The raphe-reticular connection. An experimental study using the silver impregnation and horseradish peroxidase techniques in the rat

The raphe-reticular connection in the rat was studied using the silver impregnation and peroxidase techniques. The raphe nuclei, were found to constitute two independent groups: the oblongata (ncl. raphealis magnus et parvus) and the pontomesencephalic groups (nuclei lineares, ncl. raphealis dorsalis and ncl. centralis superior). Inside each group the nuclei of either group are interconnected through the raphe-raphe projection but no such raphe-raphe connection exists between the two groups. The raphe reticular connection follows much the same pattern of development in either group: each group of raphe nuclei supplies the medial as well as the lateral reticular formation at its frontal level and at all more rostral levels. Through their efferents the raphe nuclei of the oblongata supply the ncl. ventralis medullae oblongate, the ncl. gigantocellularis, ncl. pontis caudalis et oralis, ncl. pedunculo-pontinus, ncl. cuneiformis, and area prerubralis. The raphe nuclei of the oblongata do not send any projections to the more rostral levels. In the small-cell, lateral reticular formation the raphe efferents of the oblongata contribute to the ncl. dorsalis medullae oblongate and ncl. parvocellularis. The raphe-reticular fibres run in a diffuse bundle localized in the dorsolateral parts of the reticular formation along the dividing line between the medial and lateral reticular formations. The ponto-mesencephalic raphe nuclei send their efferents into the ncl. pontis oralis, ncl. pedunculo-pontinus, ncl. cuneiformis, and into the area prerubralis. The ascending fibres then extend on rostrally into the diencephalon. The raphe-reticular bundle of these fibres passes diffusely through the dorsomedial portions of the reticular formation. Most of the efferents of the raphe nuclei of the ponto-mesencephalic group arise from the ncl. raphealis dorsalis and ncl. linearis caudalis. The ncl. centralis superior sends mainly raphe-raphe connections to the two above mentioned nuclei. The rest of the efferents of the ncl. centralis superior are merely poorly developed.