Practice question

Fast block to polyspermy in sea urchin eggs is achieved electrically rather than structurally. Within one to three seconds of sperm-egg fusion, sodium channels open producing rapid depolarization from resting -70 mV to positive +20 mV, known as fertilization potential. This voltage shift renders plasma membrane refractory to additional sperm fusions because sperm-egg fusion is voltage-dependent and favored only at negative potentials. Depolarization is transient lasting about a minute until slow block secures envelope elevation. Cortical granule discharge, protease activation, and bindin binding correspond to slow permanent block, not immediate electrical mechanism preventing supernumerary entries.