Egg membrane depolarizes within:
Upon sperm-egg fusion in sea urchin, sodium channels open causing rapid influx of sodium ions into egg cytoplasm raising positive charge. Membrane potential shifts from resting about minus seventy millivolts to positive twenty millivolts within one to three seconds, constituting fast electrical block to polyspermy. Depolarized membrane is refractory to additional sperm fusions because fusion machinery requires negative potential for electrostatic interaction. This transient block lasts about minute until cortical granule-derived fertilization envelope elevates permanently. Later cortical react
Ref: Gilbert, Developmental Biology, 12th ed., Chapter 7: Membrane depolarization within 1-3 seconds blocks polyspermy.