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Heartbeat and Heart Contraction Coordination

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  • 0:05 Cardiac Cycle
  • 0:49 Intrinsic Conduction System
  • 1:36 SA & AV Nodes
  • 3:48 Bundle of His & Bundle…
  • 4:57 Purkinje Fibers
  • 5:32 Lesson Summary
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Lesson Transcript
Instructor: Rebecca Gillaspy

Dr. Gillaspy has taught health science at University of Phoenix and Ashford University and has a degree from Palmer College of Chiropractic.

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.

Cardiac Cycle

The cells of the heart, or cardiac muscle fibers, have the unique ability to contract on their own
Cardiac Muscle Fibers

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.

SA Node

The SA node is the starting point of intrinsic conduction
SA Node

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.

AV Node

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
Bundle of His

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.

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