You’ve seen this earlier than in a TV present or a film. A affected person is on the desk within the emergency room, bleeding from a stab wound or having ODed on medication, when all of a sudden, an alarm goes off and the beep beep beep on the center monitor flatlines. It’s cardiac arrest, and you may inform simply by that sound that it’s horrible. Medical doctors swarm round, barking orders, and McDreamy or Dr. Gray — or whoever the newest cunning physician is on the TV present that you simply’re hooked on — grabs the paddles, pushes them down on the affected person’s chest, and is all CLEAR! Then the affected person’s chest jumps up, and everybody stares on the monitor ready for these regular beeps to reappear. That may be a fairly traditional scene. With regards to medical crises you see in fashionable tradition, it’s in all probability solely rivaled by the pool-side CPR scene, or somebody shaking an unconscious physique, pounding on their chest and saying “Don’t you die on me!” Which hopefully you realize isn’t any technique to maintain somebody alive. These are traditional script tropes, however they contribute to some misconceptions about how defibrillators, CPR, and the electrical energy of the center work. As a result of the reality is, CPR may help delay coronary heart operate throughout cardiac arrest, however it normally can’t save a life with out assist from a defibrillator. And when McDreamy — might he relaxation in peace — lastly bursts out these paddles, that high-voltage shock isn’t turning the center again on — it’s really stopping it. Confused? Nicely, I’m right here to get your head to know your coronary heart. We’ll get again to sizzling TV medical doctors in a coupla minutes, I promise. However with a purpose to perceive what’s really happening throughout cardiac arrest, we now have to perceive some fundamentals about your coronary heart cells. We’ve realized quite a bit about skeletal muscle tissue — the way it’s striated, and contracts utilizing the actin-myosin sliding filament dance you’ve heard a lot about. Your cardiac muscle can also be striated, and makes use of sliding filaments to contract, however the similarities finish there. For one, their cells look fairly totally different. Skeletal muscle tissue has lengthy, multinucleate cells, whereas cardiac cells are squat, branched out, and interconnected, each with one or two central nuclei. The cells are separated by a unfastened matrix of connective tissue known as the endomysium, which is chock stuffed with capillaries, to serve up a relentless provide of oxygen. Cardiac cells are additionally loaded with energy-generating mitochondria. In reality, mitochondria take up as a lot as 25 to 35 p.c of every cell, making it immune to fatigue, which is partly why your coronary heart can beat practically 3 billion occasions in a lifetime. Not surprisingly, the variations between skeletal and cardiac muscle tissues are key to understanding their features. Skeletal muscle fibers are each structurally and functionally separate from one another, that means that some cells can work whereas others don’t — that’s why you may grasp a fragile flower with the identical hand that you need to use crush a soda can. Cardiac cells, then again, are each bodily and electrically linked, all of the time. It takes exact coordination to create the excessive and low pressures required to pump your blood in spite of everything, and cardiac cells have to be linked with a purpose to have that good timing. And there’s another factor you want to find out about your coronary heart’s cells: A few of them can generate their very own electrical energy. How within the identify of Raymond de Vieussens can that be? Nicely, rewind your mind to after we explored {the electrical} marvel that’s the motion potential, and the way it triggers each neurons and muscle cells. That course of began by depolarizing the cell — that’s, pushing the cell’s membrane potential from unfavourable towards optimistic, previous a threshold that triggered voltage-gated ion channels to open. Most cells in your physique solely depolarize after being triggered by an exterior stimulus, or by a neighboring cell, in a protracted chain response of motion potentials that’s set off by the nervous system. However that isn’t the case for a particular group of cells discovered solely in your coronary heart — ones that may set off their very own depolarization. These are your pacemaker cells. Pacemaker cells are what maintain your coronary heart beating on the appropriate rhythm, and be sure that every cardiac muscle cell contracts in coordination with the others, since you don’t need your mind to must ship a sequence of motion potentials each time you want your coronary heart to beat. Your mind has obtained different stuff to do. So pacemaker cells are, in a manner, your coronary heart’s very personal mind, producing the preliminary spark that sends a present by your coronary heart’s inner wiring system, often called the intrinsic cardiac conduction system. This method transmits electrical energy alongside a precisely-timed pathway that ends with atrial and ventricular contractions — also called coronary heart beats. And it begins with pacemaker cells producing their very own motion potentials. In most cells, the motion potential begins with the resting potential, which the cell maintains by pumping sodium ions out and potassium ions in, Proper? Then, when some stimulus causes the sodium channels open up, the sodium ions flood again in, which raises the membrane potential till it reaches its threshold. Pacemaker cells function the identical manner, apart from that preliminary stimulus. They don’t want it. Their membranes are dotted with leaky sodium and potassium channels that don’t require any exterior triggers. As a substitute, as their channels let sodium ions trickle in, they trigger the membrane potential to slowly and inevitably drift towards its threshold. For the reason that leaking occurs at a gentle price, the cells hearth off motion potentials like clockwork. And the leakier the membrane will get, the quicker it retains triggering motion potentials. The pacemaker cells at first of the conduction system have the leakiest membranes, and subsequently the quickest inherent rhythms, so that they management the speed of the complete coronary heart. And people quick, leaky cells are discovered within the sinoatrial node, or the SA node, up in the fitting atria. They primarily flip the entire SA node into your pure pacemaker. After these pacemaker cells make themselves hearth, they unfold their electrical impulses to cardiac muscle cells all through the atria. The impulses leap throughout synapse-like connections between the cells known as hole junctions, and proceed down the conduction system till they attain the atrioventricular node, or AV node, positioned simply above the tricuspid valve. Now, when the sign hits the AV node, it really will get delayed for like, a tenth of a second — so the atria can end contracting earlier than the ventricles contract. With out that delay, all of the chambers would squeeze directly, and the blood would simply splash round and never go anyplace. So as a substitute, the atria contract and blood drops down into the ventricles, after which a second later, the sign strikes on and triggers the ventricles to squeeze, making the blood move out of the center. And there are two tips to a superb ventricular contraction. One, the ventricles are so massive that the sign must be distributed evenly to make sure a coordinated contraction. And two, the ventricles must squeeze like their squeezing a tube of toothpaste — from the underside up — to speed up the blood by the massive arteries on the high of the center. So from the AV node, the sign travels straight all the way down to the inferior finish of the center and will get distributed to each side. The trail {the electrical} impulse takes to the underside of the center is known as the atrioventricular bundle, additionally identified by the extra rad identify, the bundle of His, the place it branches out to the left and proper ventricles. Lastly, the sign disperses out into Purkinje fibers, which set off depolarization in all surrounding cells, inflicting the ventricles to contract from the underside up like toothpaste tubes, at which level the entire cycle begins yet again. And the whole lot I simply described to you — from when the SA node fires to when the final of the ventricular cells contract — takes about 220 milliseconds. So that’s how your coronary heart beats. However I do know what you need — you need to get again to speaking about TV reveals and McDreamy and his paddles. It’s completely comprehensible. OK, so image all of your particular person coronary heart cells as a bunch of musicians in an orchestra. All of them sound actually nice collectively, however then the conductor all of a sudden must go to the toilet. And it sounds OK at first, however then the tuba will get a bit of bizarre, and the triangle is half a beat off, and shortly everyone seems to be taking part in a distinct observe at a distinct time, jamming to their very own private rhythm. Within the coronary heart, we name this out-of-sync habits fibrillation, and it may be attributable to all kinds of issues, particularly ones that have an effect on the pacemaker cells within the SA node. In an orchestra, this simply sounds actually horrible. In a coronary heart, there’s no coordinated contraction, no lub-dub, no blood transferring by the physique. Which suggests you’ll quickly be useless. However then the conductor comes again from the toilet break, faucets her wand, and all people stops. It’s silent for a second earlier than the wand comes up, after which all of them begin taking part in once more, this time in unison. In case your coronary heart in fibrillation is an out-of-sync orchestra, then a defibrillator is that conductor. It stops the chaotic noise by overriding all of the people, and hits a form of reset button so everybody can begin once more on the identical web page. The paddles ship a lot electrical energy by the center that they set off motion potentials in the entire cells directly. Then, the cells repolarize, and begin leaking once more, after which essentially the most leaky cells, within the pacemaker SA node, attain their threshold and hearth first, re-setting the rhythm that retains everybody in concord so your coronary heart features correctly. And that’s how sizzling medical doctors and their paddles really cease hearts to save lots of lives. Now the factor about CPR — or cardiopulmonary resuscitation — is that it may possibly’t appropriate fibrillation. What these chest compressions can do is pressure a fibrillating coronary heart to maintain circulating oxygenated blood till assist arrives. But when an individual is in cardiac arrest, simply respiration into their mouth and compressing their chest gained’t ship the electrical energy wanted to offer the pacemaking cells an opportunity to reset. I feel I simply found out why they name these TV medical doctors coronary heart throbs. That’s a horrible joke. Anyway, right now you realized how your coronary heart’s pacemaker cells use leaky membranes to generate their very own motion potentials, and the way the ensuing electrical energy travels by the cardiac conduction pathway from SA Node to Purkinje fibers, permitting your coronary heart to contract. And if you happen to weren’t too busy daydreaming about TV medical doctors, you additionally realized how defibrillators work to reset the rhythm of your coronary heart. Thanks to our Headmaster of Studying, Thomas Frank, and to all of our Patreon patrons who assist make Crash Course doable by their month-to-month contributions — not simply to themselves however to everybody on this planet at no cost. Should you like Crash Course and need to assist us maintain making these movies, you may go to patreon.com/crashcourse. Crash Course is filmed within the Physician Cheryl C. Kinney Crash Course Studio. The episode was written by Kathleen Yale, edited by Blake de Pastino, and our advisor is Dr. Brandon Jackson. It was directed by Nicholas Jenkins; the script supervisor and editor is Nicole Sweeney; our sound designer is Michael Aranda, and the Graphics crew is Thought Cafe.