Summary
Two distinct types of neuron in the cat trochlear nucleus (one large, one small) are described, the α- and γ-motoneurons, respectively. Four types of terminals are observed which establish axo-dendritic synapses. Two of them (Types I and II) perform axo-somatic synapses as well. Terminals en passant (Types I and II) are predominant. The Type I terminal is long and slender with a characteristic distribution of the axoplasmic organelles and the unique feature of a relative narrowing of the synaptic cleft as compared to the width of the neighboring extracellular space. Its vesicle population is pleomorphic and a conspicuous glial barrier surrounds the synaptic zones. The Type II terminal differs slightly from Type I, revealing a wider synaptic cleft and lacking a characteristic distribution of the axoplasmic organelles. The type III terminal is rarely observed performing axo-somatic synapses, but is a common finding in the neuropil. Post-junctional dense bodies are often present in its axodendritic synapses. The Type IV nerve terminal performs axo-dendritic synapses and is characterized by a rich content of large granulated vesicles. Axo-axonal synapses are observed only very rarely.
The synaptic organization of the feline trochlear nucleus is compared with the synaptic morphology of the oculomotor nuclei of inframammalian species (Waxman and Pappas, 1971). In addition to certain similarities (e.g., richness of synapses en passant), significant differences are encountered: the present study provides no morphological evidence for electrotonic transmission in the trochlear nucleus of cat.
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Preliminary report made at the Annual German “Jahresversammlung der Anatomischen Gesellschaft”, Lausanne, Switzerland, 1973.
The authors wish to thank Professor R. Hassler for his valuable discussion. — Dr. W. B. Choi is on leave from the Department of Anatomy, Catholic Medical School, Seoul, Korea.
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Bak, I.J., Choi, W.B. Electron microscopic investigation of synaptic organization of the trochlear nucleus in cat. Cell Tissue Res. 150, 409–423 (1974). https://doi.org/10.1007/BF00220147
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DOI: https://doi.org/10.1007/BF00220147