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.^[1]

Relationship between stimulus and response

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.

The above account deals with the response of a single nerve fiber. If a nerve trunk is stimulated, then as the exciting stimulus is progressively increased above threshold, a larger number of fibers respond. The minimal effective (i.e., threshold) stimulus is adequate only for fibers of high excitability, but a stronger stimulus excites all the nerve fibers. Increasing the stimulus further does increase the response of whole nerve.

Heart muscle is excitable, i.e., it responds to external stimuli by contracting. If the external stimulus is too weak, no response is obtained; if the stimulus is adequate, the heart responds to the best of its ability. Accordingly, the auricles or ventricles behave as a single unit, so that an adequate stimulus normally produces a full contraction of either the auricles or ventricles. The force of the contraction obtained depends on the state in which the muscles fibers find themselves. In the case of muscle fibers, the individual muscle fiber does not respond at all if the stimulus is too weak. However, it responds maximally when the stimulus rises to threshold. The contraction is not increased if the stimulus strength is further raised. Stronger stimuli bring more muscle fibers into action and thus the tension of a muscle increases as the strength of the stimulus applied to it rises.

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