On electron microscopy of a skeletal muscle fibre the A-band is measured and found to be unchanged after passive stretching of the muscle. This is because the A-band represents:

• A The entire length of the thin (actin) filaments
• B The zone occupied solely by thin filaments with no myosin overlap
• C The distance between two adjacent Z-discs
• D The full length of thick (myosin) filaments, which does not change regardless of sarcomere length ✓

Explanation

In the sliding filament model, the A-band corresponds to the full extent of the thick (myosin) filaments. Because myosin filaments do not change length during contraction or stretch, the A-band remains constant (~1.6 µm) regardless of sarcomere length. The I-band (thin filaments not overlapping myosin) and H-zone (central myosin without actin overlap) decrease during contraction and increase with stretch. The sarcomere length is the Z-disc-to-Z-disc distance and represents the full contractile unit.

Reference: BD Chaurasia's Human Anatomy, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.