The all-or-none law is a principle that states that the strength of a response of a nerve cell or muscle fiber is not dependent upon the strength of the stimulus. If a stimulus is above a certain threshold, a nerve or muscle fiber will fire. Essentially, there will either be a full response or there will be no response at all.
The all-or-none law was first described in 1871 by physiologist Henry Pickering Bowditch. In his descriptions of the contraction of the heart muscle, he explained, "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."
While the all-or-none law was initially applied to the muscles of the heart, it was later found that nerves and other muscles also respond to stimuli according to this principle.
Authors Levitan and Kaczmarek explain, "The all-or-none law guarantees that once an action potential is generated it is always full size, minimizing the possibility that information will be lost along the way."
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Cannon, Walter B. (1924). Biographical Memoir, Henry Pickering Bowditch, 1840-1911. Washington, D.C.:National Academy of Sciences.
Levitan, I. B., & Kaczmarek, L. K. (1991). The Neuron: Cell And Molecular Biology. New York: Oxford University Press.