In this video, I want to describe
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.