The conduction process in the nerve fibre is essentially dependent upon changes in the electrophysiological status of the nerve membrane. When nerve is inactive there is a negative "resting" potential of -50 to -70 mV within the cell (by comparison with the exterior surface of the cell membrane). When excitatin occurs, a distinct transmembrane action potential can be recorded by means of an intracellular electrode.
The sequence of events after excitation is as follows: a relatively slow phase of depolarization occurs during which the electical potential within the nerv cell becomes progressively less negativ. When the potential difference between the interior and exterior surface of the cell membrane reaches a critical level, the "threshold potential" or "firing level", depolarization reverses the potential so that the nerve interior is positively charged by comparision with the exterior aspect of the cell membrane. At the peak of the action the intracellular positive potential reaches abaout 40 mV. Thereafter, a process of repolarization begins, continuing until the intacellular resting potential of -50 to - 70 mV ist restored.

The interior of a resting peripheral nerve cell - the cytoplasm - is possessed of a high concentration of potassium ions and a low concentration of sodium ions. This state is the opposite of that in the extracellular fluids. At rest the inside/ outside potassium ratio is about 30, and it is the gradient which accounts for the negative intracellular resting potential. At rest, the cell membrane is relatively resistent to ion passage, but, on excitation, cell membrane permeability increases and there is, initially, an influx of sodium ions into the cell. This accounts for the depolarization phase of the action potential.

When the cell is maximally depolarized, sodium ion passage is arrested, and potassium ions pass out of the cell. This effects a repolarization of the cell membrane.

This sodium and potassium movement during excitation is passive, since both ions move along a concentration gradient, but after repolarization there is an intracellular imbalance in comparison with the resting state - too many sodium ions intracellularly and too many potassium ions extracellularly. In this situation the necessary movement of ions must be active, because the movement is against the ionic concentration gradient. Sodium is extrudent by the sodium pump, and the necessary energy is derived from the oxidative metabolism of adenosine triphosphate.

A metabolic pump may also effect the restoration of the resting intracellular potassium ion concentration, because the necessary movement is also against the concentration gradient. Alternatively potassium ion transport may be effected along the electrostatic gradient between the resting cell and its milieu. This would not require energy expenditure.