Neuromuscular junction, motor end-plate - Medical Video Lecture - MBBS

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.