January 12, 2011

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.


– 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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