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The sinus node is located at the top of the right atrium and is known as the natural pacemaker of the heart because it is normally responsible for controlling the heart rate. The sinus node may also be referred to as the sinoatrial node.

When participating in physical activity (or with fright and anger) the body organs need more blood, the sinus node increases the heart rate to supply more blood. When the body is relaxed the sinus slows the heart rate.

The electrical signal from the sinus node rapidly spreads across the right and left atrium, this causes the atria to contract. The ventricles and the atrium are insulated electrically from each other by the AV groove, the AV groove is part of the valvular structural skeleton of the heart.

The only electrical connection between the atria and the ventricles is at the centre of the heart. The connection is called the Atrioventricular Node or AV Node, it acts as the main power cable of the hearts electrical system, all electrical signals between the atrium and the ventricles must pass through the AV Node. A bundle of special cells provide a path for the electrical signals to pass through the AV node to the ventricles.

The fibres of this bundle are called the bundle of His, this bundle divides down to the left and right ventricles. As the fibres branch out over the ventricles they are referred to as the Purkinje Fibres. The electrical signal passes rapidly through the Purkinje fibres causing the ventricle muscles to rapidly contract.

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