In this video, I want to describe
the graded membrane potential changes that
occur in neurons in response to input, which we just
call graded potentials, for short.
So I've drawn a blown-up neuron here.
We have a soma in red, and an axon in green,
and two dendrites in blue.
And recall that resting neurons-- that
is, neurons that aren't receiving any input-- usually
have a stable charge separation across the entire membrane,
where there is a layer of positively-charged ions,
also called cations, on the outside of the neuron membrane,
and a layer of negatively-charged ions, also
called anions, on the inside of the membrane.
And that we call the outside 0, just to set it as a reference,
and that the resting membrane potential of neurons may vary,
but it's often around negative 60 millivolts.
So let me show that on a graph, here.
Let's say we're looking at this piece of membrane.
And on the x-axis, we'll put time, and on the y-axis,
we'll put the membrane potential in millivolts.
And so let me put, right in the middle here,
this negative 60 millivolts that's
a common neuron resting potential.
And that when the neuron is at rest, without inputs,
most neurons just have a stable potential
at their resting membrane potential,
where it's not changing over time without input.
Now, inputs from certain types of stimuli
may increase or decrease the membrane potential
of the neuron a small amount, for a brief time, before it
returns back to the resting potential.
These transient membrane potential changes
are called graded potentials, and they
tend to occur in the dendrites of the neuron
and in the soma of the neuron.
And the size and the duration of the graded potentials
is determined by the size and the duration of inputs--
both excitatory inputs and inhibitory inputs.
Graded potentials do not pass into the axons
of most types of neurons.
Instead, most axons have a different membrane potential
change, called an action potential.
Action potentials start at the area called the trigger zone,
which is the initial segment, or the start, of the axon.
And they start when the combined effect of the graded potentials
at any moment in time brings the membrane of the trigger zone
across a certain value called the threshold potential.
So let me just draw that with a little dashed line here.
And this threshold potential will vary between neurons,
but somewhere around negative 50 millivolts
would be a common threshold potential.
So that if the membrane potential at the trigger zone
can be moved from the resting potential, which
is often around negative 60 millivolts, over the threshold
potential, which is often around negative 50 millivolts,
then a totally different potential change will happen,
called the action potential, that
will shoot all the way down the axon.
Now this adding together of graded potentials
is called summation.
And summation at the trigger zone
is how neurons process information from their inputs.
Most neurons respond to inputs from other neurons
in the form of neurotransmitter molecules
that are released at synapses.
So that if this is the axon terminal of another neuron,
it may release neurotransmitter at the synapse
where these two neurons come together,
which will bind to little receptors
on the membrane of this neuron-- in this case,
here, on a dendrite-- and this will
produce some kind of graded potential.
Now we'll get into the details of this more in other videos,
but this is the most common type of input
that a neuron will receive.
And depending on the neurotransmitter,
and depending on the receptor, this
may be an excitatory input, or it may be an inhibitory input.
Now some other types of neurons in neuron-like cells
that are sensory receptors may also generate graded potentials
from physical stimuli, such as light or odorant molecules.
Graded potentials produced from a synapse
are called synaptic or post-synaptic potentials.
And those generated by stimuli and sensory receptors
are also called receptor potentials.
A graded potential like this one,
that moves the membrane potential
to a less negative number, or closer to zero,
is called a depolarization, because now the membrane
is less polarized.
It has less charge separation.
These are also called excitatory potentials,
because they move the membrane potential closer
to the threshold, so they increase the likelihood
that an action potential will be started at the trigger zone.
A graded potential like this one,
that moves the membrane potential
to a more negative number, farther away from 0,
is called a hyperpolarization, because it's
increasing the polarization, or the charge separation,
of the membrane.
Hyperpolarizations are also called inhibitory potentials,
because by moving the membrane potential farther
from the threshold, they're decreasing the likelihood
that an action potential will be started at the trigger zone.
Two important properties of graded potentials
are that they decay with both time and distance,
so that their effect is brief and local.
Graded potentials decay with time,
just like I've drawn here.
The membrane potential changes for a brief time,
and then it returns to the resting potential,
unless there is more input.
And because graded potentials decay with time,
if two graded potentials happen that are separated
by enough time, they won't have any effect on each other.
For example, let's say that this depolarization happens and is
finished before a second depolarization over here
occurs.
Since this one was already done, already fully decayed,
these two had no effect on each other.
But if two depolarizations happened right
around the same time, their effects can add together.
They have additive effects.
And you can get a depolarization twice the size.
We call this process temporal summation, or adding together
of graded potentials in time.
Graded potentials also decay with distance, as well as
with time.
So let's look at this depolarization.
And let me just move it over here.
And let's say that this synaptic potential,
or post-synaptic potential, is a depolarization.
Let me say, right at this piece of membrane,
we get about this size of a depolarization.
As the depolarization spreads across the membrane,
it's going to decay in size.
So let's say, maybe, we check in with it
here, at this piece of the membrane.
Now it's a smaller size than it was when it started over here.
And as it continues spreading across the membrane,
maybe if we check in with it over here,
it's now actually quite small.
So that by the time it gets to the trigger zone, where
the decisions are made to fire an action potential or not,
the depolarization that started way over here
may not have much of an effect on the membrane at the trigger
zone.
Similar to the concept of temporal summation
is the concept of spatial summation--
that if two graded potentials happen far enough
away from each other, they may have no effect on each other.
For example, let's say that there's
another excitatory input way down here
at this dendrite, that causes a depolarization.
Just like this depolarization, as this
spreads across the membrane, it's
going to decay, so that it'll get smaller with distance.
So that maybe by the time these two reach the trigger zone,
they've decayed entirely so that they
have no effect on each other.
But if, instead, you had two kinds of excitatory input very
close to each other on the membrane,
then those two depolarizations could have spatial summation.
They can add together in space.
So that you could get a depolarization twice the size.
The same would be true for hyperpolarizations.
You can have temporal and spatial summation
of hyperpolarizations, to get hyperpolarizations
that are larger in size.
So what would happen if you had an excitatory input
and an inhibitory input at the same time and place?
Well, instead of getting both a depolarization
and a hyperpolarization, what you may get
is no change to the membrane potential.
They may cancel each other out and leave
the membrane potential at the resting potential.
Now one effect of the fact that graded
membrane potential changes decay with distance
is that the closer an input is to the trigger
zone, the greater effect it will have
on the likelihood of an action potential being fired down
the axon.
Because if a graded potential starts closer to the trigger
zone, it will decay less by the time
it gets there than a graded potential
that starts farther away and decays more
with greater distance.
Therefore a synapse that's closer to the trigger zone
will have a greater influence on the behavior
of the neuron in terms of action potentials being fired,
than the synapse that's farther away.
For example, here, way out at the end of a dendrite.
One last thing that I want to mention
is that synaptic potentials like these
tend to be quite small in size.
And in fact, I've drawn these too large,
because they're usually less than 1 millivolt in size.
Therefore most neurons require the temporal and spatial
summation of many synaptic potentials
to move the 10 millivolts or so that usually
separate a typical resting and a typical threshold
potential for any particular neuron.
So that as all the different synapses
that are connecting this neuron to lots
of other neurons in its network are
creating all these synaptic potentials,
the membrane potential of the dendrites and the soma
is constantly moving around and wiggling around
off the resting potential, until there's enough excitatory
potentials-- enough of these depolarizations-- that
are being summed in space and time,
to cause an action potential to be fired down the axon.
So some very complex processing of information
from all these inputs can occur because
of these graded potentials.