LEARNING AND GENERAL NOTIONS OF ORGANIC PLASTICITY IN THE 19TH CENTURY The present article has been divided into five sections: (1) Learning and general notions of organic plasticity in the 19th century (2) The idea of “mental” plasticity proposed by William James (1842–1910) (3) Neuronal theory and “structural” plasticity: general considerations (4) Evolutionary factors of the nervous system in Cajal’s work (5) Final considerations. However, perhaps one of the most interesting aspects of his studies was his innovative capacity to interpret the structure as being the result of evolutionary mechanisms, i.e., natural selection and his proposal that, ultimately, in relation to the histology of the nervous system, such selection might occur in the establishment of connections between cells. The sheer number of studies in favor of the neuronal theory that Cajal carried out over the course of his career was impressive. It is important to highlight that the studies in the field of neuroscience converged increasingly in the twentieth century to the point that the notion of the plastic nervous system was generally accepted as a critical mechanism for learning and memory ( Barlow, 1972, 1995 Azmitia, 2002, 2007 Fields and Stevens-Graham, 2002 DeFelipe, 2006). The notion that neuronal theory is a necessary condition to explain the plasticity of neural connections will also be explored. This paper aims to examine, at the cellular level, the notion of evolution of the nervous system presented by Santiago Ramón y Cajal (1852–1934). The overall way of thinking of academic circles in Europe in the second half of the 19th century was influenced by the theories of evolution ( Baumer, 1977), with the reach of this influence extending beyond the boundaries of biology and similar sciences.Īs a result, given the general importance placed on evolution by the academic community at the time, it follows that the study of the nervous system would be approached from an evolutionary standpoint. “ Naturalists had at the time an infinite belief in the actual existence of a straight parallel and even may be an identity between the developments of a full-grown chick from a chicken egg, for instance, and the entire species of chicks from a more primitive bird.” This idea influenced naturalistic thinking ( Gould, 2010): The relationships between the development of the organism (ontogenesis) and of the species (phylogenesis) resulted in the theory of recapitulation, which proposed that in developing from embryo to adult, animals go through stages resembling or representing successive stages in the evolution of their remote ancestors. In this early period, explanations were based on the phylogenetic evolution of organisms.
An important example is the strong influence of the theory of evolution on the embryologists ( Coleman, 1983). The origins of modern neuroscience was highly influenced by the revolutionary biological thinking that arose in the second half of the 19th century with the theory of Charles Darwin (1809–1882) concerning the evolution of the species through natural selection. The present article is divided into five sections: (1) Learning and general notions of organic plasticity in the 19th century (2) The idea of “mental” plasticity proposed by James (3) Neuronal theory and “structural” plasticity: general considerations (4) Evolutionary factors of the nervous system in Cajal’s work and (5) Final considerations. This capacity would ultimately lead Cajal to the conclusion that, in relation to the histology of the nervous system, such selection occurs in the establishment of connections between cells. It is true that he carried out a large number of studies throughout his career in favor of the neuronal theory, but perhaps one of the most interesting aspects of his studies was his innovative capacity to interpret structure as being the result of evolutionary mechanisms, i.e., natural selection. Although the roots of the term “plasticity” in reference to neuroscience are not completely clear, Cajal was an important figure in the propagation and popularization of its use. Here we develop Cajal’s notion that neuronal theory is a necessary condition to explain the plasticity of neural connections. In this article we discuss the influence of William James and Charles Darwin on the thoughts of Santiago Ramón y Cajal concerning the structure, plasticity, and evolution of the nervous system at the cellular level.
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