MadSci Network: Neuroscience
Query:

Re: why is the peripheral nervous system capable of regeneration, but not CNS?

Date: Fri Sep 11 15:41:30 1998
Posted By: James Goss, Post-doc/Fellow, Neurology, University of Pittsburgh
Area of science: Neuroscience
ID: 905127307.Ns
Message:

This is an excellent question Jeff.  When we discuss regeneration, we 
generally refer to the regrowth of neuron fiber tracts (neuronal axons) 
following a trauma severe enough to disrupt the electrical signal between 
the neuron cell bodies and the site of innervation (i.e. where the axons 
goes to).  If such damage occurs in the peripheral nervous system (PNS), 
the axons can regrow to the original target.  To take an extreme example, 
if you were to cut off your hand then reattach it, you would eventually be 
able to move your hand and recover your sense of touch.  That is because 
the nerves which go to muscles and the touch receptors in the skin are part 
of the PNS.  However, if similar damage occurs in the central nervous 
system (CNS), the neurons do not regrow.  For example, if you have a severe 
spinal cord injury, then you would be paralyzed for life.  This is because 
the spinal cord and brain make up the CNS. 
The reason that the PNS regenerates and the CNS does not has to do with the 
 local environment in which the neurons exist.  The fiber tracts which 
carry signals from point A to B in the CNS or PNS is not simply made up of 
neurons and their axons.  There are other cell types which have very 
important roles.  Chief among these cells are the oligodendrocytes in the 
CNS and Schwann cells in the PNS.  Both of these cells make a substance 
known as myelin.  Myelin is a fatty material which wraps around the axon of 
neurons and acts like the insulation on an electrical wire.  Without this 
myelin sheath, the nerves would not be able to function.  This myelin 
sheath also contains many other types of molecules, which scientists are 
just beginning to investigate.  The environment of the myelin sheath in the 
CNS and PNS are different, and many scientists believe that this difference 
explains the difference in regeneration. 
Many experiments have been performed in which sections of peripheral nerve 
have been grafted into the CNS.  When a piece of peripheral nerve is cut, 
the neuron axon inside degenerates and dies, but the myelin sheath remains 
intact.  If this myelin sheath is then grafted into the CNS (e.g. as a 
bridge between two cut ends in the spinal cord), CNS neurons will send 
axons through this graft just like PNS neurons.  However, once the CNS 
axons leave the graft and reenter the CNS they stop growing.  This strongly 
suggests that there is something in the PNS environment which allows 
regeneration and something in the CNS environment which inhibits 
regeneration.  A lot of research is currently being conducted to find what 
components of the PNS myelin sheath allow regeneration and what components 
in the CNS inhibit it.



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