Electrical activity and structural correlates of giant nerve fibers in Kuruma shrimp (Penaeus japonicus)

Morphology and recordings of electrical activity of Kuruma shrimp (Penaeus japonicus) giant medullated nerve fibers were carried out. A pair of giant fibers with external diameter of about 120 μ and 10 μ in myelin thickness were found in the ventral nerve cord. The diameter of the axon is about 10 μ. Thus there is a wide gap between the axon and the external myelin sheath. Each axon is doubly coated directly by Schwann cells and indirectly by the myelin sheath layer which is produced by those Schwann cells. Impulse conduction velocities of these giant fibers showed a range between 90–210 m/sec at about 22°C. Large action potentials (up to 113 mV, rise time of 0.16–0.3 msec, maximum rate of rise of 650–1250 V/sec, half decay time of 0.2–0.3 msec, maximum rate of fall of 250–450 V/sec and total duration of less than 1.5 msec) could be obtained by inserting microelectrodes or by longitudinal insertion of 25 μ diameter capillary electrodes into the gap but no DC-potential difference was observed across the myelin sheath. Transmyelin electrical parameters were very favorable for fast impulse conduction: myelin resistance of 3 × 10⁴ Ω cm²; time constant of 0.38 msec; myelin capacitance of 1.35 × 10^?8 F/cm²; gap fluid resistivity of 23 Ω cm. The existence of nodes of Ranvier could not be demonstrated morphologically, but electrophysiological evidence suggests that a type of saltatory conduction occurs in these giant fibers.     

A comparative study of oculomotor,trochlear and abducens nerves in Arabian foals

We investigated the microscopic structure of transverse sections of the oculomotor, trochlear and abducens nerves of Arabian foals using stereological methods. Bilateral nerve pairs from 2-month-old female Arabian foals were analyzed. The tissues were embedded in plastic blocks, then 1 µm thick sections were cut and stained with osmium tetroxide and methylene blue-azure II. Stereology was performed using light microscopy. Morphometry showed that the right and left pairs of nerves were similar. The transverse sectional areas of the oculomotor, trochlear and abducens nerves were 1.93 ± 0.19 mm², 0.32 ± 0.06 mm² and 0.70 ± 0.08 mm², respectively. The oculomotor nerve exhibited a significantly greater number of myelinated axons (16755 ± 1279) and trochlear (2656 ± 494) and the abducens nerves (4468 ± 447). The ratio of the axon diameter to myelinated nerve fiber diameter was 0.58, 0.55 and 0.55 for the oculomotor, trochlear and abducens nerves, respectively. Of the three nerves studied, the abducens nerve exhibited the greatest nerve fiber area, myelin area, nerve and axon diameters, and myelin thickness. The ratio of small myelinated nerve fibers was greatest in the oculomotor nerve.     

An image analysis morphometric method for the study of myelinated nerve fibers from mouse trigeminal root

For the morphometric light microscopic study of myelinated fibers in mouse trigeminal root, it was necessary to write: (1) an entirely automatic analysis program for the myelinated axons inside the myelin sheath, based on the detection of the myelin sheaths, and (2) an interactive analysis program for the myelinated fibers outside the myelin sheath, due to the high density of compactness of the myelinated fibers based on an indirect fiber individualization by reconstructing them from their axons. In the latter, a semiautomatic correction method (drawing the profile contours with a light pen) allowed compensation for the failures of the automatic method, except for the smallest fibers, which represented 8% of the total. Using these programs, 95% of the axons could be measured and 92% of the myelinated fibers whose axons were analyzed could be measured. The area-equivalent diameter was independent of the detection method; it is a correct-size measurement parameter for axons and fibers that is unrelated to their shape. The projected diameter, an estimation of the perimeter obtained by measurement of the profile projections, depended upon the detection method because the profile contour was influenced by the detection method; it thus takes into account the profile shape. For myelinated fibers, whose analysis program used two detection methods (automatic and semiautomatic), there was an average difference of 16% between the projected diameters obtained with these two methods, whereas the equivalent diameter value was the same. The fiber circularity factor could not be precisely estimated because of the detection error; the axon circularity factor was more reliable since the axon detection was completely automatic.     

Relative Conduction Velocities of Small Myelinated and Non-myelinated Fibres in the Central Nervous System

IN peripheral nerve, most axons with diameters of less than 1 µm do not have myelin sheaths, while most fibres more than 1 µm in diameter are myelinated^1,2. In the central nervous system, axons as small as 0.2 µm in diameter may be myelinated^2–5. In his paper on the effects of myelin on conduction velocity, Rushton⁶ concluded that 1 µm is the “critical diameter” above which “myelin increases conduction velocity” and below which “conduction is faster without myelination”. This conclusion is referred to widely (see, for example, refs. 7–9). In this communication we demonstrate that the analysis leading to this conclusion is based on morphological data¹⁰ which do not apply either to central or to peripheral fibres, so that myelinated fibres considerably smaller than 1 µm might be expected to conduct more rapidly than non-myelinated fibres of similar size.     

Galactosphingolipids and axono-glial interaction in myelin of the central nervous system

The myelin of central and peripheral nervous system of UDP-galactose-ceramide galactosyltransferase deficient mice (cgt ^-/-) is completely depleted of its major lipid constituents, galactocerebrosides and sulfatides. The deficiency of these glycolipids affects the biophysical properties of the myelin sheath and causes the loss of the rapid saltatory conduction velocity of myelinated axons. With the onset of myelination, null mutant cgt ^-/- mice develop fatal neurological defects. CNS and PNS analysis of cgt ^-/- mice revealed (1) hypomyelination of axons of the spinal cord and optic nerves, but no apoptosis of oligodendrocytes, (2) redundant myelin in younger mice leading to vacuolated nerve fibers in cgt ^-/- mice, (3) the occurrence of multiple myelinated CNS axons, and (4) severely distorted lateral loops in CNS paranodes. The loss of saltatory conduction is not associated with a randomization of voltage-gated sodium channels in the axolemma of PNS fibers. We conclude that cerebrosides (GalC) and sulfatides (sGalC) play a major role in CNS axono-glial interaction. A close axono-glial contact is not a prerequisite for the spiraling and compaction process of myelin. Axonal sodium channels remain clustered at the nodes of Ranvier independent of the change in the physical properties of myelin membrane devoid of galactosphingolipids. Increased intracellular concentrations of free ceramides do not trigger apoptosis of oligodendrocytes.     

THE FINE STRUCTURAL ORGANIZATION OF NERVE FIBERS, SHEATHS, AND GLIAL CELLS IN THE PRAWN, PALAEMONETES VULGARIS

In view of reports that the nerve fibers of the sea prawn conduct impulses more rapidly than other invertebrate nerves and look like myelinated vertebrate nerves in the light microscope, prawn nerve fibers were studied with the electron microscope. Their sheaths are found to have a consistent and unique structure that is unlike vertebrate myelin in four respects: (1) The sheath is composed of 10 to 50 thin (200- to 1000-A) layers or laminae; each lamina is a cellular process that contains cytoplasm and wraps concentrically around the axon. The laminae do not connect to form a spiral; in fact, no cytoplasmic continuity has been demonstrated among them. (2) Nuclei of sheath cells occur only in the innermost lamina of the sheath; thus, they lie between the sheath and the axon rather than outside the sheath as in vertebrate myelinated fibers. (3) In regions in which the structural integrity of the sheath is most prominent, radially oriented stacks of desmosomes are formed between adjacent laminae. (4) An ~200-A extracellular gap occurs around the axon and between the innermost sheath laminae, but it is separated from surrounding extracellular spaces by gap closure between the outer sheath laminae, as the membranes of adjacent laminae adhere to form external compound membranes (ECM's). Sheaths are interrupted periodically to form nodes, analogous to vertebrate nodes of Ranvier, where a new type of glial cell called the "nodal cell" loosely enmeshes the axon and intermittently forms tight junctions (ECM's) with it. This nodal cell, in turn, forms tight junctions with other glial cells which ramify widely within the cord, suggesting the possibility of functional axon-glia interaction.     

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.     

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.     

Axons and myelinating glia: An intimate contact

The coordination of the vertebrate nervous system requires high velocity signal transmission between different brain areas. High speed nerve conduction is achieved in the myelinated fibers of both the central and the peripheral nervous system where the myelin sheath acts as an insulator of the axon. The interactions between the glial cell and the adjacent axon, namely axo-glial interactions, segregate the fiber in distinct molecular and functional domains that ensure the rapid propagation of action potentials. These domains are the node of Ranvier, the paranode, the juxtaparanode and the internode and are characterized by multiprotein complexes between voltage-gated ion channels, cell adhesion molecules, members of the Neurexin family and cytoskeletal proteins. In the present review, we outline recent evidence on the key players of axo-glial interactions, depicting their importance in myelinated fiber physiology and disease.     

Properties of dorsal root unmedullated fibers on the two sides of the ganglion

As an aid in the interpretation of the physiological properties of unmedullated nerve fibers, particularly those having their cells of origin in the dorsal root ganglia, more precise information about their morphology has been acquired through employment of the electron microscope. The appearance of the fibers in the skin nerves is described, with special reference to the structure of their sheaths; and a notation is made about the bearing of the axon-sheath relationship on the biophysical mechanism of conduction (p. 714). There is no basic difference between the sheath systems of the d.r.C and the s.C fibers. Attention is called to a point of similarity between the sheaths of unmyelinated and myelinated axons (p. 715). An assessment was made of the likelihood of interaction between the fibers. In action potentials showing temporal dispersion at several distances, the elevations appeared in their calculated positions. A model of a group of Schwann sheaths was constructed from successive electron microscope sections, showing that the lengths of the sheath branches are short in comparison with the wave lengths of the action potentials. Supported by these and other considerations, the argument is strongly in favor of the conclusion that among d.r.C fibers, as in other fibers, there is no cross-excitation between the axons. A new analysis of the size distribution of the fibers in a sural nerve was made from electron microscope pictures; and from the measurements the action potential was constructed. The result confirmed the view, previously expressed, that the velocities of conduction in the fibers can be precisely accounted for by multiplying the diameters by a constant. In the dorsal roots, the striking change that takes place in the appearance of the fibers and their disposition in the Schwann sheaths can be seen in Fig. 11. The axons partake of the special properties of the peripheral branches, which necessitated the creation of the subdivision of d.r.C fibers. But, their diameters are much smaller. At a set of reduced conduction velocities the configuration of the compound action potential in the nerves is repeated in the roots, with the root velocities still conforming to the size-velocity rule derived from nerve axons.     

The “Lillie Transition”: models of the onset of saltatory conduction in myelinating axons

Almost 90 years ago, Lillie reported that rapid saltatory conduction arose in an iron wire model of nerve impulse propagation when he covered the wire with insulating sections of glass tubing equivalent to myelinated internodes. This led to his suggestion of a similar mechanism explaining rapid conduction in myelinated nerve. In both their evolution and their development, myelinating axons must make a similar transition between continuous and saltatory conduction. Achieving a smooth transition is a potential challenge that we examined in computer models simulating a segmented insulating sheath surrounding an axon having Hodgkin-Huxley squid parameters. With a wide gap under the sheath, conduction was continuous. As the gap was reduced, conduction initially slowed, owing to the increased extra-axonal resistance, then increased (the “rise”) up to several times that of the unmyelinated fiber, as saltatory conduction set in. The conduction velocity slowdown was little affected by the number of myelin layers or modest changes in the size of the “node,” but strongly affected by the size of the “internode” and axon diameter. The steepness of the rise of rapid conduction was greatly affected by the number of myelin layers and axon diameter, variably affected by internode length and little affected by node length. The transition to saltatory conduction occurred at surprisingly wide gaps and the improvement in conduction speed persisted to surprisingly small gaps. The study demonstrates that the specialized paranodal seals between myelin and axon, and indeed even the clustering of sodium channels at the nodes, are not necessary for saltatory conduction.     

IGF‐I deficient mice show reduced peripheral nerve conduction velocities and decreased axonal diameters and respond to exogenous IGF‐I treatment

Although insulin‐like growth factor‐I (IGF‐I) can act as a neurotrophic factor for peripheral neurons in vitro and in vivo following injury, the role IGF‐I plays during normal development and functioning of the peripheral nervous system is unclear. Here, we report that transgenic mice with reduced levels (two genotypes: heterozygous Igf1^+/− or homozygous insertional mutant Igf1^m/m) or totally lacking IGF‐I (homozygous Igf1^−/−) show a decrease in motor and sensory nerve conduction velocities in vivo. In addition, A‐fiber responses in isolated peroneal nerves from Igf1^+/− and Igf1^−/− mice are impaired. The nerve function impairment is most profound in Igf1^−/− mice. Histopathology of the peroneal nerves in Igf1^−/− mice demonstrates a shift to smaller axonal diameters but maintains the same total number of myelinated fibers as Igf1^+/+ mice. Comparisons of myelin thickness with axonal diameter indicate that there is no significant reduction in peripheral nerve myelination in IGF‐I–deficient mice. In addition, in Igf1^m/m mice with very low serum levels of IGF‐I, replacement therapy with exogenous recombinant hIGF‐I restores both motor and sensory nerve conduction velocities. These findings demonstrate not only that IGF‐I serves an important role in the growth and development of the peripheral nervous system, but also that systemic IGF‐I treatment can enhance nerve function in IGF‐I–deficient adult mice. © 1999 John Wiley & Sons, Inc. J Neurobiol 39: 142–152, 1999     

IGF-I deficient mice show reduced peripheral nerve conduction velocities and decreased axonal diameters and respond to exogenous IGF-I treatment

Although insulin-like growth factor-I (IGF-I) can act as a neurotrophic factor for peripheral neurons in vitro and in vivo following injury, the role IGF-I plays during normal development and functioning of the peripheral nervous system is unclear. Here, we report that transgenic mice with reduced levels (two genotypes: heterozygous Igf1+/- or homozygous insertional mutant Igf1m/m) or totally lacking IGF-I (homozygous Igf1-/-) show a decrease in motor and sensory nerve conduction velocities in vivo. In addition, A-fiber responses in isolated peroneal nerves from Igf1+/- and Igf1-/- mice are impaired. The nerve function impairment is most profound in Igf1-/- mice. Histopathology of the peroneal nerves in Igf1-/- mice demonstrates a shift to smaller axonal diameters but maintains the same total number of myelinated fibers as Igf1+/+ mice. Comparisons of myelin thickness with axonal diameter indicate that there is no significant reduction in peripheral nerve myelination in IGF-I-deficient mice. In addition, in Igf1m/m mice with very low serum levels of IGF-I, replacement therapy with exogenous recombinant hIGF-I restores both motor and sensory nerve conduction velocities. These findings demonstrate not only that IGF-I serves an important role in the growth and development of the peripheral nervous system, but also that systemic IGF-I treatment can enhance nerve function in IGF-I-deficient adult mice.     

Morphology and physiology of peripheral giant interneurons in the forelegs (whips) of the whip spider Heterophrynus elaphus Pocock (Arachnida: Amblypygi)

Summary The tarsi of the modified front legs (whips) of the whip spider Heterophrynus elaphus contain two afferent giant fibers, GN1 and GN2, with diameters at the tibia-tarsus joint of ca. 21 m and 14 m, respectively. The somata of these two neurons lie in the periphery, about 25 cm away from the CNS. These two neurons are interneurons which receive mechanoreceptive inputs from approximately 750 and 1500 bristles, respectively. The receptive fields of GN1 and GN2 overlap; they extend for 40 mm (GN1) and 90 mm (GN2) along the length of the tarsus. About 90% of the synapses onto the giant fibers are axo-axonic. Mechanical stimulation of a single bristle is sufficient to elicit action potentials in one or both interneurons. The response of the interneurons adapts quickly. Average conduction time from the soma to the CNS is 45 ms for GN1 and 55 ms for GN2. Mean conduction velocities are 5.5 and 4.2 m/s, respectively. Activity in the giant fibers does not elicit a motor response; hence the giant fibers do not mediate an escape response. Possible functions of these giant fibers are discussed and compared to those of giant fiber systems in other arthropods.Abbreviations GN giant neuron - S segment     

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.     

Effects of high fat diet induced obesity on peripheral nerve regeneration and levels of GAP 43 and TGF-β in rats

The increasing frequency of obesity is important because of its accompanying related health problems. The effects of obesity on peripheral nerves have not been elucidated. We investigated the effects of obesity on sciatic nerve regeneration using electrophysiology, stereology, immunohistochemistry, histopathology and functional tests. We used control, obese, control injured and obese injured groups of rats. Electrophysiological results showed that nerve conduction velocity and EMG were same in the experimental groups, but the amplitude of the compound action potential of the control group was significantly higher than that of the obese group. Examination of the nerves showed that the control and obese groups had both larger axon diameters and thicker myelin sheaths. The number of myelinated axons was decreased in both of the injured groups. Axon diameters and myelin sheath thicknesses of the control injured group were significantly greater those of the obese injured group. There were no significant differences in functional tests among the groups. Although growth associated protein 43 immunostaining in the control injured group was significantly greater than that of the obese injured group, no significant difference was observed between the control and obese groups. There was no significant difference in immunohistochemical staining for transforming growth factor beta 3 between the control injured and obese injured groups. Our results suggest that obesity may affect peripheral nerve regeneration negatively after crush injury.     

Extracellular potentials of a single myelinated nerve fiber in an unbounded volume conductor

The extracellular potentials of a single myelinated nerve fiber in an unbounded volume conductor were studied. The spatial distribution of the transmembrane potential was obtained by integrating the system of partial differential equations characterizing the electric processes in the active myelinated nerve fiber. The spatial distribution of the extracellular potentials at various radial distances in the volume conductor were calculated using the line source model. Up to a certain radial distance (500 m) the discontinuity of the action potential propagation is reflected in the extracellular potentials, while further in the volume conductor the potentials are smooth. The effect of the fiber diameter and the internodal distance on the volume conductor potentials as well as the changes in the magnitude of the extracellular potential (in the time domain) between two adjacent nodes at various radial distances were studied. The radial decline of the peak-to-peak amplitude of the extracellular potential depends on the radial coordinater of the field point and increases with the increase ofr.     

Myelin as longitudinal conductor: a multi-layered model of the myelinated human motor nerve fibre

 The myelin sheath is normally regarded as an electrical insulator. Low values of radial conductance and capacitance have been measured, and in electrical models of myelinated axons the contribution of longitudinal conduction within the sheath has been ignored. According to X-ray diffraction studies, however, myelin sheaths comprise alternate lipid and aqueous layers, and the latter may be expected to have a low resistivity. We propose a new model of myelinated axons in which the aqueous layers within the myelin provide appreciable longitudinal and radial conductance, the latter via a spiral pathway. We have investigated the likely contribution of these conductive paths within the myelin to the electrical properties of a human motor nerve fibre by computer simulation, representing the myelin sheath as a series of interconnecting parallel lamellae. With this new model, action potential conduction has been simulated along a 20-node cable, and the electrotonic responses to 100-ms depolarizing and hyperpolarizing current pulses have been simulated for a uniformly polarized fibre. We have found that the hypothesis of a longitudinally conducting myelin sheath improves our previous model in two ways: it is no longer necessary to make implausible assumptions about the resistivity or width of the periaxonal space to simulate realistic electrotonus, and the conduction velocity is appreciably faster (by 8.6%). Received: 19 April 1999 / Accepted in revised form: 11 September 2000