What is Wallerian degeneration and when does it occur?

• A. Degeneration of the distal portion of an axon after injury; distal to the injury site; PNS may regenerate with Schwann cell guidance.
• B. Demyelination of central neurons due to autoimmune disease.
• C. Degeneration of the neuronal cell body in the CNS.
• D. Regeneration of axons after CNS injury.

Answer: (A)

Wallerian degeneration is the process that occurs after an axon is cut or severely damaged: the part of the axon that is separated from the neuron's cell body—the distal segment—degenerates. The distal axon and its myelin break down, and the debris is cleared away by macrophages and glial cells. In the peripheral nervous system, Schwann cells play a key guiding role, forming pathways (bands of Büngner) that help new growth from the proximal stump reach its targets, so some recovery is possible when the environment supports it. In the central nervous system, regeneration is much more limited due to inhibitory factors and glial scarring. So the statement that best describes Wallerian degeneration is degeneration of the distal portion of an axon after injury, distal to the injury site, with potential regeneration in the peripheral nerves guided by Schwann cells.