When an action potential is not being generated the neurone is at resting potential, also known as membrane potential. This was determined to be at -70 mV by the Goldman equation. At resting potential the inside of the membrane is slightly negative compared to the outside; this is due to the amount of potassium (K+) and sodium (Na+) either side of the cell membrane. The K+ concentration is 150 mM inside the cell compared to 5 mM outside the cell, whereas the Na+ concentration is 15 mM inside the cell compared to 150 mM outside the cell. The resting potential is maintained by a number of transmembrane proteins, some of which involve active transport and others rely on facilitated diffusion. The Sodium Potassium Pump works by active transport i.e. it requires energy to do work this is provided by Adenosine Triphosphate (ATP). The Sodium Potassium Pump actively transports K+ into the cell and Na+ out of the cell; both ions move against their concentration gradient and two K+ enter the cell for every three Na+ that leave. The Sodium Potassium Pump works because it is phosphorylated by ATP which causes a conformation change in the protein which allows the ions to move across the membrane. Another ion involved in maintaining the resting potential is the K+ Leak Channel, named because it is highly permeable to K+. The K+ Leak Channels allows the movement K+ out of the cell down a concentration gradient; this is facilitated diffusion. As K+ leaves the cell the electrical gradient increases. K+ will continue to leave the cell until the electrical gradient is equal to the concentration gradient in the opposite direction. When this occurs the system is at equilibrium so there is no net movement of K+ ions the voltage that this occurs at is -87 mV, this is the Equilibrium Potential (Ek) for K+ and it can be determined by the Nernst Equation. All these factors work together to maintain the resting potential until an action potential is stimulated.