hi everyone Dr Mike here in this video we're taking a look at cardiac Action potentials taking a look at the different cells of the heart and how they depolarize how they send signals in order for the heart muscle to ultimately contract let's take a [Music] look so to begin you need to understand that the heart itself is made up of two major types of cells cells that we call contractile cells these are the cells that can actually contract to allow for the blood to be pumped and conducti cells the ones that send the signal and can set the rate and Rhythm of the heart let's take a look at the heart that we've drawn up here so what we've got just to orientate ourselves here's the apex of the heart here's the base of the heart which is the top of the heart strangely and we've got our Atria and our ventricles here first thing you need to understand is that all of this here is myocardium that is heart musle cell this is contractile tissue this is the these are the cells that when we treat trigger an action potential they'll contract so the protein filaments inside will form a crossbridge and contract so they're the contractile cells but we also have conducti cells and the conducti cells include those of the SA node which we've got here which stands for sinoatrial node and we've got the AV node here which is the Atria ventricular node these two types of cells are known as conducti cells and they send signals and they basically set that rate and Rhythm of the heart they will spread that action potential or signal throughout the tissue of the heart setting the rate and Rhythm so we've got our contractile we get got our conducti so what I've drawn up here here is a conducti cell also known as a nodal cell let's write it down so this is a conducti cell also known as a nodal cell so SA node or AV node also known as a pacemaker a cell it sets the rhythm of the heart we'll get there in a sec and here we have a contractile cell contractile cell so this is myocardium or a myosite right specifically you'd say it's a cardom myosite all right we'll talk about these graphs in a sec that's the graph for the conducti this is the graph for the contractile first thing you must understand is that all cells of the body have a charge difference across its membrane now this is really important especially for excitable cells or excitable tissues excitable tissues include nervous tissue muscle tissue endocrine tissue they're excitable meaning they can be at rest and not do anything but if you excite them or trigger them they can then do something so neurons cannot send a signal or you excite them and they'll send a signal for communication muscle cells cannot do anything you excite them your muscles contract you perform work endocrine tissues they cannot release a hormone you excite them they can release a hormone like insulin into the bloodstream right makes sense so these are muscle tissues what we got all muscle cells specifically cardiac so they're sceletal cardiac and smooth the cardiac muscle cells will contract similarly to that of sceletal muscle if you understand sceletal muscle contraction now the thing is how do we generate this charge difference across the membrane here's the first thing you must understand that sitting outside of our cells we've got sodium and sitting inside the cells we've got potassium predominantly let's have a look at this right in the membrane of every single cell of our body we have a pump this very important pump is called the sodium potassium atpa pump it's called that because it uses ATP an energy source to throw what we do is we throw three sodium outside 1 2 3 so I'll draw it up we've got 1 sodium 2 sodium 3 sodium and it exchang exchanges it for two potassium 1 2 all right three sodium outside two pottassium inside that's three positive things outside two positive things inside if you were to compare the charge difference where would most of the positive charge be at this time outside so you would say that the outside of the m mebrane here compared to the inside is slightly positive comparatively right makes sense but this only contributes a small amount of charge difference what you're also going to find embedded in the membrane is another Channel and this channel is a potassium Channel now remember most of the potassium is inside the cell right because the sodium pottassium pump and this channel is leaky so the door and the lid is just cracked open a little bit and you know diffusion diffusion is the movement of ions down their concentration gradient so that means if we've got all this potassium inside where most of it is it's going to slowly diffuse out now think about that positive potassium is diffusing out carrying its positive charge with it making the outside even more positive compared to inside and this is how we generate what we term the resting membrane potential the charge difference because there's a difference of charge from the outside compared to the inside regarding voltage we call this polarized so the cell membrane is polarized remember that point let's write this down it's polarized so if somebody has polarizing views means it's different so the cell membrane by charge is polarized all right this isn't just what happens in a conducti or nodal cell this is actually what happens in all the cells of our body especially the excitable cells of our body so now we've generated this charge difference super important so I'm going to do it like this now not only like I said the conductor cells but the contracti cells have this charge difference too so let's write that up and it's a charge difference simply across the membrane it's not within the entire cell it's just across the membrane all right now what we want to do is this remember the heart needs to contract the heart unlike neurons or sceletal muscle right which need to be told to contract the heart can do it by itself it has intrin it has the intrinsic ability to spontaneously send signals to contract so let's start with the SA node this is known as the pacemaker cells that sets the rhythm of the heart can be between 60 to 100 times it sends this signal so how does it do it without anyone telling it what's going on well we've got the graph here what you need to understand with this graph is that at rest now I'm going to clarify this point at rest a conducti cell is probably sitting at around about -60 molts but the problem here is that conducti cells are never at rest then they don't have what we call a resting membrane potential a neuron does right a neuron will either fire a signal or not and when it's not firing a signal it has its resting membrane potential that it just sits at until it gets told to send a signal same with sceletal muscle but not conduct cells so what happens with conducti cells is they have a couple of more channels so they've got a channel here which is called a funny sodium Channel weird name it's called a funny sodium Channel now the funny sodium Channel similar to that leaky potassium Channel its lid is open a little bit and what that means is cuz we know all the sodium or most of the sodium is outside that sodium is going to leak in in so sodium is slowly leaking in which means at all times these conducti cells are drifting into the positive because it's bringing the positive charge with it right so we start low at -60 because the inside's negative compared to the outside but as we bring the positive sodium in it makes it a little bit more positive and it slowly brings it up towards the positive now here's an important Point -40 is what we term the threshold now when I say threshold it's basically a key it's a voltage key that will open a new channel right so once enough sodium has leaked in through these funny sodium channels and hits ne40 it opens another type of Channel now this is a type of channel of an ion we haven't spoken about yet calcium calcium most of it sits outside the cell but we do have calcium inside the cell which we'll talk about shortly once we hit -40 we open a calcium channel now the thing is the calcium channel is voltage gated so just like you've got a gate on the front of your house that needs a key to open it while the key isn't a physical key in this case it's a charge and the key must fit -40 molts and so that's the key once we hit -4 that's the key it opens the lid for the calcium channel once it hits -40 M and calci will diffuse inside now calcium channels are relatively slow right and so what that means is once we hit that -40 the calcium doesn't rush in super quick but it does go up until we hit around about positive 10 mills which means enough positive calcium has gone inside that it's gone from being negative to positive so remember we started off as polarized if you change that right it's now depolarized cuz now it's both positive so it's now depolarized which means this first phase that we've got here of both the sodium coming in that sodium coming in and the calcium coming in we called this first phase depolarization so now we've got depolarization happening that's important once we hit around about positive1 the calcium channels close and what we have is positive 10 is another key to open another Channel this channel are potassium channels and again at around about positive 10 give or take opens those channels we's most of the potassium inside so the potassium will leak outside carrying that positive charge with with it and what do you think that means it means it makes it negative inside again and it drops back down back to it being negative inside and positive outside so we go from being depolarized to repolarized so this side is repolarization so if I were to separate that out like this this side is repolarization and this side is d polarization what's the whole point of this the whole point is this right what we've done by doing this is we've thrown positive sodium inside and positive calcium inside now here's the great thing our noal cells of our SA node are connected to the mardum the muscle cells the contractile cells so here's a contractile cell they're connected through what we call Gap Junctions Gap Junctions we got a a gap Junction there and in actual fact this contractile cell is connected to the next through a gap Junction and so forth and what that means is this positive sodium and potassium what do you think it will do it'll diffuse they'll go down their concentration gradient where there's not enough of them and they'll bring their charges with them okay now they're bringing their charges with them and what do you think that means for this polarized membrane it will become depolarized itself now here's the thing that is the action potential this whole thing is the action potential for the conducti cell but it's different for a contractile cell and we'll explain why shortly so now the sodium and calcium are come in here's the thing we've still got the sodium potassium pump throwing three sodium out and two potassium in and we've got the Leaky potassium Channel throwing the pottassium out generating that resting membrane potential here so it has a resting membrane potential unlike the conducti cell that constantly drifts right so what's going to happen after this it drifts again because of these funny sodium channels being open and then the whole thing happens again it's just how and this happens 60 to 100 times a minute right let's say 80 times a minute it's just con so what this is called spontaneous depolarization it doesn't need to be told to do this it'll just do it because of these funny sodium channels so but for the contractile cell it doesn't spontaneous depolarized it does have a resting membrane potential which is at 90 it's at90 it's here so it cannot contract it cannot send a signal it cannot have an action potential and stay there but once this calcium and sodium start moving in it's going to make this part of the membrane slightly positive now remember the threshold this threshold is different it's around about -70 the threshold is a key remember so we've got the threshold here now once enough sodium and and calcium have moved in to hit that threshold -70 is the key to open a new channel now this channel is a sodium Channel and it's really fast it's a fast sodium Channel now remember the sodium's outside so what happens sodium rushes inside really quickly really quickly carrying that charge with it which means it goes spares right up super quick now once it hits and this is variable but around about positive 20 molts is what it's going to do is those sodium channels will close and it will open two other channels two channels open up right interestingly one is a calcium channel and one is a potassium channel so remember what did I say where's Calcium mostly outside where's potassium mostly inside Oh weird Okay so the potassium is going to leak out right down its concentration gradient carrying that positive charge with it so it starts to slowly drop but then we at the calcium channels open calcium channels are slow so the calcium will slowly move in but it hits a point where there's an equilibrium of positive things in and positive things out and it starts to form a plateau like this now this lasts for around about 200 milliseconds right so it's nice and slow because the calcium's moving in really slow then what's going to happen is the calcium channels will close and once the calcium channels close the potassium channels remain open and the rest of the potassium will leak out right so we' got potassium out and then here we've got both potassium out and calcium in at the same time and here we've got sodium in so remember the depolarization repolarization phase depolarization repolarization and that's what we call the plateau the plateau phase and the plateau phase is actually quite important and especially the fact that it lasts for a while is the absolute refractory period so the absolute refractory period is the time period in which you can't send another action potential why do we want it to be quite long for contractile cells because we need the time for it to contract but not only that it's connected to the next cell and how does a heart contract do each cells contract contract contract contract contract contract or do they contract together as what we call a cisum yeah it's like an orchestra they have to play together they set get a signal and they need to be timed and this absolute refractory period is important for the timing because what we want is for the muscles the mardum to contract in unison and it's because of this Gap Junction now we've got again the positive sodium positive calcium in we need them to move and send the next action potential to the next mardum and this break allows for it all to catch up and contract as a Sensi right so Sensi means many act as one and so let's think about another Point calcium all right calcium is used here for the depolarization phase and calcium is also used here as well but it's different the calcium is used here and it's slow influx and that's important to set a rhythm if it's really fast like this one your intrinsic heart rate is going to be too quick right so to use calcium in the depolarization phase because it's slower for the influx allows for you to set a constant rate and Rhythm that isn't too quick but calcium isn't used here for the contract our cells for that purpose calcium is used here because these muscle these are muscle cells they need to contract muscle contraction won't happen without calcium calcium must go into a cell until tell this muscle cell to contract so this Plateau phase is associated with contraction so this pretty much from here to here is when it the muscle contracts so that's and that has to do with calcium influx now remember that most of the calcium required for contraction actually doesn't come from outside it actually sits inside in these membranes that we call we used well we generally call in other cells endoplasmic reticulum but in muscle cells we call it the pyo plasmic reticulum the pyo is referring to flesh so it's referring to muscle psychop reticulum and in muscle cells the psychop plasic reticulum doesn't play the role that other psychop plasmic reticul play it stores calcium and so when the cell gets depolarized it will release huge amounts of calcium into the cell where the contractile units are right actin ayin and tell them to contract so the calcium here is used for the contraction aspect and the calcium here is used to create the Rhythm and the slower signals which means think about the clinical ations of this a conducti cell or a nodal cell set in the Rhythm if you block the calcium you're going to slow the heart rate because it makes the influx even slower so the heart rate slows but if you block calcium for the myocardial cells or the contracti cells it's going to reduce the force of contraction hopefully that makes sense so this is looking at the difference of the action potentials between the conducti or nodal cells and the contractile or my cardial cell of the heart I hope you enjoyed it hi everyone Dr Mike here if you enjoyed this video please hit like And subscribe we've got hundreds of others just like this if you want to contact us please do so on social media we are on Instagram Twitter and Tik Tok at Dr Mike todorovich at d r m i k e t o d o r o v i c speak to you [Music] soon oh