Mammalian Nerve Fibres Research Articles

The mechanosensitive ion channel TRAAK is localized to the mammalian node of Ranvier.

TRAAK is a membrane tension-activated K+ channel that has been associated through behavioral studies to mechanical nociception. We used specific monoclonal antibodies in mice to show that TRAAK is localized exclusively to nodes of Ranvier, the action potential propagating elements of myelinated nerve fibers. Approximately 80 percent of myelinated nerve fibers throughout the central and peripheral nervous system contain TRAAK in what is likely an all-nodes or no-nodes per axon fashion. TRAAK is not observed at the axon initial segment where action potentials are first generated. We used polyclonal antibodies, the TRAAK inhibitor RU2 and node clamp amplifiers to demonstrate the presence and functional properties of TRAAK in rat nerve fibers. TRAAK contributes to the 'leak' K+ current in mammalian nerve fiber conduction by hyperpolarizing the resting membrane potential, thereby increasing Na+ channel availability for action potential propagation. We speculate on why nodes of Ranvier contain a mechanosensitive K+ channel.

Ephaptic interactions between myelinated nerve fibres of rodent peripheral nerves.

We report evidence that ephaptic interactions may occur between intact mammalian myelinated nerve fibres and not only between demyelinated or damaged mammalian nerve fibres or nerve cells as analysed in previous studies. The ephaptic interactions were investigated between nerve fibres traversing the lumbar dorsal roots and between bundles of fibres in the sciatic nerve in anaesthetized rats in vivo. The interactions were estimated by comparing the excitability of nerve fibres originating from one of the hindlimb nerves (peroneal or sural) under control conditions and when the stimulation of these fibres was combined with stimulation of another nerve (tibial). An increase in nerve volleys recorded from group I muscle afferents in the peroneal nerve and of the fastest skin afferents in the sural nerve was used as a measure of the increase in the excitability. The excitability of these fibres was increased during a fraction of a millisecond, coinciding with the period of passage of nerve impulses evoked by the conditioning stimulation of the tibial nerve. The degree of the increase was comparable to the increases in the excitability evoked by 1-2min lasting fibre polarization. Ephaptic interactions were found to be more potent and with longer lasting after-effects within the dorsal roots than within the sciatic nerve. We postulate that ephaptic interactions may result in the synchronization of information forwarded via neighbouring afferent nerve fibres prior to their entry into the spinal cord and thereby securing the propagation of nerve impulses across branching points within the spinal grey matter.

Modeling the interactions between stimulation and physiologically induced APs in a mammalian nerve fiber: dependence on frequency and fiber diameter.

Electrical stimulation of nerve fibers is used as a therapeutic tool to treat neurophysiological disorders. Despite efforts to model the effects of stimulation, its underlying mechanisms remain unclear. Current mechanistic models quantify the effects that the electrical field produces near the fiber but do not capture interactions between action potentials (APs) initiated by stimulus and APs initiated by underlying physiological activity. In this study, we aim to quantify the effects of stimulation frequency and fiber diameter on AP interactions involving collisions and loss of excitability. We constructed a mechanistic model of a myelinated nerve fiber receiving two inputs: the underlying physiological activity at the terminal end of the fiber, and an external stimulus applied to the middle of the fiber. We define conduction reliability as the percentage of physiological APs that make it to the somatic end of the nerve fiber. At low input frequencies, conduction reliability is greater than 95% and decreases with increasing frequency due to an increase in AP interactions. Conduction reliability is less sensitive to fiber diameter and only decreases slightly with increasing fiber diameter. Finally, both the number and type of AP interactions significantly vary with both input frequencies and fiber diameter. Modeling the interactions between APs initiated by stimulus and APs initiated by underlying physiological activity in a nerve fiber opens opportunities towards understanding mechanisms of electrical stimulation therapies.

Studying the interactions in a mammalian nerve fiber: a functional modeling approach.

Modern therapeutic interventions are increasingly favoring electrical stimulation to treat neurophysiological dis-orders. These therapies are associated with suboptimal efficacy since most neurostimulation devices operate in an open-loop manner $(i.e.$, stimulation settings like frequency, amplitude are preprogrammed). A closed-loop system can dynamically adjust stimulation parameters and may provide efficient therapies. Computational models used to design these systems vary in complexity which can adversely affect their real-time performance. In this study, we compare two models of varying degrees of complexity. We constructed two computational models of a myelinated nerve fiber (functional versus mechanistic) each receiving two inputs: the underlying physiological activity at one end of the fiber, and the external stimulus applied to the middle of the fiber. We then defined relay reliability as the percentage of physiological action potentials that make it to the other end of the nerve fiber. We applied the two inputs to the fiber at various frequencies and analyze reliability. We found that the functional model and the mechanistic model have similar reliability properties, but the functional model significantly decreases the computational complexity and simulation run time. This modeling effort is the first step towards understanding and designing closed loop, real-time neurostimulation devices.

Modeling electrical stimulation of mammalian nerve fibers: A mechanistic versus probabilistic approach.

Electrical neurostimulation is increasingly used over neuropharmacology to treat various diseases. Despite efforts to model the effects of electrical stimulation, its underlying mechanisms remain unclear. This is because current mechanistic models just quantify the effects that the electrical field produces near the fiber and do not capture interactions between stimulus-initiated action potentials (APs) and underlying physiological activity initiated APs. In this study, we aim to quantify and compare these interactions. We construct two computational models of a nerve fiber of varying degrees of complexity (probabilistic versus mechanistic) each receiving two inputs: the underlying physiological activity at one end of the fiber, and the external stimulus applied to the middle of the fiber. We then define reliability, R, as the percentage of physiological APs that make it to the other end of the nerve fiber. We apply the two inputs to the fiber at various frequencies and analyze reliability. We find that the probabilistic model captures relay properties for low input frequencies (<; 10 Hz) but then differs from the mechanistic model if either input has a larger frequency. This is because the probabilistic model only accounts for only (i) inter signal loss of excitability and (ii) collisions between stimulus-initiated action potentials (APs) and underlying physiological activity initiated APs. This first step towards modeling the interactions in a nerve fiber opens up opportunities towards understanding mechanisms of electrical stimulation therapies.

Optimization of a wearable pudendal nerve stimulator using computational models.

After spinal cord injury, lower urinary tract functions may be disrupted. Trans-rectal stimulation of the pudendal nerve may enable patients to regain these functions via minimally invasive means. Using a finite element model of a wearable trans-rectal stimulator in the pelvic region, and a computational model of mammalian nerve fiber, various electrode configurations and the corresponding required current levels were studied. A configuration requiring considerably lower current level than previously reported was identified. For this configuration, the strength-duration curve was simulated and the effect of different stimulus waveforms on the required current was studied. In addition, the study examined whether a multi-electrode device could selectively activate different terminal branches of the pudendal nerve.

Influence of Uncertainties in the Material Properties of Brain Tissue on the Probabilistic Volume of Tissue Activated

The aim of this study was to examine the influence of uncertainty of the material properties of brain tissue on the probabilistic voltage response and the probabilistic volume of tissue activated (VTA) in a volume conductor model of deep brain stimulation. To quantify the uncertainties of the desired quantities without changing the deterministic model, a nonintrusive projection method was used by approximating these quantities by a polynomial expansion on a multidimensional basis known as polynomial chaos. The coefficients of this expansion were computed with a multidimensional quadrature on sparse Smolyak grids. The deterministic model combines a finite element model based on a digital brain atlas and a multicompartmental model of mammalian nerve fibers. The material properties of brain tissue were modeled as uniform random parameters using data from several experimental studies. Different magnitudes of uncertainty in the material properties were computed to allow predictions on the resulting uncertainties in the desired quantities. The results showed a major contribution of the uncertainties in the electrical conductivity values of brain tissue on the voltage response as well as on the predicted VTA, while the influence of the uncertainties in the relative permittivity was negligible.

Fast Negative Feedback Enables Mammalian Auditory Nerve Fibers to Encode a Wide Dynamic Range of Sound Intensities

Mammalian auditory nerve fibers (ANF) are remarkable for being able to encode a 40 dB, or hundred fold, range of sound pressure levels into their firing rate. Most of the fibers are very sensitive and raise their quiescent spike rate by a small amount for a faint sound at auditory threshold. Then as the sound intensity is increased, they slowly increase their spike rate, with some fibers going up as high as ∼300 Hz. In this way mammals are able to combine sensitivity and wide dynamic range. They are also able to discern sounds embedded within background noise. ANF receive efferent feedback, which suggests that the fibers are readjusted according to the background noise in order to maximize the information content of their auditory spike trains. Inner hair cells activate currents in the unmyelinated distal dendrites of ANF where sound intensity is rate-coded into action potentials. We model this spike generator compartment as an attenuator that employs fast negative feedback. Input current induces rapid and proportional leak currents. This way ANF are able to have a linear frequency to input current (f-I) curve that has a wide dynamic range. The ANF spike generator remains very sensitive to threshold currents, but efferent feedback is able to lower its gain in response to noise.

High concentrations of dichloroacetate have minor effects on the vitality of the mammalian nerve fibers

Dichloroacetate has been used extensively in the treatment of cancer and genetic mitochondrial diseases, but there have been reports of dichloroacetate-induced peripheral neuropathy. In this study, the acute effects of sodium dichloroacetate on the peripheral nerve fibers were investigated, using an ex-vivo preparation, in the isolated sciatic nerve of the rat. The amplitude of the evoked nerve compound action potential (CAP) was measured to confirm the proper functioning of the nerve fibers. The half-vitality time [the time required to decrease the CAP to 50% of its initial value, here called inhibitory time 50% (IT50)], of the nerve fibers, which had been incubated in normal saline, was 30.4 ± 0.26 h (n=12). When the nerve fibers were incubated in 10 mmol/l of dichloroacetate, the IT50 was 29.7 ± 0.34 h (n=8), with no significant difference from the control (P>0.05). The fact that such a high concentration of dichloroacetate as 10 mmol/l had no effect on the parameters of the evoked CAP is an indication of the high tolerance of peripheral nerve fibers to this compound. When a concentration of 20 mmol/l of dichloroacetate was tested, a 15.2 ± 1.25% (n=12) inhibition in the CAP amplitude occurred, but although a relatively small population of nerve fibers was inactive, the vitality of the remaining active axons was not affected, with a final IT50 of 28.1 ± 0.64 h (n=12), with no significant difference from the IT50 of the control, which for this group of experiments was 28.1 ± 0.17 h (P>0.05). This moderate effect, with a 15.2 ± 1.25% decrease in the CAP amplitude, suggests that within the exposure limitation of the sciatic nerve preparation of 28-30 h, there could be a gradual development of certain biochemical changes leading to the early stages of dichloroacetate-induced neurotoxicity.

In vitro assessment of the effects of cadmium and zinc on mammalian nerve fibres

Zinc and cadmium are environmental contaminants that have a wide range of effects on the nervous system, but zinc is also considered to be an important metal in the human body. In this study the effect of CdCl2 and ZnCl2, at concentrations of 50,150, 250 and 500μM, on the nerve fibres of the sciatic nerve of the rat isolated in a three-chamber recording bath were studied. At the same concentrations, CdCl2 and ZnCl2 were found to have almost the same inhibitory effect on the compound action potential (CAP) of the nerve fibres. Their concentration–effect curves almost overlap and there was no significant difference in their EC50 which for CdCl2 is 250.1±18μM (n=5) and for ZnCl2 is 282.2±25μM (n=5) correspondingly (P>0.05). The no-observed-effect concentration (NOEC) was estimated to be 50–100μM for both metals. The identical inhibitory effect of both metals on the sciatic nerve fibres indicates a common mode of action which is related to their potential to generate reactive oxygen species (ROS).

Modeling the excitability of mammalian nerve fibers: influence of afterpotentials on the recovery cycle.

Human nerve fibers exhibit a distinct pattern of threshold fluctuation following a single action potential known as the recovery cycle. We developed geometrically and electrically accurate models of mammalian motor nerve fibers to gain insight into the biophysical mechanisms that underlie the changes in axonal excitability and regulate the recovery cycle. The models developed in this study incorporated a double cable structure, with explicit representation of the nodes of Ranvier, paranodal, and internodal sections of the axon as well as a finite impedance myelin sheath. These models were able to reproduce a wide range of experimental data on the excitation properties of mammalian myelinated nerve fibers. The combination of an accurate representation of the ion channels at the node (based on experimental studies of human, cat, and rat) and matching the geometry of the paranode, internode, and myelin to measured morphology (necessitating the double cable representation) were needed to match the model behavior to the experimental data. Following an action potential, the models generated both depolarizing (DAP) and hyperpolarizing (AHP) afterpotentials. The model results support the hypothesis that both active (persistent Na(+) channel activation) and passive (discharging of the internodal axolemma through the paranodal seal) mechanisms contributed to the DAP, while the AHP was generated solely through active (slow K(+) channel activation) mechanisms. The recovery cycle of the fiber was dependent on the DAP and AHP, as well as the time constant of activation and inactivation of the fast Na(+) conductance. We propose that experimentally documented differences in the action potential shape, strength-duration relationship, and the recovery cycle of motor and sensory nerve fibers can be attributed to kinetic differences in their nodal Na(+) conductances.

A specific blocker reveals the presence and function of the hyperpolarization-activated cation current I h in peripheral mammalian nerve fibres

Electrotonic responses recorded extra- or intracellularly from peripheral nerve preparations show a “sag” to hyperpolarizing current pulses. [10, 11, 20]The biophysical nature of this “inward rectification” is still under discussion since the phenomenon has not been noted at voltage-clamped single nerve fibres, and since Cs +, which reduces inward rectification, is not a specific ion channel blocker. In this study, we found that low micromolar concentrations of ZD 7288, a specific blocker of the hyperpolarization-activated cationic current (I h) in the soma of central mammalian neurones, [8]result in a complete block of inward rectification in the electrotonic responses of isolated rat spinal dorsal roots. In addition, ZD 7288 enhanced the activity-dependent slowing of conduction seen in compound C fibre action potentials of isolated rat vagus nerves and augmented the post-tetanic hyperpolarization following trains of action potentials in unmyelinated and myelinated axons. The data suggest that ZD 7288 is a potent blocker and a useful research tool for the study of hyperpolarization-activated inward rectification (I h) of peripheral nerve preparations.

Removal of retrogradely transported material from rat lumbosacral alpha-motor axons by paranodal axon-schwann cell networks

The aim of this study was to investigate the potential ability of Schwann cells to sequester axonally transported material via so called axon-Schwann cell networks (ASNs). These are entities consisting of sheets of Schwann cell adaxonal plasma membrane that invade the axon and segregate portions of axoplasm in paranodes of large myelinated mammalian nerve fibres. Rat hindlimb alpha-motor axons were examined in the L4-S1 ventral roots using light/fluorescence, confocal laser, and electron microscopy for detection of retrogradely transported red-fluorescent latex nanospheres taken up at a sciatic nerve crush, and intramuscularly injected horseradish peroxidase endocytosed by intact synaptic terminals. Survival times after tracer administration ranged from 27 hours to 4 weeks. During their retrograde transport toward the motor neuron perikarya, organelles carrying nanospheres/peroxidase accumulated at nodes of Ranvier, where they often appeared in close association with the paranodal myelin sheath. Serial section electron microscopy showed that many of the tracer-containing bodies were situated within ASN complexes, thereby being segregated from the main axon. Four weeks after nanosphere administration, several node-paranode regions still showed ASN-associated aggregations of spheres, some of which were situated in the adaxonal Schwann cell cytoplasm. The data establish the ability of Schwann cells to segregate material from motor axons with intact myelin sheaths, using the ASN as mediator. Taken together with our earlier observations that ASNs in alpha-motor axons are also rich in lysosomes, this process would allow a local elimination and secluded degradation of retrogradely transported foreign substances and degenerate organelles before reaching the motor neuron perikarya. In addition, ASNs may serve as sites for disposal of indigestable material.

A model of myelinated nerve fibres for electrical prosthesis design.

Starting with the spatially extended non-linear node model (Reilly et al., 1985), which incorporates Frankenhaeuser-Huxley non-linearities at each of several nodes in a row, a model is developed to describe many aspects of the behaviour of mammalian nerve fibres in a quantitative way. By taking into account the effects of temperature and by introducing a realistic nerve morphology, a good fit is obtained between the shape, duration and conduction velocity of simulated and in vivo action potentials in mammals. The resulting model correctly predicts the influence of physiological variations of body temperature on various aspects of nerve behaviour. It is shown that the absolute refractory period predicted by the model is within physiological ranges. Both in vivo and in the model, the spike amplitude and the spike conduction velocity are reduced in the relative refractory period. It is concluded that single-node models (although widely used) cannot replace this multiple nonlinear node model, as the stimulus repetition rates that can be followed by the simulated nerve fibre are limited by impulse conduction properties, rather than by the frequency following behaviour of a single node.

The action of flecainide acetate and its enantiomers on mammalian non-myelinated nerve fibres

The effects of flecainide acetate racemate and its two enantiomers on voltage-operated sodium and potassium channels and on the sodium pump activity of non-myelinated fibres of the guinea-pig vagus nerve were studied with the sucrose-gap method. The racemic mixture as well as the R enantiomer and S enantiomer in a concentration range of 3 x 10(-5)-3 x 10(-4) M caused suppression of the compound action potential, a diminished propagation velocity and a reduction of the post-tetanic potential (PTH), which was also observed with lidocaine. There was no significant difference in the effect caused by the enantiomers separately. The R enantiomer tended to evoke a stronger effect compared with the S enantiomer. However, the magnitude of the action is concentration-dependent. At a concentration less than 10(-4) M the action of the racemate was stronger than an equimolar concentration of the enantiomers. The activity of the sodium pump, defined by the time constant of post-tetanic potential decay, was affected at a concentration of 10(-4) M of the racemate, in contrast to lidocaine. The racemate and both enantiomers of flecainide acetate possess a similar local anaesthetic action, as reflected by the inhibition of voltage-operated sodium channels.

Ca2+-activated K+ current generating the after-hyperpolarization in mammalian nonmyelinated nerve fibres.

Results and discussion The results showed that the amplitude of the a.h.p. increased with decreasing the external concentration of potassium. The maximal amplitude was obtained in absence of K+ o, whereas at physiological concentration (5.6 mM) there remained only a slight rapid a.h.p. followed by a slower transient after-depolarization (a.d.p.). A further increase in K+ o produced an almost complete abolition of the a.h.p, and an even more pronounced a.d.p. Fig. 1 shows that both a.h.p, and a.d.p, are clearly Ca2+o-dependent. At physiological (5.6 mM) and lower than physiological concentrations of K+o (1.4 mM), the amplitude of the a.h.p, increased with increasing external Ca 2+. Decreasing Ca2+ o decreased the a.h.p, but a complete abolition of the a.h.p, by Ca2+-free solution was never obtained. In high K+o (11.2 mM), where the after-potential is composed of mainly the depolarizing component, the a.d.p, increased with increasing Ca2+ o, and was completely absent when Ca2+o was withdrawn. The Ca2+-sensitivity of the after-potential was further tested by blocking the voltage-sensitive Ca 2+ channels

Membranströme und Aktionspotentiale markhaltiger Nervenfasern und ihre Beeinflussung durch Diphenylhydantoin und Tokainid

A comparison of the ionic currents in single myelinated nerve fibres isolated from the sciatic nerve of the rat and the frog was performed. The nodal membrane of rats lacked almost totally potential dependent potassium channels, whereas in the frog large potassium currents were recorded. Therefore, it was concluded that in mammalian nerve fibres repolarization of the action potential is brought about by the unspecific leakage current and the inactivation of the sodium channels. Phenytoin and tocainide reduced the Na current in nerve fibres of both species. This reduction was due to a potential and frequency dependent block of sodium channels. 50 microM phenytoin and 200 microM tocainide shifted the inactivation curve of the sodium system by 25 mV to more hyperpolarizing membrane potentials. In addition, a decrease in the time interval between repetitive depolarizations increased the amount of Na current reduction. This suggested that in the presence of the drugs the equilibrium between resting and inactive sodium channel states was shifted to more inactivation. The results were successfully interpreted in terms of the "modulated receptor hypothesis".

Calcium efflux and intracellular exchangeable calcium in mammalian nonmyelinated nerve fibers

Calcium efflux was measured in desheathed rabbit vagus nerves loaded with 45Ca2+. The effects of extracellular calcium, sodium, phosphate, potassium and lanthanum ions on the calcium efflux were investigated and the distribution of intracellular calcium determined by kinetic analysis of 45Ca2+ efflux profiles. The 45Ca2+ desaturation curve can be adequately described by three exponential terms. The rate constant of the first component (0.2 min-1) corresponds to an efflux from an extracellular compartment. The two slow components had rate constants of 0.03 and 0.08 min-1 and represent the efflux from two intracellular pools. The amounts of exchangeable calcium in these two pools, after a loading period of 150 min, were 0.170 and 0.102 mmol/kg wet weight, respectively. The total calcium efflux in physiological conditions amounted to about 24 fmol cm-2 sec-1. The magnitude of the two intracellular compartments as well as the total calcium efflux were markedly affected by extracellular phosphate, sodium and lanthanum, whereas the corresponding rate constants remained almost unchanged. Phosphate reversed the effect of sodium withdrawal on the calcium efflux: in the absence of phosphate, sodium withdrawal increased the calcium efflux to 224%, but in the presence of phosphate, sodium withdrawal decreased calcium efflux to 44%. Phosphate also affected the increase in calcium efflux produced by inhibitors of mitochondrial calcium uptake, suggesting that two different mitochondrial pools contribute to the control and regulation of intracellular calcium and of the transmembrane calcium transport.

Neurofilament gene expression: a major determinant of axonal caliber.

Within the wide spectrum of axonal diameters occurring in mammalian nerve fibers, each class of neurons has a relatively restricted range of axonal calibers. The control of caliber has functional significance because diameter is the principal determinant of conduction velocity in myelinated nerve fibers. Previous observations support the hypothesis that neurofilaments (NF) are major intrinsic determinants of axonal caliber in large myelinated nerve fibers. Following interruption of axons (axotomy) by crushing or cutting a peripheral nerve, caliber is reduced in the proximal axonal stumps, which extend from the cell bodies to the site of axotomy. (The distal axonal stumps, which are disconnected from the cell bodies, degenerate and are replaced by the outgrowth of regenerating axonal sprouts arising from the proximal stump). This reduction in axonal caliber in the proximal stumps is associated with a selective diminution in the amount of NF protein undergoing slow axonal transport in these axons, with a decrease in axonal NF content, and with reduced conduction velocity. The present report demonstrates that changes in axonal caliber after axotomy correlate with a selective alteration in NF gene expression. Hybridization with specific cDNAs was used to measure levels of mRNA encoding the 68-kDa neurofilament protein (NF68), beta-tubulin, and actin in lumbar sensory neurons of rat at various times after crushing the sciatic nerve. Between 4 and 42 days after axotomy by nerve crush, the levels of NF68 mRNA were reduced 2- to 3-fold. At the same times, the levels of tubulin and actin mRNAs were increased several-fold. These findings support the hypothesis that the expression of a single set of neuron-specific genes (encoding NF) directly determines axonal caliber, a feature of neuronal morphology with important consequences for physiology and behavior.

Differential sensitivity of amphibian nodal and paranodal K+ channels to 4-aminopyridine and TEA

Voltage-dependent K+ channels are blocked by several drugs, including 4-aminopyridine (4-AP) and tetraethylammonium (TEA). 4-AP is most widely used to localize K+ channels in mammalian and non-mammalian nerve fibers, but 4-AP and TEA alter various K+ channels and/or preparations in specific ways. The reason is not known, in part because dissociation constants for 4-AP and TEA have not been measured for nodal and internodal K+ channels in the same fibers. Smith and Schauf showed that the density of nodal versus paranodal K+ channels in frog nerves depends on fiber diameter. The size dependence was used to determine the relative sensitivity of nodal and internodal K+ channels to 4-AP and TEA, and to compare voltage- and time-dependent activation. The results show nodal and internodal K+ channels activate similarly. However, internodal channels are selectivity blocked by 4-AP while TEA is more effective on nodal channels. A high sensitivity of internodal K+ channels may explain why 4-AP improves symptoms in diseases such as multiple sclerosis.