Synaptic plasticity is manifested by long-lasting changes in synaptic potency. Increased potency is of special importance in relation to the neural basis of memory. Even mild repetitive stimulation evokes large and prolonged potentiations of hippocampal synapses, as is revealed by electrical recording either in vivo or in vitro. More importantly there is a matching hypertrophy of the synaptic spines on the dendrites of hippocampal granule cells, as is shown by most convincing electronmicrographs. The increase is to about 40% some 2 h after a mild stimulation (900 impulses) and there was relatively little decline by 23 h. There is brief reference to the metabolic processes that could be involved in this plastic response.
Memory is dependent on long-enduring changes in the brain. There is general agreement that there are no gross structural changes, for example, in the growth of new pathways or of assemblages of nerve cells. All of these basic structures and connectivities are grown by genetic instructions prior to their usage. So in the learning process more subtle and prolonged changes in the synaptic communications (cf. Fig. 1 B) between nerve cells have to be involved. Thus in the first place the study of memory resolves itself into an investigation of synaptic transmission in order to see if bursts of activity are followed by long-lasting increases in synaptic potency that could be the elemental basis of memory. Such changes can be regarded as manifestations of synaptic plasticity. For several decades such plastic changes have been sought with little success in the central nervous system at simple levels, for example in the spinal cord. Even after most severe stimulation the increase in synaptic potency was evanescent, lasting only 6 min after 20 000 impulses and 2 h after 600 000 impulses at