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Current time:0:00Total duration:14:58

Video transcript

so one thing I wanted to do was compare two types of cells that what the ones that we've been talking about most often the pacemaker cells and right next to the myocytes and you're going to start seeing some really interesting kind of similarities and maybe some differences as well between the two so me actually on this side will we'll do our kind of usual millivolt scale positive and negative right and we'll do the same thing over here and on the left we're going to do our pacemaker cell and on the left on the right well do our myocyte so let's do our pacemaker cell first and you remember the pacemaker cell kind of starts out somewhere around negative 60 and kind of creeps up right we've drawn this a few times now and then it hits its action potential and it kind of goes up more more rapidly at that point and then finally starts coming back down again and this is kind of the the pattern that repeats over and over and on the other side we've got our our myocyte our muscle cell and this is kind of the cell that does a lot of the heavy lifting as it were for the for the heart kind of what you imagine the heart cell might be so this minus site actually starts out a little bit more negative around negative 90 let's say it kind of is more flat initially right and then it hits its action potential and it rises much more steeply than the pacemaker cell so now that they're kind of side-by-side you can see the difference right and then it comes back down as the potassium begins to leak in except there's a new phase here because the potassium and calcium kind of offset each other and they begin to kind of have this Plateau and eventually the potassium kind of wins out and then you get repolarization where it basically gets back down to where it started so this is kind of a really rough schematic of what it would look like and let me actually now bring some space onto our canvas I'm going to make some space here let's have to draw the two cells so we've got a pacemaker cell over here and I'll draw the myocyte right next to it right and this is our myocyte and I'm drawing them the same size but that's just so that you can see things clearly so what's the first thing that happens with our pacemaker cell well you oh that when it's rising when that membrane potential is rising sodium is actually coming in right I'm going to draw a little set of lines here to represent sodium coming in so sodium is coming in here and then you get a rapid rise right you get that rapid action potential and for that I'm actually going to draw white lines to represent the voltage-gated channels so this is kind of my way of representing voltage-gated channels and here you have predominantly calcium coming in right and I'm going to stick and take a quick moment to pause here and say when I say sodium is coming in at calcium is coming in I don't want you to think that that's the only ion that comes in at that point I mean when I say sodium is coming in for example that's the main ion that the cell is permeable to but it's not the only one in fact we've even talked about the fact that when sodium is coming in and phase four that sometimes a few other ions are actually kind of leaking in and out as well so just keep that in mind when I when I kind of draw one ion it's not to say that that's the only one it's just to kind of make things a little bit more clean and clear so you can kind of get a sense for what's the overall gist of what's happening so okay so back to our kind of regularly scheduled program we have now another voltage-gated channel down here and this is our potassium that's leaving right so this potassium leaves and to make it really kind of clear and parallel let me actually go one step further in and drawn here the phases so we have phase four here and this is phase zero and this is phase one right these are the phases of our action potential and phase four the main ion we said is sodium phase zero the main ion is calcium and phase one the main ion is potassium now a cardiac myocyte initially has mostly potassium leaving right so the dominant ion here is going to be potassium leaving and that's what's setting that membrane potential and then you have in the action potential actually me switch colors to a voltage-gated white channel so these are the voltage-gated channels now have sodium entering the cell so if your sodium is entering the cell write that and then you have in phase one you have now some voltage-gated potassium channel so unlike the one at the top of this cell now you've got potassium leaving just as before but these are voltage-gated so they kind of flip open and flip closed based on voltage and then you finally have another ion over here coming in which is calcium so you have calcium coming in as well and so let's do the same kind of exercise we did before where you kind of go through and label the phases so we know we have phase four down here phase zero here one two and three right so this is one two and three and what would this potassium be well this potassium is the dominant ion in phase four and sodium is happening or sodium coming in is happening during that action potential during that phase zero and then potassium these voltage-gated channels they're kind of involved in phase one two and three right that's kind of a unique property of those channels and they're not the exact same channel family there are different families of channels but there are voltage-gated potassium channels are actually kind of involved in a few different phases and this calcium is involved in Phase two so now you can see kind of how the different channels are involved and also their action potentials kind of side by side now one thing I should also kind of point out is that in the Myo site and this is less true of the pacemaker cells and I say less true because they also have this but but it's unclear what the role is they have this thing called a sarcoplasmic reticulum sarcoplasmic reticulum and a sarcoplasmic reticulum i think of as a magnifier sarcoplasmic reticulum sometimes you'll see it as just SR reticulum a magnifier well what do I mean exactly well what happens is that the sarcoplasmic reticulum I think of it it's an organelle basically it's sitting inside of a cell it's an organ and this sarcoplasmic reticulum is a bag of calcium literally is a bag of calcium so this is sitting here with all these little calcium ions inside and what it's waiting for is a signal from the cell to say that some calcium has entered so once this calcium has entered what it does is it binds to a little receptor binds to a receptor right here and when it binds to the receptor calcium from the inside of the SR or sarcoplasmic reticulum is dumped out into the cell so why is that necessary if you have calcium already coming in from the outside why would you need more calcium coming from the sarcoplasmic reticulum this bag of calcium well what happens is that this bag of calcium can basically empty out really quickly so you can have just a few ions kind of trickle in from the outside as long as they bind to that sarcoplasmic reticulum and let it dump out then you get tons of calcium kind of flooding into the cell but basically magnifies the effect of calcium right so just keep that in mind is that when we talk about Phase two and this calcium entering that the one thing that I haven't really talked to you about until right now is that there's this magnification that happens because of the sarcoplasmic reticulum all right now let's let's bring up kind of the main reason I wanted to do this video which is when you have all these ions kind of floating in and out you may be wondering well how in the world is the cell actually reset itself I mean at some point doesn't it need to kind of get things back to the way they were otherwise you'll just run out of sodium and calcium on the outside and you'll just fill up the the cell with that stuff in other words is sodium and calcium or just keep coming in and potassium keeps leaving in this pacemaker cell at some point won't won't you have no gradient left and so how do you set up those gradients I mean that's the real question right so you'll remember that there are these little ATP pumps and a pink pump is basically going to be for me my code for using energy and it's going to be throwing out it's gonna be throwing out three sodium's and bringing in two potassiums and there's also a little pump little pump over here that does something very similar but for calcium and what it does is basically kind of just boots out calcium says see you later buddy so this calcium leaves over here and actually there's another strategy there's another pump right here I'm going to draw it right here that also gets rid of calcium but this one is not pink this one does not take energy so you're thinking well how in the world did the first two take ATP how do these two take ATP and this third one not well you can think about the fact that there is a sodium gradient sodium likes to get inside the cell we know this right and it likes to be inside the cell because you set up a sodium gradient because of this right here so if you've created a sodium gradient using energy you can also use that sodium gradient to drive out calcium so you have a couple of mechanisms to take care of our ion problems so for example we needed to figure out how to get rid of this sodium and we've got our answer right there we had to get potassium back into the cell and we did that right there and we also have to get rid of all this calcium that keeps coming in and we did that right there and there so this is how our cell takes care of those ions now what about our myocyte how does that work out so how does it kind of reset all the concentration gradients after all this stuff happens is I described above well it also of course has you know our two ATP using pumps right so these two are using ATP and it's going to also drive sodium out and let me just draw that right there so three sodium's leave this is all looking kind of the same right to potassium Center and already have kind of solved some of my problems and it has just as before has this kind of sodium calcium pump so calcium exits right there and sodium enters right there so kind of the same answer is above but there's or as to the left but you also have this remember the sarcoplasmic reticulum right and the sarcoplasmic reticulum I said was loaded with calcium loaded with calcium well how did it get loaded with calcium how did that even happen well the way that it happens is that there's actually a pump here a pump let's draw it right here on this side that basically pumps using energy again using energy pumps calcium inside so you can actually pump calcium into the sarcoplasmic reticulum using another ATPase that's very similar to the one that's on the membrane they look literally the same right except this one is on the sarcoplasmic reticulum and doing our same kind of checklist you can see that look sodium is pumped out and that takes care of this guy and then we have to bring potassium back in and that's done there and then we have to get rid of all this calcium and all this calcium that kind of came in here and here how do we take care of it while we pump it out there we pump it back into the sarcoplasmic reticulum there and we can exchange it for sodium there so this is how we kind of take care of and and literally reset our cells now the final thing I want to say is that if you actually think about it if we're talking about permeability let's say we want to talk about you know whether more calcium is coming into a cell or out of a cell well usually under most circumstances we think of these guys all these kinds of energy-driven processes or or using concentration driven processes kind of operating it at a certain rate right there they're kind of always operating a certain rate and the same is true for these guys right we only have one extra thing over here but they're kind of operating a certain rate now if I increase the permeability let's talk about calcium for example of calcium let's say I increase the permeability of calcium that means more calcium is kind of entering that space and if I decrease the permeability of calcium I'm talking about this channel right here but decrease the permeability of calcium that means less calcium is entering that space now would you except the following what if I told you that I'm not even going to change let me erase that little line I drew I'm not even going to change how this permeability is going to function all I'm going to do is what if I change this this is kind of an interesting idea what if I change how fast this guy works how fast he works well if he starts working more sluggishly let's say use working more sluggishly then you have more calcium kind of hanging out over here right you have more calcium hanging out in the cell and let's say you make him work really fast really really fast so he's pumping that stuff back in well then you have even less calcium now you have the opposite you'd have less calcium kind of out here and so you can actually now see how by changing the basal rate of these pumps you can also affect the amount of ion that's hanging on in the cell and that of course is going to affect the the membrane potential of that cell so just keep that in mind is that usually we think about this we're usually thinking and talking in terms of this stuff up here what is the permeability what ions are coming in and out and we kind of assume that this is static that this stuff is not changing too much but every once in a while you'll see that resetting the membrane or resetting where the ions should be you can actually tweak those mechanisms as well you can actually make some of these pumps work harder or less hard and that's going to have an effect on the amount of ions as well