The commissural and intrinsic connections of the hippocampus were studied using the Fink-Heimer method and the horseradish peroxidase (HRP) uptake technique. A conspicuous septo-termporal gradient was found of the density of the commissural projection that passes through the psalterium ventrale to the Ammon's horn. The degeneration resulting from transection of the psalterium ventrale was most dense in the septal tip and decreased towards the temporal tip. The commissural and ipsilateral connections from the hilus fasciae dentatae (CA4) and regio inferior (CA3/CA2) were found to terminate in different parts of the hippocampus. The hilus fasciae dentatae gave rise to ipsilateral and commissural projections to the dentate area only. The regio inferior has ipsilateral and commissural projections to the Ammon's horn. A specific termination pattern was found of the projection from regio inferior to stratum radiatum of both the ipsilateral and contralateral regio superior (CA1) and regio inferior (CA2/CA3). At levels temporal to the lesion, the projection is primarily to the superficial part of stratum radiatum, while at levels septal to the lesion the terminal zone occupies the deep part of the layer. This pattern was not related to the position of the cells of origin, along the septo-temporal or subiculo-dentate axes. In general, the commissural projections showed the same degree of septo-temporal divergence as the ipsilateral projections. The only major difference in the terminal fields of the two sets projections to the Ammon's horn was that the terminal zone of the commissural projection to stratum oriens was always more dense than that of the ipsilateral projection to this layer, while an inverse gradient was seen in stratum radiatum. The projections from the septal and middle dorso-ventral parts of regio inferior differed. The temporal spread of the projections from the septal part was large while that from the projections arising at middle dorso-ventral levels was more restricted. Moreover, a longitudinal association path interconnecting different parts of the regio inferior along the septo-temporal axis was seen to arise only from the cells in the septal parts of the regio inferior. Each part of the regio inferior projected to all parts of stratum radiatum and oriens of the contralateral Ammon's horn. However, the projection to the contralateral regio inferior was most dense at the site homotopic to that lesioned. The ventricular part of regio inferior projected primarily to the contralateral stratum oriens of the Ammon's horn, while the part adjacent to the dentate area mostly supplied stratum radiatum.
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