Structure of the Muscles Quiz

See The Sarcomere. Option D is correct: the I band is the part of the sarcomere that contains only thin actin filaments, with no myosin at all. A sarcomere is the repeating contractile unit of a muscle fibre, measured from one Z line to the next, and under the microscope the alternating dark and light bands give skeletal and cardiac muscle their striped (striated) look. The I band is the light band because it holds only the thin filaments, and the Z line runs through its centre. The dark A band is the region where the thick myosin filaments lie, and inside it the H zone is the central stretch with no thin-filament overlap. The M line, where the myosin filaments are anchored, is the middle of the H zone and so the centre of the sarcomere. The Z line is where the actin filaments are anchored. When the muscle contracts, the thin filaments slide inwards, so the I band and H zone get narrower while the A band stays the same length. Two memory aids: dArker is the A band and lighter is the I band, and MHAZI (the M line is inside the H zone, which is inside the A band, while the Z line is inside the I band).

• I band: it contains only thin actin filaments, with no myosin.
• Z line: it runs through the centre of the I band and anchors the actin filaments.
• During contraction: the I band and H zone get narrower while the dark A band stays the same length.