Which protein(s) are found in thin myofilaments?

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: 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.

Correct Answer: A

Rationale: Skeletal muscle is striated (sarcomere-banded), voluntary (somatic control), multinucleate (syncytial), and named (e.g., biceps) unlike cardiac (striated, branched, involuntary, uninucleate) or smooth (non-striated, involuntary, uninucleate) muscle. This unique profile supports conscious movement, distinguishing it in structure and function, critical for anatomical classification.

Correct Answer: C

Rationale: Forearm flexion (elbow bending) involves the biceps brachii (agonist, origin on scapula, insertion on radius) and triceps brachii (antagonist, extending). Brachialis assists, not opposes. Origins aren't on radius or ulna. This agonist-antagonist pairing distinguishes movement roles, critical for coordinated arm motion.