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Video transcript
So here's a brain I sketched out ahead of time. And I wanted to label a couple parts of this brain that we're going to talk about right now. So the first is the pons, and the second is what we sometimes call the medula, or you might even hear the term medulla oblongata. And it's right next to the pons, kind of right there. And people have actually studied these parts of the brains, the pons and the medulla oblongata. And they found that they are actually little respiratory centers. And I'm actually shading them in green. And sometimes you might even see these areas subdivided. But the idea is that there are a couple of spots here where the neurons in these green locations are very, very important to our breathing. And so they call them respiratory centers. And even though I'm drawing it as kind of a green blob, these respiratory centers are really just a bunch of neurons packed together. So many, many neurons, I'm going to draw a couple of them for us just to kind of illustrate the point. And these neurons are going to put out their little feelers. And they're going to try to collect information. That's basically what they're doing. They're going to collect information about all sorts of things like pain. Are you anxious? Are you running late for an exam? What is the situation going on right now? And they're going to make a decision about how fast we should be breathing, how often we should be breathing, all that kind of stuff. So where do they get the information from exactly, the information they need? I'm actually going to draw in a couple of important neurons right here in the medulla, and they're actually going to be these neurons we are going to be talking about right now. So these neurons are called the central chemoreceptors, and that's the focus of our video today. So central chemoreceptors are what we're going to be discussing. And these guys, and let me show them a little bit bigger. They're basically neurons. Are so I'll draw a couple of them. These neurons are going to be projecting their little axons all the way over to the respiratory center, and they're going to communicate their message through neurotransmitters, which are basically the language of neurons. Neurotransmitters allow neurons to talk to one another. So what we're going to see is that these central chemoreceptors are going to collect information about a chemical. And that's why they're called chemoreceptors. And just to back up a second, they're called central because they're part of the central nervous system. They are in the medulla oblongata itself. They're physically right there in the brain. So that's why we call them central chemoreceptors. And the first chemical that they're going to recept, or receive information about, is going to be carbon dioxide. So like any cell, these neurons are making carbon dioxide. And where does it usually go? Where would you assume that this waste product would go? Well of course there's a blood vessel. This blood vessel is going to have less carbon dioxide, we presume, maybe just a couple molecules of it. And so you have this nice little gradient where the CO2 is going to go into the blood and get swept away, and of course eventually it's going to make its way to the lungs and you might breathe it out. But let's assume for a second that the levels of CO2 in the blood are very high. Let's assume that the partial pressure of carbon dioxide in the blood is really, really high. What would that mean? Well, let me draw a bunch of carbon dioxide molecules in this blood. And what that means is that of course this gradient, this wonderful little gradient that we had is going to not be so strong anymore. Now there's no strong diffusion gradient because of the differences in pressure are negligible. There's a lot of CO2 in the blood, a lot of CO2 all around the neurons in the interstitial space. That's the space right here, interstitial space. So because that gradient is not as impressive-- let me write fluid instead of space-- because the gradient is not as impressive, you're going to have more carbon dioxide kind of building up around these central chemoreceptors. In fact, you might even have some molecules of carbon dioxide that are building up within the neurons, the central chemoreceptor neurons. So these chemoreceptors are going to notice the extra carbon dioxide, and they're not going to like it one bit. So you know what they're going to do? They're going to start firing action potentials. Let's say they usually fire two action potentials let's say per one second. I'm just assuming that number. That's not the true number, but let's just assume that. Now that the carbon dioxide levels are high, they're going to fire off maybe six action potentials in that same timeframe. So all of a sudden, they're firing more action potentials, because they don't like the high CO2 levels that they're facing. So these respiratory centers are going to get this message loud and clear. And they're going to say, wow we need to do something. Maybe we need to do something in the way of making this person breathe faster, for example. So this is one of the many things that you might see happen, is breathing faster. So you can see how this signal might work. Now you remember we talked about the relationship between CO2 and water. We said that CO2 binds to water. And that they form carbonic anhydrates, H2CO3, and that is actually in turn going to form bicarbonate. So it's going to form this. So if you have high levels of CO2, you have high levels over here, you can also assume high levels of protons. And that's just another way of saying a low pH. So the two things that our central chemoreceptors respond to then are one, high levels of CO2. And the other thing they respond to are high levels of protons, or a low pH. Now what they don't respond to-- and this is actually very important-- what they don't respond to is oxygen levels. So they don't respond to low oxygen levels. And this is actually a difference between the central chemoreceptors and the peripheral chemoreceptors. So this is something to keep in mind. Now a final point I want to make is I want to show you a three dimensional view of the same thing we just talked about. So let's take this picture and kind of absorb it. This is a picture I drew out a little bit earlier. Some of the important features are going to be-- let's orient ourselves first to the central chemoreceptor. That's this guy right here. Actually I think there are two of them. So central chemoreceptors, and they have of course a starring role in this picture. That's this guy right here, and of course there is a second fellow right there. We also have our astrocytes. These are kind of important cells for structural support. And they're also important in setting up what we know as the blood brain barrier. So that's right here right. I'm going to focus in on this right here. And the blood brain barrier of course allows us to keep what's going on in the blood separate in many ways from what's going on in the interstitial fluid around the brain. So then to quickly recap, if there's a lot of CO2 in here, in this blood vessel if you see high levels of CO2, you're not going to get much diffusion into that blood vessel. So CO2 levels start going up all around our two central chemoreceptors. They're not going to be too happy. And so they are going to start firing more action potentials towards our respiratory centers down these two axons. I hope you enjoyed that.