All-or-none law
Encyclopedia
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. If the stimulus is any strength above threshold, the nerve or muscle fiber will give a complete response or otherwise no response at all.
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,
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 raise. 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.
Nerve
A peripheral nerve, or simply nerve, is an enclosed, cable-like bundle of peripheral axons . A nerve provides a common pathway for the electrochemical nerve impulses that are transmitted along each of the axons. Nerves are found only in the peripheral nervous system...
or muscle fiber responds to a stimulus is not dependent on the strength of the stimulus. If the stimulus is any strength above threshold, the nerve or muscle fiber will give a complete response or otherwise no response at all.
It was first established by the American physiologist Henry Pickering Bowditch
Henry Pickering Bowditch
Henry Pickering Bowditch was the dean of the Harvard Medical Faculty from 1883 to 1893. Following his teacher Carl Ludwig, he promoted the training of medical practitioners in a context of physiological research. His teaching career at Harvard spanned 35 years.-Early life:Henry P. Bowditch was the...
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.
Relationship between stimulus and response
The magnitude of the spike potentialAction potential
In physiology, an action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a consistent trajectory. Action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, and...
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
Electrotonic potential
In physiology, electrotonus refers to the "passive" spread of charge inside a neuron. "Passive" means that voltage-dependent changes in membrane conductance do not contribute. Neurons and other excitable cells produce two types of electrical potential. The first is a non-propagated local potential...
, 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 raise. 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.