The spinal circuitry is intrinsically capable of driving a variety of locomotor behaviors

20 Spinal locomotor circuits are intrinsically capable of driving a variety of behaviors such as 21 stepping, scratching and swimming. Based on an observed rostrocaudal wave of activity in the 22 motoneuronal firing during locomotor tasks, the traveling-wave hypothesis proposes that spinal 23 interneuronal firing follows a similar rostrocaudal pattern of activation, suggesting the presence 24 of spatially organized interneuronal modules within the spinal motor system. In this study, we 25 examined if the spatial organization of the lumbar interneuronal activity patterns during 26 locomotor activity in the adult mammalian spinal cord was consistent with a traveling wave 27 organizational scheme. The activity of spinal interneurons within the lumbar intermediate zone 28 was examined during air-stepping in sub-chronic spinal cats. The preferred phase of 29 interneuronal activity during a step cycle was determined using circular statistics. We found that 30 the preferred phases of lumbar interneurons from both sides of the cord were evenly distributed 31 over the entire step cycle with no indication of functional groupings. However, when units were 32 sub-categorized according to spinal hemicords, the preferred phases of units on each side largely 33 fell around the period of extensor muscles activity on each side. In addition, there was no 34 correlation between the preferred phases of units and their rostrocaudal locations along the spinal 35 cord with preferred phases corresponding to both flexion and extension phases of the step cycle 36 found at every rostrocaudal level of the cord. These results are consistent with the hypothesis that 37 interneurons operate as part of a longitudinally distributed network rather than a rostrocaudally 38 organized travelling-wave network. 39 40