ABSTRACT
The term neuroplasticity is generally used to refer to the capacity of the nervous system to modify its organization. However, such changes may occur as a consequence of many different events, including the normal development and maturation of the organism, the acquisition of new skills ("learning") in immature and mature organisms, following damage to the nervous system and as a result of sensory deprivation. Studies to date of the molecular and cellular events underlying neural plasticity in such different conditions have revealed a limited set of mechanisms available to induce changes in the organization of the neural networks of the brain. Such reports raise the hypothesis and the hope that the diverse phenomena referred to as neuroplasticity will be elucidated in the not-too-distant future. However, while there is evidence to suggest that there may be considerable overlap in the mechanisms that mediate developmental and adult neuroplasticity following abnormal experience or damage, we think it is important at the present time to distinguish between them. Even if some of the mechanisms are similar or even identical, the fact that these mechanisms operate on nervous systems that are structurally and physiologically different is likely to result in quantitative and/or qualitative differences in neuroplasticity in immature and mature organisms. For example, in the immature human brain, the number of synapses is 50% greater than in the adult brain (Huttenlocher, 1994; Figure 18.1). This redundant connectivity exists at different times in different brain regions and almost certainly constrains the nature and extent of modification that can occur at different ages. Similarly, the metabolic profiles of different brain structures change dramatically over the first two decades of life. It is likely that these variables give rise to different profiles of plasticity at different times and for functions linked to different structures. Indeed, neurophysiological studies of animals indicate considerable variability and specificity in the types of plastic changes
18 C H A P T E R
262 Developmental Aspects
Figure 18.1. Mean synaptic density in synapses/100 urn3 in auditory (filled circles), calcarine (open circles), and prefrontal (•) cortex at various ages. Adapted from "Regional Differences in Synaptogenesis in Human Cerebral Cortex," by P. R. Huttenlocher and A. S. Dabholkar, 1997, Journal of Comparative Neurology, 387, p. 1 70. Copyright 1997 by John Wiley. Also adapted from "Specificity and Plasticity in Neurocognitive Development in Humans," by H. J. Neville and D. Bavelier, 1999, p. 85. Copyright by MIT Press. Adapted with permission.
