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.
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18. The Heart
Electrical Conduction System of the Heart
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