Injury And Resprouting Flashcards
Antergrograde transport
Movement of materials away from the cell body
Retrograde transport
Movement of material toward the cell body
Background on axon
No protein synthesis takes place in the axon because only the cell body and the proximal dendrites contain ribosomes
If the axon is deprived of proteins it is because it is severed or crushed, the segment that is distal to the injury cannot support itself and will regenerate
Factors influencing transport
Neurotubules, or microtubules and energy
Describe anterograde transport
Rapid transport carries mainly membrane-bound materials such as plasma membrane proteins and synaptic vesicles
Slow transport carries soluble enzymes and structure proteins (tubulin )
The slow transport rate determines the rate of recovery following injury to the peripheral nerve
Describe retrograde transport
Rate is slow.. about half of anterograde
Important in regulating metabolism of the cell
When axon is cut ,signal which induces cell body to undergo chromatolysis is carried by retrograde
Some neurotrophic viruses such as poliomyelitis herpes and rabies and neurotoxins enter the peripheral nerve endings and ascend to infect the cell body via retrograde transport
Classifications of nerve injuries
Conduction block (focal demyelination) only, no axonal degeneration caused by mild compression or traction of the nerve (example: carpal tunnel syndrome) (neurapraxia) - 1st degree -
Damage to axon only (axonotmesis) - 2nd degree -
Damage to axons and endoneurium (axonotmesis) - 3rd degree -
Damage to axon, endoneurium and perineurium (axonotmesis) - 4th degree -
Complete nerve transection (+ epineurium). (example: laceration from knife, gunshot) (Neurotmesis) - 5th degree
Injury close to neuronal cell bodes
Poor prognosis for regeneration, reinnervation, fucntional recovery and neuronal survival
Injury close to target site of nerve fibres
Good prognosis for regeneration, reinnervation, fucntional recovery and neuronal survival
Describe first neural response to axotomy
Chromatolysis
Retrograde reaction: chromatolysis
- Cell body swells and become distended
- Nucleus is displaced to periphery
- Nissl bodies become dispersed into smaller ribosomal groupings
- Epigenetic changes to switch to a regeneration phenotype
Chromatolysis maximum at 12-24 hours after injury.
Chromatolysis more prolonged the closer the injury is to the cell body.
Not all neurons exhibit chromatolysis.
Describe Wallerian degeneration
When an axon is cut or crushed, the axon segments distal to the lesion begin to degenerate within 1 day.
The myelin surrounding the distal parts of the axons also begin to break down and become detached from the oligodendrocyte (CNS) or the Schwann cells (PNS).
This process of degeneration occurs in the direction of nerve impulse conduction (anterograde).
In the PNS, Schwann cells and macrophages remove the degenerating debris by phagocytosis over a period of 1-2 months.
Schwann cells proliferate to form new folds and successive layers of basement membrane, one inside the other. Forming empty endoneural tubes within Schwann cells and endoneurium known as the bands of Bungner.
The process of Wallerian degeneration is similar in the CNS except that glial cells phagocytose the degenerating debris and form glial scar tissue rather than the tubes formed in the PNS.
Describe growth cone formation
Several days after the injury to a peripheral nerve, the proximal axon stump begins to send out very thin axonal sprouts.
The tips of these axons are a specialised, amoeba-like region filled with microtubules called a growth cone.
Growth cones act as “feelers” for the intact endoneurial tubes.
A growth cone will advance down the empty tube at a rate of 1-4mm/day, about the same rate as slow axonal transport.
What is the end-organ response to enervation
Muscle is paralysed and reflexes are lost.
Hypotonia (little or no resistance to passive movement).
Atrophy of the muscle and spontaneous contraction set in fasciculations
What is synaptic stripping
Synaptic terminal withdraws from the neuronal cell bodies or dendrites of the chromatolytic neurons and are replaced by the processes of glial cells (Schwann cells in PNS and microglia or astrocytes in the CNS).
This process is called synaptic stripping. It depresses synaptic function and can impair recovery of function.
Transneuronal or transynaptic degeneration.
Describe injury to visual pathway
Damage of the retinal ganglion can lead to degeneration of the lateral geniculate nucleus and even to neurons in the visual cortex (Anterograde degeneration).
Damage to the visual cortex neurons can lead to degeneration of the lateral geniculate and then retinal neurons (Retrograde degeneration).
