What must occur for a contraction to cease and the muscle fiber relax?

Correct Answer: D

Rationale: Relaxation requires multiple steps: the nerve impulse stopping halts T-tubule signals, calcium being pumped back into the sarcoplasmic reticulum (SR) removes it from troponin, shifting tropomyosin to block actin sites, and ATP binding myosin detaches it from actin, ending cross-bridges. One alone like impulse cessation won't suffice without calcium removal and detachment. Calcium pumping alone leaves myosin bound if ATP's absent. ATP detachment needs prior steps. All must occur, ensuring contraction ceases fully, distinct from partial processes, restoring the muscle to rest, integral to its cyclic function.

Correct Answer: A

Rationale: The diaphragm, a dome-shaped muscle below the lungs, contracts to flatten, expanding the thoracic cavity for inspiration breathing's primary driver. External intercostals assist, lifting ribs, but diaphragm's action is dominant. Internal intercostals aid expiration, secondary to inspiration. Rectus abdominis flexes the trunk, not breathing. Diaphragm's central role in volume change sets it apart, essential for respiratory rhythm.

Correct Answer: D

Rationale: The quadriceps group rectus femoris, vastus muscles lies on the anterior thigh, extending the knee and flexing the hip, as in kicking. Abductors are lateral, adductors medial, hamstrings posterior. Quadriceps' anterior placement and extension role distinguish them, vital for leg straightening, unlike positional or opposing groups.

Correct Answer: C

Rationale: During muscle contraction, the cross-bridge cycle drives filament sliding. After myosin binds to actin and performs a power stroke pulling actin toward the sarcomere's center ATP binds to the myosin head. This binding reduces myosin's affinity for actin, breaking the cross-bridge and detaching myosin, allowing it to reset for another cycle. ATP is then hydrolyzed to ADP and phosphate, cocking the head into a high-energy state to reattach to actin. Calcium ions, managed by the sarcoplasmic reticulum, regulate actin exposure, not myosin detachment. Acetylcholine release occurs at the neuromuscular junction, unrelated to ATP's role here. Hydrolysis energizes myosin but doesn't cause attachment binding does the detachment. This ATP-driven detachment is critical for repeated contractions, distinguishing it from initiation or calcium-related steps, and underscores its mechanical role in muscle dynamics.

Correct Answer: C

Rationale: The anterior cranial fossa, housing the frontal brain lobes, is formed by the orbital part of the frontal bone (roof), ethmoid bone (midline, cribriform plate), and sphenoid bone (lesser wings, posteriorly). These create a shallow depression. The temporal bone, lateral and forming the middle fossa, doesn't contribute anteriorly its squamous and petrous parts support the temporal lobes instead. The frontal bone's orbital plate, ethmoid's midline, and sphenoid's wings align with the fossa's anatomy, while the temporal's exclusion reflects its distinct cranial position, critical for understanding skull compartmentalization.