Of the events that lead to myofilaments sliding over each other, which of the following happens first?

Correct Answer: C

Rationale: Muscle contraction follows a precise sequence. Calcium ions first enter the cytoplasm from the sarcoplasmic reticulum, triggered by a nerve impulse depolarizing the sarcolemma. This calcium binds troponin, shifting tropomyosin to expose actin's binding sites. Only then can myosin heads, energized by prior ATP hydrolysis, engage actin, pulling filaments past each other. Calcium's entry is the initial cytosolic event, preceding troponin's action, site exposure, and cross-bridge formation. This order, rooted in excitation-contraction coupling, ensures contraction starts with a neural signal, distinguishing it from subsequent mechanical steps.

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.

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.