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.     

Effects of IDPN-induced axonal swellings on conduction in motor nerve fibers

Paranodal demyelination produces a reduction of conduction velocity and conduction block. The relative proportions of these changes appear to vary among different demyelinating disorders. In this study we have examined the effects on conduction of paranodal demyelination produced by giant axonal swellings. The axonal swellings were induced in rats by administration of beta, beta'-iminodipropionitrile (IDPN). In this experimental model synchronous axonal swellings occur in the proximal region of virtually every alpha-motorneuron without evidence of segmental demyelination or fiber loss. Conduction across the motor neuron was evaluated by two methods: a monosynaptic reflex pathway and intracellular recording from single motor neurons. Increases in the delay across the central region of the monosynaptic reflex pathway began between 2 and 4 days after toxin administration. Intracellular studies confirmed that the slowing occurred across the proximal regions of the motor axons; more distal regions of the motor axons were unaffected. The substantial reduction in conduction velocity over the swollen segment occurs with only moderate evidence of conduction block, as assayed by a reduction in the H-reflex/M-response amplitude ratio. Parallel morphological studies showed that in the enlarged fibers the myelin terminal loops maintained contact with the axon but were displaced from the paranodal region into the internode. The appearance of this "passive" paranodal demyelination correlated closely with the increase in conduction delay. We suggest that the contact maintained by the displaced myelin terminal loops with the axolemma allows saltatory conduction to continue, and explains the paucity of conduction block in this model despite the prominent conduction slowing.     

Rapid alterations of the axon membrane in antibody-mediated demyelination

Alterations of nodal and paranodal axolemma of the rat sciatic nerve were investigated in antigalactocerebroside serum-induced demyelination. A ferric ion-ferrocyanide (FeFCN) stain that appears to stain the regions with a high sodium channel density in nerve fibers was applied. When acute conduction block was initiated 20 to 180 minutes after the antiserum injection, myelin terminal loops began to be detached from the paranodal axolemma and reaction product of FeFCN stain originally localized at the nodes decreased in density and extended to the paranodal axolemma. By the time that complete conduction block was established, 5 hours after the injection, FeFCN stain was barely detectable around the nodal area. The loss of staining was associated with detachment and vesiculovacuolar degeneration of the paranodal myelin. This rapid deterioration and disappearance of normal cytochemical characteristics of the axolemma in the presence of only modest paranodal demyelination could be a morphological correlate of the loss of excitability of the axon membrane.     

Lead neuropathy. 1) Morphometry, nerve conduction, and choline acetyltransferase transport: new finding of endoneurial edema associated with segmental demyelination.

Morphometric and pathologic studies along the length of the peripheral nervous system were obtained in groups of rats fed 4% lead carbonate for 3 and 6 months and in match-fed controls. The number and diameter histograms of L6 cytons of spinal ganglia and of myelinated fibers of proximal and distal portions of peroneal and sural nerve were not significantly different from the control groups. On the other hand, segmental demyelination occurred approximately as frequently in proximal as in distal parts of nerves. At 3 months approximately 1/3 of teased myelinated fibers showed changes of segmental demyelination (Condition C), or of remyelination after segmental demyelination (Condition F) or of both segmental demyelination and of remyelination (Condition D), while at 6 months more than 4/5ths of fibers showed these changes. As expected, regression lines of axonal area on number of lamellae of myelin, were less steep in nerves of rats fed on lead for 6 months as compared to controls. Axonal transport of choline acetyltransferase in lead neuropathy did not differ from that in control rats. As expected from the studies of others, conduction velocity of myelinated fibers of caudal nerve were low. A new finding was the often quite striking increase of transverse fascicular area of peripheral nerves. This was due to edema which appeared to develop at about the time of onset os segmental demyelination. Although the edema may be an epiphenomenon, it could be an important observation bearing on the development of lead neuropathy. It would be important to know next whether or not the blood nerve barrier is altered in lead neuropathy.     

Nodes of Ranvier in skin biopsies of patients with diabetes mellitus

Paranodal demyelination has been discussed as a potential mechanism of nerve fiber damage in diabetic neuropathy (DNP). Studies on human tissue are limited, as nerve biopsies are invasive and only rarely performed in patients with confirmed DNP. Skin biopsy has recently been suggested as a tool to analyze paranodal and nodal changes of myelinated fibers. We analyzed the myelinated fibers of skin biopsies of 35 patients with DNP, 17 patients with diabetes mellitus (DM) without neuropathy, and 30 normal controls. Immunofluorescence of skin sections with antibodies against Caspr, neurofascin, sodium channels, and myelin basic protein was performed to assess paranodal/nodal architecture, segmental demyelination, and myelinated nerve fibers. Staining with antibodies against protein gene product 9.5 was used to quantify unmyelinated nerve fibers. There was an increase of elongated Ranvier nodes and a dispersion of neurofascin at the distal leg in patients with DM with and without neuropathy and at the finger in patients with DNP. An increased dispersion of Caspr was only found in biopsies of the finger in patients with DNP. Skin biopsy may be an appropriate tool to analyze nodes of Ranvier in patients with DM. Structural nodal changes are detectable in DNP and even in diabetic patients without neuropathy.     

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.     

Demyelination following diphtheria toxin in the presence of axonal atrophy

In 23 guinea-pigs a constricting ligature was placed on the tibial nerve in the thigh on one side, in order to produce axonal atrophy in the distal part of the nerve. Either before or after ligature, 10 of the guinea-pigs received subcutaneous diphtheria toxin into the abdominal wall, in a dose insufficient to cause generalised paralysis or reduced motor conduction velocity (MCV). In 7 of the 10 animals, MCV distal to the ligature fell below the range seen after ligature alone.In animals which had received toxin histological studies revealed paranodal and segmental demyelination in the distal tibial nerve, which was more extensive on the side of the ligature than in the opposite leg. Occasional paranodal but no segmental demyelination was seen distal to ligature alone.These results indicate that a small dose of systemic diphtheria toxin is more likely to produce peripheral nerve demyelination if an axonal abnormality is also present.     

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).     

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).     

Nodal and paranodal structure during Wallerian degeneration in frog spinal nerve

The nodal and paranodal regions of myelinated peripheral nerve fibers in frogs were examined at sequential times (1-24 days) during Wallerian degeneration. In the region up to 3 mm distal to the transection, paranodal demyelination and axoplasmic degeneration became apparent on day 4 and progressed to involve most of the nodes by day 8. The E-fracture face of the axolemma showed a patchy distribution of nodal particles and some paranodal demyelination on days 4 and 6. On day 8, nodal particles were evenly distributed at low concentration and the adjacent demyelinated paranodal regions showed a corresponding increase in particle density, suggesting redistribution of the nodal particles. The sequence of changes seen in comparable to that in Wallerian degeneration of central nervous system (CNS) fibers but progressed more rapidly in the peripheral nervous system (PNS). In addition a higher proportion of PNS fibers shows pathological changes at corresponding time periods.     

Uncompacted myelin lamellae in dysglobulinemic neuropathy

Uncompacted lamellae, located preferentially in inner layers of myelin sheath, were observed in biopsied sural nerves of 3 cases of dysglobulinemic neuropathy, in which the main pathological findings of myelinated fibers were those of segmental demyelination and remyelination, and axonal degeneration with concurrent marked decrease of myelinated fiber density. The presence of uncompacted myelin lamellae is well explained by the irregular distribution of adaxonal, incisural and paranodal cytoplasm of the Schwann cell.     

E-cadherin is required for the correct formation of autotypic adherens junctions of the outer mesaxon but not for the integrity of myelinated fibers of peripheral nerves

The calcium-dependent adhesion protein E-cadherin is present in noncompacted regions of myelin sheaths in the peripheral nervous system. There, it is localized to electron-dense structures between membranes of the same Schwann cell referred to as autotypic adherens junctions. It has been suggested that the failure of E-cadherin-mediated adhesion might cause demyelination that proceeds in certain pathological states. To test the requirement of E-cadherin in peripheral nerves, we used tissue-specific gene ablation techniques based on the Cre/LoxP system. We show that E-cadherin deficiency does not cause significant demyelination up to the age of 15 months. Immunostainings for nodal sodium channels, the paranodal protein Caspr1, and the juxtaparanodal potassium channels Kv1.1 and Kv1.2 revealed that E-cadherin is not necessary to maintain the general functional architecture of the nodal region. On the ultrastructural level, we detected a widening of the outer mesaxon accompanied by a loss of electron-dense cytoplasmic areas. We conclude that E-cadherin is required for the proper establishment and/or the maintenance of the outer mesaxon in myelinated PNS fibers but is dispensable for proper nerve function.     

A case of Hoffmann's syndrome with peripheral neuropathy and various pathological findings in muscle biopsy

A 58-year-old hypothyroid man developed numbness in the extremities and muscular hypertrophy without pain. Needle electromyography was normal. Nerve conduction study revealed severe entrapment and impaired conduction velocity. Muscle biopsy demonstrated small group atrophy, scattered hypertrophic fibers, interstitial fibrosis and subsarcolemmal vaculoes. These vacuoles were stained dark with NADH-TR. On electron microscopy these vacuoles were filled with aggregated mitochondria and glycogen particles. These myopathological findings have been rarely described in Hoffmann's syndrome or Kocher-Debré-Semelaigne syndrome in the literature. Sural nerve biopsy revealed a loss of myelinated fibers in large diameter and relative increase in small diameter fibers. Teased fiber examination demonstrated segmental demyelination. The nerve biopsy findings indicate slowly progressive axonopathy. In Hoffmann's syndrome, associated peripheral neuropathy may play some role in the manifestation of clinical signs and symptoms including muscle weakness and some sensory disturbance.     

Chronic inflammatory demyelinating polyneuropathy in dogs and cats

Over the past several years, we have accumulated data on a spontaneous demyelinating peripheral neuropathy that is not well identified in domestic animals. This disorder occurs in dogs and cats of either sex and does not appear breed-related. Onset of signs is usually insidious and the course is typically chronic, sometimes relapsing, and often slowly progressive. Mature animals of any age may be affected. Clinical signs include tetraparesis, sometimes progressing to tetraplegia, stumbling gait, and hyporeflexia. Motor nerve conduction velocities are decreased. Pathologically, changes in teased single fibers from peripheral nerves are dominated by multifocal paranodal demyelination. Scattered, thinly myelinated fibers are seen on semithin sections. Ultrastructural studies reveal macrophages within myelinated fibers stripping the myelin sheaths, naked and remyelinating axons, and focal/multifocal endoneurial mononuclear cells. Indirect immunofluorescence revealed positive IgG staining in peripheral nerve myelin sheaths from two dogs. The course of the disease, clinical signs, electrophysiology, and pathology have similarities to chronic inflammatory demyelinating polyneuropathy in people.     

Schwann cell proliferation and migration during paranodal demyelination

This study examined Schwann cell behavior during paranodal demyelination induced by beta,beta'-iminodipropionitrile (IDPN). The stimuli for Schwann cell proliferation, extensively studied in vitro, are less well understood in vivo. Most in vivo systems previously used to examine Schwann cell proliferation in disease are dominated by loss of internodal myelin sheaths. As used in this study, IDPN administration produces neurofilamentous axonal swellings and paranodal demyelination, without segmental demyelination or fiber degeneration. We asked whether Schwann cells would proliferate following the restricted paranodal demyelination that accompanies the axonal swellings, and if so what the sources and distributions of new Schwann cells might be. IDPN was given as a single large dose (2 ml/kg) to 21-d-old rats. Neurofilamentous axonal swellings formed in the proximal regions of motor axons, reaching their greatest enlargement in the root exit zone 8 d after IDPN administration. These swellings subsequently migrated distally down the nerves at rates approaching 1 mm/d. The axonal enlargement was consistently associated with displacement of the myelin sheath attachment sites into internodal regions, and consequent paranodal demyelination. This stage was associated with perikaryal changes, including nucleolar enlargement, "girdling" of the perikaryon, and formation of attenuated stalks separating the perinuclear region from the external cytoplasmic collar. Schwann cells proliferated abundantly during this stage. Daughter Schwann cells migrated within the endoneurial space (outside the nerve fiber basal laminae) to overlie the demyelinated paranodes of swollen nerve fibers. In these regions, local proliferation of Schwann cells continued, resulting in large paranodal clusters of Schwann cells. As the axonal calibers subsequently returned to normal, the outermost myelin lamellae of the original internodes returned to their paranodal attachment sites and the supernumerary Schwann cells disappeared. Formation of short internodes, segmental demyelination, and nerve fiber loss were rare phenomena. These results indicate that paranodal demyelination is a sufficient stimulus to excite abundant Schwann cell proliferation; neither internodal demyelination nor myelin breakdown is a necessary stimulus for mitosis. The 3H-thymidine incorporation studies indicated that the sources of new Schwann cells included markedly increased division of the Schwann cells of unmyelinated fibers and, as they formed, supernumerary Schwann cells. In addition, there were rare examples of 3H-thymidine incorporation by Schwann cells associated with myelinated nerve fibers.(ABSTRACT TRUNCATED AT 400 WORDS)     

Peripheral nerve pathology in Niemann-Pick type C mouse

The sciatic nerve of the mouse mutant with Niemann-Pick type C disease (NPC mouse) was investigated using light and electron microscopy, and teased-fiber preparations. As early as postnatal day 20, when clinical symptoms were not yet apparent, focal paranodal swellings with an accumulation of small myelin figures in the Schwann cell cytoplasm were noted. These paranodal changes were more pronounced in the distal segment and became progressively conspicuous with increasing age. The morphometric analysis revealed a hypomyelination of large myelinated fibers in the NPC nerves at 70 days, whereas an essentially similar histogram pattern was noted in both control and NPC nerves at 20 days, suggesting progressively defective utilization of cholesterol in the NPC nerves with age. Intraxonal accumulation of dense bodies was noted in older mice, but no segmental demyelination or Wallerian type of axonal degeneration was observed at any age. The changes noted in the paranodal regions in the NPC mouse closely resemble those found in rats treated with an inhibitor of cholesterol biosynthesis, as well as those seen in remodeling fibers during an early stage of peripheral nerve development. Thus, the morphological changes seen in the sciatic nerve of the NPC mouse may be an expression of perturbation in myelin maintenance as a result of defective cholesterol metabolism.Preliminary findings were presented at the Annual Meeting of the American Association of Neuropathologists, Baltimore, Maryland, June 1991     

Histology and ultrastructure of alterations in neuropathy

Histologic findings are described in nerves from men exposed to lead, from patients with discrete clinical signs of peripheral neuropathy, and from controls. Every nerve from control subjects showed an abnormality (paranodal remyelination, segmental remyelination, or regeneration) in teased fibers. The only histologic alteration in eight lead-exposed males without signs or symptoms of neuropathy was a slightly increased incidence of paranodal remyelination. Sixteen patients with discrete neurologic symptoms and signs had a loss of large myelinated fibers and an increased incidence of regenerated fibers among teased fibers. Electron microscopy of unmyelinated fibers showed an increased occurrence of Schwann-cell processes, of fibers undergoing degeneration, and of Schwann-cell subunits with many profiles as the earliest signs of abnormality. Clinically mild neuropathies may exhibit advanced regeneration in the case of unmyelinated fibers. The earliest sign of degeneration in myelinated fibers was a diminution in the number of axonal organelles.     

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.     

Sodium channel Na(v)1.6 is expressed along nonmyelinated axons and it contributes to conduction

Nodes of Ranvier in myelinated fibers exhibit a complex architecture in which specific molecules organize in distinct nodal, paranodal and juxtaparanodal domains to support saltatory conduction. The clustering of sodium channel Na(v)1.6 within the nodal membrane has led to its identification as the major nodal sodium channel in myelinated axons. In contrast, much less is known about the molecular architecture of nonmyelinated fibers. In the present study, Na(v)1.6 is shown to be a significant component of nonmyelinated PNS axons. In DRG C-fibers, Na(v)1.6 is distributed continuously from terminal receptor fields in the skin to the dorsal root entry zone in the spinal cord. Na(v)1.6 is also present in the nerve endings of corneal C-fibers. Analysis of compound action potential recordings from wildtype and med mice, which lack Na(v)1.6, indicates that Na(v)1.6 plays a functional role in nonmyelinated fibers where it contributes to action potential conduction. These observations indicate that Na(v)1.6 functions not only in saltatory conduction in myelinated axons but also in continuous conduction in nonmyelinated axons.     

Axonal attenuation and secondary segmental demyelination in myeloma neuropathies

The sural nerves of 5 patients with osteosclerotic myeloma and polyneuropathy, of 3 patients with multiple myeloma and polyneuropathy, and of 6 healthy subjects were studied by neuropathological, morphometric, and teased-fiber approaches to assess cellular (Schwann cell or axon) vulnerability and to explore the mechanism of segmental demyelination. As compared with controls, the nerves of both types of myeloma neuropathy demonstrated a statistically significant and marked loss of myelinated fibers and increased frequencies of axonal degeneration among teased fibers at statistically significant levels. The peaks of diameter histograms of myelinated fibers of osteosclerotic myeloma/polyneuropathy nerves were displaced to smaller diameter categories, suggesting fiber atrophy. Segmental demyelination and remyelination was clustered, as found in secondary demyelination. Large- and intermediate-diameter myelinated fibers of osteosclerotic myeloma/polyneuropathy nerves had significantly smaller axon calibers relative to myelin spiral length seen in electron micrographs. The loss of myelinated fiber axons, the shift of the peaks of diameter histograms to smaller sizes, the lack of noticeable increased numbers of demyelinated axons, the clustered distribution of segmental demyelination, and the axonal attenuation suggest a special axonal or neuronal vulnerability and appear to provide an explanation for the observed segmental demyelination. Whether axonal attenuation has a perikaryeal or proximal axonal genesis now needs to be determined.