Rheobase is the minimal current amplitude of indefinite duration that results in the depolarization threshold of the cell membranes being reached (i.e. an action potential or the contraction of a muscle). Chronaxie (or chronaxy) is the minimum time over which an electric current, double the strength of the rheobase, needs to be applied, in order to stimulate a muscle fiber or nerve cell. In the case of a nerve or single muscle cell, rheobase is half the current that needs to be applied for the duration of chronaxie to result in an action potential or muscle twitch. Refractory period is a sho period after a nerve or muscle cell fires during which thecell cannot respond to additional stimulation. The minimal stimulus that produces excitation of any structure, eliciting a motor response is known as threshold.