The process of muscle contraction is initiated by the release of which ion from the sarcoplasmic reticulum?

Correct Answer: C

Rationale: Muscle contraction begins when calcium ions (Ca2+) are released from the sarcoplasmic reticulum into the muscle cell cytoplasm. Calcium binds to troponin, shifting tropomyosin to expose actin binding sites, allowing myosin to attach and contract the muscle. Sodium and potassium regulate nerve signals, not direct contraction, making 'Calcium (Ca2+)' the correct answer.

Correct Answer: D

Rationale: The clavicle, or collarbone, is part of the axial skeleton in some classifications due to its role in the thoracic cage, but it’s often considered part of the pectoral girdle in the appendicular skeleton. Here, it’s listed as axial, connecting the sternum to the scapula, aiding upper limb articulation. Thus, 'Clavicle' is correct.

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 'all-or-none' principle states that a muscle fiber contracts fully or not at all when stimulated above a threshold. It doesn’t contract partially; the strength varies with the number of fibers recruited, not individual fiber response, making 'The idea that muscles can either contract fully or not at all' 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.