Experimental setup and procedure. A: The diagram illustrating the experimental set-up for dipolar cortico-muscular stimulation (dCMS). The positive and negative voltage outputs were connected to electrodes situated on the primary motor cortex (M1), and on the contralateral gastrocnemius muscle, respectively. Both gastrocnemii muscles were attached to force transducers (not shown). Recording from single motoneuron (Rec) was performed simultaneously on each side of the spinal cord below the lesion, as shown. IGM - ipsilateral gastrocnemius muscle, CGM - contralateral gastrocnemius muscle. B: The experimental procedure consisted of three phases designed to stimulate the preparation and to evaluate its reactions to dCMS. The force of muscle contraction and cortically-elicited spinal responses were evaluated before and after the application of dCMS in Pre-test and Post-test phases by application of ten monopolar pulses. The type of stimulation and location of the stimulation and recording electrodes was the same in these two phases. During dCMS phase the preparation was stimulated by application of the positive and negative pulses to the motor cortex (M1) and contralateral gastrocnemius muscle (CGM) respectively. While the number of pulses delivered during Pre- and Post-test phases was the same (10), the number of pulses delivered during dCMS was 100. The duration (1 ms) and the frequency of stimulation (1 Hz) were the same in all three phases of the experiment. The shape of the stimulating current at each phase is shown. There was a continuous recording of ipsilateral and contralateral muscle twitches and evoked and spontaneous spinal activity during the entire experiment.