Dynamics and plasticity of spinal locomotor circuits

• Spinal locomotor circuits are endowed with inherent modulatory mechanisms.
• The speed of locomotion is continuously modulated by a graded recruitment of neurons.
• Neuromodulation endows the locomotor circuits with short-term and long-term plasticity.

Spinal circuits generate coordinated locomotor movements. These hardwired circuits are supplemented with neuromodulation that provide the necessary flexibility for animals to move smoothly through their environment. This review will highlight some recent insights gained in understanding the functional dynamics and plasticity of the locomotor circuits. First the mechanisms governing the modulation of the speed of locomotion will be discussed. Second, advantages of the modular organization of the locomotor networks with multiple circuits engaged in a task-dependent manner will be examined. Finally, the neuromodulation and the resulting plasticity of the locomotor circuits will be summarized with an emphasis on endocannabinoids and nitric oxide. The intention is to extract general principles of organization and discuss some onto-genetic and phylogenetic divergences.