Motor system plasticity after unilateral injury in the developing brain

In maturity, motor skills depend on the corticospinal tract (CST) and brainstem pathways that together synapse on interneurons and motoneurons in the spinal cord. Descending signals to spinal neurons that mediate voluntary control can be distinguished from peripheral sensory signals, primarily for feedback control. These motor system circuits depend initially on developmental genetic mechanisms to establish their connections and neural activity‐ and use‐dependent synaptic refinement during the early postnatal period to enable motor skills to develop. In this review we consider four key activity‐dependent developmental mechanisms that provide insights into how the motor systems establish the proper connections for skilled movement control and how the same mechanisms also inform the mechanisms of motor impairments and developmental plasticity after corticospinal system injury: (1) synaptic competition between the CSTs from each hemisphere; (2) interactions between the CST and spinal cord neurons; (3) synaptic competition between the CST and proprioceptive sensory fibres; and (4) interactions between the developing corticospinal motor system and the rubrospinal tract. Our findings suggest that the corticospinal motor system effectively ‘oversees’ development of its subcortical targets through synaptic competition and trophic‐like interactions and this has important implications for motor impairments after perinatal cortical stroke. What this paper adds Neural activity‐dependent processes inform the brain and spinal cord response to injury. The corticospinal motor system may ‘oversee’ development of its downstream subcortical targets through activity, trophic‐like interactions, and synaptic competition. Resumen Plasticidad del sistema motor después de una lesión unilateral del cerebro perinatal En la madurez, las habilidades motoras dependen del tracto corticoespinal (CST) y las vías del tronco encefálico, que conjuntamente, hacen sinapsis con interneuronas y motoneuronas en la médula espinal. Las señales descendientes que llegan a las neuronas espinales que median el control voluntario, pueden distinguirse de las señales sensoriales periféricas, principalmente por el control de retroalimentación. Estos circuitos del sistema motor dependen inicialmente de mecanismos genéticos de desarrollo para establecer sus conexiones y refinamiento sináptico dependiente de la actividad neuronal y del uso durante el período postnatal temprano, para permitir que las habilidad