Anatomy of a muscle cell

Understanding the structure of a muscle cell

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.