hi everybody dr mike here in this video we're going to take a look at the conduction of the cardiac system this is the electrical current that moves its way through the muscle of the heart the myocardium in order to tell it to contract now the first thing you need to remember is that heart muscle is specialized and when we look at it under a microscope all the heart muscle cells are connected to one another like this and they've got these little gaps between now this is important because what you're going to find is that these cells like any excitable cell of the body like neurons for example at rest they have a negative charge associated with them interesting and what happens is if you want these muscle cells to contract you need to turn that negative charge into a positive charge and when it does so it goes from negative to positive it spreads that wave of charge change from one to the other very easily we call this depolarization this is when it goes from negative charge to a positive charge and muscles won't contract unless this happens so knowing this now what we're going to do is we're going to have a look at the heart and have a look at how this depolarization event or negative to positive charge change happens through the heart to tell it to contract it all begins here this area at the atria is a specialized type of tissue that we call the cyano atrial node also known as the sa node now cyano is referring to sinus it sets what we call the sinus rhythm this is the pacemaker of the heart sets the rhythm about 60 to 100 beats per minute now at rest it's negatively charged right but what it does is it spontaneously goes positive like i showed you before and if it spontaneously goes positive it's going to send a wave of the positive charge throughout the muscle of the heart specifically the muscle of the atria remember two atria at the top two ventricles down the bottom this is the muscle associated with it so sinoatrial node muscle tissue is negative it then turns positive and sends a wave of positive charge through the atrial muscle in that manner so it goes like this spreads this positive charge through positive positive positive positive positive moving all the way through the atriomyocardium now interestingly if you do an ekg or an ecg an electrocardiogram where you put all these sticky electrodes on the hands of the feet and across the chest it picks up this charge change right that's what an ecg or ekg does and you get a number of leads but often we have what we call the reference lead which is often lead to and lead to is usually looking at the heart with a point of view from here so think of that as a set of eyes looking at the heart from the apex up towards the base what do you see if you were this particular lead looking at the heart what you're doing is you're seeing that there's positive changes going in your direction the muscle is becoming positive in the direction of that lead and what you're going to find on an ecg and ekg is that any time a positive charge goes towards you remember we call it depolarization you get a bump up on the lead so firstly nothing's happening we've had nothing then the sa node's spontaneously depolarized sending that positive charge towards the lead and you get a positive deflection on your ecg or ekg that's the first thing then what happens is there's this fibrous tissue that separates the atria from the ventricles and it stops this positive charge so you get another flat line on the ecg then what happens is we have another specialized tissue similar to the sa node that sits here between the atria and the ventricles which we call the atrioventricular node the atrioventricular node also known as the av node now this has a little pause right little pause around about 0.2 of a second and that's why we have this little flat line but then it sends this positive charge through specialized fibers these specialized fibers are called the bundle of his bundle of his and you may just refer to them as bundle branches these bundle branches then branch off right and they're going to branch off like this and then the other one branches off like that through the heart muscle of the ventricles and again it's going to send a wave of positive charges in this manner like this like this like this like this like this like this like this and as you can see it sends the positive charge like that so now what do you see is it going towards that lead or away from the lead well predominantly what you're going to find is on the left hand side it's all going towards that lead and so what you're going to find is you get a very big bump up right a very big button you may be saying but what about over here this is going away from the lead well what you're going to find is the left hand side of the heart the left muscle is four times thicker than the right so it overwhelms the electrical activity on the left hand side overwhelms anything happening on the right hand side and so you get this huge spike up because this tissue is far thicker than this tissue so more electrical activity and a big spike then what you have at the end is the electrical activity moving away from that lead as you can see up here and as you can see up here moves away from the lead so you get a dip down and then back to that charge again that isoelectric point no change now what's happened is the whole heart muscle has depolarized it's all gone positive and remember anytime it goes positive it contracts so when you had this bump up it was immediately followed by those atria contracting pushing the blood down when you had this big bump up the immediate was immediately followed by the ventricles contracting pushing the blood out but the next thing is we need to reset the heart we need to make it negative again and to make it negative again it goes in the opposite direction now interestingly because when you have a positive thing going the direction of the lead you get a bump up what happens when you have a negative thing go in the direction of the lead the opposite a bump down but then if this is going in the opposite direction you're going to have a negative charge going away from the lead which is also a bump up so what you then get at the end is another little bump up and this is the negative resetting of that heart and now what we have is what you see on an ecg you see the particular peaks and troughs associated with it and they've all got different names you've got the p wave which is representative of atrial depolarization atrial depolarization you've then got the q r s waves cumulatively known as the qrs complex and this is ventricular that when the ventricles depolarize and then the last thing you have what do you think it is if it's p q r s t wave this was the resetting of the heart making it negative again if this is called depolarization bringing it back is called repolarization so this is ventricular repolarization you might be thinking where's atrial repolarization it happened at the same time as ventricular depolarization and therefore is hidden behind this wave and so what we have is the electrical activity of the heart remember it goes sa node through the atria contraction of the atria av node bundler hiss and then it goes through these fibers which are called picking fibers we should label them these fibers are all called purkinje fibers and then that's followed by ventricular contraction and then the whole thing resets and that's called ventricular repolarization this is the conduction of the heart