Origin and termination of the hippocampal perforant path in the rat studied by silver impregnation

Abstract A detailed study of the origin and termination of the so‐called perforant path of the hippocampal region has been made in the rat. This tract connects the entorhinal area with the hippocampus and the fascia dentata. At all dorso‐basal levels of the hippocampus, the terminal field has been shown to occupy the middle part of the molecular layer of the fascia dentata and that level of the stratum lacunosum‐moleculare of the hippocampal subfield CA3 which is close to the stratum radiatum. The outer and inner parts of the dentate molecular layer, the stratum lacunosum‐moleculare of subfield CA1, and the superficial part of the latter layer in CA3 contained fibers en passage only. The origin of the perforant path has been found to be in the medial part of the entorhinal area, from the most dorsal to the most ventral levels. The fibers arise in part at least from cells in layers I—III. Lesions of the lateral part of the entorhinal area leave the perforant path unaffected. A topical organization has been demonstrated: Lesions dorsal in the entorhinal area evoke terminal degeneration in antero‐rostral (septal) parts of the hippocampus only. More ventral lesions produce degeneration in increasingly caudal (temporal) segments of the hippocampus. Earlier descriptions of the route traversed by the perforant path have been confirmed. In addition, the axons to the most rostral parts of the hippocampus and fascia dentata have been shown to course superficially in the dorsal and rostral parts of the subiculum and CA1 which form an exposed part of the medial aspect of the hemisphere.