ABSTRACT
The concept that the central nervous system (CNS) loses its plastic characteristics after development and becomes a rigid network of permanently fixed neurones has been surrendering credibility progressively throughout the twentieth century. Hebb (1949) introduced the idea that behavioral adaptation throughout life might rely on activity-dependent modification of the configuration and strength of connections between neurones. Further experiments demonstrated that the brain’s chemistry and anatomy are indeed sculpted by the environment and experience suggesting that the adult brain
retains some of the plasticity it has widely been believed to have lost following the end of development and in the postmitotic state. However, ‘‘plasticity’’ must be defined in the context in which it is being used, because its presence or absence can be investigated at multiple levels of organization from the molecular level to changes in behavior of the whole organism. In this chapter, we examine the plasticity of the adult brain, damaged as a result of disease or injury, and we discuss the degree to which brain repair can be achieved by intracerebral transplantation of neurones from an external source into this altered and extraordinary state. By plasticity in this context, we refer to adaptive events affecting the morphology, anatomical connections, and the functions subserved by neurones. Particular attention is paid to the relationship between host and transplanted cells, the anatomical integration of the grafted cells, and how circuit reconstruction within the host brain might promote functional recovery. Furthermore, we examine the role of the environment and the experience of the individual suffering from brain damage or disease. Together, these data indicate that rehabilitative training has the potential to modify the functional plasticity both of the affected brain and of the transplants. The different mechanisms by which cells from different sources, and following different types of brain damage, can affect host function will also determine their dependence on training and experience for maximizing therapeutic outcome.
