The functional contractile unit of a myofibril is the
Correct answer: B. sarcomere
- A. myofilament
- B. sarcomere
- C. sarcolemma
- D. sarcoplasmic reticulum
Explanation
A sarcomere is the region between two Z lines, containing overlapping thick and thin filaments, and the whole muscle shortens because thousands of sarcomeres in series each shorten a little. A myofilament is a single actin or myosin strand, so it is a component rather than the unit. The sarcolemma is the cell membrane and the sarcoplasmic reticulum is the calcium store.
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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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