Video transcript

- [Voiceover] So we know that when a cerebral artery gets blocked the brain tissue that that vessel serves can die off pretty quickly. The neurons die off without the oxygen that they get from their blood supply, right? And that's what a stroke is. Well, it turns out that what we see is this interesting pattern in brain tissue that's undergoing a stroke. Essentially, we see two different almost zones of injury. There's the zone closest to the artery that's been blocked off called the ischemic core. And there's a zone sort of on the periphery of the core, called the ischemic penumbra, which is ischemic, but still viable cerebral tissue. And before I explain these two areas lets draw it on the other brain here. So notice that this here is the left hand side of the brain. Here is the right hand side. And we're actually looking at this brain from a left hand view. So that works out nicely. So on this view what we'd see is something like this, so we'd see the ischemic core here. And the distribution of the middle cerebral artery because that's what's been blocked. And then we'd see the ischemic penumbra just sort of surrounding the ischemic core. So lets talk about the core first. So in this core zone, when there's severe ischemia. Which is defined as blood flow of about 20% less than normal. You get this massive loss of two vital nutrients for our neurons and their supportive cells. So you get this loss of oxygen and glucose. Because they use up what's in the area already. And then they're not getting replenished because there's no more blood flow. So what will ultimately happen is that the severe ischemia will cause these neurons and their supportive cells to die off. Which is also called necrosis. And this actually happens in only minutes. It only takes a few minutes for the core neurons to die off. So what about the penumbra? Remember I said there are two different zones? The penumbra is this almost rim of moderately ischemic tissue that lies outside the core zone between the core and the normally profuse brain tissue. The tissue in the penumbra won't actually die right away. Because what's happening, remember I said it's moderately profuse tissue. What's happening is that it's getting some blood still from nearby arteries, referred to as collateral arteries. You can imagine that oxygen is diffusing out of these blood vessels and still serving neurons in this ischemic penumbra. These white dots are my representation of oxygen diffusing out of these collateral blood vessels. So these penumbra neurons get some glucose and oxygen from collateral arteries. But the catch is that it won't last forever. And in fact these neurons in the penumbra region will still die off if blood flow isn't reestablished pretty early on, like within a few hours of the vessel obstruction happening. And that's because this collateral circulation still isn't enough to maintain the oxygen and glucose needs of these penumbra neurons past a few hours. So just two more penumbra things I want to touch on here. So, first, the more collatoral blood vessels you have then the more penumbra neurons you have. Which is a good thing because it means that neurons aren't just outright dying off. And of course those collateral arteries have to not be blocked off as well. So for example, in this drawing here this penumbra is allowed to exist because this collateral artery is still patent, open. And this collateral artery is still open. However, if the blockage happened say down here right before each artery comes off, then, of course none of those arteries would have blood in them, so then this particular penumbra wouldn't exist. All of this would just be more dead core. Although, then there might be some other collateral vessels creating a different penumbra. Like maybe this vessel and this vessel. These would then be the new collateral arteries for that newly created ischemic core. So lets put that down here: more collaterals equals more oxygen into neurons at risk of ischemic death. The second thing is that the penumbra is the area where medications are most likely to be effective. So what does that mean? One of the goals of treatment is to break up that clot and get blood back into that ischemic area. You wanna get blood flowing into that ischemic area ASAP. What this'll do is effectively rescue neurons in the penumbra. So me erasing this blue penumbra is supposed to show them now getting enough oxygen and getting enough glucose to function properly again. It's likely that the core neurons will have died though, unfortunately. So just a quick recap now. If for whatever reason blood flow stops to some part of your brain. You develop an ischemic core and ischemic penumbra. Neurons in their supporting cells that are in the ischemic core, they'll die off within a few minutes. And neurons and their supporting cells in the ischemic penumbra, without treatment they'll die off in a few hours. So they stick around for a little bit longer than the core because they get some blood flow from collatoral ateries.