What structure stores Ca²⁺ in the muscle fiber?

Correct Answer: C

Rationale: The sarcoplasmic reticulum, a specialized endoplasmic reticulum, stores calcium ions (Ca²⁺) in muscle fibers, releasing them upon T-tubule signals to trigger contraction via troponin binding. T-tubules conduct action potentials, not storing Ca²⁺. Myofibrils, contractile units of actin and myosin, rely on Ca²⁺ but don't store it. Troponin binds Ca²⁺ to shift tropomyosin, not storing it its source is the reticulum. Sarcoplasmic reticulum's storage role ensures rapid, regulated Ca²⁺ availability, distinguishing it from conduction, contraction, or binding structures, critical for muscle activation.

Correct Answer: A

Rationale: Muscle names may reflect action, movement produced. Adductor longus indicates adduction drawing a limb toward the midline as its primary function, with longus denoting size. Temporalis refers to its temporal bone location, sternocleidomastoid to origin-insertion points, and peroneus longus to its fibular position and length. Only adductor directly names the action, a convention aiding functional understanding, distinguishing it from location- or attachment-based names, essential for predicting muscle roles in movement analysis.

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.