What transmits the signal that causes both ventricles to contract?

The signal that causes both ventricles to contract is transmitted by the Purkinje fibers, which are part of the heart's conduction system. This system includes the sinoatrial (SA) node, atrioventricular (AV) node, and the bundle of His, which together coordinate the electrical impulses that initiate and regulate the heartbeat. When the impulse reaches the Purkinje fibers, it triggers the ventricles to contract simultaneously, ensuring efficient blood ejection from the heart.