What is a sarcomere?

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: 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.

Correct Answer: A

Rationale: Acetylcholine crosses the neuromuscular junction, binding sarcolemma receptors to trigger contraction in skeletal muscle. Adrenalin and noradrenalin modulate autonomic responses, calcium acts intracellularly. Acetylcholine's somatic role distinguishes it, essential for voluntary muscle activation, contrasting with hormonal or regulatory signals.