Which of these events is necessary for the contraction of a muscle cell?

Correct Answer: B

Rationale: ATP hydrolysis to ADP and phosphate energizes myosin cross-bridges, enabling them to pull actin, driving contraction. Myosin doesn't shorten, calcium binds troponin (not tropomyosin), and calcium returns to the sarcoplasmic reticulum post-contraction. This energy step distinguishes contraction's power source, essential for filament sliding, contrasting with regulatory or recovery phases.

Correct Answer: A

Rationale: The origin, the muscle's fixed attachment point (e.g., scapula for biceps brachii), moves the least during contraction, anchoring the muscle as it pulls the insertion (movable end, e.g., radius) toward it. The insertion shifts significantly to produce movement. Ligaments , connective tissues stabilizing joints, don't move but stretch slightly under tension. Joints pivot or flex, moving more than the origin. The origin's relative stability, often proximal and attached to larger bones, ensures effective leverage, making 'a' the correct answer in muscle dynamics.

Correct Answer: B

Rationale: The masseter, a powerful jaw muscle, elevates the mandible for chewing, originating from the zygomatic arch and inserting on the mandibular ramus, key in mastication. Swallowing involves pharyngeal muscles (e.g., constrictors), not masseter. Lip movement is orbicularis oris's role. Eye closure is orbicularis oculi's function. Masseter's role in biting and grinding, a primary masticatory action, makes 'b' the correct answer.

Correct Answer: D

Rationale: The sartorius, a long, strap-like muscle from iliac spine to tibia, flexes the hip and knee while rotating the thigh laterally, enabling leg crossing (e.g., tailor's pose). Gluteus maximus extends the hip, piriformis rotates it externally, and gracilis adducts the thigh none fully replicate crossing's combined flexion/rotation. Sartorius's unique diagonal action across hip and knee, dubbed the 'tailor's muscle,' makes 'd' the correct choice.

Correct Answer: D

Rationale: The hyoid bone, in the anterior neck, is unique as it doesn't articulate with any other bone, suspended by ligaments and muscles (e.g., suprahyoid, infrahyoid) to support the tongue and larynx. The mandible articulates with the temporal bone at the TMJ. Nasal bones articulate with each other and the frontal/maxilla. The cricoid cartilage , not a bone, connects to the thyroid cartilage. The hyoid's free-floating status, critical for swallowing and speech, makes 'd' the correct answer.