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Accelerator nerve increases contractions, vagus nerve slows down and adrenaline increases. The SAN initiates an electrical impulse across the atria of the heart.

The left and right atria become depolarised and this causes them to contract. Depending on the nervous impulse or hormone varies the contraction.

The impulse then travels to the AVN there is a delay of 0.13s due to non-conducting tissue

This ensure the atria have fully contracted and the ventricles have filled up with blood before contracting

The signal then travels down the bundle of His which branches and forms separate Purkyne fibres which penetrate the ventricle walls.

This then causes the ventricle walls to contract from the bottom upwards so blood can move up into the aorta and pulmonary artety.