Myelin works by lowering the electrical capacitance of the axon membrane, which means less ionic charge is needed to transmit a signal and the charge builds up faster. Because myelin insulates most of the axon except for small gaps called nodes of Ranvier, the electrical signal jumps from node to node rather than traveling continuously, a mechanism called saltatory conduction. This makes nerve signals travel much faster than they would on an unmyelinated axon, which is why myelin is described as fundamental to fast neural communication like reflexes.
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The corticospinal tract, the motor pathway running from brain to leg, can consist of a single axon cell the entire length, which requires myelin's compartmentalization to conduct signals quickly over that distance.
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The speed difference is illustrated with a side-by-side comparison: an unmyelinated axon shows a slow-building action potential, while a myelinated axon with nodes of Ranvier shows the signal jumping rapidly node to node.
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The ion movement during signal propagation is compared to a Black Friday crowd rushing through open doors, describing how positively charged ions rush apart when membrane channels open.
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8:42and so uh getting back into some bio work I think is going to be uh refreshing for many folks who are listening. >> Yeah. And before we get into MS, I think we should first try and understand just some basics about the nervous system and what myelin actually is. So in a healthy nervous system, nerve fibers, which are these axons, so a nerve cell has a cell body. It's got dendrites where all of the inputs come in and then an axon where the output goes down to post synaptic neurons to the neurons that this particular neuron is talking to. Now that axon is usually wrapped up in something called a myelin sheath and it's useful to visualize the nervous
9:24system as kind of a vast network of electrical wiring. Okay, neurons use electricity to talk to one another. That's how they're fast. If they just use diffusion, which is just like molecules moving around and bumping into each other, it wouldn't be as fast as the reflexes that we have. But there's a caveat. Even with electricity, neurons would not be as fast as they are now. Like the reflexes that we have or the reflexes that Formula 1 drivers have, >> they're pretty insane. fractions of a second that nerve impulses are traveling down the arm or the leg to like break and things like that, right?
10:06>> Um, that's happening because of the myelin sheath. Okay. So, I want I want to get into a little bit of the physics here because it's kind of cool biohysics. It's one of the first like bioysics things that I kind of learned about where I was like this is pretty cool that like biology is using these principles. So, here's the idea. You've got the axon, right? And you've got this myelin sheath around it. It's kind of analogous to the insulation around copper wire. Okay. In some sense, the problem is that the membrane of the neuron is leaky. The membrane meaning the thing that is separating the inside to the outside. The way the neuron um discharges and fires off um a signal is
10:48you've got a bunch of ions on one side and a lack of those ions on the other side. So when you open up little holes, little membrane channels on the on the membrane, the ions sort of discharge. It's kind of like having a capacitor and you shortcircuit it. So all of the charges flow from one place to the other. >> The the visual I think about is Black Friday. The inside of the store is locked. Everyone's standing on the outside. And when you open all the doors, >> it's it's a rush. >> It's a rush. >> It's a rush, right? Usually it's this um potassium, sodium, calcium. These are the ions that we're working with. And because they're all positively charged, they want to get the hell away from each other right? >> So without insulation, the signal would
11:29actually just decay and it wouldn't actually travel all that far down the axon terminal. But because we have this insulation, you can imagine a cable. >> Okay, you've got spots where there's insulation and spots where there aren't. Okay. Now the spots where there are no insulation, those are called the nodes of ranvier. Okay. That's where the actual ion transport is happening. >> So >> into and out of the cell. >> We're constricting it to specific spots where the the the the ability to go in and out is concentrated. Uh they're like gates and then there's the protective layer. It's it visually reminds me of those at summer camp. You'd make those beads with the little plastic rope and
12:11you string the beads on top of it to put on your wrist. Exactly. And the little gaps in between the beads is where the ions could actually Yes. >> go in and out. >> Yeah. And those would be the nodes of Ranvir. Right. And the point is that without >> without without the myelin sheath, >> there would be ion channels everywhere along this cable. So the ions would come in and then they diffuse and then maybe they'd go out, right? On the other hand, if there is insulation, the ions are coming in and now they're all jam-packed into this tiny little axon. So, they're going to travel really fast to the next gate to open up that next channel. >> That makes total sense. >> Okay. It's kind of like I mean, if we go back to your Black Friday analogy, right? It's like there's a there's a entrance in the in the front and then
12:52when all the people come in, if there's an entrance all the way in the back, these people are going to Yes. They're going to diffuse way quicker than if there's entrances all over and exits all over the place. Then they're going to sort of meander meander. People start blocking and the traffic doesn't move very No, that that's and I think the So the idea is this physical structure of how this is set up >> amplifies the speed at which the signal can go from one point to the next. >> Exactly. And so this means this means you need less energy because what you've actually done is um you know for those who are electrical engineers or just like undergrad physicists um capacitors when you put them in parallel the
13:33capacitance decreases. Okay. Sorry. No no no. When you put capacitors in series the capacitance decreases. When you put resistors in parallel the the resistance decreases. It's the one over C thing. Anyways, here you've got the axon membrane, which is kind of like a capacitor because you've got charge on one side and the other side. So, it's storing some kind of voltage difference based on the charge. But if you're wrapping this myin sheath around it, that's a bunch of capacitors. And so, you're decreasing the capacitance. And what that means is you need less energy to absorb and store all of that electrical charge. Now, why is that why
14:14is that important? You can have faster charging, right? For the same amount of voltage, >> you need less ions. That's always good. >> And the signal travels a lot faster because of what we just said, right? >> Um, >> and between these nodes of Ranvir, that's where the signal is traveling. This mechanism is called saltatory conduction where it goes from one node of Ranvir to the next node of Ranvir. And here you can see a visualization of that. On the left hand side >> is an axon without any myelin. M >> notice the action potential is very slow. >> Yeah. >> On the right hand side you've got these nodes of raanvier in between these myelin cells. >> Yep. >> Boom boom boom boom boom. The signal
14:57propagates a lot faster. >> Mhm. And and so again just as a a a simple way for me to think about this, the myelin helps reduce the friction >> in some sense. >> Right. So things can just move. It it it facilitates kind of like the charge coming in and all of that positive charge wanting to get the hell away from one another. There's not any exits except for like way down there. >> Mhm. Mhm. And ultimately this I think part of the point you're trying to bring up here is it's myelin is a very fundamental benefit >> to how neurons can work so efficiently. Um, and it is a fundamental part of the
15:40effectiveness of neural communication and the signals that get sent >> that are such a big part of what make us incredible creatures. >> Yes, 100%. This is why we have fast reflexes. This is why the antelope can like see a cheetah and be like, I'm out, like immediately, right? There's no there's no delay between its brain sensing a cheetah and its legs being like, I'm out, right? um the difference between reaching out and catching a falling glass versus like the thing falling. I mean we all have myelin in our hands and in our feet that is what gives us the reflexes >> um without I mean for example the corticospinal tract right which is the
16:22motor control pathway goes from the brain all the way down to let's say the legs >> that is a single cell >> that entire way >> okay that entire thing is a single axon sometimes there are single axons that go all the way down okay incredible >> if it's a single cell you need this kind of compartmentalization in order to get the signal all the way down to your foot so quickly. Right? This is how Formula 1 drivers can reflex time. Have you seen those? Uh, by the way, have you seen those the where like uh they've got their trainer like with with balls and like they're like releasing the balls and the and the Formula 1 drivers have to like catch
17:02them right before the race. If you watch the races, right before the race, you'll see all of the Formula 1 drivers just like doing some warm-up reflex exercises. Well, they can do that because of the milein sheath. >> So, you're saying that Messi has a better milein sheath than any other footballer ever. >> Yeah, maybe Ronaldo, but you know, I'm going to get a lot of hate for that. Let's let's see what the comments say about that. Hey, let me know. I I I I do I am curious how many are Ronaldo versus Messi >> versus Messi. I I'm curious to see what the comments say. >> Some have said the goat debate is over, but I I'm not an expert. >> You know, you know, I'm certainly not an
17:43expert, but I have an opinion.
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