Re: what happens to a nerve when it is damaged?

When a nerve is damaged a clearly defined sequence of events take place.  
Severing the axon (the cable along which neural signals are transmitted) 
causes a number of degenerative changes to take place.  Firstly 
communication between the damaged neuron and all other neurons to which it 
is connected ceases.  The distal portion (or the section of the severed 
axon that is furthest from the cell body) slowly starts degenerating. The 
cells that had synaptic connections (points of cell-cell communication) 
with the damaged cell also start to degenerate. The degenerating material 
attracts scavenging cells from the nervous system (glia) and the immune 
system (macrophages).  Glia and macrophages begin the process of clearing 
up the debris caused by the injury.  
What happens next depends on whether the damaged neurons were in the 
central nervous system (brain and spinal cord) or peripheral nervous system 
(sensory and motor neurons).  In the central nervous system neurons that 
are damaged have very little ability to regenerate axons and make new 
connections.  Thus, people who have brain damage or damage their spinal 
cords often remain permanently disabled.  It is thought that there are 
signals in the central nervous system that prevent neuronal recovery from 
injury.  Some of the signals that 'stop' regrowth in its path are found in 
the scars that form from glia that remain hanging around after they have 
finished scavenging dead matter.  Much scientific research is underway to 
understand these 'stop' signals and find ways of overcoming them so the 
brain and spinal cord can heal itself after injury.
To understand the signals that prevent regrowth scientists are examining 
recovery from injury in the peripheral nervous system where neurons CAN 
regenerate.  Here there are special chemicals that prevent the degeneration 
of axons after they are damaged and promote the regrowth of new 
connections. Scientists have shown that transplanting pieces of peripheral 
nerve into a damaged central nerve can promote the regrowth of the central 
axons.