This study identified neurons in the sensory trigeminal complex with connections to the medial (MVN), inferior (IVN), lateral (LVN), and superior (SVN) vestibular nuclei or the spinal cord. Trigeminovestibular and trigeminospinal neurons were localized by injection of retrograde tracers. Immunohistochemical processing revealed gamma-aminobutyric acid (GABA)- and glutamate-containing neurons in these two populations. Trigeminovestibular neurons projecting to the MVN and the IVN were in the caudal principal nucleus (5P), pars oralis (5o), interpolaris (5i), and caudalis (5c) and scattered throughout the rostral 5P. Projections were bilateral to the IVN, with an ipsilateral dominance to the MVN, except from the rostral 5P, which was contralateral. Neurons projecting to the LVN were numerous in the ventral caudal 5P and the 5o and less abundant in the rostral 5P, 5i, and 5c. Our results suggested that only 5P and 5o project to the dorsal LVN. Neurons projecting to the SVN were in the dorsal 5P, 5o, and 5i but not in 5c. Trigeminospinal neurons were mainly in the ventral 5o and 5i and in the lateral 5c, rarely or never in 5P. Among trigeminovestibular neurons, most of the somas were immunoreactive for glutamate, but some reacted for GABA. Among trigeminospinal neurons, the number of somas immunoreactive for each of the two amino acids was similar. Trigeminal terminals were observed in contact with vestibulospinal neurons in the IVN and LVN, giving evidence of a trigeminovestibulospinal pathway. Therefore, inhibitory and excitatory facial inputs may contribute through trigeminospinal or trigeminovestibulospinal pathways to the control of head/neck movements.
The dentatovestibular connections were investigated using anterograde and retrograde tracing methods. All parts of the cerebellar nucleus lateralis (NL) or dentate nucleus sent fibers onto the ipsilateral vestibular nuclear complex. In spite of their apparently widespread area of termination, dentatovestibular fibers were distributed differentially, according to the subregion of the NL they arose from. Fibers from the main, magnocellular region and the dorsolateral hump (dlh) reached the four main vestibular nuclei, but preferentially the superior (SV) and inferior (IV) vestibular nuclei. The projections to the lateral and the medial vestibular nuclei, which were less abundant, essentially originated from neurons located in the dlh. Fibers arising from the parvocellular subregion of Flood and Jansen terminated within the SV and IV only. Some rare reciprocal vestibulodentate projections were observed. These observations suggest highly integrated activities of dentatovestibular connections related to postural, but also vestibulo-oculomotor functions.
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