It narrows rapidly as it gets further from the cell body. It physically touches the next neuron. It contains many ribosomes. It ends in multiple branches. Sodium ions enter the cell.
Potassium ions enter the cell.
Potassium ions leave the cell. Sodium ions travel down the axon. One involves hyperpolarization and the other involves depolarization. One is due to the opening of sodium channels, while the other is due to the opening of potassium channels. They differ in how many sodium channels are opened.