Final answer:
Fast fibers in cardiac action potentials serve to rapidly conduct electrical impulses throughout the heart and initiate muscle contraction, leading to synchronized heartbeats and efficient blood pumping.
Step-by-step explanation:
When it comes to cardiac action potentials, the two primary functions or roles played by fast fibers include the rapid conduction of electrical impulses and the initiation of muscle contraction. The Purkinje fibers, which are specialized myocardial conductive fibers, swiftly spread the impulse to the myocardial contractile cells in the ventricles. This facilitates an efficient and synchronous contraction, beginning at the apex of the heart and moving towards the base, essential for effective blood ejection into the aorta and pulmonary trunk.
The rapid conduction is achieved thanks to the fast fibers' ability to quickly hydrolyze ATP through the action of myosin's ATPase, leading to speedier cross-bridge cycling and therefore a more rapid contraction as compared to slow fibers. Second, the fast fibers ensure a coordinated contraction pattern by efficiently transferring the electrical stimulus from the SA node through the conduction pathway to all ventricular muscle cells in about 75 ms.
These fibers are critical in ensuring that the heart beats in a steady and coordinated fashion, responding adequately to changes in physiological demands and contributing to the heart's ability to function as a pump.