Stereoencephalotomy and control of skeletal muscle tone

Following therapeutic stereotactic lesions in the ventrolateral subthalamus and adjacent nuclei there are two phenomena concerning muscle tone: decreased muscle stiffness and failure in load compensation. In order to clarify whether hypotonia is due to decreased reflex activity and/or reduced nonreflex muscle stiffness, torque-induced stretch responses in the forearm flexor muscles and muscle spindle afferent discharge were investigated. In hypotonia following these lesions, the immediate compensation of abrupt external perturbations was diminished prior to reflex onset. A special feedback system is presented which originates in skeletal muscles and which continuously controls the muscle stiffness during movements. This afferent fiber system probably has a greater representation at the thalamic level as compared to the peripheral nerve. It is assumed that the decrease in muscle stiffness as well as the failure in load compensation are due to diminished static gamma-drive. This lack in somatosensory feedback can be compensated voluntarily by changing the mode of innervation.