- [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.