Question

During muscle contraction

• A. I band shortens
• B. A band shortens
• C. length of the sarcomere remains the same
• D. all of these

Answer 1

The correct option is A I band shortens

Striated muscles have light and dark bands known as striations. These striations were found to be generated due to the presence of actin and myosin proteins. Light bands, also known as isotropic bands (I-bands), are made up of actin proteins. Dark bands, also known as anisotropic bands (A- bands), are made up of myosin proteins. Z-line is a zigzag line that cuts through the I band. The A-band is held together by the m-line. Sarcomere is the portion between successive Z-lines. H-zone is the central part of the thick filament which is not overlapped by the thin filament.
During muscle contraction, when the myosin head binds and pulls the actin filament, the actin filament comes closer to the M-line. This causes the Z-lines to come closer together. The length of the I band shortens but that of the A band remains the same. The length of the sarcomere shortens- shortest length.