Abstract
Although the mammalian locomotor CPG has been localized to the lumbar spinal cord, the functional-anatomical organization of flexor and extensor interneurons has not been characterized. Here, we tested the hypothesis that flexor and extensor interneuronal networks for walking are physically segregated in the lumbar spinal cord. For this purpose, we performed optical recordings and lesion experiments from a horizontally sectioned lumbar spinal cord isolated from neonate rats. This ventral hemi spinal cord preparation produces well-organized fictive locomotion when superfused with 5-HT/NMDA. The dorsal surface of the preparation was visualized using the Ca2+ indicator fluo-4 AM, while simultaneously monitoring motor output at ventral roots L2 and L5. Using calcium imaging, we provided a general mapping view of the interneurons that maintained a stable phase relationship with motor output. We showed that the dorsal surface of L1 segment contains a higher density of locomotor rhythmic cells than the other segments. Moreover, L1 segment lesioning induced the most important changes in the locomotor activity in comparison with lesions at the T13 or L2 segments. However, no lesions led to selective disruption of either flexor or extensor output. In addition, this study found no evidence of functional parcellation of locomotor interneurons into flexor and extensor pools at the dorsal-ventral midline of the lumbar spinal cord of the rat.
Highlights
To understand how the neural networks implicated in locomotion might work, it is of great importance to identify their constituents and to determine their spatial organization [1]
When the cord was transected dorsal to the central canal (n = 31; black arrow Figure 1B1, 1B2), locomotor period in the transected cord (3.160.1 s; light grey bar, Figure 1C) was not statistically different from that recorded in the intact spinal cord (3.460.2 s; black bar Figure 1C)
When the spinal cord (SC) was transected just ventral to the central canal all rhythmic activity ceased (n = 12) or slowed dramatically in three cases where the sections were at the dorsal limit of the dark grey bar
Summary
To understand how the neural networks implicated in locomotion might work, it is of great importance to identify their constituents and to determine their spatial organization [1]. Molecular biological techniques have permitted a systematic classification of diverse ventral spinal cord (SC) interneuronal cell types hypothesized to be constituents of the mammalian locomotor CPG [9,10] such as Ephrine-4 positive interneurons [11], Hb9 positive interneurons [12] and neurons types designed V0 [13], V1 [14,15], V2 [16,17] and V3 [18] neurons These studies have provided a wealth of detail about the anatomical location, axonal projections and biophysical properties of constituents of these diverse cell types, they have not elucidated the global anatomical distribution of these functional subgroups. The initial theory, based on the reciprocal inhibition-based ‘‘half-center’’ CPG model [19], hypothesizes that reciprocal activation of hindlimb flexors and extensors reflects the reciprocal inhibition between rhythmogenic interneuronal networks This model has provided a fruitful basis for approaching the problem of locomotor generation in limbed vertebrates and has served as the basis for more complex models [20]. Electrolytic micro-lesions localized between T12 to L2 were performed in order to detect possible selective disruption of either flexor or extensor motor output
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