Department of Exercise Sciences


Inhibition is impaired in Focal Hand Dystonia

C M Stinear & W D Byblow.
Published in the Journal of Neurophysiology 2003, 89:2014-2020

Introduction

Focal Hand Dystonia (FHD) is a rare movement disorder that affects people who perform repetitive tasks with their hands. When people with FHD start to perform a specific task, such as writing or playing a musical instrument, they experience uncontrolled contraction of their hand muscles. This causes cramping and unwanted movement of their fingers, which prevents them from performing the task. This research has looked at the way the motor areas of the brain control hand movement in people with FHD. In particular, we have measured the activity of inhibitory cells in the brain. These inhibitory cells usually function to improve fine motor control of the hand, by helping to prevent the unwanted activation of muscles not involved in the task.

Methods

We asked subjects with and without FHD to perform a mouse-button pressing task. They rested their hand on a computer mouse, and pressed the button with their index finger, in time with a 1 Hz auditory metronome. We then used transcranial magnetic stimulation (TMS) to stimulate the motor area of their brain, both ‘on’ the beat of the metronome, and ‘off’ the beat (between button presses). We measured the activity and responses of their first dorsal interosseous muscle (FDI, on the index finger), and their abductor pollicis brevis muscle (APB, on the thumb).

Results

We found that the activity levels of the inhibitory cells changed during task performance in the control subjects, but not in the FHD subjects. As control subjects pressed the button down, the inhibitory cells controlling the FDI muscle (index finger) decreased their activity. This was as we expected, as it allowed the muscle to fire and press the button down. At the same time, the inhibitory cells controlling the APB muscle (thumb) increased their activity. This probably helped subjects to prevent the accidental activation of their thumb, and maintain it at rest. However, in FHD subjects, we found that the level of activity of the inhibitory cells did not change significantly during the performance of this task. The differential changes in inhibitory cell activity in the control subjects probably helped them to perform the task selectively, by allowing the activation of the FDI muscle, while keeping the APB muscle at rest. We suggest that the lack of significant changes in inhibitory cell activity in the FHD subjects may reduce the selectivity of their performance, and contribute to their development of symptoms when they perform particular tasks.

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For more information on this project contact:

Winston Bublow
Email: w.byblow@auckland.ac.nz