M-line

Meaning: The M-line (M for "middle") is a set of proteins at the center of each sarcomere that cross-link adjacent thick filaments and hold them in a parallel, ordered array. It sits in the middle of the H-zone, the region that contains only thick filaments. The Z-lines at the sarcomere's ends play the matching role for the thin (actin) filaments, which are anchored there.

On the MCAT: You may be asked to label a sarcomere diagram or to say which bands change during contraction. By the sliding filament model, thin filaments slide toward the M-line as myosin heads cycle through cross-bridges. The Z-lines move closer together, so the sarcomere shortens. The H-zone and I-band shrink, while the A-band stays the same width because thick filaments keep their length. The M-line stays at the center of the sarcomere throughout.

Common confusion: Students mix up the M-line and the Z-line. Remember that M is in the middle and binds myosin, and that Z marks the sarcomere's boundary and binds actin. A second error is thinking the M-line is the same as the H-zone. The H-zone is a band of thick filaments with no overlapping thin filaments, and the M-line is a thin line at its center. A third error is expecting the A-band to shorten. It does not, because filaments slide past one another and do not shorten.