Hi in this video we're gonna be talking about ion channels and membrane potentials. So first let's focus on ion channels. What are ion channels? They are proteins that form trans membrane pores which allow for passive. So keep in mind this is passive transport of small polar molecules. So ion channels are gated. So what does that mean? Well that means that they are not continuously open so they can close and open and preventing or allowing transport of molecules across the membrane. Now there are different types of gates that an ion channel can have. One is called a voltage gated ion channel and that is when the channel opens depending on the charge that exists across the membrane. There's a ligand gated which means that the channel opens in response to binding of a ligand. And then there's mechanically gated and these channels opens in response to a mechanical force. And then this is kind of the hardest one to imagine. But for instance there are ion channels that respond to the mechanical vibrations made from sound in your ear. And so those channels can open based on those sound um sort of mechanics vibrating. Um and opening those channels I think it's a good example of that one. So um ion channels are very specific and permeable usually only to a single ion. And so how they do this is they contain a filter called the selectivity filter which is inside this narrow poor and um ions have to be able to pass this filter and only ions that can you pass this filter will be passed through the channel. So how they do that is usually ions because of their charge are associated with water um usually through weak interactions with water. And so in order to move across the selectivity filter, ions must disassociate from water. Well the selectivity filter is set up in a way that it will selectively disassociate the ions from water of the ion that's supposed to pass, but not of ions that aren't supposed to pass. So only the targeted ion will be able to disassociate from water and cross through the ion channel. And so ion channels just to review move molecules in response to charges across the membrane. So here we have this here's an example of a voltage gated channel where you can see that there's all these different molecules on this side and not a lot on this side. So then it opens and passes the molecules through. But if it closes then um the molecules can't get through. And then you have your ligand gated channel which opens in response to a ligand binding. And that trance for transfers molecules across the membrane. Started with that let's now move on

Okay, so now we're gonna talk about membrane potential and membrane potential. That term just refers to the difference in the environment on either side of the cell. So the inside of the cell, intracellular or outside of the cell. Now there's gonna be lots of differences between the inside and the extra cellular environments. But we're only focused on membrane potential, only focuses on a particular type of difference. And that difference is going to be differences in concentration of certain compounds or molecules. Um and the charge of those compounds and molecules. So membrane potential is just going to refer to concentration and charge differences on the intracellular and the extra cellular sides of the cell. Now, with membrane potential, there's a really kind of buzzword here that you need to know and that's resting membrane potential. Now, when you hear rest resting, you kind of think that okay, it's not moving and that is not what resting membrane potential means. What resting membrane potential means is when the amount of positive ions that are coming in and out of this l is pretty much equal to the amount of negative ions coming in and out of the cell. What This doesn't mean? It doesn't mean that the positive and negative charges are equal on either side. It doesn't mean that nothing is moving across the membrane. It just means that the negative and positive charges that are moving across the membrane are equal. So a cell or a membrane can have resting member potential, even if the extra cellular environment is more positive than the negative and then the intracellular environment which is more negative. But there just have to be moving positive and negative ions across the membrane at the same rate. So that's resting membrane potential. So resting membrane potential doesn't mean nothing's moving, it just means it's moving at the same rate, the charge is balanced. Okay. Um so the way that we actually measure this is through a equation called the nurse equation. Now I'm not going to talk to you about how to calculate the nurse equation. That's much more chemistry and much less cell biology, but I do want you to know what this has caused because when we start talking more about individual neurons which we will do then you're going to see these negative and positive numbers that are calculated using this nurse equation. And I just want you to know what the equation is called. The nurse equation is going to calculate the difference in concentration and charge across the membrane. Now there's some important pumps and transporters and things you need to know about that often facilitate this rustic membrane potential that includes the sodium potassium pump which is a transporter. This is going to create a gradient or concentration gradient, meaning that on one side of the membrane there's gonna be more sodium and on the other side of the membrane there's gonna be more potassium and because that's a difference in concentration, it's going to be measured by the membrane potential. So the sodium potassium pump is going to make that large concentration gradient across the membrane. And you also have potassium leak channels, which we're going to talk about when we talk about neurons more and these channels are always open and they allow potassium to flow to restore the membrane potential either back to resting or back to wherever it's supposed to be, which depends on the cell type and what we're talking about. So, those are two important um pumps or transporters leak channels that you need to know about. So, here's an example of what this looks like. So um here is a cell membrane and we can see if we look on either side of the cell membrane, there's different concentrations which are shown down here. There's going to be more sodium on this side? More potassium on this side. More chlorine on this side, but there's also charge differences here. We can see that this side is more positively charged and blue. This side is more nega charged in orange. And so these um concentration and charge differences are what is measured by the membrane potential and pumps and transporters and channels can open and close or allow for the movement of various molecules across the membrane in order to get to resting membrane potential or disrupt resting membrane potential. So how do scientists know all of this information? Right. Like we can't really visualize this easily. So how do we know about it? Well, there's a technique called the patch clamp technique. Now the patch clamp technique can actually measure a single ion channel. So when we talk about potassium leak channels, what we think of as we think of, oh there's hundreds of these in a cell. But the patch clamp technique can actually just focused down on one and watch the flow of ions in and out of this one channel. So how does it do this? Well, it takes a micro pipette which is just extremely tiny pipette usually made out of glass or some similar material and it's put down straight onto a membrane with a single ion channel in it. So here we see our pipette, we can see that it is isolated this one ion channel here. So as stuff either leaves or enters into the neuron, which is what we're looking at. This micro pipette is going to send signals to a computer and it's going to monitor how many ions or molecules or whatever this is passing, how many of them are going to come in the cell and how many of them are going to be leaving the cell. So it's just analyzing that flow of ions in and out of that channel. And by looking at individual ions channels then we can really start to get a picture of what all of these individual proteins do during various activities of a cell or a neuron. Okay, so with that let's turn the page