Bernstein (1902, 1912), the founder of the modern membrane theory, regarded the action potential as the “negative wave” of the resting potential. He considered that during stimulation of a nerve or muscle fiber certain chemical changes which give rise to a reversible increase in the ionic permeability of the cell membrane take place at the point of application of the stimulus. The membrane loses its selective permeability of K+ ions, as the result of which the potential difference which exists at rest is quickly equalized, and the resting potential falls to zero. Subsequent restoration of the original ionic permeability leads to a return of the membrane potential to the level of the resting potential.