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Okay,[COUGH] so let's go in much more details into this axon.
It's a very special device, you could see that it generates electrical signals.
Let's zoom into the axon and try to understand how is it built.
So this is a schematical, typical morphology of a neuron with a focus on
the axon. So again the dendritic tree collapse just
for the presentation the cell body or soma within a nucleus the dendrites, we
just spoke about them. And then there is this process, the
branching process that is called the axon.
So I'm talking now about the axon. And you can see several interesting
elements that is building this axon. First of all there is, in the beginning,
just at the exit from the soma, this part is called the axon initial segment.
The axon initial segment. This, this region is hot, so to speak,
we'll talk about what does it mean hot. But it is hot, electrically hot because
in this region there is the initiation of the spike.
So this hot region which is a bare piece of membrane consist of very special ion
channels that makes this region hot in the sense, that this electrical ion
channels enables the generation of this spike.
So you can see that you may get, under certain condition, duck, duck, duck.
A set of in this case three spikes. So this is a very special place because
it is where the action potential is initiated if it is initiated.
Not always there's an action potential but whenever there is an action potential
it starts here. And this set of action potential one or
two or three or whatever number may propagate along the spike.
They propagate along the spike after they're initiated, so they go all over
the axon. All over the axon, and they propagate
from the initiation spot to all the branches of the axon.
So, an action potential or a spike goes, travels, propagates along the axon.
It may go to this branch and this branch and this branch.
Continues here, and this branch, and this branch.
So it's a propagating wave of activity. It's a propagating spike, and we'll talk
about spikes, and we'll talk about activity.
But this is the general, operation of a, of a, of an axon.
The action potential starts here, and then propagates, all over, and without
attenuation. Full blown action potential goes all over
the axonitry. More about that axon you can see there is
the interesting structure. There is this element, and there is this
gap which we call the node of Ranvier . So, this, this is the Node of Ranvier
here. This is another Node of Ranvier, okay.
And there is between the nodes of Ranvier, between these little gaps, there
is what we call the Internode. It's a Myelin sheath.
It's a wrapping sheet as we should see in a second is in the isolating path so this
is a non isolated path of the axon, this is the node of Ranvier and this is an
isolated path of the axon at the inter neural node which is wrapped with what we
call the myelin. This myelin is a lipid, it's a lipid
wrapping of the axon. And this wrapping of the axon
electrically isolate this piece from the outside.
In the node of Ranvier there is no isolation.
So this little very, very small gap is not isolated by the myelin.
And this region is also hot. So there are also hot ion channels, and
the spike is generated here, but also can be boosted again here, and can be boosted
again here, and so forth. And then you can see the terminals of the
axon. So the axon along the axon.
For example here, or here. Or at the end of the axon just when it
ends. You have these little varicosities that
we saw before. This protons or this varicosities,
varicosities that consist the neuro-transmitter.
It consists this chemical that will eventually interact with the next neuron.
So when the action potential goes through, travels through the axon, it
gets into a varicosity or another varicosity or another varicosity and so
on. 5,000 varicosties maybe per axon.
And each varicosity consists of this neurotransmitter that we should talk a
lot about when we talk about the synapse. And you can see that whenever there is a
varicosity there is no myelin, because you don't want to wrap, you don't want to
isolate the synapse. Because then, there will be no
communication. So there is no myelin when there is a
varicosity. The varicosity is open, is bare.
It can release without interference. So inside the the internode, there are no
synapses and outside, when you don't have a myelin, there are.
This presynaptic synapses. Let's look even deeper into the axon, so
again you can see this myelin sheath, this internode, and we know today, that
this internode is being generated by a special set of neurons, sorry, by a
special set of cells that are not neurons, that are not nerve cells, and
these cells wrap by their membrane. You see this one cell sends the branch
and then wraps, wraps, wraps, wraps. And generate the myelin, here.
Another branch wraps, and set the myelin, here.
So this very special organization, very special interaction.
Between this unique cell, sometimes called glare cells, depending on the
system. And sometimes it has different names
which you should not necessarily remember.
These cells are responsible for wrapping the axon in a particular regions.
But leaving this particular important gaps, the node of Ranvier , so that the
action potential's starting here, may so to speak, jump and being boosted here,
being boosted here, being boosted here, and eventually, when it comes to the
synaptic buton, or the varicosity release.
Release the transmitter, to talk about the release mechanism.
And if you take, if you cut the axon in this direction, you will see something
like that. So that's the inside of the axon, and the
green envelope is this wrapping that I just mentioned, sometimes with hundreds
of wrappings, wrapping, wrapping, wrapping.
And that's the, and that's the myelin. And that's the isolation part, that's the
isolating element, where current cannot flow outside from this internode, because
of this myelin sheath wrapping all around.
So that's what we call an, a myelated axon.
In our nervous system, in our brain, in the spinal cord, most of the axons are
myelinated, not all of them. But each myelinated axon also contains as
I just said, pieces that are not myelinated.
So there are the nine myelinated part of the axon but I would, I would call this
axon a myelinated axon. This is a myelinated axon because it has
a myelin. Note that the dendrites never have
myelin. So whenever I see a myelin I know that it
must be an axon. But whenever I don't see a myelin, it's
hard for me to know whether this part is dendrite, or this part is dendrite, if I
just look at that. If I look at the whole process, of
course. I know that this emerges from a
myelinated axon. Okay.
So let me go even further, zooming into the axon because it's such an interesting
electrical device. And we really really should understand
it. And there will be a whole lesson, number
four, discussing the signal, the action potential of the spike, what makes it
generated, and how does it propagates. So if I zoom into the node, this is the
node of Ranvier, this very hot, or we call it excitable region.
So you can see the myelin she, sheath here, covering this part.
And then you see a very small gap, a few microns in length.
And then, again, you see the next internode.
So myelin, no myelin, myelin. And if you look very carefully into this
node. You see that it is hot in a sense that it
has all this specific membrane ion channels.
When we talk about the spike, we'll talk about what is the role of this specific
ion channels in generating this spike. This zero, one.
This all or none. This very special phenomena that
propagates a long axon. So this node is a, node of Ranvier is a
very, very, very important part of the nervous system.
And if something goes wrong with the axon for example with multiple sclerosis and
this marrying sheet is not functioning well anymore.
There is no propagation of the action potential along the axon and we have
already difficulties in activating systems,[INAUDIBLE] muscles and so on.
So the propagation of the action potential is made possible without
attenuation due to the fact that along the axon, there are this amplification or
this boosting regions the node of Ranvier, because it's so hot that each
time a signal arrives it makes it big and then it goes there and the next node
makes it big. So this node of Ranvier, what I call hot
node of Ranvier. These are very important element in
making the propagation of the action potential successful along the axon.
So let's summarize what we said about axon, the axons.
The axon is a highly branched structure, we should not think about this as a wire.
It's starting as a little wire. And then, branch, branch, branch, branch.
It could branch locally, and it could branch distally.
It's a very thin process, so you should think about axon's diameter.
A typical diameter of an axon, like a micrometer.
A thousands of a millimeter. So that's one aspect and it's typically
starts at the soma. Not always, but typically starts at the
cell body as we saw and then goes from the cell body and start to branch.
As I said the axon starts with the hot initial segment where the action
potential spike starts. And this axon potential propagates along
the axon. The axon is covered with myelin, so a
myelin axon is covered with myelin. So this is the isolating lipid sh sheath,
isolating the axon. And then there are these intermediate
gaps, node of Ranvier . And in each intermediate gap, there is
this hot, excitable region, where these hot channels reside.
And finally, we should think about an axon, as a very, very big branching tree.
This very frequently you see on this big branching tree, this swelling, this
vericosity of this axonal bouton. And as I mentioned in each of these
little vericosity. In one single axon you may have as I said
5,000 sometimes 10,000 little vericosities.
Each of these varicosities. The neurotransmitter, this chemical in
the pre-synaptic part, in the axonal part, the pre-synaptic part.
The, the neurotransmitter hide. And we'll talk about the synapse in a
second. And this is what we call the presynaptic
part of the axon, and the axon has many many presynaptic regions, because each
local contact, each local bouton is a presynaptic region for a cell to talk to
late on, the postsynaptic cell. So the axon is an output electrical
device. It generates locally, in the initial
segment, spikes and it carries these electrical spikes along the axon to all
its branches. And these spikes, this communication
enables the communication between the axon, through the synapse to the next
stages, to the post synoptic cell, to the dendrites of the post synoptic cell.