a) Mechanism of regeneration
b) Why it doesn't always work
c) How long does it take?
a) Mechanism of regeneration
Wallerian Degeneration/ Regeneration
•Peripheral nerve injuries almost always involve axons only, not the cell bodies. This is what sets spinal cord injuries apart from them. [Module 2(b)]
•After the injury, the part of axon severed from the main cell body starts to disintegrate. This is known as Wallerian degeneration.
•A scientist named Waller discovered this process in 1862.
•Wallerian degeneration does not affect the Schwann cells (the cells providing the myelin sheath) if they are not destroyed by the injury.
•They, along with macrophage cells, clear the debris of degenerating axon
•This leaves an empty tunnel of the Schwann cells as the axon is degenerating.
•Once the damaged axon segment is removed completely, the cell body starts to grow a new axon in the empty tunnel left by Wallerian degeneration.
•This process is known as Wallerian regeneration. The success depends on the remaining Schwann cells, as they help guide the re-growing axon to its correct destination.
•Once the new axon reach the injury site, it tries to reconnect with the axon segment across the injury site. The success depends on the injury.
Nerve degeneration/regeneration
b) Why doesn't it always work
•The Wallerian regeneration can only bring a newly formed axon to the site of injury.
•The axon needs an intact myelin sheath to grow in a correct direction.
•Without it, the axon ‘gets lost’ and it wonders about. This gives rise to a messy structure made of axons looking like a spaghetti ball.
•If the damaged nerve has a large gap between the cut ends, or if the myelin sheath is lost, the regeneration mat not end in functional recovery.
•Also, if the neuron body is damaged for some reason, the regeneration may fail altogether.
•As mentioned earlier, the success of the repair depends on the extent of injury and size of the damaged area.
•There are 5 types of injuries according to Sunderland’s classification.
•First 2 types of injuries are easily healed on their own
•The third type of injury may require surgical treatment.
•The last 2 types may be difficult to heal even with a surgery
c) How Long Does It Take?
•The growth rate of a new axon is considered to be approximately 1 millimetre per day.
•In practicality, this may range from 0.5 – 1 mm/day.
•Hence, the total time needed for recovery depends upon the distance between the neuron’s cell body and the injury site.
•The factors discussed in module 9(b) may cause further delay.
•In case of severe nerve injuries, rehabilitation in form of physiotherapy may be required for functional recovery, as just the physical reconnection of the nerve fibres may not be sufficient sometimes.