Which of the following is in the correct order from large to small?

Correct Answer: B

Rationale: Muscle structure descends from large to small: myofibrils (organelles within fibers) contain sarcomeres (contractile units), which comprise thin (actin) and thick (myosin) filaments. Muscle belly, the whole muscle, is larger, so starting there misorders scale. Sarcomeres, between Z-lines, house filaments, not vice versa filaments build sarcomeres, sarcomeres build myofibrils. Starting with filaments then sarcomere reverses this hierarchy, and myofibril as largest ignores its cellular role. The correct sequence reflects anatomical organization, distinguishing myofibril as a bundle of sarcomeres, each with filaments, key to understanding contraction's structural basis.

Correct Answer: B

Rationale: Muscle cells hierarchically organize contractile units. Myofibrils, spanning the cell's length, are bundles of sarcomeres repeating units with thick (myosin) and thin (actin) myofilaments. Sarcomeres, about 2 micrometres long, link end-to-end within myofibrils, which extend tens to hundreds of micrometres. Myofilaments are shorter segments within sarcomeres, and troponin is a tiny protein on actin. Myofibrils' full-cell length distinguishes them as the longest, critical for coordinated contraction across the fibre, unlike shorter sarcomeres or molecular-scale components.

Correct Answer: C

Rationale: Ligaments, fibrous connective tissues, link bones across joints, stabilizing skeletal structure e.g., knee ligaments. Tendons and aponeuroses attach muscles to bones, facilitating movement, while sarcomeres are muscle contractile units. Ligaments' bone-to-bone connection distinguishes them from muscle-related structures, crucial for joint integrity and distinguishing skeletal support from contractile mechanisms in musculoskeletal anatomy.

Correct Answer: C

Rationale: Skeletal muscle cells feature neuromuscular junctions with acetylcholine, T-tubules for signal spread, and striations from sarcomere alignment, but aren't branched cardiac muscle is. Their long, cylindrical, multinucleate form suits voluntary movement, distinguishing them from branched, intercalated cardiac cells, key to their structural and functional identity in locomotion.

Correct Answer: A

Rationale: Acetylcholine, released from motor neurons, crosses the neuromuscular junction, binding sarcolemma receptors to trigger an action potential, initiating calcium release. It doesn't bind troponin (calcium does), supply energy (ATP does), or engage actin. As a neurotransmitter, it bridges nerve and muscle, distinguishing it from contraction's biochemical steps, essential for voluntary movement.