Distribution of commissural afferents to the hippocampus of the rabbit

Abstract

AbstractThe commissural connections of the hippocampus (fascia dentata and Ammon's horn) were investigated in the rabbit using a Fink‐Heimer silver impregnation method after the transection of either or both hippocampal commissures. In some cases additional sections were processed either with a histochemical method for acetyl cholinesterase or with the Timm‐Haug sulfide silver method.The fibers to the fascia dentata arrive through the ventral commissure and terminate in a juxtagranular zone that corresponds to a relatively pale zone in acetyl cholinesterase preparations and to a dark zone in Timm‐Haug sulfide silver preparations. The degeneration is more dense in the middle of the Ushaped cross‐sectional profile of the fascia dentata than at the edges. This is particularly evident in the ventral portion of the fascia dentata. There is no evidence of commissural terminals in the layer of polymorphic cells. The commissural fibers to the Ammon's horn pass through both commissures. The fibers in the dorsal commissure terminate in the strata radiatum, oriens and pyramidale at their transition to the subiculum, in the septal part of the Ammon's horn. The molecular layer contains relatively little degeneration after transection of the dorsal commissure with the exception of the crossed entorhinal efferents to the rostral part. Fibers from the ventral commissure terminate in both the stratum oriens and the stratum radiatum. The degeneration in the stratum oriens is dense through the full extent of the layer, while that in the stratum radiatum is maximal in the regio inferior near the fimbria and less dense close to the fascia dentata and in the regio superior, with the exception of the antero‐rostral extreme of the latter. The density of degeneration in the stratum radiatum diminishes in the postero‐ventral direction except for degeneration at the subicular and dentate ends of the layer.Evidence is presented for a powerful ipsilateral longitudinal association path terminating in the stratum radiatum of the regio inferior and in a minor adjacent portion of the dentate commissural zone.