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.