In humans, if the diaphragm is pushed upward, there is a decrease in chest volume. The decrease is followed by:

Correct Answer: B

Rationale: The diaphragm's upward movement, as in exhalation, reduces chest volume. Per Boyle's law, this increases pressure in the lungs, forcing air out as the higher internal pressure exceeds atmospheric pressure. Inhalation occurs when the diaphragm descends, expanding volume and lowering pressure, drawing air in. The upward push compresses the chest, driving exhalation, a fundamental respiratory mechanism ensuring gas exchange, distinct from inhalation's volume increase and pressure drop.

Correct Answer: B

Rationale: After nicotinic receptor activation by acetylcholine, Na⁺ ions enter, depolarizing the sarcolemma, with the action potential traveling through T-tubules to spread the signal inward. Ca²⁺ releases from the sarcoplasmic reticulum post-T-tubule signal, not depolarizing externally. K⁺ exits later to repolarize, not initiating, and sarcomeres are contractile, not conductive. Ca²⁺ in sarcomeres binds troponin, not traveling. Na⁺ via T-tubules ensures rapid, uniform activation, distinguishing it from internal Ca²⁺ or misaligned K⁺ roles, key to contraction propagation.

Correct Answer: C

Rationale: Muscle contraction follows a precise sequence. Calcium ions first enter the cytoplasm from the sarcoplasmic reticulum, triggered by a nerve impulse depolarizing the sarcolemma. This calcium binds troponin, shifting tropomyosin to expose actin's binding sites. Only then can myosin heads, energized by prior ATP hydrolysis, engage actin, pulling filaments past each other. Calcium's entry is the initial cytosolic event, preceding troponin's action, site exposure, and cross-bridge formation. This order, rooted in excitation-contraction coupling, ensures contraction starts with a neural signal, distinguishing it from subsequent mechanical steps.

Correct Answer: D

Rationale: Smooth muscle, found in organ walls, lacks striations due to unaligned actin and myosin, operates involuntarily under autonomic control, and has one nucleus per cell. Skeletal muscle is striated, voluntary, and multinucleate; cardiac muscle is striated, involuntary, and uninucleate with intercalated discs. Smooth muscle's non-striated, single-nucleus, involuntary nature suits its role in visceral functions like digestion, distinguishing it from skeletal and cardiac types in structure and regulation.

Correct Answer: C

Rationale: Actin's myosin-binding site is exposed when calcium ions bind troponin, shifting tropomyosin away via a conformational change. ATP energizes myosin post-binding, nerve impulses trigger calcium release, and acetylcholine initiates the impulse but only calcium directly uncovers the site. This regulatory step, within the sarcomere, distinguishes it from nerve or energy events, enabling cross-bridge cycling, fundamental to contraction mechanics.