In this video I explain how an action potential is triggered in a neuron when it reaches its threshold. I explain the movement of ions, resting potential, and the all or none principle of neural firing. I also cover how the message is propagated along the axon and how myelin makes this process more efficient.

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Video Transcript:

Hi, I’m Michael Corayer and this is Psych Exam Review and in this video I’m going to explain what happens when a neuron, or sends an action potential. So the first thing to know neurons firing is that we say that neurons fire in an all or none fashion.

So what this means is that it either fires or it doesn’t. It can’t fire a little bit or it can’t fire really strongly. It has one message that it sends and it either sends it or it doesn’t. In this way, you could think of a neuron like a gun, you can’t fire a gun a little bit. If you stimulate a neuron, you stimulate nothing’s happening, nothing’s happening, and then suddenly it fires. The same for a gun. You apply pressure to the trigger, nothing’s happening, nothing’s happening, and then suddenly the gun fires.

And it fires the same way every time. You can’t make it fire harder by pulling the trigger really strongly.

The same is true of a neuron. You can stimulate, you can cause it to fire, but you can’t cause it to fire a little bit or fire really hard.

So what does this stimulation actually refer to? Let’s we’re looking at part of a dendrite here. The neuron is actually surrounded by positive and negative ions; they’re inside the neuron and they’re outside the neuron.

The membrane of the neuron is semipermeable this means that some things can move in and out of the membrane. These charged particles, called ions, can move in and out of the neuron through special channels. What stimulation is, is when the dendrites are being stimulated, it’s positive ion channels are opening and they’re letting these positive ions moved inside the neuron. And if enough of them move that’s how the neuron is going to reach its threshold. That’s essentially what’s putting the pressure on the trigger, is the movement of these positive ions. So if we get enough of these to move inside we reach the threshold and that causes the neuron to fire.

Now normally if we look at a neuron, it’s at what’s called resting potential, and this means that if we look at the balance of positive and negative ions inside and outside of the neuron, the neuron has a charge of -70 millivolts. When a neuron reaches its threshold and it fires, this is gonna change. So during an action potential the positive ions are suddenly going to rush in. So if enough of them move in, it triggers a bunch of channels to open all at once, and positive ions rush inside the neuron. And this means that its charge jumps temporarily all the way up to +40 millivolts.

What happens after these ions rush in is the neuron actually wants to get back to its resting potential so it has pumps that will push these ions back out and they’ll restore it to -70. So if we look at this over time, we’re sitting at -70 millivolts, this is our charge, then we get stimulation that causes a bunch of channels to open so suddenly we jump up to +40 millivolts. And then, these pumps push those ions back out and they restore us back to -70.

Then at this point we’re ready to fire again. So if we’re stimulated again, we can jump back up to +40 and back down to -70. And then back up the +40 and back down. And it actually overshoots a little bit when it goes back to -70, it goes a little below that for very short amount of time which is why I’ve drawn this little dent there.

So when we look at this, we can see each of these jumps here is the neuron firing. And then there’s this period of time between when it’s fired and before it’s ready to fire again when it’s back at the resting potential and this is called the refractory period.

So when we stimulate a neuron, it doesn’t stay turned on, it can’t stay at +40, it has to go back to -70 and we continue our gun analogy here. If you think about a machine gun if you hold down the trigger on a machine gun, you give it constant stimulation, it doesn’t stay turned on, it doesn’t release a solid beam of metal or anything like that. Instead, it fires, and then it stops, prepares to fire again, has to move another bullet into the chamber, and then it fires again, and then it has to get ready, then it fires again.

The neuron is the same way. If you constantly stimulate a neuron, it fires but then it has to get back to -70 before it can fire again and then it fires, then it has to get back. So it has this refractory period in between.

This is very short, we’re talking about a millisecond or several milliseconds or so for a neuron firing and then stopping, and then firing again. So what’s actually happening on the axon when a neuron is firing? Well, so we said that when it reaches this threshold in the dendrites a bunch of ion channels open and what happens is they open here at the beginning of the axon. So we’ve got positive ions waiting outside the neuron here.

When the neuron fires, these channels here on this part of the axon are going to spring open. That means this ion can now move inside. And when this ion moves inside, that triggers the next channels to open. So this ion can move inside and then that triggers the next channels, and then that triggers this.

So we get this domino effect where these ions move in and that triggers the next one, the next one, the next one, so it moves down the line. The important thing to note is that nothing’s actually moving down the axon. There’s not a particle that has to, you know, actually travel down this distance. That’s not what’s happening at all.

What’s happening is each ion is just moving in, then the pumps are going to push it back out. So it moves in, then it gets pushed back out, and this one moves in and it’s going to get pushed back out. In this way we get the message all the way down to the end of the axon without anything having to actually travel. And this is very similar to if you’ve ever done the wave in a baseball stadium. So you stand up and sit down, and the next person stands up and sits down, and the next person stands up and sits down. In this way, the message can travel across the entire stadium and nobody’s actually walking that distance. Nobody has to actually move around the whole stadium but the message is able to get around the whole stadium anyway. So the way the ions move on an axon is very similar.

This one moves in, and then pushed back out, this one moves, this one moves in. Just like that. Now I said in the previous video that myelin helps to make neurons more efficient. We said it allows axons to send a message more quickly. So how does this actually happen?

So we have an axon here but now we’re going to put some myelin around it. So I’m going to wrap this axon with some myelin and now what happens is when channels open here on the axon and this positive ion can move inside that triggers this section to open.

Then that triggers this section to open and that triggers this section. So what happens, we don’t have to do it along this section of the axon, we can skip to the next gap, the next node of Ranvier. So now we’re only moving ions in a few places on the axon, instead of moving them all across the entire axon. This means that we can send the message more quickly.

Just like in the baseball stadium, if only the first person in each section of seats stood up and sat down, the message would get around the stadium more quickly and it would be more efficient. We wouldn’t need to move all those people in between in order to get the same message to the end.

Because when it gets here it’s that same message, standing up and sitting down, it’s the same ion channels opening or not opening at this end of the axon. So that’s how myelin is able to make neurons communicate more efficiently, send an action potential down the axon more quickly and more efficiently. In the next video I’ll talk about what happens when the message gets to the end of the neuron, to the terminal buttons.

I hope you found this helpful. If so, please like the video and subscribe to the channel for more. Thanks for watching!