Astrocytes | Nervous system physiology | NCLEX-RN | Khan Academy

November 11, 2019 0 By Kody Olson

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.