27 Neural Plasticity

Abstract

I. INTRODUCTION: CONNECTIVITY AND PLASTICITY
Neural plasticity refers to functional changes in the nervous system and therefore encompasses a range of phenomena from changes at synapses observed on a microscopic scale to changes in behavior observed in the whole animal. These diverse phenomena are related since changes in synapses are believed to underlie changes in an animal’s behavior (Greenough and Bailey 1988). Ideally, both the physical changes to the nervous system and the resultant behavioral changes could be identified and studied together to yield an integrated understanding of nervous system structure and behavior.

Nervous systems were once thought to be “hardwired” during development. In most vertebrate central nervous systems, cell proliferation occurs during embryogenesis and new neurons are not added to the mature nervous system (Jacobson 1991). An unusual exception to this rule is found in some species of birds in which neurons are added to the brains of juveniles to accommodate song learning (Paton and Nottebohm 1984). Sensory systems such as the visual cortex form their functional connectivity during a limited “critical period” during development, and new connections in most systems are not made after this period (Hubel and Wiesel 1970). This stability was thought to be an essential requirement for reliable processing of sensory information. However, this general stability has been shown to have its exceptions. The nervous system of even the dimmest among us can learn new tasks and thereby reveals a degree of plasticity in our brains. Even the inability of the brain to produce new...