Lecture 16 -Changing Membrane Potential PDF

Lecture 16 -Changing Membrane Potential Recall from last lecture I talked about resting membrane potential is actually present in many cells of the body. So what makes neurons, as well as muscle, electrically excitable? Or what gives them the ability to create an electrical impulse that can travel f...

Lecture 16 -Changing Membrane Potential Recall from last lecture I talked about resting membrane potential is actually present in many cells of the body. So what makes neurons, as well as muscle, electrically excitable? Or what gives them the ability to create an electrical impulse that can travel from one area of the cell to another? Well, it's their ability to change that membrane potential. So that's what we're going to be focusing on today. Slide 2 So neurons are electrically excitable because of the resting membrane potential. But in order to create an electrical signal, we need to change that membrane potential and this can be done with our gated ion channels. So I introduced gated ion channels in the last lecture, but we're going to look at them in more detail today. So there are three main types of gated ion channels. We can have ligand-gated ion channels, mechanically gated ion channels, and voltage-gated ion channels. Slide 3 So the first type are the ligand-gated ion channels. These are going to respond to chemical stimuli or a ligand that physically binds to a receptor on our ligand-gated channel. And by binding onto that receptor, it opens the gates of the channel and allows the ions to move through. So here's an example of a ligand gated channel. So in this example we have a sodium gated channel. So remember all of our ion channels are going to be specific to specific ions as well. Not only that, but they're also going to be specific to specific ligands. So only one type of ligand will be able to bind to this particular sodium channel. We might have another channel that's next to this, that might be a potassium channel for example. And it will not open in response to this ligand, it might need a different ligand. So in our example here, our ligand is acetylcholine, which is one of the neurotransmitters that we're going to talk about when we talk about chemical signals moving from one neuron to a target tissue or to another neuron. So this is a neurotransmitter. This neurotransmitter acetylcholine, binds onto the receptor site for our sodium channel. Once it binds onto the receptor site, it opens the sodium channel and sodium can now diffuse in the direction of its concentration gradient. So if you remember, sodium is always in higher concentration on the outside of the cell than it is on the inside of the cell, so when you open a gated channel, sodium is going to rush into the cell and that's what happens when we have gated sodium channels. Slide 4 The next type of ion channel are the mechanically-gated channels. These are going to respond to mechanical vibration or some sort of pressure stimuli. So essentially we physically are going to open the gates. So in our example here, we're actually looking at a mechanically gated channel that's part of our special sense of hearing as well as balance. So we are going to look at this in more detail a little bit later in the semester when we get into those special senses, but I just wanted to highlight it as an example here for our mechanically gated channel. So in this case we have a potassium channel and the gate is closed, but notice the gate is physically attached to a structure in this case called a gating spring. And that gaining spring is attached to a neighbouring cell. So what happens is when we actually have movement between these two cells, it pulls on the gating spring and that gaining spring then opens up the gates. And in this case our potassium that can then move through the cell. Now normally, potassium is in higher concentration on the inside of the cell, so it wouldn't move out of the cell, but in our inner ear we have a little bit of a different situation. So in this case, it's still moving from an area of high concentration to an area of low concentration. But you can see how this spring is actually physically opening the gates and that's what's allowing the ions to move through. Slide 5 The last of the ion channels are the voltage gated ion channels. These are going to respond to direct changes in membrane potential. So in our example here we have our voltage gated ion channel being closed and this is at our resting membrane potential of negative 70 millivolts. If we have a change in the membrane potential that's in the vicinity of this voltage gated channel. So in our example here, we go from a voltage of negative 70 millivolts to a voltage of negative 50 millivolts. Well, when we have this change in membrane potential and that is going to trigger this particular type of gated channel to open. So basically, any small change in membrane potential can open this gated channel and allow for ions to move through. And in fact, what happens is once the ions move through or we're gonna get further changes in the membrane potential. So this type is actually very important in creating electrical signals. And in fact, voltage-gated ion channels are found primarily in the axon regions of our neuron, and not so very important, we don't find them in the cell bodies or the dendrites, they're primarily in the axons and we'll also see some in the presynaptic terminals as well. In fact, the majority of the voltage gated ion channels that we see and the axons are going to be sodium and potassium voltage-gated channels, which we're going to look at more in detail today. There also are some calcium voltage-gated channels that we see in the presynaptic terminal, and there's also calcium voltage-gated channels in cardiac muscle and smooth muscle as well. But we'll be talking more about some of those changes in term two and we look at the cardiovascular system. So for now, know that voltage-gated channels are found in the axon plasma membranes only or in those pre-synaptic terminals. Whereas something like our ligand gated channels are mostly found on the cell body regions or in the dendrite regions of our neuron. Mechanically gated ion channels are going to be more associated with special sensory regions. So they're going to be primarily in specialized dendrite regions that are developed or designed for gathering sensory information from the body. Slide 6 So this is a nice little table that summarizes the different types of channels and where they're located in the body. So leak channels are the type of ion channels that are always open, that we talked about last day, and these are important for establishing resting membrane potential. They're found in all cells of the body, are almost all cells of the body, and they're found everywhere on our neuron. So the dendrite region, the cell body region, and all the axon on regions of our neurons. That's because we need the leak channels to make the resting membrane potential. Remember those potassium leak channels along with the sodium potassium pump, were required in order for us to create that resting membrane potential. So of course, we're also going to have sodium potassium pumps located on cell membranes everywhere as well. The other types of channels are our gated channels. So the ligand-gated channels and mechanically-gated channels and the voltage-gated channels. These are the ones that are going to be found primarily in neurons, but also will see in some muscle as well. And the ligand-gated channels are going to be found in the dendrite regions of some of our sensory neurons, some things like pain receptors for example. And we're also going to have them in the dendrites and cell body regions of things like interneurons or the neurons that we find in our central nervous system, as well as in the cell bodies and dendrites of our motor neurons. Mechanically gated ion channels as I said, they are specialized types of receptors that are going to be found in some sensory neurons, such as our touch and pressure receptors, pain receptors, as well as the special senses, hearing and balance. And then we have our voltage gated ion channels. Those are the most key for creating electrical signals that are going to propagate or move from one area to another along a neuron. So these are the ones that are found only in the axon region of all types of neurons. Now it's very important that you understand that these different channels are found in different locations, we don't typically mix them all up. Although I will show some examples of where we might have one near another one in some of our situations where we're creating electrical signals. So let's take a quick pause here and try out some practice questions. And then we'll return to look at how we create these electrical signals using these different gated channels. Video 2 Slide 7 So now that we have an understanding of the different types of gated ion channels which we can use to change membrane potential, let's now talk about the different types of signals that we can create in a neuron. So there are two main types of electrical signals that can be produced by these different gated channels. Graded potentials also known as local potentials, and action potentials. So we're going to talk about each of these different types of electrical signals and why they're important and where they're going to happen in the neuron. So when comparing graded and action potentials, action potentials allow for communication over short as well as long distances compared to graded potentials, or also known as local potentials, which communicate over only short distances only. Now the key important point here is the type of gated channel that's used to create each of these electrical signals. Graded potentials use ligand-gated and mechanically gated ion channels, whereas action potentials use voltage-gated channels. And this is really where the big difference between these two types comes in. Now if you think about a graded potential or a local potential, that means that if we're using a ligand-gated channel or a mechanically gated channel, then we're only going to get changes in membrane potential in and around the region where we actually have the channels open. So if you imagine we have resting membrane potential and then imagine that we have some ligand-gated channels, for example, interspersed along the membrane. Well, we're going to need the chemical stimulus to come over to that membrane, bind onto the receptors and open the ligated channel in order to get changes in membrane potential. Because remember, the change in membrane potential is going to be our electrical signal. Now if we only have a little bit of ligand or a little bit of those chemical signals, only a couple of gates might open. If we only have a couple of gates open, only so many ions are going to move through and if you think about it, only the ions are going to move through in the region directly around that gated channel. You're not going to get membrane potential changes farther across the membrane because it can only occur where the gated channels are open. So the more ligand we have floating around near the membrane, the more it can bind to ligand gated channels, the greater the response we're going to get. And that's why they're called graded potentials because they're really dependent upon how much of a stimulus we have to open the gates and how many gates we have to open. So this plays a role and it also determines why we can't really communicate over long distance with this type of electrical signal. Because we can only really cause membrane potential changes directly in and around the gates that are open. Now we could open lots of ion channels with a significant amount of, of ligand floating around, but we're still only going to get changes that are fairly localized to a specific region of the membrane. And likewise with the mechanically gated ion channels, we would only get changes where we're physically opening the gates of the ion channels. Typically we're not going to have ligand-gated channels and mechanically gated channels in the same place. So we would have one or the other. Now what makes action potentials different is that they use voltage gated channels. Now if you remember, voltage-gated channels open in response to changes in membrane potential. So voltage-gated channels are much like a line of dominoes that are all set up in a row. Now, if you want to knock over the far this domino in the row, all you need to do is flick the first domino, it will knock down all the dominoes in the row between, and eventually that last domino will fall. The same thing happens with action potentials because they use voltage-gated ion channels. You only need to have enough voltage change to trigger the first voltage gated ion channel to go off. Once that voltage-gated channel opens, ions move through the membrane, and when ions move through the membrane, they change the membrane potential around them. Now if you have another voltage-gated channel nearby, it will then trigger that voltage-gated channel to go off. It will have ions moving through it, that will change the membrane potential nearby and trigger the next voltage gated ion channel. So you can see how this works. You only need one voltage gated ion channel to go off, and it will trigger all of the other voltage-gated channels that are near it. And this is how we can create a signal that actually moves from one place to another. And essentially it's not actually moving at all, it's just triggering a series of other action potentials to occur. And that's exactly what happens along the length of our axons. So we'll talk about how this is working, but know that graded potentials are localized, they're ligand-gated or mechanically gated ion channels, and again, these are typically going to be happening in the cell body regions, in the dendrite regions of our neurons. Whereas action potentials use voltage-gated channels and that's only going to be in the axon regions of our neuron. Slide 8 So this table is giving us a summary of the characteristics of both graded potentials as well as action potentials. Now I'm not going to be going over all of this right now because we're going to be talking about it in the next few slides. But if you're unsure or you miss something, then this is a good reference for you to determine the differences between the graded potentials and the action potentials. Slide 9 So first talking about local or graded potentials. As I mentioned, local or graded potentials are going to be occurring in the cell body or dendrite regions of our neurons because this is where we're going to have the ligand-gated channels or the mechanically gated channels. So if we have some form of ligand present and it binds onto the receptor, it will cause an opening of a gated channel. That gated channel will that allow ions to move in or out of the membrane depending on what specific ion it's for. So for example, there's a couple of different situations that can happen when we have a ligand gated or mechanically gated channel open. The first of these situations is when we have a membrane potential that moves closer to 0. So essentially we get less difference between the inside and the outside of the membrane in terms of negative charge and positive charge, the type of potential change or a membrane potential change that moves closer to 0 is known as a depolarization because we're actually taking away polarized nature of the membrane by getting closer to 0. Because remember 0 means that there's no difference between the inside and the outside. The farther you get from 0, whether it's negative or positive, the greater the difference between the inside and the outside. Negative just means that the inside is negative and the outside is positive, but we could have the same thing happening in the positive range as well. So depolarization towards 0. So this is our resting membrane potential of 70 millivolts, and a depolarization would then therefore be somewhere between 0 and that negative 70 millivolts, so this is known as a depolarization. The other example is hyperpolarization. Hyperpolarization is when we open an ion channel that makes the difference between the inside and the outside of the membrane even greater than it was before, we're hyperpolarizing, we're making it even more polarized than it was before. So in this case, the number is going to get farther away from 0. So in our example here, we're getting a hyperpolarization of negative 70 millivolts to negative 75 millivolts, for example, could go down to even negative 90 millivolts in some situations. So this is a hyperpolarization, greater difference between the inside and the outside. Whereas depolarization is less of a difference between the inside and the outside. So negative 70 to negative 60 because we're getting closer to 0. Now, when we think about these two types of changes that will occur with our memory potential, think about what types of ion channels are actually going to open to cause these changes. Now, if we open a sodium channel, for example, which type of polarization do you think we're going to get? Are we going to get a hyperpolarization or a depolarization if we open a sodium channel? Well, if you said we get a depolarization, you'd be correct. So remember, sodium is positively charged and it's in high concentration on the outside of the cell. So when you open a sodium channel, sodium is drawn to the inside of the cell because it's moving with its concentration gradient, but also it's attracted to that negative charge on the inside of the cell. So it's going to rush into the cell and when it rushes into the cell, it brings its positive charge with it. So that makes the inside of the cell more positive than it was before and it makes the outside of the cell more negative than it was before, so we get last difference between the inside and the outside of the cell and we de polarize the membrane. So that sodium that will move into the cell; sodium causes depolarization. Another example would be calcium. Same thing, high concentration on the outside of the cell, low concentration on the inside, positively-charged, works very similarly to sodium, so it would cause depolarization. Now what would be an example of a hyperpolarization? Well, one thing that we've talked about already, potassium. Remember potassium is in high concentration on the inside of the cell and in low concentration on the outside of the cell. So if we open up a potassium channel, potassium wants to leave with its concentration gradient. So when it leaves with its concentration gradient, it takes its positive charge with it. So that will make the outside of the cell even more positive than it was before and it will make the inside of the cell even more negative than it was before. So when you open a potassium channel, it causes hyperpolarization because we're getting even greater difference between the inside and the outside of the membrane. The other ion that will cause hyperpolarization are chloride ions. So chloride ions are in high concentration on the outside of the cell, but remember chloride ions are negatively charged, so when we open a chloride ion channel that will allow our chloride ions to move from the outside of the cell to the inside of the cell with their concentration gradient. That will make the inside even more negative than it was before and the outside even more positive, so again, causing hyperpolarization. And remember from the last lecture I talked about the fact that when there is two different gradients going on, the electrical gradient and the chemical gradient, the ions tend to always move with their chemical gradient first. So even though the potassium is moving away from where the negative charge is and towards an area that's already positively charged. It's moving primarily with its concentration gradient and the electrical gradient isn't strong enough to really counteract that response. So that's why we get those hyperpolarizations even though they're moving against thermoelectric gradients in those cases. So those are just a couple of examples of how ions can cause hyperpolarization or depolarization. But you can see in the graph here, these changes in membrane potential are really only going to occur based on how much of the ions are moving in, in a specific region. So the more of these channels you open, the greater the ions are going to move across the membrane and the larger the change will occur. Slide 10 Now what I'm describing here is how stimulus strength is affecting our graded potential. So the amplitude of the graded potential depends on the stimulus strength. And you can think of the stimulus as being either the amount of pressure on mechanical gates or the amount of chemical signal or ligands being released. The more you have, the greater the change in membrane potential you get. So that's why they're graded. The greater the stimuli, the greater the change in membrane potential. So in this case, are we having a hyperpolarization or a depolarization? It's a depolarization, right, because we're getting closer to 0. So this would be a small stimulus and as the stimulus gets larger, we get more gated channels open, more ions moving through the membrane, a greater change in membrane potential. The other way that this system can work though, is actually independent of the stimulus strength and it's based on how close together the signals are in time. So graded potentials can be added together to become larger in amplitude. So in this case, if you notice the stimulus was coming when the graded potential had come back down to resting membrane potential, that's when we stimulated or sent the next chemical signal, if it's a ligand-gated channel for example, so we always came back down to baseline before the next signal came out. But in a situation where you have signals coming in rapid succession or close together temporarily or in time, then you can actually add this signal together. Now this makes logical sense, if you think about it, you've released a chemical or a ligand, it's bound onto an ion gated channel. You haven't really allowed the resting membrane potential to come back down to normal again before you open the gates. So you're rushing more ions into the cell before it's come back down to resting membrane potential. So in this case you can see that when you have the first signal, it can actually add on to the next signal. So if you took two of these and added them together, that's essentially what's happening here, when you have them close enough together. So if you have two signals that are very close in time, they add or they summate. So this process is known as summation, and this is how graded potentials can get larger and larger. And in fact, we take advantage of both the stimulus strength and the process of summation to make really large graded potentials because we need large graded potentials in order to create action potentials. So that's what we're going to talk about next. But before we do that, let's take a pause here and try some more practice questions. Video 3 Slide 11 So now that we know about graded potentials or local potentials, how do we incorporate the use of graded or local potentials to create an action potential? Well, if we have a large enough graded potential, it can cause the membrane to reach something known as threshold. And when we can get to that threshold level, we can get an action potential. Now if you think about what we've been describing so far, we've been talking about using ligand-gated or mechanically gated ion channels located in the cell body and dendrite regions. But remember, in our axon we have voltage gated channels. So essentially what we need to do is have a large enough change in membrane potential from those graded potentials to reach this region, if you recall, known as the trigger zone. And in the trigger zone we're going to have a greater than normal amount of voltage gated ion channels. And remember once we trigger the first voltage-gated ion channel, it will trigger all of the other channels along the entire length of the axon. So really what we need to do is get a large enough graded potential response in this area to reach the threshold, which is a level at which we trigger the very first voltage gated channel to go off. And that's basically how we can use these graded potentials to create an action potential. So in an action potential, what happens is once we trigger that very first voltage gated channel to open, it opens and we get a phase called depolarization. So in the graph here we're showing resting membrane potential, we're now getting a graded potential in the cell body dendrite region that gets larger and larger and larger, and then it reaches the point of threshold, where it opens the first of the voltage-gated channels. Then we get a rush of ions into the cell that are positively charged, causing a large depolarization. And in fact, a depolarizes so much that we actually get a greater negative charge on the outside and positive charge on the inside for a short time and we actually can get a positive voltage reading. So that's the depolarization phase. Then our action potential is followed by a repolarizing phase where we're bringing our memory potential back down to resting conditions, but in fact we actually overshoot that for a short amount of time, and that's known as the after potential phase or the hyperpolarizing phase. So this is a little bit lower than our normal resting membrane potential, but then eventually we're going to come back up and establish our resting membrane potential again. So this graph is indicating what an action potential would look like. And in fact, our action potentials work on something called an all-or-none principle. So you either get a large enough graded potential to create the action potential or you don't get an action potential at all. So when you think about this, because our threshold is between negative 70 millivolts and 0, then that means that you need to have a large enough depolarizing graded potential to reach threshold in order to get an action potential. So it has to depolarize in the graded potential to get to that threshold mark. If you had a hyperpolarizing graded potential, then you'd actually be getting farther away from threshold and you wouldn't be able to get an action potential. So if you want to send a signal on, you have to have a depolarizing graded potential occurring in the cell body region that gets large enough to reach this threshold, which again represents that first voltage-gated ion channel being triggered in that trigger zone of our axon. The other thing about the all-or-none principle is that once you open a voltage gated channel, all the membrane permeability changes that can occur, will occur. And once you open the first voltage-gated channel, all of the other voltage-gated channels will go off, you cannot stop them from going off. So because of this, our signal is going to spread over the surface of our axon. So we basically are going to have our domino effect where we triggered the first domino go off and it's going to trigger all the other voltage gated channels along the entire length of the axon until we get to the region and the presynaptic terminal. So it's not actually travelling, it's spreading across the membrane without dying out. The other thing that's important is the magnitude of the action potential always stays constant. So we're always going to go from this negative 70 millivolts up to this positive 35 millivolts, and the reason for that is what's changing or opening the gates of these ion channels. So if you think about these particular voltages as being the markers for either opening or closing gates on an ion channel. So that means that every single gate that opens around this membrane potential will be a threshold potential and it will open a particular gate, on the way back down and could potentially close that gate. So for example, at threshold, that's a particular change in membrane potential that opens a specific gate to open. Whereas up here at the high end, the positive 35 millivolts, that's a particular membrane potential that is going to close a gate, as we'll see in a second. So the amplitude of the action potential always stays exactly the same. And all the membrane potential changes that we could get are going to happen. We can't add them together like we have with a graded potential where the more stimulus you have, the larger the potential difference across the membrane. This is all-or-nothing, you either get a response and it's all the same or you don't get a response. Slide 12 So how is all of this actually occurring? Well, of course, again, we're using voltage-gated ion channels to create our action potential. So in the image here, this is representing our phospholipid bi-layer and the new voltage-gated channels that we're going to talk it out. Now remember, in any portion of our plasma membrane, we're always going to be having also the potassium leak channels, the sodium leak channels, we're also going to be having sodium potassium pump. So this is just highlighting the voltage-gated channels specifically. But know that all of those other channels are also present because they're the ones creating resting membrane potential in the first place. So in our resting condition, resting membrane potential again is established by leak channels and the sodium potassium pump. So any voltage-gated channels at this stage won't be open. So when you actually look across here, there are two different types of voltage-gated channels. We have sodium voltage-gated channels, and we have potassium voltage- gated channels. So the potassium gated channels, if you notice, I only have one gate. And again at rest, this gate is closed. For sodium gated channels, there are actually two gates. The gate that's on the extracellular side is closed and the gate on the cytosol side is actually open in the resting condition. Now the gate that's on the extracellular side is known as the activation gate. And again in a resting condition, the activation gate is closed. The other gate is known as the inactivation gate. In the resting condition, this inactivation gate is open. So right now no ions can move through because one at least of the gate is closed. So this is the resting condition. So in the resting membrane, the inactivation gate of the sodium channel is open and the activation gate is closed. Sodium cannot get in. The voltage-gated potassium channel is also closed. So this is our resting condition, resting membrane potential. So now we're going to start to get a graded membrane potential in the cell body dendrite regions. That's going to then reach the level of threshold in the trigger zone and trigger these voltage gated channels to go off. Slide 13 So this is going to start the depolarizing phase of our action potential. So graded potential reaches threshold, that begins the depolarization phase. What happens in depolarization? Well essentially at the threshold mark, that is the change in membrane potential that triggers our voltage gated sodium channel to open. And specifically, it opens the activation gate of the sodium channel. So it's that level of membrane change that is required to cause this voltage gated channel to open and specifically this specific gate, the activation gate. Now as soon as the activation gate opens, sodium rushes into the cell, because remember sodium is going to be attracted to the inside because it's moving with its concentration gradient, but it's also moving with its electrical gradient as well. So it really wants to get to the inside of the cell. So that's how we get this really, really rapid depolarization because so much sodium rushes into the cell so quickly that it makes the inside of the cell actually positive for a short time and then the outside of the cell will actually get more negative. So that will give us a millivolt difference across the membrane, which is a positive number, and in fact it's around positive 35 millivolts. At the same time, we will start to see some of the potassium gates opening as well, but these open much more slowly. So we will get some potassium channels opening in the depolarizing phase, and potassium will start to move out of the cell, but it's far less than what's happening with the sodium. So really what's happening in this depolarizing phase is the sodium rushing in. Potassium isn't really contributing very much at the stage. Slide 14 Now, once we get to this level of positive 35 millivolts, that particular membrane potential, it causes another change in our voltage gated channel. And in fact, it causes the inactivation gate to close. So remember, this one opened with threshold, this one closes when we get to the peak of depolarization or this positive membrane potential. So that causes the inactivation gate to close. Well, at the inactivation gate closes since sodium can no longer go through the cell. So inactivation gate closes, sodiums can no longer move through the membrane. Potassium channels are opening more rapidly now and potassium is starting to move out of the cell. So sodium's moved in, potassium's moving out. Now as potassium moves out, it brings its positive charge with it. So it's going to take positives from the inside of the cell, bring them out, that will start to make the inside of the cell more negative again and the outside more positive again. So that will start to bring us back down towards resting membrane potential. The other thing of course, that's happening during this time is the sodium potassium pumps are working overtime to try and re- establish resting membrane potential. So that's also occurring, the sodiums are being pumped back out and the potassiums are being pumped back in. So it's a bit of a mixed number of things that are happening at the same time. Slide 15 So that repolarizing phase is going to continue until we get down to the level of resting membrane potential again. So as we start to bring our membrane potential back down towards threshold, remember threshold was the point at which the activation gate opened, so basically when we hit threshold on the way down again, we reclose the activation gate of the sodium channel. And as we get towards resting membrane potential, the inactivation gate will reopen. So in this phase, the sodium channel has not opened again, we've at least had one of the gates closed at all times, so sodium is not moving through. But up until this point of the end of the repolarizing phase, we didn't have a condition where our sodium pump was back to the resting position because remember resting position was activation gate closed, inactivation gate opened, when we were in the repolarizing phase early on, the activation gate was open and the inactivation gate was closed. So therefore, we couldn't activate this particular voltage gated channel again because it wasn't in its resting position. So it doesn't get back down to resting position until the end of the repolarizing phase. During this phase, the potassium channels continue to be open and more and more potassium is moving out of the cell. So at the end of the repolarizing phase, the potassium has gotten our resting membrane potential back down to negative 70 millivolts and the inactivation gate reopened on our sodium channel and the activation gate closes, so we're back to the resting position, that's the end of the repolarizing phase. Now after the repolarizing phase, we sort of overshoot for a short amount of time our resting membrane potential, and we go even more negative than we were before. This phase is known as the after potential phase or the after hyperpolarizing phase. Essentially what happens is the potassium channels stay open longer than they should, and this causes a hyperpolarization. So remember with our graded potentialsm we said if you open a potassium channel, we get hyperpolarization. Essentially that's what's happening here, except it's a gated channel that's voltage-gated, just happens to stay open a lot longer and potassium continues to leave the cell and we get this hyperpolarizing situation. This will cause our membrane potential to drop to even close to negative 90 millivolts, so we get a significant drop in this time period. Slide 16 Now, eventually we'll start to close those potassium channels. Once those potassium channels close, the sodium potassium pump, which has been working really hard through this whole action potential, it now has a chance to get caught up, and it will bring our resting membrane potential back to negative 70 millivolts. So along with the leak channels, the sodium potassium pump reestablishes resting membrane potential now that all the voltage-gated channels are closed again. So following this, we have an animation that shows you all of the changes that are happening with these voltage-gated channels through the different phases of an action potential. Hopefully this will help you visualize what's happening and then try out some more practice questions. Animation No caption file Video 4 Slide 17 So before we move on to neurotransmission in our next online module, there's a couple of other concepts we need to look at for action potentials and one of those is the refractory period of an action potential. Basically, that means that after an action potential has occurred, the membrane of the axon is less sensitive to another stimulus. So essentially, we lose the ability to stimulate that axon or the voltage-gated channels within that axon for a period of time after an action potential has just moved along the axon. So there are two different types of refractory periods. The first one is known as the absolute refractory period. During the absolute refractory period, even a maximum stimulus will not begin another action potential. Now if you actually look at the graph of our action potential, notice when the absolute refractory period is, it's basically during the depolarizing phase and repolarizing phase of our action potential. Now if you think about what's happening during those phases, we either have our sodium gate open already, so if it's open already we can't activate it to open again, so that's what we see in the depolarizing phase, and then in the repolarizing phase the activation gate is still open but the inactivation gate is now closed. So we still can't activate the voltage gated channel to open because it's the activation gate that needs to open and it's still open, whereas the inactivation gate is what's blocking the sodium. So in either the depolarization or repolarization phase, essentially, the sodium voltage gated channel is not in the resting condition. Therefore, we cannot stimulate that channel to open again. So it's impossible to create another action potential because that's really what the trigger is in order to start an action potential. So that's the absolute refractory period during that phase of an action potential, we can't create another action potential. There's also another refractory period known as the relative refractory period. Now the relative refractory period is happening in that after potential phase or the after hyperpolarizing phase. Now what's a relative refractory period? Well, this is where we need a suprathreshold stimulus to start another action potential. Basically what that means is that we need a stimulus that is larger than what we typically need to reach threshold to create an even larger graded potential so that we can actually get to threshold. Now if you look at the actual graph here, the relative refractory period is in that hyperpolarizing phase. So what's happened is we're farther away from 0 than we were before, remember hyperpolarized, so in order to get to threshold in our resting condition we only needed a graded potential that was this large. Well now during the relative refractory period, we're going to need an even larger graded potential in order to get to that same threshold point. So we need to have a really strong stimulus essentially, in order to have a larger graded potential that will get us to that threshold point. So that's the relative refractory period. So absolute can't get another action potential no matter what you do, relative, you can get an action potential, but you need a really strong stimulus, larger than what you normally would need or supra- threshold, it has to be large enough to get to that threshold point. And again, in this relative refractory period, we can get another action potential because of the sodium channels are back in their resting state, even though the potassium channels aren't, the sodium channels are because they are what trigger the action potential in the first place. So as long as those are back and they're resting condition, then we can make another action potential. So those are the refractory periods. Slide 18 Now, one of the other things that we talked about was that all action potentials have the same amplitude of response. So if every action potential has exactly the same amplitude of change in membrane potential, how does our brain interpret a strong stimulus versus a mild stimulus? So the stimulus strength is determined by the frequency of action potentials until we reach the maximum rate. When you look at the graph over here you can see at first we have a sub-threshold stimulus. So that's not even enough to get the threshold, we wouldn't get an action potential in that case. If we have a threshold stimulus, that's enough to get us the threshold we'll get a single action potential. But as we increase the stimulus, we get larger and larger graded potentials. Those graded potentials then trigger more action potentials to occur and they're going to get closer and closer together in time. So when action potential moves along, we set the dominoes back up again, another one goes along, we set the dominoes backup again, essentially as fast as we possibly can. Eventually you're going to get to this maximal stimulus. That is, we are making action potentials as fast as possible. In a sense, what we're doing is as soon as we get to the relative refractory period, another action potential happens. So we can't set off another action potential during the absolute refractory period, but in the relative refractory period, see how we're supra-threshold here, that means that we can get another action potential as soon as we hit that relative refractory period. So that will be when the next one starts and we continue that, and it will just send them in rapid, rapid succession. But because we're already making action potentials as fast as we possibly can, and they're going down the axon as fast as they possibly can, any stimulus that's greater than maximal won't look any different. So notice that these two look exactly the same and that's because we're already firing action potentials as fast as we possibly can. So once you get to the point of maximal stimulus, you can't really detect anything beyond that. So in terms of this single neuron, this is the maximal stimulus that your brain will be able to detect. Conclusion So hopefully now we have a pretty good understanding of the different gated channels that we can use to change our resting membrane potential. So today we talked about ligand-gated ion channels and mechanically gated ion channels and how they can change the membrane potential in the cell body and dendrite region, specifically creating graded potentials. We also talked about voltage gated ion channels, which are located in the axon region of our neuron and can create action potentials.