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Synaptic organization of the substantia gelatinosa glomeruli in the spinal trigeminal nucleus of the adult cat

  • Published:
Journal of Neurocytology

Summary

In the spinal trigeminal nucleus of the adult cat, the neuropil of the substantia gelatinosa (SG) layer is made up of axonal endings of V nerve axons, dendrites and axons of SG neurons and dendrites of marginal and magnocellular neurons. A typical SG glomerulus consists of a centrally located V nerve ending which contains large synaptic vesicles and forms asymmetrical axodendritic synapses on two kinds of dendritic spines and on dendritic shafts. The dendritic processes of the SG glomerulus are linked by two kinds of dendrodendritic synapses: type 2 dendritic spines, containing large synaptic vesicles, form slightly asymmetrical (intermediate) synapses on type I dendritic spines. Dendritic shafts, containing clusters of small synaptic vesicles, form symmetrical dendrodendritic synapses amongst themselves and on type I dendritic spines. Small axonal endings, containing small synaptic vesicles form intermediate axodendritic synapses on type I dendritic spines and on dendritic shafts. These small axons also form symmetrical axoaxonic synapses on the V nerve ending. The V nerve endings in the SG layer are dark and differ morphologically from the pale V nerve endings in other parts of the trigeminal nuclei.

The data are interpreted based on considerations that some marginal and magnocellular neurons are projection neurons and that some SG neurons are interneurons which form inhibitory synapses. The following circuits in the SG glomeruli are suggested: V nerve endings form excitatory synapses on the dendrites of projection neurons (marginal and magnocellular neurons) and on the dendrites of inhibitory interneurons (SG neurons). The interneurons inhibit transmission through the V nerve-projection neuron circuit by means of their axoaxonic synapses on the V nerve endings. Interneurons are linked together through inhibitory dendrodendritic synapses along their dendritic shafts. Two interpretations of dendrodendritic circuits between projection neurons and interneurons are considered. The morphologically distinct V nerve innervation of the SG layer and the internal circuitry of the SG glomeruli appear to be essential for the perception of thermal and painful stimuli.

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  1. Neurobiology and Anesthesiology Branch, National Institute of Dental Research, National Institutes of Health, 9000 Rockville Pike, Bethesda, Maryland, USA

    Stephen Gobel

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Gobel, S. Synaptic organization of the substantia gelatinosa glomeruli in the spinal trigeminal nucleus of the adult cat. J Neurocytol 3, 219–243 (1974). https://doi.org/10.1007/BF01098390

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  • DOI: https://doi.org/10.1007/BF01098390

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