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Current time:0:00Total duration:10:25

Increasing ventricular contractility - inotropic effect

Video transcript

I'm going to start out by showing you the membrane potential of a cardiac cell and I know you've seen this a few times now so you might be getting kind of tired of it or might be seem kind of familiar that's good that's good that you you know this by now but let's just go over just in case you need some refreshing so if you have a heart muscle cell and let's just make sure we're totally on the same page this is a heart muscle cell or a myocyte if you have one of these cells and you're looking at it usually it kind of hangs out in a negative membrane potential meaning the cell is negative relative to its environment and it kind of hangs out there for a while and then we know that at some point it's going to get some positive charge from a neighboring cell it's going to have an action potential it's going to go really positive and then it's going to peter down a little bit as the potassium channels let out potassium and then it's going to go through this kind of interesting plateau where calcium is rushing in potassium is rushing out and finally it kind of goes back down as potassium wins the game when potassium kind of drives it back down to it's kind of happy place what it likes to be and around negative 90 or so so these are the phases of an action potential and we know that they're numbered off this is phase zero or for rather phase 0 and this is one two and three so these are the kind of the normal phases and how we kind of count off what it would look like and what I want to do now is kind of draw your attention to the heart and let's not lose sight of kind of what the whole organ looks like and this is kind of our four chambered oregon this is our heart muscle with the two ventricles down here and our two atria up top and this is our right atrium left atrium right ventricle and left ventricle so this is what it looks like right and there are actually nerves that come and sit on different parts of this so there's a nerve that might come and sit right there and this might be a parasympathetic nerve I'll write P for parasympathetic and actually parasympathetic nerves also come over to the left atrium they settle in on left atrial tissue as well and you also sympathetic nerves that come and settle in over here and on the other side as well now as far as the ventricles go you really only have sympathetic nerves down here so that's an interesting thing in order to point that out to you so you really only have sympathetic nerve stimulation you don't really have much parasympathetic activity down here so I'm going to focus in now the rest of this video is actually going to just focus in on the ventricles I'm actually going to just kind of ignore what happens in the atrium because the main point I want to make is that sympathetic activity on the ventricles is going to cause increased contractility meaning you're going to be able to cause increased force of contraction and why do I not care as much about the force of contraction of my atria well it's because the atria are going to be used to help fill up my ventricles but my ventricles the force of contraction of my ventricles is really important because that's going to affect how the blood gets to the the rest of the body into the lungs and so that's why I want to focus in on just the ventricles for the rest of this talk now I'm going to draw a ventricular cell so this is a ventricular muscle cell let's say it's right here this is this little guy over here where the X is so this ventricular cell I'm actually also going to even be more specific I'm going to focus in on what happens in phase 2 and 3 phase 2 and 3 of this cell so our ventricular cell it's going to in phase 2 have some channels it's going to have some potassium channels voltage-gated potassium leaving it's going to have some calcium channels some voltage-gated calcium channels letting calcium in and you remember actually when that calcium comes in we talked about the fact that there is this sarcoplasmic reticulum this is our sarcoplasmic reticulum and the sarcoplasmic reticulum is kind of a bag of calcium and so the sarcoplasmic reticulum is going to wait patiently for a little calcium to come and bind to its channel and the moment it does it's going to start letting calcium out so this thing is full of calcium and it's going to start shooting into the cell into the kind of cytoplasm of the cell so these are kind of the changes that that normally take place in phase 2 and 3 you have calcium rushing in and you have the sarcoplasmic reticulum dumping calcium out and you also have it when I say out out into the cell and then you also have potassium actually leaving the cell entirely now if you have sympathetics sympathetic nerves let's say this is my sympathetic nerve I'm going to write s4 sympathetic well maybe I'll just write it out just to make sure that we don't have any confusion this is my sympathetic nerve my sympathetic nerve is going to have let me make a little bit of space here is going to have some neurotransmitter neurotransmitter in this kind of space in here and that's going to bind to a receptor that receptor is going to send a message into the rest of the cell and this neurotransmitter that's doing the messaging and we actually oh by I almost switched it I was going to write a cetyl choline but I mean to write as norepinephrine norepinephrine and acetylcholine just as a point of reference is the neurotransmitter that the parasympathetic nerves use so I want to make sure I don't screw that up so norepinephrine goes into the cell and what does it do well it's going to cause an effect here it's going to make the calcium Russian even more forcefully it's going to activate these channels so that they let out more calcium when they get a chance to do that as well and so these are two major changes right it's going to activate more calcium coming in or basically activate these channels so more calcium can rush in and it's going to let more calcium out of the sarcoplasmic reticulum so my curve is going to start looking like this calcium is going to make this rise it's going to rise because remember calcium wants membrane potential to go up and the reason that it goes down eventually is because of potassium so if you have more calcium rushing in it's going to quietly start winning the battle so it won't be a plateau anymore it'll start kind of looking like I just drew it now a second thing that happens with sympathetics and this is actually very interesting is that you have these ATP controlled channels or proteins really that allow calcium back in so these are transporters are going to get calcium to come back in and of course this is going to happen when the sarcoplasmic reticulum is ready to kind of mop up all of the calcium and let it let it re-enter the SR the sarcoplasmic reticulum so when that's happening usually you get a decline like like what I've drawn in Phase three but if you're going to stimulate that and that's exactly what happens the sympathetics kind of stimulate that well all of a sudden now the calcium can get quickly put back into the sarcoplasmic reticulum and if it quickly goes back inside then the calcium current Falls more rapidly and the potassium current dominates even more than it usually does so basically what happens is instead of having kind of a slower phase three you have a rapid phase three and that is because of the fact that you're able to get the calcium more more quickly and more efficiently tucked into the sarcoplasmic reticulum so at the end of the day you see some interesting things right you see that you're able to kind of quickly get back to baseline and as a result this distance goes down so you actually have a smaller contraction in the sense that let me rephrase that completely so don't confuse you you don't have a smaller contraction you have a shorter shorter in terms of time contraction but you have more calcium that's actually coming into the cell so these are two key changes I want to point out to you the fact they have more calcium coming in but it's a shorter period of time so those are the effects of the sympathetic nerves on the ventricular muscle cells so you can see now that you're going to have a change in ina tropi minor tropi and inotropy just means a change in force of contraction or affecting the force of contraction and here we're talking specifically about the ventricles but those are the chambers I said that we care more about kind of in this scenario and so I know tropi is really shown to be affected right there so you can see right there you have more calcium and more calcium means more force a more force of contraction and that's because calcium actually directly affects the mechanism that a cell uses to contract and we'll talk about that in future videos exactly what that mechanism might look like but really that increase in calcium is is a demonstration of the inotropic effect of a sympathetic nerve and this effect is actually showing you that the ventricles can repolarize more quickly so this is more that's a rapid ventricular repolarization so that the ventricles are actually kind of reset and ready to fire again more rapidly rapid repolarization so these are the two major sympathetic nerve effects on the ventricular muscle cells