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Neuroplasticity is the ability of the central nervous system to remodel itself. In the last few decades, we have learned that neuroplasticity is not only possible but that it is also constantly occurring; the brain is always changing. Neuroplasticity is how we adapt to changing conditions, learn new facts, and develop new skills. If the brain is injured, it tries to repair itself with these normal mechanisms. If all goes well, spontaneous recovery can be excellent. Of course, the natural scope of these plastic processes is limited, and sometimes the end point of the remodeling is problematic itself. The job for those of us interested in rehabilitation is to promote plasticity in the right direction and, sometimes, to correct it if it has taken a wrong turn. We must, therefore, understand neuroplasticity and learn to control it. Fortunately, this is an active area for current research.
MECHANISMS OF NEUROPLASTICITY
Neuroplasticity can be understood at several levels. The first level is that of the individual neuron and the events responsible for remodeling that occur within the cell. The second level is that of groups of neurons and their functions that can change to alter behavior. At the cellular level, multiple processes can occur both in parallel and serially. Generally speaking, some processes are quick, but transient, and can allow for rapid adaptations. Others take longer, but are then more permanent. One principle appears to be that the more persistent an early change is, the more likely it is that it will be permanent.
The fastest type of change is a simple modulation of neuronal traffic that leads to a change in the amount of excitation and inhibition. Apparently, many neural networks are chronically and largely inactive because of tonic inhibition. Release of this inhibition will allow the networks to function; a process sometimes called "unmasking." Unmasking can be demonstrated in animal models with local delivery of gamma-amino-butyric acid antagonists, which immediately opens new functional networks [1].
Another relatively rapid change involves alteration of the strength of specific synapses. The magnitude of the response of the synapses changes as a function of the pattern of activity that they experience, both at the individual synapse and at the temporal conjunction of two or more synapses on the same...