The difference in electrical potential that exists across the plasma membrane of an excitable cell, such as a neuron, when it is in a stable, nonexcited state, i.e. its resting membrane potential. It is the result of differences in concentrations of negatively and positively charged ions and molecules between the cytosol and the extracellular fluid, and the differential permeability of the resting plasma membrane to certain ions. A principal mechanism for establishing the resting potential is the sodium-potassium pump. This actively exports sodium (Na⁺) ions and imports potassium ions (K⁺). Other cations, notably protons (H⁺) and calcium ions (Ca²⁺), may also be actively pumped out of the cell. These movements of cations create a net negative charge inside the cell, i.e. an electrical gradient is established across the plasma membrane. However, the K⁺ ions concentrated inside the cell also experience a concentration, or chemical, gradient, with a tendency to diffuse out of the cell through K⁺ leak channels in the plasma membrane. But the positive K⁺ ions are simultaneously attracted in the opposite direction by the negatively charged cell interior (due chiefly to negatively charged proteins and other large molecules). Hence, an equilibrium is established by the action of the two gradients combined, i.e. the electrochemical gradient; the electrical charge difference between interior and exterior at this equilibrium point is the cell’s resting potential. Its value is measured by inserting a microelectrode into the cell, and measuring the potential difference across the plasma membrane. The resting potential of neurons is generally –60 to –70 mV. Compare action potential; graded potential.