Smooth muscle cells may be described by which of the following?

Correct Answer: D

Rationale: Smooth muscle, found in organ walls, lacks striations due to unaligned actin and myosin, operates involuntarily under autonomic control, and has one nucleus per cell. Skeletal muscle is striated, voluntary, and multinucleate; cardiac muscle is striated, involuntary, and uninucleate with intercalated discs. Smooth muscle's non-striated, single-nucleus, involuntary nature suits its role in visceral functions like digestion, distinguishing it from skeletal and cardiac types in structure and regulation.

Correct Answer: A

Rationale: Tendons, fibrous extensions of muscle, anchor it to bone, transmitting force for movement e.g., Achilles tendon. Fasciculi are fibre bundles, sarcomeres are contractile units, and intercostals are rib muscles, not attachment structures. Tendons' role in linking muscle to skeleton distinguishes them, essential for biomechanical function and contrasting with ligaments' bone-to-bone role.

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.