A feature of skeletal muscle that is NOT shared with cardiac or smooth muscle is:

Correct Answer: D

Rationale: Skeletal muscle's multinucleate nature a syncytium from fused myoblasts contrasts with uninucleate cardiac and smooth muscles. Striations occur in skeletal and cardiac, branching and intercalated discs are cardiac-specific. Multiple nuclei support skeletal muscle's size and repair, distinguishing it in structure and voluntary function, key to its histological identity.

Correct Answer: B

Rationale: Thin myofilaments comprise actin (structure), tropomyosin (covers binding sites), and troponin (calcium receptor), regulating contraction. Thick myofilaments are myosin only, with heads forming cross-bridges not binding sites and lack tropomyosin/troponin. Myosin isn't in thin filaments. This composition distinguishes thin filaments' regulatory role, critical for controlled muscle action, contrasting with thick filaments' motor function.

Correct Answer: C

Rationale: Sarcomeres, within myofibrils, contract by sliding actin and myosin, shortening the myofibril and thus the cell. Sarcoplasm houses organelles, sarcolemma conducts signals, and sarcoplasmic reticulum stores calcium not contractile. Sarcomeres' role as the contractile engine distinguishes them, essential for muscle movement mechanics.

Correct Answer: A

Rationale: Calcium ions bind troponin, shifting tropomyosin to reveal actin's binding sites, initiating contraction. Acetylcholine triggers nerve signals, phosphate and ADP relate to ATP not regulation. Calcium's specific troponin role distinguishes it, pivotal for muscle activation, contrasting with nerve or energy molecules.