Question

In muscle, the functional unit of contraction, which is a portion of a myofibril, positioned in between two successive Z lines is?

• A. I band
• B. A band
• C. H zone
• D. I band with Z line

Answer 1

Answer: (A)

I band

• Myofibril is made of two types of filaments. They are thin filament(actin ) and thick filament(myosin).
• Each myofibril has alternate dark and light bands on it. The light bands contain actin and are called I-band or Isotropic band, whereas the dark band called ‘A’ or Anisotropic band contains myosin.
• 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 portion of the myofibril between two successive ‘Z’ lines is considered as the functional unit of contraction and is called a sarcomere.
• Hence In muscle, the functional unit of contraction, which is a portion of a myofibril, positioned in between two successive Z lines is I band.
• So, the correct answer is I band'.