This study examines the effect of high-level skilled behaviour on motor cortex representations of upper extremity muscles of ten sportswomen. We used transcranial magnetic stimulation to map proximal medial deltoid and distal extensor carpi radialis muscle representations on both hemispheres during low-level voluntary contraction. We compared cortical representation areas between two groups of subjects and between hemispheres within subjects. The first group comprised five elite volleyball attackers and the second group five runners. Four stimuli were delivered on multiple scalp sites (1.5 cm apart) to induce motor-evoked potentials recorded by surface EMG. Maps were described in terms of excitable scalp positions and of motor-evoked potentials. We observed differences in map areas between the two groups. Volleyball players had larger cortical representations of the proximal medial deltoid muscle than runners. Furthermore, the volleyball players had larger map areas for dominant muscles compared with non-dominant muscles. There was no difference, however, in map area for either muscle between the dominant and non-dominant arm in the runner group. Our results show that heavy training in a specific skill induces an expansion of proximal muscle representation in the contralateral primary motor cortex. This enlarged map area for proximal muscle is accompanied by an increase in the overlapping of proximal and distal muscle representations. This could reflect the fact that motor learning of co-ordinated movement involves a common control of both muscles. This reorganization supports the hypothesis of a cortical plasticity driven by activity.
For decades cortical representations of the parts of the body have been considered to be unchangeable. This view has changed radically during the past 20 years using new tools designed to study plasticity in the adult human brain. Transcranial magnetic stimulation (TMS) is a valuable non-invasive technique for exploring the ability of the motor cortex to change during motor skill acquisition. Results obtained with TMS in neurological patients as well as in normal subjects demonstrate that cortical plasticity is a necessity for correct adaptation to the continuously changing environment. Topographical reorganization of the motor cortex depends on the types of movements performed by the subjects. During simple training, the cortical representation is enlarged, and it returns to its initial size when the task is overlearned. These transient modifications characterize simple motor training. Motor skills in which coordination of distal and proximal muscles, precision of the task and spatio-temporal constraints are associated, has a different impact on cortical reorganization. We propose that years of practice of a complex motor skill induces a new cortical topography that must be interpreted as structural plasticity which provides the capacity to execute a plastic behaviour instead of a stereotypical movement. We review the neuronal mechanisms underlying plasticity in different types of movement. We stress new emerging notions, such as overlap of cortical maps, and system dynamics at single neuron and network levels, to explain the reorganization of movement representations that encode motor skill. Dendritic arborizations as functional computing elements, newly generated neurons in adult brain, and plastic architectures of cortical networks operating as distributed functional modules are new hypotheses for structural plasticity.
In the dystonic group, facilitation of the FDI was observed during a task involving proximo-distal coordination. No differences in silent periods were observed when the muscle was activated alone. Our results suggest that such abnormal facilitation is not only an impairment of the central inhibitory mechanisms reported for dystonic patients, but, in addition, represents true abnormality in cortical muscle activation strategies.
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