Which of the following muscles is named according to its origin and insertion?

Correct Answer: C

Rationale: Muscle names often reflect origin (fixed attachment) and insertion (movable end). Sternocleidomastoid specifies origins on the sternum and clavicle, inserting on the mastoid process of the temporal bone, guiding neck movement. Transversus abdominus denotes fibre direction and location, semimembranosus' implies membrane-like shape and location, and deltoid reflects its triangular shape. Only sternocleidomastoid explicitly ties to origin-insertion points, a naming style aiding anatomical precision, distinguishing it from shape-, action-, or location-based names in functional mapping.

Correct Answer: B

Rationale: Skeletal muscle exhibits excitability (signal response), contractility (shortening), and extensibility (stretching), but is innervated by the somatic nervous system for voluntary control, not the autonomic system, which governs involuntary smooth and cardiac muscles. This voluntary innervation distinguishes skeletal muscle's conscious movement role e.g., lifting from autonomic-regulated visceral functions, key to its physiological classification.

Correct Answer: A

Rationale: A sarcomere, a myofibril segment between Z-lines (2 micrometres), is smaller than myofibrils (cell-length), muscle fibres (cells), and fasciculi (fibre bundles). Containing myofilaments, it's the basic contractile unit, distinguishing it as the smallest listed structure, foundational to muscle shortening and force generation.

Correct Answer: B

Rationale: Skeletal muscle, voluntary and striated, includes the diaphragm (breathing), vastus lateralis (thigh), and tongue (speech, swallowing). The pyloric sphincter, controlling stomach emptying, is smooth muscle involuntary, non-striated under autonomic control. This distinction reflects tissue type and function, key to digestive versus locomotive roles.

Correct Answer: C

Rationale: Contraction requires calcium binding to troponin, shifting tropomyosin to expose actin's binding sites, enabling myosin cross-bridges. Myofilaments slide, not shorten; ATP hydrolysis (not synthesis) energizes cross-bridges; acetylcholine moves from axon to sarcolemma, not reverse. Calcium's troponin interaction is essential, distinguishing it as the regulatory trigger, pivotal for the sliding filament process in muscle activation.