Question

In the mechanism of muscle contraction or shortening of muscle, the ____ get reduced whereas the ____ retain the length.

• A. Z line, I bands
• B. A bands, Z line
• C. A bands, I bands
• D. I bands, A bands

Hint:

I bands shorten during muscle contraction, while A bands stay the same length.

Answer 1

The Correct Option is D

• Muscle Contraction: During muscle contraction, the sarcomere shortens due to the sliding filament theory.
• Structure Analysis:
  • I bands (light bands) contain only actin filaments and shorten as actin and myosin filaments slide past each other.
  • A bands (dark bands) contain the full length of myosin filaments and remain constant in length.
  • Z lines (Z discs) anchor actin filaments and move closer together but are not the primary focus of length change.
• Option Analysis:
  • (1) Z line, I bands: Z lines move, but I bands reduce, not vice versa. Incorrect.
  • (2) A bands, Z line: A bands retain length, Z lines move. Incorrect.
  • (3) A bands, I bands: A bands retain length, but I bands reduce, not the other way. Incorrect.
  • (4) I bands, A bands: I bands reduce in length, A bands retain length during contraction. Correct.
• Conclusion: The correct answer is (4) I bands, A bands, as I bands shorten while A bands remain constant.