What happens to the sodium and potassium ions when the neurons are stimulated? how does their concentration inside and outside the cell change?

The two sides of a nerve cell membrane have unequally distributed ions ( Na+ and K+). Sodium is inside, while potassium is outside. Carriers actively transport them. This active transport mechanism is called Sodium-Potassium Pump. Without stimulus, sodium channels are closed while some of the potassium channels are open.

Stimulus (depolarization) will cause the sodium channels to briefly open and the positively-charged ion diffuses into the nerve cell. The membrane potential becomes positive. The potassium channels open and positively-charged potassium diffuse out. With respect to the outside, the inside will become less positive again.

Plato

Sodium ions flood into the cytoplasm of the cell from the outside, increasing the concentration of sodium ions inside the cell. Potassium ions move outside, decreasing the concentration of potassium ions within the cell.

sheep is a herbivore animal whereas grasshopper belongs to the class chordates but both of them feed on greens.  

if grasshopper and sheep feeding on a pasture land, i do not think both of them will have any impact on each other. however, the number of the species can make difference. since sheep feed on greens, there is a possibility there left no pasture land for the grasshoppers or they may get extinct.