Effects of inferior alveolar nerve severance on the eruption rate of the mandibular incisor in the 10-day-old wistar albino rat

Abstract

Following severance of the inferior alveolar nerve on one side, the incisor tooth on that side invariably exhibited an acceleration of eruption. When the inferior alveolar nerve was severed bilaterally, both teeth showed an acceleration of eruption far beyond the rate in the control animals of the same litter.The teeth were notched in order that eruption rates could be measured and recorded. Notching was not attempted before the twenty-fifth to the thirtieth day in order to ensure degeneration of fibers in the distal segment and retrograde degenerative changes in the cells of origin of these fibers (that is, mesencephalic nucleus of the fifth cranial nerve). Degeneration of inferior alveolar nerve fibers at the periphery and chromatolytic changes in cells in the center were constant findings.Careful studies by Corbin on the peripheral distribution of fibers rising in the mesencephalic nucleus and by Corbin and Harrison on the function of the mesencephalic nucleus and root of the fifth cranial nerve had previously demonstrated the actual links in a highly stable anatomic substratum for segmental activity. This neural are served functionally to coordinate biting and chewing movements. Specialized receptors in this neural are are situated in organs of movement and their appendages (muscles, teeth, and supporting structures). Tension in these receptors largely determines muscle sense and stretch information. Large and medium-sized myelinated fibers which have their origin in the mesencephalic nucleus and their end terminals in these receptors mediate proprioceptive impulses from the muscles of mastication and deep pressure from the teeth, periodontal membranes, gingivae, and soft palate. The operation of nerve severance deliberately interrupts the afferent limb of this are. It is the sensory information from these receptors which is essential for the operation of a coordinated masticatory response. Lack of this information following on the heels of nerve severance leads to incoordination and imbalance in such vital activities as biting and chewing.From this vantage point, it is clear that the incisors and their investing structures do not stand alone. Thanks to a functional union of several related structures, the teeth are but part of a complex functional whole—a proprioceptive system. It is precisely the deep pressure sensibility information provided by this system of fibers and the inhibition conferred by the reflex which control the force of the bite and protect the teeth of the control animals and result in damage to the teeth and supporting structures of the experimental animals. Responses based upon this proprioceptive sensory information result in control of the special attritional and incisive activities of the teeth, both of which play a role in maintaining the length of the teeth and hence in their rates of eruption.