Wallerian Degeneration

Pathophysiology

Distal segment changes

1. Earliest event: disruption of anterograde and retrograde axonal transport immediately after injury.
2. Ion influx: rapid inflow of Ca²⁺ and Na⁺ through the disrupted axonal membrane activates proteases and a cell-death-like cascade.
3. Cytokine cascade: leukocyte recruitment and cytokine-mediated signaling trigger synthesis of neurotrophins, chemokines, extracellular matrix molecules, proteolytic enzymes, and interleukins.
4. Day 3: Schwann cells retract from the node of Ranvier; activated Schwann cells and macrophages begin to digest myelin.
5. ~1 week: the entire distal axonal process of Wallerian degeneration is complete.
6. Myelin breakdown: myelin fragments into blocks/ovoids ("digestion chambers of Cajal") containing axon remnants; macrophages convert these into neutral fats and cholesterol esters and transport them to the bloodstream.

Proximal segment changes

• A limited degree of axon breakdown extends proximal to the injury, up to the first node of Ranvier.
• The cell body undergoes chromatolysis: breakup/dispersion of rough ER, eccentric displacement of the nucleus, and upregulation of transcription factors that switch gene expression from axon maintenance → protein synthesis.
• Very proximal injuries (e.g., proximal arm amputation) may cause apoptosis of the cell body itself.

Key Cellular Players

Comparison with Other Nerve Injury Responses

Nerve Regeneration After Wallerian Degeneration

• Regeneration from the proximal stump begins only after Wallerian degeneration is complete.
• Schwann cells form bands of Büngner — longitudinal cords that guide the regenerating axon tip distally.
• Axon sprouts arise from nodes of Ranvier (nodal sprouts) or nerve terminals (terminal sprouts) within ~4 days.
• Rate of regeneration: approximately 1–3 mm/day.
• Collateral sprouting (from surviving axons) provides earlier functional recovery in partial injuries (3–6 months).
• New myelin is thinner, with more internodes per original internode.
• If regeneration fails, permanent denervation and target organ atrophy result.

Clinical Relevance

• Electrodiagnostics: after Wallerian degeneration, distal nerve stimulation fails to produce a CMAP/SNAP (both amplitudes fall), distinguishing it from conduction block (where distal stimulation still works).
• Sensory nerve special case: destruction of the dorsal root proximal to the dorsal root ganglion causes Wallerian degeneration of the posterior columns (central), but NOT of the peripheral sensory nerve — because the ganglion cell body maintains the distal axon's integrity.
• CNS Wallerian degeneration: also occurs in the CNS (e.g., descending corticospinal tract degeneration after stroke or motor neuron injury), but CNS regeneration is far more limited due to inhibitory myelin-associated proteins and absence of Schwann cell-mediated support.