Coloured computed tomography (CT) scan of the human heart, highlighting the cardiac conduction system (yellow). This is a series of specialised muscle and nerve cells responsible for transporting the electrical signal that regulates the heart beat. The signal begins in the sinoatrial node (top left) and travels through a muscular chamber called the right atrium (top left of image) to the atrioventricular node (centre). From here, the signals are sent along the bundle of His (centre of heart, descending) to the bottom of the heart, causing the ventricles (lower chambers) to contract, completing one heartbeat.