What triggers the release of calcium ions from the sarcoplasmic reticulum, initiating muscle contraction?

Correct Answer: A

Rationale: Electrical impulses (action potentials) from motor neurons travel to the muscle, depolarizing the sarcoplasmic reticulum via T-tubules, triggering calcium release. Acetylcholine initiates the nerve signal, but the direct trigger is the electrical impulse, making 'Electrical impulses from motor neurons' correct.

Correct Answer: B

Rationale: The human spine, or vertebral column, consists of 26 individual bones in children (33 before some fuse in adulthood), collectively called vertebrae. These include 7 cervical, 12 thoracic, 5 lumbar, 5 sacral (fused into the sacrum), and 4 coccygeal (fused into the coccyx). Ribs protect the chest, skull bones form the head, and pelvic bones support the lower body, making 'vertebrae' the correct answer.

Correct Answer: C

Rationale: At joints, cartilage—specifically articular cartilage—covers bone ends, acting as a cushion to absorb shock and reduce friction during movement (e.g., knee cartilage). Ligaments stabilize joints by connecting bones, tendons attach muscles to bones, and muscles move bones, but cartilage directly protects and cushions, making it the correct answer.

Correct Answer: B

Rationale: The diaphysis is the long, cylindrical shaft of a long bone, primarily compact bone, providing strength. The epiphysis is the end, the endosteum lines the interior, and the periosteum (misspelled here as 'periosteium') is the outer membrane, making 'diaphysis' the correct answer.

Correct Answer: C

Rationale: Osteons (or Haversian systems) are the cylindrical units of compact bone, containing lamellae, a central canal, and osteocytes. Lamellae are layers within osteons, canaliculi are channels, and trabeculae are in spongy bone, making 'osteons' the correct answer.