After the sperm’s receptors come into contact with the egg’s jelly layer and the acrosomal enzymes are released and break down the jelly layer, the sperm head comes in contact with the plasma membrane of the egg. When the two plasma membranes contact one another, signals in the egg are initiated.

First, Na⁺ channels in the egg open, allowing Na⁺ to flood into the egg. This causes a depolarization of the egg from its normal resting potential of -70 mV.

While depolarization is occurring, the remainder of the jelly layer is dissolving. With the dissolution of the jelly layer and the depolarization of the plasma membrane, the first block to preventing fertilization by multiple sperm is put into place.

Within 1/10th of a second of contact, the fast block to polyspermy is initiated. The cortical granules release its secretion causing the change in the composition of the zona pellucida membrane.

Slow block:

The cortical vesicles contain enzymes that cause a number of changes to occur in the egg.

1.  The egg’s sperm receptors are removed.

2.  H₂O is absorbed, causing the membrane to swell and push the vitelline membrane away from the plasma membrane.

3.  The vitelline membrane hardens.