Average Axon Diameter Research Articles

On the effects of a varied diffusion time in vivo: is the diffusion in white matter restricted?

The aim of this work was to study the diffusion-related signal attenuation curves (signal-vs.-b curves) measured perpendicular and parallel to the neuronal fibers of the corticospinal tract in vivo and to determine whether effects of restricted diffusion could be observed when varying the diffusion time (T(D)). A biexponential model and a two-compartment model including exchange according to the Kärger formalism were employed to analyze the signal-vs.-b curves. To validate the two-compartment model, restricted diffusion with exchange was simulated for uniformly sized cylinders, using different diameters and exchange times. The model was shown to retrieve the simulated parameters well, also when the short gradient pulse approximation was not met. The in vivo measurements performed perpendicular to the tracts, using b values up to 28000 s/mm(2) and T(D) values between 64 and 256 ms, did not show the effects of restricted diffusion as expected from previous ex vivo studies. The applied two-compartment model yielded an average axonal diameter of about 4 mum and an intracellular exchange time of about 300 ms, but did not fit statistically well to the data. In conclusion, this study indicates that if the diffusion is modeled as two compartments, of which one is restricted, exchange must be included in the model.

Chronic nerve compression induces local demyelination and remyelination in a rat model of carpal tunnel syndrome

In a previous study, we demonstrated that chronic compression of rat sciatic nerve, a model of compressive neuropathies, triggered dramatic Schwann cell proliferation and concurrent apoptosis. Importantly, this Schwann cell response occurred before there are signs of overt axonal pathology, raising the question of whether there are alterations in axonal myelination in the areas of the nerve in which Schwann cell apoptosis and proliferation occur. Here, we use nerve teasing techniques and unbiased stereology to assess myelination in nerves after 1 and 8 months of compression. Evaluations of myelin thickness and axonal diameter (AD) using design-based, unbiased stereology revealed alterations in myelin structure that indicate remyelination, specifically a dramatic decrease in the average internodal length (IL) and an increase in the proportion of axons with thin myelin sheaths. The mean IL was reduced after 1 month of chronic nerve injury with no further decrease in IL at 8 months. There was limited change in average axonal diameter at both 1 and 8 months. Measures of myelin thickness revealed not only a greater than 6-fold increase in the number of axons with very thin (<5 μm thickness) myelin sheaths, but also a proportional decrease in the number of axons with the thick myelin sheaths characteristic of normal nerve. These results confirm that an early consequence of chronic nerve compression (CNC) is local demyelination and remyelination, which may be the primary cause of alterations in nerve function during the early period post-compression.

Computer-assisted morphometric study on the postnatal growth of axon size and myelin thickness in the ventral and dorsal roots of the rabbit

The morphometric analysis of the myelinated fibers is of great value in developmental, experimental, and pathological studies. The present study was performed on the ventral and dorsal spinal roots (L7) of new-born, 10-, 15-, 20-, 30-, 60-, 120- and 240-day-old rabbits. Semi-thin cross-sections were investigated by OLYMPUS Video image analyser. The average axonal diameter was measured as an average length of Feret's diameters and the specific width of the myelin sheaths as a total width of the myelin sheath. In the ventral roots, the increase of the axon size is more intensive than the increase of the myelin thickness. In adult rabbits, the mean axonal diameter for the ventral root is approximately 15% greater than the mean axonal diameter for the dorsal root, whereas the mean myelin thickness values are practically equal for both roots. For the period newborn-adult (240-day-old) rabbits, a full correlation was found in respect to the increase of the dorsal root mean axonal diameter and mean myelin thickness (380% and 380%). In the ventral roots, the mean axonal diameter increased 420% whereas the mean myelin thickness increased 350%. It is intriguing whether the greater axonal size of the ventral roots might compensate their relatively thin myelin sheaths in comparison to the dorsal roots. Our findings of essential increase (16%) of the mean myelin thickness in the last (120-240 days) period in both, ventral and dorsal roots, points to a protracted period of myelinization in the rabbit.

Treatment of the chronic inflammation in peripheral target tissue improves the crushed nerve recovery in the rat: Histopathological assessment of the nerve recovery

An experimental study was performed to investigate the influence of subsidence of chronic inflammation in peripheral target tissue on the recovery of crushed nerve. Seventy-eight male Wistar rats weighing 300–370 g were used. The sciatic nerve was operatively crushed unilaterally with an aneurysm clip (250 gf) applied for 5 min. Chronic inflammation, localized to the ankle, was induced by intra-articular injection of complete Freund's adjuvant 1 week preoperatively. Prednisolone farnesylate (PNF-21) 1.4% gel was applied on the ankle as an anti-inflammatory agent for consecutive days after the operation. The animals were divided into five groups as follows: crush injury with ipsilateral arthritis (CIA); crush injury with ipsilateral arthritis and PNF-21 gel applied on the ipsilateral ankle (CIA+IPNF); crush injury with ipsilateral arthritis and PNF-21 gel applied on the contralateral ankle (CIA+CPNF); crush injury with contralateral arthritis (CCA); crush injury without arthritis (C). Specimens for histopathological examination were taken from the nerve at a site 5 mm distal to the crush lesion at 4 weeks postoperatively. The average axon diameter was significantly larger in the CIA+IPNF group than in the CIA group (p<0.01). No significant difference was observed between the CIA+CPNF group and the CIA group. In conclusion, chronic inflammation in peripheral target tissue suppresses recovery of the crushed nerve, and subsidence of this chronic inflammation improves this suppression histopathologically.

Automatic image analysis of the postnatal growth of axons and myelin sheaths in the tibial and peroneal nerves of the rabbit

The numerous morphometric studies on the myelinated fibers in the peripheral nerves have presented varying results. Only a few studies deal with the peripheral nerves from rabbits. In this work, a morphometric study was carried out on the tibial and peroneal nerves of new-born, 10-, 15-, 20-, 30-, 60-, 90- and 240-day-old rabbits. The bilateral proximal segments of both nerves were investigated. Negatives of semi-thin cross sections were used for myelinated fiber morphometric analysis, carried out by an OLYMPUS Video image analyser. Two morphometric parameters, the average axon diameter (AD) (the average length of Feret's diameters) and the specific width of the myelin sheaths (SWMS) (specifying the total width of the myelin sheath), were evaluated for every age group. In the tibial and peroneal nerves a bimodal distribution of the average AD appeared on the 20th day, and of the SWMS on the 10th day postnatum. A tight correlation was obtained when comparing mean AD (mAD) and mean SWMS (mSWMS) in new-born and 240-day-old rabbits. From birth to adulthood the mAD increased in both nerves by about 270% and the mSWMS by about 280%. The mAD/mSWMS ratio in both new-born and 8-month-old rabbits was found to be 4 in the tibial and 5 in the peroneal nerves. According to the available data, an approach to the measurement of AD as an average length of Feret's diameters and the measurement of the myelin thickness as a SWMS has not yet been employed. An extension of this methodological approach could help to understand the growth and myelinization of peripheral nerve fibers.

Changes in the compound action current amplitudes in relation to the conduction velocity and functional recovery in the reconstructed peripheral nerve

The average axon diameter in the proximal segment of a transected and reconstructed peripheral nerve will decrease shortly after the transection and increase again when the regenerating axons make contact with their targets. The magnetically recorded nerve compound action current (NCAC) amplitude and the conduction velocity (CV) are directly related to the axon diameters. In this experiment, the peroneal nerve was unilaterally transected and reconstructed in 42 rabbits. After 3, 4.5, 6, 8, 12, 20, and 36 weeks of regeneration time, hind leg motor function recovery, NCAC amplitude, and CV1st peak were studied. Our results demonstrate a significant decrease in signal amplitude and CV in the first 8 weeks after reconstruction. These decreases are related (P < 0.05). After 8 weeks of regeneration time, motor function and the CV of the recorded signals start to recover, but the signal amplitudes do not. Based on the correlation of the CV and signal amplitude with axon diameter, they would both be expected to increase with recovering function. As an explanation for this lack of increase of signal amplitude, we suggest that, at the same time as some axons reach their target organs and start to mature, a number of the axons which have not reached a proper target organ will lose their signal-conducting capability. This will cause a decrease in compound signal amplitude, which cancels out the expected increase in NCAC amplitude, due to axonal maturation. © 1999 John Wiley & Sons, Inc. Muscle Nerve 22: 1087–1093, 1999

Changes in the compound action current amplitudes in relation to the conduction velocity and functional recovery in the reconstructed peripheral nerve.

The average axon diameter in the proximal segment of a transected and reconstructed peripheral nerve will decrease shortly after the transection and increase again when the regenerating axons make contact with their targets. The magnetically recorded nerve compound action current (NCAC) amplitude and the conduction velocity (CV) are directly related to the axon diameters. In this experiment, the peroneal nerve was unilaterally transected and reconstructed in 42 rabbits. After 3, 4.5, 6, 8, 12, 20, and 36 weeks of regeneration time, hind leg motor function recovery, NCAC amplitude, and CV(1st peak) were studied. Our results demonstrate a significant decrease in signal amplitude and CV in the first 8 weeks after reconstruction. These decreases are related (P < 0.05). After 8 weeks of regeneration time, motor function and the CV of the recorded signals start to recover, but the signal amplitudes do not. Based on the correlation of the CV and signal amplitude with axon diameter, they would both be expected to increase with recovering function. As an explanation for this lack of increase of signal amplitude, we suggest that, at the same time as some axons reach their target organs and start to mature, a number of the axons which have not reached a proper target organ will lose their signal-conducting capability. This will cause a decrease in compound signal amplitude, which cancels out the expected increase in NCAC amplitude, due to axonal maturation.

Metachromatic leukodystrophy: molecular genetics and an animal model.

Metachromatic leukodystrophy (MLD) is a lysosomal storage disorder caused by the deficiency of arylsulphatase A (ASA; EC 3.1.6.8). Deficiency of this enzyme causes intralysosomal storage of the sphingolipid cerebroside sulphate. This lipid is abundant in myelin and it may thus not be surprising that storage mainly affects oligodendrocytes. Patients suffer from a progressive demyelination causing various neurological symptoms. The disease is fatal and treatment is not available. The human ASA gene has been cloned and more than 40 mutations have been analysed that cause metachromatic leukodystrophy. Few of these alleles are frequent among patients, whereas most mutant alleles have only been found in single families. Since MLD has only been described in humans and no naturally occurring animal model has been described, ASA-deficient mice have been generated by homologous recombination. The ASA knockout mice are unable to degrade sulphatide and store the lipid intralysosomally. The pattern of lipid storage in neuronal and non-neuronal tissues resembles that described for patients. In the nervous system, lipid storage is found in oligodendrocytes, astrocytes and some neurons. Animals display an astrogliosis and a decreased average axonal diameter. Purkinje cells and Bergmann glia of the cerebellum are morphologically aberrant. Demyelination is seen in the acoustic ganglion and occurs between the ages of 6 and 12 months. The animals are deaf at this age and display various neuromotor abnormalities. However, compared to humans the mice have a surprisingly mild phenotype, since they have a normal life span and do not develop widespread demyelination. ASA-deficient mice have been transplanted with bone marrow, which was transduced with a retroviral vector expressing arylsulphatase A. The majority of transplanted animals display sustained expression of arylsulphatase A from the retroviral construct up to 5 months after transplantation. However, preliminary data suggest that this therapeutic approach does not reduce storage material.

Synaptic responses to mechanical stimulation in calyceal and bouton type vestibular afferents studied in an isolated preparation of semicircular canal ampullae of chicken

Relationships between the response patterns of semicircular canal afferents to mechanical stimulation and the morphologies of their peripheral endings were investigated in an isolated preparation of the anterior semicircular canal ampulla of chicken, using a combination of electrical recording with intracellular injections of Lucifer Yellow CH. The hair bundle mechanical stimulus was applied in a diffuse manner by a glass rod vibrating in the nearby bathing medium. Two types of spike discharge patterns and postsynaptic potentials were recorded. One type was found exclusively in the bouton type afferent and demonstrated a phasic increase of firing frequency and transient depolarizing postsynaptic potentials at the beginning of mechanical stimulation. These synaptic potentials were also observed spontaneously and their amplitudes were increased by membrane hyperpolarization. The other type was found exclusively in afferents with calyceal endings and showed a tonic increase of spiking frequency and depolarizing DC postsynaptic potentials with superimposing AC responses at the frequency of the mechanical stimulation. Amplitudes of postsynaptic potentials were increased by hyperpolarization. Hair cells generated depolarizing DC transduction potentials superimposed with AC potentials at frequency of the mechanical stimulation. The spontaneous spike discharging patterns of afferent nerve fibres were classified either as a regular type (CV less than 0.10) or as an irregular type (CV greater than 0.25) on the basis of coefficient of variation (CV) of interspike intervals. The spontaneous firing rate of regular units was higher than that of irregular units. Several membrane characteristics are different between these two types of afferent fibers; irregular units had short membrane time constants and fast spikes associated with clear spike-afterhyperpolarization. These features fit with the fact that irregular units tend to have phasic responses to mechanical stimulation while regular units typically have tonic responses. Irregular units had bouton endings with an average axonal diameter thicker than the regular units which had calix endings.

Automated measurement of peripheral nerve fibres in transverse section

Transverse sections of rat tibial nerve were scanned with an automated flying-spot microscope and the pictures examined in a one bit (black-and-white) form. Computer programs have been developed, and are described briefly, to isolate individual nerve fibres in the field of view, and to measure axon area and myelin area, from which it is trivial to calculate average axon diameter and average myelin thickness if the axon is assumed to be circular in cross-section. At the magnification used, it was possible to measure 40–50 fibres per field and the time required to do this was 5–7 minutes. For each specimen, 7–8 fields were measured, providing data on 300–400 fibres. Measurements, made on nerves from rats between 3 and 38 weeks of age, show the rate at which nerve fibres grow and also show that the ratio between axon diameter and fibre diameter remains constant, within experimental error, over this period and is close to the predicted value.

Changes of the ratio between myelin thickness and axon diameter in the human developing sural nerve.

Axon caliber and myelin sheath thickness of individual nerve fibers were evaluated in the developing human sural nerve using three different methods of measurement: 1. ocular micrometer evaluation of large fibers, 2. photographic enlargements for evaluating large numbers of nerve fibers of all sizes, and 3. electron microscopic enlargements for more precise measurements in selected nerves. The average axonal diameter doubles from 5 months gestation to about 5 years of age. Large fiber group axons increase, during the same period, by a factor of 3--3.5 with a slight decrease thereafter. The myelin thickness increases more slowly, but continuously, between 5 months gestation until the age of 14. This asynchronous development of axons and myelin sheaths results in a statistically significant change of the ratio between axonal caliber and myelin thickness. The slope of the regression line is steeper in older than in younger individuals, and the correlation coefficient increases during development of the nerve.

Development of myelination in optic tract of the cat

The postnatal development of myelin in the optic nerve and tract of normal and dark reared cats has been studied quantitatively with light and electron microscopy. In the newborn cat few myelinated fibers (3% of the population) are seen in the optic tract. Until the end of the second postnatal week, the total number of myelinated axons in the tract remains low (23%). At this time, however, there is an explosive increase in the rate of myelination and by the end of the fourth postnatal week 80% of the optic tract axons have acquired a myelin sheath. Thereafter, the number of myelinated axons increases gradually, reaching adult levels (100%) at 12 weeks. During the initial period of myelination, the average axon diameter is 0.6 mu for unmyelinated fibers and a.2 mu for myelinated fibers. Both of these means remain substantially unchanged until myelination is completed, suggesting that initial myelination of an axon is not a continuous process but rather proceeds in a step-wise manner. Dark rearing appears to have no effect on the initiation of myelination.

A fine structural analysis of the statocyst in Aplysia californica

AbstractEach statocyst in Apylsia californica contains 13 neurons. The statocyst nerve, which connects each statocyst to the cerebral ganglion, contains only the 13 axons of the statocyst neurons. The size of the statocyst, the number of neurons in the statocyst, and the average axonal diameter does not change even though the statocyst nerve lengthens greatly as the animal enlarges. A description of the statoconia and the supporting cells in this organ has been given. This prepazation may be useful for microelectrode studies designed to test whether the gap and cytoplasmic specializations that are used to identify active synapses, are necessary for all types of chemical synaptic transmission.