Biology · Locomotion and Movement · NEET
The A band is the DARK band. A stands for Anisotropic. It contains the thick myosin filaments. Because myosin is thick, this region looks dark under a microscope. Memory trick: A band = dArk. The A band keeps its length during contraction, so its length never changes.
The I band is the LIGHT band. I stands for Isotropic. It contains only the thin actin filaments. Because actin is thin, this region looks light. Memory trick: I band = lIght. During contraction the I band gets shorter (reduces in width).
The H zone is the CENTRE of the A band. It is the part of the thick myosin filament that is NOT overlapped by thin actin filaments, so it has only myosin. The full A band contains myosin (and the overlap ends of actin), but the H zone is myosin ONLY. So H zone is a smaller region inside the A band. During contraction the H zone disappears because actin slides in and covers the myosin.
The Z line is an elastic fibre in the CENTRE of each I band. The thin actin filaments are firmly attached to the Z line. The portion between two successive Z lines is one sarcomere, the functional unit of contraction. During contraction the Z lines are pulled inwards, which shortens the sarcomere.
The M line is a thin fibrous membrane in the MIDDLE of the A band. It holds the thick myosin filaments together in the centre. Do not confuse the M line with the Z line: M line is in the middle of the A band (myosin), while Z line is in the middle of the I band (actin) and marks the boundary of the sarcomere.
Anisotropic means it bends (polarises) light in a way that makes the region look dark under polarised light, and this is the myosin-rich A band. Isotropic means light passes evenly, making the actin-rich region look light, which is the I band. For NEET you mainly need: A = Anisotropic = dark = myosin; I = Isotropic = light = actin.
During muscular contraction which of the following events occur? (a) 'H' zone disappears (b) 'A' band widens (c) 'I' band reduces in width (d) Myosin hydrolyses ATP, releasing the ADP and Pi (e) Z-lines attached to actins are pulled inwards. Choose the correct answer.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
A sarcomere is the portion of a myofibril between two successive Z lines. It is the functional (contractile) unit of a muscle. Each sarcomere has one central A band and two half I bands on either side.
The I band reduces in width and the H zone disappears; the Z lines are pulled inwards so the whole sarcomere shortens. The A band retains its length and does not change.
The H zone is a region of only myosin in the centre of the A band. The M line is a thin membrane in the middle of the A band (and inside the H zone) that holds the thick filaments together.
Actin is the thin filament and forms the light I band. Myosin is the thick filament and forms the dark A band.
The thin actin filaments are firmly attached to the Z line, which sits in the centre of the I band.
