Nonetheless, it's not sufficient.
It's not sufficient to produce the kind of movement that we are capable of,
and that we are used to.
So if all we had was the motor hierarchy,
the movement that we would produce would be unrecognizable.
And that's because there are two critical places that modulate
the motor hierarchy and the first one is the cerebellum.
The cerebellum is, as I've said, like an orchestral conductor.
It's saying, okay, muscles, all work together.
Work well together.
When the strings start to ebb I want some real energy from the woodwinds,
and then when the woodwinds ebb then we should get some percussion.
So, in order to make sure that movements that depend on multiple joints and
multiple muscles acting across multiple joints, that they actually are smooth,
we need someone organizing that, some entity organizing that.
And that's the cerebellum.
But what makes this a modulatory pathway as opposed to
a part of the hierarchy
is that the cerebellum does not talk to the motoneurons.
It only talks to these top three areas.
The cortical control motor centers, the brain stem motor control centers, and
the central pattern generators.
It's only modulating those types of neurons.
And so the central pattern generators are found, for the most part, in the brain
stem, the ones that are modulated by the cerebellum are all in the brain stem.
So the cerebellum is only modulating places in the brainstem and the forebrain.
To do this, the cerebellum gets sensory input from the periphery,
from the muscle, from the skin, as well.
From lots of places.
And it also gets input from the motor hierarchy about what we intend to do.
And the cerebellum is all about comparing intention and actual movements.
So when the cerebellum is on the fritz, when it doesn't work, what do we get?
We get ataxia.
And that looks like, if I ask a person with ataxia to place
their finger on my finger, it's gonna look like this.
They're gonna start out okay, a little slowly, but as they get close to there,
they're going to miss the target.
That's dysmetria.
In other words, they're not measuring
the, how far they should go accurately.
They typically overshoot and then they have to come back.
And they're essentially,
instead of smoothly stopping the muscle that extends the arm and
starting to activate the muscle that pulls back the arm so
that you stop on a dime on my finger.