Lecture 15 - Resting Membrane Potential PDF

NOTE: Transcripts are made from the auto-generated Lecture Captions, so are not edited for grammar/spelling. Lecture 15 - Resting Membrane Potential Video 1 Introduction Welcome to the next online lecture. In previous lectures, we talked about the structure of a neuron, but we haven't really tal...

NOTE: Transcripts are made from the auto-generated Lecture Captions, so are not edited for grammar/spelling. Lecture 15 - Resting Membrane Potential Video 1 Introduction Welcome to the next online lecture. In previous lectures, we talked about the structure of a neuron, but we haven't really talked about what's so special about that neuron that allows it to create an electrical signal. So that's what we're going to be focusing on today when we talk about membrane potential. So first we're going to define membrane potential, and then we'll talk about how we create resting membrane potential in our neuron. So let's take a look. Slide 1 The material for this online lecture module can be found in chapter 11 of your textbook. Specifically, we're going to be focusing on Section 11.5 called electrical signals. And in this module we're going to focus primarily on something called resting membrane potential. Now because we are talking a lot about membranes in this lecture, it's important to understand and remember or recall some of the key features of membranes of cells. So if you're unfamiliar or you need a quick review of the features of cell membranes. I highly recommend reading over Chapter three of our textbook where it talks about cell membranes and a lot of the protein transporters that are imbedded within those membranes and how they work. But just as a quick review here, in this image, it's showing us a blown up version of one portion of an axon cell membrane. So if you recall, membranes are made up of phospholipid bi-layers. So that's what's depicted in this blue region here and the important feature of that bi-layer is that things can't easily move through that bi-layer, so that we can control what actually leaves the cell and comes into the cell. And often the way that we do that is by using transport proteins. So that's what these coloured structures are representing. So this is the phospholipid bi-layer, the inside of the cell, the cytosol side, and this is the outside of the cell, the extracellular space. So here are transport proteins and they're going to be specific for specific molecules or ions that are moving through the phospholipid bi-layer. Now because we have this feature of controlling what moves in and out, we can actually control what's on the inside and outside of the cell, and that's going to be very important when we start to talk about resting membrane potential today. So again, if you just want a quick review of membranes and their features, I recommend just reading over chapter 3 of your textbook. Slide 2 So we're going to start by talking about what makes neurons so special in terms of creating an electrical impulse or an electrical signal that we can transmit from one part of the body to another part of the body. And one part of that feature of neurons that makes them what we call electrically excitable, is that we have a polarized membrane. So that means that we actually have a charge difference on one side of the membrane compared to the other side of the membrane or the membrane itself is polarized. So we'll talk about how that works today, but the electrical properties of these membranes result from a couple of different things. One, they result from ionic concentration differences across the plasma membrane. Now what does that mean? Well, we know that ions are molecules that are positively charged or negatively charged. So in the image here we have our bi-lipid layer and you can see some transport proteins that are embedded in that layer, on the outside of the cell we have a number of different ions. So we have some examples of chloride ions, they're negatively charged. We have some examples of sodium ions which are positively charged. On the inside of this others, phosphate ions, protein ions, both negatively- charged and some potassium ions which are positively charged. So by controlling what can move through this phospholipid bi-layer, we can set up not only concentration gradients where if you recall things or molecules always want to move from an area of high concentration to an area of low, so that's one thing that we can set up across the plasma membrane, is a concentration gradient, but we can also set up an ion gradient because we're taking ions and sequestering them on one side of the cell versus the other side of the cell, it can make a charge difference on either side of the cell. So for example, if you have lots of positively charged ions on the outside and lots of negatively charged ions on the inside, not only would you have lots of one concentration of something on the outside, but you have lots of charge on the outside as well. So that's what's important or different about ion concentration gradients, is that they also carry this positive / negative charge with them as well, and that will become important. So the plasma membrane is able to not only separate concentrations of molecules, but it can also separate charges of specifically ions. The other important feature of the neuron is how permeable the membrane is to these ions. So again, depending on what types of proteins you have in the membrane will dictate how easily these things are able to move in and out of the cell. So this will become important. So we're going to break all of this down a little bit more. Slide 3 Now, when we think about ions and we think about the fact that they're negatively or positively charged, we know that when we have ions that are of similar charge, that they're going to repel each other. So if we have two positively-charged ions, they're going to repel. If we had two negatively charged ions, they're going to repel each other. But if we have a positive and a negative, they're attracted to each other. So like charges repel and unlike charges are attracted to each other. When we're talking about ions and negatively charged ions called an anion and a positively-charged ion is called a cation. So what we're actually doing or what we're trying to do to create a polarized, plasma membrane is to separate the charges. By separating the charges, we create a potential for those charges to want to be attracted back to each other and that is going to create an electrical force. So essentially what we need to do is we need to put a whole bunch of positives on one side and a whole bunch of negatives on the other side of the plasma membrane and not let them move through the plasma membrane. They're attracted to each other, so they want to move back together, but by separating them, we actually create a voltage. If you think about, if you've ever looked at a battery before, there's always a positive end and a negative end right? Well that's because what's inside the battery is some separated charges, and by separating those charges into one end versus the other, it creates voltage. And we do the exact same thing across plasma membranes. And it's not just in neurons, most body cells actually have a separation of charge on the outside versus the inside. And specifically the outside of the cell is typically positive and the inside of the cell membrane is typically negative. So by separating ions on either side of the plasma membrane, we create a polarized membrane or a charge difference, which can create a voltage or electrical force. Now this separation of charge is known as the membrane potential because it's the potential difference between the ions that are on the inside of the cell versus the outside of the cell. The more you have on either side, the more you separate those electrical charges, the more voltage you are going to get, or the more potential difference. There's more attraction, so that if there is a way for the ions to move from one area to another, they will be more likely to move in the direction towards the other ions. So again, I know this might sound a little bit confusing, but these dissolved ions across our plasma membrane are going to create both concentration gradients because we're going to have higher amounts of one versus another, as well as electrical gradients, so that positively-charged things are going to want to move to a negatively- charged area, or negatively-charged things are going to want to move to a positively-charged area. So all of this is going to come into play when we start to talk about how resting membrane potential is created. Slide 4 So back to the original image when we're talking about the electrical nature of neurons and specifically, it's not just neurons, there's many cells of our body, Most of the cells in fact, has a charge difference across the inside and the outside of the cell. And often that charge difference is even set up the same way as we see in neurons, but there are some things about neurons that allow us to take advantage of this charge difference, and that's what's going to be different between neurons and other cells of the body. So in the image here we can see again our plasma membrane of our axon region. On the outside of the plasma membrane, you can see that it's more positively charged. On the inside of the plasma membrane, we have more negative charge, and that's because we have more positively charged ions on the outside versus the inside. If we were actually going to measure this using an oscilloscope, basically we would take a reference electrode and have it on the outside. And then we have a recording microelectrode. So if we have this microelectrode on the outside of the cell, it would be in the same region as the reference electrode and our voltage would actually read zero because there's no charge difference; they're both in the same environment. But as soon as we stick the microelectrode into the inside of the cell, and specifically very close to the, in the inner side of the membrane, we notice that we're now getting a difference in charge because this area's positively charge and this area is negatively charged. And we can actually measure that charge difference by measuring voltage essentially. So in the case of an axon, when we are in a resting condition, the inside of the cell is more negative. So that means that compared to our reference electrode are voltage will be negative. So that doesn't mean that we're less than zero voltage, it just indicates that the inside of the cell is more negative compared to the outside of the cell. So in this case, the voltage difference at rest in an neuron, on the axon region, and in fact, it's similar on the cell body and dendrite region, is negative 70 millivolts, and you do need to remember this number. So negative 70 millivolts is the membrane potential at rest in a neuron. So it's the difference in charge across the plasma membrane because we have ions of one charge more on the outside of the cell compared to the inside of the cell. So that's what's going to set up this membrane potential or this potential difference across the membrane, the polar nature of our cells. So let's take a moment to pause here and do some practice questions and then we'll come back and talk about how we set this membrane potential up. Video 2 Slide 5 So now that we know that there's a charge difference across the plasma membrane, what is it that's actually causing that charge difference? And also I keep using the term resting membrane potential, what's resting membrane potential? Well basically resting membrane potential is the voltage difference across the membrane when the cell is not activated or sending an electrical signal. So we are going to be talking about in the next lecture module, electrical signals and how we transmit those electrical signals not only within a single cell, but also between cells as well. So this is really just telling us what's happening on either side of the plasma membrane when our cell has no changes or no electrical signals going through it. So this is a table showing us the ion concentrations on either side of the cell membrane. So the intracellular fluid would be on the cytosol side and the extracellular fluid would be on the outside of the cell. It's also divided it up into cations or positively charged ions, and anions or negatively charged ions. Now you don't need to memorize all of the numbers here. What I want you to get is which ones are higher and which ones are lower? And where are they high and where are they low? Basically the concentration highlights. So in our extracellular fluid, basically outside the cell, we have a higher concentration of chloride ions, and chloride ions are negatively charge. We have a higher concentration of sodium ions which are positively charged. And we have a higher concentration of calcium ions, which are also positively charged. On the inside of the cell, on the cytosol side of the plasma membrane, we have a higher concentration of potassium and proteins which are anions or hold a negative charge. Now this is going to be important because the amount of each of these ions we have on either side of the plasma membrane is going to help set up this polarized nature of our membrane or this membrane potential. Now, when you look at the outside and the inside of the cell, you might say to yourself, Well, we have negatively charged and positively charged molecules in both situations. So how does the inside of the cell become more negative compared to the outside of the cell? In fact, you can see over here in the table that the actual total number of anions and cations are actually equal on the inside of the cell as well as the outside of the cell. So you might say, well, you just told us we have a positive charge on the outside and a negative charge on the inside of the cell. How can we have that if we have equal number of ions with opposite charges on either side of the membrane? Well in fact, it is true we have the same amount of ions on either side, what's different is the immediate inside and the immediate outside of the plasma membrane, that's where the charge difference actually occurs. So when you think about ions in total, those ions are going to span a large space outside the cell or inside the cell. So in total and you add them all together, yes, they equal out, but on the immediate inside, and the immediate outside of the plasma membrane, that's when we're going to have clumps of either positively charged ions or negatively charged ions. So I'll show you how this works when we start to talk about the ions and how they're set up when we talk about establishing the resting membrane potential in a moment. So how do we get these ion concentrations where we have sodium high outside the cell, potassium high inside the cell, we have calcium high outside the cell, we have proteins high inside the cell. What is it that's actually setting up these concentration gradients for these different types of ions? Well, these ion concentrations are actually a result of two different processes. The first one is the sodium potassium pump, and the second one is based on the permeability of the membrane. Slide 6 So let's start with the sodium potassium pump. Now I know you're probably all familiar with the sodium potassium pump, also known as the sodium pump, sometimes known as sodium potassium ATPase, because that's the enzyme that actually breaks down the ATP that's attached to the pump that's required in order to make the pump work. The sodium potassium pump creates the gradient of the sodium ions and potassium ions. So we know sodium is high outside the cell and potassium is high inside the cell. Well it's this pump that's actually setting up that concentration gradient. And in fact, the pump works by actively transporting two potassiums into the cell and three sodiums out of the cell. Now when we talk about active transport, that means that it requires ATP energy. If you are moving something against its concentration gradient, then that requires energy. Now, if you were going with your concentration gradient, it can move via diffusion or movement through an ion channel using diffusion. But if you're moving it against its concentration gradient, so for example, you move sodium out of the cell where it's already high in concentration, that actually needs energy and that's active transport. So how does this actually work? So in our first image here, this would be our sodium potassium pump or sodium potassium ATPase. And in this case, we have some sodium ions on the inside of the cell that are entering into the pump. Remember that the gradient for potassium moves towards the outside of the cell because it's in high concentration on the inside, so it wants to go where it's in lower concentration. So from high to low, this is the direction of its gradient. For sodium, it's high on the outside of the cell, so it wants to move inwards. So this is the direction of the sodium gradient. So in this case, in the beginning of the functioning of the sodium potassium pump, the sodium molecules are drawn into the sodium potassium pump. At the same time, ATP binds to the sodium potassium pump because remember it's needs active transport. Now, as we cleave off the phosphate group from the ATP, we're left with inorganic phosphate and ADP, that powers the movement or changes the shape of the pump. And that change in shape basically takes the binding sites or areas where the sodium is being held, and where it's pointed towards the inside of the cell, and moves it towards the outside of the cell. So now those sodiums can move towards the outside of the cell. That happens basically when the ATP is broken down into ADP and inorganic phosphate. The ATP doesn't stay attached to the pump with the inorganic phosphate does. Now also notice while we're now facing the outside of the cell, we've opened up those sites where the potassium can now enter into the pump. So the next thing that happens is the sodiums leave, the potassiums bind into the pump, and the inorganic phosphate now moves away from the pump. When this inorganic phosphate moves away from the pump, that brings it back to its initial shape and in its initial shape it will bend back towards having these binding sites facing the inside of the cell that will allow for the potassium to move in. So that's basically how our sodium potassium pump works. Slide 7 So here's a little simplified version of this. So this represents our cell membrane here. We have a larger number of potassium ions on the inside and a larger number of sodium ions on the outside. And this is representing our sodium potassium ATPase, our sodium potassium pump, or our sodium pump. There's many different ways that this is often represented or referred to. Here's our ATP molecule. Essentially, what happens in the resting condition is that we have some protein channels that are found in the membrane of our cell that are called leak channels. And these leak channels would eventually allow all of the potassiums to move with their concentration gradient out of the cell and sodium's to move into the cell with their concentration gradients. And eventually it would reach an equilibrium on either side of the cell. So we have these leak channels that would allow this. So this is the leakage of the sodium potassium actually needs to be addressed so that we can keep the concentration gradients of these different ions high for sodium on the outside, and high for potassium on the inside. So the way that we do this or address this leakage is bind that ATP onto the sodium potassium pump. The ATP is then converted into ADP and inorganic phosphate, and that then powers the pump or turns it on, allowing three sodiums to move out of the cell and two potassiums to move into the cell. Once those ions have moved, the inorganic phosphate and ADP disassociate and the pump stops and we no longer have movement of ions through the cell membrane anymore. So technically this process is going on all the time, this pump never really completely stops. It requires a lot of ATP energy in order to constantly maintain these ion concentration gradients. So it's constantly working. So that's one way that we set up these ionic concentration gradients. Slide 8 But the other part of this is how permeable the membrane is to these ions in the first place. So if you remember on the last slide, I said that there were channels that allow the ions to move through with their concentration gradients. And these are known as non-gated or leak channels. So this is one of the main two types of ion channels that we see in the plasma membrane. So leak non- gated ion channels are essentially always open. It's always going to allow the ions to move from an area of high concentration to an area of low concentration. So potassium would move out of the cell through potassium ion channels. Now, note that each ion channel is specific to one type of ion. So it's not like a sodium can move through a potassium channel or vice versa. So in purple we have potassium, only potassium ions can move through potassium leak channels. We also will have some sodium leak channels as well, but they're not as numerous as we have for potassium channels. You can also see in this example there's a leak channel for our chloride ions as well. Again, this is in many different cell types. We're not going to be talking too much about leak channels for chloride ions in axons because they're not as prevalent. The other type of ion channels are called gated ion channels and these are the ones that are going to be very important for creating and transmitting electrical signals. Now what makes gated ion channels different is that they have gates on them essentially that allow them to open and close on demand. So they don't always let ions move freely through the plasma membrane, but they can do it on demand. Now there are three different types of gated ion channels. We have ligand-gated ion channels, which basically require a ligand or a chemical signal to bind on to the ion channel to open the gates. So again, a signal opens the gates that allows control of when and which types of ions are going to move through the plasma membrane. We can also have mechanically gated ion channels where we physically open the gates. And we can have voltage gated ion channels, where the ion channel gates open in response to changes and the potential across the membrane. So if you remember resting membrane potential is negative 70 millivolts. So if we have a change in that membrane potential, it will signal the voltage-gated channels to open. So we're going to be talking more about gated ion channels when we start to talk about changing membrane potential in the next lecture module. But just know that these are also very important because these are the ones that are going to change the permeability of the membrane on demand. The other thing that's very important when we're talking about permeability is how many ion channels are actually open at any given time. So of course, the leak channels are always open. So those are going to help establish resting conditions. But if we have lots or few of these gated channels open, it will change the movement of ions at any given time. So always keep that in mind, the more channels are open, the more movement's going to occur. The other thing that's very important is the size of the ions and the specificity, the actual channels. So as I said, these ion channels are very specific to certain types of ions, so only those ions can move through them. So again, that's how we can control which ions are moving and when. But the size of ions also plays a role. And in this example you can see that proteins which get me on the inside of the cell are very large. And because there are very large, they can't leave this cell easily. So there are ways of packaging proteins and releasing them outside the cell, but in the majority of the cells, the proteins that are being produced inside the cell stay inside the cell. So that traps these negatively charged proteins on the inside of the cell because they can't move through the lipid bi-layer. So this is going to be really important when we start to talk about establishing resting membrane potential and how we get the inside of the cell more negative than the outside of the cell. And finally, the last point here is the number of gated channels we actually have. So the more gated channels we have, the more on- demand changes we can make to our plasma membranes, membrane potential. So when we have fewer gated channels, will have less ability to change the membrane potential. If we have lots of gated channels we'll have a greater ability to change the membrane potential. So again, we'll talk more about the gated channels when we get into the next lecture module. So for now, let's pause for another moment and try out some more practice questions before we get into establishing membrane potential. Video 3 Slide 9 So now let's take all of what we've learned and put it all together to figure out how we're creating resting membrane potential. So in our image here we have our plasma membrane or our phospholipid bi-layer. You can see the sodium potassium pump. You can see our sodium leak channel and pink and our potassium leak channels in purple. Note that we have many more potassium leak channels than we do sodium leak channels. In the cytosol side, you can see that we have a number of different circles, these are all going to represent different ions. So our pink circles represent sodium ions, our purple circles represent potassium ions, and these blue bean-shaped structures represent our proteins. So on the inside of the cell we have a higher concentration of proteins and we have a higher concentration of potassium ions. On the outside of the cell, we have a higher concentration of sodium ions. So these are going to be the main ions that we're going to focus on. So at all times our sodium potassium pump is running. It's constantly pumping out 3 sodiums and in 2 potassiums and using an ATP. So this is going to keep the high concentration of sodium on the outside and the high concentration of potassium on the inside. So assume that this is always running and this is a huge demand for ATP and that's one of the things that actually helps establish our basal metabolic rate in our body is the amount of ATP needed to keep this ionic concentration difference across the membrane. So sodium potassium pump always going keeping sodium high out here, potassium high on the inside of the cell. Now, because potassium is high on the inside of the cell and there's lots of potassium leak channels, potassium is drawn towards the outside of the cell with its concentration gradient. So remember, we're always going to be moving from an area of high concentration to an area of low concentration. But in addition, we also have to keep in mind that there's also an electrical gradient. So positively charged ions are going to be attracted to negatively charged areas and vice versa. So keep that in mind. So right now we're just moving with our concentration gradient. Now because the potassiums are leaving the inside of the cell, they're bringing their positive charge with them. So that positive charge is creating a net positive charge on the outside of the cell. So more potassiums leave more positive charge on the outside. We get a net positive charge just on the other side of the membrane. Now, that also means that as the positive charges are leaving, we start to get a net negative charge on the inside of the cell because remember what's left in here is going to be our negatively-charged proteins. And in fact, what happens is as the potassium is leaving the cell, the negatively charged proteins are drawn towards that cell membrane because they're trying to follow the positive charge as it leaves the cell. So that's going to draw this negative charge to the direct inside of the membrane. Eventually we're going to get to the point where so much of the potassium has left the inside of the cell and gone to the outside that now those potassium ions are actually drawn back into the inside of the cell, not because of the concentration gradient because it'll still be higher in concentration on the inside, but because they're attracted to the negative charge on the inside of the membrane. So potassiums are moving into the cell because they're positively charged and they're attracted to those negatively-charged proteins. So negative charge, right near the side of the membrane on the inside, positive charge right near the side of the membrane on the outside. The potassiums are drawn in with their electrical gradient and drawn out with their concentration gradient. So because of this, we're going to eventually get to the point where the amount to potassium drawn in with the electrical gradient is the same as the amount of potassium ions moving out with their concentration gradient, and this is when we reach resting membrane potential. So resting membrane potential is established when the movement of the potassiums out of the cell is equal to their movement into the cell. So again, all of this is established by the fact that we have a concentration gradient made by the sodium potassium pump that's making high concentration of potassium on the inside. It's drawn towards the outside. But then it's like, I don't want to leave because I'm still attracted to that negative charge. So it tries to go back into the cell towards those proteins with the electrical gradient and then we get this balance between the in and out of the potassium ions. So it doesn't create an equal concentration on either side of the membrane, don't think of it that way is that there are now equal. Just think about it like this for every potassium that leaves, one potassium's coming in again. So it actually establishes, with the concentration gradients still being high on the inside, it establishes this point where you don't actually have very much movement of the ions anymore. Slide 10 So in this table, it's summarizing the characteristics that are responsible for our resting membrane potential. So one, the concentration of potassium is higher on the inside of the cell and sodium is higher on the outside of the cell. We have many, many more potassium leak channels than we do sodium leak channels, in fact it makes the plasma membrane about 50 to a 100 times more permeable to potassium compared to sodium. The plasma membrane is impermeable to those large molecules like proteins, which are negatively charged and essentially get trapped inside the cell. Potassium ions tend to move from the inside of the cell to the outside of the cell with their concentration gradient. And as we'll see as we go through some of the lectures on changing membrane potential, even though the potassium ions could move in both directions, because there's two different gradients, the concentration gradient always tends to win. So if you're ever confused about where an ion is going to move when you open a channel and change the membrane permeability, it tends to always move with its concentration gradient. So just keep that in mind. Because the negatively charged molecules, those proteins, follow the positively charged potassium's towards the outside of the cell, it creates a small negative charge just on the inside of the plasma membrane. And that actually attracts the positively charged potassiums to move back towards the inside of the cell. And eventually we reach an equilibrium where the amount of potassium is leaving with the concentration gradient equals the potassiums coming back in with the electrical gradient. This charge difference can be measured as a difference in potential across the membrane, and as I said, in most of our neurons it's about negative 70 millivolts. Now when you think about that negative 70 millivolts, remember that the 70 is actually the difference between the inside and the outside, and the negative is just indicating that the inside is negative, then the outside is positive. So if you had positive 70 millivolts, for example, it's exactly the same difference across the membrane. It still 70 or 70 millivolts away from 0, which is no difference across the membrane. It's just now the inside of the cell is positively-charged and the outside of the cell is negatively charged. So don't think that positive 70 millivolts is actually higher, then negative 70 millivolts. It's really about how far away that voltage is from 0 to tell you what the potential difference is across the membrane. The negative sign just tells you that the inside is negative compared to the outside. That's important because if you're talking about increasing membrane potential, the number always has to go up. And if you're decreasing memory potential, the number always has to get closer to 0. So it's not like decreasing membrane potential, you get farther from 0 into the negatives, you wouldn't go to negative 90 and say that decreased membrane potential, it actually increased membrane potential because now you're 90 away from 0 instead of 70 away from 0. So hopefully that makes sense. And in fact, if you add more potassium ion channels, you'll get more movement of potassium outside of the cell, and that actually increases membrane potential. And we start to see that in some tissues like muscle, which has a greater resting membrane potential, in fact is around negative 90 millivolts. In this next statement here is the one that tends to confuse a lot of students because it says "the resting membrane potential is proportional to the potential for potassium to diffuse out of the cell, but not to the actual rate of flow of potassium". So that sounds confusing and that is just because when we actually look at what's happening with the potassiums, there's very little movement of potassium at this resting membrane potential because everything's at equilibrium. So the actual rate of flow for potassium is not going to be very high, but we still have a great difference across the membranes such that if you actually changed any of the channels in that membrane, say increase the number of potassium channels for example, we would have the potential for the potassiums to move. So it's about the separation of the charge, not the actual flow of the potassium through the membrane. Conclusion So hopefully now we all have a pretty good understanding of what creates the electrical signal across the plasma membrane of our neuron known as the membrane potential. In the next online lecture, we're going to look at changing that membrane potential to create an electrical signal that moves along our axon. We're also going to talk about how we can send that signal from one neuron to another neuron or to a target tissue. So until then, take care.