Which structure is the sheet of proteins on the midline of the light I band that anchors thin filaments and connects myofibrils to one another?

• A. Z disc
• B. M line
• C. A band
• D. I band

Answer: (A)

Think about where actin filaments are anchored and how sarcomeres connect. The I band contains only thin filaments, and right at its center sits a dense protein structure that acts as the boundary between sarcomeres and anchors the plus ends of the thin filaments. This midline protein sheet also links adjacent myofibrils, helping hold the structural organization together. That central, anchoring sheet is the Z-disc.

The other terms describe different parts: the M line sits in the middle of the A band and stabilizes thick filaments; the A band covers the length of the thick filaments; the I band is the region with thin filaments outside the overlapped area.