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This post was updated on April 5, 2022.
So we’ve gone over Depolarization, Repolarization and Hyperpolarization in some detail. It’s time to do an overall review. To understand what causes the action potential to happen, you have to understand what’s happening with the neuron at rest.
Where it all happens…
First off, here we have a neuron. There are a few main parts to the neuron – the dendrites, the cell body a.k.a. the soma, the axon and the axon terminals. Yes, we can get more detailed, but let’s stick with that for now.
The action potential happens in the axon. So let’s look a little more closely at it.
When a neuron is at rest, you have a few things happening. First thing, the membrane potential, that’s the charge across the membrane – that’s at -70 millivolts. And at rest, we have sodium ions more concentrated outside the axon and potassium ions that are found more inside the axon.
Why exactly that’s the case is important to understand, but that’s for another video. Sodium outside and potassium inside. That’s the key. Sweet!
How and Where the Action Potential starts
Ok, let’s say this neuron is stimulated by another neuron. That causes an increase in the membrane potential. You might get a little bump in the membrane potential. Now, if that stimulus is large enough so that the membrane potential reaches what’s called the threshold potential, we get an action potential.
That’s the key – we need a big enough stimulus. And the place this starts is called the axon hillock. That’s the part of the neuron where the axon starts.
Once we reach the threshold potential, voltage-gated sodium channels open.
Now, we said that sodium is concentrated outside the cell. What’s going to happen when those channels open? Well, since they only allow sodium ions to pass through them, sodium ions are going to start rushing into the cell.
Sodium ions have a positive charge, so as sodium ions start rushing in, the membrane potential is going to get more and more positive. This phase is called depolarization – it’s where the membrane potential is getting more positive. But there’s another fact that you need to know.
The equilibrium potential for sodium is +60 mV. That’s the membrane potential where sodium is kinda balanced. Now, that’s a simplified explanation, but it’ll work for now. This is where sodium wants the membrane potential to be. So it’s basically rushing into the cell trying to get the membrane potential to +60 mV.
But it doesn’t quite reach there, because, at this point, the membrane potential is high enough to cause voltage-gated potassium channels to open.
Now here’s the thing. The equilibrium potential for potassium is -93 mV. And as we said, potassium is more concentrated INSIDE the cell. So what’s going to happen? How do we get to that negative value?
Well, potassium is going to seize the opportunity to leave the cell. Potassium is also positively charged, so as the positive ions start flying out of the axon, the membrane potential is going to go back down.
This is called repolarization.
But, remember, potassium is trying to get to IT’S equilibrium potential. And that’s a pretty low number. So it’s going to shoot past that -70 millivolts and once it passes that resting membrane potential, it’s now in the stage called hyperpolarized.
Now here’s how I remember this. I always think of polarized as being in the negative resting state. That’s not technically true, but it helps me remember it. Here’s how.
Depolarization makes it less negative, repolarization brings it back to that negative resting state and hyperpolarized makes it even more negative than the normal resting state. It’s overpolarized.
Now remember, don’t use that as a definition, because it’s not. But it helps me to remember it, and it might help you too.
Restoring Resting Membrane Potential
Ok, the last step is this…
We said that we’re trying to get to the equilibrium potential for potassium by having potassium leave the cell. That number was -93 mV. But we never actually get there because as the membrane potential gets lower, the voltage-gated potassium ions start to close.
Once they are closed in this hyperpolarized state, the factors that are responsible for establishing the resting membrane potential do what they do, and the resting membrane potential gets restored.
These have to do with things like the presence of passive channels in the membrane, sodium potassium pump helping to redistribute sodium and potassium, and other factors. The key thing is resting membrane potential is restored and the action potential is now complete.
So there you have it, that’s an overview of what happens during an action potential.
Now, of course, there are many more details that we can get into about the different stages of the action potential, and if there’s some aspect of it you’d like me to dig deeper into, go ahead and let me know in the comments here.