Mesencephalic trigeminal sensory neurons of cat: axon pathways and structure of mechanoreceptive endings in periodontal ligament.

We injected 3H-proline into cat brainstem in order to label the entire mesencephalic trigeminal nucleus (Mes-V) for autoradiographic analysis of the size and pathways of Mes-V sensory axons and for microscopic study of Mes-V receptor structure in dental tissue. Labeled sensory axons were found in the trigeminal motor and sensory tracts and roots; approximately equal numbers of axons were found in both roots. The sensory root and all three divisions of the trigeminal nerve contained larger Mes-V axons than the motor root. Labeled Mes-V axons were found at the ganglion in the dorsomedial (infratrochlear) branch of the ophthalmic nerve but not in the ventrolateral branch. The mean diameter of Mes-V axons in periodontal ligament was 4.0 +/- 1.9 micron compared to 7.3 +/- 2.1 micron in maxillary and mandibular nerve, suggesting axonal arborization prior to innervation of ligament. Mes-V receptors in dental tissue were confined to ipsilateral periodontal ligament close to the root apex, with greater innervation on the posterior side. Receptor incidence was moderate for most teeth; however, maxillary first and second incisors and maxillary and mandibular canines had focal areas with remarkably dense innervation. No labeled axons were found in pulp of any ipsilateral teeth, and none was found in any contralateral dental tissue. EM-autoradiography demonstrated that Mes-V axons form unencapsulated Ruffini-like mechanoreceptors in periodontal ligament. The preterminal axons were small and myelinated. Neighboring bundles of unmyelinated axons and rare encapsulated endings were not labeled. The labeled mechanoreceptors branched to varying degrees among the ligament fibers; they contained numerous mitochondria and glycogen particles, as well as some vesicles and rare multivesicular bodies. They were surrounded by special Schwann cells that formed one or several layers around the ending. The endings were exposed to the basal lamina at numerous sites and occasionally extended fingers beyond the lamellar Schwann cells to contact ligament collagen.