One of the major goals of neuroscience has been the cellular mechanisms of learning and memory. Before about a century, a lot of anatomists, physiologists and psychologists suggested that the key elements concerning with the learning may be the synapses of the neurons. In the last decade, the physiological bases of synaptic plasticity have been well characterized. One of them is the plasticity of synaptic transmission, especially the long-term potentiation after tetanic stimulation of synaptic inputs. Another important nature of plasticity is the axonal sprouting and formation of new synapses, which occur not only after partial denervation of the synaptic input but also without brain lesion. Functional significance of dendritic spines has been also suggested, and it was recently discussed theoretically whether the morphorogical change in the spine produces an effective change in the synaptic potential. On such elemental basis of neuronal plasticity, the physiological studies on learing have been well developed using many preparations, for example, classical conditioning mediated by the simple, identified pathways in invertebrates and vertebrates.
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