In skeletal muscle, the 'A band' appears dark on electron microscopy. What accounts for its dark appearance and what change occurs in the A band during muscle contraction?
- A Overlap of myosin and actin; A band length remains constant during contraction ✓
- B Actin filaments only; A band shortens during contraction
- C Myosin filaments only; A band shortens because myosin shortens
- D Z discs at both ends; A band widens as sarcomere shortens
Correct answer: A. Overlap of myosin and actin; A band length remains constant during contraction
Explanation
The A band (anisotropic) corresponds to the full length of the thick myosin filaments, including the region where actin and myosin overlap. The A band appears dark because thick filaments are electron-dense. During contraction (sliding filament theory), actin filaments slide over myosin, shortening the sarcomere by reducing the I band and H zone; however, the A band length remains constant because the myosin filaments do not shorten. Only the I band and H zone decrease in width during contraction.
Reference: BD Chaurasia's Human Anatomy, 8th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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