A muscle never decides to contract on its own. It waits for a message from a nerve. This page follows that message in — through the neuromuscular junction and into the fibre — and then answers a question students rarely think about: if every fibre either fires or does not, how do you pick up an egg without crushing it?
📘 What you need to know
Skeletal muscle contracts when it receives an impulse from a motor neurone.
The message crosses at the neuromuscular junction, which works in much the same way as a synapse.
A motor unit is one motor neurone plus all the muscle fibres it supplies, and the neuromuscular junctions between them.
One motor neurone branches to many fibres, so there are many neuromuscular junctions in a single motor unit.
The strength of a contraction is controlled by how many motor units are switched on.
Low intensity = few motor units active. High intensity = many motor units active.
The message arrives: the neuromuscular junction
A motor neurone carries an impulse from the central nervous system out to a muscle. Where the end of the neurone meets a muscle fibre there is a tiny gap. That meeting point is the neuromuscular junction.
It behaves like a synapse between two neurones, with one difference: the receiving side is not another neurone, it is a muscle fibre.
The impulse reaches the end of the motor neurone.
A neurotransmitter is released into the gap and crosses to the muscle fibre membrane.
The fibre membrane is depolarised, and that signal spreads through the whole fibre.
The sarcoplasmic reticulum releases calcium ions, and the cross-bridge cycle begins.
This is why the sarcoplasmic reticulum matters so much. It reaches into every part of the fibre, so the calcium arrives everywhere at almost the same instant and the whole fibre contracts together, not in a ripple.
The motor unit
One motor neurone does not serve one fibre. Its axon branches, and each branch ends on a different muscle fibre. All of those fibres get the same message at the same time, so they all contract together.
DefinitionMotor unit = one motor neurone + all the muscle fibres it supplies + the neuromuscular junctions joining them
The fibres of one motor unit are not neatly stacked in real muscle — they are spread out, so the pull is even.
How the body controls the strength of a pull
Here is the puzzle. A muscle fibre does not do “a bit of a contraction”. It gets the message and contracts. So how can the same bicep hold a feather and also haul a suitcase?
The answer is how many motor units are switched on. Each motor unit is controlled by its own neurone, so the nervous system can call in as few or as many as the job needs.
Gentle movement — only a small number of motor units receive impulses, so only a small share of the fibres pull.
Powerful movement — many more motor units receive impulses, so far more fibres pull at once and the force is greater.
Force is controlled by counting fibres in, not by making each fibre try harder.
Worth knowing. Muscles that need fine control, like the ones moving your eye, have very small motor units — a few fibres each. Muscles built for power, like those in your thigh, have huge motor units. Smaller units means finer steps in force.
Worked examples
WORKED EXAMPLE
Outline how an impulse in a motor neurone causes a skeletal muscle fibre to contract. [4]
Step 1: cross the gap
The impulse reaches the neuromuscular junction and a neurotransmitter is released onto the muscle fibre.
Step 2: into the fibreThe fibre membrane is depolarised and the signal spreads through the whole fibre.Step 3: release the triggerCalcium ions are released from the sarcoplasmic reticulum.Step 4: the machinery startsBinding sites on actin are exposed, cross-bridges form and the filaments slidefour marks, four clear stages — do not merge them into one sentence.
WORKED EXAMPLE
A student says: “To lift something heavier, each muscle fibre must contract harder.” Explain why this is wrong. [3]
State what a fibre actually does
A fibre that receives an impulse contracts fully — it cannot contract by halves.
Give the real mechanismForce is increased by activating more motor units, so more fibres contract at the same time.Finish the comparisonMore motor units recruited, not harder work per fibreuse the word “recruited” — it is the term the mark scheme likes.
💡 Exam tip
Define a motor unit with all three parts: the neurone, the fibres it supplies, and the junctions between them.
Say neuromuscular junction, not “synapse”, when the receiving side is muscle.
Link intensity to the number of motor units, never to the effort of a single fibre.
If a question gives you a diagram, trace the axon from the cell body and count the branches — that is the size of the motor unit.
Contraction questions often want the chain from nerve to calcium to cross-bridge. Practise saying it in four steps.
⚠ Common mix-up
Thinking one neurone serves one fibre. It branches to many, and all of them fire together.
Thinking a motor unit is a lump of muscle. Its fibres are scattered through the muscle, not stacked in a block.
Forgetting the calcium step. The impulse does not touch the filaments directly — calcium is the link.
Saying the neurone contracts. The neurone signals; the fibre contracts.
Confusing intensity with speed. More motor units means a stronger pull, not a faster one.
Up next: Requirements for Movement — a muscle that pulls is useless on its own. It needs something firm to pull against.
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