Subtitles section Play video Print subtitles In the last video, I showed you what a neuron looked like and we talked about the different parts of a neuron, and I gave you the general idea what a neuron does. It gets stimulated at the dendrites-- and the stimulation we'll talk about in future videos on what exactly that means-- and that that impulse, that information, that signal gets added up. If there's multiple stimulation points on various dendrites, it gets added up and if it meets some threshold level, it's going to create this action potential or signal that travels across the axon and maybe stimulates other neurons or muscles because these terminal points of the axons might be connected to dendrites of other neurons or to muscle cells or who knows what. But what I want to do in this video is kind of lay the building blocks for exactly what this signal is or how does a neuron actually transmit this information across the axon-- or really, how does it go from the dendrite all the way to the axon? Before I actually even talk about that, we need to kind of lay the ground rules-- or a ground understanding of the actual voltage potential across the membrane of a neuron. And, actually, all cells have some voltage potential difference, but it's especially relevant when we talk about a neuron and its ability to send signals. Let's zoom in on a neuron's cell. I could zoom in on any point on this cell that's not covered by a myelin sheath. I'm going to zoom in on its membrane. So let's say that this is the membrane of the neuron, just like that. That's the membrane. This is outside the neuron or the cell. And then this is inside the neuron or the cell. Now, you have sodium and potassium ions floating around. I'm going to draw sodium like this. Sodium's going to be a circle. So that's sodium and their positively charged ions have a plus one charge and then potassium, I'll draw them as little triangles. So let's say that's potassium-- symbol for potassium is K. It's also positively charged. And you have them just lying around. Let's say we start off both inside and outside of the cell. They're all positively charged. Sodium inside, some sodium outside. Now it turns out that cells have more positive charge outside of their membranes than inside of their membranes. So there's actually a potential difference that if the membrane wasn't there, negative charges would want to escape or positive charges or positive ions would want to get in. The outside ends up being more positive, and we're going to talk about why. So this is an electrical potential gradient, right? If this is less positive than that-- if I have a positive charge here, it's going to want to go to the less positive side. It's going to want to go away from the other positive charges. It's repelled by the other positive charges. Likewise, if I had a negative charge here, it'd want to go the other side-- or a positive charge, I guess, would be happier being here than over here. But the question is, how does that happen? Because left to their own devices, the charges would disperse so you wouldn't have this potential gradient. Somehow we have to put energy into the system in order to produce this state where we have more positive on the charge of the outside than we do on the inside. And that's done by sodium potassium pumps. I'm going to draw then a certain way. This is obviously not how the protein actually looks, but it'll give you a sense of how it actually pumps things out. I'll draw that side of the protein. Maybe it looks like this and you'll have a sense of why I drew it like this. So that side of the protein or the enzyme-- and then the other side, I'll draw it like this. It looks something like this, and of course the real protein doesn't look like this. You've seen me show you what proteins really look like. They look like big clusters of things, hugely complex. Different parts of the proteins can bond to different things and when things bond to proteins, they change shape. But I'm doing a very simple diagram here and what I want to show you is, this is our sodium potassium pump in its inactivated state. And what happens in this situation is that we have these nice places where our sodium can bind to. So in this situation, sodium can bind to these locations on our enzyme or on our protein. And if we just had the sodiums bind and we didn't have any energy going into the system, nothing would happen. It would just stay in this situation. The actual protein might look like something crazy. The actual protein might be this big cloud of protein and then your sodiums bond there, there, and there. Maybe it's inside the protein somehow, but still, nothing's going to happen just when the sodium bonds on this side of the protein. In order for it to do anything, in order for it to pump anything out, it uses the energy from ATP. So we had all those videos on respiration and I told you that ATP was the currency of energy in the cell-- well, this is something useful for ATP to do. ATP-- that's adenosine triphosphate-- it might go to some other part of our enzyme, but in this diagram maybe it goes to this part of the enzyme. And this enzyme, it's a type of ATPase. When I say ATPase, it breaks off a phosphate from the ATP-- and that's just by virtue of its shape. It's able to plunk it off. When it plunks off the phosphate, it changes shape. So step one, we have sodium ions-- and actually, let's keep count of them. We have three sodium-- these are the actual ratios-- three sodium ions from inside the cell or the neuron. They bond to pump, which is really a protein that crosses our membrane. Now, step two, we have also ATP. ATP gets broken into ATP plus phosphate on the actual protein and that changes the shape. So that also provides energy to change pump's shape. Now this is when the pump was before. Now after, our pump might look something like this. Let me clear out some space right here. I'll draw the after pump right there. And so this is before. After the phosphate gets split off of the ATP, it might look something like this. Instead of being in that configuration, it opens in the other direction. So now it might look something like this. And of course it's carrying these phosphate groups. They have a positive charge. It's open like this. This side now looks like this. So now the phosphates are released to the outside. So they've been pumped to the outside. Remember, this is required energy because it's going against the natural gradient. You're taking positive charge and you're pushing them to an environment that is even more positive and you're also taking it to an environment where there's already a lot of sodium, and you're putting more sodium there. So you're going against the charge gradient and you're going against the sodium gradient. But now-- I guess we call it step three-- the sodium gets released outside the cell. And when this changes shape, it's not so good at bonding with the sodium anymore. So maybe these can become a little bit different too, so that the sodium can't even bond in this configuration now that the protein has changed shape due to the ATP. So step three, the three Na plusses, sodium ions-- are released outside. Now once it's in this configuration, we have all these positive ions out here. These positive ions want to get really as far away from each other as possible. They'd actually probably be attracted to the cell itself because the cell is less positive on the inside. So these positive ions-- and in particular, the potassium-- can bond this side of the protein when it's in this-- I guess we could call it this activated configuration.