Question
Describe the structure of a skeletal muscle fibre, focusing on the sarcomere. Explain the sliding filament theory of muscle contraction.
Solution — Step by Step
A skeletal muscle is made of muscle fascicles (bundles), which contain muscle fibres (individual cells). Each muscle fibre contains many thread-like myofibrils running along its length. Myofibrils show alternating light and dark bands — the striations that give skeletal muscle its striated appearance.
The dark band is called the A-band (anisotropic — rotates polarised light). The light band is the I-band (isotropic). A dark line called the Z-disc bisects each I-band.
The sarcomere is the unit of contraction, defined as the region between two adjacent Z-discs. A single myofibril is a long chain of thousands of sarcomeres arranged end to end.
Within a sarcomere:
- Thick filaments (myosin): run through the A-band, anchored at the centre by the M-line
- Thin filaments (actin): extend from the Z-disc into the A-band, overlapping with myosin in the outer parts of the A-band
- H-zone: the central part of the A-band where only thick filaments are present (no actin overlap)
- Zone of overlap: regions where actin and myosin filaments interdigitate
Thin filaments contain:
- F-actin (fibrous actin): two strands twisted in a helix, each made of G-actin monomers
- Tropomyosin: wraps around actin strands, blocking the myosin-binding sites at rest
- Troponin: a complex of three proteins (TnT, TnI, TnC); TnC binds
Thick filaments consist of myosin molecules. Each myosin has a tail and a globular head that can bind actin and hydrolyse ATP.
The sliding filament theory (proposed by Huxley and Hanson, 1954) states that muscle contraction occurs by the thin actin filaments sliding over the thick myosin filaments. The filaments themselves do NOT shorten — they slide past each other, pulling the Z-discs closer together.
Steps of the cross-bridge cycle:
- Stimulus — a nerve impulse releases from the sarcoplasmic reticulum
- binds to troponin-C → troponin complex shifts tropomyosin, exposing active sites on actin
- Cross-bridge formation — myosin head (loaded with ADP + Pi) binds to exposed actin site
- Power stroke — myosin head pivots, pulling actin toward the M-line; ADP and Pi are released
- ATP binding — a new ATP molecule binds to myosin, detaching it from actin
- ATP hydrolysis — ATP splits into ADP + Pi, “cocking” the myosin head back to its original position
- If is still present, the cycle repeats
During contraction:
- I-band shortens (actin slides in, reducing the light-band width)
- H-zone narrows (actin filaments slide into it)
- A-band stays the same length (myosin filament length unchanged)
- Z-discs move closer (sarcomere shortens)
The A-band remaining constant is a key observation that proved filaments slide rather than shorten.
Why This Works
The elegance of sliding filament theory is that it explains contraction at the molecular level using well-understood chemistry. ATP provides the energy; acts as the trigger; the protein machinery (actin-myosin) acts as a molecular motor. Each power stroke moves actin by about 10 nm. In a full contraction, thousands of cross-bridge cycles happen simultaneously across millions of sarcomeres — generating the macroscopic force of muscle contraction.
For NEET, the mnemonic for bands that change during contraction: “I shorten, H disappears, A stays.” I-band and H-zone both shorten; A-band does NOT change. This is a direct consequence of actin sliding into myosin territory without the myosin filaments themselves getting shorter.
Common Mistake
Students often write that “both actin and myosin filaments shorten during muscle contraction.” The sliding filament theory explicitly states that neither filament shortens — they slide past each other. The sarcomere shortens because the overlap region increases, pulling Z-discs closer. If you state that filaments shorten, you’ll lose marks in NEET and CBSE 5-mark questions.