[MUSIC]. Okay, so let's look at the blood supply of the brain. There are two sources. There is an anterior or, or forward circulation, and a posterior, or back, circulation. So, once again, to orient you, this is the front of the brain. This is the back of the brain. I, I have to turn every time [LAUGH]. This is the left side of the brain. And this is the right side of the brain. This is the bottom. And one source of the of blood to the brain is coming up through the neck and it's the internal carotids. So, you know that you have a carotid artery that comes up through the neck. And it branches off to form an internal carotid, which is going to serve the brain. The external carotid is going to serve the face. The internal carotid is right here and right here. And you can see that these are large vessels. So here, and here. And this is the source of the anterior circulation. The source of the posterior circulation are these two vessels called the vertebral arteries. And they do something that very few other arteries in the body do, which is that they join to form a, another, so two become one. Usually, one becomes, divides into two. But in this case, two becomes one. So, this one is called the basilar artery. And the basilar artery is going to split up here, and it's going to split into a couple of, sort of twice. You can see this is one artery, but we're going to worry about this artery right here, which is the posterior cerebral artery. One interesting thing before we go on from the posterior cerebral artery, is you see this nerve? This is the ocular motor nerve, which is very important in moving your eyes. It is wholly responsible for vertical eye movements for, for, for convergent eye movements. It also controls the lens and the pupil, and it also controls voluntary movement of the eyelid. And this is sandwiched between two different arteries. So a problem with either one of these arteries is going to result in problems with this ocular motor nerve and its functions. But, the important thing that we want to do, what we want to understand about the circulatory system that is serving the brain, is that it's very, it's got backup plans. So, this is the, coming in from the vertebrals, this is coming in from the internal carotids. As it turns out, we have this bizarre structure, which is a circle. So, it's called the circle of Willis. From this internal carotid, there are two vessels that split off. One, is this one. It's diving down here. It's going to serve the entire temporal lobe. It's going to serve all over here, the motor cortex, somatosensory cortex, a lot of stuff. This is called the middle cerebral artery, and then this one here is called the anterior cerebral artery. But the important thing is that between, here's one anterior cerebral artery. Do you see that? There's one anterior cerebral artery. On the other side, here's the, here's the other one. You see that? Okay. What's really cool? What's really cool is that they're joined. Do you see this tiny little artery? That is the anterior communicating artery. Because of this artery, if we lost function in one internal carotid, the other internal carotid could, could replace it. It could send it's blood all across this anterior communicating artery into these other arteries that usually this artery is feeding. So, back up plan. There's another back up plan, which is that now we're going to look back here. Remember that the vertebrals do this weird thing of joining to become the basilar. And then the basilar is going to form the is going to end and split as a T into these two posterior cerebral arteries. See that? Posterior cerebral artery on one side, posterior cerebral artery on the other side. Well, what's to join the anterior and posterior circulation? It's these things. Look at this and this, which has actually been detached, but there it is. But look at this. This is joining this to the internal carotid. So, it's joining the anterior and posterior circulations. This is a circle. This is called the posterior communicating artery, and there are two of them, one on each side. So, blood can go all the way around, across the anterior communicating artery, or around past this other posterior communicating artery and around and around. So if there's a blockage in any one of these sources, the other sources can make up for it. That's a really amazing thing. So, this is, these these vessels are the places where, we can have strokes here. We can have aneurysms here. And the effect, the problem with a stoke is cardiovascular. But the effect of the stroke is going to depend on where it where the, what part of the brain is going to lack oxygen. And so what this is is half of a brain. Here's the front. Here's the back. I've taken out brain stem. I've taken out diencephalon. And you see, what you see is actually, here's the carotid. You see the carotid coming right here? Well it branches, and here this called the anterior cerebral artery. It's this artery right here. It's this artery right here. And what this does is it serves blood to the entire medial surface, the medial surface of the brain. The middle cerebral artery is going to serve provide blood to the outer part of the brain, the convexity of the brain. And then the posterior cerebral artery is going to provide blood to the back part of the brain. And so each one of these is going to have a different effect. The most common place to have a stroke is in the middle cerebral artery, so in this area, which contains primary motor cortex. That's why under most circumstances there is going to be a problem with movement of some part of the body. Depending on, on where, you could have a, you could have a stroke right here. Or you could have a stroke way down, past in a, in a small tributary. In which case less brain will be affected than if it's at the root of the, of the vessel. In any case, the effect of a middle cerebral artery stroke is going to typically involve an inability to move voluntarily some part of the body: legs, arms face. In addition, if it is on the left side of the brain, so let's go back to our whole brain. If it affects the left hemisphere, a stroke could effect this area right here, which called Broca's area, and this is going to cause a problem with speech. So that's another very common but not inevitable accompaniment of a stroke. If the stroke is on the right hemisphere, a, a stroke in this analogous place that would be Broca's on the left side is not going to have the same effect. So, we have a lot of backup for losing blood supply but it's not always sufficient. And and so strokes will have effects. And those effects are going to be determined by what part of the brain loses its oxygen supply. [MUSIC]
