Nerve Regeneration Question

[booshwa]

So we've been studying anastomosis of CN XII to CN VII following a schwanomma of the internal acousitc meatus.....my question is how are the portions of the neurons below this tumor able to survive yet not be viable? My understanding was that the compression is analogous to the severing of the nerve in terms of the effects on the distal portion of the nerve.

From what I've read on the anastomosis, it may be done over 1 month post nerve severing (although obviously the sooner the better). Doesn't this seem like a long time for a cell to be viable if it is removed from is main source of nutrients?

Any input on the matter would be greatly helpful.... thanks

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[tofurious]

A severed nerve has some regenerating potential. The more common form of re-innervation involves sprouting of one intact nerve to cover the area previously innervated by another nerve (rather than the connection of a functional axon to an isolated degenerating segment). However, it has been shown that the proximal end of a severed axon can regenerate to reconnect with the distal segment. I don't know if the physiology of the actual reconnection has been worked out yet, but I know that glial cells play an essential role in the process. For one thing, they provide the essential nutrients for survival of the distal segment (there is also a role for target derived growth factor - whatever the end organ the nerve is innervating such as muscle releases a number of growth factors to promote the survival of the nerve that innervates it). However, more importantly (especially in myelinated axons), they release chemical signals to either promote or inhibit the extension of the proximal axonal segment through the myelin. What is interesting is that a severed axon still has directional transport of molecular packages, so whatever is going from the axonal terminal to the hillock will accumulate at the proximal end of the distal separated segment. I think a higher local concentration of some of these transported proteins will have an essential role in promoting or inhibiting the reconnection.

One thing for sure is that each peripheral nerve has a limited growth rate and if the distance between the proximal end of a potential connection is too far away from the distal end, sprouting is the more likely mechanism for re-innervation than reconnection.