Degeneration and Regeneration of Nerves

Question 1

What does a motor unit consist of?

Answer 1

• an α motorneuron

- all extrafusal skeletal muscles it supplies

Question 2

What is neuropraxis?

Answer 2

when a neurone loses its ability to function normally (loss of function only)

Question 3

What is axonomesis?

Answer 3

when an axon of a neurone is damaged, with the myelin preserved

Question 4

What is neurotmesis?

Answer 4

when the entire nerve fibre is completely severed

Question 5

What happens when there is cell body damage to the middle neurone (in a chain of three neurones)?

Answer 5

• the whole cell will die and not be replaced (neurones are post-mitotic)
• there is no neuronal cell division in the brain or spinal cord to replace the dead neurone

Question 6

What happens if the initial segment of the middle neurone is damaged?

Answer 6

• likely to lead to death of cell body (due to close proximity) and so the cell will due
• neurone therefore not replaced

Question 7

What happens if the axon of the middle neurone is severed at a point distal from the cell body?

Answer 7

• this will not likely lead to the death of the cell body so the cell will survive the injury
• the neurone could regrow a new axon to re-innervate its original targets

Question 8

What happens to the neuronal chain if the middle neurone dies?

Answer 8

• break up of neuronal chain
• pre-synpatoc neurone will withdraw its processes (and could die)
• the post-synaptic neurone will most likely die

Question 9

What is denervation?

Answer 9

depriving the muscle of its supply

Question 10

What occurs minutes after a neurone is damaged?

Answer 10

• the neurone will immediately stop conducting action potentials beyond the site of injury
• the two ends of the cut axon will be exposed and will start to leak intracellular fluid (axonal transport occurs in both directions)
• the cut ends will soon pull apart sealing themselves and swelling at the same time

Question 11

What happens an hour or so after a neurone is damaged?

Answer 11

• synaptic terminal degenerates causing the accumulation of neurofilaments and vesicles
• astroglia normally surround the terminal and react by causing the terminal to be pulled away from the postsynaptic cell

Question 12

What happens days to weeks after a neurone is damaged?

Answer 12

• the cells body of the proximal segment will undergo chromatolysis
• the distal stump of the axon at the distal segment will undergo Wallerian Degeneration

Question 13

What is the fate of the proximal segment of a severed nerve?

Answer 13

• days after the injury it begins undergoing chromatolysis
• the cell body becomes very active producing its of proteins for repairing the cell
• the volume of the cell body increases as it bloats with newly synthesised products
• the nucleus is consequently displaced from the centre to the periphery of the cell body
• the injured never soon seals the wounded stump to form a neuroma
• the segment of nerve does not die
• in some cases the nerve stump soon regenerates to innervate peripheral structures

Question 14

What is the fate of the distal end of a severed nerve?

Answer 14

• the segment of the axon distal to the site of lesion is never viable
• it soon dies as a result of loss of nutritional support from the cell body
• thus the axonal segment undergoes Wallerian degeneration
• the axon is digested by phagocytes
• tissues that may be preserved are: myelin sheaths, epineurium, perineurium, endoneurium
• these from hollow tubes to guide any new regrowth of the end of the proximal end

Question 15

What can happen to a denervated muscle of a motor unit?

Answer 15

-It might be reinnervated by a motor neurone from a different motor type

Question 16

What happens to a denervated muscle that is not reinnervated?

Answer 16

in the acute stage:

• the muscle is immediately paralysed
• the muscle will become areflexic (absent of reflexes)
• the muscle will start to fasciculate (small involuntary contractions)

in the chronic phase:
as the fasciculations subside, the muscle will:
-lose bulk due to denervation
-lose bulk due to lack of use
-the muscle will die
-muscle tissue will be replaced by connective tissue
-this is a state of fibrosis

Question 17

How do neurones in the PNS regenerate their axons?

Answer 17

1) after degeneration of distal axon and myelin, macrophages clean up debris
2) macrophages release mitogens that induce Schwann cells to divide
3) the myelin-forming Schwann cells repopulate the nerve sheaths;
4. Schwann cells make laminin
5. macrophages make interleukin which induces Schwann cells to make nerve growth factor
6. axons sprout and some sprouts enter new Scwann cell tubes
7. axonal growth cones successfully grow