In this video, I want to talk about the action
potentials of neurons.
Here we have our neuron with soma in red
and a larger than normal axon in green.
And now I've drawn a myelin sheath in yellow
around the axon and a couple of larger than normal dendrites
in blue.
And here's our graph with the membrane potential
on the y-axis and time on the x-axis.
And we've been talking about, in the absence of input,
most neurons have a stable potential
across the entire membrane that I've shown here,
which is often around negative 60 millivolts.
And without input, the resting potential
will just stay right there.
But excitatory or inhibitory inputs,
which usually come in through the dendrites,
but less often can come into the soma or the axon itself,
will cause changes to the resting potential
that we call graded potentials that
may be either a depolarization, also called
an excitatory potential, or a hyperpolarization,
also called an inhibitory potential.
And these are called excitatory or inhibitory
because they involve movement to the membrane potential
closer to or farther away from this threshold potential, which
is often around negative 50 millivolts.
And we talked about how these graded potentials
decay with both time and distance,
so that as a polarization from an excitatory input
spreads along the membrane, the size of the graded potential
gets smaller-- and the same thing
with the a hyperpolarization caused by an inhibitory input.
And we talked about how this decay, with time and distance,
involves how much effect any of these graded
potentials out here in the dendrites
or the soma will have on the trigger
zone at the initial segment of the axon
and that what's usually needed is multiple excitatory
inputs causing depolarizations to have
temporal and spatial summation so
that the size of these excitatory potentials
can be large enough when they get to the trigger
zone of the axon here to push the membrane of the trigger
zone up over this threshold potential.
And if summation of all the excitatory and inhibitory
potentials at any moment in time brings the membrane potential
of the trigger zone right here up over this threshold value,
which is often around negative 50 millivolts, then usually
an action potential will be started here
at the trigger zone and conducted all the way
down the axon.
Action potentials being conducted down
the axons of neurons are the way that neurons
can transmit information over a wide range of distances,
which may be one meter or more.
Action potentials have some big differences
from graded potentials in that they usually have the same size
and duration for any particular neuron,
and they are usually conducted at the entire length
of an axon basically unchanged, regardless
of the distance of that axon.
The shape of an action potential is fairly characteristic.
So it starts with summation of graded potentials
getting it to the threshold value.
But then instead of just decaying back down
to the resting potential like graded potentials
do, it has this huge rise to a very positive membrane
potential, where it's become more positive inside the neuron
membrane than outside, which is the reverse of normal, where
normally it's more negative inside the membrane
than outside.
After this part that's called the rising phase of the action
potential, it has a short plateau.
Then it has a rapid falling phase back down
to the resting potential.
But it actually doesn't stop there.
It keeps going more negative than the resting potential.
Then that plateaus, and then it has
a bit of a slower kind of return to the resting potential
from there.
So the exact values may vary between neurons,
but common numbers are that it might
go from around negative 50, if that's the threshold potential
for that neuron, all the way up to somewhere around positive 40
or so.
Then it may come all the way down to around negative 70
or so before it kind of more slowly
settles back into a typical resting potential
of around negative 60 or so.
The total size of the action potential
may vary between neurons, but for any particular neuron,
the size is usually the same, which
is called the all-or-none property of an action
potential.
You either get an action potential
or you don't get an action potential as opposed
to graded potentials, where the size varies depending
on the size of the input, so that if a depolarization
leading to an action potential is just slightly over threshold
like this, you'll get the same-sized action
potential as a graded potential that's way over threshold.
It doesn't make any difference how far over threshold you get.
The size of the action potential will usually be the same.
That's the all-or-none property of action potentials.
The duration of an action potential
is also usually consistent for any particular neuron.
It's usually pretty quick, just a few milliseconds.
Graded potentials can also be pretty fast.
They can be a few milliseconds as well.
But they can also be much, much longer than this.
Graded potentials have a wide range
of durations based on the duration of their inputs.
Another big difference is that action potentials usually
do not decay with distance like graded potentials do.
Action potentials are usually conducted
down an axon unchanged no matter how long the axon is, so
that if we look right here at the trigger zone
where the action potential starts,
it'll have this kind of shape to its waveform.
Then if we check maybe halfway down the axon,
it'll have the exact same shape.
And then if we check way down at the end of an axon,
even if this is a meter or more, it'll
have the exact same shape again.
It doesn't decay with distance.
The speed that action potentials are conducted down axons
is often very fast.
It can be anywhere from 1 meter per second
to up to 100 meters per second.
A couple of things go along with faster conduction speeds
for action potentials.
Larger-diameter axons conduct action potentials
faster than smaller-diameter axons.
And myelinated axons-- axons that have a myelin sheath--
conduct action potentials faster than axons
that don't have a myelin sheath.
And that often goes hand in hand.
We usually see myelin sheaths on larger-diameter axons.
With myelinated axons, the speed an action potential
that is conducted down the axon is not consistent.
We actually see that the action potential is conducted faster
through the myelinated segments than through the gaps
between the myelinated segments called the nodes of Ranvier,
so that if we look here at the trigger zone,
the action potential travels a little slower through there
and then much more quickly through
this myelinated segment.
And then it travels a little more
slowly through that node of Ranvier,
then much faster through this next myelinated segment.
This phenomenon is called saltatory conduction.
And this word "saltatory" comes from a Latin word
for jumping because the action potential appears
to jump from node to node instead of having
a nice, smooth conduction along the axon.