Source: Video and Images Created by Amanda Soderlind
Welcome to this lesson today entitled membrane potential. In this lesson, we will be describing how the differences in concentration of ions across the plasma membrane can set the stage for a nerve impulse. Sodium and potassium are the two ions that are very important for setting the stage for a nerve impulse. Sodium and potassium ions, or the differences in the concentration of these ions across the membrane, produce an electrical charge. And the difference in the electrical charges across the membrane of a neuron are what set the stage for a nerve impulse or an action potential to actually occur.
So we are going to be taking a look at this diagram right here, and this information here, to describe how the stage is set for an action potential. So our diagram here is just illustrating a small part of the plasma membrane of a neuron. If we take a look, we have a couple different things going in here. We have these little proteins right here which are our sodium channels. We have something here called a sodium potassium pump. And if we take a look on this side of the membrane, we're going to pretend it's the outside of the neuron, and we're going to pretend this side of the membrane is the inside of the neuron. Now you'll notice that the inside of the neuron is more negative relative to the outside of the neuron. And that has to do with the concentrations of sodium and potassium ions across the membrane. So on the outside of the membrane, we have a higher concentration of sodium ions. Whereas inside, we have a higher concentration of potassium ions. And what this does, this difference in concentration of ions across the membrane, sets up a voltage difference.
In a resting membrane, that voltage difference is about negative 70 millivolts. So the inside is more negative relative to the outside as I had mentioned. Now normally these gated sodium channels are closed in a resting membrane. And when I'm speaking of a resting membrane, I'm talking about a neuron that is not stimulated or not experiencing any activity. This is what the membrane will look like in a resting neuron for example. The sodium channels are generally close in a resting neuron. So what that means is that sodium is not allowed to pass through this membrane easily because the sodium channels are generally closed. Therefore the membrane is usually more permeable to potassium. So we'll have a little bit of potassium leaking out; sometimes we'll have a little bit of sodium leaking in other channels that are sometimes open. But generally we have a higher concentration of sodium out here and a higher concentration of potassium in here. We have this concentration gradient that has developed. That concentration gradient meaning higher concentration sodium here, higher concentration potassium here.
When a voltage or when some sort of disturbance occurs, what happens is it causes the sodium channels to open. And because we have this concentration gradient of sodium out here, once those channels open, sodium is going to flow through those channels. It's going to flow from an area of high concentration to an area of low concentration and then sodium will leak out as well. So those concentration gradients build up setting the stage for this nerve impulse, so that when there's a disturbance and those gates open, they're going to flow with their concentration gradient. So sodium will flow in and potassium will flow out reversing the voltage across the membrane. Therefore, allowing an action potential to occur. So the difference in charge across the cell membrane is called the resting membrane potential. And it has potential for the launch of a nerve impulse. This lesson has been an overview on membrane potential and what happens in order to set the stage for a nerve impulse.
Source: Video and Images Created by Amanda Soderlind
In this lesson today, we will be discussing nerve impulses and action potentials. So an action potential is basically just another way of saying a nerve impulse. And an action potential occurs when a stimulus causes the voltage difference across the cell membrane to shift. And this voltage difference is caused by concentrations of potassium and sodium ions.
So potassium and sodium are very important in setting the stage for an action potential. And the threshold is the minimum shift needed for an action potential to occur. So in order for this action potential to occur, the voltage difference across the cell membrane has to shift by a certain amount. And once it's shifted by that amount, an action potential can happen.
So we're going to take a look at these steps right here that set the stage for an action potential. They're the steps that occur in an action potential. So first an electrical disturbance has to occur. So signals will reach the input zone of a neuron, and those signals will change what occurs in the cell membrane of that neuron.
So this electrical disturbance will occur, causing sodium gates in the membrane of the cell to open. And what this does is allow sodium to rush into the cell. Now, normally, in a resting membrane, the outside of the cell is positive relative to the inside of the cell. So as these sodium gates open, sodium will rush in, causing more gates to open until that threshold, we discussed, is reached. And then the voltage difference across the cell membrane is reversed.
Now in order for another action potential to occur, we have to restore our resting membrane potential. And this is done so by sodium-potassium pumps. So in just a few minutes, we're going to take a look at a diagram that explains these steps right here. But before we do that, I want to discuss the structure of a neuron here.
OK, so a neuron is a nerve cell. And it's made up of dendrites, which are part of the input zone. So information will move through the input zone, through the dendrites, to the cell body. And you'll notice, we have the nucleus in here as well. From there, the signal will travel along this long, narrow part of the neuron called the axon, and then down to the axon endings.
So information goes through the input zone, through the cell body, along the axon to the axon endings. And then from there, that signal will be sent either to another neuron or to a muscle or a gland cell.
So let's take a look at this diagram here. I'm going to zoom out just a little bit. So we're going to take a look at what happens in an action potential. So this here is going to be our resting membrane. As I had mentioned, the outside of the neuron is generally positive relative to the inside.
And we have our cell membrane right here. And then embedded within that, we have something called a sodium-potassium pump. And then we have our gated sodium channels. And normally these gated sodium channels are closed, making the cell membrane, more or less, impermeable to sodium. So sodium is not allowed to just flow through freely.
But when a disturbance happens, and it causes an action potential, sodium gates will open. So the sodium gates will open, allowing sodium to flow into the cell. And then as that happens, more and more of these gates will open, allowing more and more sodium to flow in. And this will continue to happen, as I mentioned earlier, until the threshold is reached and the voltage difference is reversed.
Now this will actually happen in patches across the membrane. So this isn't all happening throughout the whole membrane at the same time. It occurs in patches of the membrane.
So as sodium moves into the cell in one patch, the previous patch of that membrane will allow potassium to leak out. So this is happening, reversing that voltage difference across the cell membrane. And this will cause the impulse to propagate along that cell membrane in patches.
Now, eventually, that resting membrane potential has to be restored. So that's where our sodium-potassium pumps, in pink here, come into play. So our sodium-potassium pumps will restore that resting membrane potential. But in order to do that, it has to use the cell's ATP. So it's a form of active transport because it's using ATP in order to restore this membrane potential.
So what the sodium-potassium pump will do is it will pump sodium back out and potassium back in. So as that action potential was happening, we had our sodium leaking in, potassium leaking out, which was reversing our voltage difference across the membrane. But in order to restore that resting membrane potential, we have to get the sodium back out and the potassium back in. And then our resting membrane potential can be restored.
So this lesson has been an overview on nerve impulses and action potentials.
