IMAT Biology Practice Exam

The boundary of a sarcomere is defined by the Z-discs, which anchor the thin (actin) filaments and mark the ends of the repeating unit along a myofibril. The region between two successive Z-discs is one sarcomere. The I-band lies adjacent to a Z-disc and contains only thin filaments, the A-band spans the length of the thick filaments and the overlapping region, and the M-line sits at the center where thick filaments connect. During contraction, the sarcomere shortens as Z-discs move closer together, while the A-band length stays constant.