Some connections of the entorhinal (area 28) and perirhinal (area 35) cortices of the rhesus monkey. III. Efferent connections.

In this investigation the efferent projections of the entorhinal and prorhinal cortices relative to their sites of termination in the hippocampus and fascia dentata were investigated in the rhesus monkey using experimental silver impregnation methods. Contrary to the often cited observations of Lorente de No, all entorhinal areas, including the laterally lying prorhinal cortex, were found to give rise to the perforant pathway, and furthermore, each cytoarchitectonically defined subarea was found to contribute a unique component. These perforant pathway components terminate in distinct regions of the dendritic zones of the fascia dentata granule cell and the hippocampal pyramidal cell. A previously undescribed projection to the prosubiculum and hippocampus has been found to originate from the prorhinal cortex which forms the medial wall of the rhinal sulcus along the lateral-most portion of the entorhinal cortex in the rhesus monkey. These results, in conjunction with our previous observations regarding differential afferents to the entorhinal cortex, indicate that specific afferent and efferent connections characterize each cytoarchitectonically definable subareas of this periallocortical region. Additionally, they indicate that the perforant pathway might be conceptualized as the final link in a multisynaptic series of connections instrumental in providing the hippocampus with potential modality specific and multimodal input.