Which of the following supports the sliding filament theory of muscle contraction?
A The I band increases in width during contraction.
B The distance between Z lines increase during contraction.
C The distance between the Z lines decreases during contraction.
D The A band decreases in width during contraction.
E The A band increases in width during contraction.
The correct answer and explanation is:
The correct answer is C: The distance between the Z lines decreases during contraction.
The sliding filament theory of muscle contraction explains how muscles contract at the microscopic level. According to this theory, the interaction between two types of protein filaments, actin (thin filaments) and myosin (thick filaments), leads to muscle shortening. This process occurs within the muscle fibers, specifically in the sarcomere, the structural unit of muscle contraction.
During muscle contraction, the actin filaments slide over the myosin filaments. This sliding action brings the Z lines (the boundaries of each sarcomere) closer together. The sarcomere shortens, which results in the overall shortening of the muscle. As the Z lines come closer, the I band (which consists of thin actin filaments) also decreases in width, while the A band (the region containing both thick myosin filaments and overlapping thin actin filaments) remains relatively unchanged in length.
To clarify the incorrect options:
- A: The I band does not increase during contraction; it actually shortens as the actin filaments slide towards the center of the sarcomere.
- B: The distance between the Z lines does not increase during contraction; rather, it decreases as the sarcomere shortens.
- D: The A band does not decrease in width during contraction. It remains roughly the same length since it spans the length of the myosin filaments, which do not change in length.
- E: The A band does not increase in width during contraction; the length of the myosin filaments remains constant.
In summary, the key feature supporting the sliding filament theory is the decrease in the distance between Z lines, as actin and myosin filaments slide past each other during contraction. This leads to the shortening of the muscle fiber, which is essential for muscle contraction.