All-or-none law

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 the muscle at the time.”

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

The magnitude of the action potential set up in any single nerve fiber is independent of the strength of the exciting stimulus, provided the latter is adequate. An electrical stimulus below threshold strength fails to elicit a propagated spike potential. If it is of threshold strength or over, a spike (a nervous impulse) of maximum magnitude is set up. Either the single fiber does not respond with spike production, or it responds to the utmost of its ability under the conditions at the moment. This property of the single nerve fiber is termed the all-or-none relationship. This relationship holds only for the unit of tissue; for nervous tissue the unit is the nerve cell, for skeletal muscle the unit is the individual muscle fiber and for the heart the unit is the entire auricles or the entire ventricles.

Stimuli too weak to produce a spike do, however, set up a local electrotonus, the magnitude of the electronic potential progressively increasing with the strength of the stimulus, until a spike is generated. This demonstrates the all-or-none relationship in spike production.

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Wikipedia