An autoradiographic study of the organization of intrahippocampal association pathways in the rat

Abstract The longer associational connections of the hippocampal formation have been studied autoradiographically in a series of adult rats after small injections of 3 H‐amino acids into each of its various cytoarchitectonic fields. The major findings can be summarized as follows. The dentate gyrus projects in a topographically ordered manner upon the pyramidal cells of the regio inferior by way of the supra‐ and infrapyramidal bundles of mossy fibers. Certain cells in the hilar region of the dentate gyrus (which operationally may be defined as constituting field CA 4 of Ammon's horn) give rise to a hippocampodentate projection to the inner one‐quarter of the molecular layer of the dentate gyrus. Either the same or closely related cells give rise to fibers which join the Schaffer collateral system from field CA 3 to the stratum radiatum and stratum oriens of the regio superior . The regio inferior is also characterized by a longitudinally directed associational bundle which runs throughout the septo‐temporal extent of the hippocampus and is centered in the region of subfield CA 3a . The regio superior has no reciprocal projection to the regio inferior but sends a substantial projection back to the subiculum and to the entorhinal area. There is also a projection to the subiculum from the regio inferior , and the subiculum itself probably contributes significantly to the projection to the entorhinal and perirhinal cortices. There is a striking parallelism between certain of these associational connections and the commissural projections to the hippocampus and dentate gyrus. Each cytoarchitectonic field that contributes a commissural projection also gives rise to an ipsilateral associational pathway which in its intrahippocampal course and its mode of termination exactly matches that of the commissural projection, although in general, the associational connections are more extensive in their distribution along the septo‐temporal extent of the hippocampus than the corresponding commissural connections. The reverse is not true; there are a number of associational projections which are not paralleled by commissural projection. All of the associational projections are topographically arranged, but those which extend across the transverse axis of the hippocampus usually show considerable divergence so that afferents from different levels overlap fairly considerably within their respective projection fields.