The Quick and Dirty Guide to Cardiology III: Basic Cardiac Electrophysiology

That's a big scary phrase, isn't it! We hear, all the time, how difficult this is supposed to be. Don't sweat it. This is going to be cake. Instead of rushing through it and only remembering a few key phrases, we're going to teach it to you in a better way, so that its easier for you to understand and teach it to others!

The Basic Concept

The heart is just another muscle. Like all muscle cells, it squeezes when a signal from your body tells it to do so. Your brain tells your bicep to flex. All of the little muscle cells squeeze (contract) and BAM! your whole bicep muscle is flexed. Your heart is basically the same thing. A signal tells your the muscle in your left atrium to squeeze. It does, and all of the blood in it gets pushed into the ventricle. When that same signal travels down to the ventricle a little later, it tells all THAT muscle to squeeze, and then the blood shoots over the aortic arch into the rest of your body. These cells can also pass on the signal once it is received. So, its like each cell is told "Hey, I want you all to squeeze, so you squeeze and then pass it on." So, its a chain reaction where each cell is squeezing(contracting) and then passing it on(conducting) to the next cell. That's really it.  Pretty easy huh?  Lets put some fancy names on it so we can talk like doctors now...

4 Things Heart Cells Can Do

1. React to a signal.   This is called Excitablity. It means a heart cell has the ability to be excited into some action by a signal.
2. Squeeze.   Muscle cells shorten, or contract. This is called Contractility. This is the 'action' that you want a muscle cell to do. Contractility just means the ability to contract...or squeeze.
3. Pass it on.   After the muscle cells squeezes, it passes the signal on to the cell that is next to it. This is called Conductivity.  It means "pass on the signal you just received."
4. So what happens if it doesn't receive a signal? If your bicep muscle doesn't receive a signal, it doesn't flex... ever. But, the heart is magic! Cardiac cells are special; because they have a superpower that other muscle cells don't. If the cardiac muscle isn't getting a signal, it can GENERATE ITS OWN!  That superpower is called "Automaticity."  This just means 'the ability to generate its own signal' without the brain being involved at all. This is why a person can be brain dead and still have a working heart!

Where does the signal come from?

In the upper right corner of the heart, there is a little bundle of cells that happen to have a TON of parasympathetic nervous system fibers. This is called the Sinoatrial node. (Sinus just means pocket, atrial refers to the location, node just means a knot of tissue. So, Sinoatrial Node (or SA node) literally means a pocket of knotted tissue in the atrium. This bundle of tissue is really good at generating signals  because they really have no 'downtime' in their action cycles. Because there is no rest time needed for SA node signal generation, they can fire signals at a faster rate than the rest of the heart. The  SA node normally sends a 60-100 signals per minute! Because it is the first to do so, it tells the heart cells nearest it to squeeze, and then pass it on! Then, its a chain reaction throughout all of the muscle fibers of the wall, creating a giant muscle squeeze! Thus, the SA node is called the primary "pacemaker" of the heart. It sets the beat in a normal situation!

Slowing down the Signal

If the heart cells just squeeze and then pass it on, the ventricles and atria would contract at the same time!

So how does the atria squeeze first, then the ventricles? The signal generates in the SA node, tells all the atrial muscle cells to squeeze, and then travel down pathways toward the ventricles. The junction between the electrical pathways down the atria and the electrical pathways down the ventricles is called the AtrioVentricular junction. The job of the AV junction is to slow down the signal, so that the signal pauses long enough for the atria to squeeze before sending that signal on to make the ventricles squeeze. This gives time for blood to be pushed from the atria to the ventricles.

The AV Junction is made up of the AV node and the AV bundle.

The AV, or atrioventricular node, is the gateway between the atria and the ventricles.  It is the next stop on the electrical pathway and it serves two very important functions in the process of circulation.  The first is that it has a built in delay for the transmission of the electrical signal.  This 150-200 millisecond delay is what allows the atria to contract before the ventricles do, which is critical to proper heart function.  It is the reason why blood flows in the direction that it does, as you learned in Cardiology I.

The AV bundle is also called the "Bundle of His" We wish they would just say AV bundle, because its easier to teach and remember, but the guy who discovered it was named Wilhelm His (pronounced 'hiss') so everyone says 'bundle of His.'  The function of the bundle of His is to pass the signal to the ventricles. Since the ventricles are so large, the signal splits into two branches; one for each ventricle. The branch that carries the signal to the left ventricle is called the Left Bundle Branch. The branch that carries signals to the right ventricle is called the Right Bundle Branch. The bundle branches go down to the apex of the heart, then curl around the ventricular walls and then branch off into nerve fibers that are embedded into the muscle tissue of the ventricular walls. These smaller branches of fibers are called Purkinje fibers. Like the bundle branches, Purkinje fibers are just heart cells that are specialized for speed of conductivity rather than contraction. They connect the bundle branches to the actual muscle walls of the ventricles. They are named after the dude who discovered them.

So lets look at the path the signal travels again...

1. SinoAtrial Node (SA node) - Signal is generated 60-100 times a minute and passed via internodal pathways to the right atrium and via another bundle that tells the left atrium to contract. (Bachman's bundle)
2. Atrial muscle squeezes. Atria contracts. Sends blood down the AV valves (tricuspid and bicuspid) into the ventricles.
3. Signal is slowed down in the AV junction just long enough to allow the blood to get into the ventricles after atrial squeeze.
4. Signal speeds up again once it gets out of the junction, so that it can travel throughout the ventricles rapidly. It then travels down the left and right bundle branches to the Purkinje fibers, where it tells ventricular muscle walls to squeeze!
5. Ventricular wall muscle squeezes and the blood gets pushed over the aortic arch into the systemic circulation!

Polarization and Depolarization

We will touch on this again, but we want to demystify these terms today so that we can use them in the next lesson.

Depolarization and Repolarization Simplified

1. Polarized means kept apart.  The cell membrane keeps all of the ions apart (sodium, calcium, potassium etc.) They are separated! The muscle is at rest.
2. When the electrical signal hits the cell membrane, the membrane opens and allows the ions to mingle! Now they are not separated. They are UN-SEPARATED.  In other words: DE-POLARIZED.
3. The movement of the ions across the cell membrane causes the cell to contract  (squeeze).
4. The signal passes on ot the next cell, and acts as a wave: spreading out across the wall of the heart, causing ALL THE MUSCLE cells to contract in turn!
5. After the signal has passed, the ions separate again across the cell membrane to their normal resting state, which is APART from each other. In other words, they are getting RE-POLARIZED!
6. As the ions go back to being separated (polarized) the muscle cell relaxes again.

During rest, potassium channels are open and sodium/calcium channels are closed. During depolarzation, its the opposite!

Polarized = Kept Apart. ie. Resting state.
De-polarization = the process of ions moving across the cell membrane, causing the muscle cell to shorten (contract.)
Re-polarization = the process of ions moving across the cell membrane again, becoming separate, causing the muscle cell to relax.
The cell becomes completely polarized again and starts over.

Something to think about and/or answer in comments:

If there is an imbalance in the ions (K+, sodium or calcium) would that weaken or stop depolarization and contraction? How or why?

Summary:

1. Cardiac cells are special cells that can do 4 things: Generate its own signal, react to a signal, squeeze, and pass it on!
2. The signal normally is generated in the SA node and goes through the atria to the AV bundle, which lies between the atria and ventricles, where the signal is paused.
3. After the signal is finished pausing, it speeds back up and races through the ventricular walls, causing ventricular contraction, pushing blood out in the circulation.
4. Polarization, depolarization, and repolarization are just explanations of how a cell reacts to a signal, squeezes, and goes back to resting.
5. Polarized means apart. Depolarized means making things NOT-apart, or together. Re-polarization means going back to being apart, or resting.
6. Depolarization makes the muscle shorten, or contract. Afterwards, they re-polarize until they are totally back to their polarized resting state.

Hungry for More?!?

Paramedic ECG Cheat Sheet

The Quick and Dirty Guide to Cardiology I

The Quick and Dirty Guide to Cardiology II

The Quick And Dirty Guide to Congestive Heart Failure and Pulmonary Edema

The Quick And Dirty Guide to Heart Tones 

Other links for further study

Short video of basic electrocardiology in 1 minute.
EXCELLENT video of electrocardiology in 6 minutes!

This has been the Quick and Dirty guide to Cardiology III.  We now return you to your regularly scheduled dialysis run.

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