In a nervous cell, some ions are kept in specific positions on each side of the cell membrane. But ions tend to drift around so that they become spread out and balanced, that is, they will diffuse until their concentration is equal, and any charge difference has been eliminated.
Ions and molecules dissolved in solution move randomly. This temperature dependent, constant movement will tend to distribute the ions throughout the solution. In this manner, there will be movement of ions from regions of high concentration to regions of low concentration. This movement is called diffusion. As an example, add a spoon of milk to a glass of water. The milk spreads through the water solution.
 
 

Molecules in solution move randomly. In this beaker full of water, a membrane divides the beaker in half. On the left side there is a collection of molecules. A membrane prevents movement of the water and the molecules from crossing from one side of the beaker to the other Dark red bars means the concentration substance. (falar que o canal mostrado não é permeável aos íons).

When the membrane is removed, it allows the water and molecules to move freely.