I think we have a respectable sense of how muscles contract
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.
You have the action potential.
The action potential travels along the membrane, really in
all directions.
And then it eventually, if we look at it from this view,
they have those little transverse or T-tubules.
They essentially go into the cell and continue to propagate
the action potential.
Those trigger the sarcoplasmic reticulum to release calcium.
The calcium attaches to the troponin that's attached to
these actin filaments that moves the tropomyosin out of
the way, and then the crawling can occur.
The myosin can start using ATP to crawl
along these actin filaments.
And so as you can imagine, as they crawl along, their power
stroke is going to push-- you can either view it as the
actin filaments in that way or you can say that the myosin is
going to want to move in that direction, but you're pulling
on both sides of a rope, right?
So the myosin is going to stay in one place and the actin
filaments are going to be pulled together.
And that's essentially how the muscle is contracting.
So we've, hopefully, in this video, connected the big
picture from the flexing muscle all the way over here
to exactly what's happening at the molecular level that we
learned in the last few videos.
And you can imagine, when this happens to all of the
myofibrils inside of the muscle, right, because the
sarcoplasmic reticulum's releasing calcium generally
into the cytoplasm of-- which is also called myoplasm,
because we're dealing with muscle cells-- the cytoplasm
of this muscle cell.
The calcium floods all of these myofibrils.
It's able to attach to all of the troponin-- or at least a
lot of the troponin on top of these actin filaments and then
the whole muscle contracts.
And then when that's done, each muscle fiber, myofiber,
or each muscle cell will not have that
much contracting power.
But when you couple it with all of them that are around
it-- if you just have one, actually, working, or a few of
them, you'll just have a twitch.
But if you have all of them contracting together, then
that's actually going to create the force to actually
do some work, or actually pull your bones together, or lift
some weights.
So hopefully you found that mildly useful.