heads stay bound to actin until

Correct Answer: D

Rationale: Myosin heads stay bound to actin post-power stroke until another ATP binds, detaching myosin by altering its conformation, allowing the cycle to repeat or relax. More Ca²⁺ affects troponin, not detachment Ca²⁺ sustains contraction. Troponin doesn't bind actin directly for release. More acetylcholine restarts excitation, not detachment. ATP's binding to myosin triggers release, distinguishing it from Ca²⁺ or neurotransmitter roles, key to cross-bridge cycling.

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.

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.