Muscle shortens because sarcomeres shorten
A skeletal muscle fibre contains parallel myofibrils built from repeating sarcomeres. Each sarcomere lies between two Z lines. Thin actin filaments extend inward from the Z lines, while thick myosin filaments occupy the centre.
During contraction, the filaments do not become shorter. Actin slides past myosin, increasing overlap and pulling Z lines closer together. The I band and H zone become narrower, while the A band—the length of the thick filament—remains constant.
| Feature | During contraction |
|---|---|
| Sarcomere length | Decreases |
| I band | Decreases |
| H zone | Decreases and may disappear |
| A band | Remains constant |
| Actin and myosin filament length | Remain constant |
Calcium removes the molecular blockade
At rest, tropomyosin lies over myosin-binding sites on actin. When an action potential travels along the sarcolemma and down T-tubules, the sarcoplasmic reticulum releases calcium ions into the cytosol.
Calcium binds troponin, changing the troponin–tropomyosin complex and exposing actin sites. Myosin heads can then form cross-bridges. Calcium does not supply the mechanical energy; it permits the interaction to occur.
A muscle-fibre action potential begins.
Depolarisation enters the fibre.
The sarcoplasmic reticulum releases calcium.
Tropomyosin shifts away from actin sites.
The cross-bridge cycle uses ATP repeatedly
An energised myosin head carrying ADP and phosphate binds exposed actin. Phosphate release strengthens binding and initiates the power stroke; the head pivots and pulls the thin filament. ADP leaves near the end of the stroke.
A new ATP binds myosin and causes detachment from actin. ATP hydrolysis then re-cocks and energises the head. The cycle repeats while calcium remains elevated and ATP is available.
Energised myosin binds exposed actin.
Phosphate release drives filament movement.
A new ATP binds myosin.
ATP hydrolysis energises the head again.
Relaxation also requires energy
When stimulation ends, calcium is actively pumped back into the sarcoplasmic reticulum. Cytosolic calcium falls, troponin returns toward its resting shape, and tropomyosin covers the actin-binding sites again.
ATP is therefore needed both for cross-bridge cycling and for calcium reuptake. Rigor mortis develops after death because ATP production stops, myosin cannot detach normally, and calcium gradients cannot be maintained.
From fibres to whole-muscle force
A motor unit consists of one motor neuron and the fibres it controls. Force increases when more motor units are recruited and when impulses arrive at a higher frequency, allowing temporal summation. Small motor units provide fine control, while large units generate greater force.
Muscle fatigue can reflect multiple factors, including metabolite accumulation, altered ion gradients, reduced substrate availability and central nervous-system effects. It should not be reduced to a single idea such as 'lactic acid stops contraction.'
The ideas to carry forward
- Actin and myosin slide; the filaments themselves do not shorten.
- Calcium exposes actin sites through troponin and tropomyosin.
- ATP enables detachment, re-cocking and calcium reuptake.
- Recruitment and firing frequency determine whole-muscle force.
Answer first. Then reveal the marking logic.
01Which sarcomere band remains unchanged during contraction, and why?2 marks · show the biological link
Answer: The A band remains constant because it represents the length of thick myosin filaments, which do not shorten.
02Why does loss of ATP produce stiffness rather than immediate relaxation?2 marks · show the biological link
Answer: ATP is required for myosin to detach from actin and for calcium to be pumped back into storage.
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