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Abstract
The excitatory synapses onto CA1 pyramidal cells have become a model system for understanding
the activity-dependent changes in synapses that underlie learning and memory. Here
we examine physiological and anatomical results that are relevant to understanding
the mechanisms of synaptic transmission and plasticity at these synapses. Three main
points are discussed. First, quantal analysis indicates a large heterogeneity of postsynaptic
efficacies for different synapses on the same cell. Reconstructions from electron
microscopy show that synapse size is also highly heterogeneous. Reasons for suspecting
a relationship between synaptic size and efficacy are discussed. Second, physiological
evidence indicates that the changes during long-term potentiation are both pre- and
postsynaptic. Similarly, several lines of anatomical evidence suggest that plasticity
affects the structure of both the pre- and postsynaptic elements. The detailed registration
of structures across the synapse and the physical linkage between pre- and postsynaptic
elements suggest a 'structural unit hypothesis' for coordinating pre- and postsynaptic
modifications. Third, quantal analysis indicates that stimulation of a single axon
can release multiple quanta. Anatomical evidence shows that cell pairs can be connected
by multiple synapses, suggesting that multiple quanta may be released at independent
sites. These results raise the possibility that one component of synaptic plasticity
is mediated by changes in the number of functional synaptic sites.