Which region of a sarcomere contains only thin filaments?

Which region of a sarcomere contains only thin filaments?

• A. Z-line
• B. I-band
• C. H-zone
• D. A-band

Answer: (B)

Understanding how a sarcomere is organized helps. Thin actin filaments extend from the Z-lines toward the center, while thick myosin filaments reside in the middle. The I-band is the light region that contains only thin filaments, with no overlapping thick filaments. It lies between the ends of the thick-filament overlap. In contrast, the H-zone within the A-band is the region with only thick filaments (no actin overlap), and the A-band spans the full length of the thick filament and includes areas of overlap with actin. Therefore, the region that contains only thin filaments is the I-band. During contraction, the I-band shortens as actin slides past myosin, while the length of the thick filaments stays the same.

Understanding how a sarcomere is organized helps. Thin actin filaments extend from the Z-lines toward the center, while thick myosin filaments reside in the middle. The I-band is the light region that contains only thin filaments, with no overlapping thick filaments. It lies between the ends of the thick-filament overlap. In contrast, the H-zone within the A-band is the region with only thick filaments (no actin overlap), and the A-band spans the full length of the thick filament and includes areas of overlap with actin. Therefore, the region that contains only thin filaments is the I-band. During contraction, the I-band shortens as actin slides past myosin, while the length of the thick filaments stays the same.