Heart Muscle (myocardium) Action Potential | Cardiology

in this video we're going to have a quick look at depolarization and repolarization of curvy myocytes so heart muscle cells and have a look at how these depolarization repolarization events give you a readout on an ECG okay now what I've drawn up here is a pretty dodgy version of a heart as though it's been coronal section so you can see the for atrium there so that two atrium and the two ventricles and you can also see you've got the myocardium all around here and I've also drawn this thing here in blue which is this fibrous tissue that separates the atria from the ventricles okay it's important a part of the heart and I'll show you why in a second now before we begin with that when this heart contracts so you need to remember that first the atria contract pushing the blood down through the trying bicuspid valves then the ventricles contract pushing the blood up and out by the arteries okay which means the heart contracts weights contracts now in order for it to contract a signal needs to be sent just like in order for me to speak to you and order you to hear me signals need to be sent through our neurons in order for the heart to contract signals also need to be sent and they're very very similar signals okay now if you take a cell in the body any cell in the body doesn't really matter what you're going to find is that there is a difference in charge from inside that cell compared to outside that cell okay so but I just take oneself what you would find is if I would have put an electrode inside and outside and measure the charge difference it's going to be a little bit negative inside the cell compared to outside the cell now why is this the case two reasons first off all the cells in your body have a pump sitting in their cell membrane now what this pub does is it takes 1 2 3 sodium from inside the cell and it pumps it outside the cell at the exact same time it takes 2 potassium from outside the cell and pumps it in now have a look at these individual ions remember ions are charged atoms or elements these are atoms sodium atom potassium atom and they're charged so they're an ion 1 2 3 positive sodium 1 2 positive potassium if you throw three positive things out but only bring two positive things back in you have a net positive charge outside the cell compared to inside that's one reason why it's slightly positive outside compared to inside but that only counts for a difference of negative five okay now the question you may be asking is what is the difference well it's a bit in a heart muscle cell the difference is negative 90 so it's negative 90 inside the cell compared to outside this pub here which we call the sodium potassium ATPase pump only contributes to around about negative 5 millivolts okay so what constitutes the other 85 millivolt as well because we now have established a gradient of sodium outside the cell and potassium inside the cell you understand diffusion and you know that all this sodium outside the cell is high concentration of sodium what's the diffuse to where there's low concentrations of sodium where is that if there's a lot of sodium outside the low concentration is inside so this sodium wants to diffuse in now but see we have now generated a high concentration of potassium inside the cell and it wants to diffuse to where there's a low concentration outside the cell so it wants the diffuse out so sodium also diffuse in potassium wants the diffuse out but remember the cell membranes are made up of fats and fats don't like charged substances so the potassium just bounces often stays in and the sodium bouncers often stays out so what that means is in order for the sodium to get in and potassium to get out that needs two big doorways there's two B channels and we do have these channels we have sodium specific channels and we have potassium specific channels now what you'll find is that these sodium specific channels at rest so what I'm saying is when this cell so remember what is this cell in this situation this cell is taken from the heart muscle okay so it's a muscle cell what does a muscle cell wanna ultimately do contract so when it's not contracting it's not doing its work it's function it's at rest what you'll find is that these sodium channels are closed okay so that means that the ceiling that's outside can't get in it's trapped outside the potassium cells are the potassium ions when this cell is at rest are also closed but some channels remain a little bit open like a door that's just creeped open a little bit and what this means is some potassium can leak out of the cell so that means that we have some positive stuff leaving the cell and because this door is open a little bit positive potassium continues to leave the cell bringing its positive charge with it making it even more negative inside compared to outside and finally giving us that negative 90 millivolts of difference that we're after now why am I telling you this I'm telling you this because first thing is this you need to understand that when a cell is at rest doesn't matter what cell in the body what a cell is at rest Eunice's particular scenario to another heart muscle cell it is negative inside the cell compared to outside this difference is called the resting membrane potential so it needs to be negative inside compared to outside when the cells are pressed this has to be the case because if it's not that a heart muscle cell cannot contract okay now what I'm going to draw up actually is if I were to take a histological sample from this heart muscle so I'll just take a biopsy I'll just have a look at it under the microscope you would see that this heart muscle is made up of many muscle cells and these muscle cells are actually connected to each other okay that'll do it and then connected through a little channels which we call gap junctions now this is important so let's just say I want to stimulate this first cell okay now remember the point of these cells is to contract and I said that all these cells need to be negative on the inside compared to the outside and they are okay that's at rest so what that means is we have a concentration gradient of sodium outside the cell and potassium inside the cell okay now if we were to stimulate this first cell okay will to do something and doesn't matter what that stimulus is yet we'll talk about in the future will stimulate this first cell excuse me what you're find is that this stingless will open up a sodium channel so remember I said all these cells have sodium channels I also said all these cells also have potassium channels which are drawn shortly but if I were to stimulate this first cell a little bit what you'll find is the lid for this first channel would pop open and that means that a little bit of sodium can trickle inside of that cell now remember sodium is positive so if that little bit of positivity moves inside the cell it makes this area that it's moved into positive ok now why is this important well I'm going to just make one line of these cells why is this important it's important because if we were to have a look at a graph which we all love doing looking at graphs but very quickly we would have a look remember I said at rest this cell is negative 90 millivolts let's just put some other values here let's put negative 70 now let's put positive 20 okay now at rest if I was to measure the voltage inside I said it was negative 90 but if I like stimulate the cell a little bit a little little bit of sodium in because it's positive it makes the inside now positive so from negative 70 it starts to move up towards the positive side and as more sodium comes in it becomes more positive now if it becomes positive enough that it hits the charge of negative 70 this is the threshold so I've stimulated it some sodium's come in it's gone from negative 90 to negative 80 and now it's negative 70 negative 70 is a key the rest of these sodium channels are gates and or gates need keys so that means these gates will only open up if the key is a charge and the charge is negative 70 so these gates will not open up unless it is negative 70 inside the cell if it wriggled in enough sodium we've now made it positive enough from negative 92 negative 70 and the channels open so it's hid negative 70 all the lids pop open for this sodium channels and that means sodium rushes into the now think about what that means we know all this positive sodium rushing into the cell so fully draw this on the graph it just shoots all the way up like that okay going from being negative inside the cell what it was originally to now being positive inside the cell this is called depolarization this is important because depolarization happens with muscle cells in a fashion like this as soon as one muscle cell depolarizes because they connect it to the next some of this sodium trickles through the gap and again that means that if it becomes a little bit positive here and hits negative 70 this cell flips all its lives and so in trickles in which means that this cell depolarizes and then some of that sodium that's come in again moves through to the next cell becomes negative 70 pops the lids soon comes in and so forth now see this domino effect one cell depolarizes and spreads a wave of depolarization through the rest of the muscle cells this is what happens in the heart we start a wave of depolarization and it spreads across the heart okay now why is this important what's this got to do with the contraction of the heart well once the polarization has occurred and it's reached around about positive 10 positive 20 positive 30 all the channels are open and all the sodium has come in and then it stops sodium channels then closed now once these sodium channels are closed it's a very positive inside these cells it's positive 20 or positive 30 so the Sun is to reset okay but in actual fact what happens is this once that's happened potassium channels which we know is inside potassium channels start to open up so once it's hit around about positive 20 positive 30 potassium channels start to open up all those limbs open up which means potassium starts to leak out and what charge is potassium it's positive which means it carries that positive charge outside making it more negative get inside which means this graph starts the drop down to the negative but something else happens what you find is that there's another channel in the walls of these cells the last channel you need to be aware of this channels for calcium okay now as this potassium leaks out these channels open up these calcium channels open up and calcium moves in now this is important you're probably asking why does calcium come in because these are muscle cells you know already that muscle cells have acted a Meissen filaments that need to come together and walk across each other to contract and the key to allow this happening is calcium so calcium must enter cells in order for these cells to be stimulated for contraction so what do we have now calcium calcium entering cells and they begin to contract so the muscle cells start to contract but if I were to draw this up because we have positive stuff coming out and positive stuff coming in so positive potassium coming out at the same time positive calcium coming in it plateaus right because positive out positive in balances each other out the calcium channels close very quickly and after a short period of time they shut off but the potassium channels remain open and potassium continues to leave the cell which means then it starts to drop down again so we have depolarization where sodium comes in and repolarization when potassium goes out so basically think about this and here at this point we have contraction so what am I trying to say I'm trying to say this in order for your heart to contract it must depolarize because depolarization precedes contraction so if we have leads on the heart we can view the events of depolarization and in what direction they're moving too and if there's any problems any disruptions with this depolarization because disruptions of depolarization are indications that there's going to be problems with the next part which will be contraction of the heart muscle cell okay once all of this has occurred once all of the heart muscle has depolarized and then contracted it needs to reset right because remember what's this depolarization can action we have depolarization sodium coming in okay can it contract again once all that sodium Zin no can't come have another depolarization event once sodium is come in needs to reset we need to bring that sodium out okay so this repolarization needs to occur we need to bring that potassium back out and then once that's happened what do we have working at all times in every one of these cells the sodium potassium ATPase pump bringing that sodium out bringing the potassium in and what does that do it resets those cells okay and so when you have an ECG what are you measuring you're measuring depolarization moving through the heart and repolarization moving through the heart okay why because I give you a good indication as to the function of the heart in regards to contraction in the next video we'll talk more about ECGs and what it's actually measuring as it moves through the heart

Channel: Dr Matt & Dr Mike

Views: 38,677

Rating: 4.9532709 out of 5

Keywords: action potential, myocardium, heart muscle action potential, myocardial action potential, myocardium action potential, heart muscle depolarisation, myocardial depolarisation, cardiology, nursing, medicine, usclme, scicomm, anatomy, heart physiology, heart conduction, heart contraction

Id: 32vOiSDfvAE

Length: 17min 28sec (1048 seconds)

Published: Mon Aug 08 2016