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See how muscle cells in the heart contract by allowing Calcium to flow inside and bringing along some positive charge with it! Rishi is a pediatric infectious disease physician and works at Khan Academy. Created by Rishi Desai.
Video transcript
Let's figure out how a heart squeezes exactly. To do that, we have to actually get down to the cellular level. We have to think about the heart muscle cells. So, we call them cardiac myocytes. These are the cells within the heart muscle, and these are the cells that actually do that squeezing. If you actually were to go with a microscope and look down at one of these cells, it might look a little bit like this- with proteins inside of it. When it's relaxed, these proteins are all kind of spread apart. When it's squeezing down, because each cell has to squeeze for the overall heart to squeeze, these proteins look completely different- they are totally overlapped. And that overlapping is really what we call "squeezing." So this is the squeezed version of the cell. The first one was a relaxed version. The trigger that kind of gets it from squeezing- and of course, actually I should probably draw that, too, in fact, at some point it has to go back to relaxed to do it again (to beat again). But the trigger, for squeezing, is calcium! So it's easy to get confused when you're thinking about all the squeezing and relaxing- and all that kind of stuff. But, if you just keep your eye on calcium and think about the fact that calcium is the trigger, then you'll never get confused. You'll always be able to kind of find your way in terms of where the heart is in its cycle. I'm going to draw for you the heart cycle- specifically, the cycle of an individual cell. This is what one cell is going to go through over time. And the heart cycle or the cycle for a cell- a heart cell- is going to be measured in milliVolts. We're going to use milliVolts to think about this. And you could use, I guess, a lot of different things. But this is probably one of the simplest things to kind of summarize what's happening with all of the different ions that are moving back and forth across that cell. Now, the major ions- the ones that are going to mostly influence our heart cell- are going to be calcium, sodium, and potassium. So, I'll put those three on here. And, I'm putting them, really, just as bench marks, just so you can kind of keep track of where things will like to be. So, calcium will like to be at 123mV; sodium at 67mV. And what that means is that if these were the only ions moving through, then sodium would like to keep things positive. Potassium, on the other hand, would like to make the membrane potential negative. So this scale is actually the scale for the membrane potential. And, if we move up the scale- if we go from negative to something positive- this process would be called depolarization. Depolarization. That just means going from some negative number up towards something positive. And if you were to do the reverse- if you were going to go from something positive to something negative- you would call that repolarization. Repolarization. So, these are just a couple of terms I want to make sure we're familiar with because we're going to be able to, then, get at some of the interesting things that happen (I'm going to make some space here) inside of this cell. So, let's start with a little picture of the cell. So, let's say that this is our cell here. I'm going to draw in little gap junctions, which are little connections between cells. So, maybe a couple there, maybe one there, maybe one over here. And let me label that. So these are the gap junctions. Gap junctions. And also, let's draw in some channels. So we have, let's say, a potassium channel right here. And we know that potassium likes to leave cells. So this is going to be the way that potassium is going to flow, and it's going to leave behind a negative membrane potential, right? Let's say, potassium is the main ion for this cell- which it is- then our membrane potential is going to be really really negative. In fact, if it was the only ion, it would be -92mV, but it's not, it's actually just the dominant ion. So it's over here, and our membrane potential is around -90mV. It continues around -90mV, so let's say, nothing changes over a bit of time. So we stay at -90mV. This is what things look like, right? With the dominant ion that our cells are permeable to being potassium. Now, a neighbouring cell, let's say, has a little bit of a depolarization- so it goes positive. And through the gap junctions, leak a little bit of sodium and some calcium. So these stuff starts leaking through the gap junctions, right? Now, what will happen to our membrane potential? Well, it was -90mV, but now that we've got some positive ions sitting inside of our cell. Our cell becomes a little bit more positive, right? So it goes up to, let's say, here. And it happens pretty quickly. So now it's at -70mV up from -90mV. So at this point, you actually get (I'm going to erase gap junctions) but now that you're at -70mV, you'll actually get new channels opening up. I haven't drawn them yet, I'm going to erase sodium and calcium just to make some space. But you get new channels opening up. And these are going to be the sodium channels (so let me draw those in). Sodium channels. There are so many of them! Lots of these fast sodium channels open up. I say fast because sodium can flow through very quickly. Sodium starts gushing in. You know that's going to happen because there is a lot more sodium outside of the cell than inside of the cell. And so, sodium gushes in. It's going to drive the membrane potential very quickly up to a very positive range. Now it would go all the way, let's say, close to 67mV, maybe not exactly 67mV because you still have those potassium ions leaving, but close to it. If not for the fact that these voltage-gated channels actually close down. So these sodium channels are voltage-gated. And they will actually close down just as quickly as they opened up. To show that, I'm actually going to do a little cut and paste. I'm going to just draw the cell here, and I'm going to move it down here. So we've got our cell just as before. And now these voltage-gated channels, they close down. So let me get rid of all these arrow heads. But we're already now in the positive range. So at this point you could say, our channels have caused a depolarization. Let me just quickly show these shut downs, so you don't get confused. There's no more sodium flowing through. You still have some potassium leaking out, but that's kind of as it has always been. And in addition to those potassium channels, that little channel I've drawn here. You have new potassium channels that open up down here. These are actually voltage-gated potassium channels. So you had them before. They existed, but they were actually not opened. So let me just draw little X's. The only reason they flipped open is because depolarization happened- you had a negative go to a positive. So now that our cell is in positive territory (actually let me write in +20 or so), our potassium voltage-gated channels open up. So these voltage-gated channels open up. You can guess what's going to happen- which direction do you think that the membrane potential will go? Well, if the sodium channels aren't gushing the sodium inwards and potassium is leaking outwards, now you're going to have a downwards depolarization or repolarization. So now, potassium is causing the membrane potential to go back down. And let's say it gets to about +5mV, and if it continued again, it would go all the way back down to -90mV. But, an interesting new development occurs. At this point, I'm going to actually cut-paste again. I'll show you what happens next, which is that calcium (this is the thing I said to keep your eye on the whole time, right?) calcium finally kind of starts leaking in. So, let me get rid of this. This is the key idea, right? So we still have..I don't want you to forget that this is potassium, so we still have potassium over here. But now calcium leaks in. Let's draw that over here. So you have these calcium voltage-gated channels that allow calcium to come in. So you've got calcium coming in, potassium leaving. Now think about what will happen in this situation. So calcium is going to want to rise the membrane potential this way. Potassium leaving is going to want to continue going down this way. Because both are happening simultaneously, you basically get something like this. So you get kind of a flat line. So because both events are happening- both potassium leaving the cell and calcium entering the cell, you get this kind of flat line. The membrane potential stays kind of around the same. So we can just write something similar. Something like +5. Just so we're clear, these are also voltage-gated. Voltage-gated calcium channels. So to round this out, then what happens after that? So you have, so far so good, we have all these channels coming in to our cell, and allowing different ions' passage. Now we get to something like this. I'm going to try to clean this up a little bit. What happens is that the calcium channels actually close just as suddenly as they opened. So now you don't have anymore calcium coming in If calcium was the only thing I was keeping this membrane potential going flat. I said that potassium makes it want to go down, but calcium was making it flat. Well, what will happen now? If again you have just those potassium channels open, well then you're going to have the membrane potential go back down. It's going to go back down to -90mV or so. This is kind of the last stage, where those potassium channels are going back down. Those voltage-gated potassium channels also close at this point. So finally, they close down as well. So now they're closed, you're going to finally get back to just your initial state, which was having a little bit of potassium kind of leaking out of the cell and those voltage-gated channels are shut down now. Now that you're -90mV, you stay down there. This process is ready to begin again. The last thing I want to say is the stages- how they are named. So this is Stage Four, this kind of baseline negative state that the relaxed muscle cell is in. This action potential, when it finally fires and hits that -70mV, is actually considered the threshold. This is our threshold. When it gets to that point, we call that Stage Zero. On the other side of Stage Zero, you have Stage One, Two, and Three. Stage One is that point when just the potassium channels first open up- the voltage-gated ones. Stage Two is when they're balanced with the calcium channels. Stage Three is, again, when you have just potassium channels (voltage-gated ones) that are open. Then you get back to Stage Four again- so this would be Stage Four. Because Stage Zero is happening so rapidly, because this is so fast, we actually call this a fast action potential. So compare that to how the action potential goes in the pacemaker cells, where it's much slower, this fast action potential is the result of those really really amazingly quick voltage-gated sodium channels.