 apologize again for going over time last time and forgetting started late. We are going to talk about action potentials just a bit more today. We were in the process of simulating the propagation of an action potential along the squid axon and somebody asked how come it looks backward and the answer is that when you're sitting watching an action a wave which is asymmetric it looks backward if you're watching from right to left but if you were watching from left to right it looked fine. So let's move on now to an interesting topic which is the enormous variation in the shapes and kinds of action potentials caused by the enormous variation in the kinds of voltage gated channels. Here for instance is a heart and how many of you have taken the physiology course here at Caltech taught by Chase Lydell. One of you, two of you. Okay well Chase will tell you all about this wonderful topic of cardiac electrophysiology and I can assure you that by the time you get to my age or actually much younger you may or may not have had an electrocardi... an electroencephalogram or deep and I hope that you will not have had deep brain stimulation or brain surgery but everybody will have had an electrocardiogram and so this is a topic that is extremely well researched among cardiologists and among electrophysiologists and we see here the chambers of the heart and the various nodes of the heart there's the sinoatrial node which is located between the sinus and the atrium the atrium the atrioventricular node which is located between the atrium and the ventricle the Purkinje fibers who discovered the Purkinje fibers good good and the endocardial fibers and mid-myocardial and the epicardium so the action potential as it progresses from the top to the bottom of the heart changes its waveform those waveforms actually have significant benefit selective advantage for the function of the various regions of the heart you can see here at the sinoatrial node which is a pacemaker organ the waveform looks as though it's getting ready to fire another spike here and indeed it is likewise at the atrioventricular node there is also a pacemaker in dodge and is firing that's not true in most of the left ventricle which generates the major force the left ventricle needs to be triggered by the atrioventricular node so the left ventricle pumps against this greatest resistance and therefore it has the thickest walls and therefore its integrated currents are largest and therefore it contributes most of the electrocardiogram now to give you an idea of the pervasive power of Ohm's law in the activity of not only the nervous system but the rest of the body we're going to put up one of these equivalent circuits again with the macroscopic sodium conductance and the macroscopic potassium conductance and chloride conductance and the capacitance now these even though there are two of these equivalent circuits those are not little gammas those are not little ion channels they are in fact just different regions of the heart and so because they are in different regions of the heart one of the things that can happen is that current flows into one region and out another still preserving charge but having a complete circuit now it turns out that to first approximation and so the current flows previously we said well actually there isn't much resistance in the extracellular medium since it's saltwater and there isn't much resistance in the intracellular medium since it too is saltwater but actually there is enough resistance so that when current flows from one region of the heart to another there's a little resistor there see our external current flowing across a resistor produces a voltage drop equal to the current times the resistance and so if you have electrodes near the heart you can record that voltage drop and in fact near the heart in the body means practically anywhere on the body and so you can yourself take a electrode in one hand a graphite pencil would be fine and in the other hand put it through a sensitive amplifier and watch the heartbeat which is a couple of millivolts you can get iPhone apps to do this so you can record the electrocardiogram and as it turns out we will describe later that the actual currents which flow during the heartbeat are rather small but at certain points in the heartbeat the rate of change of voltage is very large and so what we did last time which was to forget about the capacitive currents because the rate of change occur because the large changes occurred very briefly actually is large enough so that the dominant currents recorded by the EKG by your iPhone app I promise to get one of those for next year are CDV by DT so an extra cellular electrode pair records IR drops proportional to the first derivative of the membrane potential now I'm a little confused and no no cardiac physiologist has been able to tell me why the currents are proportional not to the actual first derivative but to the absolute value of the first derivative and I remain confused about that but it's true so if we looked at the small fraction of the resistance between two electrodes for instance the chest and the limb the voltages that we remember are a thousand times smaller than the trans membrane potential but we can measure them anyway so these are small resistances that we are way far away from the current sources but you can measure them anyway and so let's look at the electrocardiogram here is the action potential that you might measure in the date in the ventricle with an intracellular electrode now one of the fascinating aspects of the heartbeat is that it the action potential associated with the heartbeat is a good fraction of a second about half a second so that's much longer than the action potential associated with a nerve which is about a millisecond and in fact the heart contracts for about half a second then let's go contracts again and so these action potentials last for about half a second but the rate of greatest the points of greatest change are the upstroke and part of the downstroke and sure enough the first derivative of this guy up here is this guy down here which is the electrocardiogram except for this annoying absolute value that I don't understand and so if you get one of these iPhone apps it'll look like this and you will be recording your own EKG or ECG so here the upstroke is because the sodium channels are conducting rapidly giving the upstroke of the action potential and then as we learn last time we return to the resting potential because the potassium channels are conducting outward currents here inward currents here and we get the EKG and the at the beginning of the 20th century when the EKG was first recorded using really cute instruments back there people didn't know what to call all these waves so they started in the middle of the alphabet and called them the PQR S and T waves and that has stuck so it won't help for me to ask you who discovered the P wave okay so one of the most common anomalies in our heartbeat is depression between the S and the T waves it's called ST depression and it implies that additional current flows between sections of the heart during the plateau which means that the heart is using more energy in this sometimes not a good thing but because the EKG is so far removed from the actual events in the heart this is just an indicator not a predictor of heart trouble okay so what did we learn last time we learned that the frequency of the impulses of action potentials represents signaling among cells in the nervous system from sense organs to the brain within the brain from the brain to muscles even in a muscle or in the heart and in fact even in the pancreas even that organ that releases insulin is doing so under the command of voltage changes that look a lot like action potentials so why do we call these action potentials not impulses well this is it has historic reasons for many years up until sort of the middle of the 20th century scientists believed that there was a resting that believed in the nurse potential believed in the nurse potential the resting potential and then when the first blips were recorded in its century organ in the 20s people said oh there's not only a resting potential but there are also blips and therefore we'll call them action potentials and the resting potential was measured several decades and explained several decades before the action potential so instead of calling this stylized thing an impulse which is what an engineer or a physicist would call it physiologists proudly called it when they first measured it and action potential okay now let's have a quiz so we do quizzes I do quizzes a little bit differently from the way Ralph does quizzes days while I'm looking for the quiz would you hand out the cards please one per person