In this lesson you will learn about your heart's very own regulatory system, the intrinsic conduction system, and how it sets the pace for a normal, healthy heartbeat. You will discover that your heart moves to its own beat thanks to the natural pacemaker of the heart called the SA node.
The cells of the heart, or cardiac muscle fibers, have the unique ability to contract on their own
During a normal, healthy heartbeat, or what we call a cardiac cycle, the top two chambers of the heart, called the atria, contract simultaneously. Then, as they relax, the bottom two chambers, called the ventricles, contract. This explains what happens during a cardiac cycle, but what it doesn't tell us is how the atria and ventricles know when it's time to contract. In other words, what's controlling the beating of the heart?
As it turns out, the cells of the heart muscle, otherwise called cardiac muscle fibers, have a unique ability that makes them unlike any other muscle cells in your body. What's so unique about cardiac muscle fibers is that they can contract on their own, even if the nerves to these muscle fibers are severed.
Intrinsic Conduction System
This ability is thanks to the intrinsic conduction system, which is the internal regulating system that causes the heart to contract rhythmically. This term is easy to recall if you remember that 'intrinsic' means 'internal.'
The specialized tissue of the intrinsic conduction system is unlike any other tissue in your body because it has both nervous and muscular characteristics. This is what allows this tissue to conduct impulses throughout the heart muscle. We should note that the heart rate can be influenced by nerves of your autonomic nervous system, and this can cause your heart to speed up and slow down, but in this lesson, we'll focus on the rhythmic internal control of the heartbeat.
The SA node is the starting point of intrinsic conduction
The starting point of the intrinsic conduction system is the sinoatrial node, or SA node. A node is a mass of cells, somewhat like a knot, and the SA node is a mass of cells that set the pace of the heart. The SA node is very important, and it's often referred to as the pacemaker of your heart. In fact, if something goes wrong with this node, a person might need to have an artificial pacemaker implanted to keep their heart's steady beat.
The cells of the SA node can depolarize on their own without any external influence. We remember that depolarization is a change in the cell's membrane potential, making it more positive on the inside, and it's this switch to a more positive state that sparks the electrical impulse needed to start each heartbeat. Once the impulse is generated, it travels throughout the heart, kind of like an electrical current through a wire when you flip on a light switch. The SA node is actually only found in your right atrium, but its signal quickly spreads to the left atrium, and this allows both atria to contract at the same time.
However, the impulse cannot spread directly to the ventricles. The impulse must be conducted by more of this specialized tissue, namely called the atrioventricular node, or AV node. This name is easy to recall because this node is found between your atria and ventricles, and its job is to transmit the electrical impulse from the atria to the ventricles. You can think of the AV node as a traffic cop, guiding traffic through a busy intersection. The AV node moves the impulse from the SA node through to the rest of the heart.
But, there's another important function of the AV node, and that is that it causes a delay between the contraction of the atria and the contraction of the ventricles. This delay is very brief - it's only about a tenth of a second - but it's enough time to ensure that the atria have expelled their blood into the ventricles before the ventricles contract. This delay is very important, because if the atria and ventricles contracted at the same time, they would be pushing against each other and blood would not be able to move through the heart in a coordinated way.
Bundle of His and Bundle Branches
The bundle of His is a collection of cardiac muscle fibers transmitting electrical impulses
After the brief delay at the AV node, the impulse moves on to more conducting tissue called the bundle of His. This looks like it would be pronounced bundle of 'his,' but it's actually pronounced 'hiss,' as in the hiss of a snake, because it's named after the German cardiologist who discovered it, and his name was Wilhelm His.
The bundle of His is simply a collection of cardiac muscle fibers that transmit the electrical impulse from the AV node. The bundle of His is sometimes called the AV bundle, and it looks like a trunk in the upper part of the wall that separates the two ventricles. It runs only a short distance before sprouting into two branches.
These branches are known as the right and left bundle branches. They are a continuation of the conduction fibers that transmit the electrical impulse down the ventricles. Even though these are just conducting fibers, they can be injured due to heart disease, a heart attack, or surgery. If the branches are injured, they might slow down or block the conduction of the electrical impulse to the ventricles, making it harder for the heart to pump blood efficiently through the body.
Electrical impulses move through the Purkinje fibers very rapidly
The impulse is now passed from the bundle branches to the terminal branches of the intrinsic conduction system known as the Purkinje fibers. These are conducting fibers that relay the electrical impulse throughout the ventricles. The electrical impulse moves through the Purkinje fibers very rapidly, and this impulse is what causes the ventricles to contract simultaneously. This brings us to the end of the electrical impulse, and even though the impulse traveled to every cell of your heart, it's interesting to note that this entire process took less than a second, the time it takes for a single heartbeat.
Let's review. The heart is a unique muscle because its muscle fibers have the ability to contract on their own. This ability is thanks to the intrinsic conduction system, which is defined as the internal regulating system that causes the heart to contract rhythmically.
The sinoatrial node (SA node) is the starting point of the intrinsic conduction system, and it's often referred to as the pacemaker of your heart because its cells can depolarize on their own and generate the impulse that begins your heartbeat. It's found in the right atrium, and when it fires, it quickly sends a signal to the left atrium, causing the atria to contract simultaneously.
The impulse then reaches the atrioventricular node (AV node), which is the traffic cop of the heart and transmits the electrical impulse from the atria to the ventricles. The AV node creates a delay between the contraction of the atria and the contraction of the ventricles. This delay allows the atria to contract and expel all of their blood into the ventricles before the ventricles contract. This delay is important because it allows blood to move through your heart in a coordinated way.
Following this lesson, you should be able to:
- Describe the intrinsic conduction system
- Explain how an impulse travels through the heart