Figure 1. Organization of the locomotor system in vertebrates.

(a) Schematic of the rodent central nervous system showing the neural structures that are part of in the motor pathways controlling simple behaviors such as mastication, respiration and locomotion. Adapted from Ref. 5. (b) Motor pathways in aquatic and terrestrial vertebrates share a similar neuroanatomical structure. Local control of muscle movements is effected by pools of motor neurons in the spinal cord that are part of a dispersed locomotor CPG network. The motor commands are modulated by proprioceptive sensory feedback via sensory afferents. Descending reticulospinal, rubrospinal and vestibulospinal pathways control the locomotor network in the spinal cord, although the reticulospinal pathway is the primary pathway for intiating locomotion. The reticulospinal pathway can be activated by the mesencephalic locomotor region (MLR), which has inputs from the basal ganglia and thalamus. The cerebellum coordinates motor behaviors by mediating sensory and internal feedback and optimizing the motor pattern to the task at hand. It also coordinates spinal motor actions with the supraspinal motor pathways. Connections from the motor cortex refine and initiate motor actions. The black lines indicate direct command pathways, the grey lines indicate feed-back pathways. VS, vestibulospinal; RbS, rubrospinal.