Question

Identify the events occurring in muscular contraction;

(i) H-zone disappears

(ii) A band widens

(iii) I-band reduces in width

(iv) The width of A band is unaffected

(v) M-line and Z-line come closer

• A. i, ii, v
• B. i, ii, iii
• C. i, iii, iv, v
• D. ii, iv, v

Answer 1

Answer: (C)

i, iii, iv, v

• 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 thick filaments in the ‘A’ band are also held together in the middle of this band by a thin fibrous membrane called’ line.
• Utilizing the energy from ATP hydrolysis, the myosin head now binds to the exposed active sites on actin to form a cross bridge.
• This pulls the attached actin filaments towards the centre of ‘A’ band.
  
• The ‘Z’ line attached to these actions are also pulled inwards thereby causing a shortening of the sarcomere, i.e., contraction.
• Hence the events occurring in muscular contraction are H-zone disappears, I-band reduces in width, The width of A band is unaffected and M-line and Z-line come closer.
• So, the correct answer is 'i, iii, iv, v