Waller's degeneration
As Wallerian degeneration or Wallerian degeneration the complex molecular events are referred to, according to a damage to a nerve in the peripheral nervous system (PNS) or fibrous webs of the central nervous system occur (CNS) and viewed from the location of the damage from the fall of the distally from Soma lying nerve portion to lead.
This degeneration of the axon with “collapse” and “disintegration” follows a similar time course in the PNS and CNS. The myelin sheath itself is broken down more slowly in the CNS, however, since there are hardly any phagocytic macrophages there.
Possible causes are traumatic interruptions in continuity such as sharp severance ( axotomy ) or contusion , ischemia , bleeding , viral inflammation, or toxic damage. If the first motor neuron is damaged, e.g. B. in the context of amyotrophic lateral sclerosis, in advanced cases this can be shown as hyperintensity in the coronal T2-weighted MRI scan.
The dissolution of the myelin sheath and the axon and phagocytosis can be demonstrated histologically . In proximally located from the site of injury and somanahen portion of the nerve cell it comes to chromatolysis .
Clinically, flaccid motor paralysis including a possible bladder and rectal disorder and / or loss of sensitivity can occur in the originally innervated areas . The evidence of failure depends on the type and number of axons involved. Damage to the first motor neuron results in spastic motor paralysis.
consequences
Such damage has different consequences in the PNS than in the CNS:
Peripheral nervous system
If the medullary sheath of the axon of a peripheral nerve is preserved, the axon stump above ( proximal ) the damage can grow into the denervated myelin sheath at a rate of 1 mm per day and reinnervate the target organ, which enables function ( regeneration ) to be restored . If the sprouting proximal axon end with its so-called growth cone does not find the myelin sheath as a guide structure, this outgrowing axon end can proliferate aimlessly and form a traumatic or amputation neuroma as a hyperregenerate of the peripheral nerve.
Central nervesystem
As early as 100 years ago, based on Ramon y Cajal's transplantation experiments, it was possible to show that, in contrast, damaged CNS fibrous tracts do not regenerate. If a piece of an optic nerve - which is assigned to the CNS as a cranial nerve and whose nerve sheath is formed by oligodendrocytes - was attached to the proximal end, the proximal end of the axon did not grow into this lead structure.
Augustus Volney Waller (1816-1870) was an English physiologist in Birmingham who first described these degenerating nerve fibers in his histological slides in 1850.