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Summary: Synaptic contacts between neurons in the central nervous
system exhibit a highly specialized structural arrangement
in which postsynaptic receptors are arranged in tight
clusters, closely apposed to the presynaptic sites of quantal
neurotransmitter release (Siekevitz, 1985; Walmsley, 1991;
Kirsch et al. 1996; Nusser et al. 1997; Colledge et al. 1998;
Walmsley et al. 1998). Considerable variability in the size
and shape of these active zones has been observed, which
may relate to differences in the functional parameters of
individual synapses (Nusser et al. 1997; Schikorski & Stevens,
1997; Walmsley et al. 1998). A fundamental measure of
synaptic function is provided by the postsynaptic current
generated in response to the presynaptic release of a
quantum of neurotransmitter. At an individual synapse, the
amplitude of this quantal postsynaptic current depends
theoretically on the concentration and time course of neuro-
transmitter in the synaptic cleft, and on the transmitter
binding properties of the postsynaptic receptors (Clements,
1996). If the released neurotransmitter approaches a
saturating concentration for the receptors, then the total
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