Bilaterally symmetrical effects of high threshold afferents from the masseteric muscle on the jaw movement

The location of brainstem neurons with bilateral projections to the motor nuclei of jaw openers in the cat

Symmetrical motor output is the rule in the masticatory system. We examined morphologically how this functional symmetry might be reflected in the organization of premotor neurons that could mediate excitation of jaw-opener motoneurons. Premotor neurons projecting bilaterally to jaw-opener motoneurons by way of axon collaterals were identified by retrograde dual-labeling with cholera toxin B-conjugated fluorescein isothiocyanate (CTb-FITC) and tetramethylrhodamine (TMR). In each cat, CTb-FITC and TMR were injected into the digastric motoneuron pools, respectively, on the left and right sides. In three animals, 69-147 neurons were labeled with both tracers, comprising approximately 44% of all retrogradely labeled cells. Double-labeled cells were located bilaterally in the trigeminal oral nucleus (Vo) and the adjacent reticular formation (RF), the former containing a larger number of cells. Neurons labeled with only one tracer were also distributed bilaterally in the Vo and RF. The results indicated that the bilaterally projecting premoter neurons identified mainly in the Vo and RF represent neuronal substrates for the symmetry that characterizes most jaw movements.

GABAergic and glycinergic neurons projecting to the trigeminal motor nucleus: a double labeling study in the rat

The distribution of GABAergic and glycinergic premotor neurons projecting to the trigeminal motor nucleus (Vm) was examined in the lower brainstem of the rat by a double labeling method combining retrograde axonal tracing with immunofluorescence histochemistry. After injection of the fluorescent retrograde tracer, tetramethylrhodamine dextran amine (TRDA), into the Vm unilaterally, neurons labeled with TRDA were seen ipsilaterally in the mesencephalic trigeminal nucleus, and bilaterally in the parabrachial region, the supratrigeminal and intertrigeminal regions, the reticular formation just medial to the Vm, the principal sensory and spinal trigeminal nuclei, the pontine and medullary reticular formation, especially the parvicellular part of the medullary reticular formation, the alpha part of the gigantocellular reticular nucleus, and the medullary raphe nuclei. Some of these neurons labeled with TRDA were found to display glutamic acid decarboxylase (the enzyme involved in GABA synthesis)-like or glycine-like immunoreactivity. Such double-labeled neurons were seen mainly in the supratrigeminal region, the reticular region adjacent to the medial border of the Vm, and the dorsal part of the lateral reticular formation of the medulla oblongata; a number of them were further scattered in the intertrigeminal region, the alpha part of the gigantocellular reticular nucleus, the nucleus raphe magnus, the principal sensory trigeminal nucleus, and the interpolar subnucleus of the spinal trigeminal nucleus. These neurons were considered to be inhibitory (GABAergic or glycinergic) neurons sending their axons to motoneurons in the Vm, or to local interneurons within and around the Vm.