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.
The difference in the charge across a cell membrane which has the potential for an action potential to occur.