The fine structure of the "spoon" type synaptic endings of the chick tangential nucleus was studied with the electron microscope. These endings often measure ∼18 µ in length by ∼3–4 µ in width. The axoplasm of the endings contains very few synaptic vesicles, a large number of neurofilaments oriented parallel to the long axis of the nerve fiber, and microtubules and numerous mitochondria. The synaptic membrane complex shows areas of localized occlusion of the synaptic cleft with the formation of an external compound membrane. It has not been decided whether these areas have a disc shape; their length measures between 0.04 and 0.47 µ. The five-layer pattern characteristic of an external compound membrane is shown in specimens fixed with formalin—OsO4, glutaraldehyde—acrolein—OsO4, and acrolein KMnO4 but it does not appear in the glutaraldehyde-OsO4-fixed specimens. The over-all thickness of the external compound membrane varies depending upon the fixative used. The synaptic clefts in the regions between the external compound membrane discs are widened and measure ∼300 A. A condensation of dense material occurs in pre- and postsynaptic cytoplasms all along the synaptic membrane complex. The morphological relationships described in the spoon endings are suggestive of electrical transmission.

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