Journal Section: Behavioral/systems/cognitive Title: Recovery of Hindlimb Locomotion after Incomplete Spinal Cord Injury in the Affiliation: Multidisciplinary Team in Running Head: Changes in Locomotor Pattern after Sci Groupe De Recherche Sur Le Système Nerveux Central

33 After incomplete spinal cord injury (SCI), compensatory changes occur 34 throughout the whole neuraxis including the spinal cord below the lesion as suggested by 35 previous experiments using a dual SCI paradigm. Indeed, cats submitted to a lateral 36 spinal hemisection at T10-11 and trained on a treadmill for 3-14 weeks re-expressed 37 bilateral hindlimb locomotion as soon as 24 hours after spinalization, a process which 38 normally takes 2-3 weeks when a complete spinalization is performed without a prior 39 hemisection. Herein, we wanted to ascertain whether similar effects could occur 40 spontaneously without training between the two SCIs and within a short period of 3 41 weeks in 11 cats. One day after the complete spinalization, 9 of the 11 cats were able to 42 re-express hindlimb locomotion either bilaterally (n=6) or unilaterally on the side of the 43 previous hemisection (n=3). In these 9 cats, the hindlimb on the side of the previous 44 hemisection (left hindlimb) performed better than the right side in contrast to that 45 observed during the hemispinal period itself. Cats re-expressing the best bilateral 46 hindlimb locomotion after spinalization had the largest initial hemi-lesion and the most 47 prominent locomotor deficits after this first SCI. These results provide evidence that 1) 48 marked reorganization of the spinal locomotor circuitry can occur without specific 49 locomotor training and within a short period of 3 weeks; 2) the spinal cord can reorganize 50 in a more or less symmetrical way; 3) the ability to walk after spinalization depends on 51 the degree of deficits and adaptation observed in the hemispinal period. 52 53