024 Re: nicodube23 How Myelin sheaths Speed up the Action Potential

In this video, Leslie clarifies how the myelin sheaths speed up the conduction of the action potential, in response to nicodube23’s question posted on YouTube.

Enjoy!

Transcript of Today’s Episode

Hello and welcome to another episode of Interactive Biology TV, where we’re making biology fun! My name is Leslie Samuel. Inside this episode, Episode 24, I’m going to be talking about how myelin sheaths increase the speed of the action potential.

This is actually in response to a question that was asked by nicodube23 on YouTube. I’m not sure if I’m saying your name right, but if I’m not, please forgive me. This video is in answer to the question that you left. His question was placed on Episode 15 on YouTube where I spoke about saltatory conduction. This is what he says: “Why would the steps be bigger in myelinated versus unmyelinated axons? That’s the real question. What is the conceptual explanation for insulation increasing speed of conduction?” That is an excellent question, such a good question, that I felt the need to make this video to answer the question.

So this is what he’s referring to. Here we have a neuron, and the axon has these myelin sheaths, so I’m going to write here myelin sheath. Those are made by the Schwann cells. This is a Schwann cell that’s actually surrounding the axon and forming that myelin sheath. With saltatory conduction, I spoke about how the action potential jumps from one node of Ranvier to the next node. I called that saltatory conduction, and I said that is responsible for speeding up the signal. Since you’re taking bigger steps, it’s going to travel faster.

What I want to do is explain how that happens. When a stimulus comes along and causes the membrane potential to reach threshold, I said that voltage-gated sodium channels open. I’m going to say that this is a sodium channel. We know that we have a bunch of sodium ions on the outside, and when those channels open, sodium is going to rush in. When sodium rushes in, it doesn’t just stay here. Sodium has a positive charge, and that’s going to cause the positive charges that are close to it to be repelled, and sodium is actually also going to rush down the axon.

Now, this process of the charges moving along the axon, this is called electrotonic conduction. So what your have is a positive charge moving in, repelling all of the positive charges, and the positive charges are just travelling along the axon. One of the main benefits of this type of conduction is that it’s extremely fast. And that’s a good thing, you want it to be fast. However, we can’t really lie solely on electrotonic conduction. The reason for that is this also dies down, so the charge dissipates.

Let’s say the threshold is -55 millivolts. The membrane potential reaches the threshold, sodium rushes in, causing it to become very positive. That positive, I’ll put some pluses here, is going to repel the other positives and those charges are going to move along the axon extremely fast. Let’s say it goes down here. However, as it moves, that charge dies down. If we were to rely only on electrotonic conduction, if we have a long axon, the signal wouldn’t reach all the way to the end because it would die down until it gets beneath the threshold.

That’s a problem. So what we’re going to have here is, right here we have more voltage-gated sodium channels, we actually have them all along, but here they’re covered up. So even though there are sodium ions on the outside, they can’t get in because these voltage-gated sodium channels here are blocked.

As the charge moves down and it dies down, before it dies down too much, we have more voltage-gated sodium channels here, and those voltage-gated sodium channels are going to open and, of course, sodium ions are going to rush in. The charges can move again via electrotonic conduction. Before it does down too much, we can have more sodium ions rushing in here, and charge moving down by electrotonic conduction.

Now, the problem with this process is that it’s much slower. And if we were to rely on the voltage-gated channels opening to cause the action potential to go all the way down the axon, that would take much longer. And of course, if you put your hand on a hot stove, for example, you want that signal to travel extremely quickly. The good thing here is that it helps to increase the membrane potential, so I’m going to put an arrow going up, and Em stands for membrane potential. That causes a boost in the signal, so that this process can continue.

So we have an exchange of this fast process, with this slow process. But the way it’s paired makes it so that the signal can jump quickly from one node to the next node. So nicodube, to answer your question, the reason why it makes it faster is because electrotonic conduction is fast. Voltage-gated sodium channels opening is slower, so we’re pairing them up so we can have the perfect combination of a fast-moving charge and the boost to the membrane potential so that the fast-moving charge can continue until we reach all the way down the axon.

I hope that makes sense to nicodube and anybody else who has questions about saltatory conduction and how the myelin sheaths increase the speed of conduction. That’s it for this video, and I’ll see you in the next one.



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  • Wouldn’t electrotonic conduction exist in unmyelinated axons? If so, it
    would make sense for unmyelinated to move faster since there is no myelin
    sheath preventing Na to enter. Therefore, there would be an endless amount
    of Na and the electrotonic conduction would not slow down.

  • @pirateXhunterXzoro Yes, it would still exist. The myelin sheaths just
    gives it the perfect balance of electrotonic conduction and the boosting
    effect of the V-gated Na channels. In cases where the axons are
    unmyelinated, Voltage-gated channels are opened all along the axon, and
    that is slower. When one channel opens, that triggers the one next door to
    open. In addition to the speed benefits, it also uses more energy to open
    v-gated channels.

  • @MaxPayne909 A lot of people seem to be curious about that. All that I know about the topic is in the video and in the comments below, including what Nicodube23 explained. Unfortunately, I don’t have anymore to add.

  • This relates to some questions I’ve had about neuron membranes (Mb). The Mb acts like an RC circuit ie input resistance (R) relates to the # of leak channels, diameter, length of the axon and the capacitance (C) relates to the circumfrence of the mb. and distance between inside and outside of the cell. Is it that the C of the Mb “holds” the ions along the Mb slowing their movement? So, myelin reduces the C by creating more distance between in- & outside of the cell so ions don’t get stuck?

  • (cont.) If that is the case, that would explain what happens in demyelinating diseases where the myelin is attacked but the neuron remains intact. if the myelin is destroyed, nerve conduction slows or stops. If the myelin is completely gone, perhaps the ions leak out through the leak channels (LC) that were previously covered, but it seems logical that the myelin might still cover the LCs but the capacitance is higher so electrotonic conduction slows. Any thoughts?

  • @krudolph2000 When you put it like that, it makes even more sense. That would definitely cause an increased capacitance, and since it’s more likely to hold the charge, there’s more charge to travel, which would help the signal travel as quickly as possible.

  • As I understand it, in unmyelinated neurones electrotonic conduction causes localised circuits of +ve ions between neighbouring V-gated Na+ channels (which relies on ion exchange between axon and extracellular fluid), causing them to open as the p.d surpasses the threshold potential and they open. However, in myelinated neurones, there can be no ion exchange under the myelin sheath, so localised circuits can only be established between adjacent nodes of ranvier, and it is this that causes the a

  • (continued) action potential to almost instantly jump between the nodes at about every 3mm, hence the overall speed of transmission is greatly increased. I hope that helps with any general confusion 🙂

  • The voltage gated ion channel density is much larger in nodes of ranvier than in unmyelinated axons. What effect does this have?

    Is it really so that beneath the myelin sheat there are voltage gated ion channels? Isn’t the myelin sheat there to reduce the leakage that occurs outside of the voltage gated channels?

  • The voltage gated ion channel density is much larger in nodes of ranvier than in unmyelinated axons. What effect does this have?

    Is it really so that beneath the myelin sheat there are voltage gated ion channels? Isn’t the myelin sheat there to reduce the leakage that occurs outside of the voltage gated channels?

  • I guess during the impulse travelling along the axon the strength of the impulse need to be kept the same. As we know that is an all-or-none response. so at each node of Ranvier (the no myolinated point the sodium channel will be widely open for sodium NA+ to get in the membrane. I would really like to c a clip about the absolute and relative refractory period. thank you so much.

  • @Djalitana Hi, I see that you already found the refractory period video, so you should be good. In terms of suggestions about videos, I make them as I need them for my classes. That’s all I have the time to do. I receive a lot of great suggestions, but unfortunately, I’m not able to do them unless they are relevant to my classes that I’m preparing for.

  • Hi, I see that you already found the refractory period video, so you should be good. In terms of suggestions about videos, I make them as I need them for my classes. That’s all I have the time to do. I receive a lot of great suggestions, but unfortunately, I’m not able to do them unless they are relevant to my classes that I’m preparing for.

  • I guess during the impulse travelling along the axon the strength of the impulse need to be kept the same. As we know that is an all-or-none response. so at each node of Ranvier (the no myolinated point the sodium channel will be widely open for sodium NA+ to get in the membrane. I would really like to c a clip about the absolute and relative refractory period. thank you so much.

  • Hi, I see that you already found the refractory period video, so you should be good. In terms of suggestions about videos, I make them as I need them for my classes. That’s all I have the time to do. I receive a lot of great suggestions, but unfortunately, I’m not able to do them unless they are relevant to my classes that I’m preparing for.

  • @InteractiveBiology Hello. Thank you for your nice videos. I would like to state something which was not so clear from the video. Regarding conduction within the sheathed part of the membrane, it should be emphasized that this takes place at speeds comparable to the speed of light. ie: it is an electromagnetic disturbance (spike) in a medium.

  • Hello. Thank you for your nice videos. I would like to state something which was not so clear from the video. Regarding conduction within the sheathed part of the membrane, it should be emphasized that this takes place at speeds comparable to the speed of light. ie: it is an electromagnetic disturbance (spike) in a medium.

  • @InteractiveBiology (contd from prev post) The actual charged particles move extremely slowly. This electromagnetic disturbance (spike) dies out very quickly. The myelin sheath helps the spike last for a longer distance, but it will still eventually die out within a few millimeters, unless it is able to trigger a new spike.

  • (contd from prev post) The actual charged particles move extremely slowly. This electromagnetic disturbance (spike) dies out very quickly. The myelin sheath helps the spike last for a longer distance, but it will still eventually die out within a few millimeters, unless it is able to trigger a new spike.

  • @InteractiveBiology (contd from prev post) A new spike cannot be generated until there is a gap in the myelein sheath, so that the spike can trigger an ion exchange which in turn triggers a new electromagnetic spike. In this way a signal is propagated. I emphasize again that the intra-sheath signal is virtually instantaneous. Most of the time is spend in the gaps.

  • (contd from prev post) A new spike cannot be generated until there is a gap in the myelein sheath, so that the spike can trigger an ion exchange which in turn triggers a new electromagnetic spike. In this way a signal is propagated. I emphasize again that the intra-sheath signal is virtually instantaneous. Most of the time is spend in the gaps.

  • @InteractiveBiology (contd.) Thus the fewer the gaps, the faster the overall speed of the signal. On the other hand you cannot have too few gaps since there is a limit to how long the electromagnetic spike can propagate without decaying completely (even with the insulation). So there needs to be just the right number (as you mentioned) of gaps per unit length to optimize the speed of the signal. END

  • (contd.) Thus the fewer the gaps, the faster the overall speed of the signal. On the other hand you cannot have too few gaps since there is a limit to how long the electromagnetic spike can propagate without decaying completely (even with the insulation). So there needs to be just the right number (as you mentioned) of gaps per unit length to optimize the speed of the signal. END

  • Hello. Thank you for your nice videos. I would like to state something which was not so clear from the video. Regarding conduction within the sheathed part of the membrane, it should be emphasized that this takes place at speeds comparable to the speed of light. ie: it is an electromagnetic disturbance (spike) in a medium.

  • (contd from prev post) The actual charged particles move extremely slowly. This electromagnetic disturbance (spike) dies out very quickly. The myelin sheath helps the spike last for a longer distance, but it will still eventually die out within a few millimeters, unless it is able to trigger a new spike.

  • (contd from prev post) A new spike cannot be generated until there is a gap in the myelein sheath, so that the spike can trigger an ion exchange which in turn triggers a new electromagnetic spike. In this way a signal is propagated. I emphasize again that the intra-sheath signal is virtually instantaneous. Most of the time is spend in the gaps.

  • (contd.) Thus the fewer the gaps, the faster the overall speed of the signal. On the other hand you cannot have too few gaps since there is a limit to how long the electromagnetic spike can propagate without decaying completely (even with the insulation). So there needs to be just the right number (as you mentioned) of gaps per unit length to optimize the speed of the signal. END

  • @InteractiveBiology hey, just something about terminology – in class our prof stressed that the sodium ions were not literally “jumping” from node to node, rather just moving very quickly. is this correct?

  • @InteractiveBiology hey, just something about terminology – in class our prof stressed that the sodium ions were not literally “jumping” from node to node, rather just moving very quickly. is this correct?

  • Semantics my friend. He says moving quickly, I say jumping. We both mean the same thing!

  • So, is it that the Na + moves down the axon following the negative charges distributed along the length of the axon?

  • Consider the assault to our myelin sheaths by the globalist bankster and corporate controlled and corrupted criminal US political elite onto the American people today. In the form of Chemtrails that contain aluminum oxide and barium oxide. Fluoridated water and vaccines that contain mercury. GMO foods etc. The list of assaulting substances goes on and on. To dumb us down while they conspire to cull us all to a manageable number.

  • Consider the assault to our myelin sheaths by the globalist bankster and corporate controlled and corrupted criminal US political elite onto the American people today. In the form of Chemtrails that contain aluminum oxide and barium oxide. Fluoridated water and vaccines that contain mercury. GMO foods etc. The list of assaulting substances goes on and on. To dumb us down while they conspire to cull us all to a manageable number.

  • hey, I really appreciate this! it helped me out a lot and I had the same type of question. I will watch episode 3 now. Do you also get into the CNS and PNS?

  • @airfergz18 Glad to know that it helped 🙂 Yes, there are more videos in the website at the Interactive Biology site. And, there are those focused on the nervous system touching on topics about the CNS and PNS… We hope you’ll find what you need 🙂 And, stay tuned because we have more coming soon!

  • Glad to know that it helped 🙂 Yes, there are more videos in the website at the Interactive Biology site. And, there are those focused on the nervous system touching on topics about the CNS and PNS… We hope you’ll find what you need 🙂 And, stay tuned because we have more coming soon!

  • hey, I really appreciate this! it helped me out a lot and I had the same type of question. I will watch episode 3 now. Do you also get into the CNS and PNS?

  • Glad to know that it helped 🙂 Yes, there are more videos in the website at the Interactive Biology site. And, there are those focused on the nervous system touching on topics about the CNS and PNS… We hope you’ll find what you need 🙂 And, stay tuned because we have more coming soon!

  • Hi good video. i have a different way of thinking about why it speeds up conduction. basically in myelinated neurones the actional potentials only has to depolarise the nodes of ranvier and not the mylein sheath since the mylein sheath is impermeable to Na+ ions and node of ranvier are premeable. Therefore allows for saltatory conduction. However in unmyleinated neurons the action potential must depolarises the entire area of the axon. therfore resulting in slower conduction. is this right?

  • Hi good video. i have a different way of thinking about why it speeds up conduction. basically in myelinated neurones the actional potentials only has to depolarise the nodes of ranvier and not the mylein sheath since the mylein sheath is impermeable to Na+ ions and node of ranvier are premeable. Therefore allows for saltatory conduction. However in unmyleinated neurons the action potential must depolarises the entire area of the axon. therfore resulting in slower conduction. is this right?

  • @2ManuBoy2

    No, see, Na+ wouldn’t ‘need’ to wait around to depolarise every Voltage Gated Sodium Channels in order, but would travel down the axon triggering all of VGSC’s as the ions diffused along. What slows it down is that Na+ actually leaks back out in uninsulated areas, shortening the effectiveness of each action potential event, slowing down transmission despite the fact that more VGSC’s are available to provide a Na+ infusion.

  • @2ManuBoy2

    No, see, Na+ wouldn’t ‘need’ to wait around to depolarise every Voltage Gated Sodium Channels in order, but would travel down the axon triggering all of VGSC’s as the ions diffused along. What slows it down is that Na+ actually leaks back out in uninsulated areas, shortening the effectiveness of each action potential event, slowing down transmission despite the fact that more VGSC’s are available to provide a Na+ infusion.

  • Hi,
    i think the Schwann cells are responsible for Mylein sheath in PNS and in CNS its a sheet of Oligodendrocyte plasma membrane wrap around the axon.
    I’m not sure…

    thanks ..

  • i mentioned a comment in ep 15 where i said that the Na+ move really fast and spreads across the axoplasm by simple diffusion,until it reaches a nearby Node and causes threshold.The main diff between ur concept and mine is that u say the Na+ ions are REPELLED away.That’s okay,the video here- watch?v=DJe3_3XsBOg
    says that the same thing happens in UNMYELINATED AXONS too.Why then the diffusion process faster here than in unmyelinated axons???Pls explain

  • i mentioned a comment in ep 15 where i said that the Na+ move really fast and spreads across the axoplasm by simple diffusion,until it reaches a nearby Node and causes threshold.The main diff between ur concept and mine is that u say the Na+ ions are REPELLED away.That’s okay,the video here- watch?v=DJe3_3XsBOg
    says that the same thing happens in UNMYELINATED AXONS too.Why then the diffusion process faster here than in unmyelinated axons???Pls explain

  • The purpose of the myelin sheath is to allow impulses to transmit quickly and efficiently along the nerve cells.

  • hello
    aren’t those two different things ?
    1) Sheet of Oligodendrocyte plasma membrane wrap around the axon are glia-cells, you can say these are SUBSTITUES for Schwann cells as these dont occur in CNS level

    2) Schwann cells only occur in PNS ..

    or have i got this wrong
    Best regards Martin (Denmark)

  • Wow, you’re awesome!! I totally get how this works from your video, and I can’t believe there’s a whole website here of other free neuro explanations. it’s a treasure trove! i need to know this stuff for my grad school (psych) entrance exam, and i want to be quite competent in it for its own sake, but one biopsych class in undergrad isn’t conducive to that! this is really amazing. you’re helping promote scientific progress with what you’ve done here!

  • Actually, the signal that your talking about (the “Action Potential”) does not “die down and die down” as it propagates down the axon. Once the membrane potential reaches ~50-55 mV threshold is reached and the Action Potential travels without signal loss (it cannot fall below threshold. An action Potential is an “All-or-Nothing”.

  • Actually, the signal that you’re talking about (the “Action Potential”) does not “die down and die down” as it propagates down the axon. Once the membrane potential reaches ~-50 to -55 mV, threshold is reached and the Action Potential travels without signal loss (it cannot fall below threshold) because an action Potential is an “All-or-Nothing” process.

  • firstly i want to pay a big thanks to you Mr.SAMUEL.. ! well.. will you please upload some video about detail of spinal cord?? its structure, function???

  • woooow thank u soo much i have been having problems understanding it by my self from my physiology book .. u helped alot !!

  • I’m so glad I understand this now! But I have another question… How is it that with more myelin, you obtain a faster signal?

  • @InteractiveBiology I dont understand a thing. In unmyelinated axons, what prevent charge to rush through the inside of axon like in myelinated axons happens? The rush should be even more fast, why didn’t?

  • Hi! I have a question that not even my teachers at the university (in Stockholm) was able to answer and I hope that you could. Why is it said that neurons with thicker myelin sheath send signals faster? Is that even true?
    Thanks for some great videos!

  • So in other words.. Conduction in myelinated axons is faster because electrical impulses are accelerated by repulsive force of sodium ions transported into the cell by sodium channels at each nodes?

  • Well um writing a test on biological communication which covers action potential and nerve next week (University of Bradford). Anyway um sure u know that the myelin sheath provides electrical insulation. If thick, impulses traveling in the axon will experience less electrical resistance in its motion cause by charged ions outside the surface of the axon. Thats basically it n i hope it helps u 😉

  • Maybe a poor analogy as i’m only just learning this stuff myself but think of a room full of people and a waiter trying to cross the room, the wider the room the easier it is to cross it as there is less resistance.

  • Wow..thats kinda works like an elector megnetic on an alternator, iron around the Myelin sheet will push the electron to move faster. Am I right?

  • what i honestly think makes this guy so great at his job is that he listens to not only his own students but the online community who asks questions i am totally subscribing

  • @interactivebiology, Thanks for your clarification as I have the same question! However, I wonder if Schwann cell helps the neurons to speed up the signal transmitting, why wouldn’t the Schwann cell develop a longer in length to cover the whole axon, so that the signal can transmit from axon hillock to the end of axon?

  • if there isn’t any myelin sheath,won’t there be more sodium channels present along the membrane? so there is a possibility of the process being more faster without the myelin sheath right? please help..

  • or is it a case of more resistance to movement of the impulse from other positive charges resisting the travelling of the impulse?

  • Could having a thin Myelin Sheath contribute to experiencing behaviors of Obsessive Chronic Disorder (OCD) ?

  • In behavioral neurobiology, OCD stands for obsessive/compulsive disorder. Is this what you meant to say?

  • In behavioral neurobiology, OCD stands for obsessive/compulsive disorder. Is this what you meant to say?

  • Thanks you so so so much! its very helpful for my learning, i can grab the concept in 7 mins instead of digging into the textbook all day long!!

  • Thanks very much made great sense, however could have been explained in thirty seconds!

  • But I dont understand, why the voltage gated sodium channels is slow in entering sodium.. it’s characteristic that it’s fast opening and fast inactivated in contrast to potassium channels..?

  • if there were no myelin sheeths, wouldnt there be more sodium gated channels thus increasing the speed of the neuro transmission?

  • if there were no myelin sheeths, wouldnt there be more sodium gated channels thus increasing the speed of the neuro transmission?

  • Nice explanation but I think a point could have been clarified about how the myelin sheath helps speed the conduction. The axon has other pore-like channels that sit in the membrane that would allow the Na ions to leak out and dampen down the voltage sweeping down so that the impulse would ‘peter out’ as it travelled down the axon. The tightly wound layers of myelin prevent Na leakage out of the cell through the axon membrane allowing the charge to reach the node to cause another flood of Na ions into the axon. This new inflooding of Na opens up more voltage gated channels and starts the cycle over again. Great site!

  • what exactly is the role of myelin sheath in conduction as Na channels are present every where in the axon whether it is myelinated or non myelinated ?

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