January 5, 2011

009 Depolarization: Phase 1 of the Action Potential

The action potential can be a complicated thing to understand, unless you are dealing with little white plusses on a table 🙂

In this video, I help you visualize the first phase of the action potential – the Depolarization phase.

Go ahead and watch the video and you should get a clear understanding of the events that cause depolarization of the neuron.

– Leslie Samuel

Transcript of Today’s Video

Hello and welcome to Interactive Biology TV, where we’re making biology fun! In this episode, we’re going to be talking about depolarization, which is the first phase of the action potential. We’re going to go into some more detail than we’ve been doing in the past when it comes to the action potential. It’s going to be a little different. You’re not going to be looking at me talking about something. You’re actually going to be looking at another one of my elaborate set-ups, and I’m going to call this set-up the “Action Potential Simulator.” I hope that gets you excited about it.

Alright, so what we have here is, this side represents outside the cell, outside the axon, and this side represents inside the axon. Here we have a bunch of pluses, and these pluses are representing sodium ions. Now, sodium ions have a positive charge, and that’s why I chose these very attractive pluses to represent sodium ions.

Now, here in the center, we have what we’re going to refer to as voltage-gated sodium channels. What happens is, you have all these sodium ions on the outside of the cell, and these sodium ions want to get into the cell, but they cannot get into the cell. Why can’t they get into the cell? Because the voltage-gated sodium channels are closed. In order for them to get into the cell, this channel needs to open.

Now, why does sodium want to get into the cell? Well, if you go back to Episode 6, this can refresh your memory a little bit, we spoke about Donnan equilibrium. And we spoke about the fact that the membrane potential at rest is somewhere between -50 to -70, -80 millivolts. At that negative charge, sodium ions are not happy. In order for these guys to be happy, the membrane potential needs to be around +58. And on the inside of the cell, we have a negative charge, so this is not something that sodium likes.

So how does sodium want to counteract this? Well, sodium wants to rush into the cell so that it can make the membrane potential more positive, which will be closer to the Donnan equilibrium potential for sodium ions. I hope that makes sense, if not, once again, go back to Episode 6 and refresh your memory on how the Donnan equilibrium works.

So, someone touches you or there’s some stimulation and there needs to be an action potential that’s sent along the axons of the neurons that are stimulated. What happens is, when the membrane potential reaches threshold, that is enough charge to cause voltage-gated sodium channels to open. Once those voltage-gated sodium channels open, sodium ions can then go into the cell.

Now, what is this going to do to the charge inside the cell? What is it going to do to the membrane potential? Well, now you see you have all these positives on the inside, so that’s going to make the membrane potential more positive. This is the process of depolarization. It is the first phase of the action potential, and this is how the charge starts that gets sent along the axon.

So once again, when the membrane potential reaches threshold, voltage-gated sodium channels open and once those open, sodium ions are going to rush into the cell, making the membrane potential more positive, and that is depolarization. I hope that makes sense to you. If you have any questions, as usual, feel free to ask them in the comments below, and I’ll be happy to answer your questions. And who knows, maybe even make a video answering your specific question. That’s it for this video, and I’ll see you in the next one.

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