the neuron resting membrane potential, which we often

just call the resting potential for short.

So first, let me just draw a neuron that'll

be a little distorted, just so I have room to draw.

So we'll draw this soma here and a really big axon coming out

of the soma-- and normally an axon

is a thin, long process coming out of the soma,

but I just need a little room to draw.

So I'll draw a big, thick one.

And this will be the other part of the soma, or the cell body.

And then I'll just draw one really big dendrite.

And like the axon, of course, these

are normally just these little thin processes coming out

of the soma.

But I just need some space.

So most neurons at rest, meaning when they're not

receiving any input, have a stable separation

of charges across the cell membrane called

the resting potential.

And that consists of more positive charges in a layer

on the outside of the membrane, and more negative charges

in the layer along the inside of the membrane.

And these charges are ions.

So the negatively charged ions that

are in a layer along the inside of the membrane we also

call anions.

And the positively charged ions in a layer

on the outside of the membrane we call cations.

And this layer of anions on the inside

and cations on the outside goes all over the neuron cell

membrane.

All through the membrane of the dendrite,

and the soma, and all along the membrane of the axon.

And just to be clear, there is a mix

of anions and cations on both sides of the membrane.

And I've just drawn plus signs on the outside of the membrane

to represent that in the layer against the outside

of the membrane, there are more cations and anions.

And I have drawn negative signs on the inside of the membrane

to represent that in that layer, there

are more anions than cations.

And talk about the size of the difference in the separation

of charges, the convention is to call the outside zero.

So we just say the outside is zero,

and we just kind of set that as the reference.

And then we just refer to a single number

on the inside of the membrane, which

is the difference between the voltage on the outside

and the inside, or the difference in the strength

of the charge separation.

And this difference can vary between neurons,

but around negative 60 millivolts

would be a really common resting potential for a neuron.

So I'll just write a little m and a big V for millivolts.

That's the value we use to quantify

this difference in charge separation.

And around negative 60 would be a really common

resting membrane potential for a neuron.

The resting potential of neurons is

related to concentration differences, which are also

called gradients, of many ions across the cell membrane.

So there's lots of different ions

that have high concentrations outside the neuron compared

to lower concentrations inside the neuron, or vice versa.

But a few of these ions are the most important

for neuron function.

The cations, or the positive charged ions that are most

important for neuron function are potassium--

and I'll just write that as a K+, sodium,

which I'll write as an Na+, and calcium,

which I'll write as a Ca2+.

Because each calcium ion has two positive charges.

And the most important anions for neuron function,

or negatively charged ions, are chloride,

which I'll write as Cl-, and then there are multiple organic

anions.

And so I'll just write OA- to stand for organic anions.

And there a bunch of different organic

anions inside neurons and other cells.

Most of these are proteins that carry a net negative charge.

Now, these five kinds of ions are

going to have concentration differences across the cell

membranes, which we also call concentration gradients.

And it's different for the different ions

if they have a higher concentration

inside or outside the neuron.

The organic anions and the potassium ions

have a higher concentration inside the neuron than outside.

So I'll just represent that by having these letters written

large inside the neuron.

And then I'll write a small OA- to show

that there's a smaller concentration of organic anions

outside the neuron than inside.

And the same for potassium.

I'll write a small K+ outside the neuron compared to a large

K+ inside, because the concentration of potassium is

higher inside the neuron that outside the neuron.

And the opposite is true for these other three ions.

So the concentration of sodium is much higher

outside the neuron than inside the neuron,

as is the concentration of calcium.

There's much more calcium outside the neuron than inside.

And the concentration of chloride ions

is also much higher outside the neuron than inside the neuron.

Each of these ions, therefore, is

going to be acted on by two forces that

try to drive them into or out of the neuron.

The first is an electrical force from the membrane potential.

Because each ion will be attracted

to the side of the membrane with the opposite charge,

opposite charges attract each other and like charges

repel each other.

So if we look at each of these ions in turn,

the organic anions are negatively charged,

so they will be attracted to the outside of the neuron

where there are more positive charges.

So the electrical force acting on the organic anions

will try to drive them out of the neuron.

Potassium is the opposite.

It's positively charged.

So it will be attracted to the inside of the membrane

where it's more negative.

So it's electrical force will try

to drive it into the neuron.

Sodium is the same as potassium.

It's positively charged, so it will

be attracted to the more negative inside of the neuron.

Chloride is an anion like the organic anions,

so its electrical force will try to drive it out of the neuron.

Calcium is a cation like potassium and sodium,

so it's electrical force will also

try to drive it into the neuron.

But now the second force acting on these ions

can be thought of as a diffusion force,

or it's often called a chemical force, related

to the concentration gradients across the neuron membrane.

Because particles in solution will always

try to move from an area of higher concentration

to an area of lower concentration.

So if we look at the organic anions,

they're in a higher concentration

inside the neuron than outside.

So their diffusion force will be trying to drive them out

of the neuron, just like their electrical force is.

Now, potassium is a little confused.

Its electrical force is trying to drive it into the neuron,

but it has a higher concentration

inside the neuron.

So it's diffusion force is actually

trying to drive it out of the neuron.

Sodium has matched electrical and diffusion forces,

because it has a higher concentration

outside the neuron than inside.

Chloride's electrical force is trying

to drive it out of the neuron.

But because it has a higher concentration

outside the neuron, it's diffusion force

will be trying to drive it into the neuron.

And calcium is just like sodium.

Both its electrical and its diffusion force

are trying to drive calcium into the neuron.

These forces we often call electrochemical driving forces

for short.

And neurons are going to use these forces to perform

their functions.

But before we talk about that, in the next video,

let's talk about how the resting membrane potential is created

and how it's related to the concentration

differences of some of these key ions.