MadSci Network: Neuroscience |
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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