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.     

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.     

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     

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.     

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.     

Ultrastructural changes of human sural nerves in the neuropathy induced by intrauterine methylmercury poisoning (so-called fetal Minamata disease).

Biopsy of the sural nerve was performed on three patients with severe Minamata disease of more than 10 years duration. There were so many unmyelinated and poorly myelinated nerve fibers that myelinated fibers scattered irregularly in small numbers or in groups of peculiar features in the intraneural bundle. Abnormaly thin or poorly formed myelin sheaths were noticed. Incomplete myelination and abnormal myelination varied in size and shape appeared as fetal anomaly. Regenerated axons extremely small in size remained singly or in groups following regenerative sprouting. Sometimes, extremely small axons with normal myelination were noticeable, while the axons were lost, leaving myelin sheaths. Axons occasionally contained increased neurofilaments. Schwann cells were not so increased as in adult Minamata disease. Degenerative changes of nerve fibers still proceeded, presumably because the patients lived in the mercury-contaminated district. Myelin degenerations and glycogen deposits in the axoplasm were identified.     

Myelinated synapse-bearing cell bodies in the central nervous system of Scyliorhinus canicula (L.)

Summary Some neurons of the anterior lateral-line lobe of dogfish (Scyliorhinus) have synapse-bearing perikarya enclosed by layers of compact myelin. The identity of these cells, which have myelinated axons and unmyelinated, synapse-bearing dendrites, is unknown.     

Morphometric analysis of the sciatic nerve and its principal branches in the pigeon (Columba livia)]

The methodological approach used in this study is to characterize the number, the density and the diameter distribution of myelinated fibers (MFs) and unmyelinated fibers (UMFs) in sciatic nerve and its main branches of pigeon. The results have shown that the fiber composition is quite variable because in pigeon there are relatively MF with thin myelin sheaths and MF with thicker sheaths. Our data suggest that morphometric analysis could represent a helpful methodological approach to better characterize these systems.     

Compact myelin dictates the differential targeting of two sodium channel isoforms in the same axon

Voltage-dependent sodium channels are uniformly distributed along unmyelinated axons, but are highly concentrated at nodes of Ranvier in myelinated axons. Here, we show that this pattern is associated with differential localization of distinct sodium channel alpha subunits to the unmyelinated and myelinated zones of the same retinal ganglion cell axons. In adult axons, Na(v)1.2 is localized to the unmyelinated zone, whereas Na(v)1.6 is specifically targeted to nodes. During development, Na(v)1.2 is expressed first and becomes clustered at immature nodes of Ranvier, but as myelination proceeds, Na(v)1.6 replaces Na(v)1.2 at nodes. In Shiverer mice, which lack compact myelin, Na(v)1.2 is found throughout adult axons, whereas little Na(v)1.6 is detected. Together, these data show that sodium channel isoforms are differentially targeted to distinct domains of the same axon in a process associated with formation of compact myelin.     

Morphological and morphometric studies of early myelination in the cervical dorsal funiculus of the newborn mouse spinal cord

The early myelination of the dorsal funiculus at the level of the 4th cervical spinal cord was ultrastructurally studied in the one-day-old mouse. It was found that the fibers were mainly unmyelinated. However, some early myelinated fibers were scattered among unmyelinated fibers. In the initial stage of the myelination, the axon was partially contracted by a piece of cytoplasmic process of the oligodendroglial cell. The two lips of the oligodendroglial process then extended and converged, enwrapping the axon completely and forming the first contact point. With the anchorage of that contact point, the two lips of the process became elongated and enfolded by each other, and produced the internal and external tongues of the future myelin sheath. More contact points were formed at a regular interval by the regional fusion of the two external surface layers of the opposed cytoplasmic membranes of adjacent tongue processes. With the advanced bidirectional spiralization of the two tongue processes, many contact points were found between the adjacent lamellae of the concentrically arranged oligodendroglial process; simultaneously, the cleft between the neighboring contact points disappeared and formed the initial sites of the intraperiod line. During the early myelination, one single axon ensheathed concentrically by two different oligodendroglial processes as well as several axons enwrapped by a continuous spiral myelin sheath of one oligodendroglial cell were frequently observed. The cross-sectional areas of unmyelinated axons varied from 0.01 to 0.2 micron 2, with a median of 0.07 micron 2; whereas, that of promyelinated axons ranged from 0.09 to 1.4 micron 2, with a median at 0.61 micron 2. These data support the suggestion that the axon calibre is a critical factor for the initiation of central myelination.     

Pathological changes of human sural nerves in Minamato disease (methylmercury poisoning). Light and electron microscopic studies.

Biopsy of the sural nerve was performed on six patients with relatively mild Minamata disease of 10-year or longer duration. All of the six patients presented characteristic pathologic changes. Light microscopy disclosed the formation of irregularly shaped myelin sheaths and fine axons, an increase in them, which is suggestive of incomplete regeneration, cicatrization following the loss of nerve fibers, increase in Schwann's nuclei, and formation of Büngner's bands. Electron microscopy revealed incomplete myelinated fibers and ultrafine unmyelinated fibers associated with incomplete regeneration, formation of regeneration units, and collagen increase. The laminar encapsulation with the processes of Schwann's cells were often observed in ultrafine fibers. In view of the fact that small quantities of mercury-contaminated fishes are still being caught in the Minamata district, myelin degeneration, glycogen deposits and appearance of dense bodies in axons, and vesiculation and fragmentation of endoplasmic reticulum were observed as degenerative changes due to the effects of mercury accumulation.     

Morpho-functional characteristics of the optic nerve of the steppe turtle Agrionemys horsfieldi]

The optic nerve of the tortoise Agrionemys horsfieldi contains about 400,000 fibers (90% unmyelinated and 10% myelinated ones). the diameter of unmyelinated fibers varies from 0.3 to 1.1 mu, mean value being equal to 0.5 mu; fibers with a diameter 0.4-0.7 comprise 77%. The diameter of myelinated fibers varies within 0.3-3.0 mu, average value being 0.5-0.8 nu; fibers with a diameter 0.5-0.9 mu amount to 62%. Electrogram of the optic nerve consists of two components which are equal in their amplitudes. These components presumably reflect summary firings of modal groups of unmyelinated and myelinated fibers. The velocity of propagation of excitation along the fibers producing the first component is equal to 1.3 m/sec, wheras that in fibers producing the second component - to 0.5 m/sec. The data obtained are compared with those related for the other tortoise - Emys orbicularis.     

Acetylcholinesterase: a histochemical identification of motor and sensory fascicles in human peripheral nerve and its use during operation

To evaluate the precision of acetylcholinesterase histochemical identification of motor and sensory fascicles, this study presents a systematic observation of human peripheral nerves by Karnovsky and Roots' histochemical method. The results indicate that either of the enzymatic activities of myelinated and unmyelinated fibers was different between motor and sensory fascicles. Fifty-seven percent of the myelinated fibers showed enzymatic activity in the motor fascicles, while none of the myelinated fibers in the sensory fascicles showed enzymatic activity. The unmyelinated fibers showing enzymatic activity in the sensory fascicles were far denser than those in the motor fascicles. Our study demonstrated that the unmyelinated fibers were sympathetic postganglionic unmyelinated fibers. From these results it is concluded that the motor and sensory fascicles may be identified not only according to the enzymatic activities of the myelinated fibers, but also according to the enzymatic activities of the sympathetic postganglionic unmyelinated fibers. An improved histochemical method was suggested for its applicability as a method of intraoperative nerve fascicle identification. Simulated experiments were done on the radial nerves and the median nerves in human cadavers. This improved histochemical process can be completed within 50 minutes and can be used in intraoperative nerve fascicle identification.     

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.     

Ways to approximate current-distance relations for electrically stimulated fibers

The behaviour of myelinated and unmyelinated fibers is simulated by a model for extracellular stimulation in order to confirm current-distance phenomena known from experiments. Lower and upper limits for cells stimulated with cathode current are calculated for unmyelinated fibers. In the case of myelinated fibers the threshold depends on the distance from the axon and from the nodes.     

Localization of Phospholipid Synthesis to Schwann Cells and Axons

Quantitative electron microscopic autoradiography was used to detect and characterize endoneurial sites of lipid synthesis in mouse sciatic nerve. Six tritiated phospholipid precursors (choline, serine, methionine, inositol, glycerol, and ethanolamine) and a protein precursor (proline) were individually injected into exposed nerves and after 2 h the mice were perfused with buffered aldehyde. The labeled segments of nerve were prepared for autoradiography with procedures that selectively remove nonincorporated precursors and other aqueous metabolites, while preserving nerve lipids (and proteins). At both the light and electron microscope levels, the major site of phospholipid and protein synthesis was the crescent-shaped perinuclear cytoplasm of myelinating Schwann cells. Other internodal Schwann cell cytoplasm, including that in surface channels, Schmidt-Lanterman incisures, and paranodal regions, was less well labeled than the perinuclear region. Newly formed proteins were selectively located in the Schwann cell nucleus. Lipid and protein formation was also detected in unmyelinated fiber bundles and in endoneurial and perineurial cells. Tritiated inositol was selectively incorporated into phospholipids in both myelinated axons and unmyelinated fibers. Like inositol, glycerol incorporation appeared particularly active in unmyelinated fibers. Quantitative autoradiographic analyses substantiated the following points: myelinating Schwann cells dominate phospholipid and protein synthesis, myelinated axons selectively incorporate tritiated inositol, phospholipid precursors label myelin sheaths and myelinated axons better than proline.     

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.     

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.     

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.     

Label-free detection of peripheral nerve tissues against adjacent tissues by spontaneous Raman microspectroscopy

Detection of peripheral nerve tissues during surgery is required to avoid neural disturbance following surgery as an aspect of realizing better functional outcome. We provide a proof-of-principle demonstration of a label-free detection technique of peripheral nerve tissues, including myelinated and unmyelinated nerves, against adjacent tissues that employ spontaneous Raman microspectroscopy. To investigate the Raman spectral features of peripheral nerves in detail, we used unfixed sectioned samples. Raman spectra of myelinated nerve, unmyelinated nerve, fibrous connective tissue, skeletal muscle, tunica media of blood vessel, and adipose tissue of Wistar rats were analyzed, and Raman images of the tissue distribution were constructed using the map of the ordinary least squares regression (OLSR) estimates. We found that nerve tissues exhibited a specific Raman spectrum arising from axon or myelin sheath, and that the nerve tissues can be selectively detected against the other tissues. Moreover, myelinated and unmyelinated nerves can be distinguished by the intensity differences of 2,855 cm^?1, and 2,945 cm^?1, which are mainly derived from lipid and protein contents of nerve fibers. We applied this method to unfixed section samples of human periprostatic tissues excised from prostatic cancer patients. Myelinated nerves, unmyelinated nerves, fibrous connective tissues, and adipose tissues of the periprostatic tissues were separately detected by OLSR analysis. These results suggest the potential of the Raman spectroscopic observation for noninvasive and label-free nerve detection, and we expect this method could be a key technique for nerve-sparing surgery.