Question

Contractile unit of muscle is part of myofibril between

• A. Z-line and I-band
• B. Z-line and Z-line
• C. Z-line and A-band
• D. Z-band and Z-band

Answer 1

The correct option is B Z-line and Z-line

• The light bands contain actin and are called I-band or Isotropic band, whereas the dark band called ‘A’ or Anisotropic band contains Myosin. Both the proteins are arranged as rod-like structures, parallel to each other and also to the longitudinal axis of the myofibrils.
• Actin filaments are thinner as compared to the myosin filaments, hence are commonly called thin and thick filaments respectively.
• In the centre of each ‘I’ band is an elastic fibre called ‘Z’ line which bisects it. The thin filaments are firmly attached to the ‘Z’ line. The thick filaments in the ‘A’ band are also held together in the middle of this band by a thin fibrous membrane called 'M’ line. The ‘A’ and ‘I’ bands are arranged alternately throughout the length of the myofibrils.
• The portion of the myofibril between two successive ‘Z’ lines is considered as the functional unit of contraction and is called a sarcomere.
• Contractile unit of muscle fibre lies between Z-line and Z-line as, during muscle contraction, length of sarcomere between two Z-line decreases and they are brought closer. This takes place due to sliding of thin filaments over thick filaments.