Xenotransplantation of transgenic pig olfactory ensheathing cells promotes axonal regeneration in rat spinal cord

Here we describe transplantation of olfactory ensheathing cells (OECs) or Schwann cells derived from transgenic pigs expressing the human complement inhibitory protein, CD59 (hCD59), into transected dorsal column lesions of the spinal cord of the immunosuppressed rat to induce axonal regeneration. Non-transplanted lesion-controlled rats exhibited no impulse conduction across the transection site, whereas in animals receiving transgenic pig OECs or Schwann cells impulse conduction was restored across and beyond the lesion site for more than a centimeter. Cell labeling indicated that the donor cells migrated into the denervated host tract. Conduction velocity measurements showed that the regenerated axons conducted impulses faster than normal axons. By morphological analysis, the axons seemed thickly myelinated with a peripheral pattern of myelin expected from the donor cell type. These results indicate that xenotranplantation of myelin-forming cells from pigs genetically altered to reduce the hyperacute response in humans are able to induce elongative axonal regeneration and remyelination and restore impulse conduction across the transected spinal cord.     

Local anesthetic action of phentolamine on insect mechanoreceptors

1. The effect of phentolamine on the response properties of insect mechanoreceptors and on the conduction in their axons was examined using electrophysiological techniques. 2. Phentolamine blocked conduction of action potentials along axons, an effect which exhibited 3 characteristics typical of local anesthetics: the effect was frequency-dependent, reversible and varied for nerves with different diameters. 3. The concentration of phentolamine required to block axonal conduction (1-2 x 10(-3) M) was significantly higher than that required to abolish the response of receptors to mechanical stimulation (3-5 x 10(-4) M). 4. All mechanoreceptors that were examined in Locusta migratoria and Periplaneta americana were inactivated by phentolamine (Table 1). The type I receptors (chordotonal, campaniform and hair sensilla) were inactivated within 5-15 min following phentolamine application. The only type II receptor examined (forewing stretch-receptor) underwent a phase of repetitive discharge before being inactivated. 5. Tolazoline and metoclopramide inactivated, like phentolamine, mechanoreceptors at lower concentrations than necessary to block axonal conduction. However, yohimbine and chlorpromazine inactivated mechanoreceptors and blocked axonal conduction at similar concentrations. 6. These findings suggest that phentolamine affects sense-organ specific ionic processes that are more sensitive to the drug than the ionic processes along the axons.     

Quantification of concealed conduction in the atrioventricular node

Twenty-eight anaesthetized open-chest mongrel dogs were used. Programmed atrial pacing was used and Hisian electrograms recorded through endocavitary electro-catheters to study and quantify the concealed conduction of non-transmitted atrial impulses in the A-V node. An exponential model was used in three situations to quantify the nodal conduction during incremental atrial pacing: a) during 1:1 conduction, b) during 2:1 nodal block, and c) during pacing, coupling an atrial impulse delivered at fixed intervals and blocked in the A-V node to each transmitted impulse. The relation between intranodal conduction times was analyzed both with and without the presence of blocked impulses, and the quotient between the obtained functions in situations b, c and situation a was determined. In a subgroup of 13 dogs the study was repeated following pharmacological block of the autonomic nervous system. In dogs with autonomic block, this relation always tended to decrease when the atrial pacing rate increased. The variations in the group of dogs with intact autonomic nervous systems were not homogeneous. During pacing with coupled block impulses, the progressive removal of conduction curves obtained for each coupling interval with respect to those obtained during 1:1 transmission, expresses the interval with respect ot those obtained during 1:1 transmission, expresses the lesser influence of the blocked impulses on decreasing their coupling interval.     

Propagation failure of action potentials at the bifurcation point of the slow axon innervating the extensor tibiae muscle ofDecticus albifrons (Orthoptera)

Summary Failure of conduction of nerve impulses has been observed at the bifurcation point of the metathoracic slow extensor tibiae motor axon (SETi) ofDecticus albifrons. Records from the region proximal and distal to the bifurcation point of the axon showed that during prolonged and repetitive stimulation and after a certain number of stimuli, proportional to the stimulating frequency, some SETi action potentials failed to cross this point (Fig. 1).Cross-sections of the metathoracic extensor motor nerve ofD. albifrons show that at the region of axonal bifurcation, both the neural lamella and the layer of glial cells (the sheath) around the SETi axons became thinner than the region proximal and distal to the bifurcation (Fig. 2).The possible role of the conduction block in the neuronal control of the muscle has been discussed.Abbreviations ETi extensor tibiae - SETi slow extensor tibiae - PE proximal electrode - DE distal electrode - SE stimulating electrode     

Changes in interspike interval during propagation: Quantitative description

The intervals between nerve impulses can change substantially during propagation because of conduction velocity aftereffects of previous impulse activity. Effects of such changes on interval histograms and on statistical parameters of spike trains were evaluated for Poisson spike trains and for trains generated by a clock with added random delays. The distribution of short intervals was significantly changed during propagation for these spike trains. Substantial changes in serial correlation coefficients were found in trains with certain initial interval distributions. The relevance of these effects to neural coding is discussed.     

Mechanism of conduction block in amphibian myelinated axon induced by biphasic electrical current at ultra-high frequency

The mechanism of axonal conduction block induced by ultra-high frequency (≥20 kHz) biphasic electrical current was investigated using a lumped circuit model of the amphibian myelinated axon based on Frankenhaeuser-Huxley (FH) equations. The ultra-high frequency stimulation produces constant activation of both sodium and potassium channels at the axonal node under the block electrode causing the axonal conduction block. This blocking mechanism is different from the mechanism when the stimulation frequency is between 4 kHz and 10 kHz, where only the potassium channel is constantly activated. The minimal stimulation intensity required to induce a conduction block increases as the stimulation frequency increases. The results from this simulation study are useful to guide future animal experiments to reveal the different mechanisms underlying nerve conduction block induced by high-frequency biphasic electrical current.     

Early glial responses in murine models of multiple sclerosis

Investigations of functional interactions among axons and glia over the last decade have revealed the extent and complexity of glial-neuronal and glial-glial communication during development, adult function and recovery from injury. These data have profound implications for the understanding of central nervous system (CNS) disorders, which until recently, have been classified as either neuronal or glial diseases. Re-evaluation of the pathological processes in a number of conditions has clearly shown involvement of both neurons and glia in early pathology. In multiple sclerosis (MS), the myelin sheath has traditionally been regarded as the primary target. However, recent evidence has clearly demonstrated axonal damage in new lesions. We have addressed the question of the role of axonal pathology in early MS by using well-characterized murine models for the relapsing-remitting (RR) or the primary progressive (PP) forms of the disease. We performed a histopathological survey of the CNS, following induction of the disease, to determine the timing of appearance, as well as the development of lesions. Then we analysed the relationship between inflammation, demyelination and axonal damage together with responses from astrocytes and microglia in each model from the earliest evidence of inflammation. We found that axonal damage begins well ahead of the appearance of motor symptoms. Pathology appears to be more closely related to the degree of inflammation than to demyelination. We also show that early astrocyte responses and the degree of axonal loss are markedly different in the two models and relate to the severity of pathology. These data support the now widely accepted hypothesis that axonal damage begins early in the disease process, but also suggest modulation of axonal loss and disease progression by the astrocytic response.     

Stochastic simulations on the reliability of action potential propagation in thin axons

It is generally assumed that axons use action potentials (APs) to transmit information fast and reliably to synapses. Yet, the reliability of transmission along fibers below 0.5 μm diameter, such as cortical and cerebellar axons, is unknown. Using detailed models of rodent cortical and squid axons and stochastic simulations, we show how conduction along such thin axons is affected by the probabilistic nature of voltage-gated ion channels (channel noise). We identify four distinct effects that corrupt propagating spike trains in thin axons: spikes were added, deleted, jittered, or split into groups depending upon the temporal pattern of spikes. Additional APs may appear spontaneously; however, APs in general seldom fail (<1%). Spike timing is jittered on the order of milliseconds over distances of millimeters, as conduction velocity fluctuates in two ways. First, variability in the number of Na channels opening in the early rising phase of the AP cause propagation speed to fluctuate gradually. Second, a novel mode of AP propagation (stochastic microsaltatory conduction), where the AP leaps ahead toward spontaneously formed clusters of open Na channels, produces random discrete jumps in spike time reliability. The combined effect of these two mechanisms depends on the pattern of spikes. Our results show that axonal variability is a general problem and should be taken into account when considering both neural coding and the reliability of synaptic transmission in densely connected cortical networks, where small synapses are typically innervated by thin axons. In contrast we find that thicker axons above 0.5 μm diameter are reliable.     

Nerve conduction blocks and peripheral neuropathies

A motor nerve conduction block is defined as a reduction of either amplitude or area of the compound motor action potential elicited by proximal vs. distal motor nerve stimulation. The pathophysiological mechanisms leading to a figure of conduction block include segmental demyelination, recent axonal interruption, or various axonal excitability abnormalities due to ion channel dysfunction or membrane potential changes. These processes can be related to compressive, ischemic or dysimmune inflammatory causes. The etiologic diagnosis is established on the combination of clinical, electrophysiological, and biological data. Among the neuropathies that feature nerve conduction blocks, there is a group of particular dysimmune multifocal neuropathies characterized by long-term persistent conduction blocks, including pure motor forms and sensori-motor forms. The clinical, electrophysiological, biological, and therapeutic specificities of these two types of neuropathy will be discussed.     

Electrical signs of impulse conduction in spinal motoneurons

An analysis has been made of the electrical responses recorded on the surface and within the substance of the first sacral spinal segment when the contained motoneurons are excited by single and repeated antidromic ventral root volleys. A succession of negative deflections, designated in order of increasing latency m, i, b, d, has been found. Each of those deflections possesses some physiological property or properties to distinguish it from the remainder. Indicated by that fact is the conclusion that the successive deflections represent impulse conduction through successive parts of the motoneurons that differ in behavior, each from the others. Since the spinal cord constitutes a volume conductor the negative deflections are anteceded by a positive deflection at all points except that at which the axonal impulses first enter from the ventral root into the spinal cord. Frequently two or more negative deflections are recorded together in overlapping sequence, but for each deflection a region can be found in which the onset of that deflection marks the transition from prodromal positivity to negativity. Deflection m is characteristic of axonal spikes. Latent period is in keeping with known axonal conduction velocity. Refractory period is brief. The response represented by m is highly resistant to asphyxia. Maximal along the line of ventral root attachment and attenuating sharply therefrom, deflection m can be attributed only to axonal impulse conduction. Deflection i is encountered only within the cord, and is always associated with a deflection b. The i,b complex is recordable at loci immediately dorsal to regions from which m is recorded, and immediately ventral to points from which b is recorded in isolation from i. Except for its great sensitivity to asphyxia, deflection i has properties in common with those of m, but very different from those of b or d. To judge by properties i represents continuing axonal impulse conduction into a region, however, that is readily depolarized by asphyxia. Deflection b possesses a unique configuration in that the ascending limb is sloped progressively to the right indicating a sharp decrease in velocity of the antidromic impulses penetrating the b segment. A second antidromic volley will not conduct from i segment to b segment of the motoneurons unless separated from the first by nearly 1 msec. longer than is necessary for restimulation of axons. This value accords with somatic refractoriness determined by other means. Together with spatial considerations, the fact suggests that b represents antidromic invasion of cell bodies. Deflection d is ubiquitous, but in recordings from regions dorsal and lateral to the ventral horn, wherein an electrode is close to dendrites, but remote from other segments of motoneurons, d is the initial negative deflection. In latency d is variable to a degree that demands that it represent slow conduction through rather elongated structures. When associated with deflection b, deflection d may arise from the peak of b with the only notable discontinuity provided by the characteristically sloped rising phase of b. Deflection d records the occupation by antidromic impulses of the dendrites. Once dendrites have conducted a volley they will not again do so fully for some 120 msec. Embracing the several deflections, recorded impulse negativity in the motoneurons may endure for nearly 5 msec. When the axonal deflection m is recorded with minimal interference from somatic currents, it is followed by a reversal of sign to positivity that endures as long as impulse negativity can be traced elsewhere, demonstrating the existence of current flow from axons to somata as the latter are occupied by impulses. Note is taken of the fact that impulse conduction through motoneurons is followed by an interval, measurable to some 120 msec., during which after-currents flow. These currents denote the existence in parts of the intramedullary motoneurons of after-potentials the courses of which must differ in different parts of the neurons, otherwise nothing would be recorded. The location of sources and sinks is such as to indicate that a major fraction of the current flows between axons and somata. For approximately 45 msec. the direction of flow is from dendrites to axons. Thereafter, and for the remaining measurable duration, flow is from axons to dendrites.     

Analysis of nerve conduction block induced by direct current

The mechanisms of nerve conduction block induced by direct current (DC) were investigated using a lumped circuit model of the myelinated axon based on Frankenhaeuser–Huxley (FH) model. Four types of nerve conduction block were observed including anodal DC block, cathodal DC block, virtual anodal DC block, and virtual cathodal DC block. The concept of activating function was used to explain the blocking locations and relation between these different types of nerve block. Anodal/cathodal DC blocks occurred at the axonal nodes under the block electrode, while virtual anodal/cathodal DC blocks occurred at the nodes several millimeters away from the block electrode. Anodal or virtual anodal DC block was caused by hyperpolarization of the axon membrane resulting in the failure of activating sodium channels by the arriving action potential. Cathodal or virtual cathodal DC block was caused by depolarization of the axon membrane resulting in inactivation of the sodium channel. The threshold of cathodal DC block was lower than anodal DC block in most conditions. The threshold of virtual anodal/cathodal blocks was about three to five times higher than the threshold of anodal/cathodal blocks. The blocking threshold was decreased with an increase of axonal diameter, a decrease of electrode distance to axon, or an increase of temperature. This simulation study, which revealed four possible mechanisms of nerve conduction block in myelinated axons induced by DC current, can guide future animal experiments as well as optimize the design of electrodes to block nerve conduction in neuroprosthetic applications.     

Promoting myelin repair and return of function in multiple sclerosis

Multiple sclerosis (MS) is an inflammatory demyelinating disease of the CNS. Conduction block in demyelinated axons underlies early neurological symptoms, but axonal transection and neuronal loss are believed to be responsible for more permanent chronic deficits. Several therapies are approved for treatment of relapsing-remitting MS, all of which are immunoregulatory and clinically proven to reduce the rate of lesion formation and exacerbation. However, existing approaches are only partially effective in preventing the onset of disability in MS patients, and novel treatments to protect myelin-producing oligodendrocytes and enhance myelin repair may improve long-term outcomes. Studies in vivo in genetically modified mice have assisted in the characterization of mechanisms underlying the generation of neuropathology in MS patients, and have identified potential avenues for oligodendrocyte protection and myelin repair. However, no treatments are yet approved that target these areas directly, and in addition, the relationship between demyelination and axonal transection in the lesions of the disease remains unclear. Here, we review translational research targeting oligodendrocyte protection and myelin repair in models of autoimmune demyelination, and their potential relevance as therapies in MS.     

The facilitated probability of quantal secretion within an array of calcium channels of an active zone at the amphibian neuromuscular junction

A Monte Carlo analysis has been made of the phenomenon of facilitation, whereby a conditioning impulse leaves nerve terminals in a state of heightened release of quanta by a subsequent test impulse, this state persisting for periods of hundreds of milliseconds. It is shown that a quantitative account of facilitation at the amphibian neuromuscular junction can be given if the exocytosis is triggered by the combined action of a low-affinity calcium-binding molecule at the site of exocytosis and a high-affinity calcium-binding molecule some distance away. The kinetic properties and spatial distribution of these molecules at the amphibian neuromuscular junction are arrived at by considering the appropriate values that the relevant parameters must take to successfully account for the experimentally observed amplitude and time course of decline of F1 and F2 facilitation after a conditioning impulse, as well as the growth of facilitation during short trains of impulses. This model of facilitation correctly predicts the effects on facilitation of exogenous buffers such as BAPTA during short trains of impulses. In addition, it accounts for the relative invariance of the kinetics of quantal release due to test-conditioning sequences of impulses as well as due to change in the extent of calcium influx during an impulse.     

Effect of non-symmetric waveform on conduction block induced by high-frequency (kHz) biphasic stimulation in unmyelinated axon

The effect of a non-symmetric waveform on nerve conduction block induced by high-frequency biphasic stimulation is investigated using a lumped circuit model of the unmyelinated axon based on Hodgkin-Huxley equations. The simulation results reveal that the block threshold monotonically increases with the stimulation frequency for the symmetric stimulation waveform. However, a non-monotonic relationship between block threshold and stimulation frequency is observed when the stimulation waveform is non-symmetric. Constant activation of potassium channels by the high-frequency stimulation results in the increase of block threshold with increasing frequency. The non-symmetric waveform with a positive pulse 0.4–0.8 μs longer than the negative pulse blocks axonal conduction by hyperpolarizing the membrane and causes a decrease in block threshold as the frequency increases above 12–16 kHz. On the other hand, the non-symmetric waveform with a negative pulse 0.4–0.8 μs longer than the positive pulse blocks axonal conduction by depolarizing the membrane and causes a decrease in block threshold as the frequency increases above 40–53 kHz. This simulation study is important for understanding the potential mechanisms underlying the nerve block observed in animal studies, and may also help to design new animal experiments to further improve the nerve block method for clinical applications.     

Remyelination — An effective means of neuroprotection

Remyelination following central nervous system (CNS) demyelination restores rapid saltatory conduction of action potentials and contributes to the maintenance of axonal integrity. This robust regenerative phenomenon stands in contrast to the limited repair capacity that is characteristic of CNS neuronal injury. However, despite its efficiency in experimental models and some clinical diseases, remyelination failure becomes an increasingly pronounced feature of the pathology of chronic multiple sclerosis (MS) lesions. Chronic demyelination predisposes axons to atrophy, an irreversible event that is a major pathological correlate of progressive functional decline. This has created a compelling case for developing therapies that promote remyelination: evidence from experimental animal models suggests that hormones may have a beneficial role to play in this regard.     

First node of Ranvier facilitates high-frequency burst encoding

In central neurons the first node of Ranvier is located at the first axonal branchpoint, ～ 100 μm from the axon initial segment where synaptic inputs are integrated and converted into action potentials (APs). Whether the first node contributes to this signal transformation is not well understood. Here it was found that in neocortical layer 5 axons, the first branchpoint is required for intrinsic high-frequency (≥ 100 Hz) AP bursts. Furthermore, block of nodal Na(+) channels or axotomy of the first node in intrinsically bursting neurons depolarized the somatic AP voltage threshold (～ 5 mV) and eliminated APs selectively within a high-frequency cluster in response to steady currents or simulated synaptic inputs. These results indicate that nodal persistent Na(+) current exerts an anterograde influence on AP initiation in the axon initial segment, revealing a computational role of the first node of Ranvier beyond conduction of the propagating AP.     

Effects of bupivacaine on the membrane properties of nerve cell soma

While the effects of local anaesthetics on axonal conduction and axonal membrane have been extensively studied, there is little information about the actions of these agents on nerve cell soma. Therefore, the effects of the amide local anaesthetic bupivacaine on the electrophysiologic properties of the nerve cell soma were studied on isolated superfused superior cervical ganglia of rats. Administration of 100-200 nM of bupivacaine to the preparation produced marked changes in membrane properties of the cell soma. The resting membrane potential did not change, but the membrane resistance decreased 20% (P less than 0.01). The firing threshold, the action potential duration at 50% of maximal amplitude, and the intracellular current threshold for firing the cells increased significantly (P less than 0.01), while the action potential amplitude decreased significantly (P less than 0.01), before its complete blockade. The results show that the cell soma is a major site of action of local anaesthetics. The implication of the results is that when local anaesthetics are applied to areas where cell bodies and processes (axons and dendrites) are present together, such as during celiac plexus block, lumbar sympathetic block, stellate ganglion block, etc., they will all be effectively depressed and/or blocked.     

Cytoskeletal transition at the paranodes: the Achilles' heel of myelinated axons

Myelination organizes axons into distinct domains that allow nerve impulses to propagate in a saltatory manner. The edges of the myelin sheath are sealed at the paranodes by axon-glial junctions that have a crucial role in organizing the axonal cytoskeleton. Here we propose a model in which the myelinated axons depend on the axon-glial junctions to stabilize the cytoskeletal transition at the paranodes. Thus paranodal regions are likely to be particularly susceptible to damage induced by demyelinating diseases such as multiple sclerosis.     

Conduction in bundles of demyelinated nerve fibers: computer simulation

This study presents a model of action potential propagation in bundles of myelinated nerve fibers. The model combines the single-cable formulation of Goldman and Albus (1967) with a basic representation of the ephaptic interaction among the fibers. We analyze first the behavior of the conduction velocity (CV) under the change of the various conductance parameters and temperature. The main parameter influencing the CV is the fast sodium conductance, and the dependence of CV on the temperature is linear up to 30 degrees C. The increase of myelin thickness above its normal value (5 microm) gives a slight increase in CV. The CV of the single fiber decreases monotonically with the disruption of myelin, but the breakdown is abrupt. There is always conduction until the thickness is larger than 2% of its original value, at which with at this point a sharp transition of CV to zero occurs. Also, the increase of temperature can block conduction. At 5% of the original thickness there is still spike propagation, but an increase of 2 degrees C causes conduction block. These results are consistent with clinical observations. Computer simulations are performed to show how the CV is affected by local damage to the myelin sheath, temperature alterations, and increased ephaptic coupling (i.e., coupling of electrical origin due to the electric neutrality of all the nerve) in the case of fiber bundles. The ephaptic interaction is included in the model. Synchronous impulse transmission and the formation of "condensed" pulse states are found. Electric impulses with a delay of 0.5 ms are presented to the system, and the numerical results show that, for increasing coupling, the impulses tend to adjust their speed and become synchronized. Other interesting phenomena are that spurious spikes are likely to be generated when ephaptic interaction is raised and that damaged axons suffering conduction block can be brought into conduction by the normal functioning fibers surrounding them. This is seen also in the case of a large number of fibers (N=500). When all the fibers are stimulated simultaneously, the conduction velocity is found to be strongly dependent on the level of ephaptic coupling and a sensible reduction is observed with respect to the propagation along an isolated axon even for low coupling level. As in the case of three fibers, spikes tend to lock and form collective impulses that propagate slowly in the nerve. On the other hand, if only 10% of fibers are stimulated by an external input, the conduction velocity is only 2% less than that along a single axon. We found a threshold value for the ephaptic coupling such that for lower values it is impossible to recruit the damaged fibers into conduction, for values of the coupling equal to this threshold only one fiber can be restored by the nondamaged fibers, and for values larger than the threshold an increasing number of fibers can return to normal functioning. We get values of the ephaptic coupling such that 25% of axons can be damaged without change of the collective conduction.     

A comparison of the effect of experimental general anaesthetics on nerve impulse blockade and on a proteinaceous target site

While it has been reported that general anaesthetics inhibit the enzyme luciferase and thus reduce the light output of the reaction with luciferin, we find that in squid giant axons injected with luciferin and luciferase, treatment with experimental general anaesthetics at concentrations sufficient to block axonal conduction leads to an increase in the light production by the reaction. This potentiation of the protein activity is best observed when luciferin concentration is above the apparent association constant. Our findings raise doubts regarding the suitability of luciferase as a model for the target region of general anesthetic action.