Internodal Membrane Research Articles

P-NE006. Atypical presentation of Guillain-Barré syndrome (GBS) in acute inflammatory demyelinating polyneuropathy (AIDP) with conduction block and its association with anti-sulfatide antibody: A case report

Introduction. AIDP represents a subtype of GBS, whereby demyelination impairs transmission of impulse via altered paranodal and internodal membranes. Conduction blocks and velocity reduction occurs as a result in which our case illustrates. Results. A 63-year-old Chinese retired salesman with dyslipidemia presented with sudden limb weakness and slurring of speech. He experienced fever, lethargy and flu-like symptoms two days prior. On arrival, temperature was 38.8 degrees Celsius while other vital signs were within normal limits. He was oriented with left lower motor seventh nerve lesion and dysarthria. Other cranial nerve functions were intact. Both upper limbs had power of 4/5 except for a left wrist drop while lower limbs had power of 3/5 proximally and 4/5 distally except for a right foot drop. Reflexes were present and sensation was intact. Computed tomography of the brain with perfusion study was normal. He was treated as lacunar stroke with NIHSS 8/42 and initiated on antiplatelet. Nerve conduction study (NCS) done in view of patchy clinical distribution shows evidence of generalized motor demyelinating polyneuropathy with multifocal conduction block suggestive of AIDP variant of GBS. A lumbar puncture performed revealed cytoalbuminologic dissociation with cerebrospinal fluid (CSF) protein of 726 mg/L and absence of cells. Meningitis panels were negative and his antiganglioside antibodies demonstrated borderline positivity of anti-sulfatide IgG. Intravenous immunoglobulin (0.4 g/kg) was administered over 5 days. He was discharged and regained full power of both upper and lower limbs. Ensuing NCS at three weeks revealed minimal improvement with return of F-waves recording. Conclusion. GBS presents in myriad of ways underlining the importance of electrophysiological studies. Our patient presented with debilitating symptoms but regained full function after intravenous immunoglobulin. Sensory impairment of varying degree has been observed in raised titres of anti-sulfatide antibodies. Further studies to define phenotype of GBS with anti-sulfatide antibodies are recommended.

Simulation of impedance changes with a FEM model of a myelinated nerve fibre

Objective. Fast neural electrical impedance tomography (EIT) is a method which permits imaging of neuronal activity in nerves by measuring the associated impedance changes (dZ). Due to the small magnitudes of dZ signals, EIT parameters require optimization, which can be done using in silico modelling: apart from predicting the best parameters for imaging, it can also help to validate experimental data and explain the nature of the observed dZ. This has previously been completed for unmyelinated fibres, but an extension to myelinated fibres is required for the development of a full nerve model which could aid imaging neuronal traffic at the fascicular level and optimise neuromodulation of the supplied internal organs to treat various diseases. Approach. An active finite element method (FEM) model of a myelinated fibre coupled with external space was developed. A spatial dimension was added to the experimentally validated space-clamped model of a human sensory fibre using the double cable paradigm. Electrical parameters of the model were changed so that nodal and internodal membrane potential as well as propagation velocity agreed with experimental values. Impedance changes were simulated during activity under various conditions and the optimal parameters for imaging were determined. Main results. When using AC, dZ could be recorded only at frequencies above 4 kHz, which is supported by experimental data. Optimal bandwidths for dZ measurement were found to increase with AC frequency. Significance. The novel fully bi-directionally coupled FEM model of a myelinated fibre was able to optimize EIT for myelinated fibres and explain the biophysical basis of the measured signals.

The Axon-Myelin Unit in Development and Degenerative Disease.

Axons are electrically excitable, cable-like neuronal processes that relay information between neurons within the nervous system and between neurons and peripheral target tissues. In the central and peripheral nervous systems, most axons over a critical diameter are enwrapped by myelin, which reduces internodal membrane capacitance and facilitates rapid conduction of electrical impulses. The spirally wrapped myelin sheath, which is an evolutionary specialisation of vertebrates, is produced by oligodendrocytes and Schwann cells; in most mammals myelination occurs during postnatal development and after axons have established connection with their targets. Myelin covers the vast majority of the axonal surface, influencing the axon's physical shape, the localisation of molecules on its membrane and the composition of the extracellular fluid (in the periaxonal space) that immerses it. Moreover, myelinating cells play a fundamental role in axonal support, at least in part by providing metabolic substrates to the underlying axon to fuel its energy requirements. The unique architecture of the myelinated axon, which is crucial to its function as a conduit over long distances, renders it particularly susceptible to injury and confers specific survival and maintenance requirements. In this review we will describe the normal morphology, ultrastructure and function of myelinated axons, and discuss how these change following disease, injury or experimental perturbation, with a particular focus on the role the myelinating cell plays in shaping and supporting the axon.

BK Channels Localize to the Paranodal Junction and Regulate Action Potentials in Myelinated Axons of Cerebellar Purkinje Cells.

In myelinated axons, K(+) channels are clustered in distinct membrane domains to regulate action potentials (APs). At nodes of Ranvier, Kv7 channels are expressed with Na(+) channels, whereas Kv1 channels flank nodes at juxtaparanodes. Regulation of axonal APs by K(+) channels would be particularly important in fast-spiking projection neurons such as cerebellar Purkinje cells. Here, we show that BK/Slo1 channels are clustered at the paranodal junctions of myelinated Purkinje cell axons of rat and mouse. The paranodal junction is formed by a set of cell-adhesion molecules, including Caspr, between the node and juxtaparanodes in which it separates nodal from internodal membrane domains. Remarkably, only Purkinje cell axons have detectable paranodal BK channels, whose clustering requires the formation of the paranodal junction via Caspr. Thus, BK channels occupy this unique domain in Purkinje cell axons along with the other K(+) channel complexes at nodes and juxtaparanodes. To investigate the physiological role of novel paranodal BK channels, we examined the effect of BK channel blockers on antidromic AP conduction. We found that local application of blockers to the axon resulted in a significant increase in antidromic AP failure at frequencies above 100 Hz. We also found that Ni(2+) elicited a similar effect on APs, indicating the involvement of Ni(2+)-sensitive Ca(2+) channels. Furthermore, axonal application of BK channel blockers decreased the inhibitory synaptic response in the deep cerebellar nuclei. Thus, paranodal BK channels uniquely support high-fidelity firing of APs in myelinated Purkinje cell axons, thereby underpinning the output of the cerebellar cortex.

Internodal mechanism of pathological afterdischarges in myelinated axons

Recent optical recordings of transmembrane potentials in the axons of pyramidal neurons have shown that the internodal action potentials (APs) predicted in our previous studies do exist. These novel processes are not well understood. In this study we aim to clarify electrical phenomena in peripheral myelinated axons (MAs). We used a multi-cable Hodgkin-Huxley-type model to simulate MAs with potassium channels that were either normal or inhibited along a short region of the internodal membrane. A brief stimulus was applied to the first node. We demonstrated peculiarities in the internodal APs induced by a saltatory AP: They existed across internodal membranes, were detectable in periaxonal space but not in intracellular space, propagated continuously, collided near the mid-internodes, and produced internodal sources of afterdischarges. These results highlight the importance of the MA internodal regions as new therapeutic targets for avoiding afterdischarges provoked by reduced axonal fast potassium channel expression.

A possible mechanism of repetitive firing of myelinated axon

A unique mechanism is proposed according to which processes within the internodal axolemma are responsible for repetitive activation of myelinated axon with deficit of internodal potassium conductance. A numerical simulation of activity in axon with 21 nodes was performed. The axon was represented by cables for axoplasmic and periaxonal spaces. Accumulation and diffusion of ions were taken into account. Fine segmentation of each internode (338 segments) allowed simulation of internodal activation in response to a normal saltatorial action potential initiated by a short stimulus. The internodal membrane without potassium conductance experienced considerable depolarization. This resulted in formation of a transition zone and significant currents that caused repetitive activation of the internode and neighbor node. Decline of periaxonal sodium concentration during the spike production or lowering of sodium channel density decreased the sodium currents. As a result, the interspike intervals increased up to cessation of the burst. The cessation was reversible.

Type II brain 4.1 (4.1B/KIAA0987), a member of the protein 4.1 family, is localized to neuronal paranodes

Histochemical analyses of type II brain 4.1/4.1B/KIAA0987, a member of the protein 4.1 family, were carried out in rat brain. In situ hybridization (ISH) showed that type II brain 4.1 mRNA is expressed in a variety of neuronal cells. In particular, type II brain 4.1 mRNA was actively transcribed in the cells of the mesencephalon and the brainstem, which have large myelinated nerve fibers. Expression of type II brain 4.1 mRNA was not observed at least in glial cells distributed in nerve fiber tracts. In immunohistochemical studies using anti-type II brain 4.1-specific antibody, the major immunosignals appeared as brilliant pairs of dots along nerve fibers. Such immunosignals were detected throughout the brain, but were highly concentrated in nerve fiber tracts. These data suggested that type II brain 4.1 is predominantly localized to neuronal paranodes. Detailed analysis concentrating on the nodal region indicated that type II brain 4.1 is present at the paranodal membrane but not in the axoplasm. Weaker type II brain 4.1-specific immunosignals were observed along the internodal membrane of myelinated axons and in the cytoplasm of some neuronal cells. Finally, comparative immunohistochemical studies using antibodies against the other three protein 4.1 family members, type I brain 4.1/4.1N/KIAA0338, erythroid type 4.1 (4.1R) and 4.1G, demonstrated that each of these proteins is distributed in a unique pattern in the cerebellum. Our results are the first to show that type II brain 4.1 is the only member of the protein 4.1 family localized to neuronal paranodes.

Composition and biophysical properties of myelin lipid define the neurological defects in galactocerebroside- and sulfatide-deficient mice.

Oligodendrocytes and Schwann cell-specific proteins are assembled with a highly ordered membrane lipid bilayer to the myelin sheath of axons, which functions as an insulator and allows rapid saltatory conduction. We approached the question of the function of the CNS and PNS myelin-specific galactospingolipids cerebrosides and sulfatides by generating a ceramide galactosyltransferase null allelic mouse line (cgt-/-). Galactocerebroside- and sulfatide-deficient myelin loses its insulating properties and causes a severe dysmyelinosis that is incompatible with life. Here, we describe the biochemical and biophysical analysis of the myelin lipid bilayer of cgt-/- mice. The lipid composition of CNS and PNS myelin of cgt-/- mice is seriously perturbed and the sphingolipid biosynthetic pathway altered. Nonhydroxy and hydroxy fatty acid-substituted glycosylceramides (GlcC) are synthesized by oligodendrocytes and sulfated GlcC in addition in Schwann cells. The monogalactosyldiglyceride fraction is missing in the cgt-/- mouse. This new lipid composition can be correlated with the biophysical properties of the myelin sheath. The deficiency of galactocerebrosides and sulfatides leads to an increased fluidity, permeability, and impaired packing of the myelin lipid bilayer of the internodal membrane system. The loss of the two glycosphingolipid classes causes the breakdown of saltatory conductance of myelinated axons in the cgt-/- mouse.

Strength-duration properties of human peripheral nerve.

The strength-duration time constant (tau SD) is a property of nodal membrane and, while it depends on a number of factors, its measurement may shed light on axonal properties when taken in conjunction with measurements of axonal excitability. For example, tau SD increases with demyelination as the exposed membrane is enlarged by inclusion of paranodal and internodal membrane, it decreases with hyperpolarization and it increases with depolarization. The present study was undertaken in 20 normal volunteers to compare strength-duration curves for compound sensory and muscle action potentials, to determine the most appropriate curve fitting equation for the data, and to examine the reproducibility of the calculated time constant on different days, for potentials of different amplitude and at different sites along the nerve. Using a computerized threshold-tracking system, stimulus intensity was adjusted to produce an antidromic compound sensory action potential (CSAP) or an orthodromic muscle action potential of 30% of maximum. Stimulus duration was increased every minute in 20 microseconds steps from 20 microseconds to 1 ms. The time constant for compound sensory potentials (665 +/- 182 microsecond) was longer than that for compound EMG potentials (459 +/- 126 microseconds). Weiss's formula, which relates threshold charge to stimulus duration, provided an accurate fit for the experimental data, and the study validated that, using it, relatively few experimental measurements were required to calculate the time constant. In repeated studies on the same subject, time constants usually differed by < 400 microseconds for sensory axons and < 250 microseconds for motor axons. They were identical at different sites along the nerve and did not alter with the size of the compound action potential. These characteristics suggest that the determinations of strength-duration time constant could be suitable for clinical usage.

Action potential refractory period in axonal demyelination: a computer simulation.

Axonal demyelination leads to an increase in the refractory period for propagation of the action potential. Computer simulations were used to investigate the mechanism by which changes in the passive properties of the internodal membrane increase the refractory period. The properties of the voltage dependent ion channels can be altered to restore conduction in demyelinated nerve fibers. The ability of these alterations to decrease the refractory period of demyelinated model nerve fibers was compared. The model nerve fiber contained six nodes. The action potential was stimulated at node one and propagated to node six. The internode between nodes three and four was demyelinated in a graded manner. The absolute refractory period for propagation of the action potential through the demyelinated internode increased as the number of myelin wraps was reduced to less than 25% of the normal value. The increase in refractory period was found to be due to a reduction in the rate or repolarization of the action potential at node three. The delay in repolarization reduced the rate of recovery of inactivated Na channels and slowed the closing of K channels. The rate of repolarization of node three was reduced by the conduction delay for the depolarization of node four caused by demyelination of the preceding internode. In these simulations the increase in refractory period due to demyelination was eliminated by slowing the onset of Na channel inactivation. A small reduction of the K conductance also decreased the refractory period. However, larger reductions eliminated this effect.

Ultrastructure of afferent axon endings in a neuroma

Injured sensory axons with endings trapped in a nerve-end neuroma become a source of abnormal impulse discharge and neuropathic pain. We have examined the ultrastructure of such endings anterogradely transported WGA-HRP and freeze-fracture replication, with emphasis on the postinjury period during which the abnormal neural discharge is maximal. Most axons ended in a terminal swelling, depleted of myelin but surrounded by Schwann cell processes. These 'neuroma endbulbs' were richly packed with membrane-bound organelles, and had a smoothly undulating surface with (in neuromas of several weeks standing) a moderate number of short filopodia. Massive sprouting did not occur until several months postinjury. Both p- and e-faces of endbulb axolemma had larger intramembranous particles, on average, than corresponding internodal membrane of control axons. This change, interpreted as indicating remodelling of axolemmal channel (and perhaps receptor) content, may be related to the abnormal electrical behavior of neuroma afferents.

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.

Macromolecular structure of axonal membrane in the optic nerve of the jimpy mouse.

The macromolecular structure of the axon membrane in 26-28-day-old Jimpy mice and control optic nerve were examined with quantitative freeze-fracture electron microscopy. Premyelinated and myelinated axons were observed in control optic nerves, with axonal diameters of premyelinated axons being generally smaller than that of myelinated axons (approximately 0.2-0.4 micron vs approximately 0.5-1.5 micron, respectively). Axon membrane from control optic nerves exhibited an asymmetrical partitioning of intramembranous particles (IMP). P-faces of internodal membrane displayed nearly twice as many IMP as the premyelinated axolemma (1,731 vs 893 micron-2, respectively). E-faces of internodal and premyelinated axolemma exhibited IMP densities of 124 and 157 micron-2, respectively. Few myelinated axons were apparent in optic nerves from Jimpy mice. The amyelinated axons of Jimpy mice displayed a spectrum of axonal diameters, ranging from approximately 0.2 to 1.5 micron. P-face densities of amyelinated axons, considered as a group, exhibited a wide range (600-2,100 micron-2). However, large diameter (greater than or equal to 0.5 micron) axons exhibited a significantly greater P-face IMP density than that of small caliber (greater than 0.5 micron) axons (1,525 vs 1,032 micron-2, respectively). Aggregations of E-face IMP were not observed along amyelinated axons of Jimpy optic nerves. The results demonstrate that the changes in P-face IMP density that occur during development of normal myelinated axons also occur in developing axons of Jimpy optic nerve, irrespective of a lack of normal glial cell association, and provide further evidence that the primary defect of hypomyelination within Jimpy mice is not attributed to the neuron.

Unmyelinated and myelinated axon membrane from rat corpus callosum: differences in macromolecular structure

The macromolecular structure of unmyelinated and myelinated internodal axon membrane was examined with freeze-fracture electron microscopy. Unmyelinated axons exhibited a gradient of axonal diameters, generally ranging from 0.1 to 0.5 μm, with some unmyelinated axons of up to 0.7 μm diameter. Myelinated fibers also displayed a range of axonal diameters, with axons generally 0.3–1.0 μm. The overlap in diameters, between unmyelinated and myelinated fibers, permitted a comparison of membrane structure in myelinated and unmyelinated axons of the same diameter. Small (<0.5 μm) diameter unmyelinated axons exhibited a moderate density (≈700/gmm 2) of P-face intramembranous particles (IMPs), while large (⪖0.5 μm) caliber unmyelinated axons displayed a significantly greater P-face IMP density (≈1100/μ m 2). Internodal membrane of both small (<0.5 μm) and large (⪖0.5 μ m) diameter myelinated fiber exhibited densities of P-face particles (≈1400/μm 2) that were similar to each other, but significantly different from unmyelinated fibers. These results demonstrate that there are differences in membrane structure between unmyelinated and myelinated axons of similar diameter. These findings also demonstrate that membrane structure of unmyelinated axons is not invariant for all unmyelinated fibers within a given CNS tract but, on the contrary, is related to diameter.

Macromolecular structure of axonal membrane during acute experimental allergic encephalomyelitis in rat and guinea pig spinal cord.

Axonal membrane structure during acute experimental allergic encephalomyelitis (EAE) was examined with freeze-fracture electron microscopy. Axons without myelin sheaths were prevalent within EAE spinal cords. Often these axons were associated with astrocytic processes, though membrane specializations were not observed at these sites. The demyelinated axons exhibited a highly asymmetrical partitioning of intramembranous particles (IMP), with approximately 2,000 particles/micron2 on P-faces and approximately 150/micron2 on E-faces. This distribution and density of IMP is similar to myelinated internodal membrane. The IMP were generally randomly distributed along the axons. However, in some regions, E-faces of demyelinated axons without paranodal-like membrane specialization in the vicinity displayed a greater than normal (approximately 500/micron2) particle density. Many of the IMP in these regions of increased density were of a large (greater than 10 nm) diameter. Axonal membrane bounded by a single set of paranodal oligodendroglial loops ('heminodal') was also observed, and the axolemma adjacent to the terminal glial loop exhibited a gradient of morphologies. The E-faces of presumed heminodal membrane most often displayed a moderately low density of IMP. However, in several instances, heminodal membrane exhibited a moderately high IMP density (approximately 1,100/micron2), similar to that observed within normal nodal membrane. In all cases, a high percentage of the E-face IMP within heminodal membrane were large. The results demonstrate that acute demyelination is associated with a maintenance of the integrity of certain components of the axolemma and an apparent dedifferentiation in other constituents.

Properties of potassium and sodium channels in frog internode.

1. Voltage-clamp experiments were performed on single frog internodes after acute demyelination with lysolecithin. The action of lysolecithin was stopped by washing out the lysolecithin with normal Ringer solution containing bovine albumin when the first delayed current was observed. After washing, the temperature was lowered from 25 to 15 degrees C. These procedures greatly prolonged the survival of the demyelinated internode up to 1 h. 2. External tetraethylammonium chloride (TEA+, 110 mM) reduced the K+ current in the internode only to 11% of the control value. 110 mM-TEA+ increased the time constant tau n of K+ activation by a factor of two in the node and by a factor of four in the internode. 120 mM-CsCl at the cut ends of the fibre also reduced the delayed outward current recorded at 60 mV in the internode to 11% of the control value, hardly changing the time constant tau n. 3. After a depolarization, the K+ tail current decayed in two phases, suggesting that the K+ conductance of the internodal membrane may be composed of at least two components, a slow one (gKs) and a fast one (gKf). As in the node, the fast K+ conductance of the internode can be further decomposed into two components (gKf1 and gKf2) with different activation potential ranges. The fast phase of the tail current was blocked by external application of 1 mM-4-aminopyridine (4-AP). The slow phase was almost unaltered by 1 mM-4-AP. The extrapolated slow tail current was 33% of the total tail current in the internode and 15% at the node, i.e. the proportion of slow K+ channels is larger in the internode than in the node. 4. Tetrodotoxin (TTX)-sensitive transient inward currents could be measured in the demyelinated internode, provided the large K+ currents were blocked by internal Cs+. The time course, TTX sensitivity, reversal potential and steady-state inactivation of the transient early inward current indicate that this current is caused mainly by Na+ passing through Na+ channels. 5. The density of K+ and Na+ channels in the demyelinated internode is estimated from the size of the K+ and Na+ current, respectively, and the capacity of the demyelinated segment. The K+ channel density of the internode seems to be about 20 times smaller than in the node, whereas the Na+ channel density in the internode appears to be about 500 times smaller than in the node.

Effects of delayed myelination by oligodendrocytes and Schwann cells on the macromolecular structure of axonal membrane in rat spinal cord.

The macromolecular structure of axonal membrane from dorsal funiculi of control and irradiated spinal cord of 45-day-old rats was examined with freeze-fracture electron microscopy. In control spinal cords, virtually all myelination is mediated by oligodendrocytes, and the internodal axonal membrane of these fibres displays highly asymmetrical partitioning of intramembranous particles (IMPs). The internodal P-face particle density is approximately 2350IMPs per micron 2, whereas the E-face IMP density is approximately 150 per micron 2. In control dorsal spinal roots, myelination is mediated by Schwann cells, and the ultrastructure of the internodal axolemma of the myelinated fibres is similar to that displayed by myelinated fibres of dorsal funiculi. On the internodal P-face of Schwann cell-myelinated fibres the IMP density is approximately 2350 per micron 2, whereas on the E-face the density is approximately 175 per micron 2. Irradiation of the lumbosacral spinal cord at 3 days of age results in a glial cell-deficient region within the spinal cord such that myelination in irradiated dorsal funiculi is delayed and subsequent myelination is mediated by both oligodendrocytes and Schwann cells. By 45 days of age, dorsal funiculi of irradiated spinal cords are well populated with fibres myelinated by oligodendrocytes and Schwann cells. However, fibres myelinated by oligodendrocytes display very thin myelin sheaths whereas Schwann cell-myelinated fibres exhibit myelin sheaths with normal thicknesses. Internodal membrane of fibres myelinated by Schwann cells and oligodendrocytes exhibit similar macromolecular structure, with approximately 2400 IMPs per micron 2 on P-faces and approximately 150 IMPs per micron 2 on E-faces. Occasional large (greater than 1.5 micron diameter) axons without glial-Schwann cell ensheathment are observed. These axons display a high density of P-face particles (approximately 2000 per micron 2) and a moderate density (approximately 350 per micron 2) of E-face IMPs on their fracture faces. These results demonstrate that CNS fibers exhibit similar axonal membrane ultrastructure irrespective of whether they are myelinated by Schwann cells or oligodendrocytes, or whether myelination is delayed. Moreover, when myelination does not occur, the axolemmal E-face IMP density, which may be related to the density of voltage-sensitive sodium channels, is not reduced.

The plasma membrane of young Chara internodal cells revealed by rapid freezing

Young elongating internodal cells of Chara globularis var. capillacea (Thuill.) Zanev. were rapidly frozen and freze-fractured in order to observed transient events occurring within the plasma membrane. Several structures have been observed. Relatively small depressions, varying in depth, are prolific and scattered at random over the plasma membrane. Charasomes and clusters of particle rosettes are common. Arrays of intramembrane particle lines are a characteristic feature of the internodal cell plasma membrane. The charasomes and the arrays of particle lines occupy a considerable proportion of the plasma membrane. In these young cells, substantial movement must take place across this membrane and its basic structure must fluctuate accordingly. The innumerable small depressions may represent pinocytotic and secretory processes. The array of intramembrane particle lines may represent stages in fusion between the membranes of vesicles within the cytoplasm and the plasma membrane. The technique of ultra-rapid freezing allows these events and their intermediate stages to be visualised; some features of the membrane may only be seen by this method.

Lack of excitability in the internodal membrane of myelinated nerve fiber in frog.

Monophasic action potentials of about 70 and 10 mV were recorded by inserting a microelectrode into the axon and the myelin sheath of an intact myelinated fiber, respectively. When the intra-axonal or the intra-myelinic microelectrode was used for both stimulation and recording, only the anodal current was effective in inducing action potentials. The inter-nodal membrane was, therefore, intrinsically inexitable.

Impulse conduction in inhomogeneous axons: Effects of variation in voltage-sensitive ionic conductances on invasion of demyelinated axon segments and preterminal fibers

Conduction in inhomogeneous axons may be blocked by several mechanisms. Conduction in demyelinated axons may fail since normal internodal membrane is inexcitable, because values of sodium conductance are too low to support impulse conduction. In addition, focal loss of myelin causes increased current leakage which slows or blocks invasion of impulses into the demyelinated zone due to inadequate current density. Similar considerations apply to the invasion of non-myelinated preterminal axons from myelinated parent fibers, where conduction can be blocked as a result of inadequate current density. A cable model of an axon is presented which allows myelinated regions, regions without myelin, and variable length transition zones of redistributed channel densities, to be studied. Action potentials and membrane currents were studied. Computer simulations using this model show that the safety factor for invasion is dependent on temperature. These studies also show that small changes in axon membrane properties, at the transition region between the myelinated zone and the region without myelin, may promote invasion of the region without myelin. In particular, increasing sodium conductance (g Na) or decreasing potassium conductance (g K) promotes invasion. Because of the non-linear behavior of excitable membranes the spatial distribution of channels is shown also to have significant effects on invasion. Thus, relatively small degrees of membrane reorganization may lead to functional changes with respect to the invasion of demyelinated axon regions. Similarly, the properties of the heminode at the distal part of the parent myelinated fiber may determine the invasion characteristics of non-myelinated terminal axons.