The cytoplasm of a neutron contains many negatively charged proteins, which give the cell a slightly negative charge at rest. What do you think happens to this charge when the neuron is stimulated?

In the stimulation, there's an influx of sodium ions. So, the cell acquires a positive charge. But when the positively charged potassium ions move outside of the cell, the neuron returns to its previous state.

On stimulation, energy level of negatively charged protein increases, hence negative charge increases.