Locomotor-related V3 interneurons initiate and coordinate muscles spasms after spinal cord injury

Spinal cord injury leads to a devastating loss of motor function and yet is accompanied by a paradoxical emergence of muscle spasms, which often involve complex muscle activation patterns across multiple joints, reciprocal muscle timing, and rhythmic clonus. Here we investigated the hypothesis that spasms are a manifestation of partially recovered function in spinal central pattern generating (CPG) circuits that normally coordinate complex postural and locomotor functions. We focused on the commissural propriospinal V3 neurons that coordinate inter-limb movements during locomotion, and examined mice with a chronic spinal transection. When the V3 neurons were optogenetically activated with a light pulse, a complex coordinated pattern of motoneuron activity was evoked with reciprocal, crossed and intersegmental activity. In these same mice, brief sensory stimulation evoked spasms with a very similar complex pattern of activity to that evoked by light, and the timing of these spasms were readily reset by act...