Outline the mechanisms that control the heartbeat including the SA node, AV node and conducting fibres in the ventricular wall.

The heart beat is referred to as myogenic because it originates in the heart. It is controlled primarily by the “pacemaker”, which is located in the upper right hand corner of the right atrium. This area is known as the sinoatrial node (SA node). Cells in the SA node generate electrical signals that are sent to the atria. This induces the atria to contract. The impulses are prevented from spreading directly into the walls of the ventricles by a layer of fibrous tissue. Instead, when these electrical impulses reach the division between the atria and the ventricles, they excite the atrioventricular node (AV node) which sends impulses to the ventricles. However, these signals are delayed by the AV node during 0.09 seconds to allow for the atria to completely contract. When the impulses are sent, they travel along the septum through the bundle of His nerves until they are dispersed along the Purkinje cells which are spread out across the ventricles. These impulses cause the muscles of the ventricles to contract.
**Animation: The Cardiac Conduction System**

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