A level-readyAll subjects · Biology · WBI15 (Unit 5)
The sliding filament theory
Muscle contracts when myosin heads pull actin filaments towards the centre of each sarcomere. Calcium ions and ATP control the process.
Starting contraction
1. An impulse arrives at the neuromuscular junction and depolarisation spreads along the sarcolemma and down the T-tubules.
2. The sarcoplasmic reticulum releases calcium ions (Ca2+) into the sarcoplasm.
3. Ca2+ binds to troponin, which changes shape and pulls tropomyosin away from the myosin-binding sites on actin.
The cross-bridge cycle
1. Myosin heads (carrying ADP and Pi) bind to the exposed sites on actin, forming cross-bridges.
2. The heads tilt, pulling actin towards the M line: the power stroke. ADP and Pi are released.
3. A new ATP binds to each myosin head, which detaches from actin.
4. Myosin heads act as ATPase: ATP is hydrolysed and the energy returns the head to its original position, ready to bind again further along.
The cycle repeats as long as Ca2+ and ATP are available.
Relaxation and evidence
When stimulation stops, Ca2+ is actively transported back into the sarcoplasmic reticulum (using ATP). Tropomyosin covers the binding sites again and the muscle relaxes; the antagonistic muscle pulls it back to its original length.
As the sarcomere shortens: the I band and H zone get shorter, the Z lines move closer, but the A band stays the same length, because the filaments slide past each other without changing length.
Worked example
A sarcomere shortens from 2.4 µm to 2.0 µm. Calculate the percentage decrease in length.
- Decrease = 2.4 - 2.0 = 0.4 µm
- Percentage = 0.4 / 2.4 x 100 = 16.7%
Answer: 16.7%
Check yourself
- A sarcomere contracts from 2.5 µm to 2.1 µm. Calculate the percentage decrease to 1 decimal place.
Show answer
16.0 %. 0.4 / 2.5 x 100 = 16.0%.
- What is the role of calcium ions in muscle contraction?
- They bind to myosin heads and detach them
- They hydrolyse ATP
- They break actin filaments
- They bind to troponin, causing tropomyosin to move off the binding sites
Show answer
They bind to troponin, causing tropomyosin to move off the binding sites. Ca2+ unblocks the myosin-binding sites on actin.
- What is ATP needed for in the cross-bridge cycle?
- To attach troponin to actin
- To detach myosin heads from actin and return them to their original position
- To release calcium from the blood
- To make the A band shorter
Show answer
To detach myosin heads from actin and return them to their original position. ATP binding detaches the head; hydrolysis recocks it.
- Which band stays the same length when a sarcomere contracts?
- I band
- H zone
- A band
- Distance between Z lines
Show answer
A band. The A band is the length of the myosin, which does not change.
- In resting muscle, what blocks the myosin-binding sites on actin?
- Tropomyosin
- Calcium ions
- ATP
- Myoglobin
Show answer
Tropomyosin. Tropomyosin lies along actin covering the sites.
- What is the power stroke?
- Actin pushes myosin outwards
- Calcium is pumped back into the sarcoplasmic reticulum
- Myosin heads tilt, pulling actin towards the M line
- The Z lines move apart
Show answer
Myosin heads tilt, pulling actin towards the M line. The tilt slides the thin filaments inwards.
- How does a muscle fibre relax after stimulation stops?
- Ca2+ is actively pumped back into the sarcoplasmic reticulum
- Myosin is broken down
- ATP is no longer made anywhere
- Troponin is destroyed
Show answer
Ca2+ is actively pumped back into the sarcoplasmic reticulum. Without Ca2+, tropomyosin blocks the sites again.
Sources
Last updated 7 November 2026. Spotted a mistake? Tell us.