07 The sliding-filament theory of muscle contraction (Huxley)
- The most acceptable explanation is that of Huxley.
- Using the electron microscope to compare a contracted fibre with a relaxed fibre, he concluded that the protein filaments slide over each other; neither filament itself shortens.
- Cross-bridges: projections ("heads") on the myosin filaments, which can attach to the actin filaments with the help of calcium ions and ATP.
- Contraction: the cross-bridges attach to actin and, using energy from ATP, pull the actin filaments from both sides towards the centre of the sarcomere. As a result the Z-lines come closer, the sarcomere shortens, the I-band and H-zone become narrower (the A-band does not change in length), and the whole fibre shortens.
- Relaxation: when the stimulus stops, the cross-bridges detach from actin (this also needs ATP), the Z-lines move apart again, and the sarcomere returns to its original length.
- So ATP is needed twice: once to give the energy for the pulling stroke when the bridges attach and bend, and once for detaching the bridges from the actin.
- If ATP is missing, the bridges stay attached, the muscle stays contracted and cannot relax (as in rigor mortis after death, when ATP production stops and the muscles stiffen).
1Sarcomere length
Relaxed
- Long
Contracted
- Short
2Z-lines
Relaxed
- Far apart
Contracted
- Near each other
3I-band
Relaxed
- Wide
Contracted
- Narrow
4A-band
Relaxed
- Unchanged
Contracted
- Unchanged
5H-zone
Relaxed
- Present
Contracted
- Narrow/disappears
6Cross-bridges
Relaxed
- Detached
Contracted
- Attached to actin
✍️ Worked examples
Worked example 8 (identifying a band).
- In a micrograph, a band that stays the same length when the muscle contracts is the A-band (myosin length is constant).
- The band that disappears when the muscle is maximally contracted is the H-zone, because actin filaments from both sides meet in the middle.
Worked example 9 (ATP shortage).
- A person dies; after a few hours the body stiffens.
- Why?
- ATP is no longer produced, so the cross-bridges cannot detach from actin; muscles remain in a fixed contracted state.
- After tissues break down, the stiffness disappears.