The control of isometric contractions in patients suffering from different types of hypotonia

Different forms of hypotonia, resulting from various lesions in the somatosensory system, are discussed. In order to investigate the causes of hypotonia the forearm flexors of patients were stretched and the segmental EMG responses studied. Special attention was given to parkinsonian patients who were stereotaxically operated for the relief of tremor. These patients were studied before and after their operation and their reflex responses were compared to those of normal subjects. It was found that the M2 component was always significantly larger before operation when compared to normal subjects. This could be due to high-frequency firing of single motor units during the M2 interval, as shown with single unit analysis. After operation the M2 component was significantly reduced; clinically hypotonia was observed. It is thus suggested that hypotonia observable in parkinsonians following subthalamotomy and/or thalamotomy could be the result of the decreased M2-component. Pathways which may be interrupted by the lesion are discussed and possible mechanisms mediating unusually high M2-components are mentioned. The hypothesis is forwarded that the stereotaxic lesion may selectively interfere with the static gamma drive to muscle spindles.