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George E. Carvell, PhD, PT   University of Pittsburgh
 
PERIPHERAL NERVE DYSFUNCTION VI: WALLERIAN DEGENERATION
Ben.  fig 13-8 p 392
Peripheral nerve disorders may include one or more type of injury. Classic Wallerian degeneration occurs when an axon is cut or crushed. The axon distal to the injury site degenerates and the cell body undergoes a chromatolysis where it gears up for the repair process (axonal regeneration). Other types of neuropathic disorders have less drastic effects on the neuron. Axonal dystrophy (dying-back phenomenon) occurs when the distal axon begins to retract from the target organ (muscle or receptor). The most distal part of the axon may narrow but the cell does not undergo chromatolysis since not all axonal connections are lost. This may be due to diseases that effect the nutrition or microvasculature of the most distal axons. Conduction velocity is normal for remaining axon branches. Segmental demyelination is a process where myelin is lost at one or more internodes but the axon remains intact. Conduction will continue if sufficient nodes remain but conduction velocity will be slowed. If too many
Normal Motor Neuron
Central Chromatolysis
A = Nucleus
B = Nucleolus
C = Nissl-filled
      Cytoplasm
D = Axon Hillock
E = Dendrite
N = Eccentric
      Nucleus in
    Chromatolysis
Ben.  Fig 4-3 p 59
Ben.  Fig 4-18 p 72
Ben.  fig 4-19 p 73
Schwann cells are busy after axonal damage; they assist in clean-up, form Bands of Bunger for regenerating axons and remyelinate regenerated axons.
Chromatolysis: cell swells, Nissl disperses & Nucleus is eccentric
Note: not all axon terminal connections are lost
internodes are lost there will be a conduction block.