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Current time:0:00Total duration:15:40

Video transcript

so we're going to talk about pacemaker cells and these are actually really really cool cells because these are the cells in our heart that basically keep all of the heart beating both in a certain rhythm and at a certain pace right so these cells are going to work from the moment that you're conceived and a little fetus in in the womb all the way to the point where you die so these pacemaker cells have a property and we call that property automaticity automaticity actually has the word automatic right in it right so automatic and all that means is that you don't actually need a neighboring cell to tell this guy that he needs to fire an action potential pacemaker cells can do it themselves such a pretty pretty neat thing and when you think about it these are the cells where everything really begins in so there's three clumps of pacemaker cells or three groups that we talk about one would be the group of cells sitting in what we call the sinoatrial node SA node and this is probably the most common group that we think of but there's also some pacemaker cells in the atrial ventricular node or AV node so this is another group of potential pacemaker cells and finally there's a third group in what we call the bundle of His and Purkinje fibers now I know this sounds like I'm speaking a new language because I'm throwing out a lot of words that really may not make a lot of sense but all that you need to know about these three are that they are just in different parts of the electrical conduction system of the heart so they're all parts of the electrical conduction system and they're just a little different locations in the heart so they make up part of the electrical conduction system and they are all kind of given this special property of having the ability to paste the heart let me make a little bit of space now so really understand these three groups of cells and what you know the kind of magic they can actually bestow upon us let's let's talk in terms of how heart cells really talk and think and and they don't really think kind of the way you and I might think they think in terms of voltage this is this is kind of their language and so to understand them let's let's use it so this is millivolts and let's say this is positive and let's say this is negative because that's by most intuitive we have a few ions that are going to pass into and out of cells and you know that ions are going to help determine the voltage of a cell and we've - we've talked about that in other videos and so let's say this is calcium right here this is calcium so why am i drawing an arrow with a voltage up there 123 what that means is that if calcium was the only ion moving into and out of a cell then the cell's voltage would be 123 so it really just tells you what would happen if that was the only ion that had the ability to permeate a cell now let's say sodium was the only ion able to permeate a cell go into and out of a cell then the membrane potential would be 67 so it'd actually be almost a little bit more than half and then finally if let's say potassium was the only ion that could get into and out of a cell that's potassium down here then the membrane potential would be negative 92 so potassium likes things to be more negative now in real life you would have cells and I'll draw a cell right here that actually are permeable to multiple things right there not just permeable to one ion and let's say for argument's sake that you know it's half permeable to calcium in half permeable to sodium well if it's exactly half in half then your membrane potential would be somewhere in the middle somewhere like there and that works out to about what does that let's see if I can do my math quickly ninety seven or so so around ninety seven maybe 96 so that would be about 96 millivolts because it would be half in half so it would be split between the two now let's say that it was only permeable to or let's say 99 and permeable to potassium and 1% permeable to sodium well then it would be down here very very close to potassium is probably like 90 negative 91 so depending on how permeable it is to what ion you can kind of predict roughly what the membrane potential is going to be now let's start out let's say that our cell here is going to be one of these pacemaker cells and it's permeable to just salt actually you know what before I do that let me tell you what its voltages let's say it's voltage is negative 60 and we won't tell you how it got there we'll figure that out later but it's negative 60 and I tell you that it's permeable to just salt or maybe not just salt but predominantly salt and we know salt is going to want to rush into the cell because there's a lot more salt on the outside than on the inside now if that was the case if it wasn't negative 60 and never and put aside the thought of how did it get there in the first place but let's say it was there what would happen if it was permeable to to primarily salt well it's going to want to eventually get up here right it might take some time which is why I draw drew it all the way out there and remember this is our time access might take some time to get there but it will eventually want to get there right it'll eventually want to get over to close to positive 67 if sodium is the major ion that it's permeable to so it's going to start in that direction and actually that's about exactly what happens it starts kind of marching towards that point now it gets a little bit further along and from negative 60 so it's like let's say negative 40 and then interesting thing happens it doesn't just continue to that purple dot let me erase that purple dot now it doesn't continue there but it actually hits a threshold now when I say threshold you'll see in just a few moments what I mean but it hits this threshold and this threshold is for a new type of ion so let me actually switch it up I'm going to actually save myself some time by just cutting and pasting this like that I'm going to move it over here so this is my cell right and now I got to negative 40 and a new channel emerges so we have this channel here for calcium and we have a bunch of them so calcium starts dumping into the cell and calcium just like sodium loves to be inside of the cell so now I've got a lot of calcium and what opened up these channels even actually voltage-gated channels that's actually why I said that there was a threshold because these channels I didn't actually draw them before they're there it's not like it's not as if the cell just made these channels out of thin air they were there the whole time but they were closed they were closed literally gated shut and so now that you hit negative 40 that's their ticket now they open up and they let all the calcium in so that's why we call it voltage-gated and that's why we say that there's a threshold this is the word threshold it really is talking about what is the voltage needed to open those calcium gates so now calcium pours in so now you can kind of step back and think about what will happen to our white line if calcium is the major ion that this cell is permeable to now I mean it's still a little bit permeable to salt you can see that but mostly calcium it's going to want to rise up to calcium's resting potential which is even higher than sodium's so instead of going kind of chugging along slowly it's going to start moving up in a nice kind of steady clip now it's going to go up nice and quick right so it's going to start getting more steep so it was going slowly and now it's going more steep and it gets to let's say positive 10 and now the next interesting thing happens so we said that these calcium channels are voltage-gated and that's what makes them open well the cool thing is that that's not only what makes them open it's also what makes them shut and so let me actually draw this one more time I'll just cut and paste it and now I'm put it over here and if that's what makes them shut then watch watch what happens now I'm going to actually erase this calcium because now these voltage channels are going to close down and we're going to have to show them close so this draw a little X's here so no more calcium can get in so you've got just that sodium and at the same moment that the calcium gated channels close that same moment the potassium voltage-gated channels open so now you have some potassium channels here that open up and the potassium is going to escape right it's going to leave because potassium loves to leave the cell it likes to get outside because that's the direction of its concentration gradient and so just like we said that the calcium has voltage-gated so does the potassium so these are voltage-gated as well and they're voltage-gated to open when it's a little bit more positive and just like I said you know earlier that these voltage-gated channels exist they certainly exist I just didn't draw them but they're closed so they were closed up until this point they were there the whole time so they were there in both scenarios right they're there and they just stayed shut so if the potassium is escaping what happens to our white line think about that well it's going to go towards now that potassium is the dominant ion always think in terms of what's the dominant ion in terms of permeability our cell is mostly permeable to potassium right now so the membrane potential is going to go towards potassium and potassium is way down here so it's going to start going down so the membrane potential starts creeping down creeping down creeping down and stops stops right here negative 60 well why did it stop why didn't it just go all the way down closer to negative 92 well just as the calcium ion a calcium gated channel shut so do the potassium so these ones actually shut down as well and I'm just going to erase this potassium at this point because now they're shut and I'm even going to put little X's through them so basically these are not open for business closed for business so now we have just the sodium entering well this looks a lot like how we started right and so what happens is that this process repeats itself so it basically will just rise up until thresholds the calcium gated channels flip up and open then they close and the potassium gated channels open and they close and we're back to just sodium so this is how we get our action potential this is how it forms and you can see that I didn't talk about any other cells this is a cell doing it all by itself and because the channels are constantly opening and closing we don't really ever think of this cell as having a resting potential it's just it has a membrane potential but it's never really resting anywhere it's always kind of on the move right it's always either rising or falling so now if my heart rate let's say my heart rate is 60 beats a minute right 60 beats per minute then what that means is that this right here this is one heart beat one heart beat is happening in one second right which i think is pretty sweet I mean all this stuff is happening in one second the sodium is coming in then the calcium is coming in and then that stops but then the potassium rushes out and then that stops and then the whole thing happens again and again and again every single second so this is how our heart is beating this this little cycle keeps going on and on and on and now people will actually talk about different phases of this or they're obviously three basic phases right that I've drawn out for us so there's this phase four this is called phase zero and this is Phase three now you're thinking wait a second this sounds totally wacky why would you call it phase four zero three what what sense does that make and I'll draw for you an example of how the heart muscle actually the action potentials in the heart muscle look and you'll see how this naming system came about I'm not trying to defend it because I don't think it's the best but this is at least how it came about so when heart muscle beats it looks like that doesn't look the way that we've drawn this one no get into that one some other time but that's what it looks like and if you were to number the different parts the numbers would be basically this down here is phase 4 and this up here is phase or this is phase zero and then there's phase 1 2 and then 3 and so someone kind of step back and took a look at this and said well this phase 4 looks a lot like this guy and this phase 0 this upward swing looks like this upward swing and this downward swing looks like this so that's how the phase 4 0 & 3 come about and they said well I guess these pacemaker cells they don't have phase 1 & 2 so let's just ignore those two numbers so that's why those two numbers are not included when I number 4 0 & 3 but but there is something actually kind of important I want to point out when comparing the phase zero here so in the pacemaker cell the phase zero right here it might seem pretty fast you know to you and I happens you know let's say about in a tenth of a second or in about the 2/10 of a second but in fact it's actually a little bit slower it's let me write it right here it's actually a little bit slower than what happens with the heart muscle this one is actually faster I'm talking specifically about phase zero so because it's slower and that phase zero is called the action potential this is a slower action potential and the other one is considered a faster action potential so sometimes you might hear that term the slow action potential cells or something like that and they're referring to the pacemaker cells when they say that last thing I should probably mention is that anytime you go up like this and you actually become less negative that's called depolarization I want to make sure that's really really clear so anytime you become less negative that's considered depolarization and anytime you become more negative like that that's considered repolarization repolarization so in this case we have phase 4 and zero are kind of slowly depolarizing and then phase three is repolarizing or so