Which of the following muscle structures is the smallest?

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: 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.

Correct Answer: D

Rationale: Myofibrils, within muscle fibres, are chains of sarcomeres Z-line-to-Z-line units containing myofilaments (actin, myosin). Myofilaments are filament strands, motor units are neuron-fibre groups, and myosin is a protein. Sarcomeres' serial linkage enables myofibril contraction, distinguishing them as the repeating structural unit, key to muscle shortening.

Correct Answer: D

Rationale: Tendons, fibrous muscle extensions, anchor to bone, transmitting force e.g., patellar tendon. Menisci cushion joints, ligaments link bones, and cartilage covers bone ends. Tendons' muscle-bone role distinguishes them, essential for movement mechanics, contrasting with joint-support structures.