Some cortical and subcortical fiber projections to the accessory optic nuclei in the cat

Abstract

The terminal nuclei of the accessory optic system receive a dense fiber projection from the contralateral retina and have recently been implicated in visuo-vestibular aspects of oculomotor function. The present anatomical study, based on anterograde tracer experiments with 3H-labeled amino acids or horseradish peroxidase, provides evidence for a diverse set of non-retinal inputs to the accessory optic nuclei arising in both the neocortex and the brainstem. Corticofugal fiber projections to the terminal nuclei originate in several areas of the visual cortex; those from the medial Clare-Bishop area may be especially dense. Subcortical inputs to the terminal nuclei include fiber projections arising in ( a) the pretectal region; ( b) the ventral mesencephalon, probably including the substantia nigra; and ( c) more dorsal or caudal parts of the mesencephalic reticular formation. The source of the pretectal projection to the terminal nuclei appears to be the nucleus of the optic tract or the underlying posterior pretectal nucleus. Fiber projections from the ventral mesencephalic tegmentum were labeled by injections infiltrating the substantia nigra, but these deposits all involved, at least marginally, either interstitial cells of the transpeduncular limb of the accessory optic tract or the perinigral tegmentum. The part of the midbrain reticular formation giving rise to an accessory optic projection was not precisely delimited, but it appears to lie outside the substantia nigra proper. Although these afferent projections to the accessory optic nuclei appear individually to be much weaker than the input from the contralateral eye, their presence clearly suggests that transmission of visual information through the terminal nuclei may involve a complex modulation of retinal input by non-retinal pathways.