Final answer:
Opening sodium ion channels allows sodium ions to enter the cell, causing depolarization and the propagation of nerve impulses.
Step-by-step explanation:
When a nerve impulse reaches a neuron, it opens sodium ion (Na+) channels, which allows sodium ions to enter the cell. This process causes a depolarization of the membrane, where there is a decrease in the voltage difference between the inside and outside of the neuron. This depolarization and entry of sodium ions is necessary for the propagation of nerve impulses and the transmission of signals.