NeuroPlasticity
by
Roger Harris
Associate Professor of Neurology at Washington State University
Hi Trond Ruud!
All right, I will take up your challenge and try to
explain neuroplasticity, and why it is so crucial to us.
Neuroplasticity is our ability to change the circuitry in
our central nervous system in the face of damage to a
part of it, so that pathways which were used for other
purposes, or not used much at all, can be can be used to
carry out the functions lost by the damage. The reason we
need this is that when there is damage to the central
nervous system, as in a stroke, the major effect is to
damage groups of axons, which are carrying messages from
one part of the CNS to another. For example, the pathway
that carries motor commands from your motor cortex down
to motorneurons in the spinal cord. If that pathway is
damaged, the motor commands don't get relayed, and the
result is paralysis.
If this happens in the peripheral nervous system - say
you have severed a nerve in your arm - the axons are
capable of "regeneration", which means they can
grow right back to their previous connections, and once
that connection is reformed, recovery occurs. In the
central nervous system- the brain and spinal cord- this
regeneration of damaged axons does not occur, for a
number of reaons. Therefore, the only way for recovery to
occur in the CNS is for neighboring, undamaged axons to
"rewire" themselves to take over the functions
that were lost by the damage. This rewiring takes place
by several mechanisms, such as "axonal
sprouting", where undamaged axons "sprout' new
endings to innervate the neurons which have lost their
input due to the damage. This neuroplasticity occus at a
number of levels. At the lowest level, as we've seen, it
can involve undamaged axons sprouting new connections to
drive the neurons which have lost their input due to the
damage. At a higher level, it can involve activating
different pathways to accomplish the same funtion in a
slightly different way. And at the highest, behavioral
level, it can involve just learning to do something in a
different way- like learning to do with the right arm
what you used to do with the left, when the left arm
becomes paralyzed or non-funtional. There are lots of
ways around a disability! The reason this all works, is
that our central nervous ststem is designed with a lot of
redundancy- pathways that are present, but not really
needed for our normal functioning. These pathways are
then available to take over for the function of our
normal pathways when these are damaged. Redundancy is the
heart of neuroplasticity.
Another point is that the neuroplasticity we have
available is very much a function of age. In infants and
children, there is a large amount of neuroplasticity in
the system, because this is also used in the normal
development of the nervous system. So a stroke in a young
child can often have relatively good recovery. As we get
older, neuroplasticity is less and less available, and so
recovery takes loger, and is not as complete. This gets
worse at an old age (I'm not sure what that would be),
where neuroplasticity mevhanisms are no longer as
available, and recovery is worse.
The moral of the story is, if you're going to have a
stroke, have it young!
I hope this is somewhat understandable...
roger
