What is "all-or-none law"

n. (neurophysiology) a nerve impulse resulting from a weak stimulus is just as strong as a nerve impulse resulting from a strong stimulus

The all-or-none law is the principle that the strength by which a nerve or muscle fiber responds to a stimulus is independent of the strength of the stimulus. If that stimulus exceeds the threshold potential, the nerve or muscle fiber will give a complete response; otherwise, there is no response.

It was first established by the American physiologist Henry Pickering Bowditch in 1871 for the contraction of heart muscle. According to him, describing the relation of response to stimulus,

“An induction shock produces a contraction or fails to do so according to its strength; if it does so at all, it produces the greatest contraction that can be produced by any strength of stimulus in the condition of the muscle at the time.”

The individual fibers of both skeletal muscle and nerve respond to stimulation according to the all-or-none principle.