Membrane of a nonconducting neuron is positive on the outside and negative inside. This difference in charge is called as resting potential and the membrane is said to be polarized. In the external medium (tissue fluid), sodium ions (Na+) and chloride ions (Cl−) predominate, whereas within the fibre (intracellular fluid), potassium ions (K+) and large negatively charged organic (protein) ions predominate. The steady state of a resting nerve fibre, is maintained by the active transport of sodium and potassium ions against the concentration and electrochemical gradients. The process of expelling out sodium ions and drawing in potassium ions against concentration and electrochemical gradients is termed sodium pump, or better sodium-potassium-exchange pump. This requires energy (ATP), which is provided by metabolic processes within the nerve fibre itself with the help of enzyme, sodium and potassium - dependent adenosine triphosphatase, present in the plasma membrane. When a stimulus is applied, the permeability of the membrane to Na+ ions is greatly increased at the point of stimulation. It is due to the fact that the Na+ channels open and the K+ channels remain closed. As a result, sodium channels permit the influx of Na+ ions by diffusion. This results in the positive charge inside and negative charge outside. The change in polarity across the plasma membrane is known as action potential and the membrane is said to be depolarised.