In this video, I want to talk about the effects of axon
diameter and myelination because it turns out
that larger diameter axons conduct action potentials
faster than smaller diameter axons.
And axons with a myelin sheath on them
also conduct action potentials faster.
So first let's consider the diameter of an axon
and how that affects the speed of action potential conduction.
An axon with a larger diameter offers less resistance
to the movement of ions down the axon,
causing ions to move faster down the axon
and causing the action potential to be conducted faster.
Let me show you the way I think about this by just considering
a single sodium ion that's entering
an axon through a voltage-gated sodium channel.
So we'll say we just have a single sodium ion in here we're
going to look at, even though there's many sodium
ions flowing in through these voltage-gated sodium channels.
And let's consider this for both-- the large diameter
axon and the small diameter axon.
Here's our sodium ion in the large diameter axon.
Now, both of these sodium ions, once they're inside the axon,
could move in, really, an infinity of directions.
They could kind of go in any direction and any degree
of direction so that there's an infinity of pathways
these ions could travel.
Now, if they happened to travel backward
in the axon, back toward the soma,
or if they happen to travel perpendicular
to the long axis of the axon, that won't really contribute
much to the action potential other than the effects
they'll have on the other sodium ions that are coming in.
But if they're heading in any of these directions
down the length of the axon, that's
going to contribute to moving the action potential down
the axon.
And the same is going to be true for this sodium
ion in the small diameter axon.
It could go in an infinity of directions
as well, just like in a larger diameter axon.
But now let's consider the obstacles
to this sodium ion moving down the axon.
First, there's the membrane of the axon.
And then there are all sorts of structures
in the cytoplasm of the axon, such as vesicles
or large proteins.
And then, there are filaments, and there are tubules.
And there are all sorts of structures
in the cytoplasm that would pose an obstacle
to the movement of this sodium ion.
And the concentration of these obstacles
would be the same in the smaller diameter axon.
But of course, there's less cytoplasm.
So there would be fewer of these obstacles,
but the same number for any given volume of cytoplasm.
So if we consider this to represent the obstacles
in the way of these sodium ions moving down the axon,
what we see is that there are fewer potential pathways
for the sodium ion to move down the axon in the smaller
diameter axon before it runs into something
in a fairly short distance.
So let's say this sodium ion heads this way.
That gets it a pretty good ways until it
collides into the axon membrane.
Or if it goes this way, it also gets it a pretty good ways
until it collides with the cell membrane.
But if it heads this way, it collides
into one of these cytoplasmic structures pretty quickly.
Or if it heads in any of these directions,
it runs in the axon membrane very quickly.
Now, if we compare that to the sodium
ion in the large diameter axon, it
has more potential pathways to travel
before it collides into something.
I mean, it certainly has the probability
to collide into things in a pretty short distance.
But there's a higher probability that it
will travel a farther distance at a faster speed
because there are just so many more pathways that it can take
to make it a farther distance before it collides
into something, so that for each individual sodium ion,
the probability is higher that it will travel faster
over a longer distance than the probability of this sodium
ion in the smaller diameter axon.
And then when you consider that many, many sodium ions are
coming in and you average all those average speeds together,
it's going to turn out that, in the larger diameter axon,
the ions are going to move faster on average down the axon
because they have more potential pathways to travel
before colliding into things.
And since the speed of action potential conduction
is related to the average speed of ions moving down
the length of the axon, then action potentials
will be conducted faster down a larger diameter axon
than down a smaller diameter axon.
At least this is the way I like to think about it.
But now let's consider the other thing that really speeds up
the conduction of action potentials
down axons, which is the presence of a myelin
sheath around the axon.
The speed of action potential conduction
is faster in myelinated axons, like I've drawn here
with the myelin sheath in yellow,
because the capacitance of the membrane
is reduced in the myelinated segments, which decreases
the number of ions and the time needed to change
the membrane potential in these areas.
In this context, the word "capacitance"
refers to the number of ions that
can be stored in the layers on both sides of the membrane
at any given membrane potential, because the membrane potential
reflects the strength of the charge
separation for any particular charge carrier,
like a single cation or a single anion.
But the total number of charges along the membrane
is the "capacitance" of the membrane, a word derived
from the same word as "capacity" or the amount of charge
that can be stored.
One of the principles of a capacitor like the cell
membrane is that the closer the charges are to each other,
the more charges can be stored on both sides of the capacitor.
The way I like to think about that for this
is that at the nodes of Ranvier, there's
only the normal thickness of the cell membrane, which
is relatively thin, so that an anion
in the layer against the inside of the membrane
is strongly attracted to a cation
in the layer on the outside of the membrane.
Because they're very close to each other,
they're attracting each other very strongly.
And because this attraction is so strong,
it's able to overcome the repulsion that the like charges
feel to each other on either side of the membrane.
So all these positive charges-- these cations--
repel each other because like charges repel.
But with this very small distance
between the opposite charges on the other side of the membrane,
this strong attraction overcomes the repulsion
that the like charges are feeling
on either side of the membrane so
that on the thin membrane at the nodes of Ranvier,
you can pack in lots of charges.
You can pack lots of anions on the layer along the inside
and cations in the layer along the outside,
at least at the resting potential when
it's more negative inside than outside.
But the myelin sheath you can think
of, really, as making the membrane much, much thicker
because the myelin sheath is just membrane that's wrapped
around the neuron membrane many, many, many times.
So now you have a really thick membrane.
So in terms of a capacitor, the distance
is now much greater between the opposite charged ions
on either side of the myelin sheath membrane.
So while they're still attracting each other--
so there will be a cation out here and an anion right here--
and they are still attracting each other.
They're trying to pull toward each other
across the membrane that won't let them pass.
But the strength of this attraction
is much less because the distance
is much farther between these ions
so that now you can put less like charges on the same side
of the membrane from each other because there's
less of an attraction across the membrane
to make up for the repulsion that these like charges are
feeling toward each other.
They want to move as far away from each other as they can,
and so they're spread out much farther.
You can store less charges or less ions
along both sides of the membrane because the distance
between the opposite charges is farther away.
So in a myelinated axon like this,
there will be this alternation at the nods of Ranvier, which
is high-capacitance.
There will be lots of charges.
And in the myelinated segments, which is low capacitance,
there will be less charges.
For unmyelinated axons, they're basically all just
like the nodes of Ranvier.
They're high-capacitance, so they
store lots of charges on both sides of the membrane.
Now consider when the action potential starts
and this voltage-gated sodium channel opens,
sodium will flow into the axon, bringing positive charges
inside, so this area of the membrane
is going to depolarize.
And to do so, some of these negative charges
are going to need to leave the inside of the membrane and mix
in with the rest of the ions in the cytoplasm.
And on the outside, some of these charges
will have to leave and mix in with the ions
in the interstitial fluid.
And this will take a little time.
And it'll take more time here on the high-capacitance membrane
at the nodes of Ranvier, or the trigger zone,
than it will here in the myelinated
segments just because there are less charges that
have to move off the membrane to change the membrane potential.
So because fewer ions and less time
is needed to discharge or depolarize
the membrane in the myelinated segments, which has a lower
capacitance, or to recharge the membrane
or repolarize the membrane, when potassium starts flowing out
during the falling phase of the action potential,
the action potential is able to travel faster
through these myelinated segments than it can
through the nodes of Ranvier, which
is why we see slower conduction of the action potential
through the nodes of Ranvier than through the myelinated
segments, which we call "saltatory conduction," where
it looks like the action potential is jumping
from node to node.
Myelination also decreases the membrane permeability to ions
so that fewer ions in total cross the membrane
during an action potential.
Therefore, fewer ions in total need
to recross the membrane after the action potential
through the sodium potassium pump that's
going to be pumping back out all the sodium ions that came in
through the voltage-gated sodium channels
and pumping back in all the potassium ions that
left during the action potential.
And since this process uses energy,
myelination increases the efficiency
of action potential conduction in terms of the energy needed
to maintain these ion concentrations after action
potentials.
Myelinated axons have most of their voltage-gated ion
channels at the nodes of Ranvier,
so that while an action potential is conducted faster
through a myelinated segment, it actually
does decrease in size a little bit as it's going along.
The nodes of Ranvier, therefore, are
necessary to regenerate the full size of the action potential
so that it can continue all the way down the axon.