Electromyography
Recordings were obtained with a standard electromyography apparatus (Dantec, Les Ulis, France), in accordance with the guidelines of the American Association of Electrodiagnostic Medicine. Mice were anesthetized as indicated above and kept under a heating lamp to maintain a physiological muscle temperature (at about 31°C). A concentric needle electrode (no. 9013S0011, diameter 0.3 mm; Medtronic, Minneapolis, MN) was inserted in the gastrocnemius, and a monopolar needle electrode (no. 9013R0312, diameter 0.3 mm; Medtronic) was inserted into the tail of the mouse to ground the system. Each muscle was monitored in four different regions, and the degree of denervation was scored as the number of regions with spontaneous activity expressed in percentage. Spontaneous activity was differentiated from voluntary activity by visual and auditory inspection. Only spontaneous activity with a peak-to-peak amplitude of at least 50 μV was considered to be significant.