Cardiac Conduction System

This lesson is on the cardiac conduction system and covers:

1. Structure
2. Function

1. Structure

About 1% of the cells of the heart are apart of the cardiac conduction system. This group of cells are self-exciting and produce electrical signals, or electrical impulses, that stimulate heart contractions. They act as a pacemaker, and drive our heart contractions. More importantly, they do so independently of the nervous system. This is an interesting fact because, even if all nerves that were leading to the heart were cut, the heart would still be able to beat.

• Cardiac Conduction System
• A system of specialized cells and nodes in the heart that rhythmically pace the heart and time when the chambers contract. Its major components are the sinoatrial node (SA node), atrioventricular node (AV node), bundle branches, and Purkinje fibers.

There are two nodes in the heart made up of these self-exciting cells:

• The sinoatrial node
• Atrioventricular node

Connected to these nodes, we have conducting muscle fibers that spread throughout the atria and the ventricles.

2.Function

Our heart beats because of the self-exciting cells in the two nodes of the heart. The sinoatrial node (also known as the SA node) produces an excitation wave that will spread over both of the atria. This signals for contraction and spreads so fast that the cardiac muscle cells will contract together almost as one unit. When our atria contract, this allows blood to flow into our ventricles.

This wave will start to slow down when it reaches our atrioventricular node, which is the second node. The slower conduction in the atrioventricular node is important because it allows for the atria to have time to finish contracting before the wave travels from the atrioventricular node to the ventricles. Once the atria are done contracting and the blood has filled out the ventricles, then the wave will stimulate the ventricles to contract.

• Sinoatrial node (SA node)
• The SA node is known as the pacemaker of the heart due to its ability to depolarize and repolarize faster than the rest of the cardiac conduction system. It is located in the right atrium of the heart.
• Atrioventricular Node (AV Node)
• Known as the “gateway” between the atria and ventricles; the AV node slows down the rate at which the action potential travels from the atria to the ventricles allowing for more filling time.

Have you ever known someone who has had an artificial pacemaker before? Why do you think they needed this?

It is because there is some sort of malfunction in their own SA node. The artificial pacemaker is implanted so they still have the stimulus for these heart contraction.

The cells in the heart that cause the heart to contract are self-exciting, meaning they work independently of the nervous system. The structure of the cardiac conduction system includes the sinoatrial node and the atrioventricular node. Connected to these nodes are conducting muscle fibers. The function of the sinoatrial node is to set the pace of the heart by generating an impulse that spreads like a wave through the heart along the conducting muscle fibers. The atria contract almost as one from this signal, but the wave is slowed down by the atrioventricular node before going to the ventricles. This allows the atria to finish contracting, and the ventricles to fill with blood before contracting themselves.

Keep up the learning, and have a great day!