How do the muscle cells help in movement?

Correct Answer: D

Rationale: Muscle cells, or fibers, enable movement through their ability to contract and relax, a process driven by actin and myosin interactions. In skeletal muscles, nervous system signals trigger contraction: myosin heads pull actin filaments, shortening the muscle and moving attached bones. Relaxation follows when signals cease, allowing fibers to lengthen, returning bones to rest. This applies to voluntary actions (skeletal muscle), heart pumping (cardiac), and organ functions (smooth muscle). Blood flow delivers oxygen and nutrients, supporting energy production, but doesn't directly cause movement. Cell layer thickness influences strength, not the mechanism. Vessel linings (endothelium) regulate flow, not muscle action. Contraction and relaxation are the core processes, fueled by ATP and coordinated by nerves, making them the fundamental drivers of bodily movement across muscle types.

Correct Answer: D

Rationale: Facial expression muscles, like orbicularis oris or zygomaticus, uniquely insert into the skin, allowing fine movements like smiling or frowning, unlike skeletal muscles attaching to bones via tendons. They don't all originate from the scalp many arise from facial bones (e.g., zygomaticus from zygomatic bone). Cartilage insertions are rare and incorrect facial cartilage (e.g., nasal) isn't a primary target. Insertion solely onto facial bones applies to some but not all (e.g., masseter), missing the skin's role. Skin insertion enables their role in subtle, dynamic expressions, distinguishing them from scalp-origin, cartilage, or bone-only attachments, key to facial muscle function.

Correct Answer: D

Rationale: Abductor names a muscle moving the leg away from the midline, like gluteus medius abducting the thigh. Flexor (e.g., biceps femoris) bends joints, decreasing angles. Adductor (e.g., adductor longus) pulls the leg inward. Extensor (e.g., quadriceps) straightens joints, not moving laterally. Abduction's outward action aligns with the term, distinguishing it from bending, inward, or straightening motions, key for leg positioning.

Correct Answer: A

Rationale: Axial (e.g., serratus anterior) and scapular (e.g., trapezius, rhomboids) muscles stabilize the pectoral girdle (scapula, clavicle). Axial alone omits scapular roles. Appendicular includes limb movers, not all stabilizers. Combining both captures serratus anchoring scapula to ribs and trapezius/rhomboids positioning it, distinguishing them, vital for shoulder stability.

Correct Answer: A

Rationale: Gram-negative bacteria possess a unique cell wall with an outer membrane containing lipopolysaccharides (LPS). These LPS molecules, termed endotoxins, are integral structural components released upon cell damage or death, triggering strong immune responses like fever or septic shock. Exotoxins are secreted proteins, not wall components, and 'toxins' is too vague. 'None of the above' ignores LPS's role. Endotoxins' presence in the gram-negative wall, distinct from gram-positive peptidoglycan, and their release mechanism underline their identity as key immunological triggers, differentiating them from secreted toxins.