so with our last lecture we learned about the structure of the heart and what I want to do now is kind of build on that to start to understand how the heart actually goes about Contracting in order for you to best be able to understand that one of the things that you need to know is that the heart is actually composed of two different types of cardiac muscle cells so the first of these these autorhythmic cells are what we're going to be focusing on today a little bit about these they make up only one percent of the Heart by volume so they're really in the minority as far as cardiac muscle cells but they do make up a system which is known as the intrinsic conduction system so it's these autorhythmic cells that actually allow the heart to be able to contract and that's where they're getting their name from so Auto means self and Rhythm or rhythmic of course means Rhythm so these are self Rhythm cells or cells that generate the heartbeat one of the things that's kind of unique about the heart is if you were to cut it out of the heart let's say you're doing a transparent transplant and you cut the heart out of somebody's chest cavity even when you've severed all of the connections between the heart and the Brain it can continue to contract because of these autorhythmic cells and the fact that these Auto rhythmic cells are what is actually generating the heart rate the other cells that make up the heart are what are known as non-autorhythmic cells and they make up 99 of the Heart by volume so if you look at this diagram over here of a heart you'll notice we've got a few areas of cells that are represented in yellow these are areas where we find these autorhythmic cells in the heart but you'll notice most of the heart wall is actually this pinkish color and that's the non-autor rhythmic cells that make up 99 of the heart the non-auto rhythmic cells as their name implies are not responsible for generating the electrical signals that allow the heart to contract rather they're just cells that do the contraction so they're the actual cells and that have the actin and the myosin and that can track down to allow the heart to be able to beat so with that having been said what I want to do now is talk a little bit more about these autorhythmic cells because again they're really kind of the focus of this particular lecture so I mentioned with the last slide that the autorhythmic cells are the cells that are represented in these areas that are yellow and you'll notice that they make a pathway that kind of starts up here and then it spreads down through the wall of the heart on either side so these different areas where we have autorhythmic cells are named and the first of them this one that's right up here in kind of the upper corner of the right atrium we've got a cluster of autorhythmic cells there which is known as the SA node and the SA node is capable of generating Action potentials electrical signals about 75 times a minute so this is often known as the pacemaker of the heart because this is the area from which the heart speak is actually being generated so about 75 times a minute on average and this will vary a little bit from person to person these cells these autorhythmic cells that are located right here in this upper corner of the right atrium send out an action potential and what that action potential then does is it travels across the right atrium okay and it also travels all the way over to the left atrium now remember that this electrical signal that's being generated is the signal that tells the skeletal muscle cells to contract this signal travels extremely rapidly from the SA node to the left atrium and extremely rapidly from the SA node across the right atrium and basically what happens because it's traveling so quickly is all of these cells that make up the right and left Atria get that signal to contract add effectively the same time that allows the right and left Atria to be able to contract together so they are Contracting at the same time in response to that signal that's generated by the SA node down in here so this is kind of at the base of the right atrium so this little area that's represented in yellow and we have another cluster of autorhythmic cells and this cluster rhythmic cells is what's known as the atrioventricular or the AV node so what happens with the signal right here in about the area of the AV node is it was traveling really rapidly before it slows down and it moves through this area really slowly before speeding back up and going through both of the walls of the ventricles really quite rapidly the reason for this is super important so the reason that we need that signal to slow down right here in the area of the AV node is that we need to be able to give the Atria time to get the signal and to contract before we send that signal to contract down into the ventricles so that's why we get a slowing and that action potential or that electrical signal that tells the heart muscle to contract right through here we want to give again the Atria both the right and the left time to get the signal time to contract time to finish Contracting before we send that signal for contraction down into the right ventricle and also into the left ventricle if that were not the case if we didn't slow down that signal right here what would happen is that all areas of the heart because that signal moves so quickly up through here all areas of the heart would effectively get that signal to contract at the same time that means that the ventricles would be contracting at the same time that the Atria are Contracting and of course when the ventricles contract they are generating a back pressure and when the Atria can track they're generating a forward pressure and so if they are Contracting at the same time based basically happen is those chambers would be working against each other and so we slow down that signal right there at the AV node again so the Atria can finish Contracting before the ventricles get the signal and then contract in response to it so we've talked about the essay no the pacemaker that's kind of setting the pace for the heart it sends out signals 75 times a minute causing the Atria to contract as a unit and then that signal gets picked up down in here by what's known as the AV node the signal slows down so that those Atria have an opportunity to contract before moving down through these autorhythmic cells that are located throughout the rest of the heart wall so the next area where we have autorhythmic cells is right through here they form a pathway which is known as the AV bundle it's also sometimes referred to as the bundle of hiss so despite the fact that this is spelled like bundle of His it's pronounced bundle of hiss so this area is through there and then you'll notice that this bundle of hiss right in this area right here divides into two different Pathways so down here where we've got the bundle that has divided into these two different Pathways of autorhythmic cells we have what are known as the bundle branches so you'll sometimes hear these referred to as right bundle branches and left bundle branches where we can just collectively call them the bundle branches you'll notice that this left bundle branch goes down and then it goes up into the wall of the left ventricle so out in here we've also got these Pathways of autorhythmic cells which are known as purkinje fibers the right bundle branch if you follow it down goes up into the wall of the right ventricle where we have again autorhythmic cells that are represented in yellow that are also known as purkinje fibers but what these bundle branches do because they split is they allow us to send that signal to the left ventricle and send that signal to the right ventricle at the same time so that these ventricles can contract as a unit so that they're Contracting at the same time the other thing that you should know is once the signal has slowed down up in here Atria finished Contracting and now that signal Zips very rapidly through the ventricles so that basically all of the ventricles on both the right side and the left side are getting that signal to contract at the same time I talked about previously the idea that the SA node which remembers our pacemaker it's this area of autorhythmic cells up here in the corner of the right atrium and that it has an intrinsic rate which means that 75 times a minute on average it's generating electrical signals and those electrical signals are spreading across the Atria and across the ventricles you'll notice that the AV node and the AV bundle and the bundle branches and actually the purkinje fibers as well all have an intrinsic rate also so they are autorhythmic cells which means if they need to they can generate their own electrical signals to cause the heart to contract however because the SA node has the fastest intrinsic rate usually what happens is that the electrical signals that are coming from the SA node override signals that would be generated by any other area of the intrinsic conduction system and so normally the AV node the AV bundle the bundle branches and the purkinje fibers aren't generating their own signals they are just transferring signals 75 times a minute in response to what's happening with the SA node however there are situations where you can have damage to the SA node or damage to some other area along the intrinsic conduction system so I want to go with the idea of having damage to the a node let's say somebody has a congenital birth defect or maybe they've had a heart attack and this area of tissue up here in the SA node has died now all of a sudden what we lose is the function of the SA node and its ability to generate an electrical signal that spreads across the Atria 75 times a minute if that happens then remember the AV node is still autorhythmic cells it's part of the intrinsic conduction system and it also has an intrinsic rate and it's capable of generating electrical signals or action potentials to tell the heart to contract albeit at a slower pace so the intrinsic rate of the AV node is about 50 times a minute so if your SA node were to be damaged for whatever reason what would generally happen is the AV node would take over as the pacemaker of the heart it would continue to generate electrical signals and it would send those signals out to the ventricles about 50 times a minute so your heart would still be able to beat or at least the ventricles would be able to beat it just wouldn't be as rapid as you would see happening if the SA node were not damaged here's another scenario that I want you to think about because this one actually comes up on the activity that you do for today so let's imagine that the SA node is fine um the bundle of hits and bundle branches and purkinje fibers are all fine but we've had a heart attack or we've had some type of damage that takes out the AV node so now all of a sudden we have a situation where the SA node is working just fine it's sending electrical signals across the Atria 75 times a minute the Atria are Contracting in response to that 75 times a minute but there's nothing down here because it's been damaged to pick up that signal from the SA node and transfer it down into the ventricles if that were to happen What would then occur in the heart is the AV bundle would take over so if the AV node can't pass the signal from the SA node to the AV bundle then the AV bundle okay remember it's intrinsic cells too it will start generating its own electrical signals and passing them basically Downstream from where the AV bundle is so the intrinsic rate of the AV bundle is even slower than the intrinsic rate of the AV node the AV bundle is capable of generating electrical signals telling the heart to contract about 30 times a minute so 30 times a minute in this particular scenario you would get a signal telling the ventricles to contract but remember 75 times a minute you were getting a signal that was telling that Atria took contract so the Atria are Contracting almost three times as much as the ventricles are Contracting in that particular scenario so I bring this up just because you're going to be playing with EKGs in the activity and you do in this particular folder and that's one of the scenarios that you actually see is a heart block where there's damage to the AV node and now you've got signals being generated here that are spreading to the ventricles and signals being generated up in here that are spreading to the Atria but the Atria and the ventricles are not coordinated and there not Contracting in a rhythmic pattern with each other so the last thing that I want to look at today is just some very brief introductory stuff on EKGs so an EKG is a reading of the electrical activity in the heart I see a lot of confusion with this so some people think it's like actually measuring the heartbeat or it's measuring the opening and closing of valves or it's detecting heart sounds or something like that it is not what an EKG is measuring and you should be very very clear on this is that electrical activity that's occurring within the heart so those Action potentials that are being generated by the SA node and then spreading throughout the rest of the heart like we talked about in the last slide that's what an EKG is measuring if the electrical signals are messed up if there's something that's weird about them because they are the signal to contract there's likely going to be a problem with the contraction but an EKG or an ECG whatever you prefer is not actually measuring the contraction it's measuring the electrical signals as they spread through a heart so most of you are probably somewhat familiar with EKG readings so here's one here just a normal regular heartbeat and what you'll notice is we've got kind of this pattern of waves that repeats and this would be considered normal what I want to do is take this pattern of waves and break it down so that you understand what each one of these waves is representing so we've got one complete heartbeat okay represented in this little area here here is a second complete heartbeat okay here's a third one here or at least this is representing the waves that are causing that heartbeat and so over here we've got one complete heartbeat or again the waves that are causing that heartbeat representing us it's kind of this repeating pattern so the first wave is what's known as the P wave and then we've got this what's known as a QRS complex which is actually three separate waves QR and S but we refer to them usually together as the QRS complex and then a little bit later over here we've got a T wave and we're not even going to worry about the U wave for the sake of this class okay so if you take this back to this here's a P wave QRS complex T wave P wave QRS complex T wave P wave QRS complex T wave and that just repeats over and over and over as these electrical signals move through the heart causing the contraction of the heart to actually occur now what I want you to notice is if you look at this diagram where we've got the P QRS complex and T wave represented if you go just below it we've got an artist representation of a heart and and there's a portion of that heart that's highlighted so in this one here the Atria the walls of the Atria are highlighted and that's matched up with the P wave and the reason for this is it is during the P wave that the Atria are actually depolarizing so that electrical signal that's being generated by the SA node is spreading across the Atria during the time of the P wave if you look over here at the QRS complex and you match that up with what's going on here we've got the ventricles highlighted and that's because the QRS complex represents the time when the electrical signal is traveling through the ventricle so specifically when the depolarization of that electrical signal is traveling through the ventricles if you look over here at the T wave the T wave represents the repolarization or the recovery wave of the ventricles so we've talked about depolarization of the Atria depolarization of the ventricles repolarization of the ventricles you may remember from 227 that anytime you've got an electrical signal or you've got an action potential that that consists of a depolarization followed by a repolarization but we haven't talked about where on an EKG the repolarization of the Atria is actually represent said the reason for that is repolarization of the Atria happens during the QRS complex so it happens at the same time that the ventricles are actually depolarizing this is a much bigger electrical activity this ventricular depolarization then is the atrial repolarization and for that reason this what's going on in the ventricles kind of masks what's going on in the Atria and you don't have a wave that shows up that represents atrial repolarization but this should help you to kind of match up these different waves with what's actually going on with electrical signals moving across the heart itself and with this having been said one of the things that you can get from an EKG now that you know kind of what it's representing that it's looking at the electrical signals and where the electrical signals are happening is you can also get the timing of everything so back in the day before this was computer automated what used to happen with an EKG is you had this grid paper and it would scroll through a machine and as it was scrolling through a machine it would take a certain amount of time for the paper to go from there to there and from there to there and that was a consistent set time and based on that you could start getting the timing that it's taking for the Atria to depolarize how long it takes for the ventricles to depolarize how long it takes for the ventricles to repolarize and so forth here's why if you think about each one of these squares being equal to 0.1 seconds and I'm just making up time here what that means is for this P wave if you took each one of these squares and added them up we've got one two three four five six squares basically that this P wave took we know each one of those squares was 0.1 seconds now we know that the P wave took 0.6 seconds to occur so it took six seconds for the Atria to actually depolarize sometimes if somebody has heart damage you'll see this P wave being really extended and that basically means it's taking a longer period of time for um The Wave the electrical signal to actually move across the Atria this may happen because there's scar tissue or something like that that's actually slowing down the wave another thing that you'll see commonly is the waves will be inverted so the P wave is normally um in this direction Q dips down R goes back up S dips down and then T goes back up sometimes you'll start to see inverted waves so for example after a heart attack it's very common to see a T wave that actually is upside down so rather than having it go up like it does you might see a T wave that's now starting to do this that's an inverted wave and that is just an indication that there has been damage to the tissue and because of that damage the electrical signals having to take a different pathway than it normally would that change in pathway is going to change the deflection wave and the shape of that wave and what it actually looks like so if you think about this let me go back again to this normal heartbeat okay we've got a P wave QRS complex T wave here's another P wave QRS complex T wave we know that each one of these is we'll say 0.1 seconds I don't even know if that's true back in the day when they used to run EKGs through this graph paper but let's say it is um we could figure out how often the heart is beating by calculating the amount of time that it is required for these electrical signals to move fully through the heart in a depolarization and repolarization so we start here and here's where those signals have finished and the heartbeat should have finished and here's where those signals have finished again and the heartbeat should have finished it's kind of hard to go from P wave to P wave what I like to do actually is go from the tip of the r to the tip of the r because those are a lot easier to see so if you look at that we've got one two three four squares in there we said that each of those were 0.1 one second so basically every 0.4 seconds the heart is Contracting according to this particular EKG now if you look at this one down here you'll notice that those waves are much closer together so it's not taking as much time for them to happen this is what you would see with a faster heartbeat on an EKG here they're more spaced out from each other so it's taking a longer period of time um for the electrical signals the hearts beating slower in response to that and down here we've got just an irregular heart rate so sometimes they're really close together sometimes it's taking longer sometimes they're kind of average and because the heart isn't really beating with a consistent rhythm in this particular situation so this is some really kind of Baseline 101 introduction to learning about an EKG and the type of information that it's going to be able to tell you and you'll need this as you're working on your activity for this week