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.