Degeneration and Regeneration of Nerves Flashcards
What is neuropraxis?
Temporary loss of neuronal function.
What is axonotmesis?
When an axon is damaged but the myelin is preserved.
What is neurotmesis?
Complete severance of a nerve fibre.
Why can neurones not be replaced?
They are post-mitotic.
How do pre-synaptic neurones respond to the death of their post-synaptic counterparts? What about neurones that follow this post-synaptic neurone?
The pre-synaptic neurones withdraw the axons that were innervating the post-synaptic neurone.
Other neurones innervated by the post-synaptic neurone will most likely die.
Where in the nervous system are the chances of nerve regeneration the highest?
In the PNS.
How does a neurone respond to neurotmesis minutes after an injury?
•Synaptic transmission stops:
- Pre-synaptic terminal retracts.
- Glial cells ensheath the pre-synaptic terminal. Usually astroglia.
•Cut ends pull apart
-The cut ends seal and swell.
How does a neurone respond to neurotmesis hours after an injury?
•Post-synaptic terminal degenerates:
- Neurofilaments accumulate
- Vesicles enclose the axon.
How does a neurone respond to respond to neurotmesis days after an injury?
- The cell body undergoes chromatolysis.
- The distal stump of the axon undergoes Wallerian Degeneration.
What happens during chromatolysis?
- The cell body becomes very active, producing lots of proteins (to meet demand for axonal regeneration).
- The volume of the cell body increases.
- The nucleus of the cell body is displaced to the periphery.
- The wounded stump is sealed to form a neuroma.
Why does the section of the axon that is distal to the lesion soon die after injury?
- Loss of nutritional support from the soma.
- Therefore undergoes Wallerian degeneration.
What happens during Wallerian degeneration?
- The axon is digested by phagocytes.
- Some tissues such as epi/peri/endoneurium may be preserved as hollow tubes.
- Preserved tissues guide new regrowth of the proximal end.
How do type S and type FF motor units differ?
- Type S motor units contract slowly whereas type FF contract quickly.
- Type S motor units exert relatively small forces whereas type FF motor units exert relatively large forces.
- Type FF muscle fibres source their energy from glycolysis only. Type S muscle fibres source their energy from oxidative means.
*Similar to Type I, IIa and IIb muscle types.
What happens to the fibre type of a muscle when the characteristics of the motoneurone that innervates it changes?
The fibre type of the muscle switches to match the characteristics of the motor nerve that innervates it.
Define denervation.
Depriving an effector organ of its nerve supply.
How will a muscle react in the acute phase to denervation if it is not re-innervated?
- Paralysis
- Areflexia
- The muscle will fasciculate
How will a muscle react in the chronic phase to denervation if it is not re-innervated?
- Fasciculations subside.
- The muscle loses bulk due to denervation (denervation atrophy).
- The muscle loses bulk due to lack of use (disuse atrophy).
- The muscle dies.
- The muscle is replaced with connective tissue. This is a state of fibrosis.
Summarise how neurones in the PNS regenerate their axons in 5 steps.
1 - After degeneration of distal axon, macrophages clean up debris.
2 - Macrophages induce Schwann cell division.
3 - The Schwann cells reform the nerve sheaths.
4 - Macrophages produce interleukin, stimulating Schwann cells to produce nerve growth factor.
5 - Axons sprout, some sprouts enter new Schwann cell tubes.
What is rhabdomyolysis?
- Crushing injury to muscle fibers leading to ischaemic damage and subsequent release of toxic metabolites into the bloodstream.
- Can lead to renal failure, haematuria, fluid and blood loss.
What is crush syndrome / compression syndrome / Bywaters syndrome?
Traumatic rhabdomyolysis due to crushing.
