Pacemaker Cells of the Heart | Cardiology

[Music] hi everybody dr. Mike here in this video we're gonna take a look at how the SA node which is the sinoatrial node and the AV node which is the atrioventricular node how they send their signal their electrical signal that we term an action potential to the rest of the muscle of the heart to tell it to contract so what we need to go through is firstly if I were to draw the heart up itself and just very quickly I've got the heart here we know that there's a fibrous bit of tissue here that separates the atria the two chambers at the top and the ventricles the two chambers at the bottom and we know that there is a specialized type of myocardium which is heart muscle cell a very specialized type that sits at the back of the right atrium that we termed the SA node now these cells spontaneously depolarize what's that mean it means they spontaneously send an electrical signal they actually do this around about seventy to a hundred times a minute and when they send this signal they spread the electrical signal to the surrounding heart muscle tissue in a fashion like that now as it spreads that signal through what we termed the myocardium the heart muscle of the atria that then tells the heart muscle to contract and they squeeze and push the blood down into the ventricles now the electrical signal wants to spread down into the ventricles but it can't because of this fibrous bit of tissue here but the electrical signal can bottleneck itself through another specialized group of myocardium that we now turn the AV node the atrioventricular node and the action potentials are being pushed through this AV node and therefore the heart muscle doesn't contract for 0.1 of a second there's a break between the atrial contraction and the ventricular contraction of 0.1 of a second this gives the heart enough time for the ventricles to fill with blood after the atria contract once that point one of a second has occurred and the action potential spreads down the AV node it goes down into we call the bundle of His and that spreads to to the left and right bundle branches and then they send electrical signals through the ventricular heart muscle and as it spreads through that action potential then tells the muscle to contract and blood gets squirted out if it's the right ventricle it goes to the lungs if it's the left ventricle it goes to the body so what we're talking about is today the AV node sorry the SA node and the AV node how do they send their electrical signals if I said that AV nodes spontaneously depolarize they just basically stimulate this action potential all by itself how does it do it well let's first have a look here actually let's have a look here at a cell of the SA node so I said they're specialized myocardium so it looks a little bit different here compared to normal myocardium but it's connected you can see there's a conversation that can be had between the SA node and the myocardium now the first thing you need to be aware of is that all cells of the body have a very special pump called the sodium potassium ATPase pump let's draw this sodium potassium ATPase pump right here and what it does is it uses ATP we know ATP is the energy currency of the body uses ATP to throw 3 sodium out of the cell and throw 2 potassium into the cell now I want you to think about that if we throw 3 positive sodium out of the cell and throw 2 potassium into the cell where do we have a net positive charge well we've got three positive things outside two positive things inside the net positive charges outside which means the net negative charge is inside and if you were to compare those charge differences what you would find is inside compared to outside outside is negative 5 millivolts okay so on this table here we've got zero millivolts down to negative 90 millivolts up to positive 20 negative 5 millivolts is only around about here now this isn't all that's happening but what is happening due to this is we're accumulating a whole bunch of sodium outside the cell and were accumulating a whole bunch of potassium in the cell that's the first thing second thing is this this cell is quite leaky to sodium there side to potassium and if it's leaky to potassium we're gonna have potassium leak out of the cell very slowly now if positive potassium is leaking out of the cell it becomes even more negative inside right because positive things are going out so it goes from negative five actually all the way down to around about negative fifty-five and so we've now got around about negative fifty-five a charge difference inside compared to outside so insides negative fifty-five compared to the positive outside now another thing that's happening is this these SA node cells are leaking not just to sodium or potassium but they're also leaking to sodium and calcium so what you'll find is this you're gonna have some cells here let's draw this as a square you're gonna have some channels here sorry that Aliki to calcium and I want you to think about this calcium sits predominately outside the cell and it will leak into the cell which brings a positive charge inside an actual fact there's also leaky sodium channels which bring positive sodium into the cell as well they probably getting confused here we've thrown three sodium out to potassium in but potassium is leaking out making it negative inside but we've also got calcium and sodium leaking inside making it positive inside it doesn't know what charge it is in actual fact what that means is this it starts its baseline is around about negative 55 but because of the leaky calcium and sodium it actually is always drifting drifting drifting drifting up towards the positive because all this calcium and sodium is slowly drifting in now in actual fact there is a mark right here at around about negative 40 now this market negative 40 is what we termed the threshold and it's the threshold for what another group of channels what are these channels they're another type of calcium channel at negative 40 this is the key that unlocks this new type of calcium channel a faster calcium channel and when that opens up more calcium rushes in and when more calcium rushes in what we get is a fast stimulus up to around about positive 20 now when it hits as this is happening going up to positive 20 what you'll find is that at around about positive twenty more potassium channels open up remember these were just leaky potassium channels now we've got very specific potassium channels opening up and heaps of potassium start to leak out of the cell now if heaps of potassium leak out of the cell at positive twenty the positive is going back out again and it drops back down to around about negative fifty-five to negative 60 now if it's dropping back down to here what happens again the whole thing occurs leaky calcium leaky sodium and then it goes back up to negative 40 which then opens the calcium channels and that sends the signal back up which closes the channels but opens the potassium channels and it drops back down so what do we have here basically to summarize we've got at this point here it's sodium and calcium leaking in at this point we've got calcium shooting in and at this point we've got potassium going out now what we term this is this part is depolarization this part is real or zation depolarization repolarization and this is what's happening at both the SA and AV node and as you can see it just spontaneously does this and it does it at the SA node it does it around about 60 to 100 per minute and at the AV node it does it around about 40 to 60 times per minute and this is what we term the pacemaker cells of the heart so these are the action potentials of the SA node and AV node

Channel: Dr Matt & Dr Mike

Views: 77,173

Keywords: cardiac action potential, action potential, sa node, av node, pacemaker cells, heart action potentials, how does the heart send a signal, how does the heart fire off, rythym of the heart

Id: pXvhTFpMOTw

Length: 9min 10sec (550 seconds)

Published: Thu Jul 25 2019