[0:14]What's up Ninja Nerds? In this video today we're going to talk about resting membrane potentials, graded potentials, and action potentials of neurons. Guys, before you guys watch this video, please hit that like button, hit that subscribe button, comment down in the comment section and all of the information for all our social media platforms, Instagram, Facebook, Patreon, all of that will be listed down in the description box. Go check that out. All right, let's go ahead and get started.
[0:36]All right, Ninja. So what we have to do when we're talking about all of these membrane potentials within a neuron is we have to zoom in on a neuron. Really talk about all that cellular processing and ion movement that is occurring here.
[0:48]So first thing we have to talk about since we're talking about all these potentials of a neuron, we have to start with resting membrane potential. Now, first thing we have to do is come up with just a just a basic definition of resting membrane potential. So how would you describe resting membrane potential? What is it?
[1:07]Well, resting membrane potential is the voltage difference across this cell membrane when the cell is at rest. That's all it is.
[1:14]So it's the voltage difference across cell at rest.
[1:27]And the next thing that you have to remember is is yes, we're talking about this resting membrane potential existing in neurons, but resting membrane potentials can exist in every single cell.
[1:37]So it exists in all cells. That's very important to remember.
[1:43]We're just referring to it in this case in the neurons, okay?
[1:46]The next thing that you have to know is what is this actual voltage? If we could put a value, a number on this voltage difference across the cell membrane at rest in a neuron, what would it be?
[2:00]And it's actually a range. So I'm going to abbreviate resting membrane potential. Generally, this is a range. Now, most textbooks like Meria say it's around negative 70 millivolts. That's kind of like the average.
[2:10]Other textbooks will give it a little bit further from that. The best way to kind of just cover all grounds is to say, generally, it could be somewhere between negative 70 millivolts to negative 90 millivolts with most textbooks supporting negative 70 millivolts is that kind of average number.
[2:29]Okay. So that's kind of what we know about resting membrane potentials. Now, what we need to do is I want you to understand what we're looking here because we're zooming in on a neuron.
[2:37]So what I'm actually doing here, is I'm taking a neuron, right? We're looking at a neuron like this. Here is going to be your axon, your cell body, and then here we're going to have the axon terminal. What I'm doing is, is I'm zooming in on this portion of the cell membrane, and we're looking at that, okay? That's what we're doing here.
[3:00]So we're really zooming in on the cell membrane of this neuron and looking at the activity. So you have to have one question here. How in the heck does your resting membrane potential get to negative 70 to negative 90 millivolts? How do we get it there? There's three ways that we get it there.
[3:14]One of the ways that we get it to that voltage, that negative 70 to negative 90, is sodium potassium ATPAs. These sodium potassium ATPAs, what they do is, they're so interesting and they're so intelligent, and they pump three sodium ions out of the cell. So three cations or positive ions out of the cell.
[3:34]And then they pump two potassium ions or two cations into the cell. Now, take a look at this. Let's pretend for a second, we're starting at a particular voltage. Let's say we're starting at 0 millivolts. That's our imaginary start point of how we're going to get to negative 70 millivolts. So we're starting at 0 millivolts.
[3:56]Now, when these sodium potassium ATPAs are working, they're pumping three positive ions out and only bringing two positive ions in. Because of that, that makes the inside of the cell just a little bit more negative. Not significant, just a tanty bit negative. Maybe it only takes it from 0 millivolts to negative 5 millivolts. So not a big change, but that's obviously due to the sodium potassium ATPAs. So these are going to be one of the reasons, okay?
[4:26]So these are going to be one of the reasons, okay? So what are these called here? These are called your sodium, potassium, ATPAs. Now, that's one function. One of the functions of the sodium potassium ATPAs is to help to make the inside of the cell just slightly negative.
[4:45]The second reason that these are so important is that they establish the concentration gradient for sodium and potassium. Now, what is it doing to the sodium, these pumps? It's pushing sodium out.
[4:58]So what is that doing? That's increasing the concentration of sodium outside the cell. And by contrast, the sodium concentration inside of the cell will be lower, okay?
[5:11]Now, it's also concentrating potassium into the cell. It's pushing lots of potassium in. So what that's going to do is that's going to increase the potassium concentration inside the cell and in contrast, there'll be less potassium outside the cell. So there's two functions of these sodium potassium ATPAs.
[5:28]One is they generate a small negative charge inside of the cell at rest. The second reason, the second thing that they do is they generate concentration gradient for these ions to move and that's going to be important for the next two things that contribute to the resting membrane potential.
[5:46]Okay, beautiful. Now, the second thing that contributes to the resting membrane potential are over here, these blue channels, okay?
[5:54]And that leads us to our next discussion. There's going to be lots of different channels within a neuron that contributes to all these different potentials, resting, graded, action. When we're talking about resting membrane potential, these blue channels here are very special types of channels. They are called leaky potassium channels.
[6:21]And what that means is, is that there're these little proteins embedded in the cell membrane and they're always open. And they allow for ions like potassium in this case to move in and out of the cell freely and keyword here passively, okay?
[6:32]Now, these potassium channels are super leaky. So it's going to allow for ions like potassium to move. Which direction would the potassium want to move? Well, remember, what did we just say over here with the concentration gradients? Potassium is higher inside of the cell because of these sodium potassium ATPAs.
[6:49]So if potassium is higher outside the cell and it's lower outside the cell, where is it going to want to go? It's going to want to leave this cell and exit. So all of this potassium is going to start leaving. So let's have this showing here that the potassium is leaving the cell. And how is it leaving? It's leaving moving what? Down its concentration gradient, from high concentration to low concentration. From the intracellular fluid to the extracellular fluid.
[7:22]Okay, beautiful. Now, as these positive ions, these potassium ions are leaving, what's happening to the inside of the cell? Great question, guys. Potassium is actually bound normally inside of the cell. Potassium is bound to an anion.
[7:38]You see this? I'm going to represent it as A, okay? A is just your general, your non-specific anion, negatively charged ion. What are these anions and why am I mentioning it? These anions can be of two types.
[7:59]One is they can be phosphates. You know, phosphates are just really negatively charged ions, very difficult for them to move outside of the cell because of that charge. The other thing, oh, let me get my marker here. The other thing here is going to be proteins. Proteins. You know, proteins are made up of amino acids, right? Tons of amino acids.
[8:22]And these amino acids have lots of negative charges on them. Well, that's another reason they can't exit the cell. But what did I say was the common thing with these? This has negative charges, this has negative charges. That's what makes it an ion. They love to interact with potassium, which is a cation.
[8:40]So whenever potassium leaves, you would think, oh, the anions also going to leave. No, it's too big and too charged to leave the cell. So because of that, whenever potassium leaves, it leaves behind an unoccupied anion. And now every time the potassium leaves, it leaves behind an unoccupied anion and makes the inside of the cell more and more and more negative. How much negative? What voltage?
[9:11]You're not going to believe this, but if potassium could, it would move outside of the cell until you got that voltage somewhere around, let's say negative 90 millivolts.
[9:22]To negative 90 millivolts.
[9:31]That was because of these leaky potassium channels. Now, the last one that contributes, so we had the sodium potassium ATPAs, the leaky potassium channels. The last one that is going to contribute here to this resting membrane potential is your leaky sodium channels. Now, these are leaky. Again, they allow for sodium to move in or out of the cell. But again, where is the concentration gradient of sodium?
[10:24]We already said, it's higher outside the cell. So if the concentration of sodium is higher outside the cell, in contrast, it's lower inside the cell. So where will the sodium want to move? The sodium will then want to move into the cell down its concentration gradient. As sodium moves into the cell down its concentration gradient, it makes the inside of the cell positive.
[10:49]But here is the big thing, I can't stress this enough. This cell, in this case, a neuron, is so many more times permeable to potassium than it is to sodium. It'll allow for tons and tons of potassium to leak out of the cell, but only allow a little bit of sodium to come into the cell.
[11:11]So because of that, we have to, let's actually write this down. that potassium, when we talk about permeability, we'll kind of put like a little heading here. Permeability factors, right? When we're talking about that with the cell. Potassium is significantly more permeable. This cell is way more permeable to potassium than it is to sodium.
[11:32]So potassium will make a big change like go from negative 5 to negative 90, but the sodium, not much of it moves in. So because of that, it might not make a significant as a change. Maybe it only takes it from negative 90 to negative 70 millivolts. And we've reached our resting membrane potential.
[11:54]So, to recap, what are the three components that are actually helping us with this? Sodium potassium ATPAs, the leaky potassium channels, the leaky sodium channels. And if you really wanted to add other ones in, leaky calcium and leaky chloride channels, but the same concept applies.
[12:10]Last thing that we have to talk about this, because this does come up on your exams a lot, is learning how to calculate what's called the Nernst potential for sodium, potassium, calcium, chloride. We're literally just going to go through the equation quickly.
[12:24]All right, Ninja. So when we talk about Nernst potential, really I only want you to know the equation and then really it's a plug and chug thing from here. There really isn't much to this. It's more important for you to know when you use the Nernst potential and and what that Nernst potential like formula is, okay?
[12:40]So Nernst potential. The first thing that you need to know is when do you use it? So when do you use it? And that's a very important question. Let's take for example, potassium in this case, okay?
[12:51]Potassium, we know, is moving out of the cell down its concentration gradient. But as it moves outside of the cell, the inside of the cell becomes more negative, right? That negative charge inside the cell wants to pull some of the potassium back into the cell. That's called the electrostatic gradient. So potassium moves out of the cell down its concentration gradient, but kind of gets pulled back into the cell down its electrostatic gradient. The point in time in which potassium is moving equally or it's there's kind of like no net movement of potassium moving out of the cell down its concentration gradient or moving into the cell down its electrostatic gradient. Whenever those two are equal, that movement, you've reached Nernst potential. So we kind of write it like this, whenever the potassium is moving out of the cell, equals the potassium moving into the cell. And moving out is via its concentration gradient, right? It's concentration gradient, and moving into the cell is via its electrostatic gradient. Then we can use the equation. Well, then the next question is, what is the equation? That equation is like this.
[14:00]We're going to write it for potassium. E for voltage, okay? The equilibrium or the voltage that potassium is able to generate across that cell membrane at rest is equal to 61.5, which is a constant, divided by Z, which is just basically the charge of the ion. In this case, what's the charge of potassium? Plus one. What's 61.5 divided by plus one? 61.5. So we can just get rid of that. Multiplied by log base 10, the concentration of potassium ions outside of the cell. And again, this is a value that you would get from a table or a textbook. And we're going to put down here 5 is the potassium concentration outside the cell. And over the potassium concentration inside the cell. And again, this could be 150, right? If you calculate all of this out, that'll come somewhere around negative 90 millivolts. So that tells you that potassium will move outside of the cell until, it'll move outside of the cell down its concentration gradient until the inside of the cell becomes negative 90 millivolts and then that movement down its electrostatic gradient keeps it in that kind of equilibrium point. Now, the same concept goes for sodium. If I wanted to calculate for sodium, I say equilibrium potential of sodium is equal to 61.5 divided by Z. It's a plus one, so I don't need to. times log base 10, and then again, I'd have to kind of pull this number from a table. And generally, that's like 140 for for the sodium concentration outside of the cell. And then about 10 for the sodium concentration inside the cell. And then again, if you calculate all of this, you're going to get your equilibrium potential of sodium is somewhere around positive 70 millivolts. Now, if you added both of these up, negative 90 and positive 70, you're basically saying that your resting membrane potential is the equilibrium potential of potassium and the equilibrium potential of sodium. And if you did that, what would you get? Positive, you get negative 20 millivolts. Negative 90 plus 70. But remember, what did I say? It's a permeability thing. So potassium, there's going to be so much more potassium moving out of the cell.
[16:29]So the cell's voltage will actually be closer to this equilibrium potential of potassium. So whenever you actually calculate this out, if you were to take a percentage and say, well, let's say that this cell is 90% permeable to potassium, and only 10% permeable to sodium. If you calculated all of this out, and then added them together, you would probably get somewhere approximately around negative 70 millivolts.
[16:59]And that is kind of how we really get down into the nitty gritty of how to calculate out these voltages. All right, ninjas. That covers everything that you'll need to know about action potentials and all these other things that we talked about.
[17:03]All right, Ninja. In this video, we talk about resting membrane potentials, graded potentials, and action potentials. I hope you guys liked this video and I hope it helps. All right, Ninja. You guys know what to do. As always, until next time.
