1. Factors contributing to the efficacy of transmission in synapses located in the middle third of the molecular layer of the rat fascia dentata have been investigated using combined intra- and extracellular recording in the in vitro hippocampal slice preparation. 2. The mean excitatory postsynaptic potential (EPSP) size necessary to discharge a granule cell was 24 ± 9 mV. The mean EPSP resulting from single afferent fiber activation was estimated as 0.1 mV. 3. The EPSP at the granule cell discharge threshold was corrected for nonlinear summation by extrapolating the initial slope of the relation between EPSP and presynaptic fiber potential (a measure of the number of active fibers contributing to the EPSP). The ratio of the corrected EPSP amplitude at threshold to the mean single-fiber response gave an estimate of 400 as the number of medial perforant path fibers required to discharge the granule cells. 4. EPSP quantal contents were estimated using the statistical characteristics of the EPSP amplitude fluctuations and by a method involving Fourier analysis of the EPSP amplitude histograms. It was concluded from these analyses that the mean quantal content of the average single-fiber EPSP was close to 1. 5. Evidence is presented that the summation of quantal EPSPs is much more nonlinear for quanta released at a single synaptic site than for quanta released at different sites.
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