Individual neurones may show plasticity or die during the course of the normal development of the nervous system as well as in disease or following injury. In this final section we bring some of these issues together. Thus Diamond and Gloster describe influences on the plasticity of peripheral nerves, which they argue represent, at least in part, influences at work in the normal CNS. The chapters by Johnson and by Munson and Nishimura also concern influences from the periphery, but this time largely on the properties of the central parts of the neurones, either their connections or their ultrastructure: an important issue in both cases is the restitution of function following injury. In the next chapter Nicholls describes a new preparation: the CNS of the neonatal opossum maintained in tissue culture. The opossum emerges from the womb at a very immature stage, in which it seems that central nerve fibres can regenerate following injury, like peripheral nerves but unlike those of the adult CNS. This preparation should provide a new base for direct study of the influences operating on plasticity in the CNS.

The final two chapters, by Pullen and by Krieger, extend the theme of neuronal survival from Johnson's chapter to the context of motor neurone diseases, conditions characterized by the selective degeneration of neurones in cortico-motor pathways. It is likely that only by understanding those factors that normally allow these neurones to survive and to repair following injury or insult will we understand why in these devastating diseases they do not.