The Sarcomere

Each myofibril is not uniform along its length but is built of a repeating unit called the sarcomere, laid end to end — it is the sarcomere that gives skeletal muscle its striated (banded) appearance under the microscope. A sarcomere runs from one Z-line (Z-disc) to the next. Immediately either side of the Z-line is the I-band, a lighter zone containing only thin (actin) filaments anchored to the Z-line. Occupying the centre of the sarcomere is the A-band, a darker zone that marks the full length of the thick (myosin) filaments; toward its edges the thick and thin filaments overlap, but at its very centre lies the H-zone, a narrower, lighter region where only thick filaments are present with no overlapping thin filaments. Running through the middle of the H-zone is the M-line, which anchors the thick filaments in place and keeps them centred. During contraction the I-band and H-zone narrow as the thin f …