Electrical Conduction System of the Heart: Study with Video Lessons, Practice Problems & Examples

The intrinsic cardiac conduction system initiates and conducts action potentials, ensuring coordinated heart contractions. Key components include the sinoatrial (SA) node, which acts as the primary pacemaker, and the atrioventricular (AV) node, which regulates ventricular contractions. Action potentials spread through specialized conducting fibers, including the AV bundle and Purkinje fibers, facilitating efficient blood flow. Heart rate is modulated by intrinsic pacemaker cells and extrinsic factors from the autonomic nervous system, with sympathetic stimulation increasing heart rate and contractility, while parasympathetic activity decreases it.

Intrinsic Cardiac Conduction System

As we think about the physiology of the heart, we want to consider 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 going to go into the details of the anatomy and physiology, following an action potential throughout the heart, and seeing how that leads to a coordinated contraction. Alright. But first, let's define it. The intrinsic cardiac conduction system, we're going to say, initiates contraction 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 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 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 receive action potentials from the nervous system telling the muscle to contract, and we're going to 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, start 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 going to 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 going to 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 going to spread very, very fast, and you're going to 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 going to 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 potentials. So this is where the action potentials start. So we have a couple of places, 2 of them, the sinoatrial node, which sort of starts all of the 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 action potentials out. So once these action potentials start, well, then they spread through 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 going to 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 it at regular intervals, it's going to 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 going to 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 contraction. I think it's going to be a heck of a good time. I'll see you there.

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.

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.

Anatomy of the Intrinsic Cardiac Conduction System

We want to spend some more time talking about this intrinsic cardiac conduction system, and we're going to start talking about the anatomy of the system. We're going to 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 going to 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 going to 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 signals. 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 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 going to go, we're highlighting here in orange, and we have it labeled B traveling through 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 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 going to 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 going to 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 going to 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 are 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 backup 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 going to 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 His 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 AV 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 going to 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 going to 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 point. So we're going to 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.

Electrical Conduction System of the Heart Example 1

This example says that for each structure in the cardiac conduction system, identify in which region of the heart wall it is found by writing the location in the space provided. Then answer the question below. Note: Some structures may span multiple regions. Alright. We have 5 heart wall regions here: the right atrium, the left atrium, the right ventricle, the left ventricle, and the septum. We have 6 structures here, all of which we've discussed previously, so we really just have to remember where they are.

Alright. We'll start with 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. It's near the top of the right atrium, right under the entrance of the vena cava in that right atrium wall. Remember the sinoatrial node or the SA node initiates the action potential, which then spreads out through the heart and causes the heart to contract.

Next, we have the atrioventricular node, also called the AV node. Where is that AV node located? Remember, the atrioventricular node 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 at the base of the atrial wall there, kind of sitting on top of the septum a little bit.

As we continue, then we have the AV bundle, also sometimes called the bundle of His. Where is that AV bundle? We mentioned that the action potential goes down the septum through that AV bundle. So, this AV bundle is going to be in the septum.

Next up, we have the right bundle branch and the left bundle branch. Where are those right and left bundle branches located? Well, they have the names right and left in them, but these are still in the septum. Remember that the AV bundle goes down, splits into the right and left branch, and those right and left branches continue all the way down the septum to the apex of the heart.

Finally, we have the subendocardial conducting network, also sometimes called the Purkinje fibers. Where are those Purkinje fibers located? From the apex of the heart, those Purkinje fibers spread out, and they conduct that signal through the ventricular walls. So, we're going to say that these are in both the left and right ventricles.

The question here asks: 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. What do you think allows them to spread that action potential from one to another? Well, that's going to be the gap junctions. Remember, these cardiac cells are branching muscle cells that are attached to each other and have gap junctions. So when an action potential stimulates one, it spreads from one to another like a wave through this cardiac muscle.

Note that this is very different from skeletal muscle, where each individual muscle fiber or muscle cell needs to be stimulated, and the action potential does not spread from cell to cell. In cardiac muscle, you just need to stimulate some or a subset of this cardiac muscle, and the action potential then spreads like a wave from cell to cell because of those gap junctions.

With that, we've finished our example. We've got more practice problems after this. I'll see you there.

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.

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.

Conduction Pathway and Contraction

We're going to continue talking about the intrinsic cardiac conduction system. Previously, we looked at the structures of this conduction system. Now, we want to follow the conduction pathway and see how it actually leads to contraction in more detail. To orient ourselves, we have this image of the heart with the conduction system laid over it. Before we dive into it, I want to set up two main goals that this conduction system is trying to achieve. First, we want to pump blood properly, which requires the atria to contract first. The atria need to squeeze the blood downwards towards the ventricles. Secondly, the ventricles need to contract. The ventricles wait for the atria to pump blood into them and then contract upwards. The exit to the heart, the aorta, and pulmonary artery are at the top. So, the blood needs to move down first and then get pushed up.

To make this happen, there's an important structural feature in the heart. It's stated that connecting all these cells are gap junctions, and that's how the action potential gets passed from cell to cell. Importantly, there are no gap junctions between the cardiomyocytes of the atria and ventricles, keeping the contractions separate. When considering the spread of action potentials through the heart, we can visualize a dividing line. The action potentials spread through the atria but don’t cross over to the ventricles, necessitating the use of the conduction system to pass the signals from one to the other.

Now looking at the heart more closely, we see the cardiac conduction system outlined in yellow. The conduction starts with the pacemaker cells in the SA node, labeled 1, which I'll highlight in pink. These pacemaker cells initiate the action potential, which then spreads out through the atria, labeled 2, highlighted here. The action potential travels across the atria through the conducting fibers and contractile cells. These fibers spread the signal from the SA node down to the AV node while also connecting to the muscle cells that squeeze.

Once these cells are stimulated, they pass the action potential downwards as the heart squeezes. The action potential then reaches the AV node, labeled 3, highlighted in yellow. This node is responsible for transmitting the signal to the ventricles. The AV node introduces a delay of approximately 100 milliseconds to ensure that the chambers' contractions remain separate. Given a normal heart rate of 75 beats per minute, this pause represents a significant fraction of the cardiac cycle.

After the pause, the action potential moves down the septum through the AV bundle (or bundle of His) and through the left and right bundle branches, shown with arrows in green and blue. These are fast-conducting fibers that rapidly transmit the action potential to the apex of the heart. None of the contractile cells in the ventricles have begun contracting yet; we're just moving the action potential. As it reaches the apex, the signal spreads out through the subendocardial conducting network, the Purkinje fibers, labeled 5. This spread stimulates the contractile cells starting at the apex, and the action potential gets passed cell to cell in the ventricles. The ventricles contract from the apex upward, achieving our goal of an upward contraction.

We have successfully passed the action potential through the heart, having the atria contract first and the ventricles second. More practice problems to follow. I'll see you there.

Electrical Conduction System of the Heart Example 2

Our example says that the steps of electrical conduction in the heart are listed below in the incorrect order. We want to 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 initiating an action potential, and we see 7 steps leading all the way to the ventricles contracting. Alright. So take a second, look at those 7 steps, and think which of those 7 steps is going to come after the pacemaker cells initiate that action potential. Alright. Well, the action potential is going to be initiated in the SA node, the sinoatrial node. And then from there, the action potential is going to 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, the action potential is passed through the atria. That's what I think is going to come next. So I'm going to 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 are even conducting fibers that get that action potential there very, very fast. So I think I'm going to 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 slowdown, 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 going to come next. I'm going to 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 AV node, the action potential goes down the AV bundle. That AV bundle, conducting fibers in the septum, that's where it goes next. So I'm going to 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 AV 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 out that action potential all through the ventricles. So I'm going to put b on my line down here and I'll cross it out up here. And then finally, 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 going to 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.

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.

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.

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.

Control of Heart Rate

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 going to say here that there are 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 going to 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 this is done by the nervous system, and the way that works is through the medulla oblongata. The medulla oblongata is responsible for chronotropic control of heart rate by the CNS. Now remember, the depolarization and spread of these action potentials through the heart, that's intrinsic. That's all happening within the heart. Here, we're going to 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 are 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 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 center in the medulla oblongata. And we can 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 going to 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 going to 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 the muscle, though, it's going to increase 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 going to turn up the dial on the rate. It's going to get that heart beating faster. But with the muscle cells, it's going to 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 going to decrease your heart rate, and this is going to be controlled by the cardioinhibitory 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 nervous system, the vagus nerve. It's going to travel down the vagus nerve, and it's going to innervate in 2 places. It's going to 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 going to turn up the rate and the contractility, how hard the heart beats. The parasympathetic nervous system is just going to sort of turn down that heart rate and let the contractility go back to its sort of default setting. With that, we have examples and practice problems to follow. You should give them a try.

Electrical Conduction System of the Heart Example 3

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 want to 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 exercise I'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 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.

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.

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.

Here’s what students ask on this topic:

What is the intrinsic cardiac conduction system and how does it function?

The intrinsic cardiac conduction system is a network of specialized cardiac muscle cells that initiate and conduct electrical impulses, ensuring the heart contracts in a coordinated manner. Key components include the sinoatrial (SA) node, which acts as the primary pacemaker, and the atrioventricular (AV) node, which regulates ventricular contractions. Action potentials generated by the SA node spread through the atria via internodal pathways, reach the AV node, and then travel down the AV bundle (bundle of His) and Purkinje fibers to the ventricles. This system allows the heart to beat independently of the nervous system, maintaining a regular and coordinated heartbeat.

How do pacemaker cells regulate the heart's rhythm?

Pacemaker cells, primarily located in the sinoatrial (SA) node, regulate the heart's rhythm by spontaneously depolarizing at regular intervals. This depolarization initiates action potentials that spread through the heart, causing it to contract. These cells have a unique ability to generate electrical impulses without external stimuli, thanks to their unstable resting membrane potential. The regular depolarization of pacemaker cells ensures a consistent heart rate, which can be modulated by the autonomic nervous system. Sympathetic stimulation increases the rate, while parasympathetic stimulation decreases it.

What role do the SA node and AV node play in heart contractions?

The sinoatrial (SA) node and atrioventricular (AV) node play crucial roles in heart contractions. The SA node, located in the right atrium, acts as the primary pacemaker, initiating action potentials that cause the atria to contract. These impulses then travel to the AV node, located at the junction of the atria and ventricles. The AV node delays the impulse slightly, ensuring the atria have time to fully contract and empty blood into the ventricles before they contract. This delay is essential for efficient blood flow and coordinated heart function.

How does the autonomic nervous system influence heart rate?

The autonomic nervous system (ANS) influences heart rate through its sympathetic and parasympathetic branches. The sympathetic nervous system, via the cardioacceleratory center in the medulla oblongata, increases heart rate and contractility by releasing norepinephrine, which acts on the SA and AV nodes and the heart muscle. Conversely, the parasympathetic nervous system, through the cardioinhibitory center and the vagus nerve, decreases heart rate by releasing acetylcholine, which primarily affects the SA and AV nodes. This dual innervation allows the ANS to finely tune heart rate in response to the body's needs.

What are Purkinje fibers and their function in the heart?

Purkinje fibers are specialized conducting fibers located in the subendocardial layer of the heart's ventricles. They are part of the intrinsic cardiac conduction system and play a crucial role in ensuring efficient ventricular contractions. After the action potential travels down the AV bundle and bundle branches, it reaches the Purkinje fibers, which rapidly distribute the electrical impulse throughout the ventricles. This rapid conduction ensures that the ventricles contract in a coordinated manner, starting from the apex and moving upwards, effectively pumping blood out of the heart.