In the sliding filament theory of muscle contraction, what causes the myosin heads to bind to actin filaments?

Correct Answer: A

Rationale: In the sliding filament theory, calcium ions trigger myosin binding to actin. Released from the sarcoplasmic reticulum, calcium binds to troponin, shifting tropomyosin to expose actin’s binding sites, allowing myosin heads to attach. Sodium and potassium regulate nerve signals, not this process, making 'Calcium ions' the correct answer.

Correct Answer: C

Rationale: The skull, part of the axial skeleton, forms a bony encasement around the brain, protecting it from injury. The vertebral column protects the spinal cord, the rib cage shields thoracic organs, and the pelvic girdle is appendicular, making 'Skull' the correct answer.

Correct Answer: B

Rationale: When muscle fibers contract, sarcomeres (the units between Z-lines) shorten as actin and myosin filaments slide past each other, per the sliding filament theory. This reduces sarcomere length, contracting the muscle, making 'Sarcomeres shorten' the correct answer.

Correct Answer: A

Rationale: Bones require protection and nutrition to stay healthy. Wearing a helmet (Choice A) prevents skull fractures during activities like biking. Other correct options from the document include wearing pads, using sports equipment, avoiding trampolines, eating calcium-rich foods, and being active. Sitting all day, eating candy, and skipping exercise weaken bones, making 'wear a helmet' a representative correct choice.

Correct Answer: D

Rationale: Bones are classified by shape: long (e.g., femur), short (e.g., carpals), flat (e.g., scapula), irregular (e.g., vertebrae), and sometimes sesamoid (round, e.g., patella). 'Square,' 'cubed,' and 'triangular' are not standard categories; 'round' aligns with sesamoid bones, making 'long, short, flat, irregular and round' the most accurate.