Fast and Slow Fibers

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From the course by The University of Chicago

Understanding the Brain: The Neurobiology of Everyday Life

688 ratings

The University of Chicago

Understanding the Brain: The Neurobiology of Everyday Life

688 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!"

Fast and Slow Fibers5:12

Motoneuron Types4:11

Orderly Recruitment7:40

Exercise and Muscle Mass2:48

Meet the Instructors

Peggy Mason
Professor
Neurobiology

[MUSIC]

Okay, so let's start with the two different types of skeletal muscles.

There are fast skeletal muscles and slow skeletal muscles.

And they differ on a few criteria.

So first of all the slow, skeletal

muscles, use oxygen as their energy source.

And because they use oxygen, they need to get it from the blood.

They need to be, vascularized, so there's lots of

capillaries that are going into the, slow skeletal muscle.

And it gives the, this muscle the appearance of red.

It gives it a red appearance.

And because it's, it's deriving its, its energy

resource from blood, it can't be too far away from a capillary.

That puts a, an upper limit on how big this muscle can be.

It can be, it can't be very big.

So, it, the upper limit is, is if we looked at the cross section, a, a

muscle fiber looks like this, but if we took a cross section, we'd see that.

This can be that big and in contrast, the fast skeletal muscles can be much bigger.

The fast skeletal muscles can be much bigger because

they do not depend on oxygen as their energy source.

They depend on glycogen which is contained within the muscle fiber.

Now the, the other big difference between these two muscles.

Types.

Muscle fiber types.

Is that the slow muscle fibers can

be contracted for hours, and hours, and hours.

They take a little while, they take a few seconds to.

Get going, but, once they're going, they can go for hours.

The fast muscle fibers, they, they go really fast, they, they're,

if you contract the fast muscle fiber, it goes like this.

If you contract a slow muscle fiber, it goes like that.

But once that fast muscle fiber is contracted, it can only

stay contracted for a few minutes before it runs out of glycogen.

Oxygen going to keep coming.

Bloods going to keep coming.

Glycogen is a limited resource.

And so, as you may imagine different types of, different muscles that we have.

Contain different proportions of fast and slow muscle fibers.

So let's take two,a couple of examples.

One is the calf muscle which is involved in allowing us to stand.

We can stand for hours.

We could stand for a full day.

And that's because.

Standing depends in large part on just the, the fact that the ankle

is, the ankle joint is, is very well built, and it takes a lot of the strain.

But the, as far as muscle power put into standing, it's mostly

the calf muscle, and it's mostly the part of the calf muscle called the soleus.

Which is contain, contains about 90% slow muscle fibers.

So these slow muscle fibers are good to go for a long, long time.

So, you can go and stand in a marathon.

Now on the other hand, if you want to go hurdle,

if you want to go jump, that takes a lot of power.

Need a bigger muscle.

You need it to happen fast, so you get

across that hurdle, you don't want bump into it.

And, but the problem is, that it can't stay contracted for very long.

You're only going to, if you hurdling, you're going to

use it for a few seconds and then not.

And then for a few seconds and then not.

So, these are very different types of, of muscle fibers.

Another, let me just give one more example of of

a fast, muscle, which is the muscle that's involved in blinking.

So the muscle that's involved in blinking, if this

is the eye, the muscle, it actually circles the eye.

It's a circular muscle called the orbicularis oculi.

And we can blink really fast.

So that blinking is is mostly fast muscle fibers.

So the orbicularis oculi like some of the muscles

that are involved in, in doing things like jumping.

Contain mostly fast fibers, whereas the soleus and

other postural muscles are going to contain mostly slow fibers.

Okay.

In the next segment what we're going to look

at is the motor neurons that enervate these muscles.

[MUSIC]

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