Monosynaptic Reflex

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From the course by The University of Chicago

Understanding the Brain: The Neurobiology of Everyday Life

608 ratings

The University of Chicago

Understanding the Brain: The Neurobiology of Everyday Life

608 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

Voluntary Movements

"Voluntary movements are how we interact with the world. The organization of the motor system has implications for how we move, and the types of movements we can make. This module we'll start to explore the organization of the motor system, from neurons to muscles. This module contains a lot of material, so be sure to start early so that you have time to finish!"

Monosynaptic Reflex5:28

Co-activation4:04

Gamma Reflex Loop5:12

Meet the Instructors

Peggy Mason
Professor
Neurobiology

[MUSIC]

So now we're going to talk about reflexes, and we're going to

spend some time on a very basic reflex, the stretch reflex.

This is the reflex that the physician does in

the office with tapping your, your, your knee there.

There's a lot of complexity to this.

This is actually, if you understand this, if I can explain this to you, then

you have understand, probably, understood that probably the

most complicated circuit that you could possibly construct.

So before we get to the reflex we have to understand

one thing about muscles, because a reflex means

that there is a sensory input [SOUND] and a motor output.

And the sensory input for the monosynaptic

stretch reflex comes from the muscle itself.

So, if this muscle, this muscle has all these

muscle fibers, and when they contract, the muscle shortens.

And these muscle fibers, are big and they produce force.

But sitting in and amongst these muscle fibers, are little

packages called muscle spindles, and in these muscle

spindles, if we look at one of these muscle spindles, we see that

it's made up of, about five to ten very thin muscle fibers.

These muscle fibers, if, if we cut across like this, that

muscle fiber is that thin, and this muscle fiber is that thin.

Is that, so this is the relative diameter of these two muscle fibers.

When this muscle fiber, which is what we traditionally think of

as a muscle fiber, when that contracts, there is force generated.

When this muscle fiber contracts, nothing happens.

In fact, this muscle fiber is serving a sensory role.

So, if we look at this so, let's say that we stretched this muscle.

This muscle fiber would be stretched, and we're going to

name these two muscle fibers, just for ease here, the

small ones that don't produce force are called intrafusal, and

the larger ones that produce force are called extrafusal.

So, the monosynaptic reflex is pretty simple.

What happens is, there is a sensory neuron that wraps itself

around the interfusal fiber and it is, it's activated

whenever the interfusal fiber is, is stretched.

And so, this is activated.

It's a primary afferent that has a cell body and a dorsal root ganglion.

Here's the spinal cord.

It comes in and it goes directly to a motor neuron.

And this motor neuron comes out and

innervates a muscle fiber in the same muscle.

So, when this muscles stretched, this,

this motor neuron is activated to contract.

So you stretch it, you contract it.

So it's a corrective mechanism.

It's a feedback.

You stretch, the reaction is to, to contract it.

Now we name this, muscle this reflex, the monosynaptic reflex,

because there's one synapse, and what we mean by one synapse is we mean

that there's one synapse that's in the central nervous system.

We don't count this synapse.

So there's one synapse.

This is a really fast reflex, it happens in a matter of a few milliseconds.

And it is also called the stretch reflex, because the

thing that starts it is a stretch of the muscle.

These intrafusal fibers, these muscle spindles,

are distributed throughout the the muscle.

And so any time the muscle is stretched, either because somebody extends

your arm that you weren't aware of, because you trip for any type of

reason you pick up something and it has liquid in it, and you

didn't know it has liquid, it's a little bit heavier than you thought.

Whatever load, whatever weight you put on this

muscle is going to evoke this monosynaptic reflex.

Now, in the next segment, what we're going to look at is,

how does the monosynaptic reflex work if we're actively contracting our muscles?

[MUSIC]

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