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.     

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.     

Conduction velocity and spike configuration in myelinated fibres: computed dependence on internode distance.

It has been argued theoretically and confirmed experimentally that conduction velocity (theta) should be proportional to nerve fibre diameter for myelinated fibre tracts, such as normal peripheral nerve, exhibiting 'structural' similarity'. In some axons, however, the nodes of Ranvier are more closely spaced than in normal peripheral nerve. Analytic arguments have suggested that when internodal distance (L) alone is changed, the plot of theta versus L should have a relatively flat maximum. This was confirmed by several previous computer simulations of myelinated axons, but internode lengths of less than half the normal case were not examined. In order to gain insight into impulse propagation in myelinated and remyelinated fibres with short internodal lengths, the present study examines the conduction velocity and spike configuration for a wide range of internodal lengths. As L becomes large, theta falls and finally propagation is blocked; as L becomes small, theta decreases more and more steeply. From this, it is predicted that for fibres with very short internodal lengths, small local changes in L should affect substantially the conduction velocity.     

Late changes in human sural nerves in minamata disease and in nerves of rats with experimental organic mercury poisoning

Summary The sural nerves of 2 human cases with Minamata disease and poisoned rats were examined histopathologically. Both showed similar findings: the myelinated nerve fibres were decreased in number, but small myelinated nerve fibres were increased: The latter were irregular in shape and their Schwann cells showed regressive changes, with high electron density of the cytoplasms and many glycogen granules. Onion, bulb formation was not found. According to fibre diameter histograms, the ratio of small myelinated nerve fibres of 2–5 m showed a high percentage. A large number of the small myelinated nerve fibres were presumed to be regenerated nerve fibres. These findings are different from other peripheral neuropathies and may be characteristics of the late changes of the sural nerve induced by organic mercury compound.     

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.     

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.     

Polyneuropathy. Facts and fancies.

Some conclusions are drawn from findings in 167 consecutive patients with the ordinary "garden variety" of polyneuropathy; the aetiology was unknown in 15%. Histological findings in sural nerves were related to clinical and electrophysiological abnormalities. In some patients with discrete clinical abnormalities, sensory and motor conduction and amplitudes of evoked sensory and muscle action potentials were normal, whereas the nerve biopsy showed slight but definite abnormalities. The reverse, abnormal nerve conduction and normal histological findings, did not occur. Histological findings were rarely, and electrophysiological findings were not, specific for the aetiology or type of a neuropathy. Thus, neither conduction studies nor conventional or single fibre electromyography can identify the underlying pathology: loss of large myelinated fibres (greater than 7 micrometers) was equally prominent in nerves with de- and re-myelination as in those without them. Paranodal and segmental demyelination in less than 20% of the teased fibres occurred as often in nerves with as in those without disproportionate slowing in conduction. When the recorded conduction velocity was equal to that to be expected from the fibres with the largest diameter, slowing in conduction could be explained by axonal degeneration ("proportionate" slowing, 79% of the nerves). When the recorded velocity was disproportionately slower than that expected from fibre diameter (21% of the nerves), causes other than loss of the largest fibres must be assumed to explain the slowing in conduction. Myelin abnormalities in more than 50% of the teased fibres were found only in nerves from patients with the hypertrophic type of peroneal muscular atrophy and in postgastrectomy neuropathy and can probably explain the marked disproportionate slowing in conduction. The material contained, however, only one patient with acute idiopathic polyradiculoneuropahy. In diabetic neuropathy, segmental demyelination was present in only 8 of 502 teased fibres (9 nerves), remyelination was present in 135 fibres, and could not explain the disproportionate slowing in conduction. The mechanism of disproportionate slowing, when it is not due to demyelination, is still obscure.     

A micro-electrode study of peripheral neuropathy in man. Part 1. Responses to single graded stimuli.

Multi-unit micro-electrode recording were obtained from sensory fascicles of the sural and median nerves of 12 control subjects and of 28 patients with peripheral neuropathy. Spontaneous activity and mass responses to mechanical and electrical stimuli were examined. Mechanoreceptor function appeared normal but there was a reduced number of responsive receptors in peripheral neuropathy. The electrical activation threshold of nerve fibres of all conduction velocities was increased in neuropathy and a greater number of fibres needed to be activated for preception to occur. Clinical sensory impairment was associated with a reduction in size of the initial compound action potential of the maximal evoked neurogram and with dispersion of fibre responses. Pathological slowing of fibre conduction velocity was demonstrated in demyelinating neuropathy but in most cases of axomal degeneration the changes in velocity could have been due either to a reduced number of fast conducting fibres, or to conduction block. No changes were observed in C-fibre activity in these patients.     

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.     

Accuracy of sampling methods in morphometric studies of human sural nerves.

The aim of this study was to ascertain the minimum sample required to accurately measure the total number of myelinated fibres, mean myelinated fibre density (MFD), myelinated fibre diameter (Ds) and axonal diameter (Da) in morphometric studies of sural nerve biopsies. Measurements were obtained by sampling a single fascicle or systematic sampling of up to 50% of the total transverse fascicular area of two control and eighteen pathological sural nerves showing varying degrees of demyelination and axonal degeneration. MFD and fibre size were heterogeneous between fascicles in both control and pathological sural nerves, and morphometric results from one fascicle and systematic sampling of up to 50% of the total transverse fascicular area did not accurately represent the whole myelinated fibre population in the sural nerve. For accurate morphometric data it is necessary to quantitate all the myelinated fibres in the sural nerve.     

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.     

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.     

A micro-electrode study of peripheral neuropathy in man. Part 2. Responses to conditioning stimuli.

Surface, needle and micro-electrode recordings were obtained from sensory nerves of patients with various types of peripheral neuropathy. Changes in amplitude and conduction velocity of nerve action potentials were measured after a single conditioning stimulus and after tetanic stimulation for 2 min. In patients with hereditary forms of axonal degeneration (AD), recovery processes of nerve fibres of all conduction velocities were normal; in acquired forms of AD fibres with conduction velocity less than 30 m/sec had greater and more prolonged post-tetanic depression than control nerves of similar conduction velocity. Where neuropathy was associated with segmental demyelination (SD), fibres of all conduction velocities had prolonged recovery processes after both single and tetanic stimulation. The changes were especially marked at higher skin temperature, and were greater than the changes seen in nerves with acquired forms of AD. Finally, 2 sural nerves were studied during the process of Wallerian degeneration after a biopsy had been obtained proximally, and recovery processes did not change during the period of degeneration. Perceptual abnormalities were similar in AD and SD. It is suggested that changes in recovery processes of nerve fibres with segmental demyelination or regeneration after injury contribute to the perceptual abnormalities which occur in clinically encountered peripheral neuropathies.     

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.     

The effects of temperature on human compound action potentials.

The upper limbs of 10 healthy subjects were cooled and then warmed over physiological temperature ranges. The compound action potentials of median digital nerves, median sensory nerve at the wrist, radial sensory nerve at the wrist, and median thenar muscle, all showed progressive reduction in latency, amplitude, duration and area during rising temperature. Our studies suggest that the sensory compound action potential changes occur predominantly because of the summated effects of reduction in the duration of the action potentials of single myelinated fibres, although disproportionate increase in the conduction velocity of larger myelinated fibres also plays a role.     

Inter-nerves and intra-nerve conduction heterogeneity in CMTX with Arg(15)Gln mutation.

OBJECTIVE: In X-linked Charcot-Marie-Tooth disease (CMTX), electrophysiological and histopathological studies have suggested either a demyelinating or an axonal polyneuropathy. We report a CMTX family with a striking heterogeneity of nerve conductions between and within nerves. METHODS: Two men and one woman have been studied by conduction velocities, sural nerve biopsy with morphometry (one man) and DNA analysis. RESULTS: In both men motor conduction velocities were slowed in the demyelinating range, conduction velocity differences among nerves in the same subject varied from 13 to 24 m/s, and distal median compound muscle action potential (CMAP) amplitudes were 3-5 times reduced compared to ulnar CMAPs. Abnormal area reduction or excessive temporal dispersion of proximal CMAP was present in at least two nerves in all patients. Sural nerve biopsy showed reduction of large myelinated fibres, cluster formations, occasional onion bulbs. Teased fibres study revealed short internodes for fibre diameter, enlarged Ranvier nodes but no evidence of segmental demyelination and remyelination. DNA analysis showed an Arg(15)Gln mutation in connexin32 gene in all patients. CONCLUSIONS: In this family conduction slowing and segmental conduction abnormalities, in absence of morphological evidence of de-remyelination, may be related to short internodes, widened Ranvier nodes and the specific effect of the mutation. The occurrence in some CMTX patients of a non uniform involvement between and within nerves, as in acquired demyelinating neuropathies, should be kept in mind to avoid misdiagnoses.     

Abnormalities in the vagus nerve in canine acrylamide neuropathy

Dogs exposed to acrylamide develop a sensorimotor peripheral neuropathy and megaoesophagus. The presence of neuropathy was confirmed electrophysiologically and histologically. Hindlimb motor conduction velocity was reduced and there was a loss of large diameter myelinated fibres in the dorsal common digital nerve and the tibial nerve. The conduction velocity of vagal motor fibres innervating the thoracic oesophagus was not decreased; there was a reduction in the conduction velocity of the mixed nerve action potential of the vagus. Degenerating nerve fibres were observed in the vagus in the midthoracic region. The damage to vagal nerve fibres may be an important factor in the causation of megaoesophagus.     

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.     

A clinico-pathological and follow up study of 10 cases of essential type II cryoglobulinaemic neuropathy.

Ten patients with essential cryoglobulinaemia type II were examined for peripheral nerve damage. In six cases distal symmetrical nerve involvement was present, while in three other cases abnormalities restricted to single nerves were found. Electrophysiological and morphological data were consistent with axonal damage, the larger myelinated fibres being most affected. Although active signs of vasculitis and immunoperoxidase staining for immunoglobulins were not present, endoneurial vessels were widely damaged, with abnormally thick endothelial cells and redundant basal membranes. These findings, together with a patchy distribution of myelinated fibre loss, suggest ischaemia as a cause of peripheral neuropathy during essential cryoglobulinaemia type II. A follow up examination, performed one year after haematologial remission, revealed that no further peripheral nerve damage had occurred.     

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.