Electrotonic and action potential

We've already seen that when a
neuron is in its resting state

there's a voltage difference
across the membrane.

And so in these diagrams right
over here, this right over here

is the membrane.

This right over here is
the inside of the neuron,

and this right over
here is the outside.

That's the outside and of
course this is the outside.

This is the outside as well.

So if you had a
voltmeter measuring

the potential difference
across the membrane,

so if you took this voltage
minus this voltage right

over here, the voltage
between this and that,

you would get negative-- let's
say for the sake of argument,

let's say it would
measure, it would

average about negative
70 millivolts.

So this is in
millivolts, negative 70.

And I'll do it actually
for both of these graphs.

We're going to use both
of these to describe

slightly different, or actually
quite different, scenarios.

And you could have another
voltmeter out here in yellow,

and that's a little further
out, but that's also

going to register
negative 70 millivolts.

Now let's make something
interesting happen.

Let's say that, for
some reason, let's

say that the membrane
becomes permeable to sodium.

So sodium just starts
flooding through.

It's going to flood
through for two reasons.

One, it is a positive ion.

It's more positive
on the outside

than the inside, so positive
charge will want to flood in.

And the other reason why
it'll want to flood in

is because there's a higher
concentration of sodium

on the outside
than on the inside.

So it'll just go down its
concentration gradient.

And the reason why we have a
higher concentration gradient

on the sodium on the
outside than the inside,

we've already seen, is because
of the sodium potassium pump.

But anyway, so you're going
to have this increase.

You're going to really have
this spike in positive charge

flowing.

And then what's going to be the
dynamic then inside the neuron?

Well, if you have all this
positive charge right over here

the other positive
charge in the neuron

is going to want to
get away from it.

And this is not just in
the rightward direction.

It's really going to
be in all directions.

In all directions
the positive charge,

they're going to want to
get away from each other.

So this one's going
to move that way,

and then that's going
to make that one

want to move that
way, which is going

to make that one want
to move that way.

So if we let some
time pass, what's

the voltage going to look
like on this blue voltmeter?

Well after some time, because
more and more positive

charges are trying to get
away from these other ones

right over here as
the concentration

of these positive
charges spread out,

you're going to see the
voltage start to increase.

And then as they fully
get spread out then

it might return to
something of an equilibrium.

And then if we go a little
bit further down the neuron

a little more time
will pass before you

see a voltage increase, but
because this thing is just

getting spread out across
more and more distance,

the effect is going
to be more limited.

You're not going to
see as much of a bump

in the voltage over here
than you saw over here.

And this type of spread of, I
guess you could say a signal,

is called electrotonic spread.

Let me write that down.

Or this is the spread of
an electrotonic potential.

So there's a couple of
characteristics here.

One, it's passive.

This part that we
drew right here,

this isn't the
electrotonic spread.

The electrotonic spread is
what happens after that.

Once you have this high
concentration here,

the fact that a few
moments later you're

going to have a
higher concentration

of positive charge here, and
a few moments later a higher

positive concentration here.

This is a passive phenomenon.

So this thing right over
here, it is passive.

And it also dissipates.

The signal gets weaker and
weaker the further and further

you get out because this stuff
just gets further and further

spread out.

So it's passive
and it dissipates.

Now let's play out
this scenario again,

but let's also throw in some
voltage-gated ion channels

right over here.

So let's say this right
over here that I'm drawing,

let's say this is a
voltage-gated sodium channel.

Let's say it opens at
negative 55 millivolts.

So that would be
right around there.

So that is when it opens
at negative 55 millivolts.

Let me draw that
threshold there.

And let's say it closes at
positive 40 millivolts, right

over there.

I'm just trying to
show the threshold.

And let's say we also have
a potassium channel too,

right over here.

So this is a potassium channel,
the infamous leaky potassium

channels, which are
the true reason why

we have this voltage
difference across the membrane.

But this potassium
channel, let's

say it opens when
this one closes.

So it opens, just for
the sake of argument,

these aren't going to be the
exact numbers but to give you

the idea, at positive
40 millivolts.

And let's say it closes
at negative 80 millivolts.

So that one opens up here,
and then it closes down here.

Now what is going to happen?

Well just like we saw before--
Let's let our positive charge

flood in here at the
left side of this neuron,

I guess we could say, and then
because of electrotonic spread,

a little bit later
you're going to have

the potential across the
membrane at this point

is going to start to
become less negative.

The potential
difference is going

to become less negative, just
like we saw right over here.

So it's going to
become less negative.

But it's not just going
to be just a little bump

and then go back down,
because what happens right

when the potential hits
negative 55 millivolts?

Well then it's going to trigger
the opening of this sodium

channel.

So the sodium channel is going
to open because the voltage got

high enough, and so you're going
to have sodium flood in again.

So what's that going to do?

Well that's going to
spike up the voltage.

So it's going to look
something like that.

It's going to keep flowing
in, keep flowing in.

The voltage is going to
get more and more positive.

Because remember, this
is going to be flowing in

for two reasons.

One, there's just more charge.

It's more positive
outside than the inside

so it's going to go
across a voltage gradient,

or go down the voltage gradient,
or the electro potential

gradient, but also there's a
higher concentration of sodium

out here than there is in here
because of the sodium potassium

pump, and so it'll also want
to go down its concentration

gradient.

So it's just going to keep
flowing in even past the point

at which you have
no voltage gradient,

but because of the
concentration gradient

it's going to keep going.

But then, as you get to
positive 40 millivolts,

this channel is going to close.

So that's going to
stop flooding in.

And you also have the
potassium channel opening.

And the potassium
channel, now you're

more positive on the inside than
the outside, at least locally

right over here.

And so now you're going to
have this positively-charged

potassium ions want
to get out, want

to get out from this
positive environment.

And so the voltage is going
to get more and more negative,

and it's going to go beyond
neutral because potassium

is going to want to go down,
not just its voltage gradient,

it's going to do that while
it's positive on the inside

and negative on the outside,
or more positive on the inside

than it is on the
outside, but it'll also

want to go down its
concentration gradient.

There's a higher
concentration of potassium

on the inside than on the
outside because of the sodium

potassium pump.

So the potassium will
just keep going out,

and out, and out, and out,
and then at negative 80

millivolts the potassium
channel closes, and then we

can get back to our
equilibrium state.

Now why is this interesting?

Well we had the electrotonic
spread up to this point.

But the signal would just
keep dissipating and keep

dissipating, and if
you get far enough

it would be very hard
to notice that signal.

And so what this
essentially just did

is it just boosted
the signal again.

It just boosted the signal,
and now, a few moments later,

if you were to measure
the potential difference--

because these things are trying
to get away from each other

again, once again you have
electrotonic spread-- if you

were to measure the potential
difference across the membrane

where this yellow voltmeter
is, then you're going to have--

So where that yellow one is,
before it had just a little

dissipated bump
here, but now it's

going to have quite a nice bump.

And if you actually had
another voltage-gated channel

right over here, then
that would boost it again.

And so this kind of very
active boosting of the voltage,

this is called an
action potential.

You could view this as the
boosting of the signal.

The signal is spreading,
electrotonic spread, then you

trigger a channel, a
voltage-gated channel,

then that boosts
the signal again.

And as we'll see, the neuron
uses a combination, just

the way we described it here,
in order to spread a signal,

in order for it to have
the signal spread, in order

to obviously to spread
passively, but then

to boost it so that the signal
can cover over long distances.