Understanding Action Potential in Neurons

Hi everyone, Dr Mike here. In this video we're going to go through a concept that many students find difficult when they go through neuroscience. That's the concept of the action potential. Now simply put, any time one of your neurons or nerves sends a signal, it does it via an action potential. Now you always think of your nerves sending electrical signals, but in actual fact they send electrical chemical signals. What are these electrical chemical signals? Simply ions moving into or out of a neuron. Now a neuron is just a cell, so all we're talking about with action potentials are ions, which are charged atoms or elements, they could be positive or negatively charged, moving into or out of a cell. Now remember that when you have a cell, so I've got a neuron drawn up here, right? I've got the body of a neuron, the axon of a neuron, but I haven't drawn the rest of it. Remember, a couple of things, that ions being charged atoms or elements include positively charged sodium, and positively charged sodium sits predominantly outside of the cell, that's important. And we've got positively charged potassium, which sits predominantly inside the cell. Now, we don't really need to talk about the other ions in this video. Remember, there's chloride, Cl-, magnesium, Mg2+. There's so many different types. But these are the two that we need to focus on for an action potential. And remember that I spoke about in a previous video that our excitable tissues, being our muscles and our neurons, they have what we call a resting membrane potential. It means when they're at rest, not doing anything. There is a charge difference between the outside of the cell compared to the inside of the cell. It's the resting membrane potential. You can see here, for the neuron, it's predominantly negatively charged inside the neuron compared to outside the neuron. In actual fact, if I were to put an electrode inside and outside and measure the charge difference from the two, what you'd find is that the inside of the neuron sits at about negative 70 millivolts. So at negative 70 millivolts, that's where the neuron is resting and nothing's happening. Now that we've got a charge difference and a chemical difference, most sodium outside, most potassium inside, we can now stimulate a neuron to send a signal. Okay, how does this happen? First thing is this. Firstly, you need something to excite the neuron. In this case, I've got some neurotransmitters that have been released from a neuron, and these neurotransmitters are going to be excitatory neurons like glutamate. And what these glutamate neurotransmitters will do is they'll bind to glutamate receptors. Now when they bind to glutamate receptors, they'll open up, they'll flip their lid open, and they allow for sodium to move through. Remember diffusion, where an ion or a substance moves from its high chemical concentration to its low? Well if most sodium is outside, it means it wants to go inside. So when glutamate binds to these receptors, sodium will start to move inside the cell. And remember, sodium has a positive charge to it. So if sodium goes into the cell, it's taking its positive charge with it, and it makes this small part of the neuron more positive. Now, if more glutamate binds to these receptors and lets more sodium in, it becomes even more positive inside the neuron. Now, if we look at this graph here, if we let enough positive sodium inside the neuron that it drifts up, up, up, up, up to Negative 55 millivolts which we term the threshold. So if we let enough positive sodium in that it goes from negative 70 to negative 55. So negative 55 is more positive than negative 70. If it hits negative 55, then what we've done is something very special. If enough positive sodium has come in that it's now negative 55, this first sodium channel is going to flip its lid. This sodium channel is what we call a voltage-gated sodium channel. Voltage-gated. Think of a gate. A gate needs to be either open or closed and some of these gates require keys. In this case, it's a voltage gated channel which means the key is a charge and the key charge is negative 55. So as soon as it hits negative 55 in this area of the neuron, the lid flips open, sodium rushes into the cell and takes its positive charge with it. That now means this part of the neuron becomes more positive. And if this part of the neuron becomes more positive and hits negative 55... It opens the next sodium channel and sodium rushes in. And the same thing happens. Hits negative 55, next sodium channel opens up. Now what you can see is what we've done is in a domino-like fashion, we've opened voltage-gated sodium channels and sodium goes boom, boom, boom, boom. All that is, is an electrical signal. That is any time you think, any time you move, any time you feel an object, all that's happening is sodium is rushing in. That is the signal that's being sent. That's the electrical. signal. If we were to draw it up on the graph, like I said, enough positive sodium goes in, it hits negative 55, then we open up all these voltage-gated sodium channels and sodium rushes in, taking its positive. charge with it, it becomes so positive inside the neuron that this graph spikes up to positive 30. Now once it hits positive 30, these sodium channels close. So that means the sodium is now trapped inside of the neuron and it's positive 30 inside the cell. But something else happens that's special is that these green channels here, they're voltage gated potassium channels. And again, the key to open this gate is positive 30. So now, all these voltage-gated potassium channels flip their lids open, and potassium will diffuse from inside the cell, down its concentration gradient, outside the cell, taking its positive charge with it. That means it becomes negative again inside that neuron. And that means this graph starts to drop back down again. In actual fact, so much positive potassium leaves that it goes below the resting membrane potential of negative 70. and goes to negative 90. Why does it go down this far? That's called hyperpolarization, and it means that you can't stimulate this neuron to send another signal too quickly. In epilepsy, you don't hyperpolarize. It doesn't go that far down, which means you can trigger the neuron to fire, fire, fire, fire, fire, and that's the misfire, and the abnormal overexcitement of a neuron. So, couple of phases when we look at an action potential. The first phase is the sodium coming into the cell, Sodium coming in and that next phase is potassium going out. When sodium goes in, that's called depolarization. Now remember, at rest, negative inside, positive outside, that's called polarized. Polarized means differing, okay? Differing on one side to another. So that's a polarized membrane. But when you throw the sodium in, you make it positive again inside the neuron and that's depolarization. So when sodium comes in... We term that depolarization. Then it hits positive 30 and the sodium channels shut. Then potassium channels open and potassium leaves the cell making it negative again inside the cell. That's called repolarization. So when potassium goes out, that's called repolarization. And so much potassium goes out that it hyperpolarizes the cell. Now, you're probably thinking, now we've got... all this sodium in, all this potassium out, that's the opposite of what it was in the beginning. How do we reset the neuron? Well, remember embedded in the membranes of all these cells, you've got a pump that throws three sodium out and it throws two potassium in and it resets the membrane. This is an action potential.

Hi everyone, Dr Mike here. In this video we're going to go through a concept that many students find difficult when they go through neuroscience. That's the concept of the action potential.

Now simply put, any time one of your neurons or nerves sends a signal, it does it via an action potential. Now you always think of your nerves sending electrical signals, but in actual fact they send electrical chemical signals. What are these electrical chemical signals?

Simply ions moving into or out of a neuron. Now a neuron is just a cell, so all we're talking about with action potentials are ions, which are charged atoms or elements, they could be positive or negatively charged, moving into or out of a cell. Now remember that when you have a cell, so I've got a neuron drawn up here, right?

I've got the body of a neuron, the axon of a neuron, but I haven't drawn the rest of it. Remember, a couple of things, that ions being charged atoms or elements include positively charged sodium, and positively charged sodium sits predominantly outside of the cell, that's important. And we've got positively charged potassium, which sits predominantly inside the cell.

Now, we don't really need to talk about the other ions in this video. Remember, there's chloride, Cl-, magnesium, Mg2+. There's so many different types.

But these are the two that we need to focus on for an action potential. And remember that I spoke about in a previous video that our excitable tissues, being our muscles and our neurons, they have what we call a resting membrane potential. It means when they're at rest, not doing anything.

There is a charge difference between the outside of the cell compared to the inside of the cell. It's the resting membrane potential. You can see here, for the neuron, it's predominantly negatively charged inside the neuron compared to outside the neuron. In actual fact, if I were to put an electrode inside and outside and measure the charge difference from the two, what you'd find is that the inside of the neuron sits at about negative 70 millivolts. So at negative 70 millivolts, that's where the neuron is resting and nothing's happening.

Now that we've got a charge difference and a chemical difference, most sodium outside, most potassium inside, we can now stimulate a neuron to send a signal. Okay, how does this happen? First thing is this. Firstly, you need something to excite the neuron. In this case, I've got some neurotransmitters that have been released from a neuron, and these neurotransmitters are going to be excitatory neurons like glutamate.

And what these glutamate neurotransmitters will do is they'll bind to glutamate receptors. Now when they bind to glutamate receptors, they'll open up, they'll flip their lid open, and they allow for sodium to move through. Remember diffusion, where an ion or a substance moves from its high chemical concentration to its low?

Well if most sodium is outside, it means it wants to go inside. So when glutamate binds to these receptors, sodium will start to move inside the cell. And remember, sodium has a positive charge to it.

So if sodium goes into the cell, it's taking its positive charge with it, and it makes this small part of the neuron more positive. Now, if more glutamate binds to these receptors and lets more sodium in, it becomes even more positive inside the neuron. Now, if we look at this graph here, if we let enough positive sodium inside the neuron that it drifts up, up, up, up, up to Negative 55 millivolts which we term the threshold.

So if we let enough positive sodium in that it goes from negative 70 to negative 55. So negative 55 is more positive than negative 70. If it hits negative 55, then what we've done is something very special. If enough positive sodium has come in that it's now negative 55, this first sodium channel is going to flip its lid. This sodium channel is what we call a voltage-gated sodium channel. Voltage-gated. Think of a gate.

A gate needs to be either open or closed and some of these gates require keys. In this case, it's a voltage gated channel which means the key is a charge and the key charge is negative 55. So as soon as it hits negative 55 in this area of the neuron, the lid flips open, sodium rushes into the cell and takes its positive charge with it. That now means this part of the neuron becomes more positive. And if this part of the neuron becomes more positive and hits negative 55... It opens the next sodium channel and sodium rushes in.

And the same thing happens. Hits negative 55, next sodium channel opens up. Now what you can see is what we've done is in a domino-like fashion, we've opened voltage-gated sodium channels and sodium goes boom, boom, boom, boom.

All that is, is an electrical signal. That is any time you think, any time you move, any time you feel an object, all that's happening is sodium is rushing in. That is the signal that's being sent. That's the electrical.

signal. If we were to draw it up on the graph, like I said, enough positive sodium goes in, it hits negative 55, then we open up all these voltage-gated sodium channels and sodium rushes in, taking its positive. charge with it, it becomes so positive inside the neuron that this graph spikes up to positive 30. Now once it hits positive 30, these sodium channels close.

So that means the sodium is now trapped inside of the neuron and it's positive 30 inside the cell. But something else happens that's special is that these green channels here, they're voltage gated potassium channels. And again, the key to open this gate is positive 30. So now, all these voltage-gated potassium channels flip their lids open, and potassium will diffuse from inside the cell, down its concentration gradient, outside the cell, taking its positive charge with it. That means it becomes negative again inside that neuron. And that means this graph starts to drop back down again.

In actual fact, so much positive potassium leaves that it goes below the resting membrane potential of negative 70. and goes to negative 90. Why does it go down this far? That's called hyperpolarization, and it means that you can't stimulate this neuron to send another signal too quickly. In epilepsy, you don't hyperpolarize.

It doesn't go that far down, which means you can trigger the neuron to fire, fire, fire, fire, fire, and that's the misfire, and the abnormal overexcitement of a neuron. So, couple of phases when we look at an action potential. The first phase is the sodium coming into the cell, Sodium coming in and that next phase is potassium going out. When sodium goes in, that's called depolarization.

Now remember, at rest, negative inside, positive outside, that's called polarized. Polarized means differing, okay? Differing on one side to another. So that's a polarized membrane. But when you throw the sodium in, you make it positive again inside the neuron and that's depolarization.

So when sodium comes in... We term that depolarization. Then it hits positive 30 and the sodium channels shut. Then potassium channels open and potassium leaves the cell making it negative again inside the cell.

That's called repolarization. So when potassium goes out, that's called repolarization. And so much potassium goes out that it hyperpolarizes the cell.

Now, you're probably thinking, now we've got... all this sodium in, all this potassium out, that's the opposite of what it was in the beginning. How do we reset the neuron? Well, remember embedded in the membranes of all these cells, you've got a pump that throws three sodium out and it throws two potassium in and it resets the membrane. This is an action potential.

Transcript for:Understanding Action Potential in Neurons

Transcript for:
Understanding Action Potential in Neurons