Afferent inputs to mid- and lower-lumbar spinal segments are necessary for stepping in spinal cats

Afferent inputs are known to modulate the activity of locomotor central pattern generators, but their role in the generation of locomotor patterns remains uncertain. This study sought to investigate the importance of afferent input for producing bilateral, coordinated hindlimb stepping in adult cats. Following complete spinal transection, animals were trained to step on the moving belt of a treadmill until proficient, weight-bearing stepping of the hindlimbs was established. Selective dorsal rhizotomies of roots reaching various segments of the lumbosacral enlargement were then conducted, and hindlimb stepping capacity was reassessed. Depending on the deafferented lumbosacral segments, stepping was either abolished or unaffected. Deafferentation of mid-lumbar (L3/L4) or lower-lumbar (L5-S1) segments abolished locomotion. Locomotor capacity in these animals could not be restored with the administration of serotonergic or adrenergic agonists. Deafferentation of L3, L6, or S1 had mild effects on locomotion. This suggested that critical afferent inputs pertaining to hip position (mid-lumbar) and limb loading (lower-lumbar) play an important role in the generation of locomotor patterns after spinal cord injury.