Reorganization of motor circuits after neonatal hemidecortication.

It is well recognized that a juvenile brain is more plastic than an adult brain and often undergoes better functional recovery following cortical injury. Infants treated with hemispherectomy to cure intractable epilepsy often exhibit restored normal motor function in the extremities contralateral to the lesion. Neuronal mechanisms of functional recovery after such a large cortical damage at a young age have been studied using animals with a similar lesion, hemidecortication. In such animals, descending pathways from the undamaged sensorimotor cortex to the ipsilateral forelimb motoneurons are reorganized as restoring normal motor function of the forelimb contralateral to the injury. Similar aberrant pathways from the motor cortex to the ipsilateral motoneurons are also generated following suppression of cortical activity in the other hemisphere, suggesting the development of contralateral connections in an activity-dependent manner in normal animals. Thus, formation of ipsilateral descending pathways following neonatal hemidecortication might be due to a loss of balance in cortical activity between the two hemispheres. Studies using animal models of neonatal cortical injury can reveal mechanisms of neural development and may help to establish therapeutic strategies to facilitate recovery from human juvenile cortical injury.