Involuntary muscular contractions are

Correct Answer: B

Rationale: Spasms are involuntary, sudden muscle contractions, often brief and uncontrolled, like twitches. Cramps are painful, prolonged involuntary contractions, a subset but not the broad term. Strains are muscle overstretching injuries, not contractions. Sprains affect ligaments, not muscles. Spasms' involuntary nature encompasses various unintended movements, distinguishing them from specific pain, injury, or ligament issues, common in muscle dysfunction.

Correct Answer: B

Rationale: The clavicle is a slender, S-shaped bone extending horizontally from the sternum to the scapula, forming part of the shoulder girdle. Known as the collar bone, it links the arm to the axial skeleton, providing support and mobility while protecting underlying vessels and nerves. Its superficial position above the chest makes it palpable and prone to fracture. The shoulder blade is the scapula, a flat, triangular bone posteriorly. The jaw bone is the mandible, unrelated to the shoulder. The breast bone, or sternum, is the central chest bone connecting ribs. The clavicle's distinct role in shoulder articulation and its colloquial 'collar bone' name reflecting its necklace-like placement clearly identify it, distinguishing it from other skeletal structures in anatomical context.

Correct Answer: B

Rationale: The sarcomere is the basic functional unit of skeletal muscle tissue, defined as the segment between two Z-lines where actin and myosin filaments overlap to produce contraction via the sliding filament mechanism. This repeating unit shortens during muscle action, driving movement. Muscle fibers, single cells containing many sarcomeres, are structural units, not the smallest functional ones. Myofibrils, bundles of sarcomeres within fibers, organize contraction but aren't the base unit sarcomeres are their components. The sarcoplasmic reticulum stores calcium, enabling contraction, but doesn't contract itself. The sarcomere's role as the contractile core distinguishes it, fundamental to muscle function, unlike larger structural or supportive elements, aligning with its physiological definition.

Correct Answer: A

Rationale: A synergist assists an agonist, the prime mover, by enhancing its action or stabilizing joints, like brachioradialis aiding biceps in elbow flexion. A fixator stabilizes the origin bone, indirectly supporting but not directly helping the agonist's motion. An insertion is the muscle's moving attachment, not a helper muscle. An antagonist opposes the agonist (e.g., triceps vs. biceps), hindering, not aiding. Synergists' cooperative role boosts efficiency and precision, distinguishing them from stabilizing, structural, or opposing functions, essential for coordinated movement.

Correct Answer: C

Rationale: Sternocleidomastoid, from sternum/clavicle to mastoid, flexes the head forward when both sides contract, a prime mover in neck flexion. Occipitofrontalis raises eyebrows and wrinkles forehead, not flexing. Corrugator supercilii furrows brows, a facial expression muscle. Masseter elevates the mandible for chewing, not head flexion. Sternocleidomastoid's bilateral action drives head bending, distinguishing it from facial or jaw muscles, critical for neck movement.