on the molecular level.

Let's take a step back now and just understand how muscles

look, at least structurally, or how they relate to things

that we normally associate with muscles.

So let me draw a flexing bicep right here.

That's their elbow and let's say that's

their hand right there.

So this is their bicep and it's flexing.

I think we've all seen diagrams of what muscles look,

at least on kind of a macro level and it's connected to

the bones at either end.

Let me draw the bones.

I'm not going to detail where-- so it's connected to

the bones at either end by tendons.

So this right here would be some bone.

Right there would be another bone that it's connected to.

And then this is tendons, which connects the bones to

the muscles.

We have the general sense-- connected to two bones, when

it contracts it moves some part of our skeletal system.

So we're actually focused on skeletal muscles.

The other types are smooth muscles and cardiac muscles.

Cardiac muscles are those, as you can imagine, in our heart.

And smooth muscles are-- these are more involuntary, slow

moving muscles and things like our digestive tract.

And I'll do video on that in the future, but most of the

time when people say muscles, we associate them with

skeletal muscles that move our skeletal system around, allow

us to run and lift and talk and do and bite things.

So this is what we normally associate-- let's dig in a

little bit deeper here.

So if I were to take a cross section of this bicep right

there-- if I were to take a cross section of that muscle

right there-- so let me do it big.

And then it looks something like this.

This is the inside of this muscle over here.

Now I said back here, we had our tendon.

And then there's actually a covering; there's no strict

demarcation or dividing line between the tendon and the

covering around this muscle, but that covering is called

the epimysium and it's really just connective tissue that

covers the muscle, kind of protects it, reduces friction

between the muscle and the surrounding bone and other

tissue that might be in this person's arm right there.

And then within this muscle, you have connective tissue on

the inside.

Let me do it in another color.

I'll do it in orange.

This is called a perimyseum, and that's also just

connective tissue inside of the actual muscle.

And then each of these things that the perimysium is

dividing off-- let me say if we were to take one of these

things and allow it to go a little bit further-- so if we

were to take this thing right here-- what this perimysium is

dividing off-- and if we were to pull it out-- actually, let

me do this one right here.

If we were to pull this one out just like that-- so you

have the perimysium surrounding it, right?

This is all perimysium, and it's just a fancy word for

connective tissue.

There's other stuff in there.

You could have nerves and you could have capillaries, all

sorts of stuff because you have to get blood and neuronal

signals to your muscles of entry so it's not just

connective tissue.

It's other things that have to be able to eventually get to

your muscle cells.

So each of these-- I guess you'd call it subfibers, but

these are pretty big subfibers of the muscle.

This is called a fascicle.

The connective tissue inside of the fascicle is called the

endomysium.

So once again, more connective tissues, has capillaries in

it, has nerves in it, all of the things that have to

eventually come in contact with muscle cells.

We're inside of a single muscle.

All this green connective tissue is endomysium.

And each of these things that are in the endomysium are an

actual muscle cell.

This is an actual muscle cell.

I'll do it in purple.

So this thing right here-- I can pull it out a little bit.

If I pull this out, this is an actual muscle cell.

This is what we wanted to get to, but we're going to go even

within the muscle cell to see, understand how all the myosin

and the actin filaments fit into that muscle cell.

So this right here is a muscle cell or a myofiber.

The two prefixes you'll see a lot when dealing with

muscles-- you're going to see myo, which you can imagine

refers to muscle.

And you're also going to see the word sarco, like

sarcolemma, or sarcoplasmic reticulum.

So you're also go see the prefix sarco and that's

flesh-- so sarcophagus-- or you can think of other things

that start with sarco.

So sarco is flesh.

Muscle is flesh and myo is muscle.

So this is myofiber.

This is an actual muscle cell and so let's zoom in on the

actual muscle.

So let me actually draw it really a lot bigger here.

So an actual muscle cell is called a myofiber.

It's called a fiber because it's longer than it is wide

and they come in various-- let me draw the

myofiber like this.

I'll take a cross section of the muscle cell as well.

And these can be relatively short-- several hundred

micrometers-- or it could be quite long-- at least quite

long by cellular standards.

We're talking several centimeters.

Think of it as a cell.

That's quite a long cell.

Because it's so long, it actually has to

have multiple nucleuses.

Actually, to draw the nucleuses, let me do a better

job drawing the myofiber.

I'm going to make little lumps in the outside membranes where

the nucleuses can fit on this myofiber.

Remember, this is just one of these individual muscle cells

and they're really long so they have multiple nucleuses.

Let me take its cross section because we're going to go

inside of this muscle cell.

So I said it's multinucleated.

So if we kind of imagine its membrane being transparent,

there'd be one nucleus over here, another nucleus over

here, another nucleus over here, another

nucleus over there.

And the reason why it's multinucleated is so that over

large distances, you don't have to wait for proteins to

get all the way from this nucleus all the way over to

this part of the muscle cell.

You can actually have the DNA information close to where it

needs to be.

So it's multinucleated.

I read one-- I think it was 30 or so nucleuses per millimeter

of muscle tissue is what the average is.

I don't know if that's actually the case, but the

nucleuses are kind of right under the membrane of the

muscle cell-- and you remember what that's called from the

last video.

The membrane of the muscle cell is the sarcolemma.

These are the nucleuses.

And then if you take the cross section of that, there are

tubes within that called myofibrils.

So here there's a bunch of tubes inside

of the actual cell.

Let me pull one of them out.

So I've pulled out one of these tubes.

This is a myofibril.

And if you were to look at this under a light microscope,

you'll see it has little striations on it.

the striations will look something like that, like

that, like that, and there'll be little thin ones

like that, like that.

And inside of these myofibrils is where we'll find our myosin

and actin filaments.

So let's zoom in over here on this myofibril.

We'll just keep zooming until we get to the molecular level.

So this myofibril, which is-- remember, it's inside of the

muscle cell, inside of the myofiber.

The myofiber is a muscle cell.

Myofibral is a-- you can view it as a tube inside of the

muscle cell.

These are the things that are actually doing the

contraction.

So if I were to zoom in on a myofibril, you're going to see

it-- it's going to look something like that and it's

going to have those bands in it.

So the bands are going to look something like this.

You're going to have these short bands like that.

Then you're going to have wider bands like that, like

these little dark-- trying my best to draw them relatively

neatly and there could be a little line right there.

Then the same thing repeats over here.

So each of these units of

repetition is called a sarcomere.

And these units of repetition go from one-- this is called a

Z-line to another Z-line.

And all of this terminology comes out of when people just

looked under a microscope and they saw these lines, they

started attaching names to it.

And just so you have the other terminology-- we'll talk about

how this relates to the myosin and the actin in a second.

This right here is the A-band.

And then this distance right here or these parts right

here, these are called the I-bands.

And we'll talk about really in a few seconds how that relates

to the mechanisms or the units that we talked-- or the

molecules that we talked about in the last video.

So if you were to zoom in here, if you were to go into

these myofibrils, if you were to take a cross section of

these myofibrils, what you'll find is-- if you were to cut

it up, maybe slice it-- if you were slice it parallel to the

actual screen that you're looking at, you're going to

see something like this.

So this is going to be your Z-band.

This is your next Z-band.

So I'm zooming in on sarcomere now.

This is another Z-band.

Then you have your actin filaments.

Now we're getting to that molecular level

that I talked about.

And then in between the actin filaments, you have your

myosin filaments.

Remember, the myosin filaments had those two heads on them.

They each have two heads like that, that crawl along the

actin filaments.

I'm just drawing a couple of them and then they're attached

at the middle just like that.

We'll talk about in a second what happens when the muscle

actually contracts.

And I could draw it again over here.

So it has many more heads than what I'm drawing, but this

just gives you an idea of what's happening.

These are the myosin, I guess, proteins and they all

intertwined like we saw in the previous video and then

there'll be another one over here.

I don't have to draw in detail.

So you can see immediately that the A-band corresponds to

where we have our myosin.

So this is our A-band right here.

And there is an overlap.

They do overlap each other, even in the resting state, but

the I-band is where you only have actin

filaments, no myosin.

And then the myosin filaments are held in place by titin,

which you can kind of imagine as a springy protein.

I want to do it in a different color than that.

So the myosin is held in place by titin.

It's attached to the Z-band by titin.

So what happened?

So we have all of these-- when a neuron excites-- so let me

draw an endpoint of a neuron right here, the endpoint of an

axon of a neuron right there.

It's a motor neuron.

It's telling this guy to contract.