Introduction

Functional recovery or compensation following nervous system injury may be facilitated by plasticity within the central nervous system. For example, activation of the visual cortex that occurs during Braille reading in the early blind (under 14 years of age) is of functional importance (Cohen et al., 1997) and there is convincing evidence that plasticity can play an adaptive role following deafferentation (Pascual-Leone & Torres, 1993). Whether this kind of functional reorganization occurs in the motor system is less clear.

In the motor system, the skilled use of our muscles requires the integrative actions of sensory feedback and descending motor commands, which result in appropriate activation of motoneurones through activation of spinal interneurons, i.e. sensorimotor integration (Baldissera et al., 1981).The corticospinal system, the vital component of volitional movement, controls spinal motoneuronal activity through interneuronally mediated pathways (Lundberg & Voorhoeve, 1962; Pierrot-Deseilligny, 1996; Alstermark et al., 1999), and through their direct monosynaptic contacts with spinal motoneurons (Jankowska et al., 1975). The alterations in the control of the corticospinal system have received the greatest attention in TMS studies of plasticity in humans.

The importance of tonic sensory input in regulating cortical excitability and cortical body part representation in the motor cortex was initially shown in animal studies wherein peripheral nerve injury triggers a massive reorganization in the rat (Sanes et al., 1990).