After the receptor is activated, ion depolarizes the muscle fiber cell and travels through the

Correct Answer: B

Rationale: After nicotinic receptor activation by acetylcholine, Na⁺ ions enter, depolarizing the sarcolemma, with the action potential traveling through T-tubules to spread the signal inward. Ca²⁺ releases from the sarcoplasmic reticulum post-T-tubule signal, not depolarizing externally. K⁺ exits later to repolarize, not initiating, and sarcomeres are contractile, not conductive. Ca²⁺ in sarcomeres binds troponin, not traveling. Na⁺ via T-tubules ensures rapid, uniform activation, distinguishing it from internal Ca²⁺ or misaligned K⁺ roles, key to contraction propagation.

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: 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: 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.