Final answer:
The fast block to polyspermy results from a change in oocyte membrane potential, preventing the fusion of additional sperm cells.
Step-by-step explanation:
The phenomenon known as the fast block to polyspermy is indeed caused by a change in the oocyte membrane potential. When the first sperm binds to the oocyte, there is an instantaneous alteration in sodium ion permeability, leading to the depolarization of the oocyte plasma membrane. This electrical shift prevents additional sperm from fusing with the oocyte. Following this, a slow block to polyspermy, also called the cortical reaction, ensures a more permanent prevention of additional sperm entry. The fast block is a critical step to assure that only a single sperm cell fertilizes the egg, enabling normal embryonic development.