Electrical Conduction System of the Heart - Video Tutorials & Practice Problems
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1
concept
Intrinsic Cardiac Conduction System
Video duration:
7m
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As we think about the physiology of the heart, we wanna think about how the heart contracts in a coordinated way. How does blood get pushed into the ventricles and then those ventricles contract to push the blood out through the arteries? Well, a major driver of that process is going to be the intrinsic cardiac conduction system. Here, we want to give you an overview of that system and some of the basic workings of it. Then we're gonna go into the details of the anatomy and physiology, following an action potential throughout the heart, and seeing at how that leads to a coordinated contraction. Alright. But first, let's define it. The intrinsic cardiac conduction system, we're gonna say, initiates or just starts. It initiates and contract and conducts action potentials through the heart. Alright. This is very different from skeletal muscle. Remember, in skeletal muscle, every muscle fiber is connected to a neuron, and it only contracts when a neuron sends it a sends it a signal, sends it an action potential telling it to. In the heart, this is all happening in the heart muscle cells. These heart muscle cells start action potentials and conduct those action potentials from cell to cell. Now that's where we get this word intrinsic. Intrinsic means sort of like built within or essential to. And when we say it's the intrinsic cardiac conduction system, what we're really saying is that it does not require the nervous system to function. This does not get receive action potentials from the nervous system telling the muscle to contract, and we're gonna say it's contained entirely within the heart. Now when I think of this, I think of some kind of gruesome movie scenes that I've seen before where a heart will get ripped out of somebody's chest and it keeps beating. It still beats even outside the body. Now, while that's kind of gruesome, it can happen. And that's because these action potentials, the signal for the muscle to contract, starts within the heart, and they spread through the heart muscle. It does not need to be connected through the rest of the body for the heart to beat. Now, obviously, outside of the body, it won't keep beating for very long, but it can keep beating. Alright. That fact results in heartbeats that are coordinated and regular. Alright. These words here, coordinated and regular, these aren't real technical terms, but when I think about what this intrinsic cardiac conduction system is doing and how it works, these words help sort of break it up in my mind. And so we're gonna do it that way. Before we do that, though, let's just remember an important feature of cardiac muscles. Cardiac muscle cells are connected by gap junctions. This allows these action potentials, or as I'm just gonna write here, APs, these action potentials to be passed from cell to cell. Again, that's very different from skeletal muscle. Here, if you stimulate a cardiac muscle cell, that action potential is going to spread like a wave through this heart muscle wall. Alright. So then thinking about this coordinated and regular, well, let's start with coordinated. When we say coordinated, I mean that the cells must contract together. We want all the cells in this heart muscle wall to contract at the same time, so we get this sort of one squeeze that pushes blood. And if they don't contract at the same time, that's a major problem. We call that fibrillation. We'll talk about that more later on. But again, you can think you need the heart muscle wall to squeeze as one. Well, things that help it do that. First, we have these gap junctions that we already mentioned. This allows those action potentials to spread. So if you stimulate just a few heart cells in this muscle wall, it's gonna spread very, very fast, and you're gonna get an entire chamber of the heart contracting essentially at the same time because those action potentials are able to spread so fast from cell to cell through those gap junctions. Now we're also going to have something called conducting fibers. Conducting fibers are gonna be specialized cardiac cells with few myofibrils. Remember, myofibrils are sort of machinery of contraction, of sort of a muscle cell squeezing. So if we have very few myofibrils, that means that these cells are really just specialized for sending these action potentials. And to help with that even more, they're going to be insulated from the contractile cells. So these almost work like neurons of the heart. To be very clear, they are muscle tissue, they are not neurons. But they're these fibers that are able to send an action potential very fast from one place to another without stimulating the other cells around them. Again, this helps the heart coordinate its contraction and get action potentials to where they need to be very, very rapidly. Alright. As I think about regular, what I mean here, well, the heart must contract at the right time. Coordinated, they have to beat at the same time, but you also need them to be regular. They need to contract at the right time. You need the atria to contract first. You need the ventricles to contract second. What's coordinating this? Nodes. Alright. Nodes are small regions of the heart with just a few cells in them. Or, I mean, there's more than a few, but comparatively not many cells in them. And these initiate those action potential. So this is where the action potentials start. So we have a couple places, 2 of them, the sinoatrial node, which sort of starts all of con contraction or the also called the SA node, and the atrioventricular node, the AV node, which starts the action potentials for the ventricles. Now we'll talk about those in a little bit more detail later. Just for now know that they're nodes that start these out. So once these action potentials start, well, then they spread through the the cell and we get contraction. But we want them starting not just in one place at one time, but we also want them starting to a rhythm, because that's your heartbeat. And what gets them, contracting or initiating at a rhythm are these cells called pacemaker cells. Again, you have relatively few pacemaker cells in your heart, but they are hugely, hugely important. These are gonna be specialized cardiac cells that depolarize at regular intervals. Alright. If an action potential is the depolarization and the repolarization of the cells, well, these depolarize at regular intervals, so these start these action potentials. It will depolarize. That will stimulate cells around it. It can spread throughout the heart. That stimulates the contraction. And then, again, because it's doing at regular intervals, it's gonna depolarize again, start another wave of action potentials that leads to contraction, and then it will do it again and again. And that's why we get a heartbeat. Okay. Again, we're gonna next look at the anatomy of this intrinsic cardiac conduction system, and then we'll follow these action potentials through that intrinsic cardiac conduction system and see how it leads to con contraction. I think it's gonna be a heck of a good time. I'll see you there.
2
Problem
Problem
Which feature of cardiac tissue allows for the rapid spread of action potentials through the heart?
A
Nodes.
B
Myelin sheaths.
C
Pacemaker Cells.
D
Gap junctions.
3
Problem
Problem
Which statement best describes intrinsic conduction of the heart?
A
Cells within the heart can initiate and transmit action potentials without nervous system input.
B
Cells in the heart can beat continually without fatigue.
C
Cells in the heart follow a specific rhythm that is set by the brain stem.
D
Cells in the heart pass action potentials between cells using gap junctions instead of neurotransmitters.
4
concept
Anatomy of the Intrinsic Cardiac Conduction System
Video duration:
8m
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Video transcript
We want to spend some more time talking about this intrinsic cardiac conduction system, and we're gonna start talking about the anatomy of the system. We're gonna do that by following one action potential through all the structures, and we're going to use this diagram of the heart to do that. Now we're going to talk about the physiology of the system and exactly how that action potential gets passed coming up. Right now, I want you to focus on the names and the locations of these structures and understand the basic pathway that this action potential takes. Again, we'll be using this diagram of the heart to do so and just orient ourselves there before we get started. You can see we have a cross section of the heart. You can see the four chambers. You can also see some of the blood vessels that are connected to the heart, and on there, you can see this cardiac conduction system drawn in yellow. That's what we're gonna go through and label. Now, if you're following along in your PDF, you'll see it's already labeled for you, but we're gonna build it up piece by piece here. All right. So let's dive in. But first, let's just remind ourselves that the intrinsic conduction system consists of specialized myocytes, those are just muscle cells, They initiate and conduct electrical signal. Now, again, remember this is very different from skeletal muscle. In skeletal muscle, every muscle cell is connected to a neuron, and every muscle cell contracts only when that neuron tells it to. In the heart, the action potentials start in the muscle cell, and they spread from cell to cell. Alright. So to see how that works, first, we are going to look at this structure labeled a there in the top corner there. We highlighted it in pink. That is going to be the sinoatrial node or often just referred to as the SA node. Sinoatrial, it's located in the sinus of the atrium, But this is just a very small group of cells in the superior right atrial wall, sort of just inferior to or just below the vena cava there. Now, again, not many cells here, but hugely important. This contains pacemaker cells. Remember, those pacemaker cells are what depolarize on their own to start the action potential. So the action potential that tells the entire heart to beat, it starts at these very few cells in this SA node. Alright. It then has to spread out from the sa node, and the next place it's gonna go, we're highlighting here in orange, and we have it labeled B traveling through the, the atria there. This is called the internodal pathways, or also sometimes called the atrial conducting fibers. Now I just want to note not all classes make you responsible for knowing those terms. Sometimes this part of the conduction system is just sort of skipped over. So if it's not in your notes, that's why. But it is there, and some classes want you to know it, so we're going over it. Alright. So this internodal these internodal pathways, or the atrial conducting fibers, these connect this sinoatrial node, this SA node, and the AV node or the atrioventricular node. Now, we haven't talked about that yet. That's coming up. But so this conducts this signal through the atria very quickly to the next stop on this pathway. But it also is going to distribute this action potential, labeled here as an ap, through the atria. So you can see that they spread out through the atria, and there's even one pathway that goes over to the left atrium there. And so as it spreads out, it passes the signal to the actual contractile cells, the cells that are gonna do the squeezing. They pass it from one to another, and that's when the heart contracts. Alright. So we've now passed it very quickly through these conducting fibers, through the atria. And the next thing this action potential is gonna reach is going to be what we highlighted there in yellow and labeled c. That's going to be the atrioventricular node or the AV node. And this AV node and again, it's a small group of cells, and it's gonna be located on the inferior right atrial wall. It was called atrioventricular because it's at the bottom of this atrial wall, sort of right sitting on top of those ventricles. The AV node gets this signal from the conducting fibers, and then it is going to initiate an action potential that initiates ventricular contractions. So it gets the action potential, and then it is responsible for telling the ventricles to contract. Now it also contains some, what we're going to say here, are backup pacemaker cells. Now the heart's pace is set by the pacemaker in the SA node, but the ventricles beating are much more important than the atria beating. So if for some reason that signal does not make it to this atrioventricular node, to this AV node, there's backup pacemakers here that will start an action potential. So at least the ventricles will contract. Your body can live at least, you know, at rest if you're not doing too much if the atria aren't functioning properly. If the ventricles aren't functioning properly, you can't live. Now they're back up because they're timed just slightly slower than those SA nodes. So they normally never fire because the signal from the SA node gets to them before their timer is up, we could say. But they're there just in case. All right. So this AV node sends off its action potential down through the ventricles, but it doesn't go through the contractile cells just yet. The next place it's gonna go, it's going to go down this sort of green pathway in the middle there that we've labeled d. That is the atrioventricular bundle or the AV bundle, and it's also sometimes referred to as the bundle of His. Now, typically, we like to think of to use the anatomical terms. Those terms like bundle of hiss are after somebody's name, and those are sort of falling out of favor. But you should be familiar with it because it is used sometimes. All right. So this a v bundle, it's going to be in the superior portion of the septum. Remember, the septum is that dividing wall between the right and the left ventricles. And it is gonna be made of these conducting fibers that can pass this action potential very quickly. And they really don't do any contracting. They really just pass the action potential. Now, you also have here some more backup pacemaker cells. If the sa node doesn't fire, that's bad. But the av node, it has backup pacemaker cells to cover for it. If the av node's backup pacemaker cells don't fire, well, that's getting really bad. But even to cover that, we have more pacemaker cells. So if the action potential never actually gets here, it will start in this atrioventricular bundle. Alright. From the atrioventricular bundle, it's gonna get passed down the septum, and the bundle then splits into what we have labeled here in blue as e, and it's going to split into the right and left bundle branches. These right and left bundle branches serve the right and left ventricle, but it's still going down through the septum at this port point. So we're gonna see here this is the inferior portion of the septum, and it is also made of these conducting fibers conducting this signal very quickly through the heart. Alright. The final place we're gonna go in this conduction system, all labeled here in purple. You can see it spreading out all through the ventricles. We call this the subendocardial conducting network, or often just called the purkinje fibers. Now I've said, right, those sort of anatomical terms are usually preferred over these terms that are named after somebody like purkinje fibers. But purkinje fibers is a lot easier to remember than subendocardial conducting network, so a lot of people still use it. Alright. So these Purkinje fibers, these are the smallest conducting fibers. And you can see how they sort of spread out all through the muscle wall of those ventricles, and they're going to be what actually connects to those contractile cells of the heart wall in the ventricle. So when the signal reaches the end of all those Purkinje fibers, that's when the heart muscle itself actually gets those that signal, starts passing it from cell to cell, and these ventricles will contract. Alright. Again, here we went over the anatomy and very basically what's going on. We're gonna look at the physiology and exactly how this works in more detail coming up. I will see you there.
5
example
Electrical Conduction System of the Heart Example 1
Video duration:
4m
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Video transcript
This example says that for each structure in the cardiac conduction system, identify in which region in the heart wall it is found by writing the location in the space provided, Then answer the question below. And we have a note: Some structures may span multiple regions. All right. So, it gives us 5 heart wall regions here. We have the right atrium, the left atrium, the right ventricle, the left ventricle, and the septum. And we have 6 structures here, all of which we've talked about previously, so we really just gotta remember where they are. Alright. So we'll start out the sinoatrial node, also called the SA node. Do you remember where that is located? Well, there's a little bit of a clue in the name there, right? Sinoatrial. So we know it's in the atria somewhere, and specifically it's going to be in the right atrium. So I'll write ra in this space here. It's kinda near the top of the right atrium, kinda right under the entrance of the vena cava there in that right atrium wall. Remember the sinoatrial node or the SA node that initiates the action potential, which then spreads out through the heart and causes the heart to contract. Alright. Next, we have the atrioventricular node, also called the AV node. So where is that AV node located? Well, remember the atrioventricular node, it starts the action potential, which then spreads down the septum causing the ventricles to contract. But it's actually still located in the right atrium. It's in the right atrial wall, sort of right on the base of the atrial wall there, kind of sitting on top of the septum a little bit. Alright. As we go down then, then we have the AV bundle. Remember, this is also sometimes called the bundle of His. Where is that a v bundle? Well, we said that action potential goes down the septum and it goes down the septum through that a b bundle. So this a b bundle is going to be in the septum, so I'm going to write an s on the line there. Alright. Next up, we have the right bundle branch, and we'll do the left bundle branch at the same time here. Where are those right and left bundle branches located? Well, they have the names right and left in there, but these are still in the septum. Remember that AV bundle goes down, it splits into the right and left branch, and those right and left branches then continue all the way down the septum to the apex of the heart. So I'm gonna put an s here in both these two spaces. Then finally, we have this subendocardial conducting network. Remember, that's also sometimes called the Purkinje fibers. Where are those Purkinje fibers located? Well, we said from the apex of the heart, then those Purkinje fibers then spread out, and they conduct that signal all through those ventricular walls. So we're gonna say that these go up through the left ventricle, LV, and the right ventricle. Alright. So we found all the places. Let's see what this question in here is. It says in order to contract the contractile cells, the heart must be stimulated by an action potential, but the conducting fibers of the heart do not directly connect to all contractile cells do you think allows them to spread that action potential from one to another? Well, that's gonna be the gap junctions. Remember, these cardiac cells are these branching muscle cells that are attached to each other and have gap junctions. So when an action potential stimulates 1, it spreads from 1 to the other like a wave through this cardiac muscle. Now, remember, that's very different from skeletal muscle. Skeletal muscle, each individual muscle fiber or muscle cell needs to be stimulated, and that action potential does not spread from cell to cell. In cardiac muscle, you just need to stimulate some or some subset of this cardiac muscle, and that action potential then spread like a wave from cell to cell because of those gap junctions. All right. With that, we've finished our example. We got more practice problems after this. I'll see you there.
6
Problem
Problem
The intrinsic conduction system ensures a coordinated and efficient heartbeat. If the sinoatrial (SA) node malfunctions, which part of the heart’s conduction system is most likely to take over as the pacemaker?
A
Atrioventricular (AV) node.
B
Bundle of His.
C
Purkinje Fibers.
D
Atrial muscle cells.
7
Problem
Problem
Which answer choice below correctly matches the cardiac conduction structure to where it’s found in the heart?
A
Atrioventricular node: left ventricle.
B
Purkinje fibers: left atrium.
C
Sinoatrial node: left atrium.
D
Right and left bundle fibers: septum.
8
concept
Conduction Pathway and Contraction
Video duration:
7m
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Video transcript
We're gonna continue talking about this intrinsic cardiac conduction system. And previously, we looked at the structures of this conduction system. And now we want to follow this conduction pathway and see how it actually leads to contraction in more detail. Now, to to orient ourselves, we have this image of the heart there with the conduction system laid over it. And before we really dive into it though, I really just want to set up two main goals that this conduction system is really trying to achieve. We want to say that to pump blood properly, the conduction system must cause first, well, the atria to contract first. Right? You need the atria to contract, and when they do that, they need to squeeze the blood sort of downwards towards the ventricles. So they need to conduct contract downwards, at least in a sort of general fashion, squeezing blood down. And then 2, well you need the ventricles to contract. 2nd, the ventricles need to wait for the atria to pump the blood into the ventricles, then the ventricles contract, and they need to contract upwards. Right? The exit to the heart, the aorta and the pulmonary artery, that's at the top of the heart. So the blood needs to kind of first come down, and then it needs to squeeze and get pushed up. Now, to make this happen, there's an important structural feature in the heart. We said that connecting all these cells are gap junctions, and that's how the action potential gets passed from cell to cell. But really importantly, there are no gap junctions between the cardiomyocytes through those heart muscle cells of the atria and the ventricles. That keeps the contractions separate. So when you're thinking about these action potentials spreading through the heart, we can even sort of just draw a dividing line on the heart. The action potentials in the atria will spread through the atria, but they won't go to the ventricles and vice versa. That means that it must use the conduction system to pass from one to the other. And that's going to allow us to achieve those first two goals that we talked about. All right. So let's take a look. We're now going to look at this heart a little more zoomed in here. Again, this is the same heart that we've looked at before. We have the cross section seeing all the chambers, and we have that cardiac conduction system laid over it in yellow. Alright. So the steps of the cardiac conduction system, well, we said that it starts with the pacemaker cells in the SA node. And so, here we have labeled 1, and I'll highlight it in pink there, that sinoatrial node, the SA node. Now, these pacemaker cells initiate the action potential. We're gonna talk in more detail how these pacemaker cells actually work. Right now, we just wanna follow the action potential, so we need to know that it starts here. It starts here, and then it spreads out through the atria. So we have that labeled too, and we can highlight it here. It's gonna, spread out through the atria, across the atria, through the conducting fibers and the contractile cells. Right. So remember, those conducting fibers are gonna spread it from one atrium to the other, from the SA node down to the AV node, but they're also connecting to those contractile cells, the muscle cells that actually squeeze. Now once those cells are stimulated, they pass it from one to the other, and we get here, we'll highlight it in red, The atria will contract. Now, remember, we want it to contract first. We did that. We want it to contract downwards, at least generally. Well, that's not too hard to do because the sa node is located at sort of the top of the atria. So it gets sort of spread across the top, and as those contractile cells spread it from one to another, they start contracting, and they pass it downwards, and the heart squeezes downwards. Well, that action potential though, now it's reached the AV node. And so we have that labeled 3 here, and we'll highlight it in yellow. Now that AV node is responsible for passing this signal to the ventricles. Because remember, you can't pass the signals from an atrial cell to a ventricle cell. It has to go through the conduction system. Now one of our goals, though, was to make sure that the ventricle beats second. So the AV node is just gonna slow things down a little bit. We're gonna say after a 100 millisecond pause, the AV node that atrioventricular node initiates a new action potential. And that wait is to keep the contractions of the two chambers separate. Now a 100 milliseconds, that's a tenth of a second, that doesn't sound like very long. But if you think about it, a heart rate of 75 beats per minute, which is a pretty normal heart rate, that means that your heart is beating every 8 tenths of a second. So a 100 milliseconds or 1 tenth of a second, that's 1 eighth of that time. That's a pretty significant pause that's going on here. Alright. It pauses, waits, then it sends on the action potential, and that action potential is going to move down what we have labeled in 4 here, move down the septum through the a v bundle or the bundle of His, and through the left and right bundle branches, as we see through those arrows in green and blue there. Alright. This is conducting fibers, so it's passing this very quickly from that AV node down to the apex of the heart here. Going very quickly, and remember those conducting fibers are insulated from the contractile cells. So none of the contractile cells in the ventricles have started squeezing yet. We're just sending an action potential. We have not started any contraction. Well, once it gets down to the apex of the heart here, then it's going to spread out through the subendocardial conducting network, those Purkinje fibers that we have labeled here in 5. So you can see they're gonna spread out through all the walls of those ventricles, and that is what's gonna stimulate the contractile cells. Starting at the apex, it's going to pass that signal out through the contractile cells. Now, as the contractile cells get stimulated, well, the action potential now can spread through the contractile cells, through those gap junctions from cell to cell and the ventricle. Ventricles. Gotta spell it right. The ventricles contract. And we'll simulate that through this red wave going up, causing the ventricles con to contract. Now, again, because it got so quickly down that septum and the purkinje far fibers start spreading out from the apex of the heart, and they spread sort of back upwards through that heart muscle, we saw there the ventricles contracted sort of generally in an upwards direction. And, again, that was our goal. Alright. So now we have passed this action potential through the heart. We had the atria contract first. The ventricles contract second. We did it. We got more practice problems to follow. I'll see you there.
9
example
Electrical Conduction System of the Heart Example 2
Video duration:
3m
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Video transcript
Our example says that the steps of electrical conduction in the heart are listed below in the incorrect order. We wanna fill in the blanks with a letter corresponding to each step to put the pathway in the correct order. Alright. So we have 7 steps here that you can see, labeled a, b, c, d, e, f, and g. And then down here, we see it starts with pacemaker cells initiate an action potential, and we see 7 steps leading all the way to the ventricle's contract. Alright. So take a second, look at those 7 steps, and think which of those 7 steps is gonna come after the pacemaker cells initiate that action potential. Alright. Well, the action potential is gonna be initiated in the SA node, the sinoatrial node. And then from there, the action potential is gonna spread out through the atria, through the conducting fibers in the atria, and through those contractile cells of the atria causing that atria to contract. And here we see e, action potential is passed through the atria. That's what I think is gonna come next. So I'm gonna put e on this first line here, and I can cross it out. We've done e. Alright. Now after it passes through that atria, what do you think will happen next? Take a look. Well, that action potential goes through the atria, and it very quickly gets to the AV node. The AV node, that atrioventricular node. Remember, there's even conducting fibers that get that action potential there very, very fast. So I think I'm gonna say d is next, and I'll cross that out to know that I've done it. Alright. So after that action potential makes it to the AV node, what do you think happens after that? We said in that AV node, there is going to be that 100 millisecond delay. We sort of put that slow down, that pause on things to give a chance for those atria to fully contract before we pass the message on to the ventricles. So I think that's gonna come next. I'm gonna put c here, and I'll cross out c up here because I know I've put it down there already. All right. After that 100 millisecond delay, what's next? Take a look. Well, from the a v node, the action potential goes down the a v bundle. That a v bundle conducting fibers in the septum, that's where it goes next. So I'm gonna put g on this line here, and I'll cross out g up here because I've now done that one. Alright. As it moves down the a v bundle, what's the next part that it hits? Well, here we see a says the action potential moves down the right and left bundle branches. Remember that bundle splits in the septum into 2 branches, the right and left bundle branches. That's what comes up next. So I'm going to put a on my line here I'll cross out a up here. All right. Not many options left after this. All right. So from those right and left bundle branches, where does that action potential go? We got 2 options. What do you think? Well, it goes to the purkinje fibers. The purkinje fibers, those spread though that action potential out all through the ventricles. So I'm gonna put b on my line down here and I'll cross it out up here. And then finally well, we got one more step. We got one more option. Let's see if it makes sense. From the Purkinje fibers, the action potential is passed through the contractile cells of the ventricles. That sounds correct. So I'm gonna put f on my line. I'll cross it out up here. And is that right? After the action potential moves through those contractile cells, well, the ventricles contract. Looks like we did it. Alright. More practice problems to follow. Let's give them a try.
10
Problem
Problem
The AV node has fewer gap junctions than the SA node, leading to slower conduction. How does this slower conduction help the heart function?
A
Ensures that the ventricles have enough time to fill with blood before they contract.
B
Initiates the electrical impulse in the heart.
C
Conducts the impulse rapidly to the bundle of His.
D
Allows the ventricles to beat at a slower rate than the atria.
11
Problem
Problem
What is the primary function of the pacemaker cells in the SA node in the heart?
A
Slow the action potentials to allow for a 100 ms delay.
B
Regulation of blood pressure in the right atrium.
C
Rhythmic generation of action potentials.
D
Conduction of action potentials throughout the atria.
12
Problem
Problem
Which structure or structures are most directly responsible for allowing contraction of the ventricles to begin at the apex of the heart rather than in the septum closer to the AV node?
A
Purkinje fibers.
B
Sinoatrial and atrioventricular nodes.
C
AV bundle and the left and right bundle branches.
D
Atrial conducting fibers.
13
concept
Control of Heart Rate
Video duration:
6m
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Video transcript
We've been talking about the intrinsic cardiac conduction system and how that intrinsic cardiac conduction system is able to initiate and spread action potentials through the heart so that the heart contracts. And, of course, it does that set to a rhythm, which is your heart rate. But your heart rate changes, and that's what we want to talk about now. How do we control the heart rate? So we're gonna say here that there's sort of 2 major ways that heart rate is controlled. The first we've been talking about as part of that intrinsic cardiac conduction system, and that's the pacemaker cells. These are intrinsic rhythmic initiation of action potentials. They start the action potentials on their own set to a rhythm, but they don't change the rhythm. What changes the rhythm is gonna be things called chronotropic factors, and we can break down the word chronotropic. Chrono, that means time. Tropic well, the root tropic means to change something. That's like in the endocrine system, we talked about tropic hormones. It's the same thing here. These chronotropic factors are the extrinsic factors that affect heart rate. And we normally think of these things working as either positive or negative chronotropic factors. A positive chronotropic factor or a positive chronotrope will increase heart rate. A negative chronotropic factor or a negative chronotrope will decrease heart rate. Now anything that does that is a chronotropic factor. So there are drugs that are chronotropic factors that may increase or decrease the heart rate, for example. But here we want to think about how is this done by the nervous system, and the way that works is through the medulla oblongata. Medulla oblongata is responsible for chronotropic control of heart rate by the CNS. Now remember, the depolarization and spread of these acts action potentials through the heart, that's intrinsic. That's all happening within the heart. Here, we're gonna talk about how we turn the dial on that rate, how we speed it up or turn it down. Alright. So to do that, we have dual innervation of the heart, and dual innervation is something that you should remember from when you talked about the autonomic nervous system. So that means that there's 2 controls on this that sort of work in opposition, the sympathetic nervous system and the parasympathetic nervous system. And if you remember your sympathetic and parasympathetic nervous systems, you should probably be able to predict generally what they're going to do here. Alright. Before we dive in here, let's just orient ourselves to this image that we have. We have a brain, we see the brain stem here, and medulla oblongata is down here. We have some nervous, some nerve fibers coming down, one through the spinal cord and one more directly to the heart, and we see them innervating at the heart in different places. And again, we'll break down that more specifically in just a second. So let's start with the sympathetic nervous system. The sympathetic nervous system, remember, this is usually associated with, like, your fight or flight response, your sort of get up and go. So I'm going to say that this is going to increase heart rate, and I'll just indicate that with a sort of up arrow there. So the sympathetic nervous system turns up your heart rate. This is controlled by the cardioacceleratory, acceleratory center in the medulla oblongata. And we can follow this down. We follow this nerve fiber down from the medulla oblongata. We see that it goes down through the spinal cord, and then it comes over to the heart, and it actually splits and it innervates in sort of two places. Actually, 3 places, but sort of 2 types of places. We're gonna say that it innervates first at the nodes, the SA node and the AV node. That's how it's going to affect the rate of the heart, but it's also going to innervate with the heart muscle. And it's doing different things in these two places. So as I just said, at the nodes, it's gonna increase the heart rate. And so we can see this nerve fiber come in and innervate with the AV and the SA nodes. So that's going to be a little dial on those nodes that speeds up the heart rate. With a muscle, though, it's going to increase the strength. Spell strength correctly. The strength of the heartbeat, or what we call contractility. Contractility is sort of how much these cells are contracting. So the sympathetic nervous system, it's gonna turn up the dial on the rate. It's gonna get that heart beating faster. But with the muscle cells, it's gonna get them to contract with more force. So it's beating faster and it's beating harder. Now contrast, we have the parasympathetic nervous system. And parasympathetic, that's usually associated with, like, rest and digest. So this is going to decrease or turn down or well, indicate with a down arrow here. It's gonna decrease your heart rate, and this is going to be controlled by the cardioinhibitory. Inhibitory center. I hope I spelled that right. Inhibitory center. And we can see this in the starting in the medulla. It's going to travel down this yellow nerve here, and it's going to innervate in 2 places here on our nodes. So the signal is going to travel down that major nerve of the parasympathetic parasympathetic nervous system, the vagus nerve. Alright? It's gonna travel down the vagus node nerve, and it's gonna innervate in 2 places. It's gonna innervate at the SA node and the AV node, not the heart muscle. So here, this is just turning down that rate. It does not affect contractility. That means that, well, the sympathetic nervous system turns up contractility. So if there aren't any signals from the sympathetic nervous system, that means that that heart muscle just goes sort of back to its default contractility, which is how hard the heart is contracting at your resting heart rate. Alright. So again, to sum this up, sympathetic nervous system, it's gonna turn up the rate and the contractility, how hard the heart beats. The parasympathetic nervous system is just gonna sorta turn down that heart rate and let the contractility go back to its sort of default setting. Alright. With that, we have examples and practice problems to follow. You should give them a try.
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example
Electrical Conduction System of the Heart Example 3
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4m
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Video transcript
Here, it tells us that without any extrinsic factors, the SA node will set a heart rate of about 100 beats per minute. The typical resting heart rate is about 75 beats per minute. And during exercise, heart rates are often in the range of 120 to 150 beats per minute. Knowing this, what would you expect the effect to be if nerves of a, the sympathetic, and b, the parasympathetic nervous system were severed? Consider the effect both on one, resting heart rate, and 2, on heart rate during exercise. So really, we just need to fill in this table down here. In our table, we have 3 columns, the effect of severing, either the sympathetic nerve fibers or the parasympathetic nerve fibers, and then we wanna say what that effect would be on 1, resting heart rate, and 2, on heart rate during exercise. So let's dive in. Sympathetic nerve fibers. If we severed those sympathetic nerve fibers, what effect do you think that would have on resting heart rate? Well, remember, the sympathetic nerve fibers, the sympathetic nervous system, we say, is part of that sort of, fight or flight or that sort of get up and go. So these nerve fibers are going to turn heart rate above that rate that is set intrinsically by the SA node. But the resting heart rate is 75 beats per minute and the intrinsic heart rate is sort of 100 beats per minute, so that's less than. Again, the sympathetic nerve fibers are responsible for turning it above 100 beats per minute, But here we're trying to go lower, so what effect would you would what what would the effect be of severing those sympathetic nerve fibers? I'm gonna say it would have no effect. Alright. But what effect would it be have on the heart rate during exercise? What do you think? Well, again, these sympathetic nerve these nerve fibers are responsible for turning that heart rate up above 100 beats per minute. And during exercise, we're talking about heart rates in the 120 to 150 range often. So if you can't turn it up, what's your max heart rate gonna be? Well, your max heart rate during exerciseI'm gonna say max is gonna be, I'll say, about equal to 100 beats per minute. Without the sympathetic nerve fibers, you can't go above that intrinsic heart rate set by the SA node. Alright. Let's do the same thing now for the parasympathetic nerve fibers. Alright. So what do you think the effect of severing the parasympathetic nerve fibers and maybe that vagus nerve? We're gonna sever that on first on resting heart rate. What do you think? Well, remember, parasympathetic nervous system that's responsible for our rest and digest, so So that's gonna be slowing the heart rate down, and sure enough, at rest, your heart rate is about 75 beats per minute. That's lower than 100. So it's these parasympathetic nerve fibers that are sorta setting your resting heart rate. So at rest, if you were to sever those parasympathetic nervous fibers, I'm gonna say resting heart rate will then equal about 100 beats per minute. If you sever the parasympathetic nerve fibers, you can't go below that sort of intrinsic heart rate set by the SA node, so you're stuck at 100. Now what effect would that have on heart rate during exercise? Well, again, heart rate during exercise, we need to turn it up. The parasympathetic nerve fibers do not turn things up, but turn it down. So what effect would it have on the heart rate during exercise? I'm gonna say no effect. Alright. Important things to remember here. That intrinsic heart rate, that is set by the SA node. The heart does not need stimulation to depolarize. It will do it all on its own. But the sympathetic nervous system and the parasympathetic nervous system, they're the dials that turn that heart rate up and turn it down. Alright. More practice problems after this. You should give them a try.
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Problem
Problem
Which center in the medulla oblongata controls the sympathetic neurons that stimulate the heart?
A
Cardioinhibitory center.
B
Cardiorespiratory center.
C
Vagus center.
D
Cardioacceleratory center.
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Problem
Problem
Which statement best describes a difference between how the sympathetic and parasympathetic nervous system affects the heart?
A
The sympathetic nervous system affects heart rate, while the parasympathetic nervous system affects contractility.
B
The sympathetic nervous system affects contractility and heart rate, while the parasympathetic only affects heart rate.
C
Both the sympathetic and parasympathetic nervous systems affect contractility, while the parasympathetic also affects heart rate.
D
Heart rate is controlled by the parasympathetic nervous system, while the sympathetic nervous system controls contractility.
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