AQA · A-Level · Physical Education
Course version: 7582 · Version from Sept 2016
Neuromuscular system: All or none law.
The all or none law states that once a stimulus reaches threshold at a muscle fibre, that fibre contracts to its maximum possible extent for its current condition; a sub-threshold stimulus…
Checked against the AQA 7582 specification (opens in a new tab) on .
- Point 15 of 16
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
The all or none law states that once a stimulus reaches threshold at a muscle fibre, that fibre contracts to its maximum possible extent for its current condition; a sub-threshold stimulus produces no contraction. The action potential is propagated along the sarcolemma and T-tubules, releasing calcium so that cross-bridge cycling occurs fully. Gradation of whole-muscle force therefore comes from recruiting more motor units and from increasing impulse frequency, not from a single fibre contracting partially. A concrete method: apply increasing voltages to an isolated muscle preparation and note that the twitch appears suddenly at threshold and does not grow further with stronger single stimuli.
What earns the marks
- A muscle fibre either contracts maximally or not at all once threshold is reached.
- A stimulus below threshold produces no action potential and no contraction.
- Above threshold, a single fibre's twitch force is not graded by stimulus strength.
- Whole-muscle force is graded by recruiting additional motor units and by increasing impulse frequency.
- The law applies to individual muscle fibres, not to the whole muscle as a single unit.
Common mistakes and how to put them right
Applying the law to a whole muscle.
Put it right: It applies to individual fibres, while whole-muscle force varies with recruitment and frequency.
Thinking a stronger stimulus makes one fibre contract harder.
Put it right: Above threshold the single-fibre response is maximal.
Believing sub-threshold stimuli cause weak contractions.
Put it right: They produce no contraction at all.
Confusing the all or none law with wave summation.
Put it right: The law concerns whether a fibre contracts, while summation concerns overlapping twitches increasing force.
Exam tips
- State clearly that the law refers to individual muscle fibres, then explain how whole-muscle force is graded.
- Use threshold as the key term and explain what happens below and above it.
- Link the law to recruitment and frequency to show how the neuromuscular system controls force.
Try the lesson
The start of the Neuromuscular system: All or none law. lesson, made from checked teaching notes for this specification point.
Have a go first
According to the all or none law, what happens when a motor neurone is stimulated at a strength above the threshold needed to trigger a response?
Listen
The all or none law states that once a stimulus reaches threshold at a muscle fibre, that fibre contracts to its maximum possible extent for its current condition; a sub-threshold stimulus produces no contraction. The action potential is propagated along the sarcolemma and T-tubules, releasing calcium so that cross-bridge cycling occurs fully. Gradation of whole-muscle force therefore comes from recruiting more motor units and from increasing impulse frequency, not from a single fibre contracting partially. A concrete method: apply increasing voltages to an isolated muscle preparation and note that the twitch appears suddenly at threshold and does not grow further with stronger single stimuli. A strong answer earns marks for these points. A muscle fibre either contracts maximally or not at all once threshold is reached. A stimulus below threshold produces no action potential and no contraction. Above threshold, a single fibre's twitch force is not graded by stimulus strength. Whole-muscle force is graded by recruiting additional motor units and by increasing impulse frequency. A common slip: Applying the law to a whole muscle. Instead, it applies to individual fibres, while whole-muscle force varies with recruitment and frequency. Exam tip: State clearly that the law refers to individual muscle fibres, then explain how whole-muscle force is graded.
See it
Key words
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Threshold
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All or none law
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Gradation of whole-muscle force
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Motor unit recruitment and frequency
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See every definition
- Threshold
- The stimulus level at which a muscle fibre contracts to its maximum possible extent for its current condition.
- All or none law
- Once a stimulus reaches threshold at a muscle fibre, that fibre contracts to its maximum possible extent for its current condition.
- Gradation of whole-muscle force
- Comes from recruiting more motor units and from increasing impulse frequency, not from a single fibre contracting partially.
- Motor unit recruitment and frequency
- The means by which whole-muscle force is graded, through recruiting additional motor units and increasing impulse frequency.
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