Resting Membrane Potential - Video Tutorials & Practice Problems

Resting Membrane Potential

All right. 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, 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. Um So we're gonna 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. All right. So that's what resting potential is. Now, how do we get there? What creates it? Right. So resting potential is really created by two 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 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 two potassium leak channels here in pink and we have one 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. All right. And so these two factors combined mean that we have a lot of sodium outside the cell. So its concentration gradient and 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. All right. 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 three sodium ions, which means that it helps maintain that concentration where we have more sodium outside the cell and it's pulling back to potassium ions to help maintain that concentration of potassium inside the cell. All right. So that is resting potential in a nutshell. Um 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 stop 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 gonna go over some common questions that students have about resting potential. All right. 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. Um 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. Um 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 site is all. So um potassium and sodium play a really big role in creating rusting potential, but they are not the only factor that, that, that contribute to that negative cell interior, there's some other things going on as well. All right. But that's a really great um question. So um 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. Um 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? Um 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 stabling the rate of the diffusion. So if our pump was to stop working, for example, that rate of diffusion would also become unstable, it wouldn't work the right way anymore. Um 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. Um 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 um 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.

Resting Membrane Potential Example 1

OK. So let's jump into an example here. So our question reads which of the following is the most accurate regarding the sodium potassium pump. All right. So option A has the sodium potassium pump creates resting potential by ejecting three potassium ions and transporting two sodium ions into the cell. And whew that's a doozy of incorrect information, right. So the sodium potassium pump does not create resting potential. No, no, it stabilizes it and we are ejecting three sodium ions. So this is wrong and we should be transporting two potassium ions. So this is also wrong. So A is out right. B reads the sodium potassium pump is the main factor in creating resting potential. Now it's that same word again, creating it does not create it, it stabilizes it. So B is out C reads, the sodium potassium pump directly impacts resting potential by allowing more negatively charged ions into the cell. Now, we know that the sodium potassium pump works with sodium and potassium, both of which are positively charged ions. So C is out. So it's probably d but let's just read it. The sodium potassium pump stabilizes there's that word resting potential in a neuron and that is absolutely correct. So, our answer is D and there you go.

Which of the following is the MOST important factor in generating resting membrane potential?

• A. Na+ concentration gradient.
• B. K+ concentration gradient.
• C. K+ electrical gradient.
• D. Na+/K+ ATPase.