To prevent more than one sperm fertilizing the egg (polyspermy), fast block and slow block to polyspermy are used. Fast block, the membrane potential rapidly depolarizing and then returning to normal, happens immediately after an egg is fertilized by a single sperm. Slow block begins the first few seconds after fertilization and is when the release of calcium causes the cortical reaction, various enzymes releasing from cortical granules in the eggs plasma membrane, to expand and harden the outside membrane, preventing more sperm from entering.