During muscle cell contraction, what happens because of $\mathrm{Ca}^{++}$binding to troponin?

Correct Answer: A

Rationale: Calcium drives contraction by binding troponin, a thin filament protein. This binding alters troponin's shape, shifting tropomyosin away from actin's myosin-binding sites, exposing them for cross-bridge formation. Acetylcholine triggers the initial nerve signal, not calcium's role. Cross-bridge disengagement follows ATP binding, not calcium's action, and ATP hydrolysis energizes myosin beforehand. Calcium's specific effect on troponin unveils actin sites, enabling the sliding filament mechanism, a pivotal step distinguishing it from nerve initiation or energy cycles in muscle physiology.

Correct Answer: A

Rationale: Deltoid reflects shape triangular, like the Greek delta not location, unlike extensor carpi ulnaris (wrist), rectus abdominus (abdomen), and biceps femoris (thigh's femur). Located on the shoulder, deltoid's name emphasizes form over position, distinguishing it from location-specific terms, useful for anatomical recognition beyond regional cues.

Correct Answer: A

Rationale: The leg (knee-to-ankle) extends straightens via the quadriceps, on the thigh's front, contracting to pull the tibia. Hamstrings flex the knee, gluteals extend the hip, and calf muscles (soleus, gastrocnemius) plantarflex the foot, with tibialis anterior dorsiflexing. Quadriceps' specific leg-extension role distinguishes it, critical for standing or kicking motions.

Correct Answer: C

Rationale: In skeletal muscle, cytoplasm is termed sarcoplasm, housing myofibrils and organelles, distinct from sarcolemma (membrane), sarcomere (contractile unit), and fasciculus (fibre bundle). The sarco prefix denotes muscle specificity, and sarcoplasm's role in supporting contraction distinguishes it, key to cellular anatomy and function.

Correct Answer: A

Rationale: Calcium ions bind troponin, altering its shape to displace tropomyosin, exposing actin's binding sites for myosin. They don't detach cross-bridges (ATP does), propagate action potentials (acetylcholine initiates), or join ADP in respiration. This regulatory role distinguishes calcium as contraction's trigger, essential for sarcomere shortening, contrasting with energy or nerve functions.