Spatial organization of premotor neurons related to vertical upward and downward saccadic eye movements in the rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF) in the cat.

The rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF) contains premotor neurons that are related to the control of vertical and torsional saccadic eye movements. In the present study, complimentary light microscopic anterograde biocytin and retrograde horseradish peroxidase experiments have been performed to determine the organization of premotor neurons in the riMLF in the cat that are related intimately to the vertical motoneuron populations in the oculomotor and trochlear nuclei. The results indicate a rostral-caudal topographic arrangement of neurons in the riMLF that is related to the target projections to vertical downward (inferior rectus and superior oblique) and vertical upward (superior rectus and inferior oblique) motoneurons, respectively, in the oculomotor and trochlear nuclei. Both the anterograde and the retrograde studies are consistent, in that they demonstrate the tendency for downward and upward riMLF neurons to be separated spatially by a distance of approximately 0.5 mm in the rostral-caudal axis of the nucleus. The riMLF projections to inferior oblique and superior oblique motoneurons are predominantly ipsilateral. Projections to inferior rectus and superior rectus motoneurons, however, are bilateral, and, presumably, they provide one means for assuring the conjugacy of vertical saccadic eye movements. Because premotor burst neurons that encode parameters for upward or downward saccades are intermingled within the riMLF, and excitatory and inhibitory premotor neurons also coexist in this region, the findings from this study suggest that subregions of the riMLF contain coexistent populations of excitatory and inhibitory neurons that are related to opposite directions of vertical eye movements. The spatial segregation of excitatory premotor neurons in the riMLF that are related to vertical upward vs. downward movements, furthermore, provides a basis for the interpretation of vertical upward and/or downward gaze palsies that might result from discrete lesions at the mesodiencephalic junction in humans.