Development and functional organization of spinal locomotor circuits.

The coordination and timing of muscle activities during rhythmic movements, like walking and swimming, are generated by intrinsic spinal motor circuits. Such locomotor networks are operational early in development and are found in all vertebrates. This review outlines and compares recent advances that have revealed the developmental and functional organization of these fundamental spinal motor networks in limbed and non-limbed animals. The comparison will highlight common principles and divergence in the organization of the spinal locomotor network structure in these different species as well as point to unresolved issues regarding the assembly and functioning of these networks.