- [Instructor] So in this video,
we're going to talk about two systems of supplying nutrients
and removing waste from the brain.
One is cerebrospinal fluid.
The other is blood.
So when we look at how the brain sits within the skull,
you have to remember that the brain
is going to be floating in cerebrospinal fluid.
It helps give the brain buoyancy.
It makes it feel a little or seem a little lighter.
And in order to prevent the brain
from bouncing around too much inside of the skull,
we're going to see that there are
two very tough connective tissue membranes,
one called the falx cerebri.
And this is the falx cerebri here.
And the purpose of the falx cerebri,
as you can see in this model, is to be able to separate
the left and the right hemispheres, and to prevent the brain
from moving too much from side to side.
In the next specimen,
we'll see that there is another thick membrane
that you have to look deep inside here.
And this membrane rests right here,
right above the cerebellum.
And it's called the tentorium cerebellum.
And what it does is it creates
an upper cranial cavity and a lower cranial cavity.
The upper cranial cavity is going to contain my cerebrum,
and the lower cranial cavity here is going to have
my brainstem and cerebellum.
This is not the foramen magnum.
This space is not the foramen magnum.
If I keep going to the opposite side,
you can see the foramen magnum on the bottom.
So the upper cranial cavity contains the cerebrum.
The lower cranial cavity contains
the brainstem and cerebellum.
So this is another sagittal cut of the brain.
And again, we can see
what we call the falx cerebri
that separates the two hemispheres,
but something that we couldn't see on the last specimen
was the tentorium cerebellum.
The tentorium cerebellum essentially separates
my occipital lobe from my cerebellum.
So the other thing that's good about this specimen
is that we can see the meninges.
The meninges, the dura mater that we saw in the spinal cord
is this outer meningeal layer.
There's three layers of meninges: dura mater.
Attached to it, so I really can't separate the two,
attached right to it is the arachnoid mater.
Then under the arachnoid mater, there is this space.
It's called the subarachnoid space.
And this entire subarachnoid space is filled
with cerebrospinal fluid,
and that's how the brain floats in.
The third meningeal layer you can't see
because it's a single cell layer thick.
It's called the pia mater,
and the pia mater surrounds all the cerebrum,
dipping into all of the sulci.
Now something that you might see right here
is this little hole.
This is a sinus.
We're going to call this,
these were dural septa.
Septa separate the brain into cavities.
This dural sinus is going to have venous-filled blood,
low oxygenated, nutrient-low blood
that's going to go down through my jugular vein
in order to go to the heart and be pumped to the lungs,
reoxygenated, and then go out to the body.
Not only is the brain surrounded
by the three meninges, the dura mater,
the arachnoid mater and the pia mater,
the spinal cord is also surrounded by the dura.
And you can't see it here,
but the inside would be the arachnoid.
And underneath that would be the subarachnoid space
filled with cerebrospinal fluid.
So just like the brain
floats in CSF,
the spinal cord floats in CSF.
So in the brain, we have two dural septa.
We have the falx cerebri and the tentorium cerebellum
that prevents the brain moving around too much.
Here in the spinal cord,
remember that we called this connective tissue structure
that went from the conus medullaris,
the end of the spinal cord to the end of the dural sac
the filum terminale,
and the filum terminale
is like the dural septa of the spinal cord.
So now let's take a look at the ventricles from the inside.
So we said that the subarachnoid space
had cerebrospinal fluid that allowed the brain to float in.
We also are going to have these holes in our head
called the ventricles,
and these ventricles are going to be filled
with cerebrospinal fluid.
Now this model is showing the fluid.
What's solid here would actually be liquid inside the brain.
You have two lateral ventricles,
one for your right hemisphere,
and I know this is my right hemisphere
'cause again, this is my temporal lobe.
And look at the curved part of my temporal lobe.
Since it's on the right, this would be a right hemisphere,
and we have a lateral ventricle for the left hemisphere.
We have a ventricle for the frontal lobe, the parietal lobe,
the occipital lobe and the temporal lobe.
And that's because cerebrospinal fluid is one of the systems
that's going to provide nutrients to my brain.
The lateral ventricles are going to drain
through a small structure here
called the interventricular foramen.
Inter means between, ventricular,
and foramen means a hole.
And where that's going to drain is it's going to drain
into a small ventricle right here.
This ventricle is called the third ventricle.
And the third ventricle is the perfect landmark
for your thalamus and hypothalamus.
It will then drain through an even smaller,
I'm not sure if you'll be able to get it on the video here,
an even smaller canal
called the cerebral aqueduct.
And from the cerebral aqueduct,
it will drain into this structure here
called the fourth ventricle.
And the fourth ventricle is a landmark
for the pons and the medulla.
The cerebral aqueduct
is a perfect landmark for the midbrain,
but the fourth ventricle is a landmark
for the pons and the medulla.
So now let's take a look at this in a real brain specimen.
So I've got my corpus callosum here,
and there is a membrane, it's called the septum pellucidum.
We won't learn it in this class,
but the septum pellucidum is a membrane
that separates my left ventricle from my right ventricle.
So if I looked inside here, so if I put this probe here,
that would be my interventricular foramen.
And the space here
would actually be filled with cerebrospinal fluid.
So I can't see my lateral ventricle here
because it's behind this membrane.
We'll look at it in another brain specimen.
The lateral ventricles drain into the third ventricle.
And I said that the third ventricle was a great landmark
for thalamus and hypothalamus.
And you might remember that as my mammillary body.
So anything below my probe
would be hypothalamus.
Anything above my probe would be thalamus.
The third ventricle drains
through this really thin cerebral aqueduct.
And that's a landmark for my midbrain.
It then drains into this larger fourth ventricle here
between my brainstem and my cerebellum.
And the fourth ventricle is a landmark
for my pons and my medulla.
After the CSF flows through here,
it will go to a big cavity right behind my cerebellum
called the cisterna magnum.
Cistern means vase.
So it's a really big vase.
From there, it's going to go
through my subarachnoid space
'cause I said that I had fluid all the way to the outside.
It's then going to go to specialized structures
which we'll see in the blood supply part of this video
called arachnoid granulations.
That will then go into that venous-filled cavity
that we talked about, the dural sinus, right?
So if I've got all this fluid going through my brain,
where is it coming from?
So if we go back and look at this model here,
we see these pink structures.
And what these pink structures represent
is a specialized group of blood vessels and cells
called choroid plexus.
Choroid plexus is what produces
the cerebrospinal fluid.
It will then travel from the lateral ventricle
through the third ventricle,
through the cerebral aqueduct,
the fourth ventricle, to the cisterna magna,
the subarachnoid space,
and eventually up to arachnoid granulations
that will then take the waste products
into your venous circulation.
So this is what the ventricles look like whole.
What do the ventricles look like
when I actually look inside of the brain?
So this is what your lateral ventricles look like.
So this should look very similar to that.
And if this is,
these are my two lateral ventricles,
then this space right here must be my third ventricle.
In order for me to find my fourth ventricle,
I'd have to be way down in my brainstem at the cerebellum.
So we said that this was the septum pellucidum,
that we couldn't actually see
the cavity of the lateral ventricle.
So let's pull the septum pellucidum back,
and now you can actually see the space that would create
the hole that we see here.
And more importantly, what you can see now
is there's this thin structure here
and I'll pull, try and pull most of it out.
This right here is the choroid plexus.
And what the choroid plexus is is a collection
of blood vessels and specialized ependymal cells
that are going to secrete cerebrospinal fluid
into your ventricular system.
So cerebrospinal fluid or CSF is one way
of bringing nutrients to the brain.
This blood supply tree or the vascular tree
is going to be the main way of supplying blood
to your brain and brainstem.
So we're gonna talk about two types of circulation:
posterior circulation coming from my vertebral arteries
and basilar arteries,
and another circulation here coming from my carotid.
And unlike the vertebrobasilar system
where you only have one,
two vertebral arteries come together to form the basilar.
We'll see that on a real brain.
But what I want to point out is that you have
an internal carotid for the left,
and an internal carotid for the right.
So when you look at the blood supply to the brain,
you can see that there is a hemispheric,
left hemisphere, right hemisphere.
But the other thing I want you to focus on is
that you've got a white vascular region here
and an orange vascular region.
This white vascular region is going to come
from the anterior cerebral artery.
So the more medial
is anterior cerebral,
and the more lateral
is going to be middle cerebral.
And there's going to be an itsy bitsy, tiny blood vessel,
I'm not sure you're going to be able to see,
that actually connects my posterior circulation
with my anterior circulation.
So now let's take a look
at the blood vessels on a real brain.
This is the basilar artery.
There is only one.
Blood coming up comes into the basilar artery
from two vertebral arteries.
Here's one vertebral artery.
The other one you can see is broken off here.
So the vertebrobasilar system
is responsible for providing all the blood
to the entire brainstem and cerebellum.
But most importantly,
the very last branch of my vertebral artery
is called the posterior cerebral artery.
And the posterior cerebral artery
goes back here to provide blood to my occipital lobe.
So if you have a stroke
in the vertebrobasilar system,
you could either have clinical deficits
in your brainstem, cerebellum,
or visual deficits from lack of blood supply
to your occipital lobe.
That's my posterior circulation.
My anterior circulation is going to come
from my carotid artery.
So there is my carotid artery
for the right hemisphere.
And here is my carotid artery
for the left hemisphere.
Notice you have one carotid for each hemisphere,
and that there are two major vessels
that come off the carotid artery.
One is going to be the anterior cerebral.
And the anterior cerebral runs
right down the interhemispheric fissure.
And that's why the anterior cerebral artery
basically supplies blood
to only the medial aspects of your cerebrum.
And when I told you about the homunculus,
I said that the medial aspect of my homunculus
was essentially my leg and a little bit of my trunk.
So now we've learned that this more medial artery
called the anterior cerebral
only provides blood to the middle part of the cerebrum,
including the leg and trunk.
The other artery that comes off of the carotid
actually goes deep down here
between my temporal and frontal and parietal lobes.
So what it's going to do is it's going to come out here.
This is part of my middle cerebral artery,
and it is going to provide blood
for the overwhelming majority of my cerebrum.
It's going to be responsible for receiving sensory
and sending motor information
to the arms and the face.
So if you have a stroke, you want to test,
you want to act fast by testing
the face, the arms, speech,
because both Broca's area and Wernicke's area
are going to be in this territory.
So let's take a look and see again,
how my common carotid artery here,
my common carotid artery here gives rise to a branch,
the anterior cerebral.
And the anterior cerebral runs
right down the medial part of my brain.
And the other branch it gives rise to is
the middle cerebral artery.
And the middle cerebral artery is going to provide blood
to the vast majority of the cerebrum.
The posterior cerebral artery that does my occipital lobe
and a little bit of the inferior part of my temporal lobe
is actually coming from the vertebrobasilar system.
Now, I said there was a way
that the anterior and the posterior circulation
was able to come together.
And there is one really, really small blood vessel
that connects the anterior and posterior circulation.
And in class, we're going to talk
about how this creates a little circuit
so that if you have a blood clot,
you can actually reroute the blood
around this closed circuit.
We talked about blood flowing through the vascular tree,
and there being a rerouting system
to get blood around some clots.
But unfortunately, if you have a stroke in your brain,
you might not be able to reroute around.
So stroke, or sometimes people will call it a brain attack
because it's kind of like a heart attack,
comes in two varieties:
ischemic strokes and hemorrhagic strokes.
In an ischemic stroke,
what happens is basically a clot most often
or narrowing of the artery ends up reducing blood flow.
And if you don't have blood flow, you have a lack of oxygen
and you have a lack of glucose.
So what ends up happening is the neural tissue dies.
And here is an example of where somebody had a stroke,
and this is the area
where all the neural tissue died
from a lack of oxygen and glucose.
And this is in the most common area
'cause I said out here more laterally,
more laterally, it was going to be
the middle cerebral artery.
Because it's in an area that isn't high functioning,
it's an association cortical area,
this person probably didn't show
any clinical signs or deficits
until it started impinging in areas like Wernicke's area,
the receptive component of speech.
Here's another person that had a stroke.
And again, this would be an ischemic stroke,
and this is all the area of tissue
that died from a lack of blood supply.
And unlike the last brain,
you can see that this brain is in the more medial aspect.
So this would have been due to a stroke
to the anterior cerebral artery.
Now sometimes you can have more profound ischemic events,
and these more profound ischemic events can actually result
in the loss of tissue in multiple areas.
And I think this gives you an idea
that the brain has an amazing ability,
a sense of what we call plasticity,
to be able for other surrounding areas
to be able to take over the function of these neurons
so that you don't have a clinical deficit
until you lose so much more of your brain.
Now here's three brains that had ischemic events.
The fourth brain I want to show you
is the second type of stroke.
And the second type of stroke is a hemorrhagic stroke.
This is where you actually ruptured a blood vessel
and bled into the brain.
Ischemic strokes, if they're caught early,
you can give clot-busting drugs, kind of like Drano,
and break open the clot and give blood.
But unfortunately, there's no way
of controlling the bleeding inside the brain.
And this largely is why this person died.
So now, instead of talking about what could happen
in terms of strokes to the vascular tree,
now we need to consider not the internal vessels,
but some of these more superficial vessels
that are associated with the brain.
Now, I said that the cerebrospinal fluid
would eventually drain
through something called an arachnoid granulation,
and that these arachnoid granulations would then go
through venous structures.
And we've got here,
if I pull the dura back,
you can see these veins that are being pulled,
and that these veins will go
into what's called a sinus.
So right running right down the middle here
is going to be a venous-filled cavity
called the dural sinus.
And from the dural sinus,
it'll go into the jugular vein and back to the heart.
Now, if somebody falls, if somebody hits their head
and their brain bounces too much
and they end up damaging these bridging veins,
what they could have
is called a subdural hematoma.
It's like a bruise that you might get in your body,
but it is a bruise on the brain.
A subdural hematoma isn't that bad
because it's low pressure venous blood
returning back to the heart.
This is opposed to another type of damage
that you could get.
And in this case, this is a subarachnoid hemorrhage,
under the arachnoid in that space that has your CSF.
And because the subarachnoid space
has arteries and arterioles,
this is going to be high pressure blood
coming from your heart to your brain.
Somebody that has a subarachnoid hemorrhage like this,
or like this is going to describe it
as the most excruciating headache of my life.
And chances are even before they can make it
to the emergency department,
they will bleed out
because this is high pressure arterial blood.