Degeneration and Regeneration of Nerves Flashcards

1
Q

What is neuropraxis?

A

Temporary loss of neuronal function.

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2
Q

What is axonotmesis?

A

When an axon is damaged but the myelin is preserved.

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3
Q

What is neurotmesis?

A

Complete severance of a nerve fibre.

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4
Q

Why can neurones not be replaced?

A

They are post-mitotic.

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5
Q

How do pre-synaptic neurones respond to the death of their post-synaptic counterparts? What about neurones that follow this post-synaptic neurone?

A

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.

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6
Q

Where in the nervous system are the chances of nerve regeneration the highest?

A

In the PNS.

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7
Q

How does a neurone respond to neurotmesis minutes after an injury?

A

•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.

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8
Q

How does a neurone respond to neurotmesis hours after an injury?

A

•Post-synaptic terminal degenerates:

  • Neurofilaments accumulate
  • Vesicles enclose the axon.
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9
Q

How does a neurone respond to respond to neurotmesis days after an injury?

A
  • The cell body undergoes chromatolysis.

- The distal stump of the axon undergoes Wallerian Degeneration.

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10
Q

What happens during chromatolysis?

A
  • 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.
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11
Q

Why does the section of the axon that is distal to the lesion soon die after injury?

A
  • Loss of nutritional support from the soma.

- Therefore undergoes Wallerian degeneration.

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12
Q

What happens during Wallerian degeneration?

A
  • 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.
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13
Q

How do type S and type FF motor units differ?

A
  • 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.

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14
Q

What happens to the fibre type of a muscle when the characteristics of the motoneurone that innervates it changes?

A

The fibre type of the muscle switches to match the characteristics of the motor nerve that innervates it.

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15
Q

Define denervation.

A

Depriving an effector organ of its nerve supply.

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16
Q

How will a muscle react in the acute phase to denervation if it is not re-innervated?

A
  • Paralysis
  • Areflexia
  • The muscle will fasciculate
17
Q

How will a muscle react in the chronic phase to denervation if it is not re-innervated?

A
  • 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.
18
Q

Summarise how neurones in the PNS regenerate their axons in 5 steps.

A

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.

19
Q

What is rhabdomyolysis?

A
  • 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.
20
Q

What is crush syndrome / compression syndrome / Bywaters syndrome?

A

Traumatic rhabdomyolysis due to crushing.