What is the function of the M-line within the sarcomere?

• A. Serves as a central anchoring point for thick filaments.
• B. Marks the boundaries of the sarcomere.
• C. Contains thin filaments.
• D. Regulates calcium release from the SR.

Answer: (A)

The M-line sits at the center of the sarcomere and its main job is to stabilize and organize the thick filaments. Proteins like myomesin form a lattice there, cross-linking adjacent myosin filaments so they stay aligned with one another as the muscle shortens. This central anchoring helps maintain the structure and spacing of the thick filaments during contraction, keeping the sarcomere’s architecture intact.

The M-line isn’t where the sarcomere boundaries are set—that’s the Z-disc. It also doesn’t contain thin filaments; actin-based thin filaments reside in the I-band and extend into the A-band. Calcium release from the sarcoplasmic reticulum is a separate process involving calcium channels and pumps, not the M-line.