Plasticity in the human cerebral cortex: lessons from the normal brain and from stroke.

The adult brain maintains the ability for reorganization or plasticity throughout life. Results from neurophysiological and neuroanatomical experiments in animals and noninvasive neuroimaging and electrophysiological studies in humans show considerable plasticity of motor representations with use or nonuse, skill learning, or injury to the nervous system. An important concept of reorganization in the motor cortex is that of a distributed neuronal network in which multiple overlapping motor representations are functionally connected through an extensive horizontal network. By changing the strength of horizontal connections between motor neurons, functionally different neuronal assemblies can form, thereby providing a substrate to construct dynamic motor output zones. Modulation of inhibition and synaptic efficacy are mechanisms involved. Recent evidence from animal experiments indicates that these functional changes are accompanied by anatomical changes. Because plasticity of the brain plays a major role in the recovery of function after stroke, the knowledge of the principles of plasticity may help to design strategies to enhance plasticity when it is beneficial, such as after brain infarction.