Nerve injury and regeneration (Wallerian degeneration) !

Process

When a nerve is cut or crushed, the part beyond the injury breaks down and is cleared; peripheral axons can then slowly regrow, central ones mostly cannot.

Updated 2026-11-01

See it in 3D: Median nerve · Ulnar nerve · Radial nerve · Sciatic nerve

What it is

Wallerian degeneration is the orderly breakdown of an axon and its myelin distal to an injury, followed by clearing of the debris. In peripheral nerves Schwann cells and blood-borne macrophages clear the way and Schwann cells line up into guiding tubes, so the proximal stump can sprout and regrow. In the brain and spinal cord debris is cleared slowly and myelin inhibitors and the glial scar block regrowth, so recovery is poor.

Step by step

The axon is cut or crushed; the distal segment, cut off from its cell body, swells and fragments within days, and its myelin breaks into ovoids.
Schwann cells drop their myelin, divide and recruit blood-derived macrophages, which clear axon and myelin debris over the following weeks.
The cell body reacts (chromatolysis): it swells, Nissl substance disperses and the nucleus moves to the edge as the cell switches to making repair proteins.
Schwann cells line up inside the preserved basal lamina tubes, forming bands of Büngner.
Growth cones sprout from the proximal stump and follow the bands toward the target.
Regrowth is slow, roughly a millimetre a day, so distant targets wait months; the muscle may atrophy before the axon arrives.
Once reconnected, the new axon is remyelinated with shorter internodes and somewhat slower conduction.

Seddon classification of nerve injury

Grade	What is damaged	Outlook
Neurapraxia	Myelin only (focal block); axon intact; no Wallerian degeneration	Full recovery in days to weeks
Axonotmesis	Axon cut, connective sheaths intact; Wallerian degeneration	Regrowth along intact tubes; good but slow recovery
Neurotmesis	Whole nerve divided, including sheaths	No useful recovery without surgical repair; neuroma may form

Clinical notes

Mid-shaft humerus fractures can injure the radial nerve, giving wrist drop; most are neurapraxia or axonotmesis and recover.
An advancing Tinel sign (tingling when tapping along the nerve) tracks the regenerating axon front.
Spinal cord injury: CNS axons do not regenerate well because of oligodendrocyte myelin inhibitors, the astrocytic glial scar and slow debris clearance.
Chronic carpal tunnel compression moves from demyelination (reversible) to axonal loss with Wallerian degeneration (slower, incomplete recovery).

Test yourself

Think of the answer, then tap the question to see it.

What is Wallerian degeneration?

The breakdown and clearing of the axon and its myelin distal to the site where the axon was cut or crushed.

What are bands of Büngner?

Columns of proliferated Schwann cells lined up inside the old basal lamina tubes, which guide regrowing axons to their target.

What changes does the neuron cell body show after its axon is cut?

Chromatolysis: the cell body swells, the Nissl substance disperses and the nucleus moves to the periphery.

Why do central axons regenerate poorly compared with peripheral ones?

In the CNS, oligodendrocyte myelin proteins inhibit growth, reactive astrocytes form a glial scar and debris is cleared slowly; there are no Schwann-cell guiding bands.

Which Seddon grade leaves the axon intact?

Neurapraxia: only myelin is injured, conduction is blocked locally and recovery is complete without Wallerian degeneration.

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See it in 3D

The structures of Durgical's 3D anatomy where it is found.

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Educational content on normal microscopic structure and its clinical relevance; no substitute for studying slides or for a health professional's advice.

Sources: The Role of Macrophages in Wallerian Degeneration (Brain Pathology (PubMed Central), 1997); 12.2 Nervous Tissue (OpenStax, 2022); Peripheral Nerve Disorders (MedlinePlus, U.S. National Library of Medicine, 2024); Carpal Tunnel Syndrome (MedlinePlus, U.S. National Library of Medicine, 2024).

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