What is a small bundle of nerve tissue that produces an electrical stimulus which causes the ventricles of the heart to contract?

The small bundle of nerve tissue you are referring to is the Purkinje fibers, part of the heart's conduction system. These fibers conduct electrical impulses from the atrioventricular (AV) node to the ventricles, leading to their contraction. This process ensures coordinated and efficient pumping of blood throughout the body. The Purkinje fibers play a crucial role in maintaining the heart's rhythm and overall function.