Three types of muscle | Circulatory system physiology | NCLEX-RN | Khan Academy

Let's talk about muscles.

And I've drawn the
human body on the right,

kind of a figure of it.

And I want to talk about the
three major types of muscles.

And I thought it would
be helpful to have

a picture, because then we
can actually draw on there

and show where the different
types of muscles might be.

So when I mention
muscles, the word

I want you to start thinking
about in your head is movement.

So think about all
the different types

of movements that might
happen in your body.

Just be really
creative and start

thinking of all the
different movements.

You might have, for example--
a really easy one would be,

maybe, let's say
your leg is moving.

I'm going to just draw on
our picture as we talk.

But let's say your leg is moving
because you're playing soccer.

And so you've got this
giant muscle in here,

and this muscle is
attached to a bone.

Right?

There's a little bone here.

I guess not so little, right?

This as the largest
bone in the body.

It's called the femur.

And so this muscle is
attached to the femur.

And this muscle is going to
be attached by way of tendon.

It's going to have
tendons on both sides.

And so this tendon is
attaching it to the bone

and allowing it to
act on the bones.

So this is an example
of skeletal muscle.

Right?

So this skeletal
muscle is going to be

attached to a tendon and bone.

Now, that brings
up the question--

does every skeletal
muscle have to be

attached to a tendon and bone?

Well, the answer
is no, actually.

There are some
muscles that really

aren't attached
to tendons at all.

In fact, right above
the muscle we just drew

is a muscle called the
external oblique muscle.

And don't worry so
much about the names.

But the idea here is that
this muscle is actually not

attached to a tendon.

Well, in a sense, I guess, you
could think of it as a tendon,

but it's like a flat tendon.

Basically a giant kind of
sheet of fibrous tissue.

And this fibrous tissue,
is it floating in midair?

No.

It's going to be connected
to fibrous tissue

on the other side, because, of
course, your body is symmetric

and so you've got fibrous
tissue on the other side.

And you guessed it,
on the other side

of that you've got
another external oblique.

So you've got these muscles that
are kind of coming in to not

really a tendon but
really a flat tendon,

or something that looks
like a flat tendon,

and we call that an aponeurosis.

You might hear these words.

I just want you to be
familiar with them.

And now if someone asks
you, is every muscle

in the body attached
to a tendon and bone?

You can say no.

Some are attached to a flat
tendon called an aponeurosis.

The idea here is
that you can kind of

start identifying
skeletal muscles.

They're usually the muscles
that you can see on your body.

Actually, I don't even
need to put quotes.

That's the actual name for it.

No need for quotes there.

So you can identify skeletal
muscles pretty easily.

But what about the other two?

What about the cardiac
and smooth muscle?

I mean, you might wonder,
does cardiac mean heart?

And is that the only type
of cardiac muscle out there?

And the answer is yes.

This is your heart
muscle right here.

And the only type of cardiac
muscle that we have in our body

would be related to the heart.

So in the heart, you can
find specialized cells that

were so interesting
and different

from skeletal and
smooth muscles,

they got their own
name and category.

These are the cardiac cells.

And you can only find
them in the heart.

I guess we're making a column of
where you can find these cells.

So what about smooth muscle?

Where can you find
smooth muscle?

Well, for smooth muscle,
think about any hollow organ.

Any organ that's got space on
the inside and blood vessels.

Those are the two
major categories.

Those aren't the only ones,
but those are the major ones.

That'll get you about
95% of the way there.

So blood vessels
and hollow organs

are what you should think about.

And hollow organs could be
anything from-- let's say,

your stomach would
be a hollow organ.

Let me just put
these examples here.

Or your bowels would be a hollow
organ, anything like that.

So I'm just going to
write stomach here just

to jog your memory.

Where there's basically some
empty cavity on the inside.

Right?

And then as for blood
vessels, just remember

one of the largest blood
vessels, for example,

is the aorta.

And the aorta kind of comes
up and over like that.

And it's kind of like a
hollow organ, as well.

Right?

I mean, there's a space on the
inside of that blood vessel.

And blood is usually
flowing through that space,

but at least it's hollow.

So it's really not that
different conceptually

from the hollow organ.

And just like in
the hollow organ,

the smooth muscle is in
the walls of these things.

So think about where the
smooth muscle would be.

It would be in the walls
of the hollow organ

or in the walls of
the blood vessel.

So that tells you where to find
these different muscle types.

Right?

And thinking about
movement, smooth muscle

can help the stomach, for
example, move food forward.

Cardiac muscle is going
to help your heart beat.

That's a pretty
important movement.

And skeletal muscle, I mean,
we use that every single day.

Every time you give
your friend a high five

or give your mom a hug,
those are skeletal muscles

that are helping your
body move around.

Right?

So let's move on.

Let's think about
some other differences

between these categories.

Let's talk about now
the movement control.

So who controls the movement?

Do you control it, or is
it automatically done?

So smooth muscle is what I
would consider automatic,

or I'm going to call it
involuntary because you'll

probably see that
word more often.

Involuntary just means that
your body is automatically

taking care of it.

And the same is true for your
cardiac muscle-- involuntary.

Meaning, you don't
have to actually think

about the next heartbeat.

It just happens automatically.

Right?

And skeletal muscle is
the opposite-- there,

it's voluntary.

Meaning if I didn't want to get
up, then I would not get up.

Or if I didn't
want to go running,

then I wouldn't go running.

All of those movements in my
body are under my control.

I can decide when
to do those things.

Right?

Actually, maybe I'll draw little
arrows here-- what about speed?

Which ones are fast,
and which ones are slow?

So up here, the smooth
muscle is the slowest

and the skeletal muscle
would be the fastest, which

is pretty cool because
the voluntary stuff--

the stuff you control
yourself-- is the fastest.

And the stuff that's happening
automatically is pretty slow.

And actually it's nice, because
cardiac muscle is somewhere

in between the two.

Somewhere in the middle.

So when your blood
vessels get tinier

or they get big and
vasodilate, all that stuff

is happening on a pretty slow
time scale as compared to,

let's say, I jump and
try to catch a ball.

That's all happening
really, really quickly.

Thousands of little
muscle movements

are happening really
lightning quick.

And so those would
be the fastest.

Now the final thing
I'm going to draw

is what these things look like.

So how do they look?

If you actually take a
look at these cells-- let's

actually look at each
of these one by one

and figure out what
they would look like.

So the smooth muscle actually
looks like a little eye,

or like an almond-- sometimes
it's described that way.

But I think of it as an eye.

One single eye.

And you can see that
the edges, or the ends,

are kind of tapered like that.

And so sometimes
you'll see that these

are described as spindle shaped.

I think that's kind of a
holdover from a time period

long ago when people
thought about spindles more

than they do now.

And the other thing,
it's got one nuclei.

Drew that right in the middle.

One nuclei.

And it's in the
middle of the cell.

So that's basically what a
smooth muscle cell looks like.

What about a cardiac cell?

Well, this cell is branched.

That's actually one of the
most interesting hallmark

features of it.

Now, not every single
cardiac cell is branched.

Some are actually just kind
of humdrum-looking, normal,

maybe like this.

But the fact that you
can find branched ones

is what really makes these
so easy to recognize.

If you look at a
whole bunch-- I'm

going to erase this guy now
that you know he exists,

but I'm going to focus
on the branched one

because these are the ones that
make them very easy to spot.

And they also have nuclei.

Sometimes one,
but sometimes two,

which is interesting
because, you know,

usually you think,
one cell, one nuclei.

But the reason I had to point
that out for the smooth muscle

cell, that there's only
one, is that sometimes

these cardiac cells
have more than one.

So the two features-- I'm
going to just write out

here-- branched and
one or two nuclei.

Not always two, but
they can have two.

And they're also located kind
of in the middle of this cell.

And I'll show you
what I mean by middle

when I draw the skeletal muscle.

I'll do that now.

This is the skeletal muscle, and
it's got something like this.

It's got these
little outpouchings

I'm trying to draw for you.

And you'll see in just a
second what I'm drawing.

These are little spots on the
edge, or on the periphery,

for nuclei.

And notice that there's not
one nuclei, not two nuclei,

but bunches of nuclei.

So these cells are actually
working as a giant cell,

in a sense.

So these are actually, first
of all, they're straight.

They're not branched.

So straight.

And they've got many nuclei.

This is actually really,
really important,

and you can see how it would be
easy to spot these guys, right?

Because they've got many
nuclei, and the nuclei

themselves are in the
periphery, kind of on the edges.

That's why I wanted to point
out that the other two are

in the middle.

Now, kind of a final
point is that if you

were to look at these under
a microscope-- and actually,

this is something that was
noticed a long time ago--

they would look
something like this.

And this is called striated.

So they basically
have these striations.

But notice that the smooth
muscle cells don't have this.

It's really just the skeletal
muscle and the cardiac muscle

that has these striations.

Sometimes you'll hear
about striated muscle,

and they could be talking
about either of the two.

Right?

They could be talking
about cardiac or skeletal,

but you know that
they're not talking

about the smooth muscle.

So this is striated.

And striated just
refers to those stripes.

And that's what it looks
like under a microscope.

And we'll talk about
exactly why they're

striated what that would
imply about the cell

in another video.

But I just want
you to get a kind

of a rough lay of the land.

And now you can see
there's actually

some interesting stuff here.

You have some similarities
between the cardiac

and the smooth muscle.

They're both involuntary.

You've got some similarities
between the skeletal

and the cardiac.

They're both striated.

And so you can see how all
three are somehow similar,

but also somehow different
from one another.