When not conducting any signal or impulses, the neurons are said to be in resting state. In that case, the membrane of the axon is more permeable to potassium ions and impermeable to sodium ions and the negatively charged proteins present in axoplasm. The plasma in the axon contains a high concentration of potassium ions and proteins and low concentration of sodium ions. Whereas the fluid outside the axon contains a high concentration of sodium ions and low concentration of potassium ions. Due to this, a concentration gradient is formed. Active transport of ions occurs across the membrane by the sodium-potassium pump where three ions of sodium are transported outwards and two ions of potassium move into the cell. Because of this, the outer surface of the membrane becomes positively charged while the inner surface is negatively charged. The cell is said to be in a polarised state. The electrical potential difference across the resting plasma membrane is known as resting potential.

When we apply a stimulus at a site on the polarised membrane, the membrane at the site becomes freely permeable to sodium ions. Due to this, sodium ions move inside the cell and the outer side of the membrane becomes negatively charged and the inner side becomes positively charged. The membrane is said to be in depolarised state. The electrical potential difference generated across the plasma membrane at this site is known as action potential or nerve impulse. This area becomes a stimulus for the neighboring area of the membrane which becomes depolarised. The former membrane becomes repolarised due to the movement of sodium ions outside the cell.  This is how impulses are conducted.