Which band is primarily associated with thin filaments?

• A. A Band
• B. I Band
• C. Z Disc
• D. H Band

Answer: (B)

Thin filaments are actin, and the I band is the region between Z discs that contains only these thin filaments. This is why it’s associated with thin filaments the most: you don’t have the thick myosin filaments present in this zone, so the light region reflects the presence of actin alone. The A band, by contrast, spans the full length of the thick filaments and includes overlapped regions where thin filaments intersect with thick ones, so it isn’t primarily thin. The H band lies within the A band and contains only thick filaments with no overlap from thin filaments, so it’s not the area characterized by thin filaments. The Z disc marks the boundaries of each sarcomere and serves as the anchor point for thin filaments. During contraction, the I band shortens as thin filaments slide inward, while the overall length of the A band remains roughly constant.