One of the remarkable characteristics of animals is that much of their complex behaviour can be modified as a result of experience. In ourselves we see this in such diverse activities as avoiding foods that we do not like, learning to ride a bicycle, being able to identify new faces and new voices, memorizing a new telephone number, remembering what happened last week or many years ago, and so on. Our learned capabilities and our memories are the basis of our individual personalities.

What happens in the nervous system when such changes take place? What is the cellular basis of learning and memory? Since the work of Ramon y Cajal (1911), it has seemed likely that modifications of the effectiveness of transmission at synapses might provide the answer to this question. Hebb (1949) produced a particular model for this: he assumed that repetitive activity at a particular synapse could produce lasting cellular changes. More precisely, as Hebb put it, ‘when an axon of a cell A is near enough to excite a cell B and repeatedly or persistently takes part in firing it, some growth process or metabolic change takes place in one or both cells such that A's efficiency, as one of the cells firing B, is increased’.

Learning-related changes in nerve cells have been investigated in a number of model systems. We begin by considering in some detail one system that has proved particularly fruitful, and this is followed by a briefer treatment of some other aspects.