rep polarizes and then it just starts going back up again. So we're gonna actually say step number four is go back to one. We're gonna say here there is no resting potential as soon as this is rep polarized. Well, that pacemaker potential starts again. So the pacemaker potential starts again. Those sodium uh potassium channels are open. They ca you have that slow ramp up at a threshold. The calcium channels open. You have depolarization that sets an action potential off through the heart. You re polarize, go back again, slow, ramp up depolarization, action, potential repolarization again and again and again and again, that's your heartbeat. All right. The last thing that we just wanna 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 know your heart's not normally beating at 100 beats per minute. If you're exercising, it's gonna be above that. If you're rest at rest, it's likely below that. Remember, we have extrinsic factors, the autonomic nervous system, which is gonna sort of turn up and turn down this heart rate, do that by affecting that pacemaker potential. How quickly that depolarization happens is gonna spread out those action potentials or have them be closer together. Importantly, though the actual rate is kept in these pacemaker cells and the actual potential starts in these pacemaker cells. All right. With that, we're gonna take a deeper dive and look at these cardiac contractile cells, the pumper next. But before that, we have examples and practice problems that fall. I'll see you there.
5
example
Cardiac Action Potentials Example 2
Video duration:
4m
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Our example says that the graph below shows the membrane potential of a typical pacemaker cell. All right. So we look down here, this is our normal pacemaker cell here. We should be familiar with this grasp 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 re polarization and then that pacemaker potential starting again. All right. A says draw how you think the curve would look if the permeability of the sodium potassium channels is increased. All right. So in a here, we're gonna have to redraw this curve but that special sodium potassium channel, the permeability of that channel is gonna 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. So we're talking about the pacemaker potential here just to make that clear, I'm gonna highlight this part of the graph. So it was those ions again going in opposite directions. They kind of 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 gonna think if the permeability increased, I'm gonna be changing that part of the ground. And specifically, well, I'll start, I'll uh start with it going down a little bit. It hits that negative 60 mill volts threat threshold. Now, if the permeability is increased, that means it's gonna depolarize more rapidly. So this part of the graph should be steeper, then it hits our negative 40 milli threshold. It's gonna depolarize using those calcium channels, re polarize, using the sodium channels and then hit that threshold when it's re polarized and start again. But again, this part is gonna be nice and steep because we've increased the permeability of those channels and we'll just keep going there. All right. 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 per 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. All right. 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 gonna slow down this pacemaker potential even more. So I would draw it and start down and then I'm gonna 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. All right. So again, I'll highlight that part. That's different. That part that's different is gonna be right there. OK. So then it says c for both cases indicate how this change would generally affect heart rate. Remember it's that pacemaker potential that is spreading out the heartbeat 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 sort of 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 gonna say it's gonna 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 gonna be spread out more. How, how is that gonna affect the heart rate? I'm gonna say it's going to decrease or slow down heart rate. All right, with that, we've answered the questions. Remember for those pacemaker potential, 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 kind of slow everything down, more questions to follow. Give them a try.
6
Problem
Problem
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.
7
Problem
Problem
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.
8
concept
Contractile Tissue: Molecular Physiology
Video duration:
9m
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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 gonna be looking at what ions are crossing the membrane. When in a way that gives us a graph, it looks like this. All right. 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 depolarized and then the graph kind of plateaued and they stayed depolarized for a little while before they rep polarized again. Now the other thing to remember like all cardiac cells we're dealing with three ions here. We have our sodium ions, we have our calcium or C A plus two ions. I'm gonna write and we have the potassium ions. Now remember these contractile cells, these are our pumpers. So our goal here, they're gonna squeeze force the blood out of the heart and then they're gonna 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 rep polarized. Now, this is actually kind of a simple concept, right, an action potential is the depolarization and then repolarization of the cell. Well, if a cell is depolarized, when it's an 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 rep polarize again. Well, when we look at the graph of these cardiac contractile cells, remember they had that plateau phase and they stayed depolarized for a little while. The fact that they stay depolarized prolongs or it lengthens that absolute refractory period and that forces relaxation of the heart muscle. Now, why it forces relaxation? We'll take a look at it 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 stage 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 gonna break down these graphs. We're actually gonna break down three graphs here. We're gonna spend the most time on this first one though. And this one, we've uh looked at this, 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 gonna 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 gonna have time. All right. So the first thing you're gonna note here, we're gonna highlight it in pink label. At number one, we have a rapid depolarization of the cell. All right. So let's look at what's going on there. Well, this is basically like your standard old action potential in your skeletal muscle or in your neuron. We're gonna 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 two of these potassium channels drawn in green. We'll talk about about why there's two of those in just a second. But we're gonna 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. All right. 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 two. 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 gonna say here that calcium or C A plus two channels and potassium channels open. But importantly, those potassium channels open slowly. So if we look over at our image here are, well, we can see now those sodium channels are closed and now the calcium channels open, calcium is coming into the cell. And remember, calcium is a positively charged ion. So it's bringing its positive charge into the cell. But potassium, well, these channels are open as well. Now this is why we have two 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 cause it like to become even more depolarized, you could say, but here the potassium is canceling and out going in the other direction. So we have the calcium C A plus two into the cell. Potassium out. That means that the cell lactose and it just sort of stays sort of at this kind of level depolarized state. All right. 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. All right. Well, we had that plateau. Now we've stretched out this action potential. Well, now it's time to re polarize. And we're gonna highlight that in green and label it in number three on the graph here. And again, this is gonna happen basically like your standard old action potential. Well, we're gonna have those calcium channels closed. C A plus two channels closed. So now all the other channels are closed, but now the potassium or plus 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 rep polarizes again, basically like a regular old action potential. All right, 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 uh at the absolute refractory period. Here it starts when the cell depolarizes and now this cell cannot be stimulated again until it's rep polarized. Now, why does that matter to look at this? We're gonna look at the tension in a cardiac cell that's gonna be this second graph here. So again, on the y axis, we have tension and on the x axis we have time. All right, 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 gonna 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 re polarized 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 it'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 two or three milliseconds. That's how long that uh 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. All right, we're gonna compare these action potentials in uh 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 him a try.
9
example
Cardiac Action Potentials Example 3
Video duration:
3m
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Our example tells us that two tension grafts 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. All right. So it says here a identify which graph shows cardiac contractile tissue and which shows skeletal tissue. Well, first off, we'll just 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, just 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. All right, 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 gonna say is skeletal 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 gonna 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. All right. So it says here for b the skeletal muscle shown is exhibiting a titanic contraction. How does the molecular physiology of contractile tissue prevent titanic contractions in heart muscle? All right. So again, remember titanic contraction, those contractions that squeeze and hold. We said that's important for skeletal muscle because if you're holding se something heavy or holding your baby, you don't want those muscles to just 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 titanic contractions, we said that they have a prolonged or a longer, I 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. All right, that we've mess, we've matched up our muscles, we've explained why they work that way. More practice to follow. Give them a try.
10
Problem
Problem
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.
11
Problem
Problem
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.
12
concept
Comparing Action Potentials in Pacemaker and Contractile Cells
Video duration:
6m
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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 and two cell types and those cells work differently. So there's a lot that could potentially get crossed in your mind. Here, we wanna go through these side by side one more time just to make sure that we have everything nice and straight. All right. 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 auto rhythmicity. 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 gonna be mills the X axis is time. All right, 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 called the pacemaker potential that slow ramp up, followed by a more rapid depolarization followed by repolarization. Now, in contrast, this contractile cell is gonna look very different. We had that rapid depolarization and then it kind of plateau is it just sort of stays depolarized for a little while and then it rep polarizes again. Now, when we went through these, previously, we sort of assigned three basic steps to each one of these cells, we're gonna go through those steps again. Now side by side as we do it for each, each time, for each step, I'm gonna 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 to 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 a de polarizing 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 gonna 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. All right now because these are positive ions going in opposite direction. We're gonna say here that the opposite flow of ions slows this depolarization. Now, remember for each one of these cells, we're gonna have one step that we're trying to slow down and kind of stretch out. And when that happens is because we have ions flowing in opposite directions and it's always gonna involve potassium flowing out. So to remind you of that here, we've put that potassium flowing out and that opposite flow of ions in those green boxes just to call that out. All right, next. So next, both steps two 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. All right, that contractile cells, it's gonna be more complex. Here, we have that plateau phase where it sort of just stays depolarized for a little while. Well, again, we're gonna have calcium channels, open calcium channels are gonna 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 rep polarization 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 gonna do the same thing. Now, they both need to re polarize and they're gonna do that. We see we're gonna first highlight that in green on our graphs here. And then we're gonna say that they open up those potassium channels. So on the top here we see potassium flowing out of the cell and that's gonna 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 two ion channels here. Uh just to indicate that in our previous step, we also had pota 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 uh pacemaker cells, the pacemaker cells. Well, I'll highlight it in here in pink. It's gonna start depolarizing again. So I'm gonna add one more step here. Now, previously, we just said, go back to one. But here we're gonna remind ourselves we have no resting potential that depolarization, that slow depolarization, that pacemaker potential starts again. All right. 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. All right, you have more practice problems to follow and I will see you there.
13
example
Cardiac Action Potentials Example 4
Video duration:
4m
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Our examples say that the graphs below show action potentials for cardiac contractile and cardiac pacemaker cells. Different sections of the attributed the 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. All right. So we look down here, we have these two graphs of membrane potential. On the left, we see cardiac contractile cells. This graph here, I'm gonna 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 gonna 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 crossing the membrane in what direction causes that rapid depolarization? That rapid depolarization is gonna cause by sodium or N A plus 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. All right next, when 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 C A plus two, but we're trying to slow things down and spread things out. So whenever that's happening, we're gonna have potassium ions come uh I'm sorry, flow out of the cell at the same time, those two positive ions going in opposite directions kind of cancel each other out and we get this very long plateau phase. All right, that brings us to see our repolarization. What's gonna cause that? Well, to re polarize, we're gonna have potassium ions move out of the cell, bringing their positive charge with them. All right, 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 gonna be caused by sodium ions coming in, but it's gonna 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. All right, that brings us to BB. We have this more rapid depolarization. What's gonna cause that well, after the sodium channels, then calcium channels open. So C A plus two 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 see we're gonna re polarize well, to rep polarize, we're just always gonna use potassium ions. Our potassium ions channels open potassium with its positive charge goes out of the cell and rep polarizes the cell. All right. 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. And 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 all right practice is more and more problems. Give them a try. I'll see you there.
14
Problem
Problem
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.
15
Problem
Problem
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.
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