Tremor and skeletal muscle tone.

Stereotactic interventions at thalamic/subthalamic levels for parkinsonian tremor give the chance to investigate the influence of the supraspinal proprioceptive loops on the tonic component of motor tasks. Studies demonstrate not only a normalization of the late stretch reflex component and a decrease of the pre-reflex stiffness in muscles contralateral to the side of the lesion, but also a reduction of low threshold motor unit firing frequency in the elbow flexors during isometric contractions. These data indicate a shift from a more tonic to a more phasic mode of innervation, which we interpret as the cause of hypotonia.