Because the genetic characteristics of several inbred strains of mice are well identified, their use is becoming increasingly popular in spinal cord injury research. In this context, it appears particularly important to document adequately motor patterns, such as locomotion in normal mice, to establish some baseline values of locomotor characteristics. It also seems crucial to determine the extent to which mice can express a locomotor pattern after a complete spinal transection to establish a baseline on which one can evaluate the effects of treatments after spinal injury. Therefore, we have used conventional techniques to document the kinematics of treadmill locomotion in intact mice (n = 11) and in mice with a complete section of the spinal cord at T8 (n = 12). The results show that the kinematics and EMG of adult normal mice can be adequately monitored with such conventional equipment and that mice can re-express hindlimb locomotion within 14 d after spinalization, without any pharmacological treatments. The angular excursions of the hip, knee, and ankle are similar to those of the intact mice, although the joints are sometimes more flexed. After spinal cord transection, out-of-phase alternation between the homologous limbs recovered, whereas the timing between homolateral limbs was completely lost. This remarkable ability of mice to express hindlimb locomotion after a complete spinalization should be taken into account in the evaluation of various procedures aimed at promoting the functional recovery of locomotion after spinal lesions.

The re-expression of hindlimb locomotion after complete spinal cord injuries (SCIs) is caused by the presence of a spinal central pattern generator (CPG) for locomotion. After partial SCI, however, the role of this spinal CPG in the recovery of hindlimb locomotion in the cat remains mostly unknown. In the present work, we devised a dual-lesion paradigm to determine its possible contribution after partial SCI. After a partial section of the left thoracic segment T10 or T11, cats gradually recovered voluntary quadrupedal locomotion. Then, a complete transection was performed two to three segments more caudally (T13-L1) several weeks after the first partial lesion. Cats that received intensive treadmill training after the partial lesion expressed bilateral hindlimb locomotion within hours of the complete lesion. Untrained cats however showed asymmetrical hindlimb locomotion with the limb on the side of the partial lesion walking well before the other hindlimb. Thus, the complete spinalization revealed that the spinal CPG underwent plastic changes after the partial lesions, which were shaped by locomotor training. Over time, with further treadmill training, the asymmetry disappeared and a bilateral locomotion was reinstated. Therefore, although remnant intact descending pathways must contribute to voluntary goal-oriented locomotion after partial SCI, the recovery and re-expression of the hindlimb locomotor pattern mostly results from intrinsic changes below the lesion in the CPG and afferent inputs.

During overground or treadmill walking, the stance phase and cycle durations are reduced as speed increases, whereas swing phase duration remains relatively invariant. When the speed of the left and right sides is unequal, as is the case during split-belt locomotion or when walking along a circular path, adjustments in stance and swing phases are observed, which could alter the phase/cycle duration relationships. Here, we tested this hypothesis in the left and right hindlimbs of four intact and two chronic spinal-transected adult cats during tied-belt (i.e., equal left and right speeds) and split-belt (i.e., unequal left and right speeds) walking. During split-belt walking, one side (i.e., constant limb) walked at a constant speed while the other side (varying limb) varied its speed from 0.3 to 1.0 m/s. We show that the phase/cycle duration relationships differed in both hindlimbs concurrently during split-belt walking. Specifically, the slope of the phase/cycle duration relationships for the stance/extension phase increased in the varying limb from tied-belt to split-belt walking, whereas that of the swing/flexion phase decreased. In contrast, in the constant limb, the slope of the phase/cycle duration relationships for the stance/extension phase decreased, whereas that of the swing/flexion phase increased. The results were qualitatively similar in intact and spinal-transected cats, indicating that the modulation was mediated within the spinal cord. In conclusion, we propose that neuronal networks within the spinal cord that control left and right hindlimb locomotion can differentially and simultaneously modulate phase variations when the two sides walk at different speeds.

Martinez M, Delivet-Mongrain H, Leblond H, Rossignol S. Recovery of hindlimb locomotion after incomplete spinal cord injury in the cat involves spontaneous compensatory changes within the spinal locomotor circuitry.