By sodium and potassium pump.
The sodium-potassium pump is observed in various cellular membranes. They are fueled by ATP, the pump moves potassium and sodium ions in reverse orientation, each against its concentration gradient. In a single cycle, two potassium ions are imported into the cell, and three sodium ions are extruded out.
These pumps are utilized to produce gradients of sodium and potassium ions. These gradients are used to proliferate electrical signals, which travel along the nerves.