Which region is the light area consisting of thin filaments only?

Which region is the light area consisting of thin filaments only?

• A. A band
• B. I band
• C. H zone
• D. Z disc

Answer: (B)

The light region that contains only thin filaments is the I band. It lies between two Z discs and is made up mainly of actin filaments (thin fibers) with no thick myosin filaments overlapping in that space, which is why it looks lighter under a microscope. In contrast, the A band contains the entire length of thick filaments and the overlapping thin filaments, and the central part of the A band—the H zone—has only thick filaments. During contraction, actin filaments slide inward, shortening the I band while the A band stays the same length.

The light region that contains only thin filaments is the I band. It lies between two Z discs and is made up mainly of actin filaments (thin fibers) with no thick myosin filaments overlapping in that space, which is why it looks lighter under a microscope. In contrast, the A band contains the entire length of thick filaments and the overlapping thin filaments, and the central part of the A band—the H zone—has only thick filaments. During contraction, actin filaments slide inward, shortening the I band while the A band stays the same length.