Answer
Once a sperm makes contact with the egg and penetrates it, other sperms are excluded by two mechanisms , namely, the slow block to polyspermy and the fast block to polyspermy.
Work Step by Step
The fast block to polyspermy occurs within 0.1 second after the sperm membrane contacts the egg membrane . Enzymes secreted by the acrosome increase the permeability of the egg membrane. This lets Na+ ions stream into the egg cytoplasm. These cations cause a depolarization of the egg membrane from a resting potential (-70 mv)
The slow block takes place after 30 to 60 seconds. It involves the acrosomal reaction during which sperm receptors are removed from the surface of the egg membrane so that other sperms do not bind. Depolarization allows Ca++ cations to enter the egg ER and stimulate reactions in which cortical vesicles fuse and water is admitted into the cell. This causes the zona pellucida to swell and harden. This has two effects: it makes the zona more difficult to penetrate, and also increases the distance between the egg membrane and other sperm that may have fertilization in mind.
