What marks the boundaries of a sarcomere?

• A. M-line
• B. Z-discs (Z-lines)
• C. H-zone
• D. A-band

Answer: (B)

Boundaries of a sarcomere are defined by the Z-discs (Z-lines). These structures sit at the ends of each sarcomere and anchor the plus ends of the thin actin filaments. The sarcomere extends from one Z-disc to the next, so the distance between adjacent Z-discs marks the functional unit’s length.

Understanding other features helps solidify the idea: the M-line lies at the center and helps align the thick filaments, but it isn’t a boundary between sarcomeres. The H-zone is the central region within the A-band where only thick filaments are present in relaxed muscle, and it changes during contraction as thin filaments slide inward. The A-band is the entire length of the thick filaments and remains largely constant in length during contraction.