Voluntary muscle is a

Correct Answer: A

Rationale: Voluntary muscles, synonymous with skeletal muscles, allow conscious control over actions like lifting an arm or turning the head. Linked to bones via tendons, they respond to brain signals via the somatic nervous system, enabling deliberate movement. No control describes involuntary muscles smooth and cardiac working automatically, like digestion or heartbeats. Automatic function fits those, not voluntary ones. Muscles fused together misrepresents anatomy muscles are distinct, not merged. Voluntary muscle's defining trait is this control, essential for daily tasks, contrasting with involuntary types' autonomic roles, highlighting its role in purposeful, cognitive-directed motion across limbs, neck, and torso.

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: A

Rationale: A muscle twitch, a single contraction-relaxation cycle from one stimulus, has three phases: latent (signal delay to contraction start), contraction (cross-bridge activity shortens muscle), and relaxation (calcium removal, filament separation). A myogram records this, not the event itself. Summation is multiple twitches overlapping, not a single cycle. A motor unit is a neuron and fibers, not a phase sequence. The twitch's distinct stages define its mechanics, distinguishing it from recordings, cumulative effects, or anatomical units, fundamental to muscle response analysis.

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.