 if you want to change the heart rate, if you want to change the rate at which the SA node fires, then you are going to have to somehow draw your brain back to our action potential graph and modify this. So here are my auto-rhythmic cell, like this is their pacemaker potential. And remember, the pacemaker potential existed because of these leaky sodium and potassium channels. And there are a couple of things that we can do. Do you agree that if we could make the channel's leakier, do you agree that we actually would increase the heart rate? Look at what I'm doing. What I'm doing is just making my slope of the pacemaker potential steeper. I'm reaching threshold faster and firing more action potentials in a given amount of time. Look at that. By changing the pacemaker potential, by changing the rate at which I can do this multiple ways, but if I can change the speed at which we reach threshold, I can fire one, two, three, four, instead of one, two action potential messages. That's going to speed up the heart rate. How does that happen? Dude, how about throwing some epinephrine or norepinephrine into this mix? If the nervous system dumps epinephrine or norepinephrine, there are going to be mechanisms that will enable this to happen. We will attempt to figure out what are those mechanisms, what are the things, how can we manipulate this to make it so that we actually depolarize sooner? Conversely, if we want to slow the heart rate down, all we have to do is increase the amount of time it takes to reach threshold. We're going to fire an action potential at the same pace, like the action potential, the size, the time that it takes to fire the action potential doesn't change, but the amount of time that it takes to reach threshold totally can change. And we can do that just by changing the permeability of the membrane. If we change that, and we can do that with drugs, and we can do that with chemicals from your nervous system. With your sympathetic input, you remember that we're dumping acetylcholine on muscarinic receptors. And so those muscarinic receptors are going to do something to change this pacemaker potential, to slow it down, to make the auto-rhythmic cells less leaky to sodium. Maybe sodium doesn't leave as fast. Maybe potassium leaves faster. And then we end up with this whole layer of manipulation that we can do. The last thing I want to do in this lecture is take a quick look at the overall cardiac cycle. Like, let's put all these pieces together and look at how the heart actually beats. And then in the next lecture, holy heart-beating fun times, we're going to look at the whole thing.