What causes the myosin binding site of an actin molecule to be exposed?

Correct Answer: C

Rationale: Actin's myosin-binding site is exposed when calcium ions bind troponin, shifting tropomyosin away via a conformational change. ATP energizes myosin post-binding, nerve impulses trigger calcium release, and acetylcholine initiates the impulse but only calcium directly uncovers the site. This regulatory step, within the sarcomere, distinguishes it from nerve or energy events, enabling cross-bridge cycling, fundamental to contraction mechanics.

Correct Answer: C

Rationale: Mastication (chewing) involves jaw-moving muscles: temporalis and masseter elevate the mandible, buccinator aids cheek tension. Sternocleidomastoid and scalene move the neck, sartorius-gracilis-soleus the leg, and orbicularis oculi-mentalis the face (eyes, chin). The mastication trio's jaw-specific role distinguishes them, essential for biting and grinding food.

Correct Answer: A

Rationale: A skeletal muscle fibre contains myofibrils, each a chain of sarcomeres repeating units between Z-lines that shorten during contraction via actin-myosin sliding. Sarcolemma is the membrane, sarcoplasm the cytoplasm, and fasciculus a fibre bundle not contractile units. Sarcomeres' role as the smallest contractile segment distinguishes them, summing to fibre-wide contraction, fundamental to muscle mechanics and force production.

Correct Answer: A

Rationale: An agonist drives movement (e.g., biceps in flexion), not opposes antagonists (e.g., triceps) do. Origin is the fixed attachment, skeletal cells are syncytial (multinucleate), and fixators stabilize. The agonist's role as prime mover distinguishes the error, key to movement dynamics.

Correct Answer: D

Rationale: Sternocleidomastoid names origins (sternum, clavicle) and insertion (mastoid process), guiding neck motion. Tibialis anterior, extensor digitorum longus, and rectus femoris denote location, action, or fibre direction not attachments. This origin-insertion naming distinguishes it, aiding precise anatomical mapping.