Aggregations or bundles of myelinated and unmyelinated axons located in the brain and spinal cord.

Correct Answer: D

Rationale: White matter is bundles of myelinated and unmyelinated axons in the CNS. Wallerian degeneration is axon breakdown, SNS is a system, and spinal nerves are PNS.

Correct Answer: B

Rationale: In the PNS, Schwann cells form a regeneration tube to guide axon regrowth after injury. Dendrites receive signals, soma is the cell body, and myelin sheath insulates.

Correct Answer: D

Rationale: Neurons consist of dendrites (receive signals), cell body (soma, with nucleus), and axon (transmits signals). Nucleus is part of the cell body, neurofibrils are internal, and nodes are axon features.

Correct Answer: C

Rationale: Photoreceptors (rods, cones) in the eye are sensory receptors, detecting light. Smooth muscle is an effector, somatic nerve controls voluntary muscles, and ganglion is a cell body cluster.

Correct Answer: D

Rationale: The autonomic nervous system (specifically sympathetic division) controls the fight-or-flight response, increasing heart rate and alertness. CNS integrates, spinal cord relays, and cranial nerves serve head.