MORPHOLOGY AND SOMA-DENDRITIC DISTRIBUTION OF SYNAPTIC ENDINGS FROM THE ROSTRAL INTERSTITIAL NUCLEUS OF THE MEDIAL LONGITUDINAL FASCICULUS (RIMLF) ON MOTONEURONS IN THE OCULOMOTOR AND TROCHLEAR NUCLEI IN THE CAT

The morphology and soma-dendritic distribution of anterograde biocytin -labelled rostral interstitial nucleus of the medial longitudinal fasc iculus (riMLF) synaptic endings in the oculomotor and trochlear nuclei have been examined by electron microscopy by using both preembedding immunoperoxidase and postembedding immunogold methods. The results ind icate that three morphological types of riMLF synaptic endings are dis tinguishable on the basis of synaptic vesicle morphology (spheroidal, pleiomorphic, or ellipsoidal) and postsynaptic membrane specialization s (asymmetrical or symmetrical). All three morphological types of riML F synaptic endings establish synaptic connections predominantly with d endrites. Synaptic endings that contain ellipsoidal synaptic vesicles have a more proximal soma-dendritic distribution than those that conta in either spheroidal or pleiomorphic synaptic vesicles. Furthermore, a ll three morphological types of synaptic endings are encountered in th e same motoneuron subdivisions of the oculomotor and trochlear nuclei in the same experiments. The findings suggest that subregions of the r iMLF contain coexistent populations of excitatory and inhibitory premo tor neurons that are related to opposite directions of vertical saccad ic eye movements but that project to the same motoneuron subgroups on the ipsilateral side. Both the morphology and the mode, pattern, and s oma-dendritic distribution of saccade-related riMLF synaptic endings t hat establish synaptic connections with vertical motoneurons differ fr om those of excitatory and inhibitory second-order vertical vestibular synaptic endings. These differences in the synaptic organization of r iMLF and second-order vestibular inputs to oculomotor and trochlear mo toneurons may be related to differences in the information transferred by each source, the riMLF input conveying eye-velocity signals, and t he vestibular input conveying eye-position signals. (C) 1996 Wiley-Lis s, Inc.