Sarcomeres are defined as the region between two Z-lines.

• A. Between two M-lines
• B. Around the M-line
• C. Between two Z-lines
• D. Across the I-band

Answer: (C)

A sarcomere is the basic contractile unit of a striated muscle, defined by the segment between two Z-lines (Z-discs). The Z-lines mark the boundaries where adjacent sarcomeres meet, anchoring the thin actin filaments and providing the repeating unit that shortens during contraction. The center of a sarcomere contains the M-line, where thick myosin filaments are anchored, but this is not the boundary. The I-band lies adjacent to the Z-lines and contains only thin filaments, while the A-band runs the length of the thick filament and overlaps with thin filaments in the middle of the sarcomere. Because boundaries are set by the Z-lines, the region between two Z-lines is the correct definition of a sarcomere.