When a muscle contracts, which protein binds to calcium ions, allowing myosin heads to attach to actin filaments?

Correct Answer: A

Rationale: In muscle contraction, calcium ions play a key role in exposing binding sites on actin filaments. Troponin, a regulatory protein on the actin filament, binds to calcium ions when they are released from the sarcoplasmic reticulum. This binding causes tropomyosin to shift, uncovering myosin-binding sites on actin, allowing myosin heads to attach and initiate contraction. Thus, 'Troponin' is the correct answer.

Correct Answer: D

Rationale: The scapula is a flat bone, characterized by its thin, broad shape. Flat bones provide protection and muscle attachment surfaces; the scapula protects the shoulder region and anchors upper limb muscles. Long bones are cylindrical, short bones are cube-like, and irregular bones have complex shapes, making 'Flat bone' 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.