Use-dependent plasticity of the human motor cortex in health and disease

The adult brain can reorganize to accommodate environmental modifications and to compensate for lost function, a process known as plasticity. The last few years have provided extraordinary evidence regarding the mechanisms underlying plastic changes and have spurred the development of new strategies to modulate these processes. The ability of the mature brain to constantly reorganize has gained further credibility with the advent of transcranial magnetic stimulation (TMS). Plasticity is thought to be influenced by neurotransmitters and decreases substantially with normal aging. TMS has demonstrated that remarkable plastic changes occur in response to pharmacological agents and peripheral or central nervous system injury. Plastic changes are also associated with intense motor training such as constraint-induced therapy (CIT). For instance, premedication with amphetamine resulted in faster induction, increased magnitude, and prolonged duration of plasticity elicited by motor training compared to a placebo. Similarly, peripheral sensory stimulation resulted in enhancement of plastic changes. Finally, TMS cortical motor maps enlarge after CIT in stroke patients. This work discusses the potential of TMS as a useful tool to probe novel strategies to enhance plasticity in humans.