Myelinated Axon Diameter Research Articles

A morphometric study of human fetal sural nerve

A morphometric study was performed on sural nerves from human fetuses at 15 to 36 weeks postovulation. There were no myelinated fibres at 15 and 16 weeks, but by 21 weeks there were 5,000/mm2, rising to 25,000/mm2 at 36 weeks. During the fetal period, the mean myelin lamellar count trebled and the g ratio (axon diameter: total fibre diameter) decreased from 0.90 to 0.75, although the axon diameter of myelinated fibres did not increase. The smallest myelinated axon diameter was 0.63 micron, whereas the largest unmyelinated axon in a 1:1 relationship with a Schwann cell was 2.83 micron, suggesting that axon size is unlikely to be the only stimulus for myelination. The density of unmyelinated axons that were the sole occupants of a Schwann cell fell considerably between 23 and 33 weeks, while the ratio of total unmyelinated axons to myelinated fibres decreased from 82:1 at 21 weeks to 6:1 at 36 weeks. Data for Schwann cell nuclear density and percentages of fibres cut through the nucleus are also presented.

Experimental study of vascularized nerve grafts: morphometric study of axonal regeneration of nerves transplanted into silicone tubes.

Rat sciatic nerves were used in a comparative study of vascularized and free (nonvascularized) nerve grafts transplanted into silicone tubes. A total of 39 sciatic nerves were used, 21 as vascularized nerve grafts and 18 as free nerve grafts. The rats were killed at intervals from the first to the 24th postoperative weeks, and biopsied sciatic nerves were processed for morphometric studies using a fiber caliber analyzer. There was a significant increase in the number of large myelinated axons (more than 5 mu) in the vascularized nerve graft models over those in the free nerve graft models at the distal ankle region. Furthermore, the diameters of myelinated axons in the vascularized nerve graft were larger than those in the free nerve graft at all specimen sites during all postoperative weeks. We suggest that the preservation of the vascular system in vascularized nerve grafts would decrease the likelihood of fibrosis and result in better regeneration of axons.

Myelogenesis and estimation of the number of axons in the anterior commissure of the chick (Gallus gallus).

By use of light- and electron microscopy the anterior commissure of the chick was studied at different times during development. Between the 19th day of incubation and the 35th day after hatching the cross-sectional area of the anterior commissure, as determined from mid-sagittal sections, undergoes a 6-fold increase in size. Thereafter the area remains fairly constant. The total number of fibres in the anterior commissure was estimated to be 89000. The full complement of fibres is already present by the 19th day of incubation. Myelogenesis occurs mainly between the 19th day of incubation and the 35th day after hatching, concomitant with the increase in cross-sectional area. From the 35th day after hatching, myelinated fibres comprise approximately 40% of the total number of fibres. The median diameter of unmyelinated fibres is about 0.35-0.40 micron. The median diameters of myelinated axons and fibres are 0.8-1.0 micron and 1.1-1.3 micron, respectively.

Composition of the supraoptic decussation of the chick (Gallus gallus). A possible factor limiting interhemispheric transfer of visual information.

In the chick the supraoptic decussation consists of dorsal, ventral, and subventral subregions. The dorsal region contains about 580000 axons of which 24% are myelinated. The mean diameters of unmyelinated and myelinated fibers are 0.31 micron and 1.0 micron, respectively. The ventral region contains about 520000 axons of which only 0.53% are myelinated. The mean diameters of unmyelinated and myelinated fibers are 0.35 micron and 1.09 micron, respectively. A small proportion of unmyelinated fibers has regions of localized expansions. The subventral region, the smallest subregion, contains about 31000 fibers of which 20% are myelinated. The mean diameters of unmyelinated and myelinated axons are 0.31 micron and 1.15 micron, respectively. On the basis of the interhemispheric channels available to the visual system it is argued that visual information must be modified to some extent prior to its transfer.

White-matter dendrites in the upper cervical spinal cord of the adult cat: a light and electron microscopic study.

The organization and structure of dendrites penetrating into the white matter of upper cervical spinal segments have been examined by means of Golgi staining techniques, intracellular horseradish peroxidase (HRP) injections, and ultrastructural studies. The Golgi studies established that several groups of neurons located in intermediate and ventral laminae of the upper cervical spinal cord have a substantial part of their dendritic tree extending into adjacent ventral and lateral funiculi. Most dendrites in white matter showed irregular varicosities along their length. They were devoid of spines and followed relatively direct paths. In contrast, grey matter dendrites were occasionally observed with spines and complex appendages frequently followed tortuous paths. The size and location of some Golgi stained neurons suggested that white matter dendrites might originate from neck muscle motoneurons. This possibility was confirmed using intracellular HRP injections. These studies also showed that the distribution of white matter dendrites of neck muscle motoneurons depended on the location of the motoneuron soma. White matter dendrites of neck muscle motoneurons located deep in the ventral horn projected into all regions of white matter surrounding the ventral horn. Other neck muscle motoneurons, located in the spinal accessory nucleus, had white matter dendrites largely confined to the lateral funiculus. White matter dendrites of motoneurons in the commissural nucleus were found to project across the ventral commissure into the contralateral spinal cord. Light microscopic studies of semi-thin sections stained with toluidine blue and electron microscopic studies of thin sections revealed that white matter dendrites were confined to special regions of the white matter. These regions resembled the grey matter neuropil and contained dendrites and unmyelinated and small diameter myelinated axons. Axon terminals were also found in white matter. These terminals contained either flattened or spherical vesicles and formed synaptic contacts on white matter dendrites. White matter dendrites, by virtue of their frequency of occurrence, distribution, and type of synaptic contacts may represent a means by which descending or ascending spinal systems can influence spinal neurons without recourse to axon collaterals which terminate in grey matter.

The ultrastructure of Ruffini and Herbst corpuscles in the articular capsule of domestic pigeon.

The present study identifies two types of sensory nerve endings in the articular capsule of the shoulder joint of domestic pigeons: Ruffini corpuscles (spray-like endings), and Herbst corpuscles. Ruffini corpuscles occur in the fibrous membrane of the articular capsule and consist of two to four branched cylindrical segments within a network of fascicles of collagen fibers. At the terminal ends of the cylinders the perineural sheaths of the capsule are deficient and surround the fascicles of collagen fibers. The axon terminals in each cylindrical segment of a Ruffini corpuscle repeatedly ramify, giving rise to delicate neurite profiles. These neurites and associated Schwann cells envelope small fascicles of collagen fibrils. Schwann cells cover only a part of the neurite profiles. The myelinated afferent axon enters the midregion of the cylinder and has a diameter of approximately 3 micrometer. Herbst corpuscles are situated in the subsynovial connective tissue and in the transition zone between the fibrous membrane and the muscular fascia. They appear as elongated ovals in longitudinal section and round in cross section. Small corpuscles measure approximately 5 micrometer x 200 micrometer in length and large ones approximately 100 micrometer x 600 micrometer. Each has a myelinated afferent axon (diameter 2.5--7.5 micrometer) that terminates in one to three inner cores. The inner core contains the nonmyelinated receptor portion of the nerve fiber surrounded by numerous cytoplasmic lamellae, a subcapsular connective tissue space, and a perineural capsule of eight to 12 layers. Avian joint receptors are similar to those present in the skin of various birds and Ruffini corpuscles resemble in fine structure equivalent receptors in joint corpuscles of the domestic cat.

Myelination of the mouse corpus callosum.

Myelination has been studied in th corpus callosum of the mouse brain between birth and 240 days-of-age. Myelin sheaths were first seen at 11 days. The most rapid phase of myelination occurred between 14 and 45 days when 13.5% of axons were myelinated, but myelination continued at a reduced rate up to 240 days when 28% of axons were myelinated. The mean diameter of unmyelinated axons was more or less constant throughout the study with an overall mean diameter of 0.25 +/- 0.01 micron. Similarly myelinated axon diameter showed little variation with age with a mean diameter of 0.46 +/- 0.01 micron. This suggests that in the corpus callosum axons do not increase in size until they begin to myelinate.

Oculomotor Nerve Regeneration After Aneurysm Surgery

A 57-year-old woman had symptoms of oculomotor nerve palsy first appearing one year before successful surgical ligation of a saccular aneurysm arising from the right posterior cerebral artery. During the subsequent postoperative period of two years, oculomotor nerve functions improved as the result of regeneration. Extensive morphometric evaluation of the regenerated nerve was compared to the normal side at the light microscopic level. The affected nerve showed a reduction in the transverse area (62%), estimated number of fibers (49%), and mean diameter of myelinated axons (23%). The normal g-ratio of axon to total diameter was almost constant at 0.64, but on the regenerated side it increased to 0.73. An increase in unmyelinated axons and connective tissue endoneurium was evident at the ultrastructural level. The significance of these marked quantitative changes was compared to the partial return of oculomotor nerve function.

Morphometric study of motoneurons in congenital nemaline myopathy and Werdnig-Hoffmann disease.

To test whether congenital nemaline myopathy (NM) might have a neurogenic basis, we evaluated the number and frequency distribution of L5 motoneuron cell bodies and their myelinated ventral root (VSR) axons in a typical case. Results were compared to those from three age-matched controls and those from a child with Werdnig-Hoffmann disease (WHD). Diameter histograms of cell bodies of L5 motoneurons from controls contained three peaks which were identified as large, intermediate, and small cytons (LC, IC, and SC). In previous work from our laboratory, the numbers of LC and IC of the L5 segment was found to correspond closely enough to the numbers of LA and IA (large and intermediate diameter myelinated axons) of L5 ventral spinal root to indicate that the majority of LC are alpha and the majority of IC are gamma motoneurons. The number of LC of L5 reference cords varied from 3508 to 4916, and of IC from 967 to 2204. The diameter histogram of cytons from the NM case differed from controls; the LC and IC peaks were displaced to smaller diameter categories, but their number was normal (4520 and 2023, respectively). In WHD, the numbers of LC and IC were 211 and 946, respectively. Axons were similarly affected. The severe decrease from normal in the number of large motoneurons in WHD, and the reduction in size without loss of motoneurons in NM, indicates a different morphologic basis for NM and WHD. In NM the reduction in size without decrease in number of motoneurons is probably secondary to congenital myopathy and does not provide evidence for a neurogenic etiology.

EFFECTS OF UNDERNUTRITION DURING THE SUCKLING PERIOD ON GROWTH OF THE ANTERIOR AND POSTERIOR LIMBS OF THE MOUSE ANTERIOR COMMISSURE

The anterior and posterior limbs of the anterior commissure were examined quantitatively in normal mice, and mice undernourished during the suckling period. There was no change in the mean length of the anterior commissure in either group. The total number of axons, the percentage of myelinated axons, and the mean number of myelin lamellae remained unchanged between each group in both limbs. There was, however, a highly significant fall in the total number of cells in both the anterior (P<0·001) and posterior (P<0·001) limbs, mainly in the oligodendrocyte fraction of the population. There was no change in the relative proportion of cells in each limb. The cross‐sectional area of both the limbs was significantly less in the undernourished animals, with the posterior limb being affected more than the anterior limb. These differences were found to be due to a significant (P<0·001) fall in the mean diameter of myelinated axons in the anterior limb and to a significant (P<0·001) fall in the mean diameter of unmyelinated axons in posterior limb. The mean diameters of the unmyelinated axons in the anterior limb, and of the myelinated axons in the posterior limb remained unchanged. The possible reasons for these changes, and their place in the general pathology of deficits and distortions following early undernutrition are discussed.

Drive decay theory of self-stimulation: Refractory periods and axon diameters in hypothalamic reward loci

In seven rats with low-threshold self-stimulation loci the diameter of the lateral hypothalamic myelinated axons were measured by light and electron microscopy (Experiment 1). In addition, the electron microscopic study of the axon diameters was continued in 8 loci after an experiment of confirming earlier demonstration of the 0.5 to 0.6 msec long absolute refractory period of the hypothalamic reward system (Experiment 2). Most axon diameters ranged between 0.6 to 1.3 micra. 96.5 and 98.4 per cent of the examined 3251 axons had diameters less than 2 and 3 micra, respectively. In 5 self-stimulation loci we did not observe any axons ranging between 3 to 6 micra in diameter. Measurements of both the axon diameters and the refractory times in the fornix and anterior commissure (Experiment 3) suggest that the absolute refractory period of 0.8 to 1.1 msec measured by Deutsch [5] can only belong to the axons in the lateral hypothalamic self-stimulation loci, but the other period of 0.5 to 0.6 msec would require myelinated axons of about 5 to 6 micra in diameter. Consequently, the findings support the view that the improvement of excitability occurring after the end of a post-stimulatory delay period of 0.5 to 0.6 msec is due to the onset of synaptic activation instead of recovery from axonal refractory state.

Characterization of two ganglion cell populations in avian ciliary ganglia

The cellular organization of the avian ciliary ganglion was investigated with electrophysiological, histological and pharmacological techniques. Nerve connections in the pigeon ciliary ganglion were studied. The two cell populations which had been described anatomically were found to distribute their axons separately in the choroid and ciliary nerves. There was no connection between the two groups of cells. These findings also apply to the chick ciliary ganglion. The ciliary population has 2953±683 (mean ± S.D.) ganglion cellswith single myelinated axons 5.4 ± 2.1 μm (median ± A.D.) in diameter. The choroid group is composed of 3084 ± 640 (mean ± S.D.) cells with 2.1 ± 1.0 μm (median ± A.D.) diameter myelinated axons. The conduction velocity of the presynaptic fibers which innervate the low threshold ciliary cells is 6.9 ± 3.04 m/sec (mean ± S.D.). The conduction velocity of the presynaptic fibers innervating the choroid cells is 3.4 ± 0.7 m/sec (mean ± S.D.) at 22–24°C. Similarly, the latency of the orthodromically initiated ciliary chemical response was 1.8 ± 0.5 msec (mean ± S.D.) shorter than that of the choroid response. The cells which send axons to the ciliary nerves posses a dual, chemical and electrical, mechanism of synaptic transmission. The cells sending axons to the choroid nerves only have chemical transmission. After sectioning of the ciliary nerves, leaving choroid nerves intact, retrograde changes occur only in the cells possessing myelin lamellae, i.e., the cliary cells. No changes were found in the choroid cells. Thus, electrical coupling is associated with the myelinated cells. Although chemical synaptic transmission is cholinergic for both cell group, the choroid cells are more suscetible to block by hexamethonium than are the ciliary cells. Conversely, the ciliary neurons are more sensitive to d-tubocurarine than are the choroid. A given concentration of dihydro-β-erythroidine reduced transmission about equally in the choroid and ciliary pathways. Such a pattern of selectivity by different blockers suggests that the properties of the cholinergic receptors of the two ganglion cell populations may differ qualitatively. Adrenergic blocking agents did not modify transmission in the avian ciliary ganglion.

Plastic Repair of Congenital Defects Associated with Spina Bifida and Cranium Bifidum

To examine the effect of topical artesunate treatment on peripheral nerve regeneration and compare it with the effects of topical tacrolimus and dexamethasone on nerve regeneration.Thirty-two Wistar albino rats were used in this study. The rats were divided into 6 groups: sham, saline, petrolatum, artesunate, tacrolimus, and dexamethasone. A compression injury was generated in the right sciatic nerve in all groups except the sham group. In the sham group, the nerve was dissected but compression was not applied. In the groups in which compression was applied, the agents were absorbed through resorbable gelatin sponges applied to the injured region. At the end of 4 weeks, walking analysis, electromyographic measurements, and histopathologic examinations were conducted.When the sciatic function index and electrophysiologic measurements were evaluated, artesunate, tacrolimus, and dexamethasone exhibited positive effects on nerve regeneration (P < .05); there were no significant differences among these 3 agents (P > .05). Histopathologic examination showed that artesunate decreased fibrosis scores and inflammation and increased the diameter of myelinated axons; tacrolimus decreased fibroblast scores; and dexamethasone only decreased fibrosis scores (P < .05). Immunohistochemical analysis showed that the artesunate and dexamethasone groups had more positive immunoreactivity to nerve growth factor than did the saline group (P < .05).Topical artesunate treatment had a positive effect on peripheral nerve regeneration. There were no relevant differences between the topical forms of dexamethasone and tacrolimus for peripheral nerve regeneration.