Electrotonic and Action Potentials

How electrotonic and action potentials propagate down cells

In the last video, we talked
about how the cell uses a

sodium potassium pump and ATP
to maintain its potential

difference between the inside
of the cell or the inside of

the neuron and the outside-- and
in general, the outside is

more positive than the inside.

You have a -70 millivolt
potential difference from the

inside to the outside.

It's minus because the outside
is more positive.

Less positive minus more
positive, you're going to get

a negative number
and it's by -70.

Now, I said that this was the
foundation for understanding

how neurons actually
transmit signals.

And to understand that, I'll
kind of lay a foundation over

that foundation.

I think then just the actual
neuron transmission will make

a lot of sense.

Even better, it'll make a lot
of sense why they even have

these myelin sheaths and these
nodes of Ranvier and why we

have all of these dendrites.

Hopefully it'll all
fit together.

So there are two types of
ways that kind of a

potential can travel.

So there's two types
of signal transfer.

I'll just call it
signal transfer.

I don't know what the
best word for it is.

The first one I'll talk
is electrotonic.

It sounds very fancy,
but you'll see it's

a very simple idea.

And the other one I'm
going to go over

is an action potential.

And they both have their own
positives and negatives in

terms of being able to
transmit a signal.

We're talking about within the
context of in a cell or across

a cell membrane.

Let's understand what
these mean.

So let me get my membrane
of a cell.

Let's say it's a nerve cell or
a neuron, just to make it all

fit together in this context.

And we know it's more
positive on the

outside than the inside.

We know that there's a lot of
sodium on the outside or a lot

more sodium on the outside
than on the inside.

There might be a little bit.

And we know there's a lot more
potassium on the inside than

the outside, but we know
generally that the outside is

more positive then the inside
because our sodium potassium

pump will pump out three
sodiums for every two

potassiums it takes in.

Now in the last video, I told
you that there are these

things called-- well, we could
call them a sodium gate.

A sodium ion gate, right?

These are all ions.

They're charged.

Now let's say that there's some
reason, some stimulus--

let me label this.

That right there is my
sodium ion gate.

And it's in its closed position,
but let's say

something causes it to open.

We'll talk maybe in this video
or maybe this video and the

next about the different
things that

could cause it to open.

Maybe it's some type of stimulus
causes this to open.

Actually, there's a whole bunch
of different stimuluses

that would cause it to open.

But let's say it opens.

What's going to happen
if it opens?

So let's say we open it.

Some stimulus opens-- what's
going to happen?

We have more positive on the
outside than the inside, so

positive things want
to move in.

And this is a sodium gate so
only sodium can go through it.

So it's kind of a convoluted
protein structure that only

sodium can make its
way through.

And on top of that, we have a
lot more sodium on the outside

than on the inside.

So the diffusion gradient's
going to want to make sodium

go through it.

And the fact that sodium's a
positive ion, the outside is

more positive, they're going to
want to run away from that

positive environment.

So if you open this, you're just
going to have a lot of

sodium ions start to
flood through.

Now as that happens, what's
going to happen if we go

further down the membrane?

Let's zoom out.

So let's say that this is
my membrane right there.

Let's say that this is my open
gate right here and that it's

open for some reason and a bunch
of sodium is flowing in.

So all of this is becoming
much more positive.

Let's say we had a voltmeter
right here.

We're measuring the potential
difference between the inside

of the membrane a
and the outside.

Let me do a little chart.

I'm going to do the chart
here on my voltmeter.

And this is going to be the
potential difference-- or

we'll call it the membrane
voltage or the voltage

difference across the
membrane-- and

let's say this is time.

Let's say I haven't opened
this gate yet.

So it's in its resting state.

Our sodium potassium
pumps are working.

Things are leaking back and
forth, but it's staying at

that minus 70 millivolts.

So that right there is
minus 70 millivolts.

Now as soon as this gate that's
way down some other

part of the cell opens, what's
going to happen?

And let's say that's the
only thing that's open.

So this, all of a sudden, is
going to become more positive.

So positive charges that's
already here-- so other

positive charges, whether
they're sodiums or potassiums,

they're going to want to run
away from that point because

this area hasn't had a flood
of positive things.

So it's less positive
than this over here.

So maybe we have some potassiums
and maybe we have

some sodiums. Everything is
going to want to move away

from the place where
this is opened.

The charge is going to
want to move away.

So as soon as this happens, as
soon as we open this gate,

we're going to have a
movement of positive

charge in this direction.

So all of a sudden-- this was
at minus 70 millivolts.

So more positive charge
is coming its way.

Almost immediately, it's going
to become less negative or

more positive.

The potential difference between
this and this is going

to become less.

So this is this point
over here.

Now if we took this point, if we
did the same thing-- if we

measured the voltage at this
point right here, maybe it was

at minus 70 millvolts, maybe a
fraction of a minute amount of

time later, the positive charge
starts affecting it so

it becomes more positive, but
the effect is diluted, right?

Because these positive charges,
they're going to

radiate in every direction.

So the effect is diluted.

So the effect on this thing
is going to be less.

It's going to become
less positive.

So an electrotonic potential--
what happens is at one point

in the cell, a gate opens,
charge starts flooding in, and

it starts affecting the
potential at other

parts of the cell.

But the positive of it is, it's
very fast. As soon as

this happens.

further down the cell, it starts
becoming more and more

positive, but the further you
go, the effect gets dissipated

with distance.

So if you care about speed,
you'd want this

electrotonic potential.

As soon as it happens, it'll
start affecting the rest of

the cell, but if you wanted
this potential change to

travel over large distances--
for example, let's say if we

got all the way to this point of
the neuron and we wanted to

measure it, it might not
have any impact.

Maybe a little bit later, but
it's not having any impact

because all of this gets diluted
by the time it gets--

it's increasing the charge
throughout the cell.

So it's a impact far away from
the initial place where the

gate opened.

It's going to be a lot less.

So it's really not good for
operating over distance.

Now let's try to figure
out what's going on

with an action potential.

And you might understand, this
might involve more action.

So let's start off with
the same situation.

We have a sodium gate that gets
opened by some stimulus.

What I'm going to do-- let me
draw two membranes here.

So this is the outside.

This is the inside.

And let me draw-- maybe we're
dealing with a-- and we'll go

in more detail.

Maybe this is an axon or
something, but let me-- let's

say we have another sodium
gate right here.

And then they're alternating,
essentially.

So they're alternating so then
I have another sodium gate.

I don't want to do
a bunch of these.

I think I just have to draw one
round of it for you to get

what's going on.

Let me draw another
potassium gate.

And let's say that they
all start closed.

So they're all in the
closed position.

Now let's say that this sodium
gate gets stimulated.

It gets opened.

Let's say that guy right
there gets opened.

It gets stimulated by something
to get opened.

We'll talk about the things
that-- let's say in particular

this thing gets opened-- let's
say the stimulus-- it has to

be a certain voltage.

And let's say they become open
when we are at minus 55

millivolts.

So when we're just in our
resting state, the potential

difference between the inside of
the cell and the outside is

minus 70, so it's not
going to be open.

It's going to be closed, but if
for whatever reason, this

becomes positive enough to get
to minus 55 millivolts, all of

a sudden this thing
will be open.

Let's write a couple of other
rules that dictate what

happens to this gate.

Let's say it closes-- and these
are all rough numbers,

but the main idea is for you
to get the general idea.

Let's say it closes at--
I don't know-- plus 35

millivolts.

And let's say that our potassium
gate opens at plus

40 millvolts, just to give
an idea of things.

Let's say it closes at--
I don't know-- minus 80

millivolts.

So what's going to happen?

Lets say that, for whatever
reason, the voltage here has

now become minus 55.

Let me do a chart just
like I did down here.

So I want to have space
to draw my chart.

This is membrane voltage.

And this is time down here.

And let's say we're measuring
it-- let's say this is the

membrane voltage at-- let's say
right by the sodium gate

right here.

So we're measuring this voltage

across this right here.

So if it's not stimulated any
way, we're just here,

flatlining at minus 70
millivolts-- and let's say

some stimulus, for
whatever reason,

makes this more positive.

Maybe it's some type of
electrotonic effect that's

making it more positive here.

Maybe some positive charges
are floating by.

So this becomes more positive.

So let's say this becomes more
positive and then the ATP

pumps-- the sodium potassium
pumps pump it out so it

doesn't get to the threshold of
minus 55, so then nothing

will happen, right?

It didn't get to
the threshold.

But then let's say there's
another electrotonic or maybe

a bunch of them and just there's
a lot of positive

charge here so we get to
the minus 55 millvolts.

Remember, when positive
charge comes by,

we become less negative.

The potential difference
becomes less negative.

We get to that minus
55 volts-- this

thing opens then, right?

This was closed before.

It was closed when we were
just at minus 70.

So let me write here.

So at this point, our
sodium gate opens.

Now, what's going to happen when
our sodium gate opens?

When that opens-- we've seen
this show before-- all the

positively charged sodium is
going to go down there, both

electric gradient and diffusion
gradient, and

there's going to flood
into the cell.

There's so much sodium out
there, it's so positive out

there, they just want
to come in.

So as soon as they hit that
threshold, even though this

might've only gotten us to minus
55 or maybe minus 50,

all of a sudden that gate opens
and we have all of this

positive charge flooding
into the cell.

So the potential difference
becomes

much, much more positive.

So they keep flooding in,
becomes much, much more

positive, but as it gets
more positive, it

closes at plus 35 millvolts.

So let's say that we're dealing
here-- let's say that

this up here is plus
35 millvolts.

So here it closes and at the
same time, that stuff I just

deleted-- I set at plus 40
millvolts-- or let's say at

plus 35, just for the
sake of argument.

Let's say at plus 45
millvolts, our

sodium gates open.

So what's happened here?

All of a sudden, we're at plus
35 or maybe plus 40 millivolts

so this is-- let's just say plus
40, I think you get the

idea either way so we'll say
plus 40-- either way.

So at plus 40, this guy's
going to close.

No more positive ions are coming
in, but now we are at

more positive inside, at least
locally at this point on the

membrane, than we are outside.

And so this gate will open.

So then our sodium
gate will open.

K-plus ion gate opens.

Now when that opens,
what happens?

We have all of these
sodium ions here.

We already saw from the sodium
potassium pump that the

potassium-- we have all of these
potassium ions here.

We saw from the sodium potassium
pump that it makes

the sodium concentration on
the outside higher and the

potassium concentration
on the inside higher.

And now that we've gotten to
this plus 40 millvolt range,

we're also now more positive
on the inside.

So this opens.

These guys want to escape
because there's

less potassium outside.

They want to go down their
concentration gradient.

It's also very positive
on the inside.

We're at plus 40 millvolts.

So they also want to escape.

So they start escaping
the cells.

So positive charges starts
exiting the cell from the

inside to the outside.

So we become less
positive again.

So let me write what
happens here.

So at this point, our sodium
gate closes and our potassium

gate opens.

And then the positive charge
starts flooding out of the

cell again and maybe it'll
overshoot because it's only

going to close maybe once we
get to minus 80 millvolts.

So maybe our potassium gate
closes at minus 80.

And then our sodium potassium
pump might get us back to our

minus 70 millvolts.

So, this is what's happening
just at this point in the

cell, just near that
first sodium gate.

But what's going to happen
in general, right?

As this became very positive--
we went to 40

millivolts over here.

We went to 40 millvolts in
this area of the cell.

Because of-- I guess you could
almost view it as a short term

or very short distance
electrotonic potential, this

area is going to become
more positive, right?

This is going to become
more positive.

These positive charges
are going to go

where it's less positive.

So this is going to become
more positive.

This was at minus 70, but it's
going to become more positive.

It'll go to minus 65, minus 60,
minus 55-- and then bam.

This guy will get
triggered again.

Then this guy gets opened.

Then this guy gets opened.

Sodium floods in through here.

So if you wanted to plot this
guy's, the potential

difference of what's going on
across this, this all happened

as soon as-- maybe as soon as a
sodium started going in this

first dude, the second guy-- he
gets triggered here because

the second guy a little bit
later in time-- because of all

this flow a little bit to
the left of him, his

potential goes up.

He gets triggered, same exact
thing happens to him, right?

When the sodium flows in here,
becomes really positive around

here, that makes the cell
around here, the voltage

around here, the charge around
here a little bit more

positive, triggers this next
sodium gate to open and then

this whole same thing
happens, same cycle.

Then the potassium gates open to
make it negative again, but

by the time that's happened,
it's become positive over here

to trigger another
sodium gate.

So one after another, you have
these sodium gates opening and

closing, but it's transmitting
that information, it's

transmitting that potential
change.

So what's going on here?

So this is slower and it
actually involves energy.

So this was-- the electrotonic
was very fast. This is slow.

An action potential is slower.

I don't want to say it's slow.

It's slower because it has to
involve these opening and

closing of gates and it
also involves energy.

It also requires more energy.

And you're also going to have to
keep changing the potential

in your cell and you actively
have your sodium potassium

pumps being very active.

But it's good.

The positive is, it's good
at covering distance.

When you have something like
this-- we saw with the

electrotonic, as we get further
and further away from

where the stimulus happened,
the change in potential

becomes more and more
dissipated.

It actually exponentially
declines.

It becomes more and more
dissipated as we get further

and further away so it's not
good for long distance.

This thing can just continue
forever because every time it

stimulates the next gate, it's
like we're starting all over

again and so this gate-- it's
going to have a flood of ions

come in and those ions are going
to make it a little less

negative over here.

Then the next gate's
going to open.

We're going to have the cycle
over and over again.

So this is really good for
traveling long distances.

So now we have really the
foundation to understand

exactly what's happening in a
neuron and I'm going to go

over that in the next video to
show you how electrotonic

potentials and action potentials
can combine to have

a signal travel through
a neuron.