Depolarization of a cell membrane occurs because

Stimulus stimulate the sodium ion voltage gated channel to open in the axon. This allow many Na ion to enter the axon and causes depolarisation as the pd in the axon getting less negative charge (from -70mV to -50mV).

imagine a line passing through the center of earth that goes through both the north pole and the south pole. this imaginary line is called an axis. earth spins around its axis, just as a top spins around its spindle. this spinning movement is called earth's rotation. at the same time that the earth spins on its axis, it also orbits, or revolves around the sun. this movement is called revolution.