Quantitative study of the non-circularity of myelinated peripheral nerve fibres in the cat

Maternal high-fat diet modifies myelin organization, microglial interactions, and results in social memory and sensorimotor gating deficits in adolescent mouse offspring

Prenatal exposure to maternal high-fat diet (mHFD) acts as a risk factor for various neurodevelopmental alterations in the progeny. Recent studies in mice revealed that mHFD results in both neuroinflammation and hypomyelination in the exposed offspring. Microglia, the brain-resident macrophages, play crucial roles during brain development, notably by modulating oligodendrocyte populations and performing phagocytosis of myelin sheaths. Previously, we reported that mHFD modifies microglial phenotype (i.e., morphology, interactions with their microenvironment, transcripts) in the hippocampus of male and female offspring. In the current study, we further explored whether mHFD may induce myelination changes among the hippocampal-corpus callosum-prefrontal cortex pathway, and result in behavioral outcomes in adolescent offspring of the two sexes. To this end, female mice were fed with control chow or HFD for 4 weeks before mating, during gestation, and until weaning of their litter. Histological and ultrastructural analyses revealed an increased density of myelin associated with a reduced area of cytosolic myelin channels in the corpus callosum of mHFD-exposed male compared to female offspring. Transcripts of myelination-associated genes including Igf1 -a growth factor released by microglia- were also lower, specifically in the hippocampus (without changes in the prefrontal cortex) of adolescent male mouse offspring. These changes in myelin were not related to an altered density, distribution, or maturation of oligodendrocytes, instead we found that microglia within the corpus callosum of mHFD-exposed offspring showed reduced numbers of mature lysosomes and increased synaptic contacts, suggesting microglial implication in the modified myelination. At the behavioral level, both male and female mHFD-exposed adolescent offspring presented loss of social memory and sensorimotor gating deficits. These results together highlight the importance of studying oligodendrocyte-microglia crosstalk and its involvement in the long-term brain alterations that result from prenatal mHFD in offspring across sexes.

Targeted neuromodulation of pelvic floor nerves in aging and multiparous rabbits improves continence

Pelvic floor muscle stretch injury during pregnancy and birth is associated with the incidence of stress urinary incontinence (SUI), a condition that affects 30-60% of the female population and is characterized by involuntary urine leakage during physical activity, further exacerbated by aging. Aging and multiparous rabbits suffer pelvic nerve and muscle damage, resulting in alterations in pelvic floor muscular contraction and low urethral pressure, resembling SUI. However, the extent of nerve injury is not fully understood. Here, we used electron microscopy analysis of pelvic and perineal nerves in multiparous rabbits to describe the extent of stretch nerve injury based on axon count, axon size, myelin-to-axon ratio, and elliptical ratio. Compared to young nulliparous controls, mid-age multiparous animals showed an increase in the density of unmyelinated axons and in myelin thickness in both nerves, albeit more significant in the bulbospongiosus nerve. This revealed a partial but sustained damage to these nerves, and the presence of some regenerated axons. Additionally, we tested whether electrical stimulation of the bulbospongiosus nerve would induce muscle contraction and urethral closure. Using a miniature wireless stimulator implanted on this perineal nerve in young nulliparous and middle age multiparous female rabbits, we confirmed that these partially damaged nerves can be acutely depolarized, either at low (2-5 Hz) or medium (10-20 Hz) frequencies, to induce a proportional increase in urethral pressure. Evaluation of micturition volume in the mid-age multiparous animals after perineal nerve stimulation, effectively reversed a baseline deficit, increasing it 2-fold (p = 0.02). These results support the notion that selective neuromodulation of pelvic floor muscles might serve as a potential treatment for SUI.

A shape-adjusted ellipse approach corrects for varied axonal dispersion angles and myelination in primate nerve roots

Segmentation of axons in light and electron micrographs allows for quantitative high-resolution analysis of nervous tissues, but varied axonal dispersion angles result in over-estimates of fiber sizes. To overcome this technical challenge, we developed a novel shape-adjusted ellipse (SAE) determination of axonal size and myelination as an all-inclusive and non-biased tool to correct for oblique nerve fiber presentations. Our new resource was validated by light and electron microscopy against traditional methods of determining nerve fiber size and myelination in rhesus macaques as a model system. We performed detailed segmental mapping and characterized the morphological signatures of autonomic and motor fibers in primate lumbosacral ventral roots (VRs). An en bloc inter-subject variability for the preganglionic parasympathetic fibers within the L7-S2 VRs was determined. The SAE approach allows for morphological ground truth data collection and assignment of individual axons to functional phenotypes with direct implications for fiber mapping and neuromodulation studies.

Axon and Myelin Morphology in Animal and Human Spinal Cord

Characterizing precisely the microstructure of axons, their density, size and myelination is of interest for the neuroscientific community, for example to help maximize the outcome of studies on white matter (WM) pathologies of the spinal cord (SC). The existence of a comprehensive and structured database of axonal measurements in healthy and disease models could help the validation of results obtained by different researchers. The purpose of this article is to provide such a database of healthy SC WM, to discuss the potential sources of variability and to suggest avenues for robust and accurate quantification of axon morphometry based on novel acquisition and processing techniques. The article is organized in three sections. The first section reviews morphometric results across species according to range of densities and counts of myelinated axons, axon diameter and myelin thickness, and characteristics of unmyelinated axons in different regions. The second section discusses the sources of variability across studies, such as age, sex, spinal pathways, spinal levels, statistical power and terminology in regard to tracts and protocols. The third section presents new techniques and perspectives that could benefit histology studies. For example, coherent anti-stokes Raman spectroscopy (CARS) imaging can provide sub-micrometric resolution without the need for fixation and staining, while slide scanners and stitching algorithms can provide full cross-sectional area of SC. In combination with these acquisition techniques, automatic segmentation algorithms for delineating axons and myelin sheath can help provide large-scale statistics on axon morphometry.

Neuroactive steroid treatment modulates myelin lipid profile in diabetic peripheral neuropathy

Diabetic peripheral neuropathy causes a decrease in the levels of dihydroprogesterone and 5α-androstane-3α,17β-diol (3α-diol) in the peripheral nerves. These two neuroactive steroids exert protective effects, by mechanisms that still remain elusive. We have previously shown that the activation of Liver X Receptors improves the peripheral neuropathic phenotype in diabetic rats. This protective effect is accompanied by the restoration to control values of the levels of dihydroprogesterone and 3α-diol in peripheral nerves. In addition, activation of these receptors decreases peripheral myelin abnormalities by improving the lipid desaturation capacity, which is strongly blunted by diabetes, and ultimately restores the myelin lipid profile to non-diabetic values. On this basis, we here investigate whether dihydroprogesterone or 3α-diol may exert their protective effects by modulating the myelin lipid profile. We report that both neuroactive steroids act on the lipogenic gene expression profile in the sciatic nerve of diabetic rats, reducing the accumulation of myelin saturated fatty acids and promoting desaturation. These changes were associated with a reduction in myelin structural alterations. These findings provide evidence that dihydroprogesterone and 3α-diol are protective agents against diabetic peripheral neuropathy by regulating the de novo lipogenesis pathway, which positively influences myelin lipid profile.

Morphometric parameters of peripheral nerves in calves correlated with conduction velocity

BACKGROUND

Peripheral nerve injuries are the most frequent neurologic disorder in cattle. So far, no physiologic values have been established for the motor nerve conduction velocity (mNCV) in this precocial species.

OBJECTIVES

The electrophysiologic and morphometric reference values of peripheral nerves in calves were determined. It was hypothesized that these parameters would correlate to the high degree of maturity in the first days of life in this species compared to other species.

ANIMALS

Twenty-six healthy calves were used in this study.

METHODS

The mNCV of the radial and the sciatic/common peroneal nerve was measured in all 26 calves. Nerve biopsies from a group of 6 calves were taken to correlate the obtained electrophysiologic data with morphological parameters.

RESULTS

The mean mNCV of the radial nerve was 48.3 ± 10.6 m/s, whereas the mean mNCV of the sciatic/peroneal nerve was with 83.8 ± 5.9 m/s significantly faster (P < .0001). The average fiber diameter was 8.40 ± 2.80 μm (range, 1.98-17.90 μm) and the average g-ratio was 0.61 ± 0.04 SD.

CONCLUSION AND CLINICAL IMPORTANCE

The established reference values for mNCV in calves correlate well with the evaluated morphometric parameters. Attributable to their comparably fast mNCV and high fiber diameters, juvenile calves appear to be much more mature individuals than other mammals. Electrophysiologic characterization of peripheral nerve injury now is feasible in this species.

Microscopic anatomy: normal structure

A peripheral nerve trunk is composed of nerve fascicles supported in a fibrous collagenous sheath and defined by concentric layers of cells (the perineurium) that separate the contents (the endoneurium) from its fibrous collagen support (the epineurium). In the endoneurium are myelinated and unmyelinated fibers that are axons combined with their supporting Schwann cells to provide physical and electrical connections with end-organs such as muscle fibers and sensory endings. Axons are tubular neuronal extensions with a cytoskeleton of neurotubules and tubulin along which organelles and proteins can travel between the neuronal cell body and the axon terminal. During development some axons enlarge and are covered by a chain of Schwann cells each associated with just one axon. As the axons grow in diameter, the Schwann cells wrap round them to produce a myelin sheath. This consists of many layers of compacted Schwann cell membrane plus some additional proteins. Adjacent myelin segments connect at highly specialized structures, the nodes of Ranvier. Myelin insulates the axon so that the nerve impulse can jump from one node to the next. The region adjacent to the node, the paranodal segment, is the site of myelin terminations on the axolemma. There are connections here between the Schwann cell and the axon via a complex chain of proteins. The Schwann cell cytoplasm in the adjacent segment, the juxtaparanode, contains most of the Schwann cell mitochondria. In addition to the node, continuity of myelin lamellae is broken at intervals along the internode by helical regions of decompaction known as Schmidt-Lanterman incisures; these are seen as paler conical segments in suitably stained microscopical preparations and provide a pathway between the adaxonal and abaxonal cytoplasm. Smaller axons without a myelin sheath conduct very much more slowly and have a more complex relationship with their supporting Schwann cells that has important implications for repair.

Neurophysiological approach to disorders of peripheral nerve

Disorders of the peripheral nerve system (PNS) are heterogeneous and may involve motor fibers, sensory fibers, small myelinated and unmyelinated fibers and autonomic nerve fibers, with variable anatomical distribution (single nerves, several different nerves, symmetrical affection of all nerves, plexus, or root lesions). Furthermore pathological processes may result in either demyelination, axonal degeneration or both. In order to reach an exact diagnosis of any neuropathy electrophysiological studies are crucial to obtain information about these variables. Conventional electrophysiological methods including nerve conduction studies and electromyography used in the study of patients suspected of having a neuropathy and the significance of the findings are discussed in detail and more novel and experimental methods are mentioned. Diagnostic considerations are based on a flow chart classifying neuropathies into eight categories based on mode of onset, distribution, and electrophysiological findings, and the electrophysiological characteristics in each type of neuropathy are discussed.

Visual system scaling in teleost fish

Teleost fish grow continuously throughout their lifespan, and this growth includes visual system components: eyes, optic nerves, and brain. As fish grow, the optic nerve lengthens and neural signals must travel increasing distances from the eye to the optic tectum along thousands of retinal ganglion cell (RGC) axons. Larger fish have better vision that enhances their ability to capture prey, but they are faced with the potential computational problem of changes in the relative timing of visual information arriving at the brain. Optic nerve conduction delays depend on RGC axon conduction velocities, and velocity is primarily determined by axon diameters. If axon diameters do not increase in proportion to body length, then absolute and relative conduction delays will vary with fish size. We have measured optic nerve lengths and axon diameter distributions in different sized zebrafish (Danio rerio) and goldfish (Carassius auratus) and find that, as both species of fish grow, axon diameters increase to reduce average conduction delays by about half and to keep relative delays constant. This invariance of relative conduction delays simplifies computational problems faced by the optic tectum.

Diabetes-induced myelin abnormalities are associated with an altered lipid pattern: protective effects of LXR activation

Diabetic peripheral neuropathy (DPN) is characterized by myelin abnormalities; however, the molecular mechanisms underlying such deficits remain obscure. To uncover the effects of diabetes on myelin alterations, we have analyzed myelin composition. In a streptozotocin-treated rat model of diabetic neuropathy, analysis of sciatic nerve myelin lipids revealed that diabetes alters myelin's phospholipid, FA, and cholesterol content in a pattern that can modify membrane fluidity. Reduced expression of relevant genes in the FA biosynthetic pathway and decreased levels of the transcriptionally active form of the lipogenic factor sterol-regulatory element binding factor-1c (SREBF-1c) were found in diabetic sciatic nerve. Expression of myelin's major protein, myelin protein zero (P0), was also suppressed by diabetes. In addition, we confirmed that diabetes induces sciatic nerve myelin abnormalities, primarily infoldings that have previously been associated with altered membrane fluidity. In a diabetic setting, synthetic activator of the nuclear receptor liver X receptor (LXR) increased SREBF-1c function and restored myelin lipid species and P0 expression levels to normal. These LXR-modulated improvements were associated with restored myelin structure in sciatic nerve and enhanced performance in functional tests such as thermal nociceptive threshold and nerve conduction velocity. These findings demonstrate an important role for the LXR-SREBF-1c axis in protection from diabetes-induced myelin abnormalities.

A semi-automated method for identifying and measuring myelinated nerve fibers in scanning electron microscope images

Diagnosing illnesses, developing and comparing treatment methods, and conducting research on the organization of the peripheral nervous system often require the analysis of peripheral nerve images to quantify the number, myelination, and size of axons in a nerve. Current methods that require manually labeling each axon can be extremely time-consuming as a single nerve can contain thousands of axons. To improve efficiency, we developed a computer-assisted axon identification and analysis method that is capable of analyzing and measuring sub-images covering the nerve cross-section, acquired using a scanning electron microscope. This algorithm performs three main procedures - it first uses cross-correlation to combine the acquired sub-images into a large image showing the entire nerve cross-section, then identifies and individually labels axons using a series of image intensity and shape criteria, and finally identifies and labels the myelin sheath of each axon using a region growing algorithm with the geometric centers of axons as seeds. To ensure accurate analysis of the image, we incorporated manual supervision to remove mislabeled axons and add missed axons. The typical user-assisted processing time for a two-megapixel image containing over 2000 axons was less than 1h. This speed was almost eight times faster than the time required to manually process the same image. Our method has proven to be well suited for identifying axons and their characteristics, and represents a significant time savings over traditional manual methods.

Variation of the neurofilament medium KSP repeat sub-domain across mammalian species: implications for altering axonal structure

The evolution of larger mammals resulted in a corresponding increase in peripheral nerve length. To ensure optimal nervous system functionality and survival, nerve conduction velocities were likely to have increased to maintain the rate of signal propagation. Increases of conduction velocities may have required alterations in one of the two predominant properties that affect the speed of neuronal transmission: myelination or axonal diameter. A plausible mechanism to explain faster conduction velocities was a concomitant increase in axonal diameter with evolving axonal length. The carboxy terminal tail domain of the neurofilament medium subunit is a determinant of axonal diameter in large caliber myelinated axons. Sequence analysis of mammalian orthologs indicates that the neurofilament medium carboxy terminal tail contains a variable lysine-serine-proline (KSP) repeat sub-domain flanked by two highly conserved sub-domains. The number of KSP repeats within this region of neurofilament medium varies among species. Interestingly, the number of repeats does not change within a species, suggesting that selective pressure conserved the number of repeats within a species. Mapping KSP repeat numbers onto consensus phylogenetic trees reveals independent KSP expansion events across several mammalian clades. Linear regression analyses identified three subsets of mammals, one of which shows a positive correlation in the number of repeats with head-body length. For this subset of mammals, we hypothesize that variations in the number of KSP repeats within neurofilament medium carboxy terminal tail may have contributed to an increase in axonal caliber, increasing nerve conduction velocity as larger mammals evolved.

Inhibition of inducible nitric oxide synthase reduces an acute peripheral motor neuropathy produced by dermal burn injury in mice

The systemic inflammatory response produced by a full-thickness dermal burn injury is associated with a peripheral motor neuropathy. We previously reported that a 20% body surface area (BSA) full-thickness dermal burn in C57BL6 mice produced structural and functional deficits in motor axons at a distance from the burn site. The etiology of the neuropathy, however, is not well characterized. Burn injury leads to an increase in production of a number of proinflammatory mediators, including nitric oxide (NO). We tested the hypothesis that dermal burn-induced motor neuropathy is mediated by increased production of NO. NO synthase (NOS) activity was inhibited following a 20% BSA full-thickness burn by injection of non-specific NOS inhibitor, nitro-L-arginine methyl ester or inducible NOS (iNOS) inhibitors, L-N6-(1-iminoethyl) lysine, and aminoguanidine. NOS inhibitors also prevented the reduction in ventral roots mean axon caliber and the decrease in a motor nerve conduction velocity (MCV) following burn. iNOS knockout mice prevented MCV decrease in the first 3 days post-burn, but iNOS knockout MCV was significantly reduced at 7-14 days post-burn. These results suggest that an increase in NO production generated by systemic inflammatory response pathways after burn injury contributes to the development of structural and functional deficits in peripheral motor axons.

Wrist and carpal tunnel size and shape measurements: effects of posture

BACKGROUND

Wrist anthropometrics and posture have been implicated in the development of carpal tunnel syndrome, yet it remains unclear how external measurements relate to carpal tunnel parameters in neutral and non-neutral postures. The purposes of this study were (i) to evaluate the effect of slice orientation on several indices of carpal tunnel size and shape and (ii) to examine the relationship between carpal tunnel and external wrist dimensions.

METHODS

Three-dimensional static models were generated to measure carpal tunnel and wrist parameters for six wrists in three wrist postures (30 degrees flexion, neutral and 30 degrees extension). A simulated imaging plane enabled measurement of four carpal tunnel dimensions and two shape indices throughout the tunnel length, using "axial" and "tunnel" slice orientations (perpendicular to forearm and tunnel, respectively).

FINDINGS

Correction for tunnel orientation eliminated posture-related changes in tunnel size and shape noted at the distal end using "axial" alignment. "Tunnel" alignment reduced average carpal tunnel area and depth by nearly 15% in extension, but generally less than 5% in neutral and 2% in flexion. Subsequently, "tunnel" alignment also decreased carpal tunnel and non-circularity ratios to reveal a flatter, more elliptical shape throughout the tunnel in extension than neutral and flexion. Wrist dimensions correlated significantly with tunnel dimensions, but not tunnel shape, while wrist shape correlated significantly with tunnel shape, area and depth.

INTERPRETATIONS

Slice alignment with the carpal tunnel may improve the consistency of findings within and between patient and control populations, and enhance the diagnostic utility of imaging in clinical settings.

Morfometría de los nervios laríngeos recurrentes de la rata

In mammals the recurrent laryngeal nerves are dissimilar in length between both sides. This asymmetry involves different time of arrival of the stimulus to the laryngeal musculature controlled by each nerve. Thus, several explanations have been addressed to elucidate the closest of the glottis at the same time despite the unlike length of the nerves. However, previous works on the topic lack of several important data. The present study compares, in two groups of 10 and 6 rats, the length and the composition of myelinated fibers in the recurrent laryngeal nerves of both sides, by means of light microscopy and a computerized morphometric analysis. The results show a mean difference of 0,84 cm longer the left than the right recurrent laryngeal nerve. No statistical differences were observed in the number of myelinated fibers between both sides. However, the myelinated fibers of the right side were statistically bigger in diameter than the fibers of the left side. The data are discussed in the context of the mechanisms for the compensation of the dissimilar length of both recurrent laryngeal nerves.

Ro5-4864, a synthetic ligand of peripheral benzodiazepine receptor, reduces aging-associated myelin degeneration in the sciatic nerve of male rats

The peripheral-type benzodiazepine receptor (PBR) is a protein predominantly located in the mitochondrial outer membrane that plays an important role in the regulation of cell survival and proliferation. Previous studies have shown an enhanced expression of PBR in the regenerating sciatic nerve, suggesting that this protein may be involved in the regenerative response. The rat sciatic nerve suffers important structural alterations with aging, including alterations in the morphology of myelin sheaths and a decrease in the number of myelinated fibers. In this study, we have assessed the effect of two PBR ligands, Ro5-4864 and PK-11195, to determine whether PBR may influence aging-associated morphological changes in the sciatic nerve. The treatment of 23-month-old, Sprague-Dawley male rats for 1 month with Ro5-4864 significantly reduced the percentage of fibers with myelin decompaction and increased the total number of myelinated fibers. In contrast, PK-11195, a PBR ligand that binds to a different site than Ro5-4864 in the PBR molecule, did not significantly affect any of the parameters analyzed. These findings support the potential role of PBR ligands to prevent aging-associated peripheral nerve degeneration.

Peripheral axon caliber and conduction velocity are decreased after burn injury in mice

Peripheral neuropathies are reported to arise as a result of the systemic inflammatory response produced by a full-thickness cutaneous burn injury. This study was designed to characterize the magnitude and time course of functional and morphological changes in peripheral axons that arise after a full-thickness dermal burn injury in an animal model. A 20% body surface area (20% BSA) full-thickness dermal burn was applied to the back of C57BL6 female mice. Longitudinal H- and M-wave recordings were used to determine the conduction velocities (CV) of large myelinated motor and sensory axons in the tibial nerve of sham control and burn-injured mice. Motor CVs were significantly reduced from 6 h to 28 days after the burn, and sensory CVs were significantly reduced from 7 to 14 days after the burn. Morphological evaluation also showed that the mean caliber of large axons in tibial nerves and L5 ventral and dorsal roots in burned mice was significantly decreased. The results demonstrate that both functional and morphological deficits may be produced in peripheral nerve axons at sites well removed from a full-thickness dermal burn injury. The neural deficits may contribute to changes in neuromuscular transmission and the development of limb and respiratory muscle weakness that also accompany burn injury.

Auditory nerve fiber differences in the normal and neurofilament deficient Japanese quail

A primary axonal disease affecting the central and peripheral nervous system was discovered in a mutant strain of the Japanese quail, named quiver (Quv). We have previously demonstrated altered auditory evoked potentials in the neurofilament (NF) deficient quail. In this current study we attempt to find relationships between the auditory evoked potential results and the histo-pathological abnormalities of the auditory neurons. No abnormalities in the external auditory meatus and tympanic cavity were observed in either Quv or control quails and the ganglion cell bodies and their nuclei appeared normal by light microscopy. The myelin staining pattern was found to be similar in both strains with hematoxylin and eosin and Klüver-Barrera staining. The frequency histograms of fiber and axonal diameters of myelinated fibers showed an unimodal pattern in both strains. In Quv quails myelinated fibers and their axoplasm were smaller in diameter than in controls resulting in smaller neural tissue mass. In electron microscopic observation the axons of the Quv quail were composed of mitochondria and microtubules and smooth endoplasmic reticuli. In Quv quail electron micrographs of cochlear nerve myelinated fibers NFs were not seen in the axons and the neuronal cell bodies. Our current findings indicate that the previously reported reduction of conduction velocity of auditory evoked potentials may be due to smaller fiber and/or axonal diameter. The g-ratio, myelin thickness and fiber circularity were found to be the same for both strains. In conclusion, loss of axonal cytoskeletal elements (NFs) correlates well with our electrophysiological findings. Reduced conduction velocity and severely distorted auditory evoked potentials in NF deficient quails seem to be primarily due to axonal hypotrophy.

Morphometric and ultrastructural changes with ageing in mouse peripheral nerve

Qualitative and quantitative information is reported on the morphological changes that occur in nerve fibres and nonneuronal cells of peripheral nerve during the lifetime of the mouse. Tibial nerves of mice aged 6-33 mo were studied. With ageing, collagen accumulates in the perineurium and lipid droplets in the perineurial cells. Macrophages and mast cells increase in number, and onion bulbs and collagen pockets are frequently present. Schwann cells associated with myelinated fibres (MF) slightly decrease in number in parallel with an increase of the internodal length from 6 to 12 mo, but increase in older nerves when demyelination and remyelination are common. The unmyelinated axon to myelinated fibre (UA/MF) ratio was about 2 until 12 mo, decreasing to 1.6 by 27 mo. In older mice, the loss of nerve fibres involves UA (50% loss of 27-33 mo cf. 6 mo) more markedly than MF (35%). In aged nerves wide incisures and infolded or outfolded myelin loops are frequent, resulting in an increased irregularity in the morphology of fibres along the internodes. In the mouse there is an adult time period, 12-20 mo, during which several features of degeneration progressively appear, and an ageing period from 20 mo upwards when the nerve suffers a general disorganisation and marked fibre loss.

Proopiomelanocortin gene expression is decreased in the infundibular nucleus of postmenopausal women

Previous studies have shown that estrogen withdrawal decreases the secretion of beta-endorphin from the monkey hypothalamus. In addition, there are consistent age-associated changes in beta-endorphin neurons in the rodent. Based on these findings, we hypothesized that the activity of hypothalamic beta-endorphin neurons would be decreased in the hypothalamus of postmenopausal women. In the present study, we examined the expression of proopiomelanocortin (POMC) mRNA, the precursor mRNA for beta-endorphin, in the medial basal hypothalamus of premenopausal and postmenopausal women. Every 20th sagittal section through the hypothalamus was hybridized with a synthetic [35S]labeled, 48-base oligonucleotide probe complementary to POMC mRNA. Labeled neurons were counted and their somatic profile areas were measured with an image-combining computer microscope system. The number of POMC mRNA-containing neurons/section in the infundibular nucleus was reduced by 65% in postmenopausal women. In contrast, there was no significant difference in the number of neurons expressing POMC gene transcripts in the retrochiasmatic region. The POMC neurons in the retrochiasmatic area were also distinct morphologically from those in the infundibular nucleus. The differences between the infundibular and retrochiasmatic regions suggest that functional subgroups of POMC neurons exist in the human hypothalamus. Our findings provide evidence that the activity of hypothalamic POMC neurons is decreased in the infundibular nucleus of postmenopausal women. Both aging and gonadal steroid withdrawal may contribute to the decline in POMC gene expression in postmenopausal women.

Complexity and scaling properties of amacrine, ganglion, horizontal, and bipolar cells in the turtle retina

In the present study we have evaluated the complexity and scaling properties of the morphology of retinal neurons using fractal dimension as a quantitative parameter. We examined a large number of cells from Pseudemys scripta and Mauremys caspica turtles that had been labeled using Golgi-impregnation techniques, intracellular injection of Lucifer Yellow followed by photooxidation, intracellular injection of rhodamine conjugated horseradish peroxidase, or intracellular injection of Lucifer Yellow or horseradish peroxidase alone. The fractal dimensions of two-dimensional projections of the cells were calculated using a box counting method. Discriminant analysis revealed fractal dimension to be a significant classification parameter among several other parameters typically used for placing turtle retinal neurons in different cell classes. The fractal dimension of amacrine cells was significantly correlated with dendritic field diameters, while the fractal dimensions of ganglion cells did not vary with dendritic field span. There were no significant differences between the same cell types in two different turtle species, or between the same types of neurons in the same species after labeling with different techniques. The application of fractal dimension, as a quantitative measure of complexity and scaling properties and as a classification criterion of neuronal types, appears to be useful and may have wide applicability to other parts of the central nervous system.

Morphometric analysis of axons myelinated during adult life in the mouse superior cervical ganglion

In experimental studies addressing the regulation of myelin formation and maintenance by Schwann cells, the sympathetic nervous system of young adult rodents has served a key role as an essentially nonmyelinated yet modifiable control tissue. Nevertheless there is clear evidence of substantial myelination in the superior cervical ganglion (SCG) of normal mice and rats of more advanced age. Against this background, interpretation of experimental outcomes in particular sympathetic tissues will require detailed quantitative control data taking account of animal age. To provide a baseline for future investigations on myelin remodelling, an ultrastructural morphometric study of myelinated fibres in the SCG was undertaken in 4 strains (QS, Balb/C, C57 and CBA) of adult male mice aged 32-72 wk. Numbers of myelinated fibres in SCG cross-sections varied substantially between individual animals, and the mean numbers for QS (132), Balb/C (165) and CBA (254) were significantly higher than that for C57 (32). Both axonal and fibre diameter were distributed unimodally (means for the 4 strains ranged from 2.3-2.4 microns and 3.2-3.6 microns respectively). Myelin spiral length was distributed unimodally and skewed to the right (range of means = 227-357 microns) and was significantly greater in QS mice as compared with the other 3 strains. While the mean g ratio (axonal diameter/fibre diameter) was significantly lower in QS mice than in the other 3 strains, the range for mean g ratio was 0.64-0.73, indicating that myelination had proceeded appropriately even though late in onset in this tissue. The index of circularity was high in all strains, both for axons (range of means = 0.80-0.88) and fibres (range of means = 0.84-0.89). The small axonal and fibre diameter and unimodal distribution are consistent with the characteristics of autonomic myelinated fibres and it is probable that most are postganglionic sympathetic fibres arising within the SCG. In terms of providing a sufficient population of myelinated fibres for future experimental studies, the QS, Balb/C and CBA strains would be preferable to C57 mice.

Nimodipine effects on cerebral microvessels and sciatic nerve in aging rats

At the ultrastructural level different anomalies of the cerebral microvasculature were encountered in the brains of aged rats. These aberrations can either be attributed to degeneration processes or to the perivascular deposition of, e.g., collagen fibrils and other, unidentified, proteinous debris. We previously reported that chronic treatment with the calcium antagonist nimodipine from 24-30 months especially reduced the incidence of aging-related microvascular deposits in the frontoparietal motor cortex of rats. The same drug treatment did not interfere with the degeneration of pericytes. The reduction of the microvascular depositions was, however, not consistent throughout different cortical layers. We now demonstrate that an earlier onset (16-30 months) of the drug application yields a prominent and consistent reduction of microvascular deposits for all cortical layers studied. The earlier onset of the drug treatment again did not influence the quantity of pericyte degeneration. The effect of long-term nimodipine treatment (16-30 months) was also examined in the sciatic nerve. Compared to young animals the sciatic nerve of aged control rats (30 months) showed a variety of alterations of myelinated fiber (MF) morphometry. Nimodipine treatment from 16-30 months did not significantly change these morphometric aging-related changes. Approximately 6% of the MF in aged rats display morphological myelin irregularities. After nimodipine application the frequency of these alterations was reduced, which was, however, only significant for partial demyelination known as myelin ballooning. These results indicate a consistent influence of nimodipine on cerebral microvessels, while there is only a moderate effect on the morphology of sciatic myelinated fibers during the aging process.

Cytoskeletal components and calibers in developing fish Mauthner axon (Salmo gairdneri Rich.)

In developing axons of many vertebrates, microtubular density is inversely correlated with fiber caliber. It is suggested that microtubules are causally related to axonal caliber. For this reason, cytoskeletal analysis during development of the fish Mauthner axon, which displays a giant caliber, is of particular interest. The Mauthner axon originates from the Mauthner cell in the medulla and runs in the fasciculus longitudinalis medialis in the spinal cord. At embryonic, larval and postlarval stages in trout (Salmo gairdneri Rich.), the following parameters were measured on conventional electron micrographs of Mauthner axon cross sections; axonal caliber, number of microtubules per axons, and microtubular and neurofilament densities. Results at each stage point to an inverse correlation between axonal caliber (x) and microtubular density (y) expressed by the equation y = axb (R = 0.932). Furthermore, three periods of Mauthner axon development are identified on the basis of the cytoskeletal content: (1) embryonic; the Mauthner axon has small caliber with a high microtubule density, (2) elongation period (larval stages); the axon enlarges and a transient peak of microtubules, corresponding to the caliber increment, is observed, and (3) postlarval; the axon enlarges still further (greater than 500 microns 2) but has the lowest microtubular content. During this period neurofilaments are the main axonal component.

Hereditary hypotrophic axonopathy with neurofilament deficiency in a mutant strain of the Japanese quail

A primary axonal disease affecting the central and peripheral nervous system was discovered in a new mutant strain of the Japanese quail, named quiver (Quv). Quv showed significantly smaller cross-sectional areas of the cervical spinal cord and the optic and sciatic nerves, when compared with controls by light microscopic morphometry. In the cervical spinal cord, electron microscopic morphometry indicated that myelinated axons in Quv were significantly smaller in size than in controls, though greater in density. The axonal circularity was not significantly different from that of controls. Electron microscopically and immunohistochemically, neurofilaments were not detected in the axons or neuronal cell bodies. Axons in Quv were composed mainly of microtubules, which were increased in number in relation to the axonal size. From these findings, the lesions observed in Quv were regarded as axonal hypotrophy (growth arrest or retardation) due to altered neurofilament expression.

Changes in nerve fiber numbers distal to a nerve repair in the rat sciatic nerve model

Changes in nerve fiber numbers distal to a nerve repair in the sciatic nerve of 48 rats were evaluated over a 1- to 24-month period. The results of the morphometric evaluation in the sciatic nerve distal to the nerve repair demonstrated an increase in nerve fiber counts as early as 1 month following the nerve repair. The number of nerve fibers in the distal nerve was greatest at 3 months and did not return to normal levels until 24 months. The results of this study will influence the timing of experimental studies in which nerve fiber counts are critical for evaluation, and provides a better understanding of the clinical events occurring following nerve repair.

A useful programme in BASIC for axonal morphometry with introduction of new cytoskeletal parameters

Interest in the structure of axons and quantification of their components has been growing over the last years. However, the existing literature contains few reports of available computer programmes to facilitate such studies. This paper presents a fully comprehensive BASIC programme for the morphometric analysis of electron micrographs of cross-sectional nerve fibres. From drawings of fibre and axonal contours and dots of the microtubules and neurofilaments, the programme calculates the following parameters: area, diameter and form factor of the fibres and axons, number and density of microtubules and neurofilaments, proportion between microtubules and neurofilaments (R-proportion), myelin thickness and the diameter of the axon relative to its sheath (g-ratio). The programme also introduces three new parameters to analyse the degree of uniformity of microtubule and neurofilament distribution: distances between microtubules and between neurofilaments, equilateral index and cytoskeletal intermingling index. The programme is written in Microsoft BASIC Interpreter for Apple Macintosh (Microsoft Corporation) but can be used on other computers. Although the programme has been tested on adult rat optic nerve fibres, it can be used for different projects concerning axonal morphometry.

Axonal atrophy in the painful peripheral neuropathy in AIDS

We compared the sural nerves from patients with AIDS; four with the painful peripheral neuropathy (PPN), three patients with non-painful distal symmetrical peripheral neuropathy (DSPN), one without clinical peripheral nerve involvement and two human immunodeficiency virus negative controls. Fibre diameter distributions showed a slight reduction in larger fibres in all patients with AIDS compared with controls. No significant difference was found in the relative frequency distribution of the lamellae counts between any of the groups. The relative frequency distribution of axonal area of myelinated fibres did not differ between controls, non-painful DSPN or the patient without peripheral nerve involvement; however, all patients with PPN showed marked reduction in the number of axons of myelinated fibres of larger area which was significant when compared to each patient from the other groups (P less than 0.0001). This indicates that the reduction in larger fibres in PPN is mostly due to axonal atrophy rather than selective fibre loss. Axonal atrophy is associated with painful peripheral neuropathy in AIDS but not with those without pain. The possible role of axonal atrophy as a pathological substrate for pain is discussed.

Conduction studies in peripheral cat nerve using implanted electrodes: III. The effects of prolonged constriction on the distal nerve segment

Electrophysiological properties were monitored in detail in chronically constricted peripheral nerves by implanted, multicontact nerve cuff electrodes and correlated with morphometric histology in selected cases. The physiological and histological responses in nerve to a range of constricting cuffs of standard sizes were readily graded. The initial response to any significant constriction was a transient, focal conduction slowing or block at the constriction, followed by more protracted distal effects; the latter ranged from loss of excitability consistent with "dying-back" degeneration to reductions in conduction velocity consistent with histologically observed atrophy. Smaller myelinated fibers tended to have similar but less pronounced changes than larger diameter fibers. Recordings from ventral and dorsal roots showed that distal degeneration was more pronounced in motor than in sensory fibers of similar caliber. Electronmicroscopical measurements showed that basal laminas were relatively preserved around even the most atrophic and demyelinated axons. Perimeter measurements of the basal lamina could be used to estimate the diameter of the original nerve fiber.

Adult pigment type (Peiffer) of sudanophilic leukodystrophy. Pathological and morphometrical studies on two autopsy cases of siblings

Two autopsy cases of siblings with the adult pigment (Peiffer) type of sudanophilic leukodystrophy (SLD), which demonstrated the full-blown stage (case 1) and early stage (case 2) of demyelination, were examined. Numerous brown pigments deposited in demyelinated cerebral areas were characterized histochemically and ultrastructurally as lipofuscin and ceroid. Under the electron microscope formation of blebs due to myelin splitting associated with deposition of multilamellar myeloid bodies within them was a prominent feature in the demyelinated cerebral areas of case 2 as compared with case 1. However, various features of myelin degradation such as thinning, partial or complete circumferential myelin loss, and deposition of electron-dense material on the interperiodic lines were found in both cases. Blebs occurred in all layers of myelin, and axons were compressed by these blebs or the hydropically swollen inner lips of oligodendroglias. Oligodendroglias were relatively well preserved in the demyelinated and nondemyelinated areas in case 2, although the cytoplasm was hydropic. Many spheroids were present in demyelinated areas and were irregularly distributed in both cases. The peripheral nerves in case 1 presented essentially the same changes as those in the brain, although those in case 2 were not affected. Morphometrically, the results showed that hypomyelination was not the mechanism for this pigment type of SLD. One possible cause may be an accelerated ageing of the metabolic process of myelin turnover.

Conduction studies in peripheral cat nerve using implanted electrodes: I. Methods and findings in controls

Silicone rubber cuff and patch electrodes with multiple contacts were implanted along the sciatic-tibial-plantar nerves in cat for repeated studies of conduction properties of normal peripheral nerve over periods of time. The contacts were used in various combinations for precise localization of changes in conduction velocities and excitability along the extent of normal nerves. In this paper the particular characteristics and limitations associated with cuff-electrode recordings of neural activity are discussed. The nerve action potential was recorded using a tripolar configuration with a central lead flanked by two shunted leads at symmetrical distances. This configuration records the spatial derivative of the action potential and rejects potentials from sources outside the cuff. The voltage changes are restricted by the silicone cuff, and the dynamic range is therefore very high, allowing detection from single myelinated fibers to whole nerve responses. The electrodes are well suited for following the development of regeneration and degeneration following experimental lesions.

Neurofilaments are spaced randomly in the radial dimension of axons

The organization of the cytoskeleton is compared in the large myelinated parasympathetic and somatic motor axons of the avian oculomotor system. Electron microscopic studies demonstrate that neurofilaments are the chief structural elements in these axons, and quantitative analyses of the distribution of neurofilaments in axonal cross-sections found that the average neurofilament packing density is 25% greater in the parasympathetic axons than in the somatic motor axons. In both types of axon the distributions of neurofilaments matched a randomly generated (Poisson) distribution. In axoplasm, a Poisson distribution could arise if the neurofilaments were distributed in the cross-sectional plane by stochastic forces operating randomly and without significant neurofilament-neurofilament interactions. Thus, in these axons, the neurofilaments behave as if they are inert 'molecules' in a dilute solution-subject to non-specific stochastic forces that tend to distribute them at random. We propose that neurofilaments normally are relatively free to move apart from each other and to fill the available space within the axon.

Characteristic variations of relative myelin sheath thickness in 11 nerves of the rat

Computer-assisted measurements of relative myelin sheath thickness (the g ratio) were made in 11 peripheral nerves of the rat. The scatter diagrams showed nerve-specific variations in the distribution of relative myelin sheath thickness. Myelinated fibers of less than 3.5 microns axon diameter had relatively thin myelin sheaths, particularly in the splanchnic, vagus and glossopharyngeal nerves. The oculomotor nerve had two fiber populations clearly set apart in terms of relative myelin sheath thickness. Thickly myelinated fibers were found in facial and hypoglossal nerves. No single functional modality was evident for the thinly myelinated fibers.

Chronic human nerve compression--a histological assessment

While compression neuropathy is a common clinical problem, the opportunity to study human nerve material is rare. A histological assessment of the superficial radial nerve of four human cases with entrapment syndrome is reported. Changes in the perineurium and the endoneurial microvessels as well as the presence of Renaut bodies were the earliest histological abnormalities noted. Connective tissue changes included epineurial and perineurial fibrosis. Nerve fibre pathology varied from fascicle to fascicle. The myelinated and unmyelinated fibre populations responded differently to this compression. In the myelinated fibre population, marked thinning of the myelin was noted. In the unmyelinated fibre population, a shift in the fibre histogram due to a new population of very small fibres was observed suggesting degeneration with subsequent regeneration of this fibre population.

The unmyelinated fibre spectrum of the main trunk and side branches of the intestinal nerve in the chicken (Gallus gallus var. domesticus)

An electron microscopy study was undertaken to determine the numbers and diameters of unmyelinated axons in the main trunk and side branches of the intestinal nerve at the level of the small intestine. Three techniques were used to determine axon diameter. These were: length of the least chord (D), diameter determined from cross sectional area (DA) and diameter determined from perimeter (DP). A measure of the degree of circularity of the axons was also made. On average, the nerve trunk contained 4729 unmyelinated axons and these outnumbered myelinated axons by 78:1. The mean number of unmyelinated axons in the side branches was 463. Myelinated axons were not seen in the side branches. When a test for circularity was applied to the data it was found that larger axons tended to be less circular than smaller ones and, because of these differences in the degree of circularity, DA and more particularly D were subject to a variable inaccuracy. The mean values for DA were 0.70 micron for the nerve trunk and 0.57 micron for the side branch; the mean values for DP were 0.78 and 0.66 microns respectively. The peak conduction velocity at 40 degrees C of unmyelinated axons in the nerve trunk was determined from the single (C-fibre) deflection in the compound action potential. The following relationship was found, peak conduction velocity CV = DA0.59 = DP0.85.

Morphometric analysis of circadian variations in the retinal photoreceptor synaptic terminals of the adult and fetal guinea pig

In order to test whether the alterations in photoreceptor synaptic terminal size and shape reported in lower vertebrates occur in a mammalian visual system, adult and fetal guinea pig retinas were exposed to an LD 12:12 lighting cycle, as well as to long-term light (LL) and long-term dark (DD) regimes. Representative random samples from all retinal quadrants, obtained at various times during these lighting regimes, were processed for electron microscopy. The synaptic terminals of all three photoreceptor cell types in this retina (alpha and paranuclear rods, and cones) were analyzed with computer-assisted morphometrics for changes in their area, perimeter, synaptic vesicle density, and the degree of plasmalemmal infolding. The data showed all three types of adult receptor terminals to have increased area and vesicle density, as well as decreased membrane infolding, during the light period, while both types of rods showed increased perimeter measurements in the dark. Results from adults maintained under extended lighting conditions (LL and DD) showed no difference when compared with sample times during a typical LD 12:12 lighting regimen where clear statistical differences existed. Data from fetal retinas showed no significant sustainable pattern in any of the measured variables. These quantitative findings have led to the conclusion that while alterations in perimeter measurements may be explained by using the vesicle recycling hypothesis, observed changes in terminal size and shape may be controlled by a light-initiated or light-enhanced mechanism and effected through an annular configuration of cross-striated fibrils found within these photoreceptor synaptic terminals.

Pyramidal tract of the cat: axon size and morphology

The purpose of this work was to determine the number and morphology of pyramidal tract (PT) axons in the cat, using electron microscopy, modern methods of fixation, and computer-assisted morphometric analysis. Sections taken at the level of the medullary pyramids in three animals were fixed and magnified up to 10,000 X to produce photomicrographs. Morphological data were entered into computer files for analysis by tracing axon perimeters on micrographs mounted on a digitizer tablet. The number of axons per PT averaged 415,000, of which 88% were myelinated and 12% were unmyelinated. 90% of the myelinated axons fell in the diameter range 0.5-4.5 microns. Axons larger than 9 microns diameter accounted for 1% of the total; the largest were 20-23 microns. Myelinated axon mean diameter was 1.98 microns; because of the skewed distribution, with many small axons and a few very large axons, median diameter was 1.60 micron. Size distribution was relatively uniform throughout the PT cross section, with all sizes represented in all regions. However, the more medial regions had a higher proportion of small fibers than the more lateral regions: mean medial diameter was 1.85 micron while mean lateral diameter was 2.09 microns. Myelin sheath thickness averaged 7.9% of fiber diameter for axons up to 11 microns, but was constant at 0.9 micron for larger fibers. Myelinated fibers were distorted from the circular shape in cross section, with a mean circularity index (or form factor) of 0.85, which implies that the fibers could swell about 15% without rupture of the cell membrane.(ABSTRACT TRUNCATED AT 250 WORDS)

Computer editing of morphometric data on nerve fibers. An improved computer program

A short guide is given for the planning of nerve fiber morphometry using computer-assisted methods. A new computer program is introduced which allows, among other factors, correction for myelin sheath shrinkage. If myelin shrinkage is not corrected for, there will be false high values for axon caliber and false low values for sheath thickness. The magnitude of the skew produced by this factor is shown on hand from computer editing.

Effects of cold adaptation and starvation on sciatic nerve fibers in the frog

The conditions under which frogs are kept prior to experimentation were found to have a measurable effect on peripheral nerve structure. Frogs kept for 12 weeks at 4 degree C had markedly shrunken sciatic nerve fibers compared with frogs kept at 19 degrees C. Intermediate fiber shrinkage was found for frogs kept at 19 degrees C without feeding. Counts of neurofilaments and microtubules showed that fiber shrinkage was from a preferential loss of filaments, indicating cold- or starvation-induced atrophy of the axon's cyto-skeleton. This effect, however, was superimposed with additional osmotic axonal shrinkage, causing filament densities to increase per area. There were no changes in myelin sheath thickness due to cold adaptation or fasting.

Combined scatter diagrams of sheath thickness and fibre calibre in human sural nerves: changes with age and neuropathy

A computer-assisted method permits collection of large numbers of measurements of fibre profiles in electron micrographs of human sural nerve biopsies. The method is based on simultaneous demonstration of fibre calibre and of sheath thickness in terms of the g-ratio (quotient axon diameter/fibre diameter), and on the recalculation of all parameters for circular fibre profiles. The fibres of small and large diameters of human sural nerves were found to form separate populations with distinctly different trends for sheath thickness and also different patterns of maturation. Preliminary data in neuropathy show that this method gives a better distinction of the extent of damage to either of the two populations; it also permits assessment of the number of regenerating or remyelinating fibres.

The conduction velocity, number, and diameter of unmyelinated fibers in Remak's nerve

Remak's nerve in the chicken was examined ultrastructurally and electrophysiologically to determine the characteristics of fibers in the nerve trunk. The ration of unmyelinated fibers to myelinated ones was 111:1. The mean number of unmyelinated fibers was 3555 plus/minus 232 (SEM, n=5) and they had a mean diameter of 0.502 plus/minus 0.034 (SEM) micron. The compound action potential consisted almost entirely of a large diphasic waveform which had a mean peak conduction velocity of 0.62 plus/minus 0.031 (SEM, n=5) m.s-1 at 37 degrees C.

Axonotmesis as treatment for neurotmesis

The effect of proximal nerve crush on regeneration after unrepaired nerve transection was studied in feline cutaneous nerves. Delays between the initial transection and the subsequent nerve crush varied between 0 (immediate crush) and 13 months. A survival time of 6 to 12 months after the crush was allowed for regeneration to complete. Data from these animals was compared with data from animals in which the nerve was simply transected. Four measures of regeneration success were used to assess the degree of functional recovery: (i) the number of fibers crossing the neuroma and regenerating down the distal stump; (ii) the conduction velocity and diameter of individual fibers in the distal stump relative to their conduction velocity and diameter in the proximal stump; (iii) the number of fibers reinnervating cutaneous mechanoreceptors; and (iv) the number of cutaneous type I mechanoreceptors present after regeneration. As a whole, the data showed a statistically significant increase in the number of fibers entering the distal stump and in the number of fibers reinnervating the skin when the nerve was crushed 6 months after transection compared with nerves that were only transected. There was also an increase in the size and conduction velocity of fibers in the distal stump of nerves that had been crushed 4 to 6 months after transection.

Control of axonal caliber by neurofilament transport

The role of neurofilaments, the intermediate filaments of nerve cells, has been conjectural. Previous morphological studies have suggested a close relationship between neurofilament content and axonal caliber. In this study, the regenerating neuron was used as a model system for testing the hypotheses that neurofilaments are intrinsic determinants of axonal caliber, and that neurofilament content is controlled by the axonal transport of neurofilaments. This system was chosen because previous studies had shown that, after axotomy, axonal caliber was reduced within the proximal stump of the regenerating nerve and, because the relative amount of neurofilament protein undergoing axonal transport in regenerating axons was selectively reduced. The relationship between axonal caliber and neurofilament number was examined in a systematic fashion in both regenerating and control motor axons in rat L5 ventral root. Reconstruction of the spatial and temporal sequences of axonal atrophy in the proximal stump after axotomy showed that reductions in axonal caliber were first detected in the most proximal region of the root and subsequently progressed in a proximal-to-distal direction at a rate of 1.7 mm/day, which is identical to the rate of neurofilament transport in these neurons. Quantitative ultrastructural studies showed that these reductions in caliber correlated with a proportional decrease in the number of axonal neurofilaments but not microtubules. These results support the hypotheses that neurofilament content is a major intrinsic determinant of axonal caliber and that neurofilament content is controlled by the axonal transport of neurofilaments. On this basis, we suggest a role for neurofilaments in the control of axonal volume.

Synaptic input from identified muscle afferents to neurones of the dorsal spinocerebellar tract in the cat

Single identified group I a and I b muscle afferent fibres were injected with horseradish peroxidase in the lumbar dorsal columns of anaesthetized cats. The morphological details of the axon collaterals and terminal boutons of these muscle afferents within Clarke's column were subsequently reconstructed. The rostro-caudal extent of synaptic terminals from a single afferent fibre within Clarke's column was found to be restricted to less than 1 mm. In the same experiments, dorsal spinocerebellar tract (d.s.c.t.) neurones were retrogradely labelled by injection of horseradish peroxidase into the cerebellum. Synaptic contacts between labelled group Ia and Ib afferent fibres and the soma and proximal dendrites of d.s.c.t. neurones were found. The synaptic contacts from both Ia and Ib fibres varied greatly in size, from 1 X 1 micron up to 'giant' synapses of 20 X 3 micron. Excitatory post-synaptic potentials (e.p.s.p.s) were evoked in d.s.c.t. neurones by impulses in single group I muscle afferent fibres. The fluctuations in peak amplitude of each e.p.s.p. were determined from e.p.s.p. and noise recordings, using a numerical deconvolution procedure. In general, these single-fibre e.p.s.p.s fluctuated between discrete amplitudes separated by an incremental amplitude which was approximately constant. This incremental amplitude did not depend on the average peak amplitude of the particular e.p.s.p. examined. Our anatomical observations of 'giant' boutons arising from Ia and Ib afferent fibres contacting d.s.c.t. neurones raises the possibility of multiple transmitter release sites within an individual synaptic bouton. It is proposed that synaptic transmission between group I muscle afferents and d.s.c.t. neurones occurs with discrete all-or-nothing e.p.s.p.s associated with transmitter release sites.

Endurance exercise does not modify nerve fibre morphology in the rat soleus nerve

The effects of the myelination and growth of axons in the nerve to the soleus muscle (NSM) was investigated in young male Wistar rats. Experimental animals were run on a treadmill for 13 weeks (70 min/day, 6 days/week), while sedentary weight-matched animals of the same age and sex served as controls. The activity of the enzymes, phosphofructokinase (PFK) and succinate dehydrogenase (SDH) were measured in the soleus muscle to assess the effects of training on the anaerobic glycolytic and oxidative capacities. Axon and myelin sheath cross-sectional areas were measured from electron micrograph montages of the whole NSM with the aid of a digitizing tablet. The exercise programme produced large adaptive increases in the capacity of the soleus muscle for both oxidative and anaerobic glycolytic metabolism. The specific activity of PFK and SDH increased in the soleus muscle of exercised animals by 42.8% and 68.2%, respectively. In the NSM, however, there were no differences between control and exercised animals concerning the total number of myelinated nerve fibers, the size of axons and myelin sheaths of nerve fibres and the degree of myelination of axons (myelin area divided by axon area). Comparison with other studies suggests that the intensity of exercise may be the critical parameter responsible for discrepancies in the literature. Reports suggesting that exercise increases the size of nerve fibres should not be generalized to all exercise programmes.