The ultrastructure of the nucleus of the oculomotor nerve (somatic efferent portion) of the cat.

The ultrastructure of the somatic efferent portion of the nucleus of the oculomotor nerve was studied in four adult cats. The neuronal population is composed of neurons of variable size. A continuous pattern of morphological aspects is evident between the large neurons, which show abundant cytoplasm with well developed organelles, and the small neurons which have a reduced amount of cytoplasm. The dendrites are generally smooth, with few short spines. Axo-dendritic synapses are numerous. Synaptic boutons are also present on the axon hillock. The neuropil is characterized by the occurrence of small groups of dendrites which may be in direct touch with their membranes. Direct membrane appositions may also occur between neighbouring neurons and between the cell somata and tangentially running dendrites. Generally beneath the site of apposition there is accumulation of mitochondria, multivesicular bodies, coated vesicles and moderately dense amorphous material. The morphological features suggest the possibility of cellular interchanges at the sites of direct membrane apposition. Five types of synaptic boutons were recognized on the basis of their vesicular content, the presence of abundant filaments in the pre-synaptic bag, the occurrence of post-synaptic specializations. The different synaptic types and their distribution are similar to those reported in the spinal motor nuclei. Many of the synapses make synaptic contacts with two or more post-synaptic elements. Axo-axonic synapses were also observed.