Connections of the parahippocampal cortex in the cat. V. Intrinsic connections; comments on input/output connections with the hippocampus

Abstract

The present report is the last in a series of papers on the connectivity of the parahippocampal cortex in the cat, which in this species is considered to be composed of the entorhinal and perirhinal cortices. Injections of anterogradely transported tritiated amino acids and the retrograde tracers HRP, WGA-HRP, fast blue, or nuclear yellow were placed within the limits of the parahippocampal cortex. An analysis was made of the resulting pattern of anterograde labeling and of the distribution of retrogradely labeled neurons within the parahippocampal cortex. It appears that within the parahippocampal cortex of the cat a framework exists, which is composed of longitudinal and transverse connections, organized according to three principles: (1) Medially directed projections originate mostly in superficial layers, whereas laterally directed fibers come from deep layers. (2) The longitudinal connections span the entire rostrocaudal extent of the parahippocampal cortex, whereas the mediolateral extent of the transverse connections is in general more restricted. Based on the organization of these longitudinal and transverse connections four longitudinal zones are recognized. (3) The lateral entorhinal cortex (LEA) projects both within the entorhinal cortex and to the perirhinal cortex, whereas the intrinsic projections of the medial entorhinal cortex (MEA) are confined to the entorhinal cortex. These results are discussed in conjunction with the main organizational features of the afferent and efferent connections of the parahippocampal cortex of the cat. The premise is made that the cytoarchitectonically defined subdivisions of the cortex can be grouped into four areas, each with its own set of fiber connections and subserving different functional roles. (a) A lateral area, constituted by the perirhinal areas 35 and 36, and the caudally adjacent postsplenial cortex, serves as a peripheral area through which the rest of the parahippocampal cortex---i.e., LEA and MEA, and ultimately the hippocampal formation---reciprocally communicates with extensive neocortical, subcortical, and thalamic regions associated with higher-order behavior. (b) The medial part of LEA, constituted by the ventrolateral (VLEA) and ventromedial (VMEA) divisions, has reciprocal connections with the hippocampal formation and with the cortex, partly via the perirhinal cortex, and is connected with a number of subcortical structures such as the amygdala and the striatum. (c) The medial entorhinal area (MEA), by having extensive reciprocal connections with the hippocampal formation and with other portions of the entorhinal cortex, may constitute an important structure throughwhich the hippocampal formation regulates its major cortical inputs. (d) The lateral part of LEA–i.e., its dorsolateral division (DLEA)-receives most of its inputs from other parts of the entorhinal cortex and the hippocampal formation and projects to the perirhinal cortex and a number of cortical and subcortical structures, which are also reached by fibers from the remaining parts of LEA. It is therefore conceivable that DLEA is involved in the regulation of parahippocampal input and/or the modulation of hippocampal outputs to the cortical and subcortical input structures.