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Current time:0:00Total duration:12:14

Changing the AV node delay - chronotropic effect

Video transcript

so I'm going to draw for you a quick figure to show you exactly what happens with the AV node so this is our AV node over time right and we've got 0 millivolts here positive and negative and you know the AV node is actually going to look really similar to what the SA node looks like but there a couple of key differences and I'm going to try to as I draw them I'm going to highlight them so one is that there's a shallow increase so this is of course the first phase this is phase 4 and it increases very very kind of subtly and then it finally gets to that threshold right so it finally reaches kind of this threshold point where the calcium channels the voltage-gated calcium channels flip open and it rises again but it's kind of shallow once again and it gets up to this point and then goes back down and so those doubts your phase zero right here and your phase one so what are the differences exactly well one difference is that there's a shallow phase for shallow phase for and why is it so shallow what's the reason behind that well remember that the AV node has a heart rate that it would like to set that's kind of relatively on the low side it's somewhere between 40 and 60 so compared to the SA node it's a lower heart rate which means it's going to be a longer heart beat so the fact that it's a shallow phase for kind of alludes to the fact that the AV node likes to keep a low heart rate right down arrow but I want to also point out that usually under normal circumstances this doesn't really matter a whole lot because the SA node is in charge of the heart rate and so even though it's a low heart rate we don't really care about this phase for usually because the SA node is in charge so unless the SA node is out on a holiday or something this doesn't matter so much because the SA node is in charge so that means that we can kind of draw our attention to the other two parts right because that means phase zero and phase one still matter and there are some or there's one key difference there that I want to point out and that is that you also have kind of a shallow shallow phase zero so think about that if the phase zero is shallow and we know this is the action potential right this is the action potential well what what implications does that have what does that mean well the slope of the action potential and this is actually kind of an interesting idea to get your head around the slope of the action potential is going to affect conduction velocity because really that's where the ions are kind of leaking into the neighbor cells so if this was really really steep you'd have a fast conduction velocity and if it's shallow like this one is you have a kind of low conduction velocity so the effect of this is going to be a low conduction velocity meaning that ions are taking kind of a while to get over to neighboring cells and trigger their action potentials so going from cell to cell to cell it's going to be kind of sluggish through that AV node and now you might be thinking we'll wait a second we've talked about this in a way before because this sluggishness is decreased conduction velocity this is the explanation we think for the delay so remember in the AV node you have a delay and usually it's about 0.1 seconds and this is the reason why because this phase is zero is going up so sluggishly so slowly that actually creates a delay between the atria and the ventricles a delay that it might seem initially that it's kind of a waste of time but really it matters because you want to create that delay so the ventricles don't squeeze too early so this is how you create that delay you have that shallow phase zero so now let me actually make a little space I'm going to ask you to think about something from the cells perspective so imagine now you have a cell I'm going to draw it out for you and this is our cell right here in our cell is kind of doing its own thing and letting we're going to actually look at phase zero actually let me write that here this is going to be phase zero so in phase zero what's happening well our cell has these voltage-gated calcium channels and that's actually really important so we talked about calcium channels and sometimes I'm not I haven't done a great job of kind of clarifying voltage-gated versus non voltage-gated but remember in phase four those calcium ions that are coming through kind of normal channels but these ones are voltage-gated meaning they kind of quickly flip on but then they also quickly flip off and so these voltage-gated calcium channels are going to let calcium in during phase zero alright so calcium is going to kind of flood inside the cell and that's the reason that you're getting that rise in the membrane potential right it's rising up up up and so the calcium is coming in and these cells interestingly have little receptors on them you might be kind of now guessing where this is all going to go these receptors are for a neurotransmitter that's coming from the sympathetic nerve so sympathetic nerves are actually coming down and landing on the AV node just as they did on the SA node right so they're kind of landing here and they're letting off their nor epinephrine and norepinephrine is coming in here and on the other side you have receptors as well so you've got little receptors on this side as well and there are also nerves here and as I said before I'm drawing it kind of as two different sides of the cell but you know that's just the way I'm drawing it that has nothing to do with the reality of it it's not like the cell actually organizes one side to be for the sympathetics and the other side to be for the parasympathetics but that's definitely how my mind kind of sees it just because I guess I'm a adversarial and you have a little signal coming in from the parasympathetic nerve on this side and a competitive signal coming in on this side and actually let me just make sure I'm super clear here this neurotransmitter is acetylcholine and so the sympathetic nerve is telling this cell to allow more calcium to come in quickly and the parasympathetic nerve is basically putting on the brakes and saying no don't let calcium come in quite so fast so these two are competing back and forth you make a little bit of space here so going back up to our picture I'm going to ignore phase four because we know again the heart rate is really going to be dominated by the SA node so we don't have to worry about phase four so much because really the interesting begins there so if the sympathetics won out then you'd have a rise I would go pretty quick like that and then it would fall like that okay and if the parasympathetics won it would actually be the opposite right it would rise more slowly because less calcium is coming in over a given period of time and then it's going to go down like that so really what we're looking at is the slope going up or the slope going down depending on whether that sympathetic or parasympathetic are in charge and now if I said that the slope is related to the conduction velocity if we remember we said that earlier the conduction velocity is going to be affected by the slope of the line well then basically what you're going to be doing is changing the amount of delay so now let me actually play this out and you'll see how cool this gets in just a moment let's make a little bit of space on our canvas so here we go if we have now three scenarios okay we're going to do a baseline scenario and we're going to do two scenarios with one sympathetics in control and one with parasympathetic in control and I'm going to show you kind of the number of heart you get so at baseline let's say you have I don't know let's say four heartbeats and this would be let's say these little white arrows represent atrial atrial systole so this is when the atria are contracting atrial systole and let's say in another color let me do blue this represents ventricular systole so this is when the ventricles are contracting so I'll be ventricular systole ventricular systole when the ventricles contract and we know that's going to happen about a tenth of a second later usually so this delay is usually about 0.1 seconds so if I was to kind of watch this over time I'd have of course another ventricle systole that there another one right there and a fourth one right there now if sympathetics are kind of driving this cell let's say I'm in a let's say actually I'm running right so this is a scenario where I'm running and I'm being chased for me I'm chasing someone and over the same timeframe what would happen well I'll have in this case I'll have more atrial systole x' right because my SA node is going to fire more frequently the heart rates going to go up so I'm going to have instead of just four maybe I'll have I don't know I'm going to off to see I guess I'll have maybe looks like six six heart beats and that's because the big difference is SA node SA node affects heart rate and if I was to draw in now my ventricles I would draw something like this and you see these ventricular systole are happening right after the atrial systole which is actually really interesting because this points out that I have kind of a smaller delay maybe my delays I don't know maybe 0.08 seconds slightly less than 0.1 second so now with parasympathetics parasympathetics you have basically the opposite problem or the opposite change shouldn't call it a problem it's not really a problem right you'd have let's say three heartbeats in the in this span of time we're following and if you were to see the ventricles they contract but they contract with a much longer delay so if you were to kind of measure out this delay instead of 0.1 seconds now this is let say 0.1 or maybe 0.2 seconds maybe it's double so you can see that the SA node had a change in heart rate and the AV node because of this sympathetic or parasympathetic Drive had a change in delay so these are the two kind of big changes that you see when sympathetic and parasympathetic nerves are acting on the SA node and AV node