Stroke is one of the leading causes of disability across the world. Although the consequences of stroke can be devastating, many patients show varying degrees of spontaneous recovery. This spontaneous recovery can be improved with rehabilitation, allowing patients to regain higher levels of quality of life following such an event. Recovery after stroke is attributed to brain plasticity, and the ability for brain circuits to be 'rewired.' A recent review, published in Nature Reviews (2009), evaluated the evidence for this remarkable ability.
The review looks at studies revealing that stroke recovery is based on both structural and functional changes in brain circuits. There are two main principles which form the basis for these changes. Firstly, the central nervous system (CNS) is composed of diffuse and extensive connections between populations of cells and many of these connections can perform the same tasks. Therefore, intact connections can often compensate for damaged ones. Secondly, new structural and functional circuits can be created through re-mapping related regions of the brain.
Initially it was believed that connections between neurons were made in early development and then refined and consolidated through the lifetime. Recent research has led to this notion being changed, due to the fact that cognitive processes, such as memory, function through widespread connections across the cortex, rather than being specific to certain areas.
During ischaemic stroke, blood supply is interrupted or cut off within a specific region. While these cells undergo neural death which is irreversible, they are bordered by relatively undamaged areas of cortex, which are the site of structural and functional remodelling after stroke.
Animal models of stroke have helped to demonstrate how this remodelling is based on competition for territory between the partially intact 'peri-infarct' cortex and healthy adjacent tissues. The undamaged areas compete to take over the activity of the region damaged by the stroke. In the case of larger strokes, as adjacent tissues may also be affected, this competition is spread over more distant connected sites.
Understanding the circuits which alter after stroke can help to inform improvements in stroke rehabilitation. The idea of competition between healthy tissues and the peri-infact cortex for cortical mapping highlights once again how the different parts of the brain work together, not only to function but also to recover. The findings also demonstrate the importance of early rehabilitation in order to encourage these natural recovery mechanisms as much as possible.
Reference
Murphy TH, Corbett D (2009) Plasticity during stroke recovery: from synapse to behaviour. Nature Reviews 10: 861-872
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