What is the role of $\mathrm{Ca}^{++}$in muscle contraction?

Correct Answer: B

Rationale: Calcium's role is binding troponin, altering its shape to shift tropomyosin, exposing actin's binding sites for myosin. It doesn't propagate action potentials (acetylcholine does), bind myosin, or engage actin directly those involve ATP and cross-bridge mechanics. This troponin interaction regulates contraction, distinguishing it from nerve or energy roles, pivotal in the sliding filament process.

Correct Answer: B

Rationale: Wrist bending, or flexion/extension, involves specific muscles. The extensor carpi ulnaris extends the wrist, increasing the angle toward the forearm's dorsal side, effectively 'bending' it backward. Extensor digitorum extends fingers, flexor digitorum profundus flexes fingers, and abductor pollicis longus abducts the thumb none target wrist extension directly. Located along the ulna, extensor carpi ulnaris' action aligns with wrist movement, distinguishing it from finger or thumb movers, key for precise hand positioning in activities like lifting.

Correct Answer: A

Rationale: Skeletal muscle is striated (sarcomere-banded), voluntary (somatic control), multinucleate (syncytial), and named (e.g., biceps) unlike cardiac (striated, branched, involuntary, uninucleate) or smooth (non-striated, involuntary, uninucleate) muscle. This unique profile supports conscious movement, distinguishing it in structure and function, critical for anatomical classification.

Correct Answer: C

Rationale: Forearm flexion (elbow bending) involves the biceps brachii (agonist, origin on scapula, insertion on radius) and triceps brachii (antagonist, extending). Brachialis assists, not opposes. Origins aren't on radius or ulna. This agonist-antagonist pairing distinguishes movement roles, critical for coordinated arm motion.

Correct Answer: A

Rationale: Acetylcholine crosses the neuromuscular junction, binding sarcolemma receptors to trigger contraction in skeletal muscle. Adrenalin and noradrenalin modulate autonomic responses, calcium acts intracellularly. Acetylcholine's somatic role distinguishes it, essential for voluntary muscle activation, contrasting with hormonal or regulatory signals.