Muscles and contraction
How muscles contract
- Striated muscle is made of long fibres.
- Inside, tiny filaments slide over each other to shorten the muscle.
- This is the sliding filament model.
Muscle structure
- Each fibre is divided into repeating units called sarcomeres.
- Inside are two filaments: thick myosin and thin actin.
- The T-tubules carry the impulse deep into the fibre, and the sarcoplasmic reticulum stores and releases calcium ions.
Practice
A sarcomere is:
Muscle fibres are divided into sarcomeres, each holding thick (myosin) and thin (actin) filaments.
Practice
The sarcoplasmic reticulum:
When the impulse arrives, the sarcoplasmic reticulum releases calcium ions to trigger contraction.
The sliding filament model
- an impulse makes the sarcoplasmic reticulum release calcium ions.
- calcium binds troponin, which moves tropomyosin to uncover the binding sites on actin.
- myosin heads attach to actin and pull it inwards, using energy from ATP.
- the thin filaments slide over the thick ones, so each sarcomere shortens — the muscle contracts.
Practice
Calcium ions cause contraction by:
Calcium binds troponin; tropomyosin shifts to expose actin, so myosin heads can attach.
Practice
In the sliding filament model, a sarcomere shortens because:
Myosin heads attach and pull actin inwards (powered by ATP), so the filaments slide and the sarcomere shortens.
You've got it
Key idea
- striated muscle = fibres divided into sarcomeres (thick myosin + thin actin)
- T-tubules carry the impulse in; the sarcoplasmic reticulum releases calcium
- calcium binds troponin → moves tropomyosin → uncovers actin sites
- myosin heads pull actin (using ATP) → sarcomere shortens (sliding filament model)