Ultrastructure and functional characteristics of the optic nerve of Rana temporaria L. in the norm and during degeneration

Results of electron microscopy and electrophysiological studies of the frog optic nerve are presented. The nerve contains 96% unmyelinated (about 210,000) and 4% myelinated (8700–14,800) fibers. The peripheral zone of the nerve (20–30 µm) has relatively few myelinated fibers, whereas in other zones these fibers are distributed uniformly (counting area 300–400 µm²). The curve of the distribution of the diameters of myelinated fibers has a number of peaks: a main peak at 1 µm and additional peaks at 0.6 and 1.6 µm (the latter is more prominent). Individual fibers have a diameter of 0.4–3.9 µm. The diameter of the unmyelinated fibers are 0.1–0.4 µm; 64% of these fibers have a diameter of 0.2 µm. Most fibers at a temperature of 18–20° conduct at 0.3–0.4 m/sec and a few (myelinated with a diameter of 1.0–1.6 µm) at 3 and 6 m/sec. After enucleation the myelinated fibers degenerate at first and are phagocytized by neuroglia; the ultrastructure and function of the unmyelinated fibers at 18–20° remain unchanged up to 100 days postoperation.A. N. Severtsov Institute of Evolutionary Animal Morphology and Ecology, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 2, No. 6, pp. 627–635, November–December, 1970.     

Characterization of the postnatal development of superior laryngeal nerve fibers in the postnatal kitten

A combined electron microscopic and electrophysiological study of the superior laryngeal nerve (SLN) was undertaken in postnatal kittens ranging in age from 1–63 days. The superior laryngeal nerve is predominantly a sensory nerve innervating the upper respiratory tract, and could play a potential role in the modulation of respiration, particularly in the infant animal. Distribution of fibers in the developing SLN indicates that within the first postnatal month, 75% of the fibers are unmyelinated, and by 42 days, the myelinated fibers increase in number to approximately 50%. Of the myelinated fibers present in the one day old kitten, 3–4% of those exceeded 4 μm in total diameter, which is the minimum diameter for normal conduction velocity of action potentials. The distribution of the diameter sizes of the myelinated fibers is bell-shaped within the first 45 days after which the curve becomes skewed to the right (43–61 days; mean 2.6 μm, range 0.5–8.0 μm) to resemble the adult distribution of myelinated fibers (mean 4.2 μm, range 1.6–13.0 μm). Two variable plots of myelin width to axon diameter suggest a steeper slope for developing fibers as compared to that of the adult fibers. Electrical stimulation of the sectioned SLN indicates that evoked potentials could be recorded from the recurrent laryngeal nerve innervating the laryngeal intrinsic muscles and from the hypoglossal nerve to the tongue musculature in the youngest kittens tested (i.e., age 9 days). Stimulation at selected frequencies of 3 and 30/sec readily evoked apnea in the youngest kitten studied (i.e., age 5 days), while swallowing was more readily evoked at 28–30 days when using electrical stimulation.     

扬子鳄视神经的研究

本文研究了扬子鳄的视神经。结果明明,视神经中可见有髓纤维和无髓纤维。有髓纤维分布均匀,无髓纤维常聚集成团;胶质细胞核,在视神经中可看到两种类型,有髓纤维总数为２００,０００－３００,０００根,纤维直径范围为０．４１－６．６６μｍ,只有一个峰值,峰直径为１．３１μｍ;纤维轴突径与纤维直径之比（ｄ／Ｄ）约为０．７３－０．７５。经统计分析,同个体左右侧神经纤维数目有差异,同一神经中周围区与中央区数目分布     

Conduction velocities and fiber diameters in a cutaneous nerve of the pigeon

Summary The characteristics of fibers of a cutaneous nerve supplying the wing skin of the pigeon have been investigated with electrophysiological and electron microscopic techniques.Recordings of the compound action potential showed four distinct peaks with conduction velocities of about 30 m/s, 12 m/s, 4 m/s and 0.5 m/s.From electron micrographs both fiber diameters and thickness of myelin sheath were assessed and used as criteria for segregating various fiber populations. Altogether four groups could be discerned: large thickly myelinated fibers, small thickly myelinated fibers, small thinly myelinated fibers, and unmyelinated or C-fibers. The subdivision of the thickly myelinated fibers into two populations is evidenced mainly by corresponding peaks in the compound action potential. The thinly myelinated fibers with a mean diameter of 2 m contributed about 90% of all myelinated fibers in this nerve.When comparing fiber dimensions and conduction velocities of this avian nerve with those of mammalian cutaneous nerves, the lower CV's of avian nerve fibers can be explained by smaller diameters and thinner myelin sheaths.The results of this investigation are a prerequisite for latency considerations in central somatosensory pathways in birds.Abbreviations CAP compound action potential - CV conduction velocity - D fiber diameter - d axon diameter - g ratio d/D - m thickness of myelin sheath     

Morpho-functional changes in the turtle midbrain tectum after enucleation

Morpho-functional changes in the tectum mesencephali during degeneration after enucleation were studied inEmys orbicularis L. Comparison of amplitude-time characteristics of evoked potentials of the visual center with degenerative changes in axon terminals and fibers of the optic nerve in the same animals revealed a "light" type of degeneration of the terminals of unmyelinated axons and a "dark" type in terminals of myelinated axons. During "dark" degeneration (4–5 months after enucleation) the low-amplitude presynaptic component of the evoked potential, reflecting excitation of large myelinated fibers, disappeared and changes occurred in the characteristics of the first high-amplitude component, the appearance of which is connected with excitation of myelinated fibers of medium diameter. The last component disappeared 7 months after the operation, along with disappearance of the "dark" degeneration. During "light" degeneration (2.5–3.5 months) changes took place in the characteristics of the second high-amplitude component of the evoked potential, which reflects excitation of thin fibers, both myelinated and unmyelinated, whose ranges of diameters overlap. This component disappeared after 6–7 months, almost simultaneously with disappearance of the first high-amplitude component, as the result of simultaneous completion of degeneration of myelinated fibers of medium and small diameter.     

The chorda tympani and glossopharyngeal nerves in the adult hamster

The numbers and diameters of axons in the intact chorda tympani(CT) and lingual branch of the glossopharyngeal nerve (GN) arequantified with the use of electron microscopic photomontages.The cross-sectional diameters of the CT and GN average 68 and86 microns, respectively. The intact CT contains {small tilde}1050 fibers, 63% are unmyelinated and 37% are myelinated. TheGN contains {small tilde} 1600 fibers, 79% are unmyelinatedand 21% are myelinated. Both nerves are made up of relativelysmall unmyelinated and myelinated fibers, although the GN showsa broader distribution of diameters for its myelinated fibersdue to the presence of general somatosensory fibers. Followingde-efferentation, there is a 48% reduction in the number ofunmyelinated fibers in the CT. Fifty-two per cent of the unmyelinatedfibers are sensory. The number of myelinated fibers is not significantlyreduced and nearly all of the myelinated fibers are sensory.Sixty-seven per cent of the fibers within the CT are sensory.The de-efferented CT contains an equal number of unmyelinatedand myelinated axons and a total of {small tilde} 700 fibers.Comparable data in the rat indicate that its intact and de-efferentedCT are organized differently in regards to the numbers of sensoryand motor, and myelinated versus unmyelinated fibers. The findingsof the present study, together with the available data fromother species, suggest that anatomical differences in the make-upof the major gustatory nerves do not contribute in any obviousway to the known differences in the response properties betweenthe rat and hamster CT, and that the number of myelinated fibersin the visceral motor component of the CT varies considerablyacross species.     

Afferent activity in thin myelinated and unmyelinated cutaneous nerve fibers during mechanical stimulation of skin receptors

Afferent activity in thin myelinated and unmyelinated cutaneous nerve fibers was analyzed by an impulse collision method and by methods improving the signal-to-noise ratio in the record of the antidromic action potential. The following groups were distinguished among the thin myelinated and unmyelinated nerve fibers on the basis of the results of investigation of conduction velocities, thresholds of electrical excitation, and response to mechanical stimulation: A ₁ (conduction velocity 30-14 m/sec) — a relatively larger number of these fibers conducts excitation in response to weak mechanical stimulation; A ₂ (14–4.0 m/sec) — the receptors of these fibers are more easily excited by a strong stimulus; a group of "mixed" fibers, containing myelinated and unmyelinated nerve fibers (4–2 m/sec), conducting excitation in response to both types of mechanical stimulation; C₁ (2.0–1.0 m/sec) — a fairly large number of these unmyelinated fibers conducts impulses in response to weak mechanical stimulation; C₂ (1.0–0.15 m/sec) the majority of fibers of this group is connected with receptors requiring strong mechanical stimulation for their excitation.Research Institute of Applied Mathematics and Cybernetics, N. I. Lobachevskii State University, Gor'kii. Translated from Neirofiziologiya, Vol. 8, No. 1, pp. 67–75, January–February, 1976.     

Composition of the medial and posterior articular nerves of the mouse knee joint

The number and the distribution of fiber size in the medial (MAN) and posterior (PAN) articular nerves of the mouse knee joint were studied by electron microscopy. The MAN contained 75 &#45 28 nerve fibers consisting of 63 &#45 24 unmyelinated and 12 &#45 6 myelinated fibers. The PAN was composed of 195 &#45 50 nerve fibers, namely 129 &#45 28 unmyelinated and 66 &#45 24 myelinated fibers. A skewed unimodal distribution of the unmyelinated nerve fiber diameters was seen in both nerves ranging from 0.1 to 1.2 &#119 m with a maximum between 0.3 and 0.6 &#119 m. The myelinated nerve fibers in the MAN ranged from 1 to 8 &#119 m with a peak between 2 and 5 &#119 m. In the PAN, their diameters ranged from 1 to 12 &#119 m with a clearly visible peak at 4-5 &#119 m and a plateau at 8-9 &#119 m that may represent a second maximum. These data show that the knee joint innervation of the mouse is comparable to those of the cat and rat concerning the types of nerve fibers and the composition of the two nerves. However, in relation to the much smaller area of tissue to be innervated the total number of primary afferents is considerable smaller in the mouse.     

The Fine Anatomy of the Optic Nerve of Anurans—An Electron Microscope Study

In the optic nerve of Anurans numerous myelinated and unmyelinated axons appear under the electron microscope as compact bundles that are closely bounded by one or several glial cells. In these bundles the unmyelinated fibers (0.15 to 0.6 µ in diameter) are many times more numerous than the myelinated fibers, and are separated from each other, from the bounding glial cells, or from adjacent myelin sheaths, by an extracellular gap that is 90 to 250 A wide. This intercellular space is continuous with the extracellular space in the periphery of the nerve through the numerous mesaxons and cell boundaries which reach the surface. Numerous desmosomes reinforce the attachments of adjacent glial membranes. The myelinated axons do not follow any preferential course and, like the unmyelinated ones, have a sinuous path, continuously shifting their relative position and passing from one bundle to another. At the nodes of Ranvier they behave entirely like unmyelinated axons in their relations to the surrounding cells. At the internodes they lie between the unmyelinated axons without showing an obvious myelogenic connection with the surrounding glial cells. In the absence of connective tissue separating individual myelinated fibers and with each glial cell simultaneously related to many axons, this myelogenic connection is highly distorted by other passing fibers and is very difficult to demonstrate. However, the mode of ending of the myelin layers at the nodes of Ranvier and the spiral disposition of the myelin layers indicate that myelination of these fibers occurs by a process similar to that of peripheral nerves. There are no incisures of Schmidt-Lantermann in the optic myelinated fibers.     

Comparative study of the lamina cribrosa and the pial septa in the vertebrate optic nerve and their relationship to the myelinated axons

The optic nerve contains the connective tissues, i.e. the lamina cribrosa and pial septa. This report presents a histological comparison of the lamina cribrosa and pial septa in the five classes (mammals, birds, reptiles, amphibians and teleosts) of vertebrates. Furthermore, the distribution of myelinated fibers was observed from the optic nerve through the retina in the same animals. The lamina cribrosa is found in mammals except for mice, and in birds. Structural complexity of the lamina was different in animals but generally dependent of the optic nerve thickness. The pial septa were present in the optic nerve proper of the mammals except for the mice, in birds and in a part of teleosts. Fasciculation of the optic nerve by the pial septa tended to be more prominent as the optic nerve become thicker. The optic nerve consisted of largely myelinated fibers in vertebrates. The retina contained some myelinated fibers in submammals but was thoroughly devoid of myelinated fibers in mammals. The borderline between myelinated and unmyelinated portions in the optic nerve of different species did not related to the lamina cribrosa. Amphibians had exceptionally only a few myelinated fibers in the optic nerve and no myelinated fibers in the retina.     

Axonal composition of the marginal and basal optic tracts in frogs

An electron-microscopic investigation was made of the medial and lateral branches of the marginal and basal optic tracts. The ultrastructure and composition of the branches of the marginal tract are similar. The basal tract has a relatively loose structure and contains a high proportion of myelinated fibers of large diamter. On average these structures contain 3900 (4.7%), 4700 (4.9%), and 700 (28%) of myelinated and 79,800 (95.3%), 91,500 (95.1%), and 1800 (72%) unmyelinated fibers respectively. The myelinated fibers in the branches of the marginal tract have a diameter of 0.4–2.6 (main maximum 1.0, additional maximum 1.6 µ), those in the basal tract from 0.4 to 4.0 µ (main maximum 1.8, small additional maximum at 3.2 µ). The diameters of the unmyelinated fibers in all three tracts are 0.1–0.5 µ; the diameter of 60% of these fibers is approximately 0.2 µ. Degeneration of 99% of myelinated fibers of the optic nerve and 100% of fibers of the basal tract was found 85 days after enucleation and keeping at 18–20°C. Many nondegenerated myelinated fibers were found in the medial and lateral branches of the marginal tract (33 and 13%, ranges of diameters 0.6–1.4 and 0.6–1.0 µ respectively). These fibers perhaps participate in the organization of ipsilateral visual projection of the tectum. The nonmyelinated fibers were unchanged in all the tracts.A. N. Severtsov Institute of Evolutionary Morphology and Ecology of Animals, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 8, No. 1, pp. 54–61, January–February, 1976.     

Functional and ultrastructural changes in the midbrain tectum of the frog after enucleation

In experiments on frogs immobilized with Diplacin at different times after unilateral enucleation, as degeneration of the optic fibers and their terminals developed, a successive disappearance of the components of evoked potential (EP) was observed; this indicates the direct dependence of the rate of degeneration on the diameters of the fibers. The nature of the ultrastructural changes also depends on the diameter of the cut axons: the terminals of all or some of the myelinated fibers of large diameter degenerated with the condensation of the endings and of all the cytoplasmatic organelles ("dark" type); the terminals of thin myelinated and unmyelinated fibers degenerated with a gradual lysis of organelles ("light" type). Unmyelinated optic fibers and their synapses survived and transmitted the excitation for more than 140 days at a temperature of 18–20°C. In the course of the survival of optic fibers and synapses, the static (latency, duration, and amplitude) as well as dynamic (lability, excitability cycle, and posttetanic changes) characteristics of the EP for electrical stimulation of the nerve changed insignificantly. Direct response of the midbrain tectum decreased in the course of the degeneration of the optic fibers, and after 280 days its amplitude was about 20% of the control value.A. N. Severtsov Institute of Evolutionary Morphology and Ecology of Animals, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 2, No. 2, pp. 180–188, March–April, 1970.     

Quantitative studies on the intermediate nerve of the mouse with the electron microscope.

Quantitative counts of the intermediate nerve fibers of the mouse with electron microscope have shown that, on an average, 1,267 (79.3%) and 301 (18.9%) of the total nerve fibers (1,597) were myelinated and unmyelinated fibers, respectively. The number of unmyelinated fibers in the intermediate nerve was less than one-half the number of unmyelinated fibers in the greater petrosal nerve found in previous studies. Therefore, it is postulated that some of the unmyelinated fibers in the greater petrosal nerve may come from the intermediate nerve, while the rest may be derived from another source.     

Composition of the medial and posterior articular nerves of the mouse knee joint

The number and the distribution of fiber size in the medial (MAN) and posterior (PAN) articular nerves of the mouse knee joint were studied by electron microscopy. The MAN contained 75 +/- 28 nerve fibers consisting of 63 +/- 24 unmyelinated and 12 +/- 6 myelinated fibers. The PAN was composed of 195 +/- 50 nerve fibers, namely 129 +/- 28 unmyelinated and 66 +/- 24 myelinated fibers. A skewed unimodal distribution of the unmyelinated nerve fiber diameters was seen in both nerves ranging from 0.1 to 1.2 microm with a maximum between 0.3 and 0.6 microm. The myelinated nerve fibers in the MAN ranged from 1 to 8 microm with a peak between 2 and 5 microm. In the PAN, their diameters ranged from 1 to 12 microm with a clearly visible peak at 4-5 microm and a plateau at 8-9 microm that may represent a second maximum. These data show that the knee joint innervation of the mouse is comparable to those of the cat and rat concerning the types of nerve fibers and the composition of the two nerves. However, in relation to the much smaller area of tissue to be innervated the total number of primary afferents is considerable smaller in the mouse.     

Neurogenic inflammation of gingivomucosal tissue in streptozotocin-diabetic rat

Previously it was assumed that nerve fibres are involved in the neurogenic inflammation induced by mechanical or chemical irriations. It has been also suggested that in diabetes mellitus the unmyelinated small diameter fibers are impaired as a result of diabetic neuropathy. Therefore, our aim was to study the alterations of the nerve processes in the gingivomucosal tissue in streptozotocin (STZ)-diabetic rats. Light- and electronmicroscopical examinations were made to analyze the changes in nerve fibres. After one week of steptozotocin treatment, the gingivomucosal tissue had inflammatory cell infiltration and some degenerated nerve fibres were also observed. Dense mitochondria, disorganization of cell organelles, and appearance of myelin-like dense bodies were found in the axons of degenerared nerve fibres. Semiquantitative analysis showed that 14 +/- 4% of the unmyelinated nerve fibres degenerated after one week of STZ treatment. However, degeneration of the myelinated nerve fibers was not observed. Two weeks after STZ treatment, most of the unmyelinated and myelinated nerve fibers showed degeneration (86 +/- 5%) and the placement of the ligature revealed a non-inflammatory connective tissue adjacent to a normal epithelium. The myelin sheath was disrupted and dark axoplasm with cytolysosomes became manifest. These findings demonstrated that both unmyelinated and myelinated nerve fibers are altered and inflammatory reaction exists in the gingivomucosal tissue only in the early stage of diabetes mellitus.     

Morphometry and ultrastructure of the squirrel monkey (Saimiri sciureus) vestibular nerve

Vestibular nerves of squirrel monkeys (Saimiri sciureus) embedded in plastics and epoxies were examined with light microscopy (LM) and transmission electron microscopy (TEM), and computerized measures were obtained and analyzed statistically. An average of 12,412 perikarya and 12,005 myelinated nerve fibers was obtained. Approximately 0.7% of the perikarya appeared unmyelinated under LM. About 500 unmyelinated fibers were counted. The cross-sectional area of 1,864 perikarya was 200-650 micron 2. The cross-sectional area of 1,346 nerve fibers was 3-11 micron 2 for the axoplasm and 11-12 micron 2 for the myelin sheath of the same fiber. Myelin thickness was directly proportional to the axoplasm cross-sectional area of the nerve fibers. The cross-sectional area of central axons and peripheral dendrites differed significantly (p less than 0.001). The initial segments of peripheral dendrites were usually smaller, but longer than the initial segments of the central axons. Both initial segments increased in diameter after the first node of Ranvier. Schmidt-Lantermann incisures were more abundant in thick and heavily myelinated fibers than in thin and lightly myelinated fibers. Larger perikarya usually had larger fibers and vice versa, within the first 100-200 micron from the first node of Ranvier. No major ultrastructural differences were found between myelinated and unmyelinated perikarya, except at the hillock region. The Nissl substance was preferentially located in the peripheral cytoplasm.     

Gliogenesis and myelination in the optic nerve of trisomy 19 mice. A quantitative electron-microscopic study

Trisomy 19 (ts19) of the mouse permits detailed studies on the influence of an extra autosome upon the postnatal development of the central nervous system. To examine gliogenesis and myelinogenesis, the optic nerves of 19 ts19 pugs aged 1-15 days have been examined by light and electron microscopy and compared to those of litter-mate controls. Differentiation of astrocytes and oligodendrocytes, myelinogenesis as well as the opening of the eyes are each delayed by about 2 days. Myelin sheaths are normally structured in ts19. There is a decrease in the percentage of myelinated fibres. The cross-sectional area of the ts19 optic nerve is reduced. The fibre density, which decreases with age both in ts19 and control mice, is higher in ts19 mice. Both with ts19 and control animals, the distribution of fibre diameters of myelinated axons overlaps with that of promyelinated and unmyelinated fibres, but myelinated axons cannot be observed below a diameter of 0.3 micron, and unmyelinated axons are always smaller than 1 micron. The mean diameter of promyelinated axons is identical in ts19 and control animals. Myelination is therefore not severely disturbed in the ts19 optic nerve. As retinal differentiation in ts19 is delayed by 2 days as well, reports on an asynchronous development of neurons and myelin sheaths cannot be confirmed for the visual system.     

A quantitative electron microscope analysis of peripheral nerve in the urodele amphibian in relation to limb regenerative capacity

An approximate 1:1 ratio of myelinated to unmyelinated fibers was established in counts from electron micrograph montages in nerves of the newt, Triturus (Notophthalmus) viridescens. The number of myelinated fibers correspond to the number counted with the light microscope after osmium fixation. Light microscope counts of silver impregnated sections yielded a value slightly higher suggesting that, except for bundles of unmyelinated fibers, the silver technique revealed mainly myelinated fibers. The results were used to reassess previous quantitative studies on the relation between number of nerve fibers and the control which nerves exert on regeneration. For a truer estimate of the number of axons affecting regeneration, fiber values previously reported should now be doubled to include the large number of unmyelinated fibers. However, calculations show that the unmyelinated fibers contribute less than 3% of the total neuroplasm in the peripheral nerve. Finally, counts made of Schwann cells and fibroblasts show that the latter are few in number.     

Quantitative studies on the motor root of the mouse facial nerve. An electron-microscopic study.

Fiber count analysis was performed with the electron microscope on the motor root of the facial nerve in six mice. On an average, 3,433 (84.9%) of the total nerve fibers (4,046) were myelinated and 592 (14.6%) unmyelinated. The motor root consisted mostly of large myelinated fibers (large fiber group), but a part of the root consisted of small myelinated and unmyelinated fibers (small fiber group). The nerve fibers of the small fiber group appear to correspond with those of the intermediate nerve, and to pass through the motor root and enter the intermediate nerve near the geniculate ganglion.     

Functional differentiation of splanchnic A delta fibres in relation to viscerosomatic reflexes.

The splanchnic afferent signal in the myelinated fibre spectrum was analysed in cats and its relationship to viscerosomatic reflex activity was studied. In addition to Abeta and Adelta components, a further elevation was observed in the sympathetic chain neurogram. According to the conduction rate (15 plus or minus 4 m/sec), it was caused by myelinated fibres 2-3 mu in diameter. The presence of this component at thoracic dorsal root level, or in the sympathetic chain in stimulation of the postganglionic part of the nerve trunk, confirmed the afferent nature of the observed activity. The threshold stimulus for these fibres, which were termed group Adelta2, was four times higher than for Abeta fibres. Correlation of the neurogram with evoked activity in the intercostal nerves confirmed the different functional role of group Adelta2 fibres. Adelta2 fibre activity evokes the early component of the two-component viscerosomatic discharge and Adelta2 fibre activity its later component. The authors discuss the functional significance of Adelta2 fibres, which probably mediate nociceptive information from the viscera.