A heartbeat starts with an electrical signal in the wall of the right atrium - called the sino-atrial node (SAN) or pacemaker. This electrical signal sets the rhythm of the heartbeat.
A wave of electrical activity spreads out from the SAN across both atria causing them to contract.
A layer of non-conducting collagen fibrous tissue (septum) prevents the wave from being passed directly from atria to inside the ventricles.
The electrical wave instead passes directly through one region only, the atrioventricular node (AVN), which is located inbetween the atria.
This AVN is connected to specialised conducting muscle fibres located in the wall in the septum called the bundle of His.
The AVN after a short delay is responsible for passing the electrical waves down the bundle of His.
The bundle of His fibres conduct the electrical wave rapidly to the base of the ventricles, where the bundle branches off into the Purkinjie fibres.
The electrical wave is released from these Purkinjie fibres causing both ventricles to contract quickly at the same time from the apex of the heart upwards.