Development and aging of nerve fibres in the rat inferior alveolar nerve

• Structural and functional properties of nerve fibres in the rat inferior alveolar nerve (IAN) were examined during development and aging.Rat molar pulpal expllints influence neurite outgrowth from perinatal rat trigeminal ganglion explants, eo-cultured in vitro. Three patterns were found: The A-pattern, where neurites grew in all directions, predominated. In some cultures, the highest density of neurites grew towards the pulpal explant (the B-pattern) or grew in the opposite direction (the C-pattern). The 3 patterns might reflect actions of different neurite-promoting or -repelling factors.Shortly after entering the mandibular canal, the IAN of the adult rat divides into the mental nerve (MN, 70% of all axons) projecting to muco-cutaneous domains of the chin and the lower lip, and the inferior dental nerve (IDN, 30% of all axons) projecting to the teeth and adjacent tissues. The IAN has achieved the adult number of axons (8,000-10,000) by 1 week. De novo myelination commences at birth and ends by three weeks. In the adult IAN 70% of all axons are myelinated. A mature bimodal size distribution of the myelinated IAN axons has been established by 2 months.During postnatal development, the IAN grows 3x in length, as indicated by the elongation of the mandibular canal. Measurements of the nodal spacing in deveoping and adult IANs shows that the largest intemodes elongate more than the nerve. This mis-match can be explained by the presence of very short internodes (VS!s, L<150 f!m) during the first few postnatal weeks. The VSis reflect a myelin sheath remodelling, through which some proportion of the newly formed internodes are eliminated. In young adult animals VSis are absent, the L/D relation is regular, and the internodes grow in size. During aging, the regular relation between L and D in the IAN breaks down due to reappearance of VSis and the emergence of unusually long intern odes.The sural nerve (SN), which shows little myelin sheath remodelling, and the MN, in which myelin sheath remodelling is frequent, are equally sensitive to K+-channel blockade with 4-aminopyridine (4-AP) during early development. While the adult SN is relatively insensitive to 4-AP, the adult MN remains sensitive. This difference can not be explained in terms of a non-uniform adult nodal structure in the two nerves.The MN component of the IAN exhibits less marked age-related changes than the IDN component. This might be related to the fact that the mucocutaneous targe! of the MN is grossly intact in old rats, while the lower jaw dentition shows marked changes.

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