Final answer:
When the membrane potential of a cell becomes less negative than -40 mV, voltage-gated sodium (Na+) channels open, allowing sodium ions to rush into the cell. This process is known as depolarization and is a key step in the generation of an action potential.
Step-by-step explanation:
When the membrane potential of a cell becomes less negative than -40 mV, voltage-gated sodium (Na+) channels open. These channels have two gates: the activation gate and the inactivation gate. When the cell reaches the threshold of -55 mV, the activation gate opens, allowing Na+ ions to rush into the cell. At the peak of depolarization, the inactivation gate closes, preventing further sodium entry. As the membrane potential repolarizes and reaches -55 mV again, the activation gate closes, and the inactivation gate re-opens to prepare for another cycle.