H-zone

Meaning: The H-zone is the central region of the A band where only thick (myosin) filaments are present and thin (actin) filaments do not reach. The M line runs through its middle. In contraction, myosin heads pull the thin filaments toward the sarcomere center, so they slide further into the A band and the H-zone narrows. In a strongly contracted sarcomere the thin filaments can meet or overlap, and the H-zone disappears.

On the MCAT: Passages show a labeled sarcomere diagram or micrograph in relaxed and contracted states and ask which regions change. On contraction, the H-zone, the I band, and the whole sarcomere (Z line to Z line) shorten, while the A band stays the same length. Stretching a relaxed muscle does the reverse: the H-zone and I band widen. This pattern is the standard evidence for the sliding filament model.

Common confusion: Students treat the H-zone as the whole A band, or think the thick filaments shorten. The A band equals the length of the thick filaments and does not change. The H-zone is only the part of the A band that thin filaments do not reach. Filaments slide past each other; they do not shorten.