1) An attempt was made to analyse the pre- and postoperative EMG in order to correlate the physiological mechanism of pyramidal and extrapyramidal systems with clinical symptoms in course of stereotaxic surgery. 2) Globus pallidus, thalamocapsular area and ventrolateral nuclei of thalamus were selected as the main anatomical locations for target. 3) The thalamocapsular lesion manifesting the shortening of the silent period of the H-wave recovery curve signifies the increased activity of the α -system and a symptom of pyramidal system disorders. 4) The prolongation of the period in the cases of pallidal and VL lesions represents the decrement of α-motoneuron excitability secondary to the functional drop of γ-system. 5) It seems to be clear the silent period shortening the lesions are involved in internal capsule and the prolonging ones are in the nuclei of the extrapyramidal system.