REVERSIBLE CHANGES OF MOTOR CORTICAL OUTPUTS FOLLOWING IMMOBILIZATIONOF THE UPPER-LIMB

We mapped the cortical representations of the abductor pollicis brevis , flexor carpi radialis, biceps and deltoid muscles in six subjects wi th unilateral wrist fractures, immediately after the removal of the sp lint. This was repeated 1 month later in three out of the six subjects . Duration of immobilization was I month. Muscle maps were obtained by delivering four focal magnetic pulses for each scalp position (1 cm a part with reference to Ct) over the contralateral hemisphere. Motor ev oked potentials (MEPs) were averaged off-line and expressed as a perce ntage of the motor action potential evoked by supramaximal peripheral nerve stimulation. Volume, area and threshold of the motor maps showed no significant hemispheric differences within each muscle in 10 contr ol subjects. In the first recording session the volume of each immobil ized muscle was distinctly higher when compared to that of controls in terms of absolute value and side-to-side ratio. This finding disappea red 1 month later. Moreover, MEP amplitude difference recorded from ha nd muscle could be reversed during a small tonic voluntary contraction . Immobilization had no significant effect on the threshold for activa tion of the target muscles and on the area of the motor map. The incre ase in MEP amplitudes occurred without changes in spinal excitability as tested by the F wave. These findings suggest that immobilization of the upper limb induces a reversible enhancement of the excitability o f structures along the corticomotoneuronal pathway. Sustained restrict ion of volitional movements and reduction in somatic sensory inputs mi ght promote this functional modulation of the motor system. (C) 1997 E lsevier Science Ireland Ltd.