The distribution of sodium and potassium channels in single demyelinated axons of the frog

Subtle paranodal injury slows impulse conduction in a mathematical model of myelinated axons

This study explores in detail the functional consequences of subtle retraction and detachment of myelin around the nodes of Ranvier following mild-to-moderate crush or stretch mediated injury. An equivalent electrical circuit model for a series of equally spaced nodes of Ranvier was created incorporating extracellular and axonal resistances, paranodal resistances, nodal capacitances, time varying sodium and potassium currents, and realistic resting and threshold membrane potentials in a myelinated axon segment of 21 successive nodes. Differential equations describing membrane potentials at each nodal region were solved numerically. Subtle injury was simulated by increasing the width of exposed nodal membrane in nodes 8 through 20 of the model. Such injury diminishes action potential amplitude and slows conduction velocity from 19.1 m/sec in the normal region to 7.8 m/sec in the crushed region. Detachment of paranodal myelin, exposing juxtaparanodal potassium channels, decreases conduction velocity further to 6.6 m/sec, an effect that is partially reversible with potassium ion channel blockade. Conduction velocity decreases as node width increases or as paranodal resistance falls. The calculated changes in conduction velocity with subtle paranodal injury agree with experimental observations. Nodes of Ranvier are highly effective but somewhat fragile devices for increasing nerve conduction velocity and decreasing reaction time in vertebrate animals. Their fundamental design limitation is that even small mechanical retractions of myelin from very narrow nodes or slight loosening of paranodal myelin, which are difficult to notice at the light microscopic level of observation, can cause large changes in myelinated nerve conduction velocity.

Dendrite-derived supernumerary axons on adult axotomized motor neurons possess proteins that are essential for the initiation and propagation of action potentials and synaptic vesicle release

Axotomy can trigger profound alterations in the neuronal polarity of adult neurons in vivo. This can manifest itself in the development of new axon-like processes emanating from the tips of distal dendrites. Previously, these processes have been defined as axonal based on their axonal morphology. This study extends this definition to determine whether, more importantly, these processes possess the prerequisite molecular machinery to function as axons. Using a combination of intracellular labeling and immunohistochemistry, we demonstrate that the distribution of voltage-gated sodium channels on these processes matches the arrangement of these channels that is necessary for the initiation and conduction of action potentials. At terminal bouton-like structures they possess key proteins necessary for the release of synaptic vesicles (SV2 and synaptophysin). Thus, axon-like processes emanating from the tips of distal dendrites represent a rearrangement of neuronal polarity whereby axotomized neurons can develop additional functional axons in vivo.

Na+ imaging reveals little difference in action potential-evoked Na+ influx between axon and soma

In cortical pyramidal neurons, the axon initial segment (AIS) is pivotal in synaptic integration. It has been asserted that this is because there is a high density of Na(+) channels in the AIS. However, we found that action potential-associated Na(+) flux, as measured by high-speed fluorescence Na(+) imaging, was about threefold larger in the rat AIS than in the soma. Spike-evoked Na(+) flux in the AIS and the first node of Ranvier was similar and was eightfold lower in basal dendrites. At near-threshold voltages, persistent Na(+) conductance was almost entirely axonal. On a time scale of seconds, passive diffusion, and not pumping, was responsible for maintaining transmembrane Na(+) gradients in thin axons during high-frequency action potential firing. In computer simulations, these data were consistent with the known features of action potential generation in these neurons.

CONDUCTION VELOCITY DISTRIBUTION: EARLY DIAGNOSTIC TOOL FOR PERIPHERAL NEUROPATHIES

In order to get early information on the functional state of smaller myelinated fibers this article investigated the applicability of conduction velocity distribution on compound action potential recorded in experimentally demyelinated frog sciatic nerve. Conduction velocity distribution histograms were estimated by using the mathematical model the authors enhanced. The results suggest that by using appropriate conduction velocity distribution model the diagnosis time in demyelinating neuropathy may be shortened at least three times as compared with conventional conduction velocity assessment. Therefore, it may be concluded that a well-defined model designed for the estimation of the conduction velocity distribution may be used as a diagnostic tool for the early phase of peripheral demyelinating neuropathies.

Na+ channels get anchored...with a little help

Neurons have high densities of voltage-gated Na⁺ channels that are restricted to axon initial segments and nodes of Ranvier, where they are responsible for initiating and propagating action potentials. New findings (Bréchet, A., M.-P. Fache, A. Brachet, G. Ferracci, A. Baude, M. Irondelle, S. Pereira, C. Leterrier, and B. Dargent. 2008. J. Cell Biol. 183:1101–1114) reveal that phosphorylation of several key serine residues by the protein kinase CK2 regulates Na⁺ channel interactions with ankyrin G. The presence of CK2 at the axon initial segment and nodes of Ranvier provides a mechanism to regulate the specific accumulation and retention of Na⁺ channels within these important domains.

Simulation analysis of intermodal sodium channel function

Although most sodium ion channels clustered in nodes of Ranvier provide the physiological basis for saltatory conduction, sodium ion channels cannot be excluded from internodal regions completely. The density of internodal sodium ion channels is of the order of 10/microm2. The function of internodal sodium ion channels has been neglected for a long time; however, experimental and theoretical results show that internodal sodium ion channels play an important role in action potential propagation. In this paper, based on the compartment model, we investigate the function of internodal sodium ion channels. We find that internodal sodium ion channels can promote action potential propagation, enlarge the maximal internodal distance guaranteeing stable action potential propagation, and increase the propagation speed of action potentials. In this paper, we find an optimal conductance of internodal sodium ion channels (4-5 mS/cm2), which accords with the active internodal sodium ion conductance in a real myelinated axon. With the optimal conductance, the average sodium ion channel conductance of the axon is minimal, and the metabolic energy consumption due to ion channels is also minimal.

Immunological aspects of axon injury in multiple sclerosis

The role of immune-mediated axonal injury in the induction of nonremitting functional deficits associated with multiple sclerosis is an area of active research that promises to substantially alter our understanding of the pathogenesis of this disease and modify or change our therapeutic focus. This review summarizes the current state of research regarding changes in axonal function during demyelination, provides evidence of axonal dysmorphia and degeneration associated with demyelination, and identifies the cellular and molecular effectors of immune-mediated axonal injury. Finally, a unifying hypothesis that links neuronal stress associated with demyelination-induced axonal dysfunction to immune recognition and immunopathology is provided in an effort to shape future experimentation.

Internodal function in normal and regenerated mammalian axons

AIM

Following Wallerian degeneration, peripheral myelinated axons have the ability to regenerate and, given a proper pathway, establish functional connections with targets. In spite of this capacity, the clinical outcome of nerve regeneration remains unsatisfactory. Early studies have found that regenerated internodes remain persistently short though this abnormality did not seem to influence recovery in conduction. It remains unclear to which extent abnormalities in axonal function itself may contribute to the poor outcome of nerve regeneration.

METHODS

We review experimental evidence indicating that internodes play an active role in axonal function.

RESULTS

By investigating internodal contribution to axonal excitability we have found evidence that axonal function may be permanently compromised in regenerated nerves. Furthermore, we illustrate that internodal function is also abnormal in regenerated human nerves.

CONCLUSION

The data suggest that persistently shorter regenerated internodes lead to increased Na+/K+-pump activity in response to increased Na+ entry during conduction. This may impair axonal function during prolonged repetitive activity and drain the energy reserves of the axons.

Assesment criteria for experimental demyelination induced in frog peripheral nerve

In ideal conditions the area under compound action potential may be used as an index for the number of activated fibers in a nerve trunk whereas peak amplitude, maximum time derivative, and duration may be used as an indicator for the rate of contribution to compound action potential and the degree of velocity dispersion. In this study, the time domain effect of demyelination on compound action potential has been investigated in experimentally demyelinated frog sciatic nerve. The results were analyzed in order to suggest criteria for demyelination. The results suggest that the changes in peak amplitude and maximum time derivative of compound action potential that is made up by the contribution of the active fibers may be more useful in the assessment of early phase of demyelination. Therefore, it may be concluded that these two parameters, intrinsically, carry augmented information on the velocity dispersion originated from larger-diameter fibers.

Kv1 Channels in Signal Conduction of Myelinated Nerve Fibers

The myelinated axon can be divided into three domains: the internodal axon, the paranodal axon and the nodal axon. The internodal axolemma contains high concentrations of K+ channels that are enriched in the juxtaparanodal region, whereas Na+ channels cluster in the node. This molecular organization of the myelinated axon membrane is critically important for the rapid and successful transmission of electrical impulses. The juxtaparanodal K+ channels are believed to be electrically inactive in adult peripheral nerves, but experiments with blocking drugs and genetic deletion have shown that they may serve important functions at earlier developmental stages, and during remyelination and regeneration.

Axonal oscillations in developing mammalian nerve axons

We study neuronal spike propagation in a developing myelinated axon in various stages of its development through detailed computational modeling. Recently, a form of bursting (axonal bursting), has been reported in axons in developing nerves in the absence of potassium channels. We present a computational study using a detailed model for a myelinated nerve in development to explore under what circumstances such an effect can be expected. It is shown that axonal oscillation may be caused by backfiring between the nodes of Ranvier or through backfiring from internodal sodium channels or by reducing the thickness of the myelin wrapping the axon between the nodes of Ranvier.

Does Collagenase Affect the Electrophysiological Parameters of Nerve Trunk?

Collegenase is widely used in the process of teasing a nerve in order to perform single fiber action potential (SFAP) recordings. In this study, the effects of collagenase on nerve conduction parameters were investigated. To accomplish this, normal compound action potentials (nCAPs) were recorded from isolated frog sciatic nerve at various distances using the suction technique. Then, the same nerve was treated with collagenased Ringer's solution (3.5 mg/ml, Sigma Type XI) for 90 minutes and action potentials (cCAPs) were recorded again. Numerical analysis of these records was performed and the results were compared. Using the nCAP and cCAP recordings, the conduction velocity distributions (CVD) of the individual nerve trunks were determined by a method that we have previously described. Statistical results indicated significant differences (p < 0.05) between the nCAP and cCAP CVD data. From these findings it is concluded that, when used for teasing the nerve fibers, collagenase may affect the nerve trunk conduction parameters. Specifically, a significant amount of decrease has been observed in conduction velocities of myelinated fibers having diameters smaller than 8 microns.

Molecular constituents of the node of Ranvier

The interaction between neurons and glial cells that results in myelin formation represents one of the most remarkable intercellular events in development. This is especially evident at the primary functional site within this structure, the node of Ranvier. Recent experiments have revealed a surprising level of complexity within this zone, with several components, including ion channels, sequestered with a very high degree of precision and sharply demarcated borders. We discuss the current state of knowledge of the cellular and molecular mechanisms responsible for the formation and maintenance of the node. In normal axons, Na+ channels are present at high density within the nodal gap, and voltage-dependent K+ channels are sequestered on the internodal side of the paranode--a region known as the juxtaparanode. Modifying the expression of certain surface adhesion molecules that have been recently identified, markedly alters this pattern. There is a special emphasis on contactin, a protein with multiple roles in the nervous system. In central nervous system (CNS) myelinated fibers, contactin is localized within both the nodal gap and paranodes, and appears to have unique functions in each zone. New experiments on contactin-null mutant mice help to define these mechanisms.

Molecular constituents of the node of Ranvier

The interaction between neurons and glial cells that results in myelin formation represents one of the most remarkable intercellular events in development. This is especially evident at the primary functional site within this structure, the node of Ranvier. Recent experiments have revealed a surprising level of complexity within this zone, with several components, including ion channels, sequestered with a very high degree of precision and sharply demarcated borders. We discuss the current state of knowledge of the cellular and molecular mechanisms responsible for the formation and maintenance of the node. In normal axons, Na+ channels are present at high density within the nodal gap, and voltage-dependent K+ channels are sequestered on the internodal side of the paranode--a region known as the juxtaparanode. Modifying the expression of certain surface adhesion molecules that have been recently identified, markedly alters this pattern. There is a special emphasis on contactin, a protein with multiple roles in the nervous system. In central nervous system (CNS) myelinated fibers, contactin is localized within both the nodal gap and paranodes, and appears to have unique functions in each zone. New experiments on contactin-null mutant mice help to define these mechanisms.

Functional localization of single active ion channels on the surface of a living cell

The spatial distribution of ion channels in the cell plasma membrane has an important role in governing regional specialization, providing a precise and localized control over cell function. We report here a novel technique based on scanning ion conductance microscopy that allows, for the first time, mapping of single active ion channels in intact cell plasma membranes. We have mapped the distribution of ATP-regulated K+ channels (KATP channels) in cardiac myocytes. The channels are organized in small groups and anchored in the Z-grooves of the sarcolemma. The distinct pattern of distribution of these channels may have important functional implications.

Dynamic potassium channel distributions during axonal development prevent aberrant firing patterns

The distribution and function of Shaker-related K+ channels were studied with immunofluorescence and electrophysiology in sciatic nerves of developing rats. At nodes of Ranvier, Na+ channel clustering occurred very early (postnatal days 1-3). Although K+ channels were not yet segregated at most of these sites, they were directly involved in action potential generation, reducing duration, and the refractory period. At approximately 1 week, K+ channel clusters were first seen but were within the nodal gap and in paranodes, and only later (weeks 2-4) were they shifted to juxtaparanodal regions. K+ channel function was most dramatic during this transition period, with block producing repetitive firing in response to single stimuli. As K+ channels were increasingly sequestered in juxtaparanodes, conduction became progressively insensitive to K+ channel block. Over the first 3 weeks, K+ channel clustering was often asymmetric, with channels exclusively in the distal paranode in approximately 40% of cases. A computational model suggested a mechanism for the firing patterns observed, and the results provide a role for K+ channels in the prevention of aberrant excitation as myelination proceeds during development.

Dynamic potassium channel distributions during axonal development prevent aberrant firing patterns

The distribution and function of Shaker-related K+ channels were studied with immunofluorescence and electrophysiology in sciatic nerves of developing rats. At nodes of Ranvier, Na+ channel clustering occurred very early (postnatal days 1-3). Although K+ channels were not yet segregated at most of these sites, they were directly involved in action potential generation, reducing duration, and the refractory period. At approximately 1 week, K+ channel clusters were first seen but were within the nodal gap and in paranodes, and only later (weeks 2-4) were they shifted to juxtaparanodal regions. K+ channel function was most dramatic during this transition period, with block producing repetitive firing in response to single stimuli. As K+ channels were increasingly sequestered in juxtaparanodes, conduction became progressively insensitive to K+ channel block. Over the first 3 weeks, K+ channel clustering was often asymmetric, with channels exclusively in the distal paranode in approximately 40% of cases. A computational model suggested a mechanism for the firing patterns observed, and the results provide a role for K+ channels in the prevention of aberrant excitation as myelination proceeds during development.

Potassium channel distribution, clustering, and function in remyelinating rat axons

The K+ channel alpha-subunits Kv1.1 and Kv1.2 and the cytoplasmic beta-subunit Kvbeta2 were detected by immunofluorescence microscopy and found to be colocalized at juxtaparanodes in normal adult rat sciatic nerve. After demyelination by intraneural injection of lysolecithin, and during remyelination, the subcellular distributions of Kv1.1, Kv1.2, and Kvbeta2 were reorganized. At 6 d postinjection (dpi), axons were stripped of myelin, and K+ channels were found to be dispersed across zones that extended into both nodal and internodal regions; a few days later they were undetectable. By 10 dpi, remyelination was underway, but Kv1.1 immunoreactivity was absent at newly forming nodes of Ranvier. By 14 dpi, K+ channels were detected but were in the nodal gap between Schwann cells. By 19 dpi, most new nodes had Kv1.1, Kv1.2, and Kvbeta2, which precisely colocalized. However, this nodal distribution was transient. By 24 dpi, the majority of K+ channels was clustered within paranodal regions of remyelinated axons, leaving a gap that overlapped with Na+ channel immunoreactivity. Inhibition of Schwann cell proliferation delayed both remyelination and the development of the K+ channel distributions described. Conduction studies indicate that neither 4-aminopyridine (4-AP) nor tetraethylammonium alters normal nerve conduction. However, during remyelination, 4-AP profoundly increased both compound action potential amplitude and duration. The level of this effect matched closely the nodal presence of these voltage-dependent K+ channels. Our results suggest that K+ channels may have a significant effect on conduction during remyelination and that Schwann cells are important in K+ channel redistribution and clustering.

Potassium channel distribution, clustering, and function in remyelinating rat axons

The K+ channel alpha-subunits Kv1.1 and Kv1.2 and the cytoplasmic beta-subunit Kvbeta2 were detected by immunofluorescence microscopy and found to be colocalized at juxtaparanodes in normal adult rat sciatic nerve. After demyelination by intraneural injection of lysolecithin, and during remyelination, the subcellular distributions of Kv1.1, Kv1.2, and Kvbeta2 were reorganized. At 6 d postinjection (dpi), axons were stripped of myelin, and K+ channels were found to be dispersed across zones that extended into both nodal and internodal regions; a few days later they were undetectable. By 10 dpi, remyelination was underway, but Kv1.1 immunoreactivity was absent at newly forming nodes of Ranvier. By 14 dpi, K+ channels were detected but were in the nodal gap between Schwann cells. By 19 dpi, most new nodes had Kv1.1, Kv1.2, and Kvbeta2, which precisely colocalized. However, this nodal distribution was transient. By 24 dpi, the majority of K+ channels was clustered within paranodal regions of remyelinated axons, leaving a gap that overlapped with Na+ channel immunoreactivity. Inhibition of Schwann cell proliferation delayed both remyelination and the development of the K+ channel distributions described. Conduction studies indicate that neither 4-aminopyridine (4-AP) nor tetraethylammonium alters normal nerve conduction. However, during remyelination, 4-AP profoundly increased both compound action potential amplitude and duration. The level of this effect matched closely the nodal presence of these voltage-dependent K+ channels. Our results suggest that K+ channels may have a significant effect on conduction during remyelination and that Schwann cells are important in K+ channel redistribution and clustering.

Effect of digitalis on conduction dysfunction in Pelizaeus-Merzbacher disease

We studied the effect of digitalis on nerve conduction dysfunction in Pelizaeus-Merzbacher disease (PMD). The patients were three Japanese boys with PMD, aged 7-10 years. Digitalis was administered orally in a daily dose of 0.06 mg/kg for 2 consecutive months, and the obtained serum concentrations ranged from 0.33 to 0.55 ng/ml. The digitalis therapy induced slight improvement of severe dysarthria and cognitive dysfunction in the two older patients. Electrophysiological examinations revealed the following results: In brainstem auditory evoked potentials (BAEPs), while waves II (or III) to V were absent before treatment, on treatment all waves of BAEPs except a wave IV were restored in all patients. While visual evoked potentials (VEPs) in response to transient flash stimulation showed markedly prolonged latencies before treatment, digitalis produced a mild, although not statistically significant, shortening of the latency of N160. There were also no significant changes in inter-peak amplitudes of VEPs. Transcranial cortical magnetic stimulation continued to fail to elicit motor evoked potentials of the first dorsal interosseous muscles in all patients. Thus, although the serum concentrations were insufficient to elicit favorable therapeutic effects, digitalis therapy provided slight relief of clinical symptoms with evidence of improvement of conduction dysfunction. It is suggested that patients with PMD may respond to symptomatic treatment modulating nerve conduction.

Persistent increased threshold of electrical stimulation selective to motor nerve in multifocal motor neuropathy

In multifocal motor neuropathy (MMN) the threshold of electrical stimulation showed a persistent, marked increase for the motor nerve which decreased after treatment with intravenous immunoglobulin or oral cyclophosphamide; whereas, the threshold was normal for the sensory nerve. This discrepancy of the thresholds for motor and sensory nerves indicates that the increased threshold for motor nerve is not caused by change in perineural capacitance, such as subperi- and endoneural edema or perineural thickening. Inching studies showed that the site of the elevated motor nerve threshold was closely associated with conduction slowing and block. For the cause of the increased threshold, therefore, we suppose the presence of a factor which interferes with reorganization of the nodal property in the remyelinative process or which directly blocks sodium channels where the blood-nerve barrier is impaired in MMN.

Control of myelination, axonal growth, and synapse formation in spinal cord explants by ion channels and electrical activity

The involvement of axonal electrical activity and ion channels as mediators of neuron-glial communication during myelin formation has been tested in explant culture. Transverse slices of embryonic mouse spinal cord were maintained under conditions normally leading to extensive myelination. Axonal conduction was measured optically through the use of a voltage-sensitive dye. Glial development was at a very early stage at the time of plating, and oligodendrocyte precursor cells had not yet appeared. Spontaneous electrical activity was blocked either by tetrodotoxin or by elevation of external K+ concentrations. Myelin development was unaffected by tetrodotoxin and was also present, though quantitatively reduced, in elevated K+. Tetraethylammonium ion (TEA+), a blocker of many K+ channels, almost entirely eliminated myelination at a concentration of 1 mM, but axonal growth and conduction were unaffected. Synapse formation was followed both morphologically and functionally, and was altered neither by conduction block nor by 1 mM TEA+. It is concluded that in the spinal cord oligodendrocyte development and myelination can proceed in the absence of axonal action potentials, but ion channels, possibly in glial membranes, play an important role in these events.

Resolving three types of chloride channels in demyelinated Xenopus axons

Axons from Xenopus sciatic nerve were demyelinated by intraneural injection of lysolecithin rendering the entire internodal axolema accessible to a patch electrode. In this region, three types of anion selective pores were found and characterized at the single-channel level. These included outwardly rectifying, inwardly rectifying, and maxi Cl- channels. The outwardly rectifying Cl- channels (24 pS) are activated by depolarization with a weak voltage dependence of 42 mV per e-fold change in open probability. The inwardly rectifying Cl- channels (27 pS) are insensitive to voltage, but can be blocked by internal application of 100 microM SITS or DIDS. The I-V curves of rectifying channels are S-shaped and can be fitted by a kinetic model with a single free energy barrier. The rectification may be related to the location of this barrier. The maxi Cl- channel (335 pS) is often open at the resting potential, but is inactivated by a large depolarization. The rectification, voltage dependence, and inactivation of these channels may contribute to the regulation of axonal Cl- balance and resting potential.

Conduction slowing without conduction block of compound muscle and nerve action potentials due to sodium channel block

We studied the effect of lidocaine on nerve conduction in vivo. Recovery of the compound muscle action potential (CMAP), sensory nerve action potential (SNAP), and single motor unit potential (MUP) of median nerve stimulation was recorded in four healthy volunteers after intravenous infusion of 20 ml of 0.5% lidocaine. During loading, CMAP and SNAP amplitudes rapidly decreased and their latencies increased. After recovery of the CMAP and SNAP amplitudes, nerve conduction velocity improved gradually over a period of 3-6 h, the amplitudes and configurations of CMAP and SNAP remaining unchanged. The conduction velocity of the single MUP markedly slowed before it is blocked. This indicates that maximum conduction velocity of CMAP and SNAP could be slowed by the partial inactivation of sodium channels without accompanying conduction block. Prolongation of the rise time of depolarization of the axonal membrane potential may be the active mechanism in this slowing because of sodium channel inactivation. Abnormalities in sodium channels at the nodes of Ranvier should be considered as a mechanism of conduction slowing even when there is no conduction block.

Axonal coding of action potentials in demyelinated nerve fibers

Conduction in individual axons of Xenopus has been measured optically in response to short trains of stimuli, following demyelination of the sciatic nerve. In many cases the initial action potential in a burst is absent. Failure may also occur later in the train, resulting in a profound alteration of signal coding by the axon. Integration leading to delayed transmission occurred at the heminode forming the proximal border of the demyelinated zone, as well as at new nodes of Ranvier forming in remyelinating axons. This process appeared to involve a depolarizing afterpotential and seemed to be analogous to the threshold changes involved in superexcitability. Axonal coding was very sensitive to small changes in the stimulus pattern. Neither 1 mM tetraethylammonium ion, which blocks nodal and Ca2+ activated K+ channels, nor 1 mM 4-aminopyridine, which blocks fast internodal K+ channels, prevented loss of the initial spike in a burst. Similarly, neither block of Ca2+ channels by Cd2+ nor lowering of Cl- had a notable effect. Ouabain, on the other hand, had small but possibly significant effects on responses to repetitive stimuli. A computational model was used to test mechanisms involving passive cable properties. Lowering the myelin resistance and the nodal leakage conductance, in accord with recent evidence from intracellular recordings, reproduced many of the results and was accurate with respect to stimulus frequency, temperature and sensitivity to average potential. The coding of action potentials observed here may have clinical consequences in demyelinating diseases such as multiple sclerosis.

A serial section electron microscope study of an identified Ia afferent collateral in the cat spinal cord

Serial section electron microscopy has been used to examine a horseradish peroxidase (HRP)-labelled group Ia afferent collateral from its entry point in the grey matter to its termination in Clarke's column of the cat spinal cord. A wide range of geometries and myelination patterns were identified along the collateral, including 1) nodes specialized to exhibit a single synaptic bouton, 2) nodes specialized to exhibit two or more synaptic boutons connected by fine, unmyelinated lengths of the collateral, 3) terminal heminodes, along which boutons were separated by unmyelinated branches, and 4) complex arrangements along which myelinated and unmyelinated branches gave rise to one or more boutons. Thirty-six synaptic boutons of varied shape and size were exhibited by this collateral. Previous studies have shown that the geometry, branching, and myelination pattern of an axon play an important role in determining the amplitude and duration of an action potential propagating along that axon. In turn, the amplitude and duration of a presynaptic action potential influence the efficacy of transmitter release. The varied axonal geometries and myelination patterns observed in the present study provide further evidence in support of our previous proposal that there may be considerable nonuniformity in the efficacy of synaptic transmission among release sites arising from the same primary afferent fiber.

A distributed-parameter model of the myelinated nerve fiber

This paper presents a new model for the characterization of electrical activity in the nodal, paranodal and internodal regions of isolated amphibian and mammalian myelinated nerve fibers. It differs from previous models in the following ways: (1) in its ability to incorporate detailed anatomical and electrophysiological data; (2) in its approach to the myelinated nerve fiber as a multi-axial cable; and (3) in the numerical algorithm used to obtain distributed model equation solutions for potential and current. The morphometric properties are taken from detailed electron microscopic anatomical studies (Berthold & Rydmark, 1983a, Experientia 39, 964-976). The internodal axolemma is characterized as an excitable membrane and model-generated nodal and internodal membrane action potentials are presented. A system of describing equations for the equivalent network model is derived, based on the application of Kirchoff's Current Law, which take the form of multiple cross-coupled parabolic partial differential equations. An implicit numerical integration method is developed and the numerical solution implemented on a parallel processor. Non-uniform spatial step sizes are used, enabling detailed representation of the nodal region while minimizing the number of total segments necessary to represent the overall fiber. Conduction velocities of 20.2 m sec-1 at 20 degrees C for a 15 microns diameter amphibian fiber and 57.6 m sec-1 at 37 degrees C for a 17.5 microns diameter mammalian fiber are achieved, which agrees qualitatively with published experimental data at similar temperatures (Huxley & Stämpfli, 1949, J. Physiol., Lond. 108, 315-339; Rasminsky, 1973, Arch, Neurol. 28, 287-292). The simulation results demonstrate the ability of this model to produce detailed representations of the transaxonal, transmyelin and transfiber potentials and currents, as well as the longitudinal extra-axonal, periaxonal and intra-axonal currents. Also indicated is the potential contribution of the paranodal axolemma to nodal activity as well as the presence of significant longitudinal currents in the periaxonal space adjacent to the node of Ranvier.

Axolemmal abnormalities in myelin mutants

Evidence is reviewed that the paranodal axoglial junction plays important roles in the differentiation and function of myelinated axons. In myelin-deficient axons, ion flux across the axolemma is greater than that in myelinated fibers because a larger proportion of the axolemma is active during continuous, as opposed to saltatory, conduction. In addition, older myelin-deficient rats that have developed spontaneous seizures display small foci of node-like E-face particle accumulations in CNS axons as well as more diffuse regions of increased particle density and number. Assuming that the E-face particles represent sodium channels, such regions could underlie high sodium current density during activity, low threshold for excitation, and increased extracellular potassium accumulation. Depending on the degree of spontaneous channel opening, they could also represent sites of spontaneous generation of activity. The appearance of seizures and their gradual increase in frequency and severity could represent an increase in the number of such regions. In addition, diminution in the dimensions of the extracellular space during maturation would result in increased extracellular resistance, which, together with increasing axonal diameter, would tend to increase the likelihood of ephaptic interaction among neighboring axons as well as the likelihood of extracellular potassium rises to levels that could cause spontaneous activity.

Remyelination of nerve fibers in the transected frog sciatic nerve

This project tests an important aspect of the cellular events controlling the processes of recovery of function and remyelination that follow demyelination in the peripheral nervous system. Frog sciatic nerves have been shown to survive and remain functional for up to 10 days following transection. We have utilized this property in order to dissociate the recovery process from possible control by the neuronal soma. Xenopus sciatic nerves were demyelinated in one branch by an intraneural injection of lysolecithin. The nerve was cut proximally to the injection site either immediately before, or several days after the lysolecithin injection. Recovery of function and remyelination were then followed by electrophysiological, optical, and ultrastructural techniques applied both to whole branches and single fibers. Controls included the cut but uninjected branch, and injected but uncut nerves. The progression of events during both demyelination and recovery in cut axons was indistinguishable from that in uncut fibers. This suggests that this process may be under local control and can be initiated and carried out in the absence of constant communication with the nerve cell body.

Colchicine prevents recovery of nerve conduction at chronic demyelination

A colchicine cuff was applied to rat sciatic nerve proximal to a demyelinating region produced by a focal injection of lysophosphatidylcholine (LPC). The colchicine cuff prevented the recovery of function normally seen within 6-8 days after LPC-induced demyelination. Colchicine blocked the delivery of sodium channels to the demyelinated region and induced their accumulation proximal to the cuff. The dual effect of colchicine in blocking both the recovery of impulse propagation through the demyelinated region and the delivery of sodium channels suggests a central role for fast axonal transport of sodium channels in the recovery of function at demyelination.

Optical measurement of conduction in single demyelinated axons

Demyelination was initiated in Xenopus sciatic nerves by an intraneural injection of lysolecithin over a 2-3-mm region. During the next week macrophages and Schwann cells removed all remaining damaged myelin by phagocytosis. Proliferating Schwann cells then began to remyelinate the axons, with the first few lamellae appearing 13 d after surgery. Action potentials were recorded optically through the use of a potential-sensitive dye. Signals could be detected both at normal nodes of Ranvier and within demyelinated segments. Before remyelination, conduction through the lesion occurred in only a small fraction of the fibers. However, in these particular cases we could demonstrate continuous (nonsaltatory) conduction at very low velocities over long (greater than one internode) lengths of demyelinated axons. We have previously found through loose patch clamp experiments that the internodal axolemma contains voltage-dependent Na+ channels at a density approximately 4% of that at the nodes. These channels alone, however, are insufficient for successful conduction past the transition point between myelinated and demyelinated regions. Small improvements in the passive cable properties of the axon, adequate for propagation at this site, can be realized through the close apposition of macrophages and Schwann cells. As the initial lamellae of myelin appear, the probability of success at the transition zone increases rapidly, though the conduction velocity through the demyelinated segment is not appreciably changed. A detailed computational model is used to test the relative roles of the internodal Na+ channels and the new extracellular layer. The results suggest a possible mechanism that may contribute to the spontaneous recovery of function often seen in demyelinating disease.

Single-channel recording in myelinated nerve fibers reveals one type of Na channel but different K channels

Amphibian myelinated nerve fibers were treated with collagenase and protease. Axons with retraction of the myelin sheath were patch-clamped in the nodal and paranodal region. One type of Na channel was found. It has a single-channel conductance of 11 pS (15 degrees C) and is blocked by tetrodotoxin. Averaged events show the typical activation and inactivation kinetics of macroscopic Na current. Three potential-dependent K channels were identified (I, F, and S channel). The I channel, being the most frequent type, has a single-channel conductance of 23 pS (inward current, 105 mM K on both sides of the membrane), activates between -60 and -30 mV, deactivates with intermediate kinetics, and is sensitive to dendrotoxin. The F channel has a conductance of 30 pS, activates between -40 and 60 mV, and deactivates with fast kinetics. The former inactivates within tens of seconds; the latter inactivates within seconds. The third type, the S channel, has a conductance of 7 pS and deactivates slowly. All three channels can be blocked by external tetraethylammonium chloride. We suggest that these distinct K channel types form the basis for the different components of macroscopic K current described previously.

Na+ channel accumulation on axolemma of afferent endings in nerve end neuromas in Apteronotus

In mammals, cut sensory axons trapped in a nerve end neuroma have been shown to develop hyperexcitability, and to become a source of ectopic afferent discharge and abnormal sensation. We have explored cellular mechanisms underlying neuroma electrogenesis. First we confirmed that ectopic neuroma discharge develops in injured afferents in the electrosensory lateral line nerve of the weakly electric fish Apteronotus, as it does in mammals. Then, using previously characterized antibodies that specifically recognize Na+ channel proteins in this species, we obtained light and electron microscopic evidence of abnormally intense immunolabelling of axolemma at the injury site. Accumulation of excess Na+ channels in afferent endings in neuromas could account for their electrical hyperexcitability.

Sodium channels in single demyelinated mammalian axons

The distribution of ionic channels is thought to play an important role in the recovery of function following demyelination. Rat sciatic nerves were demyelinated by lysolecithin and single fibers were examined with patch clamp techniques. Voltage-dependent sodium currents were measured in both internodal and nodal regions. The results suggested that while there is a sharp gradient in channel density at the node of Ranvier, the total number of internodal channels far exceeds the total number of nodal channels. The nodal channels do not diffuse freely following demyelination and the internodal channels may thus serve important functions as conduction is restored.

Ionic channels and signal conduction in single remyelinating frog nerve fibres

1. Ionic currents have been measured in single demyelinated and remyelinating frog sciatic nerve fibres by means of the loose patch clamp technique. Axons were demyelinated by a surgical intraneural injection of lysolecithin and recovery was followed for up to 5 months. 2. Removal of myelin debris continued for the first 2 weeks post-injection. Proliferating Schwann cells were then seen within the lesion. As remyelination proceeded new nodes of Ranvier were formed in regions that previously were internodal. Original nodes, marking the transition from old to new myelin, could be identified at all stages. 3. Peak amplitudes of internodal transient inward Na+ currents were constant over the first 2 months and increased by about 60% after 5 months. Internodal currents in remyelinated axons were recorded after a second injection of lysolecithin to remove the thin myelin sheath. 4. Records from paranodal sites neighbouring transition nodes contained transient outward currents that were strongly voltage dependent and seemed to reflect activation of a very high density of Na+ channels just outside the patch. This sharp gradient in channel density at original nodes persisted throughout the period of remyelination studied suggesting that lateral diffusion from these sites is limited. These currents were never seen at internodal sites nor were they found at new nodes of Ranvier. 5. Paranodal inward current amplitudes in new nodes were similar to those in original (transition) nodes. 6. No transient inward Na+ currents were detected in Schwann cells adhering to demyelinated axons or free standing within the area of the lesion. 7. Conduction in single remyelinating fibres was studied by measuring membrane currents that flowed in response to an invading propagating action potential. At 2 weeks post-injection, prior to the formation of myelin, conduction was decremental, but activation of internodal Na+ channels allowed signals to penetrate further into the demyelinated zone than would have been possible by passive spread alone. After an additional 3 weeks, following formation of thin myelin sheaths, conduction was significantly improved and the fractional activation of Na+ channels was increased. 8. The results suggest that Na+ channels at new nodes of Ranvier come neither from original nodes nor from Schwann cells. They may represent a moderate aggregation of existing internodal channels. New nodes seem to possess a gradient of Na+ channel density that is much less steep than that at original nodes. Continuous conduction appears to be limited to short (approximately 0.2 mm) lengths of demyelinated axons.(ABSTRACT TRUNCATED AT 400 WORDS)