What is the role of acetylcholine in muscle cell contraction?

Correct Answer: A

Rationale: Acetylcholine, released from motor neurons, crosses the neuromuscular junction, binding sarcolemma receptors to trigger an action potential, initiating calcium release. It doesn't bind troponin (calcium does), supply energy (ATP does), or engage actin. As a neurotransmitter, it bridges nerve and muscle, distinguishing it from contraction's biochemical steps, essential for voluntary movement.

Correct Answer: A

Rationale: Intercostal muscles external, internal, innermost fill spaces between ribs, aiding breathing by elevating or depressing the ribcage. Costal cartilage connects ribs to the sternum, 'intercostal space' is the gap they occupy, and pleura lines the chest cavity. These striated, voluntary muscles distinguish the intercostal region's role in respiration, key to thoracic mechanics, contrasting with cartilage or membrane functions.

Correct Answer: D

Rationale: ATP fuels contraction via anaerobic glycolysis (glucose to pyruvate), aerobic respiration (mitochondrial oxidation), and creatine phosphate phosphorylating ADP. 'Anaerobic digestion of lactic acid' isn't a process lactic acid forms from pyruvate, not ATP. This distinguishes valid energy pathways, key to muscle metabolism understanding.

Correct Answer: D

Rationale: Contraction shortens sarcomeres myofibril segments via actin-myosin sliding, reducing Z-line distance. Fascicles (fibre bundles) shorten collectively, but actin and myosin molecules don't; filaments overlap. Sarcomeres' shortening distinguishes them as the contractile unit, summing to muscle movement.