Practice question

Fast block is transient electrical depolarization preventing extra sperm fusion until slow block completes. Sea urchin resting potential -70 mV shifts to +20 mV within seconds after first sperm entry. This positive potential makes membrane refractory because sperm-egg fusion voltage-dependent and favored only at negative potentials. Depolarization lasts about a minute until cortical granule exocytosis elevates fertilization envelope. Voltage clamp experiments show positive-held eggs reject sperm, return to negative restores receptivity. One to two minutes ensures overlap with slow block; shorter seconds insufficient, longer many minutes would impair subsequent ionic homeostasis required for development.