The accessory optic system and the retino-hypothalamic system. A review

The introduction of new techniques in neuroanatomy has helped to clarify a number of important issues on the nature of the accessory optic and retino-hypothalamic system. In this study, an effort was made, to collect all data obtainable, and to arrange them on the basis of some common parameter, so that the various results may be directly compared. These data were then interpreted on specific issues, although only limited coverage has been given to functional aspects, as described in older publications. The absence of a universally accepted concept of the function of both systems to date suggests the need for a thorough analysis of both contemporary and older literature dealing with a characterization of the structural components of the accessory optic and the retino-hypothalamic systems. On the basis of these anatomical studies it is concluded that the accessory optic system (AOS) is much more complex than formerly believed. Its development in various vertebrae classes is quite different. Among the mammals, the AOS is very well established in rodents but in primates and man its anatomical and functional role seems markedly reduced. In the rodents it consists of both superficially and deeply arranged axons which are in close topical relation to the cerebral peduncles. The axons not only supply the classic accessory otpic terminal nuclei but also the subthalamic nucleus and the substantia nigra. The secondary projections have not yet been established. The retino-hypothalamic system (RHS) mainly but not exclusively supplies the suprachismatic nucleus (sc). Crossed as well as uncrossed retinal fibres project to the ventro-lateral and to the posterior part of this nucleus. The cells within the sc combine both secretory and neuronal properties. The possible functions exerted are discussed. Both, the AOS and the RHS, are so closely related within the suprachiasmatic area that most methods fail in correlating experimental results with either system. The numerous problems which still remain to be clarified are also discussed.