The sinoatrial node (SAN) produces a wave of electrical activity which causes both atria to contract, followed by the ventricles. The SAN initiates the heartbeat.
A wave of electrical activity spreads out from the SAN across both atria, causing them to contract.
A layer of non-conductive tissue (the atrioventricular septum) prevents the wave crossing the ventricles.
The wave of electrical activity is allowed to pass through a second group of cells called the atrioventricular node (AVN), which lies between the atria.
The AVN,after a short delay, conveys a wave of electrical activity between the ventricles along a series of specialised muscle fibres called the bundle of His.
The bundle of His conducts the wave through the AV septum to the base of the ventricles,where the bundle branches into smaller fibres.
The wave of electrical activity is released from these fibres, causing the ventricles to contract quickly at the same time, from the apex of the heart upwards.