Which of the following events that lead to muscle fiber contraction occurs first?

Correct Answer: A

Rationale: Muscle contraction begins with a nerve impulse triggering an action potential along the sarcolemma, which travels down T-tubules to penetrate the fiber. This precedes calcium release from the sarcoplasmic reticulum, which then binds troponin to initiate actin-myosin sliding. Calcium release follows T-tubule signaling, and sliding occurs after calcium's effect. ATP hydrolysis powers the cross-bridge cycle but happens after the signal, during contraction. T-tubule impulse transmission is the first step, ensuring rapid, uniform activation, distinct from subsequent calcium dynamics, filament movement, or energy use, foundational to the contraction sequence in muscle physiology.

Correct Answer: B

Rationale: Masseter and temporalis are primary mastication muscles, closing the jaw for chewing. Masseter elevates the mandible, temporalis assists and retracts. Buccinator aids chewing indirectly via cheeks, not jaw movement. Orbicularis oris moves lips, unrelated. Zygomaticus smiles, not chews. These two excel in jaw power, distinct from accessory, lip, or facial roles, essential for biting and grinding.

Correct Answer: A

Rationale: The rotator cuff comprises infraspinatus, supraspinatus, teres minor, and subscapularis, stabilizing the shoulder joint and enabling rotation. 'Intraspinatus' is a typo correct is infraspinatus. Teres major and deltoid assist shoulder movement but aren't cuff muscles. These four anchor the humerus, distinct from larger or adjacent muscles, vital for shoulder integrity and motion.

Correct Answer: C

Rationale: Fibromyalgia features chronic muscle pain, tenderness, stiffness, and elevated substance P, a pain neurotransmitter, with no clear cause. Cancer may cause pain but isn't muscle-specific or tied to substance P. Diabetes affects nerves, not primarily muscles. Tetanus causes spasms from infection, not chronic ache. Fibromyalgia's diffuse, persistent symptoms set it apart, a musculoskeletal disorder.

Correct Answer: D

Rationale: Muscle contraction and relaxation drive movement through a coordinated process. Contraction occurs when nerve impulses trigger calcium release, enabling myosin heads to bind actin and pull filaments together, shortening the muscle and moving attached structures, like bones in skeletal muscle. Relaxation follows when calcium is pumped back into the sarcoplasmic reticulum, halting myosin-actin interaction, allowing the muscle to lengthen and rest. The incomplete option likely misrepresents a molecular detail (e.g., 'Xn' for ions or proteins), but lacks clarity. Cell layer thickness affects force, not the process itself. Vessel linings support circulation, not contraction mechanics. The contraction-relaxation cycle, powered by ATP and regulated by calcium, is the correct mechanism, universally applicable to skeletal, cardiac, and smooth muscle functions, making it the accurate statement despite the truncated option.