Question
Explain the sliding filament theory of muscle contraction with a neat labelled diagram. Describe the role of actin, myosin, ATP, and calcium ions in the process.
(NCERT Class 11, Chapter 20 — Locomotion and Movement; high-weightage for NEET)
Solution — Step by Step
A sarcomere is the functional unit of a myofibril, bounded by two Z-lines (Z-discs). It contains two types of myofilaments — thick filaments made of myosin and thin filaments made of actin. The H-zone is the central region containing only thick filaments; the I-band contains only thin filaments.
At rest, the myosin head has ADP + Pᵢ bound to it and is in a “cocked” (high-energy) position. However, the troponin-tropomyosin complex on actin physically blocks the active sites where myosin would bind. This is the key lock — without calcium, the gate stays shut.
A nerve impulse triggers the release of Ca²⁺ from the sarcoplasmic reticulum into the sarcoplasm. Ca²⁺ binds to troponin C, causing a conformational change in the troponin-tropomyosin complex. Tropomyosin shifts away, exposing the active sites on actin.
The cocked myosin head attaches to the exposed actin active site, forming a cross-bridge. It then pivots (power stroke), pulling the thin filament toward the M-line — this is the actual sliding. ADP + Pᵢ are released during this stroke.
A new ATP molecule binds to the myosin head, causing it to detach from actin. ATP is hydrolysed to ADP + Pᵢ, which re-cocks the head back to the high-energy position. The cycle repeats as long as Ca²⁺ and ATP are available, shortening the sarcomere with each round.
Diagram: Sarcomere During Contraction
RELAXED:
Z ──── I-band ──── A-band (H-zone) ──── I-band ──── Z
[actin] [myosin] [actin]
CONTRACTED:
Z ── [actin overlapping myosin heavily] ── Z
(I-band narrows, H-zone disappears)Key labelling points for NEET diagrams:
- Z-line, M-line
- I-band (light), A-band (dark)
- H-zone (within A-band, centre)
- Thin filaments (actin + troponin + tropomyosin)
- Thick filaments (myosin with globular heads)
Why This Works
The “sliding” in the name is crucial — the filaments themselves do not shorten. The length of individual actin and myosin filaments stays the same. What shortens is the sarcomere, because thin filaments are pulled inward over thick filaments, reducing the I-band width and collapsing the H-zone.
Think of it like two combs sliding into each other — neither comb’s teeth get shorter, but the combined structure becomes compact. This is why the A-band width stays constant throughout contraction; only the I-band and H-zone change.
ATP has a dual role here that students often miss: it provides energy for the power stroke and it is required for detachment. This is why muscles stay rigid after death (rigor mortis) — no ATP means myosin heads cannot detach from actin.
Alternative Method: Memorising the Bands
For diagram questions, use this trick to remember which bands change and which don’t:
“I Have” shrinks during contraction.
- I-band → shortens
- H-zone → shortens (disappears in full tetanic contraction)
- A-band → stays the same (always = length of myosin)
The A-band is the anchor — its width equals myosin filament length, which never changes.
Common Mistake
Saying “actin and myosin shorten” in the exam.
This is the single most common error in NEET answers on this topic. Neither filament shortens. The sarcomere shortens because of overlap between the two. Many students also incorrectly state that the A-band narrows — it does not. Write clearly: “The I-band and H-zone decrease in width; the A-band remains unchanged.”
Summary of roles:
| Component | Role |
|---|---|
| Myosin | Motor protein; cross-bridge formation and power stroke |
| Actin | Structural track; has active sites for myosin binding |
| Tropomyosin | Blocks active sites at rest |
| Troponin | Calcium sensor; moves tropomyosin when Ca²⁺ binds |
| Ca²⁺ | Triggers the whole cascade by binding troponin C |
| ATP | Powers re-cocking of myosin head; essential for detachment |
The final answer: Muscle contraction occurs because repeated cross-bridge cycles pull thin actin filaments toward the centre of each sarcomere, shortening the sarcomere (and therefore the whole muscle fibre) without changing the length of individual filaments.