Projections from the superior colliculus to a region of the central mesencephalic reticular formation (cMRF) associated with horizontal saccadic eye movements.

Radioactive wheatgerm agglutinin (WGA) and horseradish peroxidase (HRP) were injected into portions of the mesencephalic reticular formation at sites where electrical stimulation induced either small or large contralateral horizontal saccadic eye movements. We have designated this region as the Central MRF (cMRF). It contains both cells and fiber tracts, including the efferent output of the superior colliculus (SC), destined for the dorsal tegmental decussation and the predorsal bundle. Cells labelled by WGA and HRP injections were found in the intermediate and deep layers of the superior colliculus and the adjacent central gray matter on the ipsilateral side. Injections into the dorsal cMRF, at sites where small saccades were induced, caused labelling of cells in the rostral intermediate layer of SC. Injections into the ventral cMRF, at points where large saccades were elicited, caused labelling of cells in the caudal intermediate layer of SC. The deepest layers of SC and the adjacent central gray were also labelled from the small eye movement region of dorsal cMRF. We interpret these findings to indicate that the intermediate layers of SC send axonal projections to the horizontal eye movement region of the MRF in a topographic fashion. The projection from the intermediate layer is organized so that regions in SC and cMRF related to small or to large eye movements are interconnected. The results support the hypothesis that cMRF is a topographically organized area, involved, like SC, in the control of eye movements. Since both cMRF and the superior colliculus project to areas of the pons and medulla where saccadic eye movements are produced, they could give rise to parallel pathways for the generation of contralateral saccades.