014 The Journey down the Axon

Why does the action potential travel in one direction down the axon? Why doesn’t it go in reverse? Are there features about the axon that makes that happen?

Watch the video and find out.

Enjoy!

– Leslie Samuel

Transcript of Today’s Episode

Hello and welcome to Interactive Biology TV, where we’re making biology fun! My name is Leslie Samuel. In this episode, Episode 14, we’re going to be looking at the journey down the axon. In other words, we’re going to look at how the action potential actually travels down the axon so that it reaches the axon terminals.

To illustrate this, I’m going to attempt to draw a neuron, and I’m going to start with the soma, which is the cell body. Then I’m going to draw the axon. I’m not going to draw the axon terminals, but I’ll just write here “AT” for axon terminals. Over here, we have the soma, and of course, this will be the axon. Now right here where the soma meets the axon, as we’ve seen in earlier videos, this is called the axon hillock, and this is the first place we see voltage-gated channels.

So, when a stimulus comes and causes the membrane potential to reach threshold, what we said happens is voltage-gates sodium channels open, and sodium rushes in. I’m going to write here “Na+”, so that’s sodium ions, and I’m going to just put a few of those outside the axon. So there we have it, sodium ions concentrated outside the axon.

A stimulus comes along, voltage-gated sodium channels open, and then sodium, because of its driving force, rushes into the cell. When it rushes into the cell, it just doesn’t go into the cell and stay in one place. Of course, it’s going to travel along the axon and it’s free to travel in both directions. When that comes in and it moves down the axon, that’s going to make the membrane potential more positive since sodium has a positive charge.

As it goes down here, it’s going to open more channels, and more sodium ions are going to rush in, and of course those are free to travel in either direction. The same process will continue: membrane potential goes up, sodium rushes in, travels in either direction. That’s going to continue over and over until it reaches the terminals.

Now, one of the questions you might be asking at this point is, “If the sodium ions can travel in either direction, why is it that we have an action potential that just travels in one direction?” That’s a very important concept for you to understand. We spoke about the refractory period. When the channels over here open and the sodium ions travel down, that causes these channels to open and sodium comes in and goes in either direction.

However, the voltage-gated sodium channels that are on this side are in their refractory period. If you remember what we said in the episode about refractory periods, when it’s in the absolute refractory period, you cannot stimulate the voltage-gated channels to open again. You have to wait for it to be reset to closed before you can re-stimulate it.

So all those sodium is rushing in and travelling in both directions, the signal is only going to travel down the axon because of the fact that the previous voltage-gated sodium channels, the ones that are closer to the soma in that direction, those are going to be either open or inactive. And it needs to wait for them to be reset to closed before they can be re-stimulated to fire. This is why the action potential will only travel in one direction.

That’s the entire concept for this video. As usual, if you have any questions, feel free to leave it in the comments below, and I’ll be happy to answer your question, and maybe even make a video to answer your specific question. That’s it for this episode, and I’ll see you in the next one.



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  • One question that I have for my lab is whether or not you can put the nerve
    in either direction with a stimulus and still get the same conduction
    volocity results?

  • @aburmeis Hi there. Thanks for asking the question. However, I’m not 100% sure what you mean when you say “whether or not you can put the nerve in either direction”. Can you explain? Also, check out my video on Saltatory. That might help to explain what you are trying to figure out. It’s episode 015.

  • @aburmeis Hi there. Thanks for asking the question. However, I’m not 100%
    sure what you mean when you say “whether or not you can put the nerve in
    either direction”. Can you explain? Also, check out my video on Saltatory.
    That might help to explain what you are trying to figure out. It’s episode
    015.

  • My lab is on the computer so we do not have any hands on experiements. One of the questions asked which really stumped me was involving a frog nerve. We would use a certain amount of voltage to stimulate the nerve and then with our information, then calculate the conduction volocity which is meters traveled per second. So in the experiement if the nerve reversed in it placement on the stimulating and recording electrodes, would any differences be seen in conduction velocity? TOUGH I KNOW!

  • My lab is on the computer so we do not have any hands on experiements. One of the questions asked which really stumped me was involving a frog nerve. We would use a certain amount of voltage to stimulate the nerve and then with our information, then calculate the conduction volocity which is meters traveled per second. So in the experiement if the nerve reversed in it placement on the stimulating and recording electrodes, would any differences be seen in conduction velocity? TOUGH I KNOW!

  • My lab is on the computer so we do not have any hands on experiements. One of the questions asked which really stumped me was involving a frog nerve. We would use a certain amount of voltage to stimulate the nerve and then with our information, then calculate the conduction volocity which is meters traveled per second. So in the experiement if the nerve reversed in it placement on the stimulating and recording electrodes, would any differences be seen in conduction velocity? TOUGH I KNOW!

  • I know this is probably not easy to understand without knowing everything about the experiement. If you are unable to answer that is ok 🙂 I do appreciate your trying. You helped me understand much more then I ever thought I would!

  • I know this is probably not easy to understand without knowing everything
    about the experiement. If you are unable to answer that is ok 🙂 I do
    appreciate your trying. You helped me understand much more then I ever
    thought I would!

  • @aburmeis Oh gotcha. I remember that experiment. Here’s the thing – If you stimulate it at the opposite end, you can be stimulating axons that have a different diameter. If the diameter is larger, that will have a higher velocity. If it’s smaller, the velocity will be slower. The velocity will depend on the ratio of larger to small. One end might be thicker and the other end might be thinner. That’s the only way that I can think that the velocity would be different.

  • Oh gotcha. I remember that experiment. Here’s the thing – If you stimulate it at the opposite end, you can be stimulating axons that have a different diameter. If the diameter is larger, that will have a higher velocity. If it’s smaller, the velocity will be slower. The velocity will depend on the ratio of larger to small. One end might be thicker and the other end might be thinner. That’s the only way that I can think that the velocity would be different.

  • Oh gotcha. I remember that experiment. Here’s the thing – If you stimulate it at the opposite end, you can be stimulating axons that have a different diameter. If the diameter is larger, that will have a higher velocity. If it’s smaller, the velocity will be slower. The velocity will depend on the ratio of larger to small. One end might be thicker and the other end might be thinner. That’s the only way that I can think that the velocity would be different.

  • – That is exactly what I was looking for. Thank you for all you do! You are a great person 🙂

  • Hi, i still didn’t understand how an action potential only goes in one direction.. is there any other way in explaining this?

  • You should check out the video on Refractory Periods. What happens is once the signal starts, the channels where the action potential already passed are in their refractory period, so they cannot be re-stimulated for a period of time. The only ones that can be stimulated at that point are the ones downstream.

    Hope that helps.

  • Thank you very much, I’ve just been seeing that video and it makes sense. I am loving your videos!

  • U r a wonderful teacher. Thank you very much…. I had a question. How much time does it take for an actional potential to take place….??

  • U r a wonderful teacher. Thank you very much…. I had a question. How much time does it take for an actional potential to take place….??

  • Thanks for your comment. An action potential usually lasts around millisecond or less.

  • Great series but just one question: Those Na+ ions (positively-charged) rushing across to achieve Donnan equilibrium into a negatively-charged space *and* traveling in the direction towards the axon terminals are depolarizing the membrane and making it less likely for Na+ to cross the membrane “downstream” (towards the terminals) correct? I guess I’m struggling to see how this effect can continue propagating down the axon.

  • Great series but just one question: Those Na+ ions (positively-charged) rushing across to achieve Donnan equilibrium into a negatively-charged space *and* traveling in the direction towards the axon terminals are depolarizing the membrane and making it less likely for Na+ to cross the membrane “downstream” (towards the terminals) correct? I guess I’m struggling to see how this effect can continue propagating down the axon.

  • @byerscha7015 Great question. When the Na+ crosses the membrane and travels downstream, remember that it’s traveling by diffusion. As a result of that, it’s basically spreading out, and the positive charge decreases. However, it’s still positive enough to open more channels downstream and cause more Na+ to rush in. That of course gives more of a boost. Hope that helps!

  • Great question. When the Na+ crosses the membrane and travels downstream, remember that it’s traveling by diffusion. As a result of that, it’s basically spreading out, and the positive charge decreases. However, it’s still positive enough to open more channels downstream and cause more Na+ to rush in. That of course gives more of a boost. Hope that helps!

  • Great question. When the Na+ crosses the membrane and travels downstream, remember that it’s traveling by diffusion. As a result of that, it’s basically spreading out, and the positive charge decreases. However, it’s still positive enough to open more channels downstream and cause more Na+ to rush in. That of course gives more of a boost. Hope that helps!

  • So this kind of thing happens everytime it’s simulated? If it’s not simulated what happens?

  • Behavioral Neuroscience has been the death of me this semester… these videos break it down a lot better than my text book! Thanks 🙂

  • Behavioral Neuroscience has been the death of me this semester… these videos break it down a lot better than my text book! Thanks 🙂

  • Can sodium traveling toward the soma move past channels in their inactive states (arp) and stimulate channels closer to the soma, thus getting a new action potential?

  • @jgordin1982 All questions are answered in the Interactive Biology community forums from now on. Go to the website in the description and then visit the community. This is to make it as efficient as possible as we have multiple people over there to help answer questions.

    All the best

  • Can sodium traveling toward the soma move past channels in their inactive states (arp) and stimulate channels closer to the soma, thus getting a new action potential?

  • All questions are answered in the Interactive Biology community forums from now on. Go to the website in the description and then visit the community. This is to make it as efficient as possible as we have multiple people over there to help answer questions.

    All the best

  • I want an explanation for
    Supernomal phase & Subnormal phase
    in action potential
    …….PLEASE……
    thank u 4 ur great efforts

  • @MsNany17 We’re glad to know that you are finding value in our videos. Unfortunately, Leslie is no longer taking requests for specific videos, but he will definitely get to more topics in the future. He has many to work on at the moment. So stay tuned for more.

  • We’re glad to know that you are finding value in our videos. Unfortunately, Leslie is no longer taking requests for specific videos, but he will definitely get to more topics in the future. He has many to work on at the moment. So stay tuned for more.

  • We’re glad to know that you are finding value in our videos. Unfortunately, Leslie is no longer taking requests for specific videos, but he will definitely get to more topics in the future. He has many to work on at the moment. So stay tuned for more.

  • That’s great to hear 🙂 THere are more Biology videos in the website that you may find useful. And, we have more coming soon! Have fun!

  • how are action potential propagated down an unmyelinated axon,is that the same action with the one myelinated axon

  • @InteractiveBiology @MsNany17 it would be great to have one for what she suggested. in the end we are only suggesting , to make this channel the place to find everything,instead of having to go look for another user.. subnormal/supernormal is part of Action Potential. its like you cut out part of it.. if youre gonna explain 3/4 of it , might as well continue. nonetheless i love those videos. Great work.

  • @InteractiveBiology @MsNany17 it would be great to have one for what she suggested. in the end we are only suggesting , to make this channel the place to find everything,instead of having to go look for another user.. subnormal/supernormal is part of Action Potential. its like you cut out part of it.. if youre gonna explain 3/4 of it , might as well continue. nonetheless i love those videos. Great work.

  • @shootingstars1511 That sounds nice, but it’s not possible right now. If all I did was this, then maybe, but it’s not. The best I can do is make the videos as I need them for the classes I teach. I will continue adding more, but since I’m just 1 person doing this for free, I can’t do as many as you’d like.

  • That sounds nice, but it’s not possible right now. If all I did was this, then maybe, but it’s not. The best I can do is make the videos as I need them for the classes I teach. I will continue adding more, but since I’m just 1 person doing this for free, I can’t do as many as you’d like.

  • @InteractiveBiology i do understand , i should mention that i wasnt trying to attack you in any way or well “tell” you what to do .. it is hard to make videos especially that you are a teacher. thank you for what youve put up so far though 🙂

  • i do understand , i should mention that i wasnt trying to attack you in any way or well “tell” you what to do .. it is hard to make videos especially that you are a teacher. thank you for what youve put up so far though 🙂

  • That sounds nice, but it’s not possible right now. If all I did was this, then maybe, but it’s not. The best I can do is make the videos as I need them for the classes I teach. I will continue adding more, but since I’m just 1 person doing this for free, I can’t do as many as you’d like.

  • i do understand , i should mention that i wasnt trying to attack you in any way or well “tell” you what to do .. it is hard to make videos especially that you are a teacher. thank you for what youve put up so far though 🙂

  • Why does the AP start in that part of the axon? I mean Why can’t it start like in a Na channel that is in the middle of the axon and go from there on?

  • hello, i have a question to ask if you dont mind, “Can you give three ways in which a neurone is similar to other animal cells

  • I spent 2 hours looking at different videos trying to find what i was looking for and nothing explained it the way I needed, your video did thank you!

  • I didnt really understand why does the action potential travels only in one direction if the ions Na+ can travel in both directions ? Can someone pls explain ?

  • Your videos are a life saver! I really appreciate you taking your time to share these with those of us who are trying to navigate the A&P jungle!

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