Neuronal plasticity: historical roots and evolution of meaning.

In this paper, we outline some important milestones in the history of the term "plasticity" in reference to the nervous system. Credit is given to William James for first adopting the term to denote changes in nervous paths associated with the establishment of habits; to Eugenio Tanzi for first identifying the articulations between neurons, not yet called synapses, as possible sites of neural plasticity; to Ernesto Lugaro for first linking neural plasticity with synaptic plasticity; and to Cajal for complementing Tanzi's hypothesis with his own hypothesis of plasticity as the result of the formation of new connections between cortical neurons. Cajal's early use of the word plasticity is demonstrated, and his subsequent avoidance of the term is tentatively accounted for by the fact that other authors extended it to mean neuronal reactions partly pathological and no doubt quite different from those putatively associated with normal learning. Evidence is furnished that in the first two decades of the twentieth century the theory was generally accepted that learning is based on a reduced resistance at exercized synapses, and that neural processes become associated by coactivation. Subsequently the theory fell in disgrace when Lashley's ideas about mass action and functional equipotentiality of the cortex tended to outmode models of the brain based on orthodox neural circuitry. The synaptic plasticity theory of learning was rehabilitated in the late 1940s when Konorski and particularly Hebb argued successfully that there was no better alternative way to think about the modifiability of the brain by experience and practice. Hebb's influential hypothesis about the mechanism of adult learning contained elements strikingly similar to the early speculations of James, Tanzi and Cajal, but Hebb did not acknowledge specifically these roots of his thinking about the brain, though he was fully aware that he had resurrected old ideas wrongly neglected for a long time. Lately the concept of neural plasticity has been complicated by attributing considerably different meanings to it. A scholarly paper by Paillard is used to show how an analysis in depth can clarify some confusion engendered by an unrestricted use of the concept and term of neural plasticity.