The ___ are the synergist muscles to the diaphragm during inspiration.

Correct Answer: A

Rationale: During inspiration, external intercostals act as synergists to the diaphragm, elevating the ribs to expand the thoracic cavity while the diaphragm flattens, increasing volume for air intake. External obliques and rectus abdominis compress the abdomen, aiding exhalation, not inspiration. Internal intercostals depress ribs in forced exhalation, opposing inspiration. External intercostals are the correct answer, as they assist the diaphragm's primary role in breathing, enhancing chest expansion, a critical synergy in respiratory mechanics.

Correct Answer: A

Rationale: Anaerobic respiration occurs without oxygen, breaking down glucose for energy in cells, producing lactic acid instead of carbon dioxide and water. This happens during intense exercise when oxygen demand exceeds supply, like sprinting. Aerobic respiration requires oxygen, yielding more energy efficiently in mitochondria. Mastication is chewing, unrelated to respiration. 'None of the above' dismisses the clear process. Anaerobic respiration's oxygen-free nature suits short, high-energy bursts, contrasting aerobic's sustained effort, and its lactic acid byproduct explains muscle fatigue, distinguishing it from unrelated terms or processes, aligning with cellular energy dynamics under specific conditions.

Correct Answer: D

Rationale: Muscles controlling substance flow within lumens like blood vessels, digestive tract, or airways are smooth muscles. These involuntary muscles, found in organ walls, contract or relax to regulate movement, such as peristalsis in the gut or vasoconstriction in arteries, without conscious control. The hormonal system involves chemical signaling, not muscle action. The skeletal system includes voluntary muscles for locomotion, not lumen regulation. Cardiac muscles power the heart, a specific function unrelated to general lumen control. Smooth muscles' unique ability to modulate flow across diverse tubular structures makes them the best fit, as their autonomic regulation and adaptability distinguish them from striated muscles in managing internal transport.

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: Recruitment increases contraction strength by activating more motor units neurons and their muscle fibers as effort rises. This graded response builds force, like lifting heavier weights, without changing individual fiber contraction strength. Atrophy is muscle wasting from disuse, opposite to recruitment's effect. All-or-none applies to single fiber response, not the cumulative effect of recruitment. Fatigue follows prolonged effort, not recruitment itself. Stronger contractions via recruitment reflect motor unit summation, distinguishing it from wasting, binary responses, or exhaustion, key to muscle power modulation.