Paranodal Demyelination Research Articles

Peripheral neuropathy in abetalipoproteinemia

We studied the peripheral neuropathy of three sisters with abetalipoproteinemia. Clinically, a sensory neuropathy progressively increased in severity. There was a diminution in the amplitude of sensory action potentials and a slight-to-moderate slowing in maximum sensory conduction velocity, initially most marked in distal portions of the nerves. Motor conduction was normal, although EMG indicated subclinical signs of partial chronic denervation. The sural nerves showed a decreased number of large fibers (greater than 7 micron); in the patient with the neuropathy of shortest duration, small fibers and clusters of regenerating fibers indicated regeneration. In the two patients with advanced neuropathy, one-half the segments of teased fibers showed paranodal demyelination. Also, unmyelinated fibers showed evidence of regeneration.

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 β,β′-iminodipropionitrile (IDPN). In this experimental model synchronous axonal swellings occur in the proximal region of virtually every α-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.

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.

The spatial distribution of excitability and membrane current in normal and demyelinated mammalian nerve fibres.

Thresholds to electrical stimulation have been recorded, concurrently with the membrane currents of conducted impulses, at many positions along undissected single fibres in rat spinal roots. In normal myelinated fibres, distinct threshold minima invariably coincided with sites of inward current generation, and were therefore identified as nodes of Ranvier. Between nodes, the thresholds rose by an order of magnitude. At normal nodes, the charge thresholds were linearly related to stimulus duration, as predicted by computer simulations of a model myelinated fibre (Bostock, 1983). The strength-duration time constants averaged 64.9 +/- 8.3 microseconds (mean +/- S.D.) at 37 degrees C, and had a Q10 of 1/1.39. They were relatively insensitive to changes in inter-electrode distance, or to partial anaesthetization with tetrodotoxin. In fibres treated with diphtheria toxin 6-8 days previously, to induce paranodal or segmental demyelination, threshold minima were found both at nodes and in internodal regions generating inward membrane current. In these fibres strength-duration curves were of the same general form as at normal nodes, but with strength-duration time constants increased at widened nodes (up to 350 microseconds) and at excitable internodes (600-725 microseconds). Comparison with the computer model indicated that these changes were most likely due to exposure of axon membrane with a time constant much longer than that of the normal nodal membrane. In none of the demyelinated fibres examined have we found any evidence of hyperexcitability.

Recovery of distal changes after nerve constriction by a ligature

When ligatures were used to constrict the proximal tibial nerve in rabbits, nerve fibres distal to the site of constriction underwent a reduction in axonal and external fibre diameter, and in conduction velocity. Distal changes could be detected within 7–12 days of the onset of constriction; removal of the ligatures was followed by partial recovery. When paranodal myelin damage occurred in the distal tibial nerve, it showed the typical non-random distribution of secondary demyelination, with multiple paranodal defects on some fibres and complete sparing of others. These findings emphasize the role of the axon in the genesis of paranodal demyelination.

Neuropathy-functional abnormalities in the BB rat

The spontaneously diabetic rat gets a decreased motor-nerve conduction velocity after the onset of diabetes as compared with age- and weight-matched controls. This finding has indicated that these rats develop a neuropathy; but morphological investigations have failed to show structural abnormalities that could explain the decrease in conduction velocity. Measurements of the nervous function revealed a decreased excitability of isolated fibers from sciatic nerves of rats with diabetes duration ranging from 18 days to 6 months. Voltage-clamp analysis of the membrane of the node of Ranvier showed (1) decreased Na-equilibrium potential, reflecting an axoplasmic Na accumulation; (2) decreased specific Na permeability, related both to an increased inactivation of the permeability of the resting node and also a decreased permeability when inactivation had been removed by hyperpolarization of the membrane; and (3) in some nodes a marked increase in the delayed K permeability, which is an early sign of paranodal demyelination. The decrease in Na permeability was the most important factor for the decreased nodal excitability and the decreased conduction velocity. This mechanism may also be involved in the reversible dysfunction found in human diabetic neuropathy.

Changes in peripheral nerve fibres distal to a constriction

Continued constriction of the tibial nerve of the rabbit by a ligature was accompanied by a reduction in maximal motor conduction velocity distal to the ligature, and by a reduction in axonal and total fibre diameter. From the presence of paranodal demyelination and distal fibre degeneration in severely affected nerves, it is suggested that in some instances the change in axonal calibre was part of a progressive distal atrophy which could lead to secondary demyelination and ultimately to “dying-back” of the affected axons.

Electrophysiological and morphological studies of a motor nerve in 'motor endplate disease' of the mouse.

Motor nerves of mice affected with hereditary 'motor endplate disease' were examined by means of electrophysiological and morphological techniques. Conduction velocity was slower, the refractory period was prolonged and the temperature sensitivity higher in mutants as compared to controls of the same age. Most of the axons examined in the electron microscope showed signs of paranodal demyelination. The relationship between the morphological and the electrophysiological findings is discussed. We conclude that alterations in the motor axons can account for the failures in neuromuscular transmission described previously in the med mutation.

Conduction block in rat myelinated fibres following acute exposure to antigalactocerebroside serum

1. We have observed conduction in single rat spinal ventral root nerve fibres following acute topical application of anti-galactocerebroside serum.2. Conduction of nerve impulses was initially slowed and subsequently blocked at the site of serum exposure.3. Conduction block occurred within as little as 1 hr in more slowly conducting (20-30 m/sec) myelinated fibres but occurred later in fibres conducting more rapidly.4. Conduction block was preceded by a rise in internodal conduction time from the normal 20 musec to about 200 musec.5. At nodes exposed to serum, conduction block was invariably associated with greatly decreased depolarization; this was contrasted with nodes exposed to local anaesthetic or tetrodotoxin where conduction block occurred despite nodal depolarization well beyond threshold potential.6. Nodal capacitance and resistance were estimated from simultaneous recordings of membrane current and extracellular potential at blocked nodes exposed to local anaesthetic or tetrodotoxin (normal nodes) and at blocked nodes exposed to anti-galactocerebroside serum.7. For normal fibres of internodal length 0.8-1.1 mm, an upper limit estimate for average nodal capacitance was 2.6 +/- 0.3 pF and a lower limit estimate for average nodal resistance was 55 +/- 10 MOmega. There was an order of magnitude increase in the capacitance of nodes at which conduction block occurred following exposure to anti-galactocerebroside serum.8. We conclude that the early conduction block caused by anti-galactocerebroside serum is due to paranodal demyelination and that acute paranodal demyelination is sufficient to cause conduction block.

In vivo effect of methylcobalamin on the peripheral nerve structure in streptozotocin diabetic rats.

To study in vivo effect of methylcobalamin (CH3-B12) on the peripheral nerve structures, rats with experimental diabetes induced by streptozotocin were administered with daily intramuscular injection of CH3-B12 (500 microgram/kg) for 16 weeks. By isolated nerve fiber studies, CH3-B12-treated diabetic rats showed less incidence of paranodal demyelination as an early sign of segmental demyelination than non-treated diabetic rats. From morphometrical analysis on sural nerves, the reduction in the density of myelinated nerve fibers, nerve fiber size and axon size of myelinated fibers was definitely protected in treated diabetic rats. The results suggested that continuous treatment with CH3-B12 had an ameliorative effect on the peripheral nerve lesions in experimental diabetic neuropathy.

The pathology of a sensory neuropathy affecting Long Haired Dachshund dogs.

The microscopical findings in a sensory neuropathy affecting Long Haired Dachshund dogs are described. This disorder, which may have a genetic basis, results in loss of proprioception, touch and sense and reduction or loss of nociception and urinary control. In the distal cutaneous nerves, there was a severe loss of large myelinated fibers but unmyelinated fiber density was not reduced. Many of the remaining myelinated fibers contained accumulations of axonal organelles and teased fibers showed that some were degenerating whilst others had successive areas of paranodal demyelination. There was marked pathology of unmyelinated fibers with proliferation of tubulo-vesicular profiles resulting in the formation of stacks or lamellar arrays and darkening of the axoplasm. There were numerous regenerating unmyelinated axons. In the spinal cord, axonal degeneration was scen in the distal portion of the dorsal columns.

Experimental cold injury to peripheral nerve.

Focal non-freezing injury to rat sciatic nerve resulted in nerve conduction block and cessation of axoplasmic transport. Rats showed early functional recovery but subsequently developed a slowly progressive sciatic nerve paralysis. Horseradish peroxidase studies revealed prominent nerve oedema with early enhanced pinocytosis and later, passive leakage through damaged endoneurial capillaries. Detailed microscopy indicated a striking selective vulnerability to cold based on nerve fibre diameter. Unmyelinated fibres were spared while large myelinated nerve fibres showed severe axonal degeneration. Paranodal and segmental demyelination were infrequent findings and may have resulted from early axonal swelling. Possible pathogenic mechanisms to explain the degeneration of myelinated fibres in hypothermic injury are discussed. It is concluded that the gradient of clinical severity seen in limb hypothermic injuries has a pathological correlate in peripheral nerve.

Changes in nodal function in nerve fibres of the spontaneously diabetic BB-Wistar rat: potential clamp analysis.

Single myelinated nerve fibres were isolated from the sciatic nerves of the spontaneously diabetic BB-Wistar rat after 17-185 days of overt diabetes. Pronounced demyelination and paranodal changes were found in one rat (diabetes duration 185 days), the others had no structural changes that could be detected light-microscopically. Potential clamp analysis of these fibres showed a decreased Na equilibrium potential (UNa), decreased Na currents caused by large inactivation, and a specific increase in the K-permeability. Measurements with high-K solution outside the node revealed that the decreased UNa was caused by high axoplasmic [Na]. Fibres with increased K-permeability had, in addition, increased nodal leak conductance. Decreased excitability was found in the majority of the fibres and was related to the permeability changes. The present findings indicate that axonal dysfunction (Na-accumulation and inactivation of the Na-permeability) precede the changes (increased K-permeability) related to paranodal demyelination.

Conduction velocity and conduction block after experimental ischaemic nerve injury

The effect of ischaemic nerve injury on conduction in the tibial branch of the sciatic nerve has been studied in rabbits. Ischaemic nerve damage was produced in 40 animals by ligation of the common and external or the internal and external iliac arteries. Conduction in the motor fibres to the small foot muscles was examined at intervals after operation, with nerve stimulation in the thigh and at the ankle. Ascending nerve action potentials in the tibial nerve were also studied. In 8 animals there was evidence of widespread degeneration of myelinated fibres; muscle action potentials could not be activated by nerve stimulation and nerve action potentials were absent. In contrast there were 14 animals in which arterial ligation appeared to produce little, if any, nerve degeneration; in this group there was no significant change in motor or afferent velocity. The remaining 18 animals showed changes of intermediate severity. While nerve degeneration was extensive, it was never complete, and in 9 nerves examined 5–11 days after surgery, conduction block between the thigh and ankle could be demonstrated in surviving fibres. In the other 9 nerves there was no block in surviving fibres but maximal motor velocity was significantly reduced. In no case did the conduction block affect more than a minority of the motor fibres, the remainder undergoing complete degeneration. This was in keeping with the histological appearances which showed a relatively small amount of paranodal and segmental demyelination compared with the amount of Wallerian degeneration.

Specific permeability properties of demyelinated rat nerve fibres.

The specific permeability properties of isolated rat fibres with diphtheria toxin induced paranodal demyelination were investigated in potential clamp experiments. Nodal widening (4 to 6 micrometers) and paranodal demyelination (20 to 90 micrometers) were associated with a very large increase in the normally low delayed K-permeability (greater than 10 times). The resting K-permeability (which was comparatively large in the rat fibres) was similarly increased, which decreased action potential amplitude and excitability. Fibres with paranodal demyelination had, in addition, a large increase in the maximum peak Na-permeability (2 to 5 times the normal), which indicated that excitability was upheld by formation of new Na-permeability sites. The conduction defects in demyelinated fibres can therefore not be related to the size of the demyelination, but to the specific permeability changes in the demyelinated segment.

Electrical properties of isolated demyelinated rat nerve fibres.

Isolated large rat nerve fibres with diphtheria toxin induced paranodal demyelination were investigated. These fibres had increased membrane time constant of the demyelinated nodal segment, related to a large increase in capacitance (5 to 50 times). The resting conductance was less increased (2 to 3 times), meaning that the internodal axolemma has considerably higher resting resistance than the nodal membrane. A large variation in action potential amplitude was found which was unrelated to the size of the nodal widening and the passive electrical properties.

Axonal lesions in acute experimental demyelination: a sequential teased nerve fiber study.

The relationship between axonal degeneration and primary demyelination was studied in isolated rat sciatic nerve fibers previously exposed to antiserum from rabbits with either experimental allergic neuritis or experimental allergic encephalomyelitis, or immunized with antigalactocerebroside antiserum. Continuous demyelination over one to eight or more internodes was seen in association with phagocytic cells or, later, with increased numbers of Schwann cells. Paranodal demyelination was prominent proximal ahd distal to the zone of continuous demyelination. Axonal degeneration affected 5 to 15% of myelinated fibers exposed to antiserum and was not related to the length of demyelination must proximal to the axonal changes. At times, there were seven or eight consecutive demyelinated internodes with no distal axonal degeneration; in contrast, one demyelinated internode was often associated with axonal degeneration beginning just distally. The inflammatory reaction could account for axonal degeneration in antiserum-mediated demyelination.

Axonal lesions in acute experimental demyelination

The relationship between axonal degeneration and primary demyelination was studied in isolated rat sciatic nerve fibers previously exposed to antiserum from rabbits with either experimental allergic neuritis or experimental allergic encephalomyelitis, or immunized with antigalactocerebroside antiserum. Continuous demyelination over one to eight or more internodes was seen in association with phagocytic cells or, later, with increased numbers of Schwann cells. Paranodal demyelination was prominent proximal and distal to the zone of continuous demyelination. Axonal degeneration affected 5 to 15% of myelinated fibers exposed to antiserum and was not related to the length of demyelination just proximal to the axonal changes. At times, there were seven or eight consecutive demyelinated internodes with no distal axonal degeneration; in contrast, one demyelinated internode was often associated with axonal degeneration beginning just distally. The inflammatory reaction could account for axonal degeneration in antiserum-mediated demyelination.

Evidence for the presence of potassium channels in the paranodal region of acutely demyelinated mammalian single nerve fibres.

1. A study has been made of the ionic currents in voltage-clamped single rabbit nodes of Ranvier at 22-26 degrees C both under normal conditions, and after the nerve fibres had been acutely demyelinated by a variety of treatments designed to lossen the myelin from the axonal membrane. 2. The myelin-loosening treatments included application of various combinations of: lysolecithin (to dissolve the myelin); collagenase (to loosen the connective tissue in the nodal region); high-potassium Locke solution, hypertonic and hypotonic solutions (to induce axonal volume changes). 3. At a critical stage in such treatment (usually after 15-45 min) a large outward current suddenly appeared. 4. There was no substantial change in the size of the measured inward sodium current when measured at this critical stage. 5. The outward current was blocked by internal TEA and caesium ions, had a reversal potential that became more positive when the external potassium concentration was increased, was kinetically similar to the known potassium current in frog fibres, and was therefore assumed to be a potassium current. 6. The phase of large outward current, whenever it appeared, was always accompanied by the appearance of a slow transient capacitative component in the leakage current, which indicated a marked increase in the effective nodal capacity (of 10- to 60-fold). We suggest that the slow transient capacity current reflected charging of newly exposed axonal membrane, probably in the paranodal region, which was uncovered by the various acute demyelination treatments. This internodal membrane seems to contain mostly potassium channels and few, if any, sodium channels. 7. Newly dissected fibres occasionally showed large potassium currents before treatment, particularly if they were deliberately stretched during dissection; a marked slow capacity transient current was consistently present in these fibres. 8. The effects of acute paranodal demyelination on the sodium and potassium currents, and on the transient capacity currents, can be simulated by a model in which the node is coupled to a cable-like paranode which contains Hodgkin--Huxley type potassium channels and which has a much higher leakage resistance. 9. The functional significance of the presence of potassium channels in rhe internodal region (at least in the paranode) of mammalian fibres is discussed.