![]() ![]() ![]() If a nerve trunk is stimulated, then as the exciting stimulus is progressively increased above threshold, a larger number of fibers respond. The above account deals with the response of a single nerve fiber. This demonstrates the all-or-none relationship in spike production. 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. It should be emphasized that this relationship applies only to the unit of tissue, as well as to skeletal muscle units (the unit being the individual muscle fiber) and to the heart(the unit being the entire auricles or the entire ventricles). This property of the single nerve fiber is termed the all-or-none relationship. 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. If it is of threshold strength or over, a spike (representing a nervous impulse) of maximum magnitude is set up. An electrical stimulus below threshold strength fails to elicit a propagated spike potential. The magnitude of the spike potential (impulse) set up in any single nerve fiber is independent of the strength of the exciting stimulus, provided the latter is adequate. Relationship Between Stimulus and Response The individual fibers of both skeletal muscle and nerve respond to stimulation according to the all-or-none principle. “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.” According to him, describing the relation of response to stimulus, It was first established by the American physiologist Henry Pickering Bowditch in 1871 for the contraction of heart muscle. If the stimulus is any strength above threshold, the nerve or muscle fiber will give a complete response or otherwise no response at all. The all-or-none law is the principle that the strength by which a nerve or muscle fiber responds to a stimulus is not dependent on the strength of the stimulus. ![]()
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