Okay,[COUGH] so letâs go in much more details into this axon. Itâs a very special device, you could see that it generates electrical signals. Letâs 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.
