The thin filaments of a sarcomere are made up of

Correct Answer: D

Rationale: Thin filaments in a sarcomere consist of actin, troponin, and tropomyosin, forming a complex that interacts with myosin for contraction. Actin provides the structural backbone, tropomyosin covers binding sites in rest, and troponin binds calcium to shift tropomyosin, exposing sites for myosin. Only actin oversimplifies, ignoring regulatory proteins. Only myosin misplaces it myosin forms thick filaments. Actin and myosin together suggest a mix, but they're separate filaments. The trio of actin, troponin, and tropomyosin defines thin filaments, enabling controlled contraction, distinct from incomplete or incorrect combinations, critical for the sliding filament theory and muscle movement precision.

Correct Answer: D

Rationale: Zygomaticus (major and minor) lifts the mouth's corners upward and laterally, forming a smile. Buccinator compresses cheeks, not smiling. Frontalis raises the brow, unrelated to lips. Orbicularis oris purses lips, as in kissing, not smiling. Zygomaticus' specific pull on the mouth defines smiling, distinct from cheek, brow, or lip-closing actions, central to happy expressions.

Correct Answer: A

Rationale: Latissimus dorsi, spanning the lower back, adducts and extends the arm, lowering it from a raised position, as in swimming strokes. Pectoralis major pulls forward. Serratus anterior protracts scapula. Trapezius elevates shoulders. Latissimus dorsi's posterior action sets it apart, crucial for arm return, distinct from chest, scapular, or shoulder roles.

Correct Answer: A

Rationale: Gastrocnemius, on the posterior leg, forms the calf's bulk, plantarflexing the foot, as in walking. Semitendinosus, a hamstring, is thigh-based. Tibialis anterior, anterior leg, dorsiflexes. Vastus lateralis, a quadriceps, is thigh-front. Gastrocnemius' calf prominence and action set it apart, key for locomotion.

Correct Answer: D

Rationale: Muscle cells, or fibers, enable movement through their ability to contract and relax, a process driven by actin and myosin interactions. In skeletal muscles, nervous system signals trigger contraction: myosin heads pull actin filaments, shortening the muscle and moving attached bones. Relaxation follows when signals cease, allowing fibers to lengthen, returning bones to rest. This applies to voluntary actions (skeletal muscle), heart pumping (cardiac), and organ functions (smooth muscle). Blood flow delivers oxygen and nutrients, supporting energy production, but doesn't directly cause movement. Cell layer thickness influences strength, not the mechanism. Vessel linings (endothelium) regulate flow, not muscle action. Contraction and relaxation are the core processes, fueled by ATP and coordinated by nerves, making them the fundamental drivers of bodily movement across muscle types.