Final answer:
The Refractory period is the time during the cardiac cycle when heart cells are unable to respond to any stimulus, ensuring proper heart function and efficiency in blood pumping.
Step-by-step explanation:
The period during the cardiac cycle when cells cannot respond to a stimulus, no matter how strong, is called the Refractory period. During this time, the heart muscle cells are unable to initiate another action potential. This period is crucial for the heart's functionality as it ensures that the cells can fully contract before another electrical event can occur, allowing for efficient blood pumping. The refractory period comprises two phases: the absolute refractory period and the relative refractory period. Specifically, during the absolute refractory period, not even a strong stimulus can trigger a new action potential, which is vital for the proper sequence of muscle contraction and relaxation that drives the cardiac cycle.