When a skeletal muscle fiber contracts, the The myosin fibrils slide over the actin Actin fibrils contract Zones of overlap of actin and myosin get larger. All of the answers are correct
The Correct Answer and Explanation is:
Correct Answer: Zones of overlap of actin and myosin get larger.
Explanation
Skeletal muscle contraction is a complex physiological process governed by the sliding filament theory, which explains how muscles generate force and shorten. According to this theory, muscle contraction occurs when myosin (thick) filaments slide past actin (thin) filaments, pulling the actin toward the center of the sarcomere — the basic contractile unit of a muscle fiber.
The correct answer — “Zones of overlap of actin and myosin get larger” — is accurate because, during contraction, the overlap between actin and myosin increases, allowing for more cross-bridge interactions. This means more myosin heads can bind to actin, pull it, and generate force.
To clarify the other options:
- “The myosin fibrils slide over the actin” – Incorrect. Myosin filaments do not “slide” over actin; instead, actin slides over myosin, pulled by the cross-bridge action of myosin heads.
- “Actin fibrils contract” – Incorrect. Actin and myosin filaments do not change length during contraction. They slide past one another, which shortens the sarcomere but not the filaments themselves.
- “All of the answers are correct” – Incorrect, as the first two options are factually inaccurate.
During contraction, several structural changes occur in the sarcomere:
- The I band (where only actin is present) shortens.
- The H zone (where only myosin is present) also shortens or disappears.
- The A band (length of myosin filaments) stays the same.
- The zone of overlap between actin and myosin increases, which is key to muscle force generation.
These changes are powered by ATP and regulated by calcium ions, which allow myosin heads to bind to actin. This precise coordination enables skeletal muscles to contract efficiently and produce movement.
Thus, understanding these mechanics helps clarify why increased overlap of actin and myosin is the most accurate description of what occurs during muscle fiber contraction.
