In the previous section, we went over the different parts of the heart and what each of their roles are. Now we will be looking into what goes on behind the scenes and what triggers the heart muscles to work, aka: The Cardiac Conduction System.
The Cardiac Conduction system creates and sends out electrical signals that stimulate the contraction of the Myocardium (the middle layer of the heart). The impulse starts off at the top of the heart, runs over the atria and moves down to the ventricles allowing the heart to fill up and push out the largest volume of blood possible. And all of this works in perfect harmony because of two very important cells: The Nodal cells and The Purkinje Cells.
The Cardiac Conduction System – Part 1
Together, the nodal and purkinje cells provide three things:
They can initiate an electrical impulse, just like a match can start fire. This is known as Automaticity.
They can react and respond to the impulse, which means they have Excitability
They transmit the electrical impulse from one cell to another making them Conductive.
These three points are saying that our body has a point that creates and regulates the heart’s electrical signal. And we call that point a Pacemaker, the primary pacemaker in our body is the Sinoatrial Node (SA Node) and just in case we get a flat tyre, our body has a back up called the Atrioventricular Node (AV Node). Both of which are made up of Nodal Cells
If we’re unlucky and both the SA and AV Node are damaged, we can get a Pacemaker device that creates and regulates the heart’s electrical signal.
The SA Node is found at the point where the superior vena cava meets with the right atrium. In a normal healthy adult, it gives off between 60 to 100 impulses per minute. These impulses travel over the myocardial cells of the atria with the help of Internodal Pathways.
Think of Internodal Pathways as highways or main roads that the impulses can use to travel without any speed limit.
The impulses given off by the SA Node electrically stimulate the atria causing the chamber to contract. From there the impulse continues to travel through the Internodal Pathways to reach and conduct the AV Node. Once the AV Node receives the signal it creates an intentional minor delay allowing time for the ventricles to fill up, and then shoots the impulse over the ventricles causing ventricular contraction.
Technically, we could stop here, but just to make your life a little bit more difficult, I’m going to tell you exactly how the impulse travels over the Ventricles 🙂
The Cardiac Conduction System – Part 2
When the impulse is conducted (shot) through the AV Node, it travels through a bundle of specialised conductive tissue called The Bundle of His. This then divides again into the Right bundle branch which conducts the impulse over the right ventricle and the Left bundle branch which conducts the impulse over the left ventricle.
Because the left ventricle is the largest chamber of the heart, the Left Bundle Branch breaks down even more into left anterior and left posterior bundle branches. Which ultimately reach the terminal point of the conduction system called the Purkinje Fibers.
These fibers are made up of Purkinje Cells which rapidly conduct the impulses through the thick walls of the ventricles. At this point, the myocardial cells are triggered and contract the ventricles.
The Cardiac Conduction System – Part 3: The Cardiac Action Potential
So far we’ve discussed how the Nodal and Purkinje Cells create and transmit electrical impulses across the heart, and they trigger the cardiac myocytes to contract. But what really happens when we say ‘an electrical signal passes through?’
There’s a group of particles in our bodies that have an electrical charge and these particles are called Ions.
Now these Ions are like a bunch of kids in quarantine that have way too much energy, and we’re going to name these kids Sodium, Potassium and Calcium. All these kids want to do is run around from one room to another screaming and using up their energy. Any responsible adult would stay at the door and guard the kids telling them when they can leave the room and when they can enter. This keeps them from harming themselves, or potentially destroying things in the next room.
The same thing is going inside your heart, the charged ions want to cross from one cell to another, and thanks to the channels in your cell membrane, these ions are regulated. Sodium is allowed to enter the cell rapidly through sodium fast channels, while calcium is told to enter slowly through calcium slow channels.
For the sake of not wanting to get into too much details, remember the following things:
Ions can have a positive or negative charge
During the resting period, the heart cells are filled with Potassium ions on the inside, and are surrounded by a lot of Sodium ions on the outside.
This difference gives the inside of the cell a negative charge, and slightly more positive charge outside.
We call this stage Polarisation.
So Polarisation occurs during the rest phase, when the heart cells have a negative charge.
But this state does not last for too long, in fact during cellular stimulation, sodium and calcium will cross through the cell membrane and enter the cell. While potassium ions move out of the cell into the extracellular space.
This switch creates a positive charge inside the cell and negative charge outside the cell and it’s known as Depolarization. As soon as this depolarization finishes, the ions revert back to their original starting point to switch again. The period when ions are moving back to their original spot is called Repolarization. This cycle is called the Cardiac Action Potential, and it continues to repeat itself over and over again generating the electrical impulse that travels through the heart.
I know it’s a bit foggy, but let’s break it down into phases:
The Cardiac Action Potential Phases
Phase 0: Positive Sodium and Calcium ions rush into the cell and cause Cellular Depolarization. (Sodium causes the atrial and ventricular myocytes to depolarize while calcium depolarizes the SA and AV node.)
Phase 1: Potassium leaves the cell and sits outside, but the moment this happens Early Cellular Repolarization is triggered. Which means that Sodium and Calcium start to leave the cell as well.
Phase 2: The repolarization phase slows down, because now that calcium and sodium are out, the calcium wants to get back in.
Phase 3: Sodium has now followed Calcium back into the cell, and Repolarization is complete allowing the cell to rest.
Phase 4: The cell rests before repeating the whole cycle again.
Let’s think of the kids again, and this time they’ve got a kiddy pool. Potassium is minding his own business enjoying the little pool, when Sodium jumps in and Calcium decides to join too. But Potassium doesn’t want to be in the pool now because it’s crowded, so he leaves.
Now that he left, Calcium doesn’t want to be in the pool because it’s not fun when Potassium isn’t there. So she slowly leaves the pool to be out next to Potassium. And of course Sodium doesn’t want to be alone, so he jumps back out to join them.
Potassium sees the empty pool and tries to get back in, until Sodium and Calcium join again, the whole process repeats itself over and over again.
The Cardiac Conduction Cycle – Part 4: Refractory Periods
Myocardial cells have to repolarize completely before they can depolarize again, and during this time the cells are in a Refractory Period.
The refractory period is divided into two phases:
The Effective Refractory Period – Occurs between phase 0 to phase 3. Here the cell is completely unresponsive to electrical stimuli making it impossible to initiation an early depolarization.
The Relative Refractory Period – Occurs at the end of phase 3. At this phase, the cell is only responsive to abnormally strong electrical stimulus. If that occurs, the cell may depolarize prematurely putting the individual at risk of dysrhythmias
And that’s pretty much it for the cardiac conduction system! The next chapter will be about Nursing Patients with Arrythmias. So hop on over, and let’s get started 🙂
BTW, I you’ve got any questions you should message me on Instagram @Miriana.Nurse and I’ll happily reply <3 Let’s ace those exams!