human anatomy and physiology part two this week's lectures are on the cardiovascular system with a focus on the heart here in episode three we will talk about the conduction system of the heart there are quite a lot of Concepts to get to I'm going to try to stick uh to um a script uh to make sure uh that uh we are able to understand the conduction system so so here we have uh the conduction system of the heart illustrating the various uh specialized components uh from your essay node which is responsible for initiating or that intrinsic uh Rhythm uh that can be established uh in The myocardium uh through to the internodal pathways uh including uh making its way to the AV node so uh that uh electrical signal always makes its way from the SA node to the AV node uh after which uh it makes its way to the AV bundle the right and left bundle branches and finally uh more to the myocardial uh walls uh in the uh Peri fibers so it's important to note uh that the heart is regulated by both neural and endocrine control um and yet uh it's capable of initiating its own action potential which triggers muscular ular contraction I've mentioned uh the importance of the SA node you can see in this diagram where the SA node is located uh in particular uh the right atrium and uh so uh the conductive cells within the heart will establish the heart rate and transmit it uh Through The myocardium as I uh had mentioned uh previously it is these contractile cells uh which contract and Propel the blood uh through the four chambers as we've discussed in the preceding lecture episodes the normal path of transmission uh for uh conduction is the saay node once again uh where the signal or essentially where the the rhythm is generated uh through the inter noal Pathways and to the AV node uh which uh is highlighted uh at uh the uh lower edge of that right Atri it then makes its way to the bundle branches as I mentioned the left and the right bundle branches you can see on the diagram as well as the uh Peri fibers showing uh essentially embedded within the myocardial wall and uh it's important to note that the action potential for these uh cells uh consists of a pre potential phase with a slow influx of sodium as well as a rapid influx of calcium and an outflux of potassium these contractile cells have an action potential with an extended Plateau phase that results in an extended refractory period um during which uh a a second action potential cannot be initiated but allows essentially for uh contraction of the heart and blood to pump effectively uh as we talked about in the preceding lecture with the simultaneous contraction of the Atria followed by the simultaneous contraction of The ventricle and uh so um this next slide really just highlights uh in more detail uh the cardiac muscle cells and how they are arranged uh and so they are unique in that they have sarum ears they have t- tual and it is the T tual which can transmit the impulse from the cemma or the surface of the cells to more of the interior of the cell uh you also have mitochondria which uh as you know where responsible for generating energy you also have intercalated diss which are uh distinct within our cardiac muscles and they are found at the junction of the different cardiac muscle cells you also have uh Gap Junctions and desmosomes which uh exist Within uh the cardiac muscle cells you can see that Illustrated in uh figure C on this diagram so as I've mentioned it is these contractile cells uh that essentially have an action potential uh and uh allow uh for the contraction uh of the heart muscle and that results in uh the blood pumping from the Atria to the ventricles and from the ventricles out to uh the circulation whether it be the pulmonary or Andor uh the systemic circulation as well and so this occurs in a cyclical pattern which is Illustrated on this next slide essentially uh cardiac conduction really starts uh first with the uh SA node and the remainder of the conduction system are effectively at rest it is that SA node though that initiates the action potential and that will propagate or move or sweep across the Atria and after reaching the AV node there is a little bit of a delay so it's only 100 milliseconds but that allows for the Atria to complete pumping blood before the impulse is then transmitted to the AV bundle or the atrial ventricular bundle so those are the first three steps now we move on to step four following that momentary uh delay the impulse then makes its way through the atrial ventricular bundle and the bundle branches so it's essentially going um inferiorly towards the apex of the heart and within that interventricular septum and that's what you can see in image number four before it kind of spreads uh throughout the ventricle and the ventricular walls uh with those pereni fibers the impulse will spread to the contract fibers of The ventricle and that is what is going to stimulate ventricular contraction as seen uh in the uh step six of this uh of this pattern and this diagram so how do we break this down here we go so this is just another way of thinking about it but once again the essay node is where the action potential is initiated hence we actually consider that also to be the pacemaker that once again within the right atrium and uh it is what is responsible for setting the heart rate and allowing the Atria to begin to contract simultaneously once again the Atria are going to contract simultaneously in advance of the ventricles Contracting so first we have the uh electrical signal we have the conduction uh within the SA node making its way to the AV node and being relayed then to the AV bundle and into the ventricles when uh the electrical signal is uh sent uh to the ventricles once again it makes its way through the right and left branches of the bundle fibers and through the pereni fibers and this is going to allow for once again those electric impulses to spread throughout the ventricular wall but also the muscles of the ventricle um and uh that is what is going to facilitate ventricular contraction once again simultaneously this means that the left and the right ventricle will contract at the same time so once again we've mentioned this concept of an action potential once again it initiates at the SA node and we mentioned how you have a slow influx of sodium ions and uh then that is followed by a rapid influx of the calcium ions that is what is going to uh trigger uh depolarization and also repolarization with the outflux of the potassium it is the prepotential that accounts for the membrane uh membrane threshold and will initi iate that spontaneous depolarization and contraction of the cell now as I've mentioned we also have what's known as a refractory period uh we've talked about this uh when we think about uh the nervous system and skeletal muscle but it can also refer to cardiac muscle and you can see some distinct differences between uh skeletal muscle and cardiac muscle when it comes to action potentials note there is a long Plateau phase and that is due to the influx of calcium ions there's also an Extended refractory period that allows for the cell to fully contract uh once again before another electrical event can occur you can see that the action potential and the contractile phase is much more pronounced or longer when we're talking about our cardiac muscle versus our skeletal muscle of note uh there's also an absolute versus a relative refractory period if we take a step back and look at figure a um once again absolute refractory period referring to the time period at which uh there is uh no way in which uh another electrical event can be triggered uh relative uh refractory period simply states that it's a time period in which it's unlikely to occur but it's not impossible possible so you could in theory have another electrical event but it's highly unlikely and that takes us to how exactly can we measure the conduction and the electrical activity of the heart and we have something known as an ECG or an electrocardiogram which I'm sure many of you are familiar with um but essentially um uh it uh is able uh to give us a sense of uh a of atrial and ventricular um depolarization and repolarization as well and in particular um we have different phases or different uh stages or intervals or waves of the ECG uh which uh identify uh these these different phases so uh first uh the start uh of um uh an the ECG tracing uh has the p-wave and the p-wave essentially is a a a small uh increase in electrical activity measured in Mill volts and it represents the depolarization of the Atria this will then effectively trigger at tril contraction next uh you have a much larger increase in electrical activity and that results in a greater change in voltage as you can see uh with the QRS complex and the QRS complex represents depolarization of the ventricles and somewhat buried or hidden within that you also have the repolarization of the Atria in the t-wave once again you have a small little increase in electrical activity or a change in those molts and that uh represents the repolarization of the ventricles and that in effect is one cardiac uh cycle when we think about electrical activity so if we move a step forward now and think about uh how uh the electrical activity corresponds to a cardiac cycle and this diagram really uh correlates the ECG tracing with the different mechanical events of uh contraction and so as I mentioned in the preceding slide each segment of the ECG tracing corresponds to an event in the cardiac cycle so for example when we have our p-wave and we have atrial depolarization that is also going to be associated with uh atrial uh contraction and you can see that highlighted uh in uh in steps uh two and three as the Atria uh start to contract and that's Illustrated in color here in number three uh following which uh with uh step number four and five what you can see is the QRS complex um creates uh quite a significant change in in voltage or in electrical activity and that is the stage at which the electrical activity is really propagating uh inferiorly through that interventricular septum and through the ventricles uh and so the depolarization of the ventricles is going to allow for ventricular contraction which then you see in Step number five at this time though it's also important to note that the Atria are also repolarizing and that's once again um is diminished in comparison to uh the the the substantial change that we see in voltage uh with the QRS complex so once again the QRS complex we typically think of as ventricular depolarization and then leading to ventricular uh contraction U but in fact at the same time we have atrial repolarization and the Atria are able to relax and then finally uh during The t-wave Illustrated in uh step number six uh that is the stage at which uh atrial sorry that is the stage at which uh the ventricles will uh repolarize as well and that takes us through an entire cardiac cycle