Each myosin head uses to change shape.

Correct Answer: D

Rationale: Each myosin head uses one ATP molecule, hydrolyzed to ADP and phosphate, to change shape, cocking for the power stroke and detaching post-actin binding, fueling one cycle. Ca²⁺ binds troponin, not myosin, initiating exposure, not shape change. Two ADP or ATP molecules overstate biochemistry confirms one ATP per stroke. Na⁺ drives depolarization, not myosin mechanics. One ATP's energy distinguishes it, critical for efficient contraction, unlike ions or excess molecules.

Correct Answer: C

Rationale: Thin myofilaments, in sarcomeres, comprise actin, a structural protein forming the filament backbone, tropomyosin, which covers actin's binding sites, and troponin, which binds calcium to shift tropomyosin during contraction. Myosin forms thick myofilaments, not thin ones. This trio actin, tropomyosin, troponin enables regulated contraction, distinguishing thin filaments' composition and role from thick filaments, essential for the sliding filament theory of muscle action.

Correct Answer: B

Rationale: Calcium's role is binding troponin, altering its shape to shift tropomyosin, exposing actin's binding sites for myosin. It doesn't propagate action potentials (acetylcholine does), bind myosin, or engage actin directly those involve ATP and cross-bridge mechanics. This troponin interaction regulates contraction, distinguishing it from nerve or energy roles, pivotal in the sliding filament process.

Correct Answer: C

Rationale: A sarcomere, a myofibril's repeating unit between Z-lines, contains overlapping actin and myosin myofilaments, contracting via their sliding. It's not the sarcolemma (membrane), sarcoplasm (cytoplasm), or just a filament bundle though filaments define it. Its sectional role distinguishes it, key to myofibril contraction and muscle function.

Correct Answer: B

Rationale: Wrist bending, or flexion/extension, involves specific muscles. The extensor carpi ulnaris extends the wrist, increasing the angle toward the forearm's dorsal side, effectively 'bending' it backward. Extensor digitorum extends fingers, flexor digitorum profundus flexes fingers, and abductor pollicis longus abducts the thumb none target wrist extension directly. Located along the ulna, extensor carpi ulnaris' action aligns with wrist movement, distinguishing it from finger or thumb movers, key for precise hand positioning in activities like lifting.