Which term is given to the unit of a myofibril that contracts?

Correct Answer: B

Rationale: Within myofibrils, sarcomeres are the contractile units, shortening as actin and myosin slide past each other, powered by ATP. Sarcoplasm is cytoplasm, sarcolemma the membrane, and sarcoplasmic reticulum a calcium store not contractile. Sarcomeres' banded structure, from Z-line to Z-line, enables muscle contraction, their collective action summing to fibre shortening, distinguishing them as the functional core of myofibril mechanics in muscle physiology.

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.

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.