If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content
Current time:0:00Total duration:4:57

Video transcript

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 so there's a 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 sort of explain these two areas let's draw it on the other brain here so notice that this here is the left-hand side of the brain here's 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 kind of 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 let's talk about the core first so in this corazon when there's severe ischemia which is defined as blood flow of less than about 20 percent of normal you get this massive a loss of two vital nutrients for our neurons and they're 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 anymore 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 corner ons 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 perfuse brain tissue the tissue in the penumbra won't actually die right away because what's happening remember I said it's moderately perfused tissue what's happening is that it's getting some blood still from nearby arteries referred to as collateral artery right 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 good 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 collateral blood vessels you have then the more penumbra neurons you have which is a good thing because that 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 paitent 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 in this vessel these would then be the new collateral arteries for that newly created ischemic core so let's put that down here more collaterals equals more oxygen to neurons at risk of ischemic death and 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 to get blood back into that ischemic area you want to get blood flowing into that ischemic area ASAP what this will 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 corner ons 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 an ischemic penumbra neurons and their supporting cells that are in the ischemic or they'll die off within about 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 collateral arteries