Practice question

Cortical reaction and fast electrical block together prevent polyspermy, but initial rapid depolarization provides immediate transient protection. Upon first sperm fusion, sodium influx depolarizes egg plasma membrane from negative to positive within seconds, making membrane refractory to additional sperm fusions that require negative potential. This electrical change precedes cortical granule exocytosis by about half minute. Although some textbooks attribute depolarization to cortical granule release, it represents initial phase of reaction sequence. Membrane depolarization thus serves as fast mechanism within cortical reaction leading to polyspermy prevention before permanent fertilization envelope elevation completes protection of embryo.