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
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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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