one of my very favorite lessons to teach is the conduction system and therefore let's start at the beginning what is it it's about the stimulation of the heart okay and by stimulation of course we're talking about the electrical stimulation of the heart how the heart actually goes about being stimulated to contract to produce force to eject blood from those ventricles or indeed how it manages to relax go through diastole and receive blood into those atria that's what we're interested in today and we've started with a very comprehensive image actually you could probably get a lot done with that image all by itself but i want to start this story at the beginning and that beginning is here with our number one our cyanoatrial sa node and here is that fellow right there we have a sinuatrial now please recognize that the sinoatrial node sits above the right atrium okay and this is the start of our story above the right h in the cyanogen node it's what we know is a myogenic myogenic structure in the sense it's self regulating you guys of course we'll be studying in other tutorials how uh neural control and hormonal control occur but effectively it regulates itself it's what we call a pacemaker okay that's a term you've probably come across previously it literally sets the pace and it emits a signal okay and we're going to have a look at the nature of that signal a second it emits a signal okay so it sends a signal out and what it does is that signal crosses the atria okay so that signal crosses the h remember the atria is the plural of atrium and it causes it causes atrial systole atrial systole so let's see if we can just draw on some of those points and see if we can address this so the sa node it emits a signal and there goes that signal across the atria it's also going around here it's also going around here and notice that effectively what it's doing is it's contracting it's contracting the a tree in that case so we get contraction here we get contraction here and of course what do we get we get atrial systole the forcing of blood down into the into the ventricles because the atria themselves are contracting they're going through a phase now the second thing and this is really where we get to and we can actually put that in here guys this is kind of point two of our stage impulse to left atrium so of course we get that atrial systole but what we're going to get now is we're going to get stage three of this cycle okay so stage three of this cycle we've got the atrioventricular or the av node now this is a node here now not surprisingly the av node where does it sit in between the atria and that let's put this in here it is between atrium ventricles and you'll see here that the actual av node is just between the two so it's sitting just at the bottom of the edge just at the top of the ventricles there and of course what this will do is it will receive it will receive the signal from the sa node okay from the sa node so notice that the sa node is not only stimulating the two at the two atria to cause atrial systole it's also stimulating this second node which is going to perform a different function and that function is it's going to relay the signal okay so effectively this av node receives the signal here look and it's going to relay it to here okay so there's our relay of that signal so think about the job with the sinuatrial no for a second yes it's the cause atrial systole but it's also to stimulate the av node which of course is then going to go on to stimulate the ventricles the other point i want to make is that i've been saying to you so far of course atrial systole in that sort of phase one to two right well notice that during that atrial systole we also get ventricular diastole okay in other words as the atria contracting the ventricles are relaxing now we're going to get to the ventricles contracting so we've got this new signal the signal has been emitted by the atrioventricular node it's now passing centrally into the septum now notice this structure here i'm actually going to get rid of this arrow this structure here this whole area here that area there that is what we call the septum we'll get rid of those arrows that's what we call a septum and you'll notice that the signal arrives here at point number four and this point number four is called the bundle of his now this is positioned in the septum positioned in the septum and you can see that on the image there and what this bundle of his is gonna do let me just give a little bullet point uh position the septum it's going to separate the signal which of course is a single signal this might separate the signal into left and right branches signal into left and right so i'll just put l and r so can you see here that are effectively what's happening our bundle of his is that we're now going to move to the next phase of this which is where this signal is going to pass down that's meant to be a different color it's going to pass down as two separate signals on left and right now this is the exciting bit notice that these signals pass down the septum and then up and around the ventricles why why would that be the case and the answer to that of course is that what these ventricles have to do during systole is contract this way and this way squeeze inwards and upwards why to force the blood out of the pulmonary artery to force the blood out of the aorta so you have to force upwards and outwards that can only be achieved by some specialist fibers which we're going to say are 0.5 of our system called what i refer to them at least as the picking j fiber sometimes spelled the per kind fibers and what these fibers do is they spread the signal they spread the signal to every muscular cell in this case of course cardiac muscle so that now is going to surround the ventricles cause the ventricles to go through systole squeeze upwards and outwards and those myocardial cells are going to contract now notice this word every now this relates to what all our non-law the heart contracts fully or not at all remember that our heart it either contracts entirely with full force or not at all and that's called the all or none we're going to look at that a few occasions actually uh as we progress these through these lessons but the other thing i want to stress here is that this that causes ventricular systole ventricular systole we've got here systole occurring because the picking j fibers have spread the impulse to every cell and therefore the ventricles move up and in squeezing the blood out and up the heart by the pulmonary artery and the aorta and the relative on the relevant sides now obviously when we've got ventricular systole what else is happening we have atrial diastole now this of course is now linking to our cardiac cycle but just remind yourself but for for this blood to get forced up and out these dia these atria actually have to be relaxing at this phase so start it's worth sort of realizing at this point that we've now almost well not almost we have got a two-stage contract two contractile phase of the heart itself we've got the atrial systole which is um which is simultaneous with ventricular diastole we haven't got ventricular systole which is simultaneous with um ventricular system which is simultaneous with atrial diastole why is that so hard to say one after the other okay so that's really important these phases these features and these roles are ultimately where you need to get a good understanding and connection to this topic area i hope that helps cheers