Alright. So this video is about resting membrane potential. So as you guys may recall from earlier in the chapter, membrane potential is just the voltage created from charge separation between the interior and exterior of the cell. So when I say charge separation, I literally just mean that the inside and outside of the cell have different charges. So, for example, one could be positive and one could be negative, and that difference in charge creates electrical potential. So that's kind of what we're dealing with here. Now, resting potential is the membrane potential of a cell that is not being stimulated. The neuron is literally resting. It's just not doing anything. And typical resting potential in the CNS is at approximately negative 70 millivolts. So there is a range here. It can be from between negative 40 to negative 90 millivolts depending on the type of neuron and where it is in the human body. So do keep that in mind. But for most neurons in the CNS, negative 70 is a pretty normal resting potential. So we're going to use that in all of our examples going forward, just so you know. So another way to say all of this just kind of written out in words is that when a cell is at rest, the interior of the cell is more negative than the exterior of the cell. Alright. So that's what resting potential is. Now how do we get there? What creates it? Right? So resting potential is really created by 2 factors, and that's these factors right here. So the first factor is just these differences in ionic composition of the intracellular and extracellular fluid. Now that sounds a little fancy, but you guys actually already know this. So remember, when a cell is at rest, what we see is that there's more sodium outside the cell and more potassium inside the cell. Right? And so those concentrations of sodium and potassium are a really big factor in creating resting potential. The other major factor here are the differences in plasma membrane permeability. To those ions. So the differences in the permeability to potassium and sodium. So as you guys may recall from when we first introduced ion channels, neurons have a lot more leak channels for potassium than for sodium. So if you look in our figure here, we have 2 potassium leak channels here in pink, and we have 1 sodium leak channel. Just to give you kind of a visual cue that there are more potassium leak channels than sodium leak channels. Now what this means is that our neuron is more permeable to potassium than to sodium. Alright? And so these two factors combined mean that we have a lot of sodium outside the cell, so its concentration gradient wants it to go in the cell. Right? And we have a lot of potassium inside the cell, which means its concentration gradient is going to direct it out of the cell. Now because the cell is so permeable to potassium, a lot of potassium can leak out of the cell. And because we have so many positive potassium ions leaking out that creates this negative cell interior. You'll notice I talked about potassium a lot there, a little more than than sodium. That's because potassium ions are kind of like the MVP of resting potential. Sodium is playing its part. It's doing its job, but potassium is really, like, the big factor here. Alright? So that is our resting potential. It has been created. And now that we have it, it's going to be stabilized by our trusty sodium-potassium pump, which is going to maintain those concentration gradients for sodium and potassium. And it does that, remember, by constantly ejecting 3 sodium ions Na+, which means that it helps maintain that concentration where we have more sodium outside the cell and it's pulling back 2 potassium ions K+ to help maintain that concentration of potassium inside the cell. Alright. So that is resting potential in a nutshell. What I just covered so far is what most standard a and p courses will want you to know about resting potential. So if you feel like you have a good handle on this, please feel free to pause the video right now. If you're feeling a little iffy or if you have some questions, stick around for a few minutes because I'm going to go over some common questions that students have about resting potential.
Alright. So the first very common question that I get is that if there are so many positively charged potassium ions inside the cell, why is the cell negative? Right? Which is a very intuitive and fantastic question. So it's important to keep in mind that even though we are focusing on the role of sodium and potassium here, and that's what most a and p textbooks probably do as well, there are other things happening here. There are other types of ions. Some of them are negatively charged and very importantly, there are lots of negatively charged proteins inside that cytosol. So, potassium and sodium play a really big role in creating resting potential, but they are not the only factors that contribute to that negative cell interior. There's some other things going on as well. Alright? But that's a really great question. So another question that I get fairly often is that, you know, why are we letting so much potassium leak out of the cell? Why do we have so many leak channels? If our poor pump is working so hard to bring it back in all the time, why are we letting so much of it leak out in the first place? That's also a fantastic question. So like we talked about, potassium leak channels play a really big role in creating resting potential, but they do play other roles as well. So recent research has implicated them in things like pain signaling, sleep duration, and potentially even movement. So, those leak channels do have other functions aside from just creating resting potential. The final question that I get fairly often is that if we want the interior of the cell to be negative, why is our sodium-potassium pump bringing in positively charged ions? Like, isn't that kind of counterintuitive? Also a great question, but it's important to remember the role of the pump is to maintain the concentration gradient. So what is really happening here is that the pump helps maintain the concentrations of sodium and potassium, more sodium outside, more potassium inside. Right? By stabilizing the concentrations, it stabilizes the concentration gradients, thereby, stabilizing the rate of diffusion. So if our pump were to stop working, for example, that rate of diffusion would also become unstable. It wouldn't work the right way anymore. So, one potential outcome of that could be that the diffusion of potassium could become too fast or too much potassium could leak out of the cell. If that were to happen, our cell would actually become too negative. It could get down to negative 90, negative 100, and that makes it very difficult for our neuron to communicate. So we want to be maintaining that negative 70 millivolts, and that's really kind of like a sweet spot. So, I would encourage you to think of resting potential as kind of like a Goldilocks zone where, you know, we want potassium leaking out, but not too much. We want sodium leaking in, but not too much. We want all of those concentrations to stay in balance and just be just right, if you know what I mean. So, hopefully, that helped clear some things up for you. If you do have other questions, please feel free to leave them in the comments, and we'll try and get back to you as quick as we can. And I will see you guys in the next video. Bye-bye.