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  Quantitative Analysis of Synaptic Contacts Made between Functionally Identified Oralis Neurons and Trigeminal Motoneurons in Cats

 

 

The trigeminal oral nucleus (Vo.r) contains two kinds of second-order sensory neurons which are GABAergic and/or glycinergic: Vo.r-dl neurons synapsing on jaw-closing (JC) motoneurons and Vo.r-vm neurons synapsing on jaw-opening (JO) motoneurons. The present study was, thus, aimed to determine the number and spatial distribution of synaptic contacts between single Vo.r neurons and their target motoneurons. Most of the contacts were on the soma and/or proximal dendrites of the JC or JO motoneurons (Figs. A-C). The average number of contacts between the single Vo.r-dl neurons and single JC motoneurons (Fig. A) was higher than that between the single Vo.r-vm neurons and single JO motoneurons (5.0 vs. 4.0). The average number of contacts between the single Vo.r-dl neurons and single Nissle-stained somata of JC motoneurons (Fig. B) was higher than that between the single Vo.r-vm neurons and single Nissle-stained somata of JO motoneurons (3.5 vs. 2.8). Further, the single Vo.r-dl neurons issued a higher number of boutons in the JC motor nucleus (Fig. C) than the single Vo.r-vm neurons did in the JO motor nucleus (4437 vs. 445). The single Vo.r-dl neurons were predicted to make contacts with a higher percentage of motoneurons than the single Vo.r-vm neurons (54% of JC motoneurons vs. 23% of JO motoneurons). These results suggest that inhibitory influences exerted by single Vo.r-dl neurons to the single JC motoneurons and JC motoneuron pool are stronger than those exerted by single Vo.r-vm neurons to the single JO motoneurons and JO motoneuron pool, respectively, and that the inhibitory Vo.r-dl and Vo.r-vm neurons play an important role for stopping the biting and for biting longer, respectively. The present study provides new insight in the inhibitory neuronal mechanisms underlying the sensory-motor reflexes.

*Reproduced from The Journal of Neuroscience, 21, 6298-6307 (2001) with permission of the Society for Neuroscience.

 

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