 So the electrical conduction system of your heart is how the heart has distributed the 1% of its cells that are auto-rhythmic. The auto-rhythmic cells can generate their own action potentials. So here's the scoop. I'm going to tell you about some anatomical structures that are not, it's not really gross anatomy. Like if we cut open a heart, can we really find these structures? They're embedded in the heart tissue. And I guarantee that we could evaluate the function of the heart tissue and find them. But can we actually see them? Not so much. The first structure is considered the heart's pacemaker. This is the SA node, the sinoatrial node. And it's the pacemaker of the heart. Auto-rhythmic cells have pacemaker potentials. All of them could lead the hearts, could initiate the heart's contraction if they're the first ones to fire. And here's the coolest part. The SA node fires the fastest out of all of our auto-rhythmic cell structures that we're going to talk about in this part of the lecture. The SA node fires the fastest. When it fires, everybody's got to listen. Why do they have to listen? Because they're all connected by tunnels and the action potential, which is the, okay, make something happen now. The action potential gets into the... It's not like a choice. It's not like the other contractile cells can be like, you know what, I'm sick of this bossy SA node. I'm just going to shut my gap junctions for a little while and not listen. The gap junctions are tunnels, and if the SA node says fire, that action potential is going to travel through the whole heart. So if the SA node says fire at, oh, let's say 90 beats per minute, then the heart is going to respond, and everybody's going to respond and beat 90 beats per minute. The SA node is not acting alone, and if it was acting alone, your heart would beat in an extremely inefficient and unorganized manner. So there are structures called inter-nodal pathways, and I'm drawing all of this in the right atrium, and that's because my two nodes, I'm just going to have to bear with me here. This was my SA node up at the top of the right atrium. Toward the bottom of the right atrium is an AV node. I guess I'm going to go like this, AV node. I didn't even need to write it. The AV node is connected to the SA node by little structures called inter-nodal pathways. So, okay, it makes perfect sense, right? You have one node, the SA node. You have another node, the AV node, and connecting my two nodes are inter-nodal pathways between the node pathways. Here's the part that I can't draw for you. The inter-nodal pathways do catch the left atrium as well. So, remember that my heart is just this basically expanded ball of blood vessel with all this muscle around it, and so there are pathways between the atria. We could actually connect those two atria. So we have inter-nodal pathways and take a deep breath because this just isn't going to work with this diagram, but there are pathways through the myocardium that could get us over here into the right atrium. Inter-nodal pathways are going to go around and catch all the atrial muscle, and now think about this. SA node fires its action potential. Action potential spreads all through both atria. My picture doesn't do it justice, and most pictures don't do it justice, but that's a really important concept to understand because once you have an action potential that fires, contractile cells will follow. SA node says, dude, let's go, 20 beats per minute. Boom, boom, boom. Contractile cells say they told me what to do. I always follow directions. I'm contracting. Told me to do it again, 90 beats per minute. I'm cool. I'm contracting every single time without fail unless you are not doing well, in which case then we might have some issues. Okay. So now you have the message, this is wild. The message is gathered at the AV node. So action potential traveled all through the atria, and then all of that converges back on the AV node. Does that seem weird to you? There is a functional value in having someone else deliver the message from the atria to the ventricles and controlling the pathway that we must follow to get an action potential into the ventricles from the AV node. I'm going to a new color like blue. From the AV node we pass into the ventricular septum, the interventricular septum. And again, like this is a garbage image because I'm pretending like I'm going over the valves in the pulmonary trunk and doggies, that ain't how it rolls. All of my drawings are actually in myocardium. I'm just drawing it on top of a picture that doesn't want to be drawn on top of. The AV node connects to the interventricular septum, which is this structure, this myocardium, this muscle that separates my two ventricles. And that structure right there is called the AV bundle. And it kind of makes sense. The AV node that delivers information to the AV bundle. Now, the SA node is firing. The AV node says let's hang on to this for just a minute. Let's go ahead. Yeah, you told me what to do. You told me to fire an action potential. I will. I don't have a choice about the matter, but I'm going to slow it down just a tad. And the action potential, if it's going to get into the ventricular myocardium, the action potential has to go through the AV node. So at about one-twentieth of the pace that the SA node set, the AV node says, okay, here comes your action potential AV bundle. The AV bundle passes the action potential on, and these guys are all innately slower than the SA node. Like, if they were in charge of things, their heart rate would drop dramatically. The AV bundle splits into left and right bundle branches, and these guys split into little fibers called prokinji fibers. And really, the whole thing is prokinji fibers, but we've got left and right bundle branches of strings of auto-rhythmic cells. All of these are the same thing. And then we've got these prokinji fibers. That branch from the bottom of, here come your two bundle branches down the interventricular septum, and then we have this net from the bottom of the heart up of these prokinji fibers and their auto-rhythmic. And so they're going to be able to generate their own action potentials. When they get the message from the bundle branches, they're going to fire. Now look at this. First we get the message in from SA node. SA node sends it all around the atria and comes back to converge on AV node. What does that do? Message to the atria. Dogs, let's go, contract. Atria are going to contract. AV node hangs on to it for a second. Meanwhile, your atria are contracting. The AV node finally says, okay, I think I'm cool. And sends the message through a very small relatively speaking structure, the AV bundle, and the left and right bundle branches. These are not nets. These are going specifically between left and right ventricles. And sure, we have a little bit of contraction that happens with the contractile muscle tissue in that zone, but we're not having ventricular contraction yet, not even close. As soon as the message travels super fast into those Purkinje fibers, now we've netted, now you can imagine it's like spreading out the message from the bottom up. Why from the bottom up? Dude, it's freaking brilliant. If we did not, I'm drawing, okay, I got to do it in black. That's cool. Look, this is the outlet. Blood hanging out in the ventricle. In order to get out of the ventricle, it has to go out the top. So if you contract it from the top down, you're going to squish all the blood down into the bottom of your ventricle and you're not going to be able to get it out. So what a fantastic setup. Let's contract from the bottom up. Thank you Purkinje fibers, woof woof, for spreading out and thank you AV node for slowing down the message. Thank you bundle branches for keeping the message very tight just in that interventricular septum and then spread it out, contract from the bottom up, squish the blood out of ventricles. All the blood. Okay, not all of it, but a lot of blood, most of the blood. If you end up with blood pooling in your heart, pooling blood clots. That ain't cool. If you have clotting blood in your heart, you're going to throw a clot and you're probably not going to last too long. That's going to be a sad story. So let's get as much of that blood out and send it to the body or to the lungs because that's how we function. Whoa, isn't that cool? Your heart can do this without any input from the nervous system. However, we know that if we see a bear as long as it's not a teddy bear, heart rate's going to increase. How does that happen? Sympathetic nervous system probably plays a role, right? So how? How can we increase our heart rate or when you're sleeping, if our pacemaker potential is 90 beats per minute, which it is, I know right now my heart is not beating 90 beats per minute. So there has to be some kind of parasympathetic input saying, dogs, just slow it down. There's no need to go that fast. You're just hanging out in your office by yourself. Just talk into your video camera because you know that's a fun thing to do on a Sunday afternoon. All right. So let's figure out how the nervous system gets involved in this whole thing and can affect the heart rate.