The thick filaments of a sarcomere are made up of

Correct Answer: B

Rationale: In a sarcomere, the contractile unit of muscle, thick filaments are composed of myosin, a protein with cross-bridges that pull actin during contraction. This forms the dark A band, driving the sliding filament mechanism. Actin forms thin filaments, interacting with myosin, not constituting thick ones. Myoglobin stores oxygen, unrelated to filament structure. Tropomyosin regulates actin's active sites on thin filaments, not part of thick filaments. Myosin's structural and functional role in thick filaments is key, enabling force generation, distinct from actin's thin filament role, myoglobin's metabolic support, or tropomyosin's regulatory function, fundamental to muscle contraction mechanics.

Correct Answer: B

Rationale: The frontalis, on the forehead, wrinkles the brow and raises eyebrows, creating horizontal lines, as in surprise. Buccinator compresses cheeks, aiding chewing. Orbicularis oculi closes eyes, wrinkling skin around them, not the brow. Zygomaticus lifts mouth corners for smiling. Frontalis' forehead action distinguishes it, key for facial expression above the eyes.

Correct Answer: C

Rationale: Pectoralis major, across the chest, adducts and medially rotates the arm, pulling it toward the midline, as in hugging. Biceps brachii flexes the elbow. Latissimus dorsi adducts from behind, not chest-directed. Triceps brachii extends the elbow. Pectoralis major's anterior pull distinguishes it, key for chest-centric motion, unlike elbow or posterior actions.

Correct Answer: B

Rationale: Hamstrings biceps femoris, semimembranosus, semitendinosus flex the knee and extend the hip, on the posterior thigh. Rectus femoris, an anterior quadriceps, extends the knee. Biceps brachii flexes the elbow, an arm muscle. Semimembranosus and semitendinosus are hamstrings. Rectus femoris' quadriceps role excludes it, distinct from arm or hamstring functions.

Correct Answer: B

Rationale: The joint connecting the neck to the head, specifically the atlas (C1) and skull's occipital condyles, allows rotational and nodding movements. This atlanto-occipital joint permits flexion, extension, and slight lateral tilting, while the atlas-axis (C1-C2) adds pivoting, as in turning the head side to side. This fits the pivotal joint category, where a bone rotates within a ring-like structure, exemplified by the atlas pivoting around the axis' dens. Ball-and-socket joints, like the shoulder, offer multi-axial rotation and swinging, exceeding neck motion. Hinge joints, such as the elbow, limit movement to one plane, unlike the neck's range. Fixed joints, like skull sutures, allow no motion. The pivotal joint's rotational capacity matches the neck-head junction's function, distinguishing it in skeletal articulation classifications.