Target influence on aging of myelinated sensory nerve fibres.

Large myelinated fibres in the rat inferior alveolar nerve (IAN) exhibit age-related aberrations partly similar to alterations in target-deprived nerves. Shortly after entering the mandibular canal the rat IAN splits into a mental (MN) and an inferior dental (IDN) portion. This offers a possibility to compare the occurrence of age-related aberrations in the MN and the IDN, which project to different target types. Hence, we examined the relation between internodal length (L) and fibre diameter (D) in teased fibre preparations of MNs and IDNs from adult rats of different ages. The results show that the relation L/D is similar in MNs and IDNs of 6- to 24-month-old rats and that it is significantly more irregular in IDNs than in MNs of 26- to 30-month-old rats. This difference may be related to the fact that the IDN-innervated mandibular dentition is deteriorating in rats older than 2 years, whereas the muco-cutaneous target of the MN is not. We suggest that target deterioration may be one major factor behind the increasing incidence of structural aberrations in large myelinated peripheral nerve fibres.