During skeletal muscle contraction, which sarcomeric region changes length the most?

During skeletal muscle contraction, which sarcomeric region changes length the most?

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

Answer: (B)

The key idea is how the sarcomere shortens when actin filaments slide past myosin. The length of the thick-filament region (the A-band) stays the same because the length of the myosin filaments doesn’t change. The part that changes the most is the region that contains only thin filaments—the I-band. As the actin filaments are pulled toward the center by the myosin heads, the gap that contains only actin shortens dramatically, and this region can even vanish at full contraction when actin overlaps the thick filaments. The Z-lines move closer together as the sarcomere shortens, and the M-line remains fixed at the center, but the single region that shows the greatest change in length is the I-band.

The key idea is how the sarcomere shortens when actin filaments slide past myosin. The length of the thick-filament region (the A-band) stays the same because the length of the myosin filaments doesn’t change. The part that changes the most is the region that contains only thin filaments—the I-band. As the actin filaments are pulled toward the center by the myosin heads, the gap that contains only actin shortens dramatically, and this region can even vanish at full contraction when actin overlaps the thick filaments. The Z-lines move closer together as the sarcomere shortens, and the M-line remains fixed at the center, but the single region that shows the greatest change in length is the I-band.