We've been talking about the intrinsic cardiac conduction system and how that intrinsic cardiac conduction system is able to initiate and spread action potentials through the heart so that the heart contracts. And, of course, it does that set to a rhythm, which is your heart rate. But your heart rate changes, and that's what we want to talk about now. How do we control the heart rate? So we're going to say here that there are sort of 2 major ways that heart rate is controlled. The first we've been talking about as part of that intrinsic cardiac conduction system, and that's the pacemaker cells. These are intrinsic rhythmic initiation of action potentials. They start the action potentials on their own set to a rhythm, but they don't change the rhythm. What changes the rhythm is going to be things called chronotropic factors, and we can break down the word chronotropic. Chrono, that means time. Tropic, well, the root tropic means to change something. That's like in the endocrine system, we talked about tropic hormones. It's the same thing here. These chronotropic factors are the extrinsic factors that affect heart rate. And we normally think of these things working as either positive or negative chronotropic factors. A positive chronotropic factor or a positive chronotrope will increase heart rate. A negative chronotropic factor or a negative chronotrope will decrease heart rate. Now anything that does that is a chronotropic factor. So there are drugs that are chronotropic factors that may increase or decrease the heart rate, for example. But here we want to think about how this is done by the nervous system, and the way that works is through the medulla oblongata. The medulla oblongata is responsible for chronotropic control of heart rate by the CNS. Now remember, the depolarization and spread of these action potentials through the heart, that's intrinsic. That's all happening within the heart. Here, we're going to talk about how we turn the dial on that rate, how we speed it up or turn it down.
Alright. So to do that, we have dual innervation of the heart, and dual innervation is something that you should remember from when you talked about the autonomic nervous system. So that means that there are 2 controls on this that sort of work in opposition, the sympathetic nervous system and the parasympathetic nervous system. And if you remember your sympathetic and parasympathetic nervous systems, you should probably be able to predict generally what they're going to do here.
Alright. Before we dive in here, let's just orient ourselves to this image that we have. We have a brain, we see the brain stem here, and medulla oblongata is down here. We have some nerve fibers coming down, one through the spinal cord and one more directly to the heart, and we see them innervating at the heart in different places. And again, we'll break down that more specifically in just a second.
So let's start with the sympathetic nervous system. The sympathetic nervous system, remember, this is usually associated with, like, your fight or flight response, your sort of get up and go. So I'm going to say that this is going to increase heart rate, and I'll just indicate that with a sort of up arrow there. So the sympathetic nervous system turns up your heart rate. This is controlled by the cardioacceleratory center in the medulla oblongata. And we can follow this nerve fiber down from the medulla oblongata. We see that it goes down through the spinal cord, and then it comes over to the heart, and it actually splits and it innervates in sort of two places. Actually, 3 places, but sort of 2 types of places. We're going to say that it innervates first at the nodes, the SA node and the AV node. That's how it's going to affect the rate of the heart, but it's also going to innervate with the heart muscle. And it's doing different things in these two places. So as I just said, at the nodes, it's going to increase the heart rate. And so we can see this nerve fiber come in and innervate with the AV and the SA nodes. So that's going to be a little dial on those nodes that speeds up the heart rate. With the muscle, though, it's going to increase the strength of the heartbeat, or what we call contractility. Contractility is sort of how much these cells are contracting. So the sympathetic nervous system, it's going to turn up the dial on the rate. It's going to get that heart beating faster. But with the muscle cells, it's going to get them to contract with more force. So it's beating faster and it's beating harder.
Now, contrast, we have the parasympathetic nervous system. And parasympathetic, that's usually associated with, like, rest and digest. So this is going to decrease or turn down or well, indicate with a down arrow here. It's going to decrease your heart rate, and this is going to be controlled by the cardioinhibitory center. And we can see this in the starting in the medulla. It's going to travel down this yellow nerve here, and it's going to innervate in 2 places here on our nodes. So the signal is going to travel down that major nerve of the parasympathetic nervous system, the vagus nerve. It's going to travel down the vagus nerve, and it's going to innervate in 2 places. It's going to innervate at the SA node and the AV node, not the heart muscle. So here, this is just turning down that rate. It does not affect contractility. That means that, well, the sympathetic nervous system turns up contractility. So if there aren't any signals from the sympathetic nervous system, that means that that heart muscle just goes sort of back to its default contractility, which is how hard the heart is contracting at your resting heart rate.
Alright. So again, to sum this up, sympathetic nervous system, it's going to turn up the rate and the contractility, how hard the heart beats. The parasympathetic nervous system is just going to sort of turn down that heart rate and let the contractility go back to its sort of default setting. With that, we have examples and practice problems to follow. You should give them a try.