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Here's a diagram of the four chambers of the heart, so let's name them to get started.

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We've got the right atrium up here, we've got the right ventricle down here, we've got the left atrium and the left ventricle.

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These are the four chambers and blood is going to flow through all of them and get out to the body.

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So, to do this and to do this right the heart has got to coordinate how it squeezes.

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We know that the way it squeezes down is you have a cell and that cell is usually negatively charged,

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and it will at some point become more positively charged. We call that process depolarization.

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Depolarization is the idea of going from a negative membrane potential to something much more positive.

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And when you depolarize is when the muscle cell can squeeze down.

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So, where does that begin? Let's actually draw it into our diagram.

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If you were to look there's actually an area here where little cells can actually depolarize by themselves.

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And that's actually quite unique because most of the cells inside the body are going to depolarize when the neighbor, when that neighboring cell depolarizes.

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So, these are actually really unique cells because they actually depolarize by themselves.

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We call that area the sinoatrial node, sometimes called the SA node.

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And the fact that they can actually depolarize by themselves we have a word for that, too; we call that automaticity.

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It just means that they can kind of automatically depolarize without having a neighbor do it first.

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So once they depolarize, what happens after that?

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Well, when these cells depolarize they immediately, they're connected through little gap junctions to the neighboring muscle cells.

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So, they are going to start sending out waves of depolarization in all directions.

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It's almost like going to a football game and watching "The Wave" start, you know, it just goes on and on and on.

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So, all the neighboring cells are going to start depolarizing, as well.

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That orange arrow is moving kind of slowly, right?

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That depolarization wave is moving kind of slowly relative to how fast it could be moving

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if it went through a specialized band of tissue.

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So this band of tissue I'm drawing, this blue band

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is almost like a highway compared to that orange arrow, which is like a little road.

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And that highway is going to take that same depolarization wave over to the other side,

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over to the left atrium and all of these cells begin to do the same thing, they start depolarizing, as well.

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So, you get depolarizing happening both in the right atrium and the left atrium in a coordinated way.

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It's happening all very evenly.

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This band, or bundle, is called Bachmann's Bundle. It's like a bundle of tissue, right?

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It's called Bachmann's Bundle.

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So, we've named two things: the sinoatrial node and Bachmann's Bundle.

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And actually just like Bachmann's Bundle there are actually a few more bands of tissue,

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almost like little highways, that take that signal down to another node called the atrioventricular node.

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So this right here is the atrioventricular node.

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And the atrioventricular node is really the only major connection, I shouldn't even say only major,

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only connection in most of us between the atria and the ventricles, the atrioventricular node.

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And this is actually sometimes called the AV node.

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The AV node is gonna get this signal and actually I didn't even tell you what a signal came through.