The ascending fiber projections from the mandibular subdivision in the trigeminal nuclear complex of the rat

Abstract

Many studies on the ascending pathways from the trigeminal sensory nuclear complex were reported, but a few reports dealt with the fiber projections from the subnuclear subdivisions of trigeminal sensory nuclear complex systematically were found.The purpose of the present experiment is to clarify the ascending pathways from the mandibular subdivision in the trigeminal sensory nuclear complex of the rat by using the Fink-Heimer and Marchi methods. The trigeminal sensory nuclear complex was divided into the main sensory nucleus and the nucleus of the trigeminal spinal tract (subnucleus oralis, interporalis and caudalis).The experimental lesions were produced to the mandibular division which was confirmed by the previous studies.Degenerated fibers emanated from the main sensory nucleus terminated in the contralateral thalamus via only the crossed ventral trigeminothalamic tract. The ventral tract crossed at the midline dorsal to the interpeduncular nucleus and ascended along the medial lemniscus. At the level of the superior colliculus, a few degenerated fibers terminated in the medial geniculate complex pars medialis (PO). The rest ascended rostrally and terminated in the ventral part of the zona inzerta (ZI). At this level, however, terminal degeneration in the caudal third of the ventrobasal thalamic complex pars medialis (VPM) was small amount. From the middle third of VPM heavy degeneration was produced and ended to the medial part of it. In addition, heavy terminal degeneration could be traced in the most rostral and ventromedial part of it.Degenerated fibers ascending to the thalamus from the subnucleus oralis were very small amount, and very sparse terminal degeneration were found at the caudal and middle part of VPM. On the other hand, both PO and ZI were free from terminal endings.Degenerated fibers from the subnucleus interporalis crossed immediately and ascended along the medial lemniscus to PO, ZI and VPM. Terminal degeneration was found in the all rostrocaudal level of VPM, but not found in the most medial part of VPM. Terminal endings in VPM were more dense than thost of the subnucleus oralis.Following the lesion restricted to the subnucleus caudalis, a small amount of degeneration was noticed in the contralateral PO, but not in ZI. In VPM, terminal degeneration was found in caudal two thirds of it. No degeneration could be found in the rostral one third of VPM as seen in the sections from the main sensory nucleus and the subnucleus interporalis.Judging from above-mentioned results, the thalamic afferent fibers originated from the mandibular division of trigeminal sensory nuclear complex could be found by way of the only crossed ventral tract, but the dorsal trigemino-thalamic tract and homolateral pathways could not be found. Theterminal endings from the different sites of the lesion produced some.differences in the density and terminal areas in the thalamus.In the rat, it appears that the impulses from the mandibular division to the thalamus are directly conducted only by crossed trigemino-thalmic ventral tract. But, the route to the ipsilateral thalmic neurons from the trigeminal sensory nuclear complex is thought to be multisynaptic via the reticular formation etc.