Wallerian degeneration of axons flowchart

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Wallerian Degeneration of Axons - Flowchart

Wallerian Degeneration diagram showing transection, chromatolysis, myelin debris, macrophage infiltration, and muscle degeneration

Fig. 64.4 - Bradley and Daroff's Neurology in Clinical Practice

Step-by-Step Flowchart

AXON INJURY (Grade II - V nerve lesion)
         |
         v
AXONAL TRANSECTION / DISRUPTION
  - Interruption of axoplasmic transport
  - Retrograde + anterograde flow blocked
         |
         |————————————————————————————————|
         v                                v
  DISTAL SEGMENT CHANGES          PROXIMAL SEGMENT CHANGES
  ("Dying forward")                (Retrograde changes)
         |                                |
         v                                v
Rapid Ca²⁺ & Na⁺ influx          Axon breaks down back
through disrupted plasma          to 1st node of Ranvier
membrane                                  |
         |                                v
         v                        CHROMATOLYSIS of cell body:
Activation of apoptosis-like       - Breakup & dispersion of
cascade (calpains, proteases)        rough ER (Nissl substance)
         |                           - Eccentric nucleus
         v                           - Switch: axon maintenance
Recruitment of LEUKOCYTES             → protein synthesis
+ Cytokine signaling cascade           for regeneration
         |
         v
 By Day 3:
Schwann cells retract from
nodes of Ranvier
         |
         v
Schwann cells & macrophages
activated → begin digesting MYELIN
         |
         v
Myelin breaks down into blocks/ovoids
= "Digestion chambers of Cajal"
         |
         v
Macrophages convert myelin fragments
→ neutral fats + cholesterol esters
→ carried to bloodstream
         |
         v
Entire distal axon degenerates
≈ 1 week
         |
         v
TARGET ORGAN (muscle) denervated
→ muscle degeneration / atrophy

After Degeneration: Regeneration Phase

WALLERIAN DEGENERATION COMPLETE
         |
         v
Schwann cells dedifferentiate
+ upregulate adhesion molecules
+ neurotrophins (laminin, cadherins, NCAM)
         |
         v
Bands of Büngner form
(Schwann cell cords aligned along
original basal lamina tubes)
         |
         v
AXON SPROUT grows from proximal stump
(Growth cone = filopodia + lamellipodia)
         |
         v
Growth cone guided distally
by semaphorins, ephrins, netrins
through Bands of Büngner
         |
         v
Axon reaches target muscle
→ Remyelination
→ Reinnervation
(Rate: ~1 mm/day)

Timeline Summary

Time	Event
Immediately	Ca²⁺/Na⁺ influx; anterograde/retrograde transport fails
Hours	Leukocyte recruitment; cytokine signaling
Day 3	Schwann cells retract from nodes of Ranvier
Days 3-7	Myelin digestion by Schwann cells + macrophages
~1 week	Full distal axon degeneration complete
Days 4+	Collateral sprouting from surviving axons begins
3-6 months	Clinical recovery via collateral sprouting
6-24 months	Main recovery via proximal-to-distal axon regeneration

Key Cellular Players

Cell	Role
Schwann cells	Retract from nodes; digest myelin; dedifferentiate; form Bands of Büngner; remyelinate regenerating axon
Macrophages	Infiltrate injury site; phagocytose myelin ovoids + axonal debris; clear path for regrowth
Neuronal cell body	Undergoes chromatolysis; shifts gene expression toward regeneration
Growth cone	Tip of regenerating axon; navigates via chemotrophic signals

Key distinction to remember: Wallerian degeneration is "dying forward" (distal to injury, outward) - contrasted with "dying-back" axonal degeneration in metabolic/toxic neuropathies where degeneration proceeds from distal to proximal. - Adams and Victor's Principles of Neurology, 12th Ed.

Sources: Bradley and Daroff's Neurology in Clinical Practice, Adams and Victor's Principles of Neurology 12th Ed., Neuroscience: Exploring the Brain 5th Ed.

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