Heart nerve conduction Illustration of a section through a human heart with an electrocardiogram ECG trace The heart is a hollow muscle that pumps blood around the body ECG traces show the pumping of the heart as a series of waves caused by the electrical impulses that coordinate heart muscle contraction The impulse starts at the oval sinoatrial node yellow at upper left causing the two small upper heart chambers atria to contract The impulse then runs down the nerve fibres yellow to the oval atrioventricular node green at upper centre The two large lower heart chambers ventricles dark pink then contract ECG is vital for heart monitoring