Recovery of nerve conduction after a pneumatic tourniquet: observations on the hind-limb of the baboon

A small pneumatic cuff inflated around the knee was used to produce tourniquet paralysis in baboons. A cuff pressure of 1,000 mm Hg maintained for one to three hours produced paralysis of distal muscles lasting up to three months. Nerve conduction studies showed that most of the motor fibres to the abductor hallucis muscle were blocked at the level of the cuff and that they conducted impulses normally in their distal parts. There was a significant correlation between the duration of compression and that of the subsequent conduction block. When tested two to three weeks after the tourniquet, the amplitude of the response of m. abductor hallucis to nerve stimulation distal to the cuff was usually slightly reduced compared with the precompression figure. This was assumed to mean that a small proportion of the motor fibres had undergone Wallerian degeneration as a result of compression. Maximal motor conduction velocity was reduced in recovering nerves. It was also reduced when a cuff pressure of 500 mm Hg was used, which was insufficient to produce persistent conduction block. In such cases a reduced velocity without evidence of block could be demonstrated 24 hours after compression. Ascending nerve action potentials were recorded from the sciatic nerve in the thigh, with stimulation at the ankle. Before compression the fastest afferent fibres had a significantly higher velocity than the fastest motor fibres in the same nerve trunk. Results after compression suggested that the high-velocity afferent fibres had a susceptibililty to the procedure similar to that of the fastest motor fibres.     

Tomaculous neuropathy. A histopathological study and electroclinical correlates in 10 cases

Among 980 sural nerve biopsies, the nerves of 10 patients showed a great number of focal sausage-shaped thickenings of the myelin sheaths and were investigated by light and electron microscopy, teasing and quantitative studies. Single teased nerve fibres revealed myelin thickening in more than 25 p. 100 of internodes. This condition defined the tomaculous neuropathy and differed from other degenerative or toxic neuropathies which displayed a small number of internodes with myelin thickenings, in less than 5 p. 100. Segmental demyelination and remyelination were found in 12 p. 100 to 65 p. 100 of myelinated fibres. Tomaculous swellings were observed in the internodes of these fibres. Except axonal constriction within the sausage-shaped thickenings, no fibers with axonal degeneration was observed. The density of myelinated and unmyelinated fibres was normal. The loss of large myelinated fibres was interpreted as resulting from the myelinic changes. Clinical and electrophysiological data were similar in the ten cases of tomaculous neuropathies and in hereditary neuropathy with liability to pressure palsies, i.e.: autosomal dominant inheritance, higher incidence in males, recurrent nerve trunck and/or brachial plexus involvement related to compression, slowing of nerve conduction velocities in clinically affected and unaffected nerves more pronounced in anatomical narrow sites and increased F wave latencies. One patient (case 10) showed a mixed sensory motor progressive neuropathy but signs of widespread neuropathy were noted in more advanced cases. A great number of tomaculous swellings of myelin sheaths is considered as a specific but non constant change of hereditary neuropathy with liability to pressure palsies.     

Segmental conduction abnormalities and myelin thickenings in Val102/fs null mutation of MPZ gene

The authors report in patients with Val102/fs null mutation a possibly age dependent variability of clinical and electrophysiologic phenotype, segmental conduction abnormalities mainly in ulnar nerves at the elbow, and excessive myelin foldings and thickenings. The authors hypothesize that myelin thickenings at the paranodal region, in concurrence with compression at usual entrapment sites or minor repetitive trauma, may induce segmental conduction abnormalities.     

Age-related molecular reorganization at the node of Ranvier

In myelinated axons, action potential conduction is dependent on the discrete clustering of ion channels at specialized regions of the axon, termed nodes of Ranvier. This organization is controlled, at least in part, by the adherence of myelin sheaths to the axolemma in the adjacent region of the paranode. Age-related disruption in the integrity of internodal myelin sheaths is well described and includes splitting of myelin sheaths, redundant myelin, and fluctuations in biochemical constituents of myelin. These changes have been proposed to contribute to age-related cognitive decline; in previous studies of monkeys, myelin changes correlate with cognitive performance. In the present study, we hypothesize that age-dependent myelin breakdown results in concomitant disruption at sites of axoglial contact, in particular at the paranode, and that this disruption alters the molecular organization in this region. In aged monkey and rat optic nerves, immunolabeling for voltage-dependent potassium channels of the Shaker family (Kv1.2), normally localizing in the adjacent juxtaparanode, were mislocalized to the paranode. Similarly, immunolabeling for the paranodal marker caspr reveals irregular caspr-labeled paranodal profiles, suggesting that there may be age-related changes in paranodal structure. Ultrastructural analysis of paranodal segments from optic nerve of aged monkeys shows that, in a subset of myelinated axons with thick sheaths, some paranodal loops fail to contact the axolemma. Thus, age-dependent myelin alterations affect axonal protein localization and may be detrimental to maintenance of axonal conduction.     

Nodes of ranvier above a neuroma in the rat sciatic nerve: Voltage clamp analysis and electron microscopy

Neuroma formation was induced in adult rat sciatic nerves and the animals were allowed to survive for 1-10 months. In 10 animals single large myelinated fibres from the nerve segment above the neuroma were subjected to voltage clamp analysis. Six animals were fixed by glutaraldehyde perfusion and nodes of Ranvier or large myelinated fibres above the neuroma were examined in the electron microscope (EM). Most fibres exhibited normal action potentials, but a few had a reduced excitability and small action potentials. Some fibres had increased membrane time constant and leak conductance and a markedly increased membrane capacitance. Most of the examined nodes of Ranvier exhibited abnormally large delayed K currents, which could be blocked with 4-aminopyridine (4-AP). The Na current was normal. In the EM most large cross-cut myelinated axons were markedly atrophic, particularly after long survival times. Evaginations from the paranodal region of these axons penetrated between the terminating paranodal myelin lamellae. The nodal axolemmal undercoating could be very prominent and in some cases the nodal axon was irregular. These findings show that large myelinated peripheral nerve fibres, which are chronically disconnected from their peripheral targets, exhibit specific structural and functional abnormalities of the nodes of Ranvier.     

Multifocal motor neuropathy: pathologic alterations at the site of conduction block

The pathologic changes of nerves in multifocal motor neuropathy (MMN), a rare neuropathy with selective focal conduction block of motor fibers in mixed nerves, remain essentially unstudied. Fascicular nerve biopsy of 8 forearm or arm nerves in 7 patients with typical MMN was undertaken for diagnostic reasons at the site of the conduction block. Abnormalities were seen in 7 of 8 nerves, including a varying degree of multifocal fiber degeneration and loss, an altered fiber size distribution with fewer large fibers, an increased frequency of remyelinated fiber profiles, and frequent and prominent regenerating fiber clusters. Small epineurial perivascular inflammatory infiltrates were observed in 2 nerves. We did not observe overt segmental demyelination or onion bulb formation. We hypothesize that an antibody-mediated attack directed against components of axolemma at nodes of Ranvier could cause conduction block, transitory paranodal demyelination and remyelination, and axonal degeneration and regeneration. Alternatively, the antibody attack could be directed at components of paranodal myelin. We favor the first hypothesis because in nerves studied by us, axonal pathological alteration predominated over myelin pathology. Irrespective of which mechanism is involved, we conclude that the unequivocal multifocal fiber degeneration and loss and regenerative clusters at sites of conduction block explains the observed clinical muscle weakness and atrophy and alterations of motor unit potentials. The occurrence of conduction block and multifocal fiber degeneration and regeneration at the same sites suggests that the processes of conduction block and fiber degeneration and regeneration are linked. Finding discrete multifocal fiber degeneration may also provide an explanation for why the functional abnormalities remain unchanged over long periods of time at discrete proximal to distal levels of nerve and may emphasize a need for early intervention (assuming that efficacious treatment is available).     

UNMYELINATED FIBRES IN NORMAL AND COMPRESSED PERIPHERAL NERVES OF THE BABOON: A QUANTITATIVE ELECTRON MICROSCOPIC STUDY

The calibre spectrum of unmyelinated axons in normal medial popliteal nerves of baboons, fixed by perfusion with glutaraldehyde and measured by electron microscopy, is consistently unimodal. Following acute compression with a pneumatic tourniquet sufficient to cause a local conduction block, unmyelinated fibres are usually undamaged. This offers an anatomical basis for the selective sensory sparing (particularly pain) in a neurapraxia. Following local compression severe enough to cause Wallerian degeneration in a proportion of the myelinated fibres, a new population of axonal profiles eventually grows within the unmyelinated fibres. The features of these profiles are those of axonal sprouts and their presence results in a bimodal calibre spectrum of unmyelinated axons. This suggests that in these cases the period of prolonged compression has resulted in damage to the unmyelinated fibres.     

Sulfatide decrease in myelin influences formation of the paranodal axo‐glial junction and conduction velocity in the sciatic nerve

Cerebroside sulfotransferase (CST) catalyzes the production of sulfatide, which is one of the major glycolipids in myelin. Homozygous CST knockout mice were shown to be completely deficient in sulfatide. They were born healthy but began to display progressive neurological deficits from 6 weeks of age. Severe abnormalities of paranodal regions and changes in axonal ion channel distribution were prominent in both the central and peripheral nervous systems. But whether partial decreases in myelin sulfatide levels influence paranodal formation, as well as nerve conduction velocity (NCV), is largely unknown. To determine the functional significance of sulfatide content in myelin, we performed electrophysiological, morphological, and biochemical analyses using heterozygote, homozygote, and wild‐type mouse peripheral nerves and compared the results with individual sulfatide content. NCVs were significantly reduced in homozygote animals compared with wild‐type mice. In contrast, these values were markedly varied in individual heterozygote mice. On the basis of NCV values, we divided heterozygous mice into two groups: mice with mild but significant reduction of NCV and those with normal NCV. Teased nerve fibers obtained from individual mouse sciatic nerves were immunostained, and Na⁺ channel and Caspr cluster lengths were measured to determine abnormal levels of junctional formation at the paranode. Furthermore, sulfatide content in each sciatic nerve was examined by thin layer chromatography. The results demonstrated significant correlations among sulfatide level, severity of paranodal abnormality, and reduction of NCV. Thus, the fine regulation of myelin sulfatide content by CST is important for normal function of myelinated axons. © 2013 Wiley Periodicals, Inc.     

Paranodal dysmyelination in peripheral nerves of Trembler mice

Subtle defects in paranodes of myelinated nerve fibers can cause significant physiological malfunction. We have investigated myelinated fibers in the peripheral nervous system (PNS) of the Trembler mouse, a model of CMT‐1A neuropathy, for evidence of such defects. Ultrastructural analysis shows that the “transverse bands,” which attach the myelin sheath to the axon at the paranodal axoglial junction, are grossly diminished in number in Trembler nerve fibers. Although paranodes often appear to be greatly elongated, it is only a short region immediately adjacent to the node of Ranvier that displays transverse bands. Where transverse bands are missing, the junctional gap widens, thus reducing resistance to short circuiting of nodal action currents during saltatory conduction and increasing the likelihood that axonal K⁺ channels under the myelin sheath will be activated. In addition, we find evidence that structural domains in Trembler axons are incompletely differentiated, consistent with diminution in nodal Na channel density, which could further compromise conduction. Deficiency of transverse bands may also increase susceptibility to disruption of the paranodal junction and retraction of the myelin sheath. We conclude that Trembler PNS myelinated fibers display subtle defects in paranodal and nodal regions that could contribute significantly to conduction defects and increased risk of myelin detachment. © 2014 Wiley Periodicals, Inc.     

The relationship between axon diameter, myelin thickness and conduction velocity during atrophy of mammalian peripheral nerves

The atrophy of cutaneous (sural) and muscle (medial gastrocnemius) nerves proximal to a ligation were studied in cats for periods up to 9 months, using light and electron microscopy, conduction velocity measurements and computer simulations. As atrophy proceeds, nerve fibres become increasingly non-circular. Cross-sectional areas of axons and fibres (axon + myelin) were measured. The diameters of equivalent circles (having the same axon and fibre cross-sectional area) were then calculated. A linear relation was found between axon diameter and fibre diameter, but the slope decreased as atrophy continued. This indicates that the axon cross-sectional area decreases relatively more than the total fibre area. Reduction in conduction velocity correlates more closely with reduction in axon diameter than fibre (axon + myelin) diameter.The ratio of the inner (axon) perimeter to the outer (myelin) perimeter remains constant at or near the optimal value of 0.6 for conduction in all groups of fibres at all periods of atrophy. Futhermore, the thickness of the myelin remains constant for a given perimeter over the entire period of atrophy studied. This suggests that the number of turns of myelin and the length of each turn remain unchanged during peripheral nerve atrophy. A simple geometric model explains how this can occur without gaps developing between the axon and myelin or between the turns of myelin. The Frankenhaeuser-Huxley equations for conduction in myelinated nerve fibres predict changes in conduction velocity similar to those observed, if the axons atrophy without changes in myelin. The advantages of this mode of atrophy are discussed.     

Uncompacted inner myelin lamellae in inherited tendency to pressure palsy

Nerves in patients with inherited tendency to pressure palsy (ITPP) are susceptible to degrees of traction or compression which in nonaffected persons do not induce neuropathic symptoms or deficits, conduction block of fibers, or electromyographic changes characteristic of the disorder. Two observations suggest a widespread asymptomatic abnormality of nerves: 1) low conduction velocity of clinically unaffected nerves, and 2) focal thickenings (tomacula) on teased myelinated fibers of clinically unaffected sural nerves. Sural nerves from five patients and five healthy subjects were assessed for morphologic abnormality in ITPP that might account for the susceptibility of nerves to compression. Teased nerve fibers showed a higher frequency of segmental demyelination or remyelination, or both (p less than 0.003). The mean frequency of fibers showing focal myelin thickenings was 57 +/- 10% in ITPP and 0% in controls. In electron micrographs, regions of uncompacted myelin lamellae, usually affecting the innermost lamellae and extending for a variable distance averaging 9 +/- 4 microns were seen in 11 +/- 4% of fibers in ITPP. None were found in the control nerves. The finding of uncompacted myelin lamellae may suggest an abnormality of myelin composition or of interaction of Schwann cells and axons accounting for the increased susceptibility to pressure palsy, tomaculous formation, or demyelination. From electron microscopic evaluation of serial skip sections we infer that myelin of tomaculae is in continuity with internodal myelin and is reduplicated (full-thickness or cleaved layers are longitudinally or circumferentially folded-back on themselves).     

Sensory action potentials and biopsy of the sural nerve in neuropathy.

In 167 consecutive patients with various types of neuropathy, the amplitude of the sensory potential and the maximum conduction velocity along the sural nerve were compared with conduction in other sensory nerves, and were related to structural changes revealed by nerve biopsy. Electrophysiological findings in the sural nerve were similar to those in the superficial peroneal and the median nerve, though the distal segment of the median nerve was normal in 20 per cent of the patients when it was abnormal in the sural nerve. Quantitation of histological findings was a more sensitive method than the electrophysiological study in that two-thirds of 33 patients with normal electrophysiology in the sural nerve showed mild loss of fibres or signs of remyelination in teased fibres. The amplitude of the sensory potential was grossly related to the number of large myelinated fibres (more than 7 micrometer in diameter). Considering the 95 nerves from which teased fibres were obtained, maximum conduction velocity was abnormal in half. In 18 of these nerves, slowing in conduction was due to axonal degeneration: the velocity was as to be expected from the diameter of the largest fibres in the biopsy ("proportionate slowing"). In 9 nerves slowing was severe and more marked than to be expected from loss of the largest fibres ("disproportionate slowing"); these nerves showed paranodal or segmental demyelination in more than 30 per cent of the fibres. In 16 nerves from patients with neuropathy of different aetiology neither loss of fibres nor demyelination could explain the moderate slowing. The cause of slowing in these nerves is unknown; other conditions are referred to in which slowing in conduction cannot be attributed to morphological changes. Finally, electrophysiological and histological findings are reported in some patients with neuropathy associated with malignant neoplasm, with rheumatoid arthritis, with polyarteritis nodosa, with acute intermittent porphyria and with cirrhosis of the liver.     

Peripheral nervous system and central nervous system pathology in rapidly progressive lower motor neuron syndrome with immunoglobulin M anti-GM1 ganglioside antibody

Pathological studies, including novel teased peripheral nerve fiber studies, were performed in a patient who presented with a rapidly progressive, lower motor neuron syndrome and high titer of immunoglobulin M anti-GM1 ganglioside antibody. In the central nervous system, there was a severe loss of motor neurons and central chromatolysis with ubiquitin immunopositive cytoplasmic inclusions in residual motor neurons. In the peripheral nervous system, axonal degeneration of myelinated fibers in the anterior nerve roots was evident. Pathologic evidence of sensory nerve involvement was also found despite the absence of clinical or electrophysiological sensory abnormalities. Sectional studies of single myelinated nerve fibers from an antemortem sural nerve biopsy showed remyelination and globular paranodal swellings due to focal complex myelin folding and degeneration in 13% of fibers. Postmortem studies of the sural nerves 4 weeks later showed paranodal demyelination (90% of fibers), but no paranodal swellings and similar findings were present in samples of the ulnar, radial, median, tibial, and common peroneal nerves. Paranodal abnormalities of enlargement of the adaxonal space, myelin degeneration, and axonal compaction were found on cross-sectional studies of individual teased fibers, which on conventional light microscopic assessment appeared normal. These changes suggest a disturbance of paranodal axonal-myelin adhesion due to binding of the anti-GM1 ganglioside antibody to the common epitope known to be present on the myelin sheath and nodal axolemma in the paranodal region of both motor and sensory nerves.     

Early paranodal myelin swellings (tomacula) in an avian riboflavin deficiency model of demyelinating neuropathy

Introduction

Disruption of the complex architectural and molecular organization of the paranodal region of myelinated peripheral nerve fiber may initiate the evolving time dependent process of segmental demyelination. In support of this notion was the finding of focal paranodal myelin swellings (tomacula) due to redundant folding of myelin sheaths, early in the time course of an avian riboflavin deficiency model of demyelinating neuropathy.

Methods

Newborn broiler meat chickens were maintained either on a routine diet containing 5.0 mg/kg riboflavin (control group) or a riboflavin-deficient diet containing 1.8 mg/kg riboflavin. Riboflavin concentrations in the liver were measured at postnatal day 11. Peripheral nerves were morphologically examined at days 6, 11, 16 and 21 using light and electron microscopy and teased nerve fiber techniques.

Results

Riboflavin-deficient chickens showed signs of a neuropathy from days 8 and pathological examination of peripheral nerves revealed a demyelinating neuropathy with paranodal tomacula formation starting on day 11. Paranodal tomacula consisted of redundant myelin infoldings or outfoldings, increased in size and frequency after day 11. After day 16, the paranodal swellings showed prominent degenerative changes accompanied by an increased frequency of myelinated fibers showing demyelination.

Conclusion

Tomacula due to redundant myelin folds are generally considered a remyelination phenomenon, yet in this avian riboflavin deficiency model of demyelination, the paranodal tomacula occurred early in the course of demyelination.     

Spatiotemporal ablation of myelinating glia‐specific neurofascin (NfascNF155) in mice reveals gradual loss of paranodal axoglial junctions and concomitant disorganization of axonal domains

The evolutionary demand for rapid nerve impulse conduction led to the process of myelination‐dependent organization of axons into distinct molecular domains. These domains include the node of Ranvier flanked by highly specialized paranodal domains where myelin loops and axolemma orchestrate the axoglial septate junctions. These junctions are formed by interactions between a glial isoform of neurofascin (Nfasc^NF155) and axonal Caspr and Cont. Here we report the generation of myelinating glia‐specific Nfasc^NF155 null mouse mutants. These mice exhibit severe ataxia, motor paresis, and death before the third postnatal week. In the absence of glial Nfasc^NF155, paranodal axoglial junctions fail to form, axonal domains fail to segregate, and myelinated axons undergo degeneration. Electrophysiological measurements of peripheral nerves from Nfasc^NF155 mutants revealed dramatic reductions in nerve conduction velocities. By using inducible PLP‐CreER recombinase to ablate Nfasc^NF155 in adult myelinating glia, we demonstrate that paranodal axoglial junctions disorganize gradually as the levels of Nfasc^NF155 protein at the paranodes begin to drop. This coincides with the loss of the paranodal region and concomitant disorganization of the axonal domains. Our results provide the first direct evidence that the maintenance of axonal domains requires the fence function of the paranodal axoglial junctions. Together, our studies establish a central role for paranodal axoglial junctions in both the organization and the maintenance of axonal domains in myelinated axons. © 2009 Wiley‐Liss, Inc.     

Quantitative and qualitative morphological properties of the soleus motor nerve and the L5 ventral root in young and old rats. Relation to the number of soleus muscle fibers

The motor nerve to the soleus muscle and the L5 ventral root from young adult (3-6 months) and old (20-25 months) male Wistar rats were studied with regard to total number of myelinated nerve fibres, calibre spectra of myelinated fibres and morphological properties. The soleus muscle was examined with respect to total number of muscle fibres. A significant decrease in the number of myelinated nerve fibres was found in the old soleus nerves, and was mainly confined to the large-diameter fibers. Similarly, there was a significant decrease in the number of large myelinated fibres in the L5 ventral roots of old animals. Morphological changes in old nerves and spinal roots consisted of axonal degeneration with areas devoid of myelinated nerve fibres, and also myelin sheath irregularities, including myelin splitting with myelin balloon formation, infolded myelin loops and myelin reduplication. The axonal degeneration was more pronounced in the peripheral nerves than in spinal roots. In conformity with other studies, the soleus muscle showed an age-related decrease in the number of muscle fibres. These findings indicate that the decrease in fibre number in old soleus muscle is associated with degenerative neuronal changes and loss of alpha-motoneurons.     

The nature of the nerve lesion caused by chronic entrapment in the guinea-pig

Light and electron microscopy of single myelinated fibres have been used to study chronic entrapment of the median nerve at the wrist in guinea-pigs.In mildly affected nerves it was found that both proximal and distal to the site of entrapment the internodes were bulbous at one end and tapered at the other. On the proximal side of the lesion it was always the proximal ends of the internodes which were bulbous; distal to the lesion the polarity was reversed. In more severely affected nerves paranodal demyelination occurred at the tapered ends of the internodes.It is argued that the reversed polarity of the changes in the two sides of the lesion indicates a mechanical rather than an ischaemic cause for the nerve damage.     

Paranodal pathology in Tangier disease with remitting-relapsing multifocal neuropathy.

Pathological studies of a sural nerve biopsy in a man with Tangier disease presenting as a remitting-relapsing multifocal neuropathy showed abnormalities in the paranodal regions, including lipid deposition (65%) and redundant myelin foldings, with various degrees of myelin splitting and vesiculation (43%) forming small tomacula and abnormal myelin terminal loops (4%). The internodal regions were normal in the majority of myelinated fibres. Abnormal lipid storage was also present in the Schwann cells of the majority of unmyelinated fibres (67%). The evidence suggests that the noncompacted myelin region of the paranode is a preferential site for lipid storage in the myelinated Schwann cell, and that the space-occupying effects of the cholesterol esters leads to paranodal malfunction and tomacula formation as the pathological basis for the multifocal relapsing-remitting clinical course.     

Nodal gap substance in diabetic nerve

Anoxia and KC1 have been used to inactivate peripheral nerves by depolarization conduction block. Investigation of the inactivation patterns in isolated sciatic nerves of healthy and alloxan-diabetic rats suggests that the paranodal gap substance of healthy nerve behaves as an effective periaxonal diffusion barrier. In diabetic nerve the permeability of this barrier is significantly increased. A marked reduction in the K' binding capacity of the nodal gap substance has been demonstrated in myelinated nerves of human diabetics and alloxan diabetic rats.     

Peripheral nerve abnormalities related to galactose administration in rats.

Electrophysiological, biochemical, and morphometric observations were made on the peripheral nerves of rats after galactose feeding. Motor nerve conduction velocity was found to be reduced. This was associated with an accumulation of galactitol in the peripheral nerves and a diminution in their myoinositol content. An increased water content and fascicular area, taken in conjunction with a probable increase in the area of the endoneurial spaces, indicated overhydration of the peripheral nerves. Morphometric observations on the myelinated fibre population in the tibial nerve showed no loss of fibres and although both the maximal and the average diameter of the myelinated fibres was slightly less than in age-matched controls, this was insufficient to explain the reduction in conduction velocity. Segmental demyelination was not detected and the relationship between myelin thickness and axon circumference was not altered. Electron microscope observations revealed no ultrastructural changes in the myelinated fibres and, in particular, no abnormalities at the nodes of Ranvier or indication of abnormal hydration of the Schwann cells. The relevance of these findings to the peripheral nerve changes in human and experimental diabetes is discussed.