On light microscopy of a cross-section of skeletal muscle, the I band, A band, H zone, and M line are visible. The I band contains which specific filament protein and represents which part of the sarcomere?
- A Actin thin filaments only; from the Z disc to where thick and thin filaments overlap ✓
- B Myosin thick filaments only; from the Z disc to the edge of the A band
- C Both actin and myosin; the central overlap zone
- D Titin (connectin) only; elastic filaments spanning the sarcomere
Explanation
The I band (isotropic — light under polarized light) consists of actin thin filaments only, extending from the Z disc to the point where they begin to overlap with myosin thick filaments. The I band is bisected by the Z disc. The A band (anisotropic — dark under polarized light) contains myosin thick filaments throughout, with the central H zone containing only thick filaments (no thin filament overlap). The M line in the center of the H zone connects adjacent thick filament heads. Titin (connectin) extends from the Z disc to the M line along the thick filament, providing passive elastic force and maintaining sarcomere structure.
Reference: BD Chaurasia's Human Anatomy, 8th ed.
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Written and medically reviewed by the StethoPrep medical team.