Diameter of Axons in the Lingual Nerve of the Mouse. A Preliminary Investigation

 
I.S. WATANABE
M. SEMPRINI
S.R. DE-MORAES
R.R. DE-SOUZA
 
Departamento de Anatomia, ICB, Universidade de São Paulo, São Paulo, SP, Brasil


Braz Dent J (1996) 7(2): 87-90 ISSN 0103-6440

| Introduction | References |


The lingual nerves from ten mice were examined so that normal axonal populations could be determined. After perfusion fixation, they were removed and processed, and sections were taken from nerves for transmission electron microscopy. The fiber-diameter spectrum of unmyelinated fibers for the mouse lingual nerve is characterized by a unimodal curve with the more pronounced peak in the medium-diameter fiber range (0.28 µm). The spectrum from the myelinated fibers also shows a population of axons of the lingual nerve (0.22-3.2 µm) that reflects different functional specializations. These data establish some baseline values for morphological evaluation of the effects of experimental lingual nerve damage.


Key Words: lingual nerve, axon diameter, mouse.


Introduction

It is well known that the functional specialization of a peripheral nerve axon is related to its diameter and, therefore, to the conduction velocity of the fiber (Sjoquist, 1938; Boyd and Davey, 1968; Heasman and Beynon, 1986). Thus, the proportion of fiber sizes present in a nerve may indicate the functional capacities of the nerve (Heasman and Beynon, 1986). The lingual nerve carries a wide range of functionally distinct nerve fiber types. Afferent components of the lingual nerve innervate mucous membranes of the mouth, the lingual gingiva, and the anterior 2/3 of the tongue. The nerve also carries visceral efferent (secretomotor) fibers to the submandibular and sublingual salivary glands (Hellekant and Kasahara, 1973) and conveys special afferent (taste) fibers from the mucous membrane of the tongue. The nerve also carries sympathetic fibers to vessels (Matthews and Robinson, 1980). Despite this interesting and unique composition, there are only three papers on the diameters of axons in the lingual nerve (Sakurada, 1973; Biedenbach et al., 1975; Holland and Robinson, 1992) and only one of these previous studies was at the ultrastructural level (Holland and Robinson, 1992). To our knowledge no paper is available concerning the diameter of axons in the lingual nerve of the mouse. The aim of this study was to establish the fiber-diameter distribution curve in the adult mouse by grouping axon-diameter data of random samples from several lingual nerves. Sections of lingual nerve, approximately 1 cm in length, were dissected from ten adult mice perfused through the heart with 2.5% glutaraldehyde and 2% paraformaldehyde in phosphate buffer 0.1 M (pH 7.4). Specimens were taken from the level of the mandibular foramen distal to the junction of the chorda tympani with the lingual nerve. The lingual nerves were post-fixed in 1% osmium tetroxide in phosphate buffer, processed for ultrastructural preservation and embedded in Epon 812. Ultrathin sections of the nerves were stained with uranyl acetate and lead citrate and examined in a Hitachi 12A electron microscope. Measurements of axon diameter were obtained digital by measuring the inner perimeters of the myelin sheaths of a randomly sampled population of axons from 10 electron micrographs of each nerve (final magnification 8500X) three times and, then, converting the perimeter measurements into the diameters of circles with the same cross-sectional areas as the axons measured. Fibers were classified by diameter size according to class intervals, and grouped frequency distributions of fiber diameters were calculated. In the electron microscope, a single field was selected randomly, and by moving in both vertical and horizontal directions, 9 further fields at regular intervals were selected. Samples were taken to induce all possible types of sensory and motor axons. Within the nerve many myelinated and a few groups of unmyelinated fibers could be clearly seen (Figure 1). The unmyelinated fibers were collected in bundles containing a variable number of fibers enclosed in a common sheath. The myelinated fibers were round, oval or elliptical in form, or showed an irregular contour. The populations of fiber-diameter measurements for the myelinated and unmyelinated fibers are shown independently in Figure 2. The spectrum from the myelinated fibers shows a population of predominantly medium-diameter axons (peak 1.7 µm, range 0.8-3.2 µm) (Figure 2, top). The spectrum from unmyelinated fibers shows a distribution of an almost unimodal form (peak 0.28 µm, range 0.22-0.46 µm) (Figure 2, bottom). The present investigation has provided for the first time estimations of the populations of fibers in the lingual nerve of mice. The method used to measure nerve fibers has been found suitable for the study of the lingual nerve of mice because it gave consistent results in all experiments and is ideal for the quantitative work planned. Analysis of both myelinated and unmyelinated data of fiber-diameter measurements showed diameter curves of almost unimodal form, with peaks at 1.7 µm and 0.28 µm, respectively. The same unimodal appearance of data were reported for the lingual nerves in rabbits (Sakurada, 1973) and cats (Biedenbach et al., 1975; Holland and Robinson, 1992). The variation in size within the population of axons of the lingual nerve probably reflects different functional specializations. The small-diameter group of fibers corresponds to the somatic afferent (A delta) fibers, responsible for the sensory transmission of pain and temperature. This group of fibers would also include the visceral efferent (pre-ganglionic secremotor) and special visceral afferent (taste) components of the chorda tympani (Heasman and Beynon, 1986) as well as the post-ganglionic sympathetic fibers for vessels (Mathews and Robinson, 1980) which cause vasodilatation (Erici and Uvnas, 1952). Larger diameter somatic afferents which relay modalities of touch, pressure and vibration (Heasman and Beynon, 1986) are also well-represented on the fiber-diameter spectrum of the lingual nerve. The very largest fibers in the lingual nerve may be proprioceptive (Dubner et al., 1978). The data presented here can be used in experimental studies dealing with lingual nerve damage for which knowledge of the axonal diameters in the lingual nerve is a necessary prerequisite.


References

Biedenbach MA, Beurman RW, Brown AC: Graphic-digitizer analysis of axon spectra in ethmoidal and lingual branches of the trigeminal nerve. Cell Tissue Res 157: 341-352, 1975

Boyd IA, Davey MR: Composition of peripheral nerves. Livingstone, Edinburgh 1968

Dubner R, Sessle BJ, Storey AT: The neural basis of oral and facial function. Plenum Press, New York, 1978

Erici I, Uvnas B: Efferent and antidromic vasodilator impulses to the tongue in the chorda-lingual nerve of cat. Acta Physiol Scand 25: 10-14, 1952

Heasman PA, Beynon ADG: Quantitative diameter analysis of lingual nerve axons in man. J Dent Res 65: 1016-1019, 1986

Hellekant G, Kasahara Y: Secretory fibres in the trigeminal part of the lingual nerve to the mandibular salivary glands of the rat. Acta Physiol Scand 89: 198-207, 1973

Holland GR and Robinson PP: Axon populations in cat lingual and chorda tympani nerves. J Dent Res 71: 1468-1472, 1992

Mathews B, Robinson PP: The course of post-ganglionic sympathetic fibers distributed with the trigeminal nerve in the cat. J Physiol (Lond) 303: 391-401, 1980

Sakurada Y: On the myelinated nerve fibres contained in the mandibular nerve in rabbits. Odontol Tokyo 60: 613-635, 1973

Sjoquist O: Studies on pain conduction in the trigeminal nerve. Contribution to surgical treatment of facial pain. Acta Psychiat Neurol 17(Suppl): 1-139, 1938


Correspondence: Prof. Dr. Romeu R. de Souza, Departamento de Anatomia, ICB, Universidade de São Paulo, Av. Prof. Lineu Prestes, 2415, Biomédicas III, Butantã, 05508-900, São Paulo, SP, Brasil. FAX: 818-7258. E-mail: rrdsouza@biomed.icb2.usp.br


Accepted March 1, 1996
Electronic publication: February, 1997


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