Which structure in the heart stimulates the ventricles to contract?

The Purkinje fibers are responsible for stimulating the ventricles to contract. They are specialized fibers located within the ventricular walls that conduct electrical impulses efficiently and effectively, leading to coordinated contraction of the ventricles. When the electrical signal reaches the Purkinje fibers after passing through the interventricular septum, it prompts the ventricular muscle cells to contract, resulting in the pumping action that is crucial for moving blood throughout the body.

The sinus node, also known as the sinoatrial (SA) node, serves as the primary pacemaker of the heart and initiates the electrical impulse that stimulates heartbeats, but its primary role is to generate signals for the atria rather than directly stimulating the ventricles. The atrioventricular (AV) node acts as a gatekeeper that delays the impulse from the atria before it is transmitted to the ventricles, ensuring that the atria contract first and the ventricles contract afterwards. Hence, while they are important components of the cardiac conduction system, they do not directly cause ventricular contraction as the Purkinje fibers do.