So when we're at a social gathering
and we've determined that this is the most appropriate time
to perform the worm, how does our brain
tell our muscles to contract?
Well, in this video, we'll talk about the place
where neurons talk directly to muscles.
That's the neuromuscular junction,
the junction of where motor neurons talk to muscle cells.
So that involves, first, the axon terminal.
This is the end of an axon, which
is the part of a neuron that casts a signal away.
And it looks like this.
It gets larger at the end right here.
And then it tapers back off like that.
Muscle cells sits adjacent to these axon
terminals at the neuromuscular junction,
and kind of look like a block, but not exactly.
They have these in-foldings that I'm drawing right here,
these in-pouchings.
And why does nature cause in-pouchings to occur?
What's the purpose of these guys?
What function do they serve?
Well, if you said that it serves to increase surface area,
you're absolutely right.
Because with the increased surface area,
we're going to have extra space where
we can have sodium channels present
that will help us transmit a message into the muscle cell.
And so it's not just present on the outside,
but there are a bunch of sodium channels
that are deep inside as well.
And in addition to sodium channels,
you definitely have calcium channels
that are present as well.
They are situated buried deep within your muscle cells, too,
to make sure that the most deepest parts of your muscle
cells will get an influx of calcium
when it's the right time.
And to foreshadow a point we'll discuss later,
I'm going to draw another muscle cell right here, just
kind of chilling out on its own.
So now how does the axon send a message to the muscle cell?
Well, if you recall, there's going
to be a signal that's cast away from our motor neuron
to this axon terminal.
And that signal is in the form of an influx of sodium ions.
So this is a depolarized membrane
that propagates this signal to this axon terminal.
But it's not just sodium that's influxing.
You're also going to have some calcium that's
running in as well.
And the calcium here is actually going to play a major role.
Because in our axon terminal, we have a bunch
of vesicles that are sitting around in here.
These are just little pockets that
are waiting for something to happen.
And in each of these pockets, we have a message
that's waiting to be released into the space between our axon
terminal and the muscle cell.
This message is called a neurotransmitter,
which is a very well-named scientific term.
Because all this is is a molecule
that the neuron uses to transmit a message.
And so the neurotransmitter that we use in the neuromuscular
junction is called acetyl-- like from chemistry,
acetyl-- choline, acetylcholine.
And oftentimes, you'll see it abbreviated as ACh.
So when there's an influx of calcium
into the axon terminal, what'll happen
is that the calcium will actually bind to our vesicle.
There are proteins that are on it
that'll grab onto the calcium.
And so when there's calcium attached to these proteins,
the vesicle will be drawn to this axon terminal membrane
and actually fuse with it to become one continuous membrane.
As a result, we release the acetylcholine
into this space we call the synaptic cleft.
The synaptic cleft is just the space
between our presynaptic membrane, the membrane
of our axon terminal, as well as our postsynaptic membrane,
which is just the membrane of our muscle cell.
So this is our postsynaptic membrane.
All right, and so we have a bunch of acetylcholine
that's released into the synaptic cleft
and is ready to send a message on.
But let's take a minute here.
What just happened here with the membrane?
I mean, the vesicle literally became one
with the membrane of the axon terminal.
What would you call this, if you have to give it a name?
Well, it looks like some compound within the cell
exited the cell.
So I'll say "exo."
And it exited a cell, so a "cyte"-- exocytosis.
Exocytosis, and that's the process of molecules,
or substrates, leaving a cell by vesicles fusing with membranes.
And so that's what we do when we want things to leave cells.
The exact opposite process, where
we have things enter cells by fusion of vesicles,
is called endocytosis.
And these are very important terms keep in mind.
Great, so now we've got acetylcholine
all over our synaptic cleft.
What is it going to do here?
Well, these sodium channels have receptors
that sit on them that are called nicotinic acetylcholine
receptors.
And as the name suggests, acetylcholine
can come and very snuggly sit here and send a message
to this sodium channel that it's time to open and cause
sodium to influx into our muscle cell.
And that's going to happen across the membrane,
causing a large amount of sodium to enter.
And once we've depolarized the membrane enough,
calcium will even start to enter.
And so in this way, we'll have what's
referred to as voltage-gated calcium release.
All right, and so we have all this calcium
that's entering the cell.
Now what?
Well, this is just one cell contracting.
How does this make me able to do the worm, or to kick a ball,
or to high five my best bro?
Well, we can't just have calcium entering through the membrane.
There's another reservoir within our muscle cells
that releases calcium for our disposal.
This reservoir-- kind of a large name,
and I'll write it out right here--
is called the sarcoplasmic reticulum, R-E-T-I-C-U-L-U-M,
sarcoplasmic reticulum.
This guy holds a whole bunch of calcium.
It's sitting in there waiting to be released,
waiting to do something.
So there are proteins that are attached
to the membrane of the muscle cell.
And they're just waiting for enough calcium
to be present here so that one of these calcium cations
could bind this protein complex and then effectively cause
calcium to be released from the sarcoplasmic reticulum.
This process is called calcium-induced calcium
release.
And so now we have a bunch of calcium in here,
and this muscle cell will be contracting.
But it's still just one muscle cell.
What's the big deal?
Well, this muscle cell is attached
to its neighbor right here, actually.
There are proteins that link muscle cells together.
These proteins are called gap junctions.
And they allow for cations to flow from one muscle cell
into another.
So these muscle cells aren't really separate at all.
Actually, they're connected because of this gap junction
and actually continuous.
And there's a term that we use to drive home
the fact that we can have our calcium cations
move to this other muscle cell and to start
a calcium-induced calcium release process here.
And it's that our muscle cells are in a syncytium.
What does that mean?
Well, "cyte," just like we talked about her, just
means a cell.
And "syn" means that these muscle cells
are in a synergy with each other.
When one contracts, it causes its neighbor
to contract as well, and that's how we scale up.
Because when muscle cells start contracting as neighbors,
you get the entire neighborhood contracting,
because you can then scale up and imagine that not
just muscle cells, but muscle fascicles
will also be contracting, too, to produce
a kick of a ball or the worm.
So this signal that started from our axon terminal
here that begins as just depolarization turns
into an acetylcholine molecule that undergoes exocytosis
to end up in the synaptic cleft, where it binds
a nicotinic acetylcholine receptor to cause sodium
to enter a muscle cell, and over time, calcium to enter a muscle
cell, which would then cause calcium-induced calcium
release, which can then go to an adjacent muscle cell
to cause a synergy of muscle contraction that spans
from muscle cell to muscle cell, from fascicle to fascicle.
And that's what happens at the neuromuscular junction.