A comparison of the mode of termination of the hippocampal and hypothalamic afferents to the septal nuclei as revealed by electron microscopy of degeneration

Summary Previous studies with the Nauta technique have established that fibres which originate in two important areas — the hippocampus and the hypothalamus — converge upon the cells of the septal nuclear complex. The purpose of this study was to investigate the anatomical basis of how the septal cells could differentiate between fibres from the two sources. Differences in the mode of termination of these two systems have been studied quantitatively at the electron microscope level by using the orthograde degeneration of terminals after lesions of the fimbria and the medial forebrain bundle. In the medial septal nucleus, the hippocampal fibres account for 35% of the terminals, and in the lateral septal nucleus, 43% of the terminals on the same side and a further 13% on the opposite side. These terminals are at least 98% axodendritic and 91% of them contain predominantly clear synaptic vesicles of 500 Å diameter. The hypothalamic fibres are the source of up to 19% of the axodendritic terminals in the medial septal nucleus, but considerably fewer in the lateral septal nucleus. In contrast to the hippocampal afferents, the hypothalamo-septal system has two characteristic features: firstly, the fibres give rise to up to 24% of the axosomatic terminals in the medial septal nucleus, and secondly, 63% of the terminals contain a population of vesicles with significantly higher proportions of dense centred vesicles of 800–1000 Å diameter.