Recent Research on the Centrifugal Visual System in Mammalian Species

Ample evidence indicates that both retinofugal (classical visual and the retino-hypothalamic pathways) and retinopetal connections (centrifugal visual system) are found between the eye and the central nervous system. More than hundred years ago Ramon Y Cajal and Dogiel, whose names are very well known by neuro-anatomists, described the termination pattern of the fibers deriving from the avian central nervous system. However, the location of nerve cell bodies was not known at that time. In the last century many data accumulated about these neurons not only in lower vertebrates but in mammals as well. The structures where the neurons give rise to the centrifugal visual fibres in mammals are the following: reticular formation and raphe nuclei of the midbrain, superior colliculus, pretectum, gray matter of the midbrain, dentate gyrus, CA1 and CA3 regions of the hippocampus, olfactory tubercle, habenula, indusium griseum, hypothalamic supraoptic, Paraiventricular and arcuate nuclei, and the lateral hypothalamus. The centrifugal visual fibers enter the optic nerve layer, then reach the inner plexiform layer and terminate in the inner nulear layer of the retina in the vicinity of the amacrine cells. A series of neuropeptides and neurotransmitters was described in the origin of the centrifugal visual system. These are the followings: luteinizing hormone releasing hormone, pituitary adenylate cyclase activating polypeptide, vasoactive intestinal polypeptide, serotonin, histamine and leu-enkephalin. Several hypotheses arose on the function of this system. Centrifugal visual system arising from the histaminergic mammillary neurons modifies the sleep/wake cycle. Hallucinogenic drugs through the limbic system may cause disturbance of visual function and result in seeing visual hallucinations or distorted images.