The Fast Block to Polyspermy
Upon fusion of the sperm’s and egg’s cellular membranes, fertilization has been completed (in contrast to sex, which is the fusion of the genetic material). At this point the egg is extremely vulnerable to other sperm attempting to fertilize, so immediate action must be taken. Seconds after fertilization the membrane potential of the egg undergoes a large depolarization via a massive influx of Na+ ions. This depolarization is similar to a neuronal action potential in that it breaches Vthreshold, however, the egg’s depolarization takes about a minute to repolarize via K+ leakage. This is the fast block to polyspermy: sperm cannot fuse to a membrane that is not -70mV