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.     

Conduction of nervous impulses in spinal roots and peripheral nerves of dystrophic mice

Conduction was studied in the sacral ventral roots and ventral tail nerves of dystrophic mice (dy/dy) and phenotypically normal littermates. In myelinated ventral root fibers of normal mice, conduction velocity was uniform with internodal conduction time45 ± 5 μsec (26 °C). In ventral root fibers of dystrophic mice, conduction velocity was decreased and strikingly non-uniform; both saltatory and continuous conduction were observed in different portions of the same nerve fiber. Continuous conduction with velocity <2 m/sec (26 °C) was characteristically observed in mid-root where the axons are bare; conduction was saltatory close to the exit from the spinal canal and near the spinal cord where the axons are myelinated.Maximum conduction velocity in ventral tail nerves was21 ± 3 m/sec for dystrophic mice and31 ± 4 m/sec for littermate controls (37 °C). Internodal lengths were somewhat decreased in the dystrophic peripheral nerves but there was no significant difference in maximum fiber diameters, myelin thickness or nodal morphology between dystrophic and normal nerves.     

Myelinated fibres in the human paravertebral sympathetic chain; quantitative studies on white rami communicantes

Myelinated fibres in the human sympathetic paravertebral chain were examined histologically and in single teased fibre preparations in various age groups in subjects dying from disorders not affecting primarily the autonomic nervous system. An increase in fibre density predominantly due to an increase in the number of small fibres was found in older subjects. A correlation between internodal length and fibre diameter was found but the internodes of sympathetic myelinated fibres are shorter for any given diameter than those found on fibres of comparable size in the sural nerve. A reduction in internodal lengths with advancing years was demonstrated. These observations are interpreted to show that Wallerian degeneration and segmental demyelination occur with increasing frequency in old age and that regeneration does not keep pace with successive degenerative events. The deterioration in function of the autonomic nervous system with advancing years may be attributed in part to the changes found in myelinated fibres in the paravertebral sympathetic chain.     

Internodal myelin volume and axon surface area: A relationship determining myelin thickness?

Internodes from normal, remyelinated and regenerated nerve fibres have been isolated from rat spinal roots and sciatic nerve. The internodes have been examined quantitatively by light and electron microscopy to determine their internodal length, myelin thickness, and the circumference and cross-sectional area of both the axon and fibre. Comparison of these measurements of the axon and myelin sheath has revealed a close relationship between the volume of myelin comprising the internode and the area over which the Schwann cell and axon are in close proximity, i.e. the surface area of the axolemma beneath the internodal myelin sheath. The same relationship described not only the internodes on normal nerve fibres, where internodal length is proportional to axon diameter, but also the short and thinly myelinated internodes formed in the adult animal on remyelinated and on regenerated axons. Examination of data presented by Berthold (1978) revealed that a closely similar relationship is also present in feline nerve fibres. In view of the constancy of the relationship between such different types of internode it is suggested that the regulation of myelin volume, and thereby of myelin thickness, may be mediated via the area of the axolemma or of the Schwann cell membrane beneath the myelin sheath.     

Peripheral nerve structure and function in experimental diabetes

Observations have been made on the peripheral nerves of rats in which diabetes had been induced by streptozotocin or alloxan. Motor nerve conduction velocity was found to become reduced, the reduction developing within a few days in severely diabetic animals. Conduction velocity remained diminished during survival times of up to 1 year.Histometric studies of the myelinated fibre population of the sural and tibial nerves showed no loss of fibres or reduction in their calibre. Evidence of segmental demyelination was not detected and myelin/axon ratios were normal. Cation binding at the nodes of Ranvier was unaltered. No explanation for the reduced nerve conduction velocity in morphological terms was therefore obtained.Observations on the abdominal vagus nerve revealed no loss of unmyelinated axons or reduction in their calibre. Measurements of Schwann cell and endoneurial capillary basal laminal thickness showed no differences between diabetic and control animals. Ultrastructural examination of other features of the peripheral nerves failed to define any pathological alterations in the diabetic animals. It is therefore concluded that the extrapolation of biochemical findings in experimental diabetes in an attempt to explain the origin of human diabetic neuropathy, where there are associated structural changes, should be made with caution.     

Cerebrotendinous xanthomatosis: clinical,electrophysiological and nerve biopsy findings,and response to treatment with chenodeoxycholic acid

Summary A 30-year-old patient with cerebrotendinous xanothomatosis was studied over a 6-year period. The clinical manifestations were cataracts, intellectual deterioration, ataxia, palatal and pharyngeal myoclonus, corticospinal tract damage and an electrophysiologically demonstrated sensorimotor peripheral neuropathy. Peripheral motor and sensory nerve conduction velocity was slowed. Sural nerve biopsy revealed reduced densities of both myelinated and unmyelinated axons and teased fibres showed evidence of axonal regeneration and some remyelination. The loss of myelinated nerve fibres particularly affected those of larger diameter, thus contributing to the slowing of nerve conduction. Chenodeoxycholic acid treatment for two separate periods of 10 and 6 months each increased nerve conduction velocity. This electrophysiological improvement was not matched by detectable clinical neurological improvement.     

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.     

Minocycline reduces microgliosis and improves subcortical white matter function in a model of cerebral vascular disease

Chronic cerebral hypoperfusion is a key mechanism associated with white matter disruption in cerebral vascular disease and dementia. In a mouse model relevant to studying cerebral vascular disease, we have previously shown that cerebral hypoperfusion disrupts axon‐glial integrity and the distribution of key paranodal and internodal proteins in subcortical myelinated axons. This disruption of myelinated axons is accompanied by increased microglia and cognitive decline. The aim of the present study was to investigate whether hypoperfusion impairs the functional integrity of white matter, its relation with axon‐glial integrity and microglial number, and whether by targeting microglia these effects can be improved. We show that in response to increasing durations of hypoperfusion, the conduction velocity of myelinated fibres in the corpus callosum is progressively reduced and that paranodal and internodal axon‐glial integrity is disrupted. The number of microglial cells increases in response to hypoperfusion and correlates with disrupted paranodal and internodal integrity and reduced conduction velocities. Further minocycline, a proposed anti‐inflammatory and microglia inhibitor, restores white matter function related to a reduction in the number of microglia. The study suggests that microglial activation contributes to the structural and functional alterations of myelinated axons induced by cerebral hypoperfusion and that dampening microglia numbers/proliferation should be further investigated as potential therapeutic benefit in cerebral vascular disease.     

Nerve biopsy and conduction studies in diabetic neuropathy.

Morphological findings in sural nerves were related to nerve conduction in 12 patients with diabetic neuropathy, five with mainly sensory involvement, four with severe, symmetrical sensory-motor polyneuropathy, and three with multiple mononeuropathy. All had loss of large and small myelinated and of unmyelinated fibres, even early in the disease; segmental remyelination was the most prominent myelin alteration in teased fibres, segmental demyelination was found in only a few fibres. Axonal degeneration and Schwann cell damage seem to proceed independently of each other. The relation between recorded conduction velocity and that expected from the diameter of the largest fibres indicated that slowing of 20 to 30% was due to causes other than fibre loss; a grossly diminished conduction velocity was caused mainly by fibre loss. Electrophysiological findings in the sural nerve were largely representative of findings in other nerves, though abnormalities were less marked in the median nerve. In half the endoneurial vessels from diabetic neuropathy the perivascular space was thickened or contained more layers of basal laminae than normal. The same abnormalities were found in one-quarter of the endoneurial vessels from other acquired neuropathies.     

Myelinated fibres in human paravertebral sympathetic chain: white rami communicantes in alcoholic and diabetic patients

White rami communicantes were studied quantitatively in patients with chronic alcoholism or diabetes mellitus. The fascicular area of interganglionic segments of the paravertebral sympathetic chain showed less variation than that previously reported for controls. Fibre densities were higher in the patients than in controls of similar age and the percentage of small fibres in alcoholics and diabetics is larger than in controls. Only 17% of fibres were larger than 5 μm in diameter in the patients, whereas, in controls, 33% of fibres were larger than 5 μm in diameter. No correlation between internodal lengths and fibre diameter was found in the patients who showed uniformly short internodes averaging only about half of internodal distances found in control subjects. The uniformly short internodes are attributed to complete degeneration followed by partial regeneration and are compatible with either Wallerian degeneration or a `dying back' phenomenon. The short internodal lengths should be reflected in slowed conduction velocities in white rami communicantes and could account for abnormalities in baroreflex activity, sweating, and in visceral dysfunction commonly seen in patients with chronic alcoholism or diabetes.     

Friedreich's ataxia: electrophysiological and histological findings

ABSTRACT- Electromyography was performed, and motor and sensory nerve conduction velocities were measured in 19 patients definitely affected by Friedreich's ataxia. Biopsy of the sural nerve was also performed in 9 patients.
Most patients presented a moderate to severe loss of motor units, a significant increase in mean duration of motor unit potentials, and in the incidence of polyphasic potentials. Short-lasting spontaneous activity was rarely seen. Conduction velocity along the motor and sensory fibres of the median and tibial nerves was moderately slowed, while distal conduction time to muscle was significantly increased and the sensory orthodromicallyevoked response markedly reduced. Intraoperative electrophysiological recordings obtained during biopsy of the sural nerve in 4 patients were consistent with the changes conventionally observed in the median, tibial and sural (6 patients) nerves.
Quantitative histology revealed a reduced number of total myelinated fibres with a severe loss of large fibres, and a moderate loss of fibres of less than 7 μm in diameter. In teased nerve fibre preparations, the most evident abnormality consisted of fibres with uniformly short internodal length, while signs of remyelination were less prominent. Signs of active axonal degeneration were rarely observed in electron microscopy.
Electrophysiological and histological findings were uniformly distributed, and the changes were neither related to the duration nor to the severity of the clinical condition.     

The neuropathy of Charlevoix-Saguenay ataxia: an electrophysiological and pathological study.

Two female patients aged 30 and 40 years with the Charlevoix-Saguenay ataxia were studied. Both had absent sensory action potentials in upper and lower extremities but, unlike typical cases of Friedreich's ataxia, they displayed a marked slowing of motor conduction velocities. Sural nerve biopsies taken from calf and ankle revealed a severe loss of large myelinated axons contrasting with a normal myelinated fiber density. Evidence for active axonal degeneration was scarce, with no indication of axonal regeneration. Teased myelinated fibers revealed an increased variability of internodal length but no evidence for myelin breakdown. These findings support, as a primary defect, a developmental abnormality of peripheral nerve, namely a lack of maturation of large myelinated axons and possibly a faulty myelination of nerve fibers. We think it is unlikely to represent a progressive axonal atrophic or dystrophic process, as suggested in Friedreich's ataxia.     

Early and preventable changes of peripheral nerve structure and function in insulin-deficient diabetic rats.

Structure and function of peripheral nerve in early insulin deficient streptozotocin diabetic rat were examined. Quantitative studies of myelinated fibres from electron micrographs showed that the fibre calibre and slope coefficients of regression lines between axon and myelin areas were reduced and quantitation of unmyelinated fibres revealed a 40% reduction in area fraction of smooth endoplasmic reticulum. The difference in conduction velocity between diabetic rats and control rats increased with time just as does the difference in fibre calibre. Insulin treatment was able to prevent the decrease in conduction velocity as well as the reduction in fibre calibre. It is concluded that the early structural axon abnormalities results from the diabetic state and cannot be explained by poor general condition of the animals.     

Voltage-clamp analysis of nodes of Ranvier in regenerated rat sciatic nerve

Crush-lesioned adult rat sciatic nerves were allowed to regenerate during 70-181 days. Single large regenerated nerve fibres were isolated from levels distal to the crush site, and mounted in a feedback system for potential recordings and voltage clamp. After the experiment, each isolated fibre was fixed and embedded for morphological evaluation. Internodal lengths and fibre diameters were obtained form teased preparations of regenerated and normal sciatic nerves. The isolated regenerated fibres were excitable and had action potentials of large amplitude. TEA and 4-AP had negligible effects on the action potential and excitability properties. The time constant of the nodal segment was larger than in normal fibres. The nodal Na and K permeabilities (PNa and PK) were calculated both in absolute values and relative to the leak conductance (gL). Regenerated fibres had normal PNa/gL and PK/gL ratios. The potential dependence of PNa activation and inactivation was also normal. The isolated physiologically examined fibres had internodal lengths (L) of about 300 micron and diameters (D) of about 7-11 micron. In the teased fibre preparations L was about 300 micron (range 150-600 micron) and D was 3-11 micron. In addition, all teased regenerated nerve preparations exhibited a few scattered unusually short internodes (L less than 150 micron). In control nerves L ranged from 150 micron in the lower fibre size range (D = 2-3 micron) to 1.6 mm in the upper size range (D = 15 micron).     

The normal sural nerve in man

Summary A combined light and electron microscope study of the normal sural nerve in 7 people aged 15–59 years is reported. Qualitative and quantitative studies of the Schwann cells and fibroblasts, myelinated and unmyelinated fibres are made in isolated fascicles.Schwann cells predominate over fibroblasts in the ratio of about 9-1. Most Schwann cells, almost 80%, are attached to unmyelinated fibres. Factors influencing the densities of these cells per cross sectional area are discussed.Some ultrastructural features of the myelinated fibres are described and their numbers per sq.mm and frequency distribution of their sizes are produced. An indirect method is proposed for assessing the mean internodal length for earch of the myelinated fibre size populations in cross sections of fascicles of normal nerves by estimating the proportion of myelinated segments cut through their nucleus.The ultrastructure of unmyelinated fibres is described and the identification of axons of extreme diameter is discussed. Their densities and size frequency histograms are the first to be reported in man by systematic electron microscope studies. The average ratio of unmyelinated to myelinated fibre density is about 3.7:1 though it varies in the fascicles of the different individuals.The implications of axonal diameter in the presence of myelin are commented on.British Council Scholar on leave of absence from Escuela de Medicina, Universidad Austral, Valdivia, Chile.     

Quantitation of the Schmidt-Lanterman incisures in juvenile,adult, remyelinated and regenerated fibres of the chicken sciatic nerve

Summary Morphometric studies of peripheral nerves in various species have shown that the number of Schmidt-Lanterman incisures in an internode is proportional to the fibre diameter. In the rat sural nerve it appears that following remyelination, the relationship between the numbers of incisures per internode and the fibre diameter remains unaltered despite the fact that the remyelinated internodes are uniformly short.We quantitatively examined the insertion of incisures in adult, juvenile, remyelinated and regenerated fibres of the chicken sciatic nerve. Remyelinated fibres were examined 100 and 200 days following the intraneural injection of diphtheria toxin, and regenerated fibres 200 days after a nerve crush. Our results show that the incisures are more abundant in the chicken than in all other species previously reported, and for both juvenile and adult hens the number of incisures in an internode is proportional to the fibre diameter.Following both remyelination and regeneration the internodal lengths were shorter than control fibres and the distances between the incisures were reduced by approximately 20% for all fibre diameters. Significantly, the numbers of incisures in an internode were related to the length of the remyelinated or regenerated internode and not to the fibre diameter. This finding is in marked contrast to previous reports for rat peripheral nerve.Our findings are discussed in relation to the hypothesis that the Schmidt-Lanterman incisures are vital to the maintenance of the myelin sheath and/or the associated axon.     

Abnormalities in sensory and mixed evoked potentials in ataxia-telangiectasia.

Electromography, motor, sensory and mixed nerve conduction velocity, and H reflex were studied in four patients with ataxia-telangiectasia. The earliest and most striking electrophysiologial finding was the reduced amplitude of evoked nerve potentials. In the oldest patient, findings suggestive of spinal atrophy and mild reduction of the motor and sensory nerve conduction velocities were found. Reduced amplitude in the evoked nerve potentials can be observed without clinical evidence of peripheral neuropathy. Electrophysiological abnormalities are more severe in older than in young patients. Sural nerve biopsy in one patient showed mild changes: loss of the largest myelinated fibres and demyelination of some fibres. The ratio between maximum conduction velocity of the sural nerve and the diameter of the largest fibres was in the lower limits of the normal range. The resemblance between electrophysiological abnormalities in Friedreich's ataxia and ataxia-telangiectasia is discussed.     

Sensory and motor peripheral neuropathy in olivopontocerebellar atrophy

We report the findings of an electrophysiological study in 9 patients affected by olivopontocerebellar atrophy, 4 with a dominant form and 5 with a sporadic form. Superficial peroneal nerve biopsy was obtained from 2 patients. The electrophysiological alterations were signs of collateral reinnervation and loss of motor units, decrease in sensory potential amplitude and increase in distal motor latency. Only a slight reduction in motor and sensory conduction velocity was observed in some cases. Nerve biopsy showed slight reduction of the number of myelinated fibres. In the first case, fibre diameter distribution was unimodal, due to reduction of myelinated fibres of large diameter, in the second case there was no significant alteration of the fibre distribution. In both cases short internodes were present with no signs of segmental demyelination, remyelination or axonal degeneration. The alterations observed in the peripheral nervous system are probably secondary to a lesion of the posterior root ganglion and the anterior horn cell in the spinal cord.     

Reinnervation of the canine tooth pulp after section of the inferior alveolar nerve in the cat

The apices of lower canine teeth have been examined 9 weeks after unilateral section of the inferior alveolar nerve. The teeth were extensively reinnervated although there was considerable variation in the number of axons present. The myelinated axons were smaller than normal with thinner myelin sheaths. There were fewer axons per non-myelinated fibre (Remak bundle). The proportions of myelinated and non-myelinated fibres were normal.     

Postnatal development of conduction velocity and fibre size in the rat tibial nerve

The maximum conduction velocity (CV) and fibre diameters (D) were determined in the tibial nerve of developing rats. In 1-day-old rats CV of the fastest motor and sensory fibres (assessed separately) was 1.4 m/sec on the average and increased to 35 m/sec by postnatal day 30. The maximum conduction rate in adult rats ranged from 60 to 84 m/sec. Diameters of at least 100 nerve fibres in each age group were measured in electronmicrographs. The calibre of myelinating fibres in 1-day-old rats was 0.5–1.5 μm. By day 90 after birth the range of myelinated fibre size extended to 1.5–12.5 μm. The factor relating conduction rate and total fibre diameter of the largest fibres (i.e. the value of ) was found to vary with age, increasing from 1.1 to 6.2 between postnatal days 1 and 90. These results indicate that functional and morphological properties of peripheral nerve fibres in the rat undergo considerable changes during postnatal ontogeny until they reach adult values.