0:12
Okay, so now, what we're going to do is see how in
your normal circumstance, how are we going to engage our motor neurons?
The motor neurons, motor neuron activation
is required for skeletal muscle contraction.
Well, who's going to tell the motor neurons
how, how to when, when to become activated?
Well, it depends on the type of motor neuron, and the type
of motor neuron is the same as the type of motor unit.
So, motor neuron, motor unit, same same thing.
So the slow motor neurons innervate a very small number
of muscle fibers and therefore they produce not very much force.
Now they can last for a long time, but they don't
produce much force because they don't innervate very many muscle fibers.
The fast fatigue resistant motor neurons innervate maybe 10, 30
muscle fibers, and so they produce a little bit more force.
The fast fatigables can innervate hundreds of muscle
fibers and they can produce a big force.
So, let's say that I want to start running.
I'm walking, I'm standing and as I'm standing,
I'm using slow fibers, slow motor neurons, and so
if we put force here and time is going this way.
So, I'm standing and I've got say five slow motor
units activated, but now I want to start to walk, I increase the
number of slow motor neur motor units that are activated.
And only when a lot of slow motor units are
activated do I then engage, so this is all slow, then
I start to engage the fast fatigue resistant motor neurons
and I increase my, the amount of force that I produce.
And then eventually, when I've engaged all those fast
fatigue motor units, I start to engage the fast fatigables,
and now we're going to get a big increase in in
the amount of force that I've produced from this muscle.
And so, this is good for walking, I'm sorry, this is good for standing.
2:56
Can't sustain it for very long, but I can get a lot of force out of it.
And this is called orderly recruitment and it's a very important concept.
It's essentially a, a motor version of, of Weber's Law.
So, let's, let's imagine, what would it look like if if we didn't have
orderly recruitment, let's say that it was just it was just a random.
Which type of motor neuron type, motor unit, was engaged at the beginning?
Well if I went, I was going along here, and
I had these slow motor units happening, and then I all
of a sudden turned on a fast fatigable motor unit,
I'd have this kind of thing, I'd have this jerky movement.
So that, that's not very useful, that's not very smooth.
And alternatively, if at this point, I put on, I
engage a slow motor unit, it's not going to make any difference.
It's too small an increment of, of force.
And so, orderly recruitment allows me to steadily
increase the force and to make a smooth movement.
So the smoothness of the movement is built in to orderly recruitment.
4:22
As it turns out, if you don't use it, you kind of lose it.
So, what I mean by that is if you're leave, leading a
relatively sedentary life and you ne, and you don't engage the
fast fatigable fibers in a muscle they become harder to recruit.
4:46
And so what you'll find is that if a
person that has a sedentary lifestyle, and they're asked to
make as strong a muscle contraction as they can,
they'll probably get 100% of these slow motor units engaged.
They'll get 100% of the fast fatigue resistant ones, but they'll get something
well less than 100%, maybe 70% or so, of, of the fast fatigue fast fatigables.
And so, what can you do about that?
5:33
Lift weights, lift weights, you feel your muscles.
The next day, you are able to recruit more
fast fatigable fibers in the muscle that you exercised.
And that, that ease of recruitment of fast fatigable fibers due to one
strength exercise bout lasts for weeks.
So that's a very nice story so if
you want to get more fast fatigables online lift weights.
Now one other thing that we should look at is what
happens when we lose various types of, of motor units.
If we lose these slow motor units we're going to have a problem with endurance.
And if we lose the fast fatigables or if we don't actually ever
engage them because we're leading a very,
very sedentary life we're going to lose strength.
So, you can see that these different types of motor neurons and motor
unit, motor unit types differentially effect endurance and strength.
6:47
in, in, various diseases where where we
lose motor neuron, motor units there has
been the, the, attempt to try to make the muscle fibers
actually switch from, say, slow to fast, and that's
that's an ongoing area of, of investigation.
7:15
It, it holds the promise of being able to give people who have lost strength some
more strength back if we could, for instance,
switch a slow fiber into a fast fiber.
it, it's not easily done, if at all, but
it's definitely something that holds promise for the future.
[MUSIC]
Peggy MasonProfessor
