Abstract
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, Para ventricular 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.
Highlights
It is well known that the ganglion cells of the retina send fibers to the lateral geniculate body which is an intermediate center of the visual pathway
They have stated that the centrifugal visual system is not a homogeneous entity formed by neurons with a common embryological origin, but rather a collection of at least eight distinct subsystems arising in very different regions of the neuraxis
Iontophoretic administration of the phaseolus vulgaris leucoagglutinin (PhA-L) into the Supraoptic Nucleus (SON) and paraventricular nuclei (PVN) resulted in the appearance of PhA-L containing fibers in the optic nerve. These findings indicate that vasoactive intestinal polypeptide (VIP) fibers observed in the optic chiasm and the optic nerve may originate from the hypothalamus [22]
Summary
It is well known that the ganglion cells of the retina send fibers to the lateral geniculate body which is an intermediate center of the visual pathway. A decade ago the intracerebral location of the retinopetal neurons, the mode of intraretinal arborisations of the centrifugal fibres and the nature of their targets, their neurochemical properties, and the afferent supplies of these neurons were summarized by Reperant et al [18]. They have stated that the centrifugal visual system is not a homogeneous entity formed by neurons with a common embryological origin, but rather a collection of at least eight distinct subsystems arising in very different regions of the neuraxis.
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