Which protein covers the binding sites on actin filaments and prevents myosin from attaching to them in a relaxed muscle?

Correct Answer: B

Rationale: In a relaxed muscle, tropomyosin covers the myosin-binding sites on actin filaments, preventing contraction. When calcium binds to troponin, it moves tropomyosin, exposing the sites for myosin attachment. Troponin regulates, myosin contracts, and collagen is structural, making 'Tropomyosin' the correct answer.

Correct Answer: C

Rationale: The humerus, an upper arm bone in the appendicular skeleton, facilitates arm movement by articulating with the scapula and forearm bones (radius, ulna). It doesn’t protect the heart (rib cage’s role) or support lower limbs, making 'Facilitating arm movement' the correct answer.

Correct Answer: B

Rationale: At the neuromuscular junction, the motor neuron releases acetylcholine into the synaptic cleft, stimulating the muscle fiber to contract. Calcium is stored in the sarcoplasmic reticulum, ATP is produced in mitochondria, and muscle fibers contract, not neurons, making 'Release acetylcholine' correct.

Correct Answer: A

Rationale: A joint, or articulation, is the anatomical location where two bones connect, allowing for stability or movement (e.g., knee, shoulder). Tendons connect muscles to bones, ligaments connect bones to other bones, and muscles move bones but aren’t the meeting point, making 'joint' the correct answer.

Correct Answer: D

Rationale: Short bones, like carpals and tarsals, have roughly equal length and width, providing stability and support. Flat bones are thin and broad, irregular bones have complex shapes, and long bones are longer than wide, making 'short' the correct classification.