Introduction

Neuronal plasticity may be defined as any functional change within the nervous system outlasting an (experimental) manipulation. Plasticity, by this definition, does not comprise structural changes, such as those occurring during development or repair. Although there is no universally accepted lower limit of its duration, the term ‘plasticity’ is usually only applied when neuronal changes outlast the manipulation by more than a few seconds. In experimental animals, as well as in humans, plasticity is usually defined neurophysiologically by changes in the stimulus–response characteristics (‘excitability’).

Plasticity of the central nervous system has attracted much interest because it is thought to be related to the mechanisms underlying the formation of memories and the learning of new skills. Very likely, it is also involved in restoration of brain function after its initial loss as a consequence of brain injury. Neuronal plasticity may be induced internally, such as by practising movements (see Chapter 4), or externally, for instance, by limb amputation, spinal cord injury or cerebral stroke (see Chapter 8), or by repetitive electrical or magnetic neuronal stimulation, as reviewed here. Models of plasticity relying on external stimulation may be attractive because they allow best to control for experimental conditions. Human models of central nervous system plasticity may contribute particularly relevant information to the understanding of fundamental principles of plasticity. Additionally, the neuronal changes induced in human models of plasticity may themselves prove to be therapeutically useful.