- [Voiceover] So in this video
we're gonna take a look at the indirect pathway.
That circuit between some of the nuclei,
of the basal ganglia,
that helps to make sure that we don't make any
unwanted muscle movements.
So maybe
an unwanted movement would be
throwing a hot cup of coffee
at your crush
on your first date.
That would definitely be an unwanted movement.
But before we get into all the,
all the details of this pathway
and see how we can avoid this unwanted muscle movement
that would definitely ruin our first date,
let's just take a look at this special structure here,
the thalamus.
Now, the thalamus,
it really really likes chatting
with the motor cortex.
And this is the motor cortex here.
This structure that chats with our muscles,
and helps to control them.
Now if the thalamus could have its way,
it would just send tons and tons of excitatory messages
to the motor cortex,
and just turn its activity up,
turn the activity up in the motor cortex.
And when the activity in the motor cortex is turned up,
the motor cortex chats more with our muscles.
And this turns their activity up,
and this makes us move more.
So when I think of all of this,
I like to picture the thalamus as a puppy.
A really excited puppy
that just cannot be trusted.
And it needs to be kept on a leash,
because that's what you do
with puppies that can't be trusted, right?
And we keep it on a leash
to control how much it can run around
and cause trouble.
And in this case, the trouble
is the thalamus overexciting the motor cortex,
making it too active.
Because that makes our muscles too active,
and that makes us move too much.
So we actually have this leash in the brain.
We have a leash for the thalamus,
and it's the globus pallidus internal,
one of these nuclei of the basal ganglia.
So this is the globus pallidus internal here,
and it controls the thalamus,
it keeps it on a leash
by sending it these inhibitory messages,
and this keeps the activity in the thalamus down.
So this is what the globus pallidus internal does
in the brain.
It keeps the thalamus on a leash.
And when the basal ganglia work together to
keep us from making an unwanted movement,
what they do
is they tighten the leash on the thalamus.
They make it even less active, right?
Because you pull that leash in,
and it can't run around as much.
So to do this,
all they do is they,
they chat with each other,
and ultimately they send a message
to the globus pallidus internal,
and they tell it to tighten that leash;
to send more inhibitory messages
to the thalamus.
And this turns the activity in the thalamus down.
So it can't chat with the motor cortex as much.
So that's the aim of the indirect pathway:
to tighten the leash on the thalamus
by chatting with the globus pallidus internal.
So let's go through this pathway
and have a look at how it all happens.
So the first thing that happens
is the motor cortex,
it chats with the striatum,
and it turns up activity in the striatum.
So to do that,
it sends this excitatory message
along this excitatory neuron here
that heads over to the striatum.
And so the striatum gets really excited, right?
It gets excited,
and its activity, it turns up.
And so, the striatum has these neurons here,
these red neurons, let's draw these in red,
that are inhibitory neurons that head over
to the globus pallidus external,
that other part of the globus pallidus.
So when the striatum gets excited,
these red inhibitory neurons get excited,
and they turn down activity in the globus pallidus external.
So we have these neurons that go from the
globus pallidus external to the subthalanic nucleus.
So, when the globus pallidus external's activity
is turned down,
it can't chat with the subthalanic nucleus
as much as it used to.
And what it usually does, is it normally kinda holds down
the activity in the subthalanic nucleus.
So, when it's not so active, it can't do that.
So the subthalanic nucleus, it gets more excited, right?
And this is the really important part.
So this is the part that matters,
is, the subthalanic nucleus, it gets excited,
and it's what sends excitatory messages
to the globus pallidus internal,
our leash on the thalamus.
So what happens if we excite our leash?
Well, that's gonna tighten our leash, right?
Because we have our leash, our inhibitory neurons,
that go from the globus pallidus internal
to the thalamus,
and so if we excite those neurons, we're gonna,
we're gonna tighten our leash.
Because those neurons are inhibiting the thalamus.
So all that kind of,
all that you need to kind of get from this is that,
when we don't want to move our muscles,
the motor cortex sends this little signal,
it goes through this little pathway here,
and the end result
is that we excite the globus pallidus internal,
and that tightens our leash on the thalamus.
And so you know from before that,
if we tighten that leash, if we inhibit
the activity of the thalamus even more,
we're gonna have less chatting between the thalamus
and the motor cortex,
and less chatting with our muscles,
between the motor cortex and our muscles, right?
Now, there's a little bit more to the pathway.
Remember what we said before, about how
we want to be able to adjust our leash?
Well, we have something kind of in the background
that does just that.
It adjusts our leash based on what we need to do.
So if we head back over here to the subthalanic nucleus,
it actually chats with the substantia nigra.
And, the substantia nigra, what it does in this pathway
is it, it kind of does the opposite
of what we just did.
It is our
adjustment of our leash, it lets our leash out a little bit
to allow our muscles to move a bit more.
So, we'll just go over how that happens, but that's,
that's the important part.
That's the goal of the substantia nigra
in the indirect pathway.
So the subthalanic nucleus,
it turns up activity in the substantia nigra.
So it does that by these excitatory neurons,
by sending these little excitatory messages.
And the substantia nigra,
it has these other types of neurons,
these dopamine neurons.
And these dopamine neurons head over to the striatum.
And, they chat with these excitatory neurons,
these neurons that turn up activity,
and these inhibitory neurons here,
that go to the globus pallidus external.
So when the substantia nigra gets excited,
when it's deciding that it needs to
allow the thalamus to be out a little bit more on its leash,
to allow it to have a bit more,
a bit more freedom, and allow it to play a bit more,
it sends dopamine to the striatum.
And, the dopamine binds to these little receptors here
on these excitatory neurons,
and these receptors are called D2 receptors,
and when dopamine binds to these,
it turns down activity
in the neurons, in these excitatory neurons.
And so they can't,
they can't chat with these inhibitory neurons as much
and turn their activity up.
So what happens is these inhibitory neurons that would be
inhibiting the globus pallidus external,
they're not able to do their job
as much as they were before,
when we were turning their activity up.
So they can't inhibit
the globus pallidus external as much as before.
So that's kinda the opposite of what we had before, right?
So before, we were inhibiting
the globus pallidus external,
and now the substantia nigra
is kind of taking away some of that,
it's allowing the globus pallidus external
to be a little bit more active.
So that means that the globus pallidus internal
is gonna be a little less active.
And so if the globus pallidus internal, our leash,
our leash on our thallamus,
is less active,
then that means that it can't hold the leash so tight.
It's gonna have to let it go a little bit.
And the thalamus is gonna be allowed to play a bit more,
it's gonna be allowed to be more active.
So it's gonna talk to the motor cortex more,
and the motor cortex is gonna talk to our muscles more.
And we're gonna have more muscle movement.
And the substantia nigra is smart, right?
It knows that our dog cannot be trusted,
so, we need to control how much we're letting it out.
We can't just let everything go.
So the subtantia nigra knows when enough is enough,
and it sends messages back to the subthalanic nuceus
telling the subthalanic nucleus
to stop telling it to be more active.
And the substantia nigra stops allowing
this little pathway
to let our leash out, right?
So that, that is the indirect pathway.
That's how these structures talk to each other,
to adjust the leash on the thallamus.
And that's how they adjust
how much movement we make.