The heart beats in response to electrical signals we call action potentials. We'll learn more about these in the chapter on the nervous system. For now, just know that these are electrical signals that are generated by moving ions across the membrane of cells. Now, action potentials in the heart are not transmitted by nerves, like we'll see throughout the nervous system. These action potentials actually move between the cells of the heart through what are called gap junctions. Gap junctions, which you can see right here, are basically direct cell to cell connections, and there are these channels between the cells that ions can flow through; these channels are referred to as connections. You don't need to worry about memorizing any of this; just know that the action potentials in heart cells are moving cell to cell through gap junctions. In fact, there's actually a specialized structure in heart muscle that connects neighboring cells and contains these gap junctions. We call those intercalated discs. And here is an example of some heart tissue, and if you zoom in you can see some intercalated discs between the cells. Now, here we actually have a recording of an action potential, and hopefully, this looks a little familiar to you. Right? That "bloop bloop bloop," that line that you see on the heart rate monitor, right, the thing in the hospital that we just looked at looks a heck of a lot like that. Right? It is in fact measuring what is known as the electric potential, something measured in volts; it's a type of voltage. You don't need to worry about remembering any of this. I just want you to see the similarity between this standard image of an action potential and what we see up here on the heart rate monitor because they're basically measuring the same thing.

Now, how do these action potentials get generated? Well, there is a special part of the heart, a group of cells in the right atrium that are referred to as the sinoatrial node. And basically, these cells are going to be responsible for initiating heart contractions in vertebrates. There's actually a group of cells there that are usually referred to as pacemaker cells, and these are the cells that will control the rate and timing of heartbeats, and they are going to actually start the action potential. They're going to be the initiator of the action potential. Now from the SA node, as it's often referred to, the action potential is going to propagate to what's called the Atrioventricular Node, which is a group of cells that is sort of, almost like the center of the heart. The AV node being pointed out here, so our SA node was up here, the action potential moves down to the AV node.

And at the AV node, something interesting happens. See that signal, that electrical signal is delayed. And the reason for that is we want the atria to have a little extra time to completely empty its blood into the ventricles. So, by having a slight delay in the signal at the AV node, before it moves down into the ventricles, we actually give the atria the time it needs to push all the blood it has out into the ventricles, making the work of the ventricles' contraction more efficient. Now, when this signal goes down into the ventricles, something kind of interesting happens. So, from the AV node, it actually goes down to basically the bottom of the ventricles. And from there, it's propagated up through the ventricles, through these fibers called Purkinje fibers, or Purkinje fibers. You may hear it pronounced, depending on whether you give it a soft or hard 'j'. Anyway, these Purkinje fibers will actually spread the action potential up through the ventricles from the bottom to the top.

Now, the reason for this is the arteries have their openings located kind of at the top of the ventricles. Let me use a different color here, make these arteries like red. So, by having the ventricles start their contraction from the bottom and move it up, you're actually pushing the blood up into these arteries. That's the reason for it. Now, this whole process, this whole electrical process is recorded in what's called an electrocardiogram. Sometimes it's abbreviated EKG, if you're wondering why it's not EKC. EKG comes from the German word, so, you know, kind of confusing there, but it's the same thing, EKG is an electrocardiogram. So this is going to record the electrical activity of the heart, and it's going to be output looking like this. So at the start here we have our SA node, that is going to lead, our SA node is going to initiate the action potential. This is going to be the big contraction of the ventricles, And here we have the relaxation of the muscles. So you don't need to worry about memorizing the different parts of the EKG signal; just wanted to kind of show you how these electrical signals correspond to the phenomena that we've just been talking about. So with that, let's actually flip the page.