SPATIAL-ORGANIZATION OF PREMOTOR NEURONS RELATED TO VERTICAL UPWARD AND DOWNWARD SACCADIC EYE-MOVEMENTS IN THE ROSTRAL INTERSTITIAL NUCLEUSOF 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, c omplimentary light microscopic anterograde biocytin and retrograde hor seradish peroxidase experiments have been performed to determine the o rganization of premotor neurons in the riMLF in the cat that are relat ed intimately to the vertical motoneuron populations in the oculomotor and trochlear nuclei. The results indicate a rostral-caudal topograph ic arrangement of neurons in the riMLF that is related to the target p rojections to vertical downward (inferior rectus and superior oblique) and vertical upward (superior rectus and inferior oblique) motoneuron s, respectively, in the oculomotor and trochlear nuclei. Both the ante rograde and the retrograde studies are consistent, in that they demons trate the tendency for downward and upward riMLF neurons to be separat ed spatially by a distance of approximately 0.5 mm in the rostral-caud al axis of the nucleus. The riMLF projections to inferior oblique and superior oblique motoneurons are predominantly ipsilateral. Projection s to inferior rectus and superior rectus motoneurons, however, are bil ateral, and, presumably, they provide one means for assuring the conju gacy of vertical saccadic eye movements. Because premotor burst neuron s that encode parameters for upward or downward saccades are interming led within the riMLF, and excitatory and inhibitory premotor neurons a lso coexist in this region, the findings from this study suggest that subregions of the riMLF contain coexistent populations of excitatory a nd inhibitory neurons that are related to opposite directions of verti cal eye movements. The spatial segregation of excitatory premotor neur ons in the riMLF that are related to vertical upward vs. downward move ments, furthermore, provides a basis for the interpretation of vertica l upward and/or downward gaze palsies that might result from discrete lesions at the mesodiencephalic junction in humans. (C) 1996 Wiley-Lis s, Inc.