Regulation of motor circuit assembly by spatial and temporal mechanisms.

Precision of synaptic connections in neuronal circuits is the product of an orderly assembly process during development. Circuits controlling motor behavior have been studied extensively in many animal species, allowing an assessment of evolutionarily conserved organizational principles that underlie neuronal subtype specification, connectivity and function. Across phylogenetically distant species, motor circuit assembly is based on spatial organization of interconnected circuit elements. Developmental molecular coordinate systems demarcate dendritic and axonal targeting territories, thereby regulating convergence of synaptic partners. Additional mechanisms subsequently control fine synaptic connection specificity within these domains. Spatial organization often correlates with the orderly sequence of neurogenesis contributing to the generation of distinct postmitotic neuronal subpopulations, a developmental strategy implemented far beyond motor circuits.