Final answer:
Two events occur at fertilization to block polyspermy: the fast block, which changes the oocyte's membrane potential, and the slow block, or cortical reaction, which releases proteins and forms a fertilization membrane to prevent additional sperm from fusing.
Step-by-step explanation:
At fertilization, to block polyspermy, which is the penetration of an egg by more than one sperm, two main events occur. The first is the fast block to polyspermy, a quick change in the oocyte's membrane potential caused by the influx of sodium ions when the first sperm binds, preventing additional sperm from fusing. This electrical change happens nearly instantaneously and lasts only a short time. However, the fast block triggers a second, longer-lasting mechanism, the slow block to polyspermy, also known as the cortical reaction.
During the cortical reaction, cortical granules just beneath the oocyte plasma membrane release zonal inhibiting proteins and mucopolysaccharides into the space between the plasma membrane and the zona pellucida. These proteins ensure the release of any other sperm that have bound and destroy the oocyte's sperm receptors, thereby stopping more sperm from attaching. Moreover, the mucopolysaccharides create a sturdy, impenetrable barrier around the new zygote, reinforcing the zona pellucida to become what is known as the fertilization membrane. This combination effectively ensures that only one sperm can fertilize the egg, preventing a genetically unviable triploid organism that would otherwise result from multiple sperm fusing with the egg.