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How plastic is the central nervous system?

Ramon y Cajal expressed the fatalistic view in 1928, of the central nervous system's response to damage that "...once the development was ended, the fonts of growth and regeneration of the axons and dendrites dried up irrevocably. In adult centers, the nerve paths are something fixed, ended, immutable. Everything may die, nothing may regenerate." Recently, however, new techniques for studying the nervous system have led to a reversal of thought. It is now clear the central nervous system (CNS) is remarkably plastic. While neurons cannot be replaced once they are lost through traumatic damage, the action of neurotoxins or disease processes, remaining neurons are capable of a remarkable array of responses under the appropriate conditions. The lines of research that led to this new concept are briefly explored and also current directions being pursued to understand factors that control CNS plasticity.