Introductory remarks

In every day usage to say that something shows plasticity implies that it can be moulded or readily made to assume a new shape. In relation to the nervous system, we commonly imply a further restriction of meaning when using this term ; plasticity in the nervous system means an ordered or patterned alteration of organization - one which makes some sort of sense biologically. We do not mean just any alteration; for instance, a massive and disorganized malfunction associated with extensive injury would not be called plasticity. To qualify for this term an alteration has to show pattern or order, and we would normally imply that the structures or functions under discussion alter in some way to compensate for the deficit. When the plasticity takes the form of learning or memory, the functional alteration resulting from the input experience must also be organized, this time in a functional sense; functional or structural changes that were chaotic would qualify neither for the term plasticity nor the term learning. Plasticity of neural structure is discussed in several papers at this meeting. Horder, Keating, and Mark consider some of the very dramatic alterations that may be induced in the nervous systems of lower vertebrates, while Rakic, Raisman, and Wall describe structural changes in mammals. Some of the work discussed, notably that on amphibians and fishes, as well as the comparable mammalian work of Lund and Schneider which was unfortunately not represented at the meeting, merges at the edges into the sort of changes normally considered embryological. The question how closely mechanisms underlying structural plasticity are related to those underlying, for instance, embryological regulation, remains unanswered. The nature of neither phenomenon is yet understood.