Fibre numbers and sizes in the inferior alveolar nerve of the cat.

Previous studies with the light microscope of paraffin-embedded, metal-impregnated sections have probably underestimated the narrow fibre components of the inferior alveolar nerve in the cat. In this study the inferior alveolar nerve of a young adult cat was fixed in situ with glutaraldehyde, post-fixed in osmium and embedded in plastic. A 1 micrometer section of the whole nerve, and an ultrathin section of its largest bundle were cut. Montage micrographs were made from both sections, and from tracings the number and size distribution of the nerve fibres were measured. The entire nerve contained 6856 myelinated nerves whose diameters were distributed bimodally with peaks at 3--4 and 9--10 micrometers. This total is higher than all previously published counts of this nerve, the increase being due to the detection of a greater number of narrower myelinated fibres. In the bundle examined electron microscopically 51.4% of the fibres counted were non-myelinated, four times as many as found in previous studies. The number and distribution of myelinated fibres found by electron microscopy was very similar to that found by light microscopy. The size distribution of myelinated nerves was similar in all the bundles examined. Apart from a lower incidence of non-myelinated fibres, the population of the core of the larger bundle was similar to that of the periphery.