 I love this, so take another deep breath because we're going to dive into what is a membrane potential. I'm going to orient you. Don't forget, we're talking about what's happening in a neuron. And a membrane potential is a difference in charge between the extracellular fluid and the intracellular fluid. And we just looked at a molecule that's going to help us establish a difference between the intracellular fluid and the extracellular fluid. And there are a couple of things that we need to know. Notice that I'm just doing a snapshot of a cell membrane, of a neural cell membrane. I'm just blowing up like just this part of it. So remember, we're going to have sodium potassium pumps embedded in this thing. There's going to be some other stuff embedded in here that we're going to look at in the next step. But first of all, we have to know, dude, what the heck is a membrane potential and what does it actually mean? Well, no matter what, you know that the cell membrane, it's a semipermeable structure that surrounds cell contents and separates the inside from the outside. And we already know that what's inside the cell isn't going to come out. This is all the stuff in the soup of the cell. It can't pass through the cell membrane, so it can't get out. And because of that, we can hold things in and set up a certain environment, and we do. Here are the facts. There is a high concentration of potassium inside the cell. Here is a high concentration of sodium outside the cell. Now, think about the sodium potassium pump. Three sodiums out, two potassiums in. Can you see how the sodium potassium pump is going to maintain this reality? There also is a high concentration of protein molecules inside the cell, and these things are negative. Most proteins are negatively charged. And if you think about it, in the extracellular fluid, sure, there's going to be some proteins hanging out out there. But the proteins are the machines that are making stuff happen. So, of course, inside the cell, you're going to have a ton of them because your cell is this little crazy factory where stuff is happening all the time and you need proteins to make all that action happen. So, it makes sense that we would have a high concentration of proteins inside. You do have a higher concentration of chloride ions outside, and you also have a higher concentration of calcium ions outside. So, these are just by the nature of intracellular fluid as opposed to extracellular fluid. So, you have different environments, and these are some characteristics that are going to be significant when we're talking about what happens in establishing this membrane potential. Okay, the sodium-potassium pump is at work. The sodium-potassium pump is pumping potassium in and sodium out, so it's definitely maintaining that part. When it does its job and if it's working, then here's the deal, are you ready for this? Inside, there is a net charge of negative 70 millivolts when compared to the outside. So, outside, there's a charge of zero millivolts. That's the action. I mean, that's the membrane potential. Do you agree that they're, what is a millivolt? A volt is just like a measure of potential electricity. And outside, when we calculate membrane potential, we basically set the outside to zero, and then we compare the outside to the inside, and we just test the difference. So, we say, okay, outside, we're just going to pretend like it's zero. It's not even close, like I have no idea what it actually is, but it's not even close to zero. But outside, it's more positive than inside. Inside, it's negative 70 millivolts, more negative. Why? Because you've got all these proteins in there. And you've got a lot of potassium, but you've got rid of a lot more sodium. Remember, we pumped three sodiums out and only two potassiums in, so we're pumping out more positives than we're leaving in, which also contributes to a difference in charge between our two environments. I want you to be able to visualize this whole idea of setting the outside to zero and measuring the difference between them. So, here's a little Joe cell, and you can tell that I have this electrode stuck inside the cell. I did that. And then I've got another electrode outside in the extracellular fluid, and this little voltmeter can tell you the difference in charge between them. And the voltmeter knows that it sets this one, the ground, or reference electrode, to zero. And then it says, okay, what's the difference between them? Well, inside the cell, it's much more negative. Can you visualize that? Okay, let's go back to my little picture. If the inside is super negative, and if the outside is a little more positive, can you speculate? How could you get moving charged particles? Go ahead, you speculate on that, and then I'll come back and tell you how the cell actually does it.