Model of spatio-temporal 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 arborization of CA3 pyramidal cells in the CA1 area and the low probability of finding pairs of connected CA3-CA1 pyramidal cells in vitro. These anatomical differences contribute to a connectivity argument for discrepancies between electrophysiological data recorded in vitro and in vivo. In order to investigate this issue, we have developed a realistic computer model of the Schaffer collateral pathway of the hippocampus and analyzed the spatio-temporal distribution of action potentials along this pathway following three different types of electrical test stimulus. Direct activation of mossy fibers, CA3 pyramidal cells and focal stimulation of CA1 stratum radiatum were investigated. The parameters of the model were selected from available biological data. Spikes in Schaffer collaterals were followed from their onset in the CA3 pyramidal cell initial segment to the last order branches of their axonal tree in two types of configuration: the whole hippocampus and the slice configuration. The anatomical and electropysiological characteristics of the mossy fibre and Schaffer collateral pathways were found to impose strong constraints on the spatio-temporal distribution of action potentials in the CA1 area. Specific projection zones are determined by the spatial localization of the emitting CA3 pyramidal cells. Their position also defines precise time windows during which some CA1 projection zones receive a large number of correlated signals. Moreover, the variability of the delay at the mossy fibre/CA3 pyramidal cell synapse seems to provide the CA1 projection zones with a background level of excitation. Finally, we show how the patterns of activation obtained in the whole hippocampus are different from those obtained in the slice.