So let’s take a closer look
at the action potential.
Again, this is something you
definitely going to need to know.
This is something you’re going
to visit in the physiology
and biology section as well,
so this is probably a review.
But let’s take a look.
So, we have something called a
“resting membrane potential”.
This is when a neuron
or a cell is at rest.
It’s not really doing anything
other than sitting there
and the starting
gate, ready to go.
And at this point, based on
the concentration of ions,
inside and outside of the cell,
we’ve established what we
call an electrical gradient,
an electrochemical gradient.
And it’s a referred to it such
because we have charge outside,
we have charge inside and we’ve
separated the amount of charge
on purpose by design so that we can make
the inside more negative than the outside.
So the inside is minus 70 millivolts
in relation to the outside.
And we accomplish this
by differentiating the
concentration of two specific
ions, sodium and potassium.
So sodium, we have
all heard of, is salt.
It’s abbreviated Na+.
It carries a positive
charge with it.
And then there’s potassium,
which it’s abbreviated with a K, and
that also has a positive charge.
So what we’ve done is we’ve created
-- we act like I did this.
But what evolution nature science
has done is created this gradient
with more sodium on the outside
and more potassium on the inside.
And we’ve created this electrical
potential which is at minus 70.
So, we say that the inside of the cell
is more negative than the outside.
So it seems like an odd way to say thanks
but that’s how they decided to do it.
So the inside is more
negative than the outside.
Now, how do we maintain
That’s done through a
sodium potassium pump,
which requires energy, so
it’s an active process.
Think of a sump pump or bilge pump that’s
constantly taking something and spitting
it out but it’s doing it at the expense
of energy and it uses ATP as energy.
So we call this a sodium
potassium ATPase pump.
And it does it in a relationship of
three sodiums out for two potassiums in.
And remember, I told you, there’s a higher
concentration of sodium on the outside
and there’s a higher concentration
of potassium on the inside.
And so what happens is, the basic
laws of physics tell us that,
things want to go from an area of high
concentration or low concentration, right?
That’s the definition
of diffusion basically.
So, there is a drive for
that sodium which is outside
because there’s more of it
outside to wanting to go in.
And so, some of it
does find its way in.
There is a leakage current and some
ions do sneak their way in through.
And so, that trickling effect
is that you end up getting some
more sodiums coming in, but we
want to maintain that minus 70.
And so, this pump allows
us to maintain that by
taking three sodiums out
for two potassiums in.
Now, let's do some basic math.
Three minus two leaves us one.
So we’re actually adding --
sorry, we’re removing
because it’s three out.
So we’re removing
an extra sodium.
So we’re removing an extra
positive charge out of the cell.
If you do that over and
over and over, over time,
we keep moving out
one positive charge.
So three for two, three
for two, three for two,
we continue to make the
inside more negative.
So this is one of the ways
that we actually maintain and
achieve that inside being more
negative than the outside.
in the process of an action
potential, once a stimulus is applied
and you’re going to see opening
of voltage-gated channels,
which we’ll take a look at in another figure,
but basically there’s voltage-gated.
So like the name implies.
Once there’s a change in
voltage, it’s a gate.
It’s going to open the door.
It’s going to open the ion channel
allowing whatever ion that
that channel is designed to allow
access to, to come in or out.
So, in this case, we’re talking
about a sodium voltage-gate --
So the change in voltage will open
it and it will allow sodium in.
All of a sudden, all
that sodium that’s been
waiting outside is just
waiting to get in.
It’s waiting to get
in to the party.
The doors are happen.
They’re going to go rushing in.
All sodium ions go in and they
carry with it the positive charge,
and that’s why you see
their charge going up.
So as the charge go up,
that is a sodium going in.
Eventually, all the sodium
that’s going to get in,
gets in, and that’s the peak of the
top of this action potential curve.
At the point, the
door closes, okay?
So, party is over, door closes, and now
you start to see it falling because
we at the same time are starting to now
open voltage-gated potassium channels.
So like the name implies, it’s
based on a change in voltage
is what activating it and
it’s allowing potassium in.
Now, that voltage change is because of
all that sodium that just rushed in.
So that change in voltage due
to sodium caused an opening
of the potassium channels
which are also voltage-gated.
Now, once the potassium
door is open,
they want to leave the inside
of the cell because they have a
higher concentration on the
inside and they want to get out.
Because remember what I said,
the laws of diffusion say,
you want to go from an area
of high concentration to low.
So their driver is, “We want to get
out of here. We want to go out.”
So the analogy like to use is think
of being in a crowded elevator
and it’s jammed in there, and it
says, “Maximum capacity 12 people,”
but you somehow have
squeezed out 15.
And there are a couple
large individuals that are
extremely sweaty right
now and you’re jammed.
Do you really want to
be in an elevator?
Once those doors
open, what happen?
You get out of that
So use that analogy
in your head.
We got an elevator, a
big sweaty elevator.
And as soon as that door opens, that ion
channel opens, everything is rushing out.
So the potassium goes rushing out and
that causes the voltage to fall.
And it does so quite efficiently
you can see it drop.
It does it so well that
actually dips below minus 70.
But then those voltage-gated
ion channels close as well.
And then we have this reestablishing of that resting membrane potential
to the sodium potassium pump, another
mechanisms, and then we return to rest.
So that whole process of rest, rise, fall,
and return would be an action potential.
Now, I just explain this whole thing in a
couple of minutes and it seems like a lot.
And you’re probably thinking, “Wow, this
must take couple of minutes to happen.”
Well, the answer is, this actually happens
on the order of milliseconds, milliseconds.
So you have to appreciate that.
A lot is going on and
that is how a signal is
actually passed along
the length of a neuron.
So if you think of things like, right
now, I ask you to snap your fingers.
For that to happen,
I am sending a signal from my brain
all the way down to my hand to snap.
And that is through a process
of several synapses.
That is synaptic
transmission happening live.
You put your hand on a burning stove,
and how quickly do you move your
hand away from that burning stove?
So this is the science learning
you’re going to extremely
excited about this process
because it is so fast.
It’s eloquent and it’s doing
it through so many steps.
Now, this process, we went into a
descent amount of detail here,
is going to keep coming up.
And so when I say, as an
action potential propagates
down to neuron, I’m
referring to this process.
Now, more specifically, we’re
going to have to get in
to how it actually travels
down the length of a neuron.
So that action potential
doesn’t travel exactly like a
wave if you image a notion,
but it does so kind of.
And we’ll see what
I’m talking about.
So this wave of depolarization
travels down the length of an axon,
but it doesn’t do it in a consistent
fashion in your typical myelinated neuron.
So, this is mediated by those voltage-gated
sodium channels that I’ve talked about.
The electrical potential across the
plasma membrane quickly to rest --
restored to a resting state
by the voltage-gated
potassium channels that
sort of off switch.
And the action potential travels down the
process called “saltatory conduction”.
That is a term you need to know.
So that’s what I mean by “it’s not
traveling as a consistent wave”.
If you think of an ocean
and a wave coming in,
you have this nice wave that travels
all the way down into the beach.
Now, as an action potential, we
actually have this hop, skip,
and jump method in terms of the
wave actually hops and jumps.
That process of jumping is
called “saltatory conduction”.