Cardiovascular System 3, Heart, electrical system

today we're looking at the internal electrical conducting system of the heart and to help as I've just sketched out a heart here but before we look at this it's interesting to think about patients with heart transplants now if someone donates a heart the heart is taken out of their thoracic cavity and if you think about it that's going to sever all of the nerve connections that would connect to that heart and then when it's placed into the recipient the heart will keep beating hopefully for years or decades without any external neuronal innovation so this tells us that the impulse to stimulate cardiac contraction must be intrinsic to the heart itself it's actually in the heart itself and we want to explain this using this simplified model of the heart so here we have the left side left atrium left ventricle that means that that valve they're pointing down the way is the mitral valve this is the aortic valve and this is the aorta leaving the left ventricle of the heart here we see the right atrium this represents the inferior and superior vena cava the right atrium the right ventricle that means this valve pointing down the way must be the tricuspid valve and this is the pulmonary arterial sometimes called pulmonary arterial semilunar valve guarding the entrance of the pulmonary artery dividing to each lung the top part here is the base of the heart and the bottom pointed part is called the cardiac apex and this vessel here represents the four pulmonary veins so all round about the heart we have the myocardium so there's going to be a tree or myocardium I'm Pierre the myocardium of course being the contractile muscle of the heart the middle layer round about the outside there's the pericardium on the inside there's the endocardium and here we're thinking about the myocardium so this is the ventricular myocardium here now the myocardium extends up into the cardiac septum and of course round the wall of the right ventricle now what has to happen in the cardiac cycle is we need the atria to contract first pushing the blood from the atrial chambers through the atrioventricular valves into the ventricles that has to happen first in a coordinated way then add an appropriate very short period of time but just the right period of time after that the ventricles need to contract now I think you can see that if the ventricles contracted from the top down the way the blood would just go down here which wouldn't be much use at all so we need the ventricles to contract from the bottom up forcing the blood towards the arterial valves the aortic arterial valve and the pulmonary arterial valve called the semilunar valves so the blood can go away via the aorta into all of the systemic circulation and via the pulmonary artery into the pulmonary circulation to be oxygenated so this whole thing needs to be precisely coordinated and regulated so how is this achieved well just in the right atrium somewhat near the entrance of the superior vena cava there's an area of specialized myocardial tissue this is not nervous tissue it is myocardial tissue but it's specialized to be electrically active and this is called the pacemaker of the heart or the sinoatrial node so this is the sinoatrial node of the heart the SA node sinoatrial node and this is what generates the sinus rhythm the normal rhythm of the heart so there's the sinoatrial node here specialized myocardial tissue electrically active and this is this area is what we call intrinsically unstable it is electrically intrinsically unstable so quite spontaneously all on its own without anyone telling it what to do without any hormonal influence without any neurological influence this will depolarize and it will depolarize spontaneously around about 90 times a minute sometimes 90 to 100 times a minute it will depolarize thereby generating a new electrical nerve impulse so this node will just keep depolarizing 90 to 100 times per minute generating a new electrical activity and this is the focus that's controlling the electrical activity of the heart now there are other electrically active areas in the heart for example there's an area here and that's a node between the atria and the ventricles so that's called the AV node the atrial ventricular node between the atria and the ventricles and that's also electrically active but the spontaneous depolarization rate is lower than the sinoatrial node so it's the sinoatrial node which governs the other electrical active tissues and in fact even the individual myocardial themselves will spontaneously depolarize but at a lower rate so because the sinoatrial node is depolarizing the fastest that is the origin the physiological origin of the electrical activity required to stimulate the myocardial cells in order to generate myocardial contraction now what do we need to contract first well it's the atria that need to contract first so coming from the sinoatrial node there are conducting pathways preferential conducting pathways carrying the electrical impulse to different parts of the atrium and most physiologists now agree there are three preferential pathways a specialized tissue which conducts the electrical activity freely and we have the posterior the middle and the anterior pathway and this anterior pathway also takes impulses across to the left atrium and as these tracts go through the atrial areas smaller branches come off them and these take the electrical activity to the atria myocardial cells that depolarizes the atrial myocardial cells and it is that depolarization that tells them to contract so they will contract when electrically stimulated so the electrical activity is generating a sinoatrial node rapidly goes through these tracks these conducting pathways to take the impulse to the atria to the atrial myocardium when that depolarizes it contracts and that is what causes the atrium to contract down the way ejecting the blood from the yeah down to the ventricles and these pathways converge at this node here which you might remember is the AV or the Atrio ventricular node and it's useful sometimes to describe this as a collecting node it collects the impulse from the atria and now we notice that there's a sinoatrial node there and an atrial ventricular node there so these pathways here can be referred to as the internodal tracts the internodal tracts the conducting tracks or the conducting pathways between the two nodes and this collects the impulse now we notice that the valves are more or less more or less on a plane across the middle of the heart and the valves are made of fibrous tissue they're largely collagen so what we end up with is a ring an atrial ventricular ring of fibrous tissue in this valvular plane so we have fibrous tissue in an atrial ventricular ring separating the atrial chambers from the ventricular chambers from the ventricles and the fibrous tissue does not conduct electricity and of course this is remarkably useful as well as being tough fibrous tissue for the valves the same tissue is electrically insulating so the impulse that gets into the atrial myocardium will cause the atria to contract but then it will stop at the valvular atrial ventricular ring so the impulse can't get from here down to here or from here down to here more from here to here there is a layer an atrial ventricular ring that is electrically resistant the only way the electrical impulse can get from the atria down to the ventricles in the physiological healthy situation is via the Atrio ventricular node so this collects the impulse and this impulse the same important was generated there it's gone through the internal tracts it's caused a tree or myocardial depolarization and it's collected by the atrial ventricular node it's a collecting node and of course remember we want the heart to contract basically from the bottom up the bottom being the cardiac apex now what actually happens is as the impulse is going through this node it's delayed by about 40 milliseconds something under a 20th of a second but that delay is important because during that delay as the impulse goes through the atrioventricular node slowly that gives the blood mechanically time to get through from the atria to the ventricles so once the atria is contracted we need these 40 milliseconds or so to give the blood time to get from there to there and that means that the ventricles fill up with blood prior to ventricular contraction or prior to ventricular systole now if you want the impulses more or less to start going into the ventricular myocardium down here we need to get them down there quickly so first of all we have a bundle of tissue here which will conduct the electrical impulse that's been collected by the AV node and it will start transmitting that impulse down through the cardiac septum and the impulse here is transmitted very quickly and this is called the atrial vent tricular bundle or in the old days we used to call it the bundle of hiss presumably after the guy that discovered it but these days we normally call that the AV bundle or the atrioventricular bundle taking the impulse down the way and then this quickly divides into two smaller bundles and these bundles start bringing the impulse further down through the cardiac septum this one actually branches taking the impulse into the higher parts of the ventricular myocardium there's actually a few branches here but there's two main ones the left bundle branch and the right bundle branch and this is going to go on down here taking the impulse down towards the cardiac apex and likewise with the right bundle branch coming down here again there's going to be some branches off it go into different parts of the right ventricular myocardium and it's going to go down here towards the cardiac apex so what we see now is the bulk of the impulse that's been picked up by the atrioventricular node rapidly transmitted down through the atrioventricular bundle of hiss into the left and the right bundle branch has been transmitted down towards the cardiac apex and at the cardiac apex smaller fibers start taking the impulse into the ventricular myocardium and these smaller fibers are called the Purkinje the Purkinje fibers now we don't normally in anatomy these days use the names of people that discovered things but all the textbooks still seem to call these fibers the Purkinje fibers but what they are is they are small areas again of special raised myocardium designed specifically for electrical contraption for electrical conduction rather not contraption and these will conduct the electrical activity rapidly towards the contractile myocytes so all this tissue I've drawn in red is myocardium yes but it's myocardium which is designed for electrical activity and conduction rather than contraction so the Purkinje fibers specialized myocardial cells designed for electrical activity and conduction taking the electrical impulse towards the contractile myocardial myocytes causing them to depolarize and of course when they depolarize they will contract and the impulse is then carried further up to all parts of the ventricular myocardium on both sides so what we see is the electrical activity is basically going into the myocardium here causing the depolarization here giving us almost a wave of contraction going up the way like that brilliantly directing all the blood to the bullseye if you like to the target of the aortic and pulmonary valve ejecting the blood into the systemic and the pulmonary circulation and all these fibers all these electrical fibers the bundle branches and the Purkinje fibers are going into the bundles of myocardial cells and we now know that the myocardial fibers are arranged in particular patterns around the heart because what we want is we want these chambers to contract but also the septum will shorten and go up the way so the Chamber's contract and it goes up the way so can you see that compressing the blood in the ventricles in two ways it's squeezing in that way and squeezing up that way that will increase the pressure of the blood and the ventricles that will shut the atrioventricular valves preventing regurgitation of blood back into the atria at the same time the increase in pressure in the ventricles will open the arterial valves allowing the blood to go up out of the ventricles into the arterial system without any vegetation so we've got the electrical activity the myocardial activity and the valvular anatomy and physiology all beautifully coordinated and as well as going that way and of the way the heart actually twists as well when the heart contracts it twists it's a bit like wringing your washing out you can twist it and as it twists that also increases the pressure and ejects blood out of the arteries and as it twists it also stores some energy and once the cardiac contraction is over once the ventricular systole is over that potential energy is released and springs the heart back into its resting anatomical position and that helps ventricular filling as well so we've got the contraction the of the way and the twisting all coordinated by these electrical this electrical internal conducting system now it's fairly obvious that the heart rate can vary so the heart will keep going at 90 to 100 beats per minute but if you look at your heart rate it's probably less than that and what we have coming down here is we have one of the large nerves in the body the vagus nerve the vagus nerve is the tenth cranial nerve it's the main Paris pathetic nerve in the body and branches from the vagus nerve will go off to the sinoatrial node and branches from the vagus nerve will also go off to the atrioventricular node so if you're fairly fit at the moment your heart rate might be 60 if you're very fit it might be 50 if you're athletically fit it could be even lower than that and the reason that your heart rate is not obeying the intrinsic rate of depolarization determined by the sinoatrial node is the parasympathetic innervation of the vagus nerve the tenth cranial nerve which is slowing that down so what you find is as we started this talk thinking about people with heart transplants when you take the heart out you're going to cut the nervous connections what you normally find is after a heart transplant the recipient has a heart rate of 90 to 100 beats per minute which is exactly what you would expect as a result of the physiological waves of depolarization generated by the sinoatrial node without the parasympathetic vagal inhibition and that's what we see now people who have had how heart transplants aren't very good at jumping up and doing sudden exercise but if they start walking and then walk faster they can even end up jogging or running because over minutes the heart can be stimulated via endocrine influences as well so the heart rate is stimulated by these well the heart the heart rate is generated by an assigned radial node is inhibited by the parasympathetic but it can be stimulated by hormones as well such as adrenaline or epinephrine of course springs to mind that will increase heart rate and also the heart rate can be increased by sympathetic activity so in the same way that there are parasympathetic fibers going to the sinoatrial and atrial ventricular node there's also going to be sympathetic fibers and of course the sympathetic fibers will increase the rate of contraction and increase the strength of the contraction as well so the heart has this intrinsic rate but it can be lowered by parasympathetic activity and it can be increased by sympathetic activity but still we need to learn these main bits sinoatrial node internodal tracts atrial ventricular node bundle of hiss or atrioventricular bundle left bundle branch right bundle branch akin to fibers going in to and coordinating the contraction with exactly the right amount of delay of ventricular contraction after the atria have contracted so there we have the internal electrical conducting system of the heart controlling and coordinating cardiac myocardial contraction

Channel: Dr. John Campbell

Views: 667,300

Keywords: heart, sinoatrial node, atrioventricular node, bundle of His, purkinje fibres

Id: MPZ_Am4HgQA

Length: 23min 26sec (1406 seconds)

Published: Tue Aug 04 2015