Central locomotor program for the cat's hindlimb

The central program for activation of different hindlimb muscles during ‘fictive locomotion’ in immobilized thalamic cats, expressed in the time structure of motor discharges in nerves to different muscles, was investigated. For the majority of hindlimb muscles this program was quite simple and consisted of alternating appearance of bursts of discharges in the nerves to flexors and extensors. The level of activity within individual bursts of discharges in the nerves to flexors and extensors was changed in time in an unpredictable manner. Correlation analysis of activity which appeared in the same phase in the nerves to flexors (or, respectively, extensors) showed that the maximum dependence between two concurrent bursts was observed at zero time shift. These data support Brown's hypothesis about the existence in each part of the spinal cord of two half-centers operating alternately during locomotion—one for the flexors and another for the extensors—which activate the motoneurones of the corresponding muscles. Activity in the nerves to biarticular muscles (m. semitendinosus, m. posterior biceps, m. tenuissimus) and in the nerves to m. extensor digitorum brevis and m. peroneus tertius was of a more complex nature. The central program of activation of these muscles depended on the intensity of ‘fictive locomotion’. A suggestion was made that the motor nuclei of these muscles are connected with the half-centers in a more complex manner.