Smooth muscle is responsible for

Correct Answer: B

Rationale: Smooth muscle operates without conscious control, managing essential involuntary functions. It lines the digestive tract, facilitating peristalsis to move food, and surrounds blood vessels, aiding in blood flow regulation not pumping, which is the heart's role, but still vital for circulation. Voluntary movements, like running, rely on skeletal muscle, not smooth muscle, which lacks the striations for such tasks. Claiming it controls all involuntary movements overstates its scope, as cardiac muscle handles heartbeats independently. 'None of the above' dismisses its clear role. Smooth muscle's involuntary nature and presence in visceral organs and vascular walls make it key for digestion and circulation support, distinguishing it from skeletal muscle's voluntary domain and cardiac muscle's specialized pumping, aligning with its physiological purpose across multiple systems.

Correct Answer: D

Rationale: Joints that are fibrous and immovable are classified as synarthroses, such as skull sutures, where dense connective tissue binds bones tightly, ensuring stability with no motion. Tendons connect muscles to bones, and tibia is a bone neither are joint types. Ligaments link bones, and femur is a bone, but this pairing doesn't define a joint category. Diarthrosis refers to freely movable synovial joints, like the knee, opposite to the question's intent. Synarthroses accurately describe fibrous, fixed joints, critical for structures requiring rigidity, like the cranium, distinguishing them from movable or cartilaginous joints in anatomical classification.

Correct Answer: C

Rationale: Myosin, the thick filament in sarcomeres, features cross-bridges protruding heads that bind actin during contraction. These bridges, powered by ATP, pull actin inward, driving the power stroke. Troponin, on thin filaments, binds calcium to regulate contraction, lacking bridges. Actin forms thin filaments, receiving cross-bridges, not bearing them. Tropomyosin shields actin's sites, also without bridges. Myosin's cross-bridges are unique, enabling force generation, distinguishing it from actin's structural role or troponin and tropomyosin's regulatory functions, essential for the sliding filament mechanism and muscle movement.

Correct Answer: C

Rationale: Hypertrophy is muscle size increase from training, as resistance stress thickens fibers via protein synthesis, enhancing strength and mass, like in weightlifting. Atrophy is size loss from inactivity, opposite to training's goal. Fatigue is temporary exhaustion, not size change. Hyperplasia, fiber number increase, is rare in humans, unlike hypertrophy's fiber growth. This adaptation reflects muscle's response to mechanical overload, distinct from shrinkage, energy depletion, or theoretical cell addition, central to exercise-induced development.

Correct Answer: D

Rationale: The sternocleidomastoid, from sternum and clavicle to mastoid, rotates and tilts the head side to side, as in looking over the shoulder. Arm abduction is deltoid's role. Breathing involves diaphragm and intercostals, not this neck muscle. Shoulder shrugging is trapezius. Its head-turning action distinguishes it, vital for neck mobility, unlike arm, respiratory, or shoulder functions.