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Current time:0:00Total duration:12:02

Video transcript

so let's talk about pacemaker cells I'm going to actually draw out the action potential for pacemaker cell and remember this is time over here and let's do it with millivolts this is positive up here and negative down here now our pacemaker cells let's specifically talk about the ones in the SA node these are SA node action potential and you know it starts out kind of negative and creeps up and that's mainly because of sodium sodium leaking into the cell and other ions are present as well but that's the major ion now it gets up to this point right where I'm drawing kind of a threshold and this threshold is for what well this is kind of the this dashed line kind of represents the point at which calcium channels start to open up and so they open up and causes the cell to become even more positive so it was already going positive it's going to go even more positive and it's going to get to about that point and then finally at this point those calcium channels those voltage-gated calcium channels closed down and potassium channels open up which causes the membrane to repolarize so these are the kind of three phases we've talked about right this is phase four we numbered it as phase four this is phase zero and this is phase one these are the three phases we discussed so now let's think about a little bit harder let's say that we view this and I think that's pretty reasonable thing to do view this is the heartbeat this is one heartbeat right and you know if we were to kind of keep this picture going basically you get another one of these and another one of these are just basically continue and this is what our heart rate then looks like right if you were just to look at a strip over let's say two three minutes it would basically be just one after another of these kinds of action potentials kind of stacked on each other so now if I was to take this heart beat and shorten it let's say I was to make it instead end of ending where it does let's say I ended it right there so that this whole thing kind of gets brought this way well it would crunch down my action potential and in for but what would that mean exactly I mean you might think well so what so it's a little bit more crunch down happens a little faster so what well what it means if you think about it is if the heartbeats are stacking on top of each other if you make the heart beat itself a little bit quicker slow a meaning takes less time to finish then the next one can start a little bit early and then that one will finish early and the next one will start early and basically at the end of a minute you'll have more heartbeats kind of fit in so by having a shorter heartbeat what you're really saying is that you're increasing the heart rate the number of heartbeats in a minute goes up so that's actually pretty powerful because we think about heart rates all the time but rarely do we think about exactly what that means for each individual heart beat and what it means is that each heart beat goes quicker now the opposite is true too right you can imagine actually extending this out let's say the heart beat actually goes a little bit longer you could extend it out that way and if the heart beat goes longer then that means that you can get fewer packed into one minute and that means that you're basically saying that you're reducing the heart rate so when I say I'm increasing or decreasing the heart rate really what I'm trying to say is that I'm shortening or lengthening the heartbeat so that's actually I think a pretty powerful idea now let's take it a step further let's actually do a little thought experiment let's imagine that this is one tenth of a second right here one tenth of a second and it may not be exactly one tenth of a second but let's just imagine it is and let's say I wanted to take a look at our cell at this point because that's where one tenth of a second has hit what would our cell look like let me actually just make a little bit of space on our canvas and draw out what our cell might look like at one tenth of a second and just to make sure like I keep everyone kind of on the same page this is what's happening in our cell our pacemaker cell at one tenth of a second so at this point you have a cell and so let me actually draw a blown-up version of our cell that might look like this and this cell is going to have ions flowing in it's going to have let's say sodium coming in and we know that this is the dominant ion let me draw let's say a few of them kind of get gushing into our cell and we also have some other ions coming in and you might think we'll wait a second I thought only sodium comes in and that's definitely not the case even though sodium is the dominant ion meaning it's this cell is mostly permeable to sodium actually calcium is actually leaking in and a little bit of potassium might be leaking out so you have other ions moving back and forth as well even though in this case sodium is the major contributor to the membrane potential so if that's the case now let's take another look at the membrane now let's take a look at this membrane and that's let me show you what might be out here you've got some receptors on this side and those receptors are for a neurotransmitter so there's actually nerves that come down and land right on our pacemaker cell and these are the sympathetic nerves sympathetic nerves and these nerves are releasing some neurotransmitter and this neurotransmitter I'm just going to try to label as I go is nor epinephrine norepinephrine so norepinephrine comes and lands on these receptors and is going to cause a signal into the cell and it's going to basically tell the cell to be permeable to these ions allow these ions to flow across the membrane so they say okay fair enough now on the other side you've got another set of receptors and of course it's not actually divided by one side and the other I'm just doing it just to kind of represent an idea which is that on this other receptor you've got other kinds of neurotransmitters landing and these right here are acetylcholine or acetylcholine now acetylcholine is also going to send a signal down here and this signal is coming from parasympathetic nerves parasympathetic nerve so you might have heard of sympathetic and parasympathetic nerves these are both part of the autonomic nerve system and the parasympathetic nerves are sending kind of an opposite message they're saying to the cell we'll wait a second don't allow so much permeability don't allow so many ions to go back and forth across your membrane so opposite messages coming in and as it turns out that they kind of balance and offset each other and so you get what I've shown you get some sodium coming in a little bit of calcium and a little bit of potassium leaving now if I was to actually show you now what could happen let me try to take a little shortcut here and do a little cut paste imagine that this happens something like this let's show you I'm gonna have to move this canvas up a little bit but let's say now you have more sympathetics let's say you have more sympathetics coming in than parasympathetics then you might get something like this where instead of just a little bit of neurotransmitter here let's say you get a lot more and let's say now this receptor is also firing and let's say you get a little bit of firing from this receptor well now you get all three receptors on the left and that really out balances the one receptor on the right so you're sympathetic drive here you could say is much stronger than your parasympathetic Drive and if that's the case if you're sympathetic Drive is much stronger then what's going to happen is you're going to have more sodium coming into the cell because again the sympathetics are trying to get more ion permeability so you have a lot more sodium kind of gushing in and you'll get a little bit of extra calcium too you'll get more calcium here too and you'll get more potassium leaving the cell so basically sympathetics are going to cause all of the ions to increase in the direction of movement so you're going to more sodium to come in you're going to get more calcium to come in and you're going to get more potassium to leave so that's interesting and let's actually just keep that in mind I'm actually going to do this one more time and show you what can happen if the opposite were true let's say that in this case you had more parasympathetic Drive so let's say here you have now in this third scenario remember the first scenario is kind of the baseline scenario and this third scenario now let's say you have more acetylcholine filling up these receptors and that's out doing what the sympathetic nerves are doing so now you've got a lot more parasympathetic stimulation well now this cell is going to think okay well the parasympathetic don't want as much ion movement so less sodium again this is all in one tenth of a second so if you just catch the cell at one tenth of a second less sodium has moved in maybe less calcium has gotten in and maybe less potassium has left so if you look at one tenth of a second the pictures the snapshots are really really different so in both scenarios sympathetics and parasympathetics it's the same ions they're moving in the same direction but what we're looking at is the amount of charge that's flowing over a period of time and sometimes you might even use the word current you might say well sympathetics are increasing the current and parasympathetics are decreasing the current the amount of charge that's moving over a period of time so how would this actually look on our figure so we drew a figure at the top how would this actually look on this figure well I'm going to use the colors red and green because that's kind of what we've gotten into using here so green remember that was our sympathetic scenario well what that's going to do is that's going to basically increase the amount of charge rushing in and at one tenth of a second you've got more positive ions the cell so let's say at that point you've actually already hit threshold and you might now fire in an action potential and it will come down just as before and your heart rate is basically going to go up because you've you've shortened the heartbeat and the opposite is going to happen with parasympathetic so with parasympathetics you're going to have a longer time to get to that threshold because again it's at one tenth of a second only a little bit of sodium and calcium we're inside and only a little bit of potassium at left and you're going to have the same roughly the same looking action potential as before and you've gotten a much lower heart rate now because the heart beat is much longer so you can see that the amount of current that's flowing is changing and so really we're tweaking phase four with our sympathetic and parasympathetic to change our heart rate