MODEL OF SPATIOTEMPORAL PROPAGATION OF ACTION-POTENTIALS IN THE SCHAFFER COLLATERAL PATHWAY OF THE CA1 AREA OF THE RAT HIPPOCAMPUS

There is a sharp contrast between the profuse in vivo axonal arborizat ion of CA3 pyramidal cells in the CA1 area and the low probability of finding pairs of connected CA3-CA1 pyramidal cells in vitro. These ana tomical differences contribute to a connectivity argument for discrepa ncies between electrophysiological data recorded in vitro and in vivo. In order to investigate this issue, we have developed a realistic com puter model of the Schaffer collateral pathway of the hippocampus and analyzed the spatio-temporal distribution of action potentials along t his pathway following three different types of electrical test stimulu s. Direct activation of messy fibers, CA3 pyramidal cells and focal st imulation of CA1 stratum radiatum were investigated. The parameters of the model were selected from available biological data. Spikes in Sch affer collaterals were followed from their onset in the CA3 pyramidal cell initial segment to the last order branches of their axonal tree i n two types of configuration: the whole hippocampus and the slice conf iguration. The anatomical and electrophysiological characteristics of the messy fibre and Schaffer collateral pathways were found to impose strong constraints on the spatio-temporal distribution of action poten tials in the CA1 area. Specific projection zones are determined by the spatial localization of the emitting CA3 pyramidal cells. Their posit ion also defines precise time windows during which some CA1 projection zones receive a large number of correlated signals. Moreover, the var iability of the delay at the massy fibre/CA3 pyramidal cell synapse se ems to provide the CA1 projection zones with a background level of exc itation. Finally, we show how the patterns of activation obtained in t he whole hippocampus are different from those obtained in the slice. ( C) 1997 Wiley-Liss, Inc.