Occurrence of long non-myelinated axonal segments intercalated in myelinated, presumably sensory axons: electron microscopic observations in the dog atrial endocardium

Summary Electron microscopy of serial sections revealed the occurrence of long non-myelinated segments in myelinated, presumably sensory axons running in the left atrial endocardium of normal adult dogs. Four such non-myelinated segments were analysed in three myelinated axons. They varied from 20 to 150 m in length, and differed from nodes of Ranvier in being invested by Schwarm cells in the manner of unmyelinated nerve fibres. Short non-myelinated portions (20–25 m long) were associated with a single Schwann cell, whereas the longest such segment (150 gmm) had five. The non-myelinated axonal segments were non-varicose and similar in diameter (1.2–3.0 m) to adjacent myelinated segments, which had myelin sheaths 6–25 lamellae thick. The cytoplasm of the non-myelinated axonal segments contained numerous neurofilaments and microtubules, some mitochondria and smooth endoplasmic reticulum. The short non-myelinated segments were enclosed by perineurium, whereas the long non-myelinated segment was devoid of perineurium at its mid-portion; instead fibroblast-like cells made a loose boundary around the axon at this level. The significance of these non-myelinated segments was discussed with special emphasis on the question of whether they result from focal degeneration of the myelin sheath (demyelination) or are generally present in the preterminal regions of some axons.     

Studies on unmyelinated axons and varicosities in the olfactory cortex

Summary The main afferent input to the olfactory cortex from the olfactory bulbs is via the lateral olfactory tract (LOT). The axons within the lateral olfactory tract are myelinated. On leaving the LOT, they lose their myelination as they fan out over the layer immediately beneath the pial surface to make en passant synaptic connections with dendrites from neurones within the olfactory cortex. Using the guinea-pig, a semiquantitative electron micrographical study was made of the density and dimensions of these unmyelinated axons and the varicosities they create. The unmyelinated axons were very fine (0.17 ± 0.004 m in diameter) and punctuated at 2 m intervals by varicosities containing a single type of vesicle. The electrophysiological consequences of this close varicosity spacing is that axonal and varicosity membranes behave electrically as single units.     

Further evidence for myelinated as well as unmyelinated fibre damage in a rat model of neuropathic pain

Summary A mononeuropathy, produced by ligation of the sciatic nerve in rats, has recently been proposed as an animal model of experimental pain and pain-related disorders (hyperalgesia and allodynia). We investigated quantitatively the morphological changes in myelinated and unmyelinated fibres of the sciatic nerves 2 weeks after ligation in rats exhibiting allodynia to thermal stimulation. There was a marked reduction in the number of large myelinated fibres distal to the ligature (711 ± 34 compared with 5315 ± 230 in normal nerves). We also found a significant loss of small myelinated fibres (2429 ± 109 compared with 3197 ± 308 in normal nerves), the remaining fibres of this type showing pathological properties. Finally, ultrastructural evidence of damage to unmyelinated fibres was found. The typical pattern of large clusters of normal unmyelinated axons was no longer present within most regions of the nerve. There was a significant reduction in the size of the unmyelinated fibres (0.41 m ± 0.15 compared with 0.71 m ± 0.08 in normal nerves), together with a twofold increase in their number per cluster. Hypotheses about the mechanism of thermal allodynia in this pain model therefore must take into account the fact that all fibre classes show pathological changes.     

Pyramidal tract of the cat: axon size and morphology

Summary The purpose of this work was to determine the number and morphology of pyramidal tract (PT) axons in the cat, using electron microscopy, modern methods of fixation, and computer-assisted morphometric analysis. Sections taken at the level of the medullary pyramids in three animals were fixed and magnified up to 10,000 x to produce photomicrographs. Morphological data were entered into computer files for analysis by tracing axon perimeters on micrographs mounted on a digitizer tablet. The number of axons per PT averaged 415,000, of which 88% were myelinated and 12% were unmyelinated. 90% of the myelinated axons fell in the diameter range 0.5–4.5 m. Axons larger than 9 m diameter accounted for 1% of the total; the largest were 20–23 m. Myelinated axon mean diameter was 1.98 m; because of the skewed distribution, with many small axons and a few very large axons, median diameter was 1.60 m. Size distribution was relatively uniform throughout the PT cross section, with all sizes represented in all regions. However, the more medial regions had a higher proportion of small fibers than the more lateral regions: mean medial diameter was 1.85 m while mean lateral diameter was 2.09 m. Myelin sheath thickness averaged 7.9% of fiber diameter for axons up to 11 m, but was constant at 0.9 m for larger fibers. Myelinated fibers were distorted from the circular shape in cross section, with a mean circularity index (or form factor) of 0.85, which implies that the fibers could swell about 15% without rupture of the cell membrane. Unmyelinated fibers averaged 0.18 m diameter (range 0.05–0.6 m); the largest unmyelinated axons were larger than the smallest myelinated axons. It is concluded that previous work greatly underestimated the number of axons in the cat pyramidal tract.     

Macromolecular structure of axon membrane and action potential conduction in myelin deficient and myelin deficient heterozygote rat optic nerves

Summary The macromolecular structure of the axon membrane in optic nerves from 25-day-old male littermate control and myelin deficient (md) rats and 16-month-old md heterozygotic rats was examined with quantitative freeze-fracture electron microscopy.The axon membrane of control optic nerves displayed an asymmetrical partitioning of intramembranous particles (IMPs); P-fracture faces of myelinated internodal axon membrane were more particulate than those of pre-myelinated axons (1600 1100 m^–2, respectively), while relatively few IMPs (150 m^–2) were present on external faces (E-faces) of internodal or pre-myelinated axon membrane. Amyelinated axons of md optic nerves also exhibited an asymmetrical partitioning of IMPs; protoplasmic membrane face (P-face) IMP densities, taken as a group, exhibited a wide range (600–2300 m^–2) and, in most regions, E-faces displayed a relatively low IMP density (175 m^–2). Axons of > 0.4 m diameter exhibited significantly greater mean P-face IMP density than axons < 0.4 m diameter. Aggregations of E-face IMPs (350 m^–2) were occasionally observed along amyelinated axon membrane from md optic nerves.Optic nerves from md heterozygote rats exhibit myelin mosaicism, permitting examination of myelinated and amyelinated axon membrane along the same tract. The axon membrane exhibits different ultrastructure in these two domains. Myelinated internodal axon membrane from md heterozygote optic nerves exhibits similar P- and E-face IMP densities to those of control internodal axolemma (1800 and 140 m^–2, respectively). Amyelinated axons in the heterozygote exhibit a membrane structure similar to amyelinated axons in md optic nerve. P-face IMP density of large diameter (> 0.4 m) amyelinated axons from md heterozygote optic nerves is significantly greater than that of small calibre (< 0.4 m) axons. In most regions, amyelinated axon membrane exhibits a relatively low E-face IMP density (200 m^–2); however, focal aggregations (400 m^–2) of E-face particles are present.Electrophysiological recordings demonstrate that amyelinated axons in md optic nerves support the conduction of action potentials. Compound action potentials in md optic nerves exhibit a monophasic configuration, even at 20-days postnatal, similar to that of pre-myelinated optic nerve of 7-day-old normal rats. Moreover, conduction velocities in the amyelinated 20-day-old md optic nerve are similar to those displayed by pre-myelinated axons from 7-day-old optic nerves. These results are consistent with persistence of action potential conduction in md axons, despite the absence of myelination in the optic nerves of the md mutant.     

Quantitative analysis of a cross-sectional area of the optic nerve: A comparison between albino and pigmented rats

Summary Comparative studies were made on albino and gray rats by measuring several morphological characteristics seen in cross-sections of the optic nerve. The total cross-sectional area of the optic nerve was about 30% smaller in the albino than in the gray rat, while the fiber density was 1.4 times higher in the albino than in the gray rat. The estimated total fiber count was about the same in the two strains: around 100,000 to 110,000 fibers. Except for a few unmyelinated fibers (less than 1%) all fibers were myelinated. Axon diameters of the optic nerve fibers were distributed in smaller values for the albino than for the gray rat, though the overall diameter range was similar (0.2–3.0 m). The myelin sheath was also thinner in the albino than in the gray rat.This paper is dedicated to Dr. Kitsuya Iwama, emeritus Professor of Osaka University Medical School, on his retirement     

Long-term acrylamide intoxication induces atrophy of dorsal root ganglion A-cells and of myelinated sensory axons

We have examined the effects of acrylamide on primary sensory nerve cell bodies and their myelinated axons in chronic acrylamide intoxication. The numbers and sizes of dorsal root ganglion cell bodies (L5) and myelinated nerve fibers were estimated with sterelogical techniques in severely disabled rats which had been treated with 33.3 mg/kg acrylamide twice a week for 7.5 weeks. There was no loss of dorsal root ganglion cells or myelinated nerve fibers in the roots, the sciatic nerve, sural nerve, and a tibial nerve branch. The mean perikaryal volume of A-cells was reduced by 20% (2P < 0.001) from 50000 m³ in controls (CV = 0.13) to 40000 m³ (0.12), whereas B-cell volume was unchanged. All size-frequency distribution curves of myelinated axon area of peripheral nerves and sensory roots were shifted to the left towards smaller values in rats exposed to acrylamide. In the L5 sensory root 3 mm from the ganglion, there was a significant reduction of mean cross sectional area of myelinated axons by 14% (2P < 0.05) from 7.6 m² (0.11) in controls to 6.5 m² (0.13) in intoxicated rats. The mean cross sectional area of myelinated sural nerve axons was reduced by 22% (2P < 0.001) from 8.6 m² (0.08) in controls to 6.7 m² (0.17) in intoxicated rats. We conclude that chronic intoxication with acrylamide leads to selective atrophy of type A dorsal root ganglion cell bodies and simultaneous atrophy along their peripheral axons, whereas neuronal B-cell bodies and motor axons are spared. It is suggested that the neuronal atrophy might well represent a defect of neurofilament synthesis and transport.     

Differentiation of the nodal and internodal axolemma in the optic nerves of neonatal rats

Summary Axon plasma membranes (axolemma) were studied by freeze-fracture electron microscopy at stages prior to and during myelination in the optic nerves of neonatal rats. In unensheathed axons, intramembranous particles associated with the internal (P) and external (E) leaflets of the axolemma increased in number before reaching a plateau (approximately 600/m² in both leaflets) at about 9 days postnatally. In newly myelinated fibres, by contrast, the distribution of particles was asymmetrical; fewer particles (approximately 200/m²) were found on the E-face and greater numbers (approximately 1400/m²) were present on the P-face, distributions similar to those observed in mature myelinated fibres. Node-like aggregations of particles were not found in unensheathed pre-myelinated axons nor were they present in axons presumed to be ensheathed by glial cytoplasm but not yet myelinated, although nodal specializations could be easily identified in fibres with only a few turns of compact myelin. These observations show first that there is a redistribution of particles in the P- and E-faces of the internodal axolemma coincident with the onset of myelination and secondly, that nodal specializations (represented by the increased densities of E-face particles) appear after ensheathment but before the formation of compact myelin in fibres of the rat optic nerve.This work was presented in part at the Society for Neurosciences Meeting, 1980.     

Freeze-fracture study of the mechanoreceptive digital corpuscles of mice

Summary The freeze-fracture replication technique was used to study the mechanoreceptive digital corpuscles in toe pads of mice. The axon terminal plasmalemma had intramembranous particles (IMPs) at a density of 2367 ± 517 m^–2 (mean ±s.e.m.) in the P-face and 84 ± 4 m^–2 in the E-face. Particles were 10 ± 1.8 nm in diameter in the P-face and 10 ± 1.5 nm (mean ±s.d.) in the E-face. Particle-rich and particle-free areas were noted in the P-face. The lamellar cell plasmalemma had IMPs at a density of 3359 ± 224 m^–2 in the P-face and 265 ± 95 m^–2 in the E-face. Particles were 10 ± 1.4 nm in diameter in the P-face and 10 ± 1.6 nm in the E-face. Non-terminal unmyelinated fibres in the connective tissue compartment of toe pads were also examined: the P-faces of the axolemma and Schwann cell plasmalemma had IMPs at a density of 1356 ± 283 m^–2 and 1514 ± 514 m^–2, respectively, while the E-face of these membranes had only a few particles. Particles were 9 ± 1.2 nm and 10 ± 1.6 nm in diameter in the P-faces of axon and Schwann cell plasmalemmata, respectively.The results show that the IMPs in terminal axolemma and in lamellar cell plasmalemma have a much higher density than those of non-terminal axons or Schwann cells in myelinated and unmyelinated fibres. In addition, IMPs in the terminal axolemma are larger than those in non-terminal axolemma except for the nodal axolemma. It can be said that plasmalemmata of both the axon terminals and lamellar cells of digital corpuscles are specialized in terms of IMPs, suggesting that they have specific physiological properties in mechanoreceptive functions including mechano-electric transduction.     

Degeneration of myelinated sympathetic nerve fibres following treatment with guanethidine

Summary The specificity and characteristics of the degeneration of myelinated axons after chronic guanethidine treatment have been investigated in sympathetic and non-sympathetic nerves. Adult male Sprague-Dawley rats aged approximately 43 weeks were treated with guanethidine sulphate (50 mg per kg body weight per day) for between ten days and six weeks. Tissues were examined by qualitative and quantitative light and electron microscopy. In the superior cervical (sympathetic) ganglion (SCG), guanethidine treatment produced a 78% decrease (P = 0.009) in the mean number of myelinated fibres at a standard level of section, compared to the contralateral control ganglion which was removed surgically prior to drug treatment. This reduction in the treated SCG was apparent after 10 days, though complete degeneration of nerve cell bodies was not widespread at this stage. Degeneration of unmyelinated axons was extensive. Degenerating myelinated fibres were consistently small in diameter (up to 3m). In individual myelinated fibres the earliest signs of degeneration involved disruption of axonal organelles, particularly the cytoskeleton, and focal widening of the periaxonal space. Myelin breakdown followed these events; degeneration of myelin still associated with a structurally intact axon was not observed. Myelin breakdown appeared to take place initially within the Schwann cell, at least to the stage of loosened membranes. However, infiltrating cells were also involved in myelin phagocytosis. At all stages of treatment some small diameter myelinated fibres remained intact, and there was no evidence of degeneration of the larger diameter fibres (up to 15 m) which are consistently present in small numbers in the SCG. In the cervical sympathetic trunk, which carries preganglionic axons to the SCG and the vagus and sciatic nerves, degeneration only of unmyelinated axons was detected. These results indicate that guanethidine does not exert a primary degenerative influence on myelin or myelinating Schwann cells and that the myelin degeneration observed in the SCG is a secondary result of the previously documented selectively destructive effect of guanethidine on postganglionic sympathetic neurons. Surviving, small diameter myelinated fibres in the SCG could be either preganglionic or processes of resistant postganglionic neurons, while the larger diameter fibres are likely to be somatic. While the cervical sympathetic trunk, vagus and sciatic nerves all contain postganglionic sympathetic fibres it appears that few of these are myelinated, at least at the levels sampled in this study.     

Axo-glial relations in the retina-optic nerve junction of the adult rat: freeze-fracture observations on axon membrane structure

Summary The axolemmal ultrastructure of nerve fibres within the retina-optic nerve junction (ROJ) from adult rats was examined by freeze-fracture electron microscopy. In the juxtaocular (proximal) region of the ROJ, all fibres are unmyelinated. The axons generally have a membrane ultrastructure similar to that of retinal nerve fibre layer axons, with a high density of intramembranous particles (IMPs) on the P-fracture face and a low density of IMPs on the E-face. However, along some axons in this region of the ROJ, localized aggregations of E-face IMPs are observed. At levels of the ROJ closer to the optic nerve proper, the unmyelinated fibres enter a transition zone in which the axons acquire myelin sheaths. By the distal boundary of the transitional zone (optic nerve proper), virtually all fibres are myelinated. Within the transitional zone, conventional axo-glial associations and axolemmal ultrastructure is present at nodes of Ranvier. In addition, atypical axo-glial relationships and atypical nodal segments are observed in this region. At some nodes, an isolated oligodendroglial process traverses obliquely across the nodal membrane. Beneath the oligodendroglial process, the axolemma usually displays a paranodal-like ultrastructure. Finger-like oligodendroglial processes were also observed in association with non-nodal unmyelinated axon membrane. At these sites of association, the axon membrane tends to be indented and may have a paranodal-like morphology. Nodal axolemma may exhibit several atypical forms in the transition zone. At some nodes, the nodal axolemma has a low density of E-face particles. Also, nodes of extended linear length (2m) exhibit a lower-than-normal density of P-face IMPs. At heminodes, the axolemma immediately adjacent to the terminal loops lacks the usual nodal characteristics of high IMP density and high percentage of large particles. The results show that aberrant axo-glial associations accompanied by unusual ultrastructural characteristics of the axolemma are present in the ROJ of normal adult rats.     

Axons of sacral preganglionic neurons in the cat: I. Origin,initial segment,and myelination

Parasympathetic preganglionic neurons in the cat sacral spinal cord innervate intraspinal neurons and pelvic target organs. Retrograde tracing studies have revealed little of the morphology of their axons including their origin, initial segments, or their myelin, due to methodological limitations. Intracellular labeling of single neurons with neurobiotin or HRP has overcome these problems. Axons were studied in 24 preganglionic neurons. In 21 neurons the axon originated as a branch of a dendrite, without a detectable axon hillock, at distances from the soma ranging from 10 to 110 m (average 34.1 m ). In 3 neurons the axon was derived from the soma. Initial segments, present in all cells, ranged from 15 to 40 m (average 26.8 m). Nearly all axons followed the initial segment with unmyelinated segments that varied between 59 to 630 m, followed by myelin and nodes of Ranvier. Internodal distances were variable and relatively short (average 93 m). Axonal diameters measured over the intraspinal course in 18 axons averaged 1.3 m (range 0.6–2.4 m) and were relatively constant compared with other neurons. Spine-like protrusions were observed on the initial segments of 12 cells. Axon collaterals originated from unmyelinated sections and nodes of Ranvier. Antidromic action potentials showing initial segment, soma-dendritic inflections, did not differentiate between soma-derived and dendrite-derived axons. The data suggest that axons originating from a dendrite are the normal structure of preganglionic neurons in the lateral sacral parasympathetic nucleus. It is proposed that the particular structure of these axons may be part of a timing mechanism that coordinates preganglionic neurons with other spinal neurons involved in target organ reflexes.     

A light and electron microscopic examination of the vagal hepatic branch of the rat

Summary The rat's vagal hepatic branch and associated tissues were studied using light and electron microscopy. Whole mounts, serial sections, and vascular endocasts were used to characterize the tissue from the anterior vagal trunk to the porta hepatis. Fiber number and caliber as well as intraneural organization were analyzed from complete cross-sectional electron micrographic montages of the hepatic branch sampled at its point of separation from the anterior vagal trunk.The hepatic branch ramified from the anterior vagus in one (in 47% of the specimens), two (in 37%) or three (in 16%) bundles. The single bundled hepatic branch contained 2887±287 unmyelinated fibers, and their size distribution, with a mean diameter of 0.66±0.02 m, was Gaussian. Myelinated fibers numbered only 21±4 per branch and had a complex size distribution ranging from 0.5 to 1.8 m with a mean of 1.2±0.03 m. Forty four ±6% of the myelinated fibers were found in a single subfascicle in the dorso-medial pole of the nerve. Whole mounts at this level revealed that a distinct bundle, here designated an extrinsic hepato-gastric bundle, occurred within the hepatic branch and linked the omental hepatic branch and the distal anterior gastric branch, apparently without central vagal connections. In the lesser omentum, between the esophagus and the hepatic artery proper, the hepatic branch formed a plexus which was characterized by numerous nerve divisions, anastomoses and large paraganglia (196–463 glomus cells per paraganglion). This plexiform segment ended with the recombining of the hepatic branch into 5–7 bundles which variously ascended in the porta, descended on the hepatic artery proper, or traversed the portal vein. Through its omental course, the hepatic branch traveled in close apposition to the hepato-esophageal artery and the corresponding vein as well as a prominent lymphatic vessel with associated hemolymph nodes.     

Optic nerves in plethodontid salamanders (amphibia, urodela): neuroglia, fiber spectrum and myelination

Summary In five species of lungless salamanders, family Plethodontidae, which all show highly developed visual abilities, the ultrastructure of the optic nerve was investigated and the total number of retinal ganglion cell axons, the percentage of myelinated axons, and the volume densities of glia and axons were determined. More than 80% of all axons were smaller than 0.4 m and only 2–3% were larger than 0.8 m. In individual nerves the degree of myelination varied between 1 and 9% which is in the range reported for other amphibian species. The miniaturized and highly paedomorphic species Batrachoseps attenuatus was an exception because only very few or even no myelinated axons were present in the nerve, which is unique among gnathostome vertebrates. The five investigated species had total numbers of axons ranging from 26000 in Batrachoseps attenuatus to about 50000 in Plethodon jordani. These numbers are the lowest found among vertebrates with an elaborated visual system. The amount of glial material in the optic nerve varied between 25 and 50%, with larger nerves possessing more glia than smaller ones. Ultrastructural analysis revealed that the optic nerve of each species contained both astrocytes and oligodendrocytes, although often in immature form. In Batrachoseps attenuatus the glia showed features of both astrocytes and oligodendrocytes which reflect an undifferentiated state.     

Characteristic variations of relative myelin sheath thickness in 11 nerves of the rat

Summary Computer-assisted measurements of relative myelin sheath thickness (the g ratio) were made in 11 peripheral nerves of the rat. The scatter diagrams showed nerve-specific variations in the distribution of relative myelin sheath thickness. Myelinated fibers of less than 3.5 m axon diameter had relatively thin myelin sheaths, particularly in the splanchnic, vagus and glossopharyngeal nerves. The oculomoter nerve had two fiber populations clearly set apart in terms of relative myelin sheath thickness. Thickly myelinated fibers were found in facial and hypoglossal nerves. No single functional modality was evident for the thinly myelinated fibersThis study was supported by grant 609 from the Deutsche Forschungsgemeinschaft     

Voltage-sensitive Na+ channels in mammalian peripheral nerves detected using scorpion toxins

Summary The localization of voltage-sensitive sodium channels was investigated in mouse, rat and rabbit sciatic nerves using iodinated - and -Scorpion toxins (ScTx) as specific probes. Saturable specific binding for a -ScTx was detected in mouse sciatic nerve homogenates (K_d=90 pM, binding site capacity=90 fmol mg^–1 protein). LM autoradiographic studies demonstrated that the two types of ScTx stained the Ranvier nodes of the myelinated fibres, and also showed a clear but weaker labelling of the unmyelinated Remak bundles. In the sciatic nerve, which is widely considered as a model myelinated nerve, the nodal membrane represented only a small fraction of the total axonal membranes (0.2% and 0.05% for mouse and rabbit sciatic nerves respectively). Therefore, despite their high channel density, nodal membranes contribute only a small proportion of the total labelling by -ScTx (15% and 2.3% for mouse and rabbit sciatic nerves respectively), with the major contribution to labelling arising from unmyelinated axons.The distribution of specific binding sites for a -Scorpion toxin was then analysed in cross-sections of rabbit sciatic nerve at the EM level. The quantitative analysis of autoradiograms involved three methods, the 50% probability circle method, and two cross-fire analyses using either systematically distributed hypothetical sources or hypothetical sources only located on the plasma membranes of axons and of Schwann cells associated with unmyelinated Remak bundles. No specific -Scorpion toxin binding sites were detected at the plasma membrane of Schwann cells from either myelinated fibres or unmyelinated bundles, or at the internodal surface of myelinated axons. Sites were only detected at the surface of unmyelinated axons and at nodal axolemma. Their density in unmyelinated axons was found to be in the range of 1–6 per m² of plasma membrane surface area by combining quantitative EM autoradiography and stereological measurements.     

Nodes of Ranvier and myelin sheath dimensions along exceptionally thin myelinated vertebrate PNS axons

Summary The trigeminal alveolar branch in the lower jaw of the cichlidTilapia mariae was examined by light and electron microscopy on single and serial sections, and by light microscopy on teased fibre preparations. The principal purpose was to find out if the exceptionally thin myelinated axons (d < 1 m) present in this nerve possess true nodes of Ranvier, and to determine the dimensions of their myelin sheaths. This necessitated analysis of the whole size range of myelinated fibres, with respect to nodal and internodal morphology. The results show that the exceptionally thin myelinated fibres exhibit primitive nodal regions, with patches of axolemmal undercoating, and few Schwann cell processes in the node gap. This contrasts with the more complex nodal organization seen in larger trigeminal alveolar branch fibres. For the whole population of myelinated fibres the number of myelin lamellae increases rectilinearly with axon diameter, and sheath length increases with fibre diameter according to a logarithmic expression. The myelin sheaths of the exceptionally thin trigeminal alveolar branch fibres are composed of 10–20 lamellae, and extend 35–50 m along the axon. These results show that the structural complexity of nodal regions in the trigeminal alveolar branch decreases with decreasing fibre size, that the exceptionally thin myelinated trigeminal alveolar branch fibres possess primitive nodes and that they have very short myelin sheaths. Our crude theoretical calculations suggest that these fibres might be capable of saltatory conduction.     

Architecture and synaptic relationships in the intermediolateral nucleus of the thoracic spinal cord of the rat: HRP labelling, catecholamine histochemistry and electron microscopic studies

Summary The organization of the intermediolateral nucleus (IML) of the thoracic spinal cord was examined using glyoxylic acid-induced fluorescence histochemistry, retrograde horseradish peroxidase (HRP) labelling and electron microscopy. In serial sections of T2, it was found that the distribution of catecholamine nerve terminals was intimately related to the neuronal perikarya of IML. Potassium permanganate fixation and 5-hydroxydopamine treatment revealed small dense-cored vesicles in axon varicosities with or without synaptic specializations. A gelatinous region, composed of small diameter dendrites and unmyelinated axons, formed a narrow longitudinal bundle in the centre of the nucleus. The population of the axon varicosities in the IML was 0.17 ± 0.02/m² in 75 nm sections. The average size of the axon varicosities with flat synaptic vesicles was 1.44 ± 0.05 m² and that of varicosities with spherical vesicles was 0.97 ± 0.02 m². After HRP injection into the superior cervical ganglion, ipsilateral IML neurons were labelled in T1–T3 segments of the spinal cord. Axon varicosities with flat and others with spherical synaptic vesicles synapsed on the dendrites labelled by HRP. Among axon varicosities synapsing on the preganglionic sympathetic neurons, 74.8 ± 7.1% at axo-somatic synapses and 46.0 ± 6.7% at synapses on proximal dendrites contained flat synaptic vesicles.     

Three-dimensional morphology of astrocytes and oligodendrocytes in the intact mouse optic nerve

Summary The three-dimensional morphology of astrocytes and oligodendrocytes was analysed in the isolated intact mature mouse optic nerve, by correlating laser scanning confocal microscopy and camera lucida drawings of single cells, dye-filled with lysinated rhodamine dextran or horseradish peroxidase, respectively. These techniques enabled the entire process field of single dye-filled cells to be visualized in all planes and resolved the fine details of glial morphology. Morphometric analysis showed that the processes of all astrocytes had branches ending at the pial surface, on blood vessels, and freely in the nerve; branches ending in the nerve were described to end at nodes of Ranvier in the accompanying paper. Astrocytes were classified into a single morphological population in which each cell subserved multiple functions. The results of this study do not support the contention that astrocytes can be subdivided into two morphological and functional subtypes, namely type-1 and type-2, which have processes ending either at the glia limitans or at nodes, respectively. Three-dimensional analysis of oligodendrocyte units, defined as the oligodendrocyte, its processes and the axons it ensheaths, showed the provision of single myelin segments for an average of 19 nearby axons (range 12–35) with a mean internodal length of 138 m (range 50–350 m). Mouse optic nerve oligodendrocytes were a homogeneous population and were markedly similar to those in the rat optic nerve. The results of our analysis of oligodendrocyte morphology are consistent with the view that the number and internodal length of myelin sheaths supported by a single oligodendrocyte are related to the diameter of the ensheathed axons.