If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content
Current time:0:00Total duration:12:11

Video transcript

let's figure out how a heart squeezes exactly and 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 so 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 and when it's relaxed these proteins are all kind of spread apart and when it's squeezing down because each cell has to squeeze for the overall heart to squeeze these proteins look completely different they're totally overlapped and that overlapping is really what we call squeezing so this is a squeezed version of the cell and the first one was a relaxed version and the trigger that kind of gets it from squeezing and of course actually I should probably draw that to the fact that of course at some point is to go back to relax 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 this kind of squeezing relaxing all this 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 so 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 kind of go through over time and the heart cycle or the 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 benchmarks just so you can kind of keep track of where things would like to be so calcium would like to be at 123 millivolts sodium at 60 and what that means is that if it was if these were the only ions moving through then sodium would like to keep things positive and 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 do the reverse if you're going to go from something positive to something negative you'd call that repolarization repolarization so these are just a couple of terms I wanted to make sure that we're familiar with because we're going to be able to then get it 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 and I'm going to draw in little gap junctions which are little connections between cells so maybe a couple there may be one there may be one over here and let me label that so these are the gap junctions gap junctions and all so 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 and 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 negative 92 but it's not it's actually just the dominant ion so it's over here and our membrane potentials around negative 90 and it continues around negative 90 so let's say nothing changes over a bit of time so we we stay at negative 90 so this is what things look like right with the dominant ion that our cell is permeable to being potassium now a neighboring cell let's say now has a little bit of a depolarization so it goes positive and through the gap junctions leaks a little bit of sodium and some calcium so this stuff starts leaking through the gap junctions right now what will happen to our membrane potential well it was negative 90 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 a negative 70 up from negative 90 so at this point you actually get I'm going to erase gap junctions but now that you're negative 70 you actually get new channels opening up and I haven't drawn them yet and 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 and there's so many of them lots and lots of these fast sodium channels open up and I say fast because the sodium can flow through very quickly the sodium starts gushing in and you know that's going to happen because there's a lot more sodium on the outside of a cell than the inside of a cell and so sodium gushes in and it's going to drive the membrane potential very quickly up to a very positive range now we would go all the way let's say close to 67 maybe not exactly 67 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 open up to show that I'm actually going to do a little cut paste I'm going to just draw this 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 positive range so at this point you could say our channels have caused a depolarization and let me just quickly show these shut down 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's always been and in addition to those potassium channels that that little channel I've drawn here you have new potassium channels that open up down here and these are actually voltage-gated potassium channels so you had them before they they existed but they were actually not open so let me just draw a little X's and the only reason they flipped open is because the depolarization happened you had a negative go to a positive so now that our cell is in positive territory actually me right in positive 20 or so our potassium voltage-gated channels open up so these voltage-gated channels open up and you can guess what's going to happen like 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 so a repolarization so now potassium is causing the membrane potential to go back down and let's say it gets to about positive five and if it continued again it would go all the way back down to negative 90 but an interesting new development occurs at this point I'm going to actually cut paste again and I'll show you what happens next which is that calcium this is the thing I said keep your eye on the whole time right calcium finally kind of starts leaking in so let me get rid of this and this is the the key idea right so we still have I don't want you to forget that this is potassium so you still have potassium and that's same over here but now calcium leaks in and let's draw that over here so you have these calcium voltage-gated channels that allow calcium to come in so you've got good calcium coming in potassium leaving now think about what will happen in this situation so calcium is going to make wana rise the membrane potential this way potassium leaving is going to want it to continue going down this way and because both are happening simultaneously you basically get something like this you get kind of a a flat so because both events are happening both potassium leaving the cell and calcium entering the cell you get this kind of flatline and the membrane potential stays kind of around the same and so we can just write something similar something like positive five 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 cells or all these channels coming in to our cell and allowing different ions passage and now we get to something like this and I'm going to try to clean this up a little bit and what happens is that the calcium the calcium channels actually close just as suddenly as they opened so now you don't have any more calcium coming in and if calcium was the only thing that was keeping this membrane potential going flat you know we I said that the potassium makes it want to go down but the calcium was was making it flat well what will happen now well 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 negative 90 or so so this is kind of the last stage where those potassium channels are going back down and those voltage-gated potassium channels also close at this point so finally they close down as well and 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 this cell and those voltage-gated channels have shut down now so now that you're negative 90 you stay down there and this process is ready to begin again the last thing I want to say is the stages how they're named so this is stage for this kind of baseline negative state that the relaxed muscle cell is in and then this action potential when it finally fires and it hits that negative 70 and this is actually considered a threshold this is our threshold when it gets to that point we call that stage zero and then on the other side of stage zero you have stage one two and three so stage one is that point when just the potassium channels first open up the voltage-gated ones and then stage two is when they're balanced with the calcium channels and stage three is again when you have just potassium channels voltage-gated ones that are open and then you get back to stage four again so this would be stage four and 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 a result of those really really amazingly quick sodium voltage-gated sodium channels