According to the sliding filament theory, when muscle fibers are stimulated by the nervous system, which of the following changes occurs?
Correct answer: A. I-bands shorten
- A. I-bands shorten
- B. H-zone becomes more visible
- C. Z-lines move further apart
- D. A-bands shorten
Explanation
According to the sliding filament theory, muscle contraction involves the sliding of thin actin filaments over thick myosin filaments within the sarcomere. This process brings the Z-lines closer together, shortening the sarcomere, and results in the shortening of the I-bands. The A-bands remain constant in length because they represent the region of overlap between actin and myosin. Conversely, the H-zone, which contains only myosin filaments, narrows or disappears entirely during contraction. Therefore, the correct answer is that the I-bands shorten during muscle contraction.
This question appeared on the UHS MDCAT 2015 paper, which you can sit online with every answer explained.
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About Muscle Contraction
Muscle contraction is explained by the sliding filament model, in which myosin heads pull actin filaments within sarcomeres using ATP. The topic covers calcium, troponin, tropomyosin, the neuromuscular junction and relaxation, with attention to how filament length stays almost constant while the sarcomere shortens.
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