January 4, 2011

008 The All-or-None Action Potential

Ever wondered why people refer to the action potential as “all-or-none”?

Well, I guess you’ll just have to watch the video above to understand. In this video, I use the kitchen sink, a paper towel and marbles to bring the concept home.

If you have any questions/comments, feel free to leave it in the comments field below.

– Leslie Samuel

Transcript of Today’s Video

Hey, this is Leslie Samuel again and I’m kind of excited today. I just posted a poll on my blog, and it was for you guys to help me to decide what the subtitle or slogan for Interactive Biology will be, and the one that you guys decided on was “Making biology fun.” So this is the first time I get to use this introduction. Are you ready for it?

Hello and welcome to Interactive Biology TV, where we’re making biology fun! I like the ring of that. I wasn’t going for that title before, but when you guys suggested it, when you guys voted on it, it really started getting me excited.

Anyhow, in today’s episode, we’re going to be talking about the all-or-none nature of the action potential. We’ve been talking about the nervous system, and depending on which book you read, it might say “all-or-none” or “all-or-nothing.” Either way, we’re talking about the same thing. Now, there are two terms that I want you to know for this episode. The first one is the axon hillock, and the second would be the threshold, okay? We’re in my kitchen today, and I’m going to be using a high-tech device to illustrate these concepts. So, let’s look at what we have here today.

Alright, so this is my high-tech device for today. I’m not going to be doing any dishes even though we’re by the sink, but hopefully, this will get you guys to understand the concept of the all-or-none nature of the action potential. So the two terms I said were the axon hillock and the threshold. Axon hillock is the place where the axon starts. We looked at the neuron and the parts of the neuron, and right where it goes from the soma to the axon, we have the axon hillock. That’s the first place we see voltage-gated ion channels. In a previous episode, I spoke about the proteins in the membrane, and you can go back to that and see what these voltage-gated ion channels do.

But anyhow, this is where action potentials can start. However, in order for them to start, the stimulation needs to be enough to bring it to the threshold. So this, we’re going to imagine that this is my axon hillock, and in order for an action potential to happen, this needs to break, and when this breaks, voltage-gated channels open and ions rush in, and we have that electrical signal.

So, these are my ions, the little blue marbles, and I’m going to put the first one on here and nothing happens, that’s not enough to cause an action potential. I’ll put the second one and nothing happens. This is a pretty strong piece of paper towel. It’s Brawny, and I think that’s the one where you see a really strong man on the plastic wrap, so that should say something about the strength, I think.

Anyhow, I’m going to continue putting, still no action potential. It has not reached threshold just yet, so I’m just going to continue putting these ions. I’m trying to bring it to threshold, but it’s not there as yet. And we’re going to put some more. Evidently, Brawny is pretty strong. No insult against Bounty or any of those other paper towels. But here we go, I’m going to continue putting more stimulation, more stimulation, nothing happens. It’s pretty strong, so let’s dump some more on there. At a certain point, it’s going to reach a point where…

Oh! There we go . . . it breaks and those ions get through. That point was the threshold. Now, up until that point, nothing happened. But once we reached that point, we reached the threshold, it’s enough stimulus to cause the action potential to happen. That is why we call it all-or-none.

So there you have it, it’s all-or-none. It’s either going to happen or it’s not going to happen. If there’s enough stimulation to reach the threshold, as you saw with this elaborate set-up, if there’s enough stimulation, it will cause an action potential. If there isn’t enough, it will not cause an action potential. That’s why we call it all-or-nothing, all-or-none. I hope you enjoyed this episode. If you have any questions or comments, leave them in the comments field below. I’d be happy to answer your questions. Who knows, I might even make a video to answer your specific question. That’s it for this video, and I’ll see you in the next one.

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