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Current time:0:00Total duration:11:44

Depolarization waves flowing through the heart

Video transcript

okay I'm going to draw for you a box diagram of our heart kind of a quick schematic and this is let's say divide it in half and I'll divide it in half again because we know the heart has four chambers right and the blood enters in the right atrium goes down into the right ventricle and then it goes over to the lungs comes back out to the left atrium and left ventricle and then out to the body that's kind of the flow of blood and we know that the heart is a pump and it squeezes in a very nice coordinated way and where that signal for squeezing begins is right here so in the right atrium if you were to zoom in and look very carefully you would see a little clump of cells in those cells we call them the SA node that SA node is just a clump of cells and that sends off a signal very quickly after it starts the depolarization wave it heads off a signal to the left atrium through what we call Bachmann's bundle that's just the name for that little band of tissue so Bachmann's bundle is that little band of tissue that heads over the left atrium and then there are a few other bands that actually head down this way to this node of tissue so in the second node of cells and these paths are called the inter meaning between internodal tracts so you have these three intra nodal tracts taking the message or the depolarization wave over to the atrioventricular node so we've got a basic scheme or picture of now how the electrical signal gets sent through the heart and if these are the highways just remember that there's also signal that's sent through the actual muscle right so this is all in the wall of the muscle and so if you were to actually kind of zoom in you would actually see that these tracts are buried inside of muscle cells there's some cardiac muscle cells all around here and these muscle cells also get the signal right so they have signal going through this way actually show with the yellow arrow signal going through this way and through the electrical conduction tissue but there's also signal going this way right there's also signal going into the muscle itself and it's actually leaving the SA node to go into the muscle etc so you see how there's actually signal that's going into the muscle but there's also signal going through those through those blue or those light blue lines that represent the electrical conduction tissue and the key here the key issue is that the signal the depolarization wave going through the electrical conduction tissue is much faster and that makes sense because if it was going pretty slow or if it was going the same speed as the muscle then why would you even need it you wouldn't need it you could just have muscle this it's the the fact that the signal can be sent much quicker and that's why I was using the analogy of the highway through those blue bands is why we have them we want the signal to get quickly to the left side so the left and right atrium can track together and we also want the signal to quickly get over to the AV node so that's that's kind of an overall view but let's now zoom in and actually I drew this out earlier and you can see now exactly what this could represent so in blue now we've got here this is the SA node SA node and this is our let's say Bachmann's bundle so this is kind of the the cells that are going to take our signal to the other atria Bachmann's bundle and then you've also got these three internodal tracts right internodal tracts that are going to take the message down to the AV node so this is kind of a rough diagram and on the outside you thought you've got all the cardiac muscle all the the muscle cells are there so the two things you should notice right away one is that if you actually look here these are gap junctions and you can see that I've drawn gap junctions all over this diagram so you can see little holes and connections between cells and and all that means is that ions that are in one cell will actually start leaking into the next cell so if you've got a few positive ions in one cell they'll leak into the next cell and make it slightly less negative and that's actually really important for depolarization now the way I'm going to show depolarization is that all these cells they have a little negative sign in them right so these negative signs represent the membrane potential so for example we know that the negative sign in this muscle cell probably represents somewhere around negative 90 millivolts because that's usually where these cells like to be and inside these are usually somewhere around negative 60 millivolts but in both cases they're negative and if they go positive we call that depolarization electrically shade in the cell and that's how you'll know that that cell in particular has depolarized now the fact that all these cells are interconnected through little gap junctions it means that these cells are a functional sync system and I'll actually write that out here functional syncytium and sync system is kind of a funny word to spell and that's how it's spelled so functional sync system all that means is that these cells are basically they're mechanically they're chemically they're electrically connected to one another so really in a way it kind of starts to resemble an enormous muscle cell they're not actually one cell because they all have their own nucleus and there they're actually in other ways behaving like individual cells but the fact that they've got all these little connections between them allows them to act in some ways like one giant unit and that's why we when you look at it heart it beats kind of all as one it's because it's so well coordinated so let's let's focus in now on the depolarization wave so this is the kind of the whole point of this is to to show you that I'm going to make a little space note the depolarization wave how it happens Oh me as you write that at the top d polarization wave and you're going to actually see how it goes through depolarization wave so let's say that oh I mean not waz wave so now let's say that one of our SA node cells decides to depolarize and we know that they can automatically depolarize if they want to so let's say that this cell over here depolarizes so this is our first cell depolarizing and i'm going to shade in the cells as they depolarize we know that that means that they go from negative to positive so if that cell depolarizes what's next going to happen so these little positive ions specifically calcium are going to leak into these neighboring cells right through the gap junctions and those cells if they were already negative 60 they're going to start rising right there their membrane potentials will start trickling up as the positive ions go in there and at some point they're going to hit their threshold for firing and so they're going to fire off and become depolarized and when I say fire I basically mean become depolarized so they're going to become depolarize themselves because they hit their threshold for for doing so and so they become depolarized and then they have some positive ions again depolarization means you have lots of positive ions floating around inside of you they're going to have some positive ions I kind of float into their neighboring cells and so now more cells are going to kind of feel the effects of the fact that there is this depolarization wave that's beginning and so now these cells are going to fire and the SA node cells are fantastic at conducting this wave so the depolarization wave this is all about conduction from their own cell into a neighboring cell and they're really really quick about it so now more cells are getting depolarized now I'm going to pause quickly and I'll show you what happens in just a few moments so soon you might get something like this where now you've got more of the SA node cells have depolarized you've also got a couple of the myocytes have depolarized as well and so I've shown you four of the models I said of depolarize now and so you can see again that the signal definitely does get into the myocytes but what happens after that is that the monocytes they actually don't propagate the wave as quickly as the electrical conduction cells do and so you'll see that difference when I speed this up one more time you'll see how the signal definitely keeps moving through the electrical conduction system but the monocytes aren't as fast and so it doesn't move as quickly so let me let me speed it up for you one more time so now it moves even further along and so you can see now the signal is going along the electrical conduction route but still you haven't seen the Maya sites themselves propagate the wave and so as I speed this up one final time you'll see exactly how that might happen so this is what it would look like if we let it keep going in and so you can see finally for the first time we have some cells down here I'm going to circle in white and maybe even one up here that's actually getting a signal from a neighboring myocyte so so definitely depolarization waves goes through myocytes there's no doubt about it of course but what I wanted to show you is that you can actually move much further along using just this electrical conduction signal this way and going this way and down the internodal tracts in all the directions then you would if you're just using and relying on the muscle cells because they don't conduct as quickly so the depolarization wave is going in all directions but some directions are moving more quickly than others and also just keep in mind that when I say that ions are travelling between cells when it comes to the electrical conduction tissue most of those cells are going to be sending calcium ions to their neighbors but once you get into these cells these myocytes now you've got actually for example actually draw it in maybe a different color now you've got actually some calcium but also some sodium that's leaking in so here you've got some sodium leaking into these cells too so both calcium and sodium are going to be leaking between the myocytes whereas between the electrical connection tissue it's mostly calcium that's leaking between cells so positive ions are slightly different in the two cases so this is a depolarization wave and I think now you can kind of see how it looks in a slow motion or sorry I flipped it in a sped up view and I think it's actually pretty cool