Here is a diagram of the four cavities of the heart. To begin with, let's name them. We have the right atrium up here. Down here is the right ventricle. We have the left atrium and the left ventricle. These are the four cavities. And the blood will flow through all of them, and then it will come out to the body. To do this and do it right, the heart must coordinate its contractions.
And we know that way to shorten is that you have a cell and this cell is usually negatively charged. And at some point it will become more positively charged. And we call this process depolarization. Depolarization is the process of transition from negative membrane potential to a much more positive value. Depolarization is when a muscle cell can be shortened. Where does this start? Let's draw it in our diagram. If you look, there's an area here, in which small cells can depolarize on their own. And it's pretty unique because most of the cells in the body will depolarize when an adjacent cell depolarizes.
And these are very unique cells because they depolarize completely independently. We call this area the sinoatrial node, sometimes called the CA node. The fact that they can to depolarize themselves - we have a word for that. We call it automatism. It just means they can automatically to depolarize without the need for an adjacent cell to do so first. What happens after depolarizing? When these cells depolarize, they are connected by slit contacts with neighboring muscle cells, they will start sending waves of depolarization in all directions.
And it's almost going to be like a football game when the Mexican wave begins and just keep going. And all the neighboring cells they will also begin to depolarize. And this orange arrow is moving slowly. This depolarization wave is moving slowly, compared to what the speed would be if it passed through specialized tissue. This fabric that I draw, this blue thread is almost like a highway, compared to the orange arrow, which is like a small road.
And this highway will catch the same depolarization wave and will carry it to the other side, to the left atrium and all these cells start doing the same thing. They also begin to depolarize. You have depolarization both in the right atrium and in the left atrium, in a coordinated manner. It happens very evenly.
And this bundle is called Bachmann's bundle. It is like a bundle of fabric. It's called Bachmann's bundle. We named two things - the sino-atrial node, as well as Bachmann's bundle. And, just like Bachmann's bundle, we have a few more small strips of fabric, almost like highways that take that signal and transfer it down to another node called the atrioventricular node.
This here is the atrioventricular node. And the atrioventricular node is the only main connection - I don't even have to say the only main one - the only connection for most of us between the atria and the ventricles. Atrioventricular node. And it actually is sometimes called an AV node. The AV node will receive this signal. And I didn't even tell you what this signal went through. He went through an internodal ... which means between two nodes, internodal pathways.
And this is the name of all three of them. The signal passed from the CA node through the internodal pathways, down to the AV node. And something interesting happens. If you step back and look at the AV node, let's imagine that we are focused on what exactly is going on here. And to find out what's going on here, I will give you a small script. Let's say you have a time axis here. And on this time axis are say, 1, 2, 3 seconds - three seconds.
And your job is just to look at the atria and see how they shrink. You're just looking at the atria and you see an acronym here and an acronym here, and an acronym here. The atria, as they receive their wave of depolarization, are shortened three times in three seconds. We saw three cuts for the atria. You do the same thing but for the ventricles.
You look at the ventricles and you watch exactly what happens and you notice that there is a contraction of the ventricles here and here again, and another here. Both the atria and the ventricles are reduced by the same number of times. But the unique thing is, that there is a slight delay between the two. They do not shrink at the same time.
There is a slight delay. And if you measure it, it will be about 0.1 seconds. Just a fraction of a second. But the reason for the delay is the AV node. One interesting thing about the AV node is, that it creates a slight delay between atrial and ventricular contractions. And the reason this is important is that if the atria and ventricles were reduced at the same time, they would pump blood against each other. They would do work, which does not move the blood in the right direction.
By creating the delay, the atria can be shortened, blood can pass from the atria to the ventricles. And then, 1/10 of a second later, the ventricles can be shortened. And the ventricles can move that blood forward. The reason for the delay is to provide coordinated movement of blood through the heart. The signal was delayed by 1/10 of a second. But then it moves on. He continues and goes to an area here.
And this is called a bundle of His. Funny names, I know. A bundle of His. And when you say that, Hiss, sounds almost like a snake hissing. And then it continues from Heath's bundle once down here. And this is considered the right bundle. And continues through the left bundle. And the left bundle splits. There is a front road, which goes to the front, and the part that goes back. And I will draw the back with a dotted line.
This is called the left posterior bundle. And it's called the left anterior bundle. And you can imagine, that it goes back and forth, in two dimensions it is difficult to show this. And this is called the right bundle. And so you don't get confused, this part here is called the left bundle, when combined and not split into two bundles.
You have the left and right bundles. And the left bundle splits. And all the threads are separated at the end. And they are called Purkinje's network. And this is happening on both sides. And from this point, the electrical signal can spread in all directions. Finally, all muscle cells are involved. So far, only those from the excitation-conduction system have participated, that is, these small highways. But now the waves of depolarization pass through all the small roads.
And I use the idea of roads and highways, to highlight the idea that through the excitation-conduction system the signal is moving very fast. And when you get to the muscle itself, the signal moves a little slower. But you can see that's important because this is the only way all muscle cells to participate simultaneously.
This is how the electrical signal travels from the CA-node through the excitation-conduction system so that the atria are shortened at the same time, and then goes to the AV node, where there is a slight delay, and then down the ventricular pathways and the ventricles will contract at the same time.
Learn more about Electrical system of the heart by the Expert Electrophysiologist - Dr Boon Lim
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