The function of irregular bones, such as the vertebrae, is primarily related to:

Correct Answer: C

Rationale: Irregular bones, like the vertebrae, have complex shapes and primarily function to protect internal organs. The vertebrae form the vertebral column, which encases and protects the spinal cord, a critical nervous system component. While some bones produce blood cells, protection is the primary role here, making 'Protecting internal organs' the correct answer.

Correct Answer: D

Rationale: The neuromuscular junction is the synapse where a motor neuron contacts a muscle fiber, releasing acetylcholine to initiate contraction. Sarcoplasm is muscle cytoplasm, myofibrils are contractile components, and sarcomeres are within myofibrils, making 'Neuromuscular junction' the correct answer.

Correct Answer: C

Rationale: The sarcoplasmic reticulum, a specialized endoplasmic reticulum in muscle cells, stores calcium ions and releases them into the cytoplasm upon a nerve signal. This calcium triggers contraction by binding to troponin. It doesn’t produce ATP or contract myofibrils, making 'Storing and releasing calcium ions' correct.

Correct Answer: B

Rationale: The radius and ulna, forearm bones, are part of the appendicular skeleton, which includes the limbs and girdles. They are long bones but belong to the appendicular division, not axial. 'Long bones' is a type, not a division, making 'Appendicular skeleton' correct.

Correct Answer: A

Rationale: Bones are living tissues, not inert structures. They contain cells like osteocytes, osteoblasts, and osteoclasts that maintain bone structure, repair damage, and regulate mineral content. Blood vessels and nerves within bones also support their living functions, such as growth and remodeling, making 'True' the correct answer.