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.
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18. The Heart
Electrical Conduction System of the Heart
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