Most neurons at rest (without input) have a stable separation of charges across the cell membrane called the resting potential, with more negatively charged ions (anions) in a layer against the inside of the membrane and more positively charged ions (cations) in a layer against the outside of the membrane.
By convention, the outside value is set at zero as the reference, and the value of the inside is used when discussing the membrane potential. The resting potential may vary in size between neurons, but around -60 millivolts (mV) is common. The resting potential is related to concentration differences, or gradients, of many ions across the cell membrane. The most important ions for neuron function are the cations potassium, sodium, and calcium; the anion chloride; and multiple organic anions, most of which are proteins with a net negative charge.
The concentrations of potassium and the organic anions are greater inside neurons than outside, and the concentrations of sodium, chloride, and calcium are greater outside than inside. Each of these ions, therefore, is acted on by two forces trying to drive them into or out of the neuron. An electrical force from the membrane potential attracts ions to the side of the membrane with an opposite charge.
A diffusion, or chemical, force from the concentration gradients attracts ions to the side with a lower concentration.
These forces are collectively called electrochemical driving forces for short, and neurons use these forces to perform their functions.