Which of the following muscle cell structures is the longest?

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: 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.

Correct Answer: C

Rationale: A muscle's origin is its fixed attachment, typically on a stationary bone (e.g., scapula for biceps), while the insertion moves (e.g., radius). This isn't a bone end or shaft line. The origin's stability enables leverage for movement, distinguishing it in muscle mechanics, critical for understanding contraction dynamics.

Correct Answer: A

Rationale: Intercostal muscles external, internal, innermost fill spaces between ribs, aiding breathing by elevating or depressing the ribcage. Costal cartilage connects ribs to the sternum, 'intercostal space' is the gap they occupy, and pleura lines the chest cavity. These striated, voluntary muscles distinguish the intercostal region's role in respiration, key to thoracic mechanics, contrasting with cartilage or membrane functions.