Saccade Circuits

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From the course by The University of Chicago

Understanding the Brain: The Neurobiology of Everyday Life

652 ratings

The University of Chicago

Understanding the Brain: The Neurobiology of Everyday Life

652 ratings

Learn how the nervous system produces behavior, how we use our brain every day, and how neuroscience can explain the common problems afflicting people today. We will study functional human neuroanatomy and neuronal communication, and then use this information to understand how we perceive the outside world, move our bodies voluntarily, stay alive, and play well with others.

From the lesson

The Vestibular Sense & Gaze

The vestibular system and gaze control give us so much but are grossly under appreciated. They are so fundamental that we discount them, assuming that they will always be there. When the vestibular system fails us, its importance dominates our lives. Living with either a vestibular or an eye movement disorder is very disruptive to everyday life. In this module, you will learn how the inner ear is designed to detect and respond to head movements. You will learn about the circuit that connects the inner ear to the motoneurons that control the location of your eyes, allowing us to reflexively maintain our view of an object even as we move about in the world. Finally, we will talk about how you can modify this reflexive control of the eyes and how you can control where you are looking.

Eye Movements5:05

Saccade Circuits4:47

Controlling Saccades6:03

Meet the Instructors

Peggy Mason
Professor
Neurobiology

[MUSIC]

Okay.

So how are saccades organized?

Saccades are organized in the brain stem and

we're going to look at the simplest saccad, which is a horizontal gaze shift.

So let's say, we're going to look to the right or look to the left,

how are we going to do that?

Well, there is a horizontal gaze center in the pons and

this is a is a bunch of neurons that can produce.

A a patterned, a patterned input to neurons

that results in a horizontal gaze shift.

So these two neurons here are part of this horizontal gaze center and

this neuron is the one that we're going to concentrate on,

which is a neuron that is going to send an excitatory input directly

to the motor neuron that excites the abducting muscle on the same side.

So this, this connection right here is going to get us half of a gaze shift.

When we, when we move our, in this situation, let's say that this is the left

and this is the right and we're going to make a leftward gaze shift.

Well, we have to activate this muscle and that muscle.

And so this neuron ec, exciting this motor

neuron that contracts this lateral rectus, which abducts this eye.

We're halfway there.

How we going to get the other half?

Well, it turns out that there's a, a neuron, a special neuron and

this neuron has a name which we don't, we don't is not, doesn't matter.

But this is a very special neuron that yolks the input to that neuron to

the input to the other neuron required for the same saccade.

So, it sends this same message, the same message that goes to this motor neuron.

And therefore, to this muscle, now goes to this interneuron,

which conveys the message to this motor neuron, which contracts that muscle.

And so we get contraction of this muscle and

this muscle and both eyes are moved towards the left.

Now, one of the reasons why I bring this up is because it shows you

I can show you using this circuit how circuits get built up and

how they can go wrong in, in sections.

And so, as it turns out this pathway right here is got

these neurons have very heavily myelinated axons.

And so in diseases, which are myelinated, central demyelinating diseases,

the most common example being multiple sclerosis.

Sometimes, what can happen is that these axons lose their myelin.

And when they lose their myelin, they don't work as well.

And in the extreme case, this connection is broken and

so when a person goes to look to the side,

this muscle works and this muscle doesn't.

Okay?

So what will that look like?

It's a dis, it's a symptom called internuclear ophthalmoplegia.

And if one is looking to the left, the lateral rectus works.

But that medial rectus, this medial rectus doesn't work,

so this eye stays looking forward.

And in contrast, if this entire pathway is blocked,

what you'll see is when you look the right, again,

lateral rectus will work, but the median rectus won't.

And so this is a, a really simple example of how you can take apart circuits and

parts of behaviors work or don't work.

Okay.

So, in the final segment of this module,

we're going to look at how saccades are controlled.

[MUSIC]

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