Understanding the Cardiac Conducting System

Since muscle tissue needs an electrical impulse, an electrical signal, to cause those muscle cells to contract, the electrical system in the heart that causes the heart muscle to contract itself is known as the cardiac conducting system. So the heart has its own built-in electrical system. that produces its own electrical impulses that signals its own contracting cells to contract. It can be influenced from outside the heart, for example, nervous system, hormones, things like that. It can influence the cardiac conducting system. But even without a lot of that external influence, the heart has its own system for generating its own electrical impulses. So it's made up of special kinds of cardiac muscle cells. There are actually several different types of cardiac muscle cells. It's not just the contracting type. So the two major types of cardiac muscle cells are known as autorhythmic cells and contractile cells. Autorhythmic cells are cardiac muscle cells that are actually kind of control, regulate, coordinate the heartbeat itself through this electrical activity. The contractile cells, the type of cardiac muscle cells that actually contract, to produce the force needed to propel the blood through and out of the heart. Let's first start with the autorhythmic cells. So the term autorhythmicity refers to the ability of the heart to to control itself, to contract on its own, without the nervous system or hormones making it happen. Again, the nervous system and the endocrine system can influence it, speed up, slow it down, can affect it. But even without that intervention, due to autorhythmicity, the heart is able to generate its own electrical impulses and beat on its own. There are two types of autorhythmic cells that generate and distribute these electrical impulses that cause the heart to beat. The first type are what we call pacemaker cells. Pacemaker cells are the type of autorhythmic cells that actually generate or make the electrical impulses. But they don't. they don't distribute or spread those electrical impulses. That job goes to the conducting cells. So the pacemaker cells generate or make the electrical impulses and the job of the conducting cells are then to distribute or spread those electrical impulses throughout the heart causing those contractile cells to then contract. There are different clusters of these types of cells in the heart. So the pacemaker cells are located in two areas. One area is called the SA node or sinoatrial node. The other area is the AV node or atrioventricular node. Notice that the atrioventricular AV node is located roughly between the right atrium and the right ventricle. So that's why it's an atrioventricular node. The SA node, sinoatrial node. You can think about it being kind of close to the coronary sinus, so sinoatrial node. It's a node in the atrium kind of by the coronary sinus where blood comes into the right atrium from the coronary circulation. The conducting cells that then distribute the signals from the SA and AV nodes are the internodal pathways, the AV bundle, bundle branches, and the Purkinje fibers. So starting here, the SA node is where it begins. The SA node is on the the back side of the right atrium. This is the main, most important part of the heart of the conducting system that actually will cause the heart to beat. So this is the the big important one that starts the heartbeat, the SA node. This is so important it is often referred to as the cardiac pacemaker or just the pacemaker of the heart. It sets the rhythm or the pace of the heartbeat, the SA node. So the SA node creates the electrical impulses and then those electrical impulses spread from the SA node through these purple pathways called the internodal pathways. We call them internodal pathways because inter means between. So these pathways here are the pathways in between the two nodes, SA and AV node. SA node fires, or I'm sorry, SA node creates the electrical impulse. We say it fires the electrical impulse. It then spreads through these internodal pathways. That causes the atria to contract, and then the signals end up at the atrioventricular node, AV node. The AV node there, once it gets signaled through the SA node and internodal pathways, it delays and then it fires its own electrical impulse that then spreads after that to the rest of the heart. So the way that these nodes work is through depolarizations that cause the generation of an action potential just like you learned in neurons. Remember in neurons, right, you get these depolarizations that reach threshold. So if you look here, okay, We're looking at the membrane potential of these cells. So we're at below threshold, so we're at rest. And then the membrane depolarizes, it becomes less negative until you reach a certain membrane potential known as threshold. And once the membrane potential depolarizes, it becomes less negative up to that threshold value. Then that causes the firing of an action potential. And then the action potential then... propagates or spreads through this conducting system from cell to cell to cell, just like with neurons. One difference is that these cells here, these pacemaker cells, they're on a timer. So the depolarization is automatic. They don't need to be signaled by another cell or neurotransmitters or anything like that. They will just automatically on their own depolarize gradually. It's... it's fairly slow but continuous and steady and once they reach threshold they fire an action potential that you know spreads and so it repolarizes and after it repolarizes it starts that another gradual depolarization until we reach threshold again and it fires another action potential so if you look at when we reach threshold threshold action potential okay that's going to trigger one heartbeat And then it starts depolarizing again, we reach threshold, fires an action potential which spreads to the heart, another heartbeat. Depolarization, reach threshold, fire again, action potential, spreads to the heart, another heartbeat. Now both the SA node and AV node have these pacemaker cells, but the pacemaker cells in the SA node fire faster, they're closer together, they fire faster than the AV node. So the SA node fires anywhere roughly 60 to 100 times per minute. The AV node only fires 40 to 60 times per minute. So really what happens is because the SA node fires faster and then it hits the AV node, making the AV node fire, the SA node is really the node that sets the heartbeat, the rate. your heart rate. It sets your heart rate. So we call this the sinus rhythm or your heart rhythm, basically. The electrical activity that causes your heart to beat every second or so. Firing those electrical impulses every second or a little bit more than a second, that's called the sinus rhythm or your heart rhythm. Now again, SA node fires. those signals, those electrical impulses spread through the internodal pathways to the AV node. So atria have contracted, then the signals hit the AV node. The AV node delays, it then fires its own electrical impulses, and those electrical impulses spread from the AV node through this AV bundle right here. These are conducting cells here that form this. And then the AV bundle splits, it forks, into the bundle branches. We need two of these because we have two ventricles. Those bundle branches continue down that interventricular septum between the two ventricles. And then they have branches that are referred to as the Purkinje fibers. So, SA node. Internodal Pathways, AV Node, Bundle Branches, AV bundle, bundle branches, Purkinje fibers. And these Purkinje fibers that are branching off of the bundle branches, they will spread out around both ventricles. And this activity, these signals, are what cause the ventricles to contract. So if we measure the electrical activity, these impulses are spreading over someone's heart, we can actually do this. The machine that does this is called an ECG. So ECG, EKG is another name for it because the the German word for cardio starts with the K. So either ECG or EKG, doesn't matter which one, both of them refer to the same thing. This is measuring this electrical activity as it spreads through the conducting system of your heart. So here's the SA node. Again, it's going to fire at 60 to 100 times per minute. And right before it fires, it hasn't yet fired through those internodal pathways. So the electrical activity is pretty flat. As soon as it fires, those signals travel over the surface of the atria, and that creates this first bump in the ECG. We call it the P wave. And the P wave is showing the depolarization over the atria. Okay, so we're firing, right? Those electrical impulses are firing, they're depolarizing, they're spreading over the atria. And that creates this first blip in the ECG, the P wave, for atrial depolarization as the signals spread over the walls of the atria. The electrical impulses over the atria are what cause the atria to contract, but then they reach the AV node. Because the AV node delays for a split second, then the electrical activity flatlines again. This is called the PR interval. This is where we're delaying in the AV node, and just at the very beginning, of the AV node firing just here at the AV bundle because it's not really a big strong enough signal to be to get a measurement on an ECG. Once the electrical activity passes down into that interventricular septum, it's a lot more area of the heart that's electrically active and that's something that we we pick up right here we call it the Q wave. So the Q wave signals the very beginning of the ventricle depolarization. So the conducting cells here in the ventricles, they are depolarizing. They're starting to fire their electrical impulses, and it's starting to spread rapidly down that septum of the interventricular septum. So Q wave is the beginning of this ventricular depolarization. And then we get... There's this big sweeping wave of electrical activity that starts at the apex, the bottom of the ventricles and spreads upward. This is such a strong signal because now the electrical impulses are spreading through those Purkinje fibers and there are a lot of them that cover the ventricles. So we get this big burst of electrical activity called the QRS complex. That's the spike here in the ECG. And this is where we finish up the rest of the ventricular depolarization and the ventricles contract. Once that is finished, the signal ends and then we have a T wave here, this last little hump at the end of the ECG. During the T wave, this is when the cells that contract in the ventricles, those ventricular contractile cells, they are repolarizing or resetting. getting ready for another round of contraction. So here are a bunch of leads hooked up to a patient's chest for an ECG. So we take various leads and put them on the chest in different positions so we can best measure this electrical activity of the cardiac conducting system. And you can see here, all right, this is the readout of the ECG.

It can influence the cardiac conducting system. But even without a lot of that external influence, the heart has its own system for generating its own electrical impulses. So it's made up of special kinds of cardiac muscle cells.

There are actually several different types of cardiac muscle cells. It's not just the contracting type. So the two major types of cardiac muscle cells are known as autorhythmic cells and contractile cells.

Autorhythmic cells are cardiac muscle cells that are actually kind of control, regulate, coordinate the heartbeat itself through this electrical activity. The contractile cells, the type of cardiac muscle cells that actually contract, to produce the force needed to propel the blood through and out of the heart. Let's first start with the autorhythmic cells. So the term autorhythmicity refers to the ability of the heart to to control itself, to contract on its own, without the nervous system or hormones making it happen.

Again, the nervous system and the endocrine system can influence it, speed up, slow it down, can affect it. But even without that intervention, due to autorhythmicity, the heart is able to generate its own electrical impulses and beat on its own. There are two types of autorhythmic cells that generate and distribute these electrical impulses that cause the heart to beat. The first type are what we call pacemaker cells.

Pacemaker cells are the type of autorhythmic cells that actually generate or make the electrical impulses. But they don't. they don't distribute or spread those electrical impulses.

That job goes to the conducting cells. So the pacemaker cells generate or make the electrical impulses and the job of the conducting cells are then to distribute or spread those electrical impulses throughout the heart causing those contractile cells to then contract. There are different clusters of these types of cells in the heart. So the pacemaker cells are located in two areas.

One area is called the SA node or sinoatrial node. The other area is the AV node or atrioventricular node. Notice that the atrioventricular AV node is located roughly between the right atrium and the right ventricle.

So that's why it's an atrioventricular node. The SA node, sinoatrial node. You can think about it being kind of close to the coronary sinus, so sinoatrial node. It's a node in the atrium kind of by the coronary sinus where blood comes into the right atrium from the coronary circulation. The conducting cells that then distribute the signals from the SA and AV nodes are the internodal pathways, the AV bundle, bundle branches, and the Purkinje fibers.

So starting here, the SA node is where it begins. The SA node is on the the back side of the right atrium. This is the main, most important part of the heart of the conducting system that actually will cause the heart to beat. So this is the the big important one that starts the heartbeat, the SA node. This is so important it is often referred to as the cardiac pacemaker or just the pacemaker of the heart.

It sets the rhythm or the pace of the heartbeat, the SA node. So the SA node creates the electrical impulses and then those electrical impulses spread from the SA node through these purple pathways called the internodal pathways. We call them internodal pathways because inter means between.

So these pathways here are the pathways in between the two nodes, SA and AV node. SA node fires, or I'm sorry, SA node creates the electrical impulse. We say it fires the electrical impulse. It then spreads through these internodal pathways. That causes the atria to contract, and then the signals end up at the atrioventricular node, AV node.

The AV node there, once it gets signaled through the SA node and internodal pathways, it delays and then it fires its own electrical impulse that then spreads after that to the rest of the heart. So the way that these nodes work is through depolarizations that cause the generation of an action potential just like you learned in neurons. Remember in neurons, right, you get these depolarizations that reach threshold. So if you look here, okay, We're looking at the membrane potential of these cells. So we're at below threshold, so we're at rest.

And then the membrane depolarizes, it becomes less negative until you reach a certain membrane potential known as threshold. And once the membrane potential depolarizes, it becomes less negative up to that threshold value. Then that causes the firing of an action potential.

And then the action potential then... propagates or spreads through this conducting system from cell to cell to cell, just like with neurons. One difference is that these cells here, these pacemaker cells, they're on a timer. So the depolarization is automatic.

They don't need to be signaled by another cell or neurotransmitters or anything like that. They will just automatically on their own depolarize gradually. It's...

it's fairly slow but continuous and steady and once they reach threshold they fire an action potential that you know spreads and so it repolarizes and after it repolarizes it starts that another gradual depolarization until we reach threshold again and it fires another action potential so if you look at when we reach threshold threshold action potential okay that's going to trigger one heartbeat And then it starts depolarizing again, we reach threshold, fires an action potential which spreads to the heart, another heartbeat. Depolarization, reach threshold, fire again, action potential, spreads to the heart, another heartbeat. Now both the SA node and AV node have these pacemaker cells, but the pacemaker cells in the SA node fire faster, they're closer together, they fire faster than the AV node.

So the SA node fires anywhere roughly 60 to 100 times per minute. The AV node only fires 40 to 60 times per minute. So really what happens is because the SA node fires faster and then it hits the AV node, making the AV node fire, the SA node is really the node that sets the heartbeat, the rate.

your heart rate. It sets your heart rate. So we call this the sinus rhythm or your heart rhythm, basically. The electrical activity that causes your heart to beat every second or so. Firing those electrical impulses every second or a little bit more than a second, that's called the sinus rhythm or your heart rhythm.

Now again, SA node fires. those signals, those electrical impulses spread through the internodal pathways to the AV node. So atria have contracted, then the signals hit the AV node. The AV node delays, it then fires its own electrical impulses, and those electrical impulses spread from the AV node through this AV bundle right here. These are conducting cells here that form this.

And then the AV bundle splits, it forks, into the bundle branches. We need two of these because we have two ventricles. Those bundle branches continue down that interventricular septum between the two ventricles. And then they have branches that are referred to as the Purkinje fibers. So, SA node.

Internodal Pathways, AV Node, Bundle Branches, AV bundle, bundle branches, Purkinje fibers. And these Purkinje fibers that are branching off of the bundle branches, they will spread out around both ventricles. And this activity, these signals, are what cause the ventricles to contract.

So if we measure the electrical activity, these impulses are spreading over someone's heart, we can actually do this. The machine that does this is called an ECG. So ECG, EKG is another name for it because the the German word for cardio starts with the K. So either ECG or EKG, doesn't matter which one, both of them refer to the same thing. This is measuring this electrical activity as it spreads through the conducting system of your heart.

So here's the SA node. Again, it's going to fire at 60 to 100 times per minute. And right before it fires, it hasn't yet fired through those internodal pathways.

So the electrical activity is pretty flat. As soon as it fires, those signals travel over the surface of the atria, and that creates this first bump in the ECG. We call it the P wave. And the P wave is showing the depolarization over the atria.

Okay, so we're firing, right? Those electrical impulses are firing, they're depolarizing, they're spreading over the atria. And that creates this first blip in the ECG, the P wave, for atrial depolarization as the signals spread over the walls of the atria. The electrical impulses over the atria are what cause the atria to contract, but then they reach the AV node.

Because the AV node delays for a split second, then the electrical activity flatlines again. This is called the PR interval. This is where we're delaying in the AV node, and just at the very beginning, of the AV node firing just here at the AV bundle because it's not really a big strong enough signal to be to get a measurement on an ECG. Once the electrical activity passes down into that interventricular septum, it's a lot more area of the heart that's electrically active and that's something that we we pick up right here we call it the Q wave.

So the Q wave signals the very beginning of the ventricle depolarization. So the conducting cells here in the ventricles, they are depolarizing. They're starting to fire their electrical impulses, and it's starting to spread rapidly down that septum of the interventricular septum. So Q wave is the beginning of this ventricular depolarization.

And then we get... There's this big sweeping wave of electrical activity that starts at the apex, the bottom of the ventricles and spreads upward. This is such a strong signal because now the electrical impulses are spreading through those Purkinje fibers and there are a lot of them that cover the ventricles. So we get this big burst of electrical activity called the QRS complex.

That's the spike here in the ECG. And this is where we finish up the rest of the ventricular depolarization and the ventricles contract. Once that is finished, the signal ends and then we have a T wave here, this last little hump at the end of the ECG. During the T wave, this is when the cells that contract in the ventricles, those ventricular contractile cells, they are repolarizing or resetting. getting ready for another round of contraction.

So here are a bunch of leads hooked up to a patient's chest for an ECG. So we take various leads and put them on the chest in different positions so we can best measure this electrical activity of the cardiac conducting system. And you can see here, all right, this is the readout of the ECG.

Transcript for:Understanding the Cardiac Conducting System

Transcript for:
Understanding the Cardiac Conducting System