In this video, we're going to talk about astrocytes.
And in their name comes from the Greek words for "star cell."
Astrocytes are glial cells of the central nervous system,
which are derived from neural stem cells.
Astrocytes have a soma of variable number
and branches of processes.
But they often have a lot.
They often have quite a few processes
that are highly branched, which is
how they got their name of star cells.
Because some people thought they look like stars
when they looked at them under the microscope.
And at the end of their processes,
they have special structures called end-feet.
Let me draw in a few of these end-feet that
are at terminus of the astrocyte processes.
So that's end-feet.
And that's all of these structures
at the end the astrocyte processes.
And astrocytes are work horses.
They have arguably more functions
than any other cell type of the nervous system.
So first, we can talk about how the astrocytes really
form the scaffold for the entire central nervous system.
These cells really occupy a huge amount
of the space of the central nervous system
and form the majority of the structure that actually makes
up the brain and the spinal cord.
So they provide the structural support
and the place for all the other cells to be,
like the neurons and all the other glia.
The second function we can talk about is called the glial scar.
And that refers to what astrocytes
do if there's injury somewhere in the central nervous system.
So if there is some type of injury
somewhere in the brain or the spinal cord, what we see
is that astrocytes proliferate.
They divide and form more astrocytes.
And they migrate over to the area of injury.
They then surround that area of injury.
And their processes hypertrophy.
They grow these much larger, thicker, longer processes
that actually serve to wall off that area of injury.
And between all the processes, they
form a thick tissue, kind of like scar tissue
that we see elsewhere in the body.
But that scar tissue elsewhere in the body
is formed by different cell types
because the astrocytes are only present
in the central nervous system.
So this whole process of astrocytes reacting to injury
has multiple names.
It's called gliosis, or it's called astrogliosis,
or it's called astrocytosis, or it's
called reactive astrocytosis.
And the actual scar tissue that's produced
is called the glial scar.
And the grill scar probably performs
kind of a similar structural role
to the astrocyte's role as the general scaffolding
for the central nervous system.
Because probably what they're trying to do
is wall off an injured area.
And particularly if there's a cavity, if a hole has formed,
and they're trying to shore that area of structural support
from this wall.
Another function of astrocytes is homeostasis.
And homeostasis just means trying
to keep everything in optimal conditions
in homeostasis of the interstitial fluid, that
is the fluid between all the cells
of the central nervous system.
And this is very important because the neurons require
a very fine-tuned, a very even keel environment for them
to function properly.
And if certain things like the concentrations of certain ions,
and particularly potassium ions, if those concentrations are
abnormal, the neurons can't function properly.
So one thing the astrocytes do is that they're constantly
monitoring the interstitial fluid.
And they're either taking in ions
or they're releasing ions to try to keep those ion
concentrations exactly the same all the time, in homeostasis.
Another thing they do for neuron sometimes
is they release lactate into the interstitial fluid.
And the reason they do this is because neurons have
very little internal energy stores in their cells.
Neurons are completely dependent on a continuous supply
of oxygen and glucose to have all the adenosine triphosphate
they need to perform their functions.
Now, astrocytes do have some internal energy
stores in the form of glycogen.
And they can convert some of that
to lactate and secrete that, so that the neurons can
use lactate in a pinch if they have lost access
to continuous oxygen and glucose.
Another function of astrocytes is contributing to something
we call the blood-brain barrier.
And this is a barrier that prevents large molecules
in the bloodstream-- so I'll just draw a little blood
vessel passing through the central nervous system.
And this prevents large molecules
from leaving the blood to enter the central nervous system
unless the cells actually want that large molecule to enter.
Components of the blood vessels themselves
play a role in this blood-brain barrier.
But astrocyte processes also play a role.
And, in particular, it's the end-feet.
These end-feet, on the end of their processes,
are plastered all over the blood vessels that
are passing through the central nervous system.
And they play a role in preventing
certain large molecules from leaving the bloodstream
and entering the brain.
So that between the astrocyte end-feet and components
of the blood vessels of the central nervous system,
there's a quite effective barrier function
between the blood and the central nervous system.
And this includes a spinal cord.
But we traditionally call this the blood-brain barrier.
And one more very important function that the astrocytes
perform is that they help to clear out
synapses between neurons.
And so synapses are the connections
between neurons and their target cells.
So if we drew an axon of one neuron coming down here.
And this will be the terminal of the axon.
And then this axon is forming a synapse with
say the dendrite of another neuron here.
Just like the astrocytes are using their end-feet
to surround the blood vessels that
pass through the central nervous system,
the astrocytes are also extending their processes
and placing their end-feet all over synapses
so that the end-feet are plastered all over the synapses
and they're actually helping to clear out those synapses.
And we'll cover this more when we cover synapses.
But basically they're clearing out
the molecules that communicate between neurons
and their target cell, called neurotransmitters.
And this is very important to reset the synapse
so that it can be used again for communication
between the neuron and its target cell.
Because if neurotransmitters just lingered in the synapse,
then that synapse wouldn't be functional any more.
It would just constantly be turned on.
When instead, the neurons need to be
able to rapidly turn on and off the synapses
to be able to communicate information effectively.
In addition to these functions, astrocytes
appear to influence neurons and other glia,
and vice versa, through exchange of a variety
of other substances.
So as you can see, astrocytes are very hard
working cells in the central nervous system.
No other cell of the nervous system
appears to do such a huge variety of functions
like the astrocytes do.