Summary
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1.
The analysis of serial ultrathin sections of the RPAI bursting neuron of the snailHelix pomatia reveals the presence of axosomatic contacts on its surface membrane.
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2.
These contacts have a number of specific features: (a) the presynaptic axon contains synaptic vesicles and electron-dense granules, typical of peptidergic terminals; (b) the terminal part of the axon forms many finger-like processes which invaginate the neuronal soma; (c) the width of the cleft (80 nm) in the area of the contact is larger than that in usual synaptic contacts; and (d) there is a system of lacoons in the region of the axosomatic contact; this system is formed by protrusions of the soma and it accompanies the contact along its extent.
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3.
It is suggested that the system of lacoons which communicates with the space between the terminal and the soma may serve as a ramified synaptic cleft into which the secretion from the terminal is released. This system may contribute to a considerable prolongation of the time of action of the secretory product on the membrane of the RPAI neuron.
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Koval, L.M., Kononenko, N.I. & Skibo, G.G. The axosomatic contacts on the bursting neuron of the snailHelix pomatia. I. ultrastructural features of the axosomatic contacts. Cell Mol Neurobiol 4, 31–41 (1984). https://doi.org/10.1007/BF00710940
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DOI: https://doi.org/10.1007/BF00710940