A neuron releases the neurotransmitter that initiates skeletal muscle contraction.

Correct Answer: A

Rationale: Somatic motor (efferent) neurons, part of the voluntary nervous system, release acetylcholine at neuromuscular junctions to initiate skeletal muscle contraction, driving movements like lifting. Sympathetic neurons regulate involuntary functions (e.g., heart rate), not skeletal muscle. Sensory neurons transmit stimuli to the CNS, not activating muscles. 'Muscle neuron' isn't a term neurons innervate, not reside in, muscle. Somatic motor neurons' direct, voluntary control distinguishes them, essential for skeletal muscle's deliberate action, unlike autonomic or sensory roles.

Correct Answer: A

Rationale: Muscle fibres contain nested structures. Myosin, a protein molecule, forms thick myofilaments bundles of myosin within sarcomeres, the contractile units. Sarcomeres align in myofibrils, spanning the fibre. Myosin, at the molecular level (about 160 nm long), is smaller than myofilaments (micrometres), sarcomeres (2 micrometres), and myofibrils (cell-length). Its role as a building block for thicker structures marks it as the smallest, foundational to contraction mechanics, distinguishing it from larger assemblies in muscle hierarchy.

Correct Answer: C

Rationale: Smooth muscle lacks striations banded patterns from aligned sarcomeres unlike striated skeletal (multinucleate, voluntary) and cardiac (uninucleate, involuntary, branched) muscles. Involuntary control and single nuclei are shared with cardiac muscle, and branching is cardiac-specific. Smooth muscle's non-striated, spindle-shaped cells suit its role in visceral organs, distinguishing its microscopic appearance and function from the organized contractile bands of other types, key for histological identification.

Correct Answer: B

Rationale: A fascicle, a bundle of muscle fibres (cells), is larger than a single fibre, which contains myofibrils bundles of sarcomeres. Sarcomeres, the smallest contractile units, are within myofibrils. Fascicles, wrapped in perimysium, contribute to whole-muscle structure, distinguishing them as the largest listed unit, key to muscle organization and force production.

Correct Answer: B

Rationale: Triceps brachii indicates location ( brachii, arm) and origins ( tri-, three). Sartorius is whimsical (tailor's muscle), soleus reflects shape (sandal), and trapezius shape (trapezoid). Brachii's arm-specificity marks it as location-named, distinguishing it from action or whimsical terms, aiding anatomical clarity.