In this talk, we're going to
review Parkinson's disease.
And we'll start with an introduction.
So first, let's talk about
what Parkinsonism is.
This is really a constellation of symptoms.
Classically, we see this as a chronic,
progressive neurodegenerative disorder
affecting the central nervous
system with four cardinal features.
And I want you to remember
these four cardinal features,
which are present in any patient
who presents with a Parkinsonism.
The first is bradykinesia, rigidity,
postural instability, with or without tremor.
And when you see these four symptoms,
you're going to think Parkinsonism is the cause.
And when you think Parkinsonism, these
four cardinal features will come to your mind.
Any patient presenting with
bradykinesia, rigidity, postural instability,
with or without tremor has
an underlying Parkinsonism.
Parkinsonism comes from the basal ganglia.
So let's spend a few minutes talking about
what the basal ganglia is and how it works.
The basal ganglia is involved in
both motor and non motor functioning.
We think about the motor functions,
but there's important non-motor control
that also comes from the basal ganglia.
Of the motor functions, the basal ganglia
is important in the initiation of movement,
helps us to start moving.
And so pathology of the basal ganglia
results in difficulty with initiating movement
that contributes to the bradykinesia.
And we also see postural instability and rigidity.
Let's look a little bit closer
at the basal ganglia circuitry.
How does it work?
What are the players what are the key
brain structures involved in the basal ganglia?
Well, we're going to start up with the cerebral
cortex and the cortex talks to the basal ganglia
to help with initiation of movement.
If you need to move the cortex has to tell
the basal ganglia that you need to move.
The first key players in the basal ganglia
will be the caudate and the putamen.
And inputs into the basal ganglia will come through
the striatum, which is the caudate and putamen.
Moving deeper, we find the globus pallidus.
And you can see where that
sits on the schematic here.
The globus pallidus is going to be an important
relay and output structure from the basal ganglia.
Information is going to relay in the basal
ganglia, including pathways to the substantial nigra
as well as other areas and
ultimately arrive at the globus pallidus,
which sends information
back to the thalamus.
The thalamus will be the key relay
center talking back to the cortex
and helping the body to start to move.
So when you think about basal ganglia circuitry,
there's three things I want you to remember.
First is to initiate movement, the cortex
has to tell the basal ganglia, I want to move.
The second is the thalamus.
The thalamus acts as a brake
on movement, it stops movement.
And so the goal of the basal
ganglia is to take the foot off the brake.
And we're going to talk about two
pathways that take the foot off the brake
and help to modulate
and initiate that movement.
When you think about the basal ganglia,
there's a few ways to break it down.
The first is the inputs and the outputs.
All information into the basal
ganglia come in through the striatum.
It's the major input center,
relay center for the basal ganglia.
So the cortex is going to talk to the basal ganglia
through the striatum, it's the major input pathway.
And then their outputs, all of the
output from the basal ganglia is going to
come from the globus pallidus.
There are relay circuits within the
basal ganglia that will modulate the signal,
but ultimately it will arrive at the globus
pallidus, which will talk to the thalamus.
All outputs come from the globus pallidus.
I also want you to know the direct and indirect
pathway and these two pathways are involved
in helping to take the foot
off the brake so you can move
or put the foot back on the brake if
you need to modulate that movement.
Let's first focus on the direct
pathway and see how that works.
The first step in the direct pathway is
the motor cortex has to talk to the striatum
and tell the body that it wants to move.
We also see inputs into the striatum
from the substantia nigra pars compacta.
And that's going to be
important in Parkinson's disease.
Both of those are activating the
striatum, they're excitatory signals.
The striatum talks to the globus pallidus internus and
to some degree, the substantia nigra pars reticulata
and inhibit signals since an inhibitory
signal to the globus pallidus interna.
The output from the globus pallidus interna is
to the thalamus and that's an inhibitory signal,
putting your foot on the brake, and the
thalamus then talks back to the motor cortex.
So the goal of the direct pathway
is to take the foot off the brake.
The motor cortex stimulates the
striatum, the striatum inhibits the GPI.
And the GPI is not able to inhibit the
thalamus, the result is the foot comes off the brake
and the body can move.
And that's the goal of the direct pathway.
In Parkinson's disease, we
lose that activation of the striatum,
and loss of the activation of the striatum
causes the foot to rest on the brake,
and patients are bradykinetic
rigid with postural instability.
Now let's turn to the indirect pathway.
Indirect pathway involves some of the
similar players - motor cortex, striatum and GPI,
but we have some other areas of
basal ganglia circuitry that are involved.
Again, this pathway starts in the motor cortex
and the motor cortex activates the striatum.
All inputs to the basal ganglia
are through the striatum.
But here there's some alternative circuitry involved.
The substancia nigra inhibits
the striatum in the indirect pathway.
The striatum inhibits the GPe the globus pallidus
external, which inhibits the subthalamic nucleus, a
and that sends excitatory fibers and
circuitry to the GPI, the globus pallidus internus,
which then inhibits the thalamus.
So this is a more complex circuitry.
And the key thing to remember here is the goal of the
indirect pathway is to prevent unwanted muscle movement
from getting in the way of voluntary
motor movements and tasks.
Here in Parkinson's disease, we have
loss of that inhibitory signal to the striatum.
So the striatum is free to do what it does.
And this results in an increase
in resistance from the GPe
And it's important in Parkinson's disease to
remember both the direct and indirect pathways,
one taking the foot off the brake,
the other putting the foot on the brake.
And in Parkinson's disease, we have abnormal circuitry,
resulting in less movement in these patients.