Bau und Funktion isolierter markhaltiger Nervenfasern

Modelling in vivo action potential propagation along a giant axon

A partial differential equation model for the three-dimensional current flow in an excitable, unmyelinated axon is considered. Where the axon radius is significantly below a critical value R(crit) (that depends upon intra- and extra-cellular conductivity and ion channel conductance) the resistance of the intracellular space is significantly higher than that of the extracellular space, such that the potential outside the axon is uniformly small whilst the intracellular potential is approximated by the transmembrane potential. In turn, since the current flow is predominantly axial, it can be shown that the transmembrane potential is approximated by a solution to the one-dimensional cable equation. It is noted that the radius of the squid giant axon, investigated by (Hodgkin and Huxley 1952e), lies close to R(crit). This motivates us to apply the three-dimensional model to the squid giant axon and compare the results thus found to those obtained using the cable equation. In the context of the in vitro experiments conducted in (Hodgkin and Huxley 1952e) we find only a small difference between the wave profiles determined using these two different approaches and little difference between the speeds of action potential propagation predicted. This suggests that the cable equation approximation is accurate in this scenario. However when applied to the it in vivo setting, in which the conductivity of the surrounding tissue is considerably lower than that of the axoplasm, there are marked differences in both wave profile and speed of action potential propagation calculated using the two approaches. In particular, the cable equation significantly over predicts the increase in the velocity of propagation as axon radius increases. The consequences of these results are discussed in terms of the evolutionary costs associated with increasing the speed of action potential propagation by increasing axon radius.

Spiral ganglion cell site of excitation II: numerical model analysis

An anatomically based model of cochlear neuron electrophysiology has been developed and used to interpret the physiological responses of the auditory neuron to electrical summation and refractory pulse-pair stimuli. For summation pulses, the summation time constant, tau(sum), indicates the ability of the membrane to hold charge after cessation of a pulse. When a spiral ganglion cell with a cell body was simulated, the value of tau(sum) was elevated at the peripheral node adjacent to the cell body. For refraction pulses, the refraction time constant, tau(ref), indicates the duration of the relative refractory period of the membrane. In spiral ganglion cell simulations, tau(ref) was decreased at the peripheral node adjacent to the cell body and slightly elevated at other peripheral nodes. The extent of the cell body influence on tau(sum) and tau(ref) was high localized. Excitation times for the nodes adjacent to the cell body were either simultaneous or near simultaneous resulting in similar response latencies. Results indicate that values of tau(sum) and tau(ref) may be useful for distinguishing central and peripheral excitation sites while latency measures alone are not a good indication of site of excitation.

Axonal coding of action potentials in demyelinated nerve fibers

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

Techniques for intra-axonal recording of electrical activity from single nerve fiber in vitro and in situ

The method usually applied for recording the electrical activity of single nerve fiber is extracellular recording on isolated single nerve fiber. It is difficult to obtain stable and satisfactory results from direct intracellular recording of single nerve fiber, except for some special biological materials, such as Loligo giant axon. We present in this paper a technique for recording intracellular resting membrane potential and action potential from single nerve fiber both in vitro and in situ, using glass microelectrode and a special mirror-base plate for fixing the preparation. Besides, we also report a method for labelling single nerve fiber by means of injection of HRP into the axon.

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.

Action of tocainide and mexiletine on the excitation threshold of myelinated nerves

The antiarrhythmic drugs tocainide and mexiletine increase the excitation threshold and decrease the Na permeability (PNa) in nerves. Hodgkin-Huxley model calculations suggest that the PNa decrease is not sufficient to explain the threshold increase at low concentrations (less than 0.7 mM tocainide; less than 0.1 mM mexiletine). On the other hand, a drug action on the membrane surface potential would be consistent with results of recent experiments. To clarify this point electrophysiological experiments on single nerve fibres (Rana esculenta) were carried out. Changes of the threshold interval (difference between membrane and threshold potential; Vm-Vs) were measured and compared with the corresponding changes of Vm and Vs (Vs was measured by means of the transition voltage VTr). The quantitatively good agreement between changes of Vm-Vs and those of VTr suggests that at low concentrations tocainide and mexiletine decrease nerve excitability by acting mainly on the membrane surface charge. This possibility was supported by an independent electrophoresis experiment with myelin vesicles from the same nerve material.

Computer simulation of action potentials and afterpotentials in mammalian myelinated axons: the case for a lower resistance myelin sheath

Depolarizing afterpotentials, recorded in peripheral nerves [Barrett and Barrett (1982) J. Physiol., Lond. 323, 117-144] and spinal axons [Blight and Someya (1985) Neuroscience 15, 1-12], have been interpreted as representing passive discharge of axolemmal capacitance. This interpretation requires a lower resistance pathway through the myelin sheath than previous measurements have suggested. A computer model was used to examine the contribution of the electrical characteristics of nerve fibers to action potential conduction and afterpotential generation. The model consisted of a resistance-capacitance network representing a chain of 20 internodes. The resistances of node, internode and myelin sheath, deduced from observations in the accompanying paper, [Blight and Someya (1985) Neuroscience, 15] were found to produce suitable length and time constants, and prolonged afterpotentials, when inserted into the model. Similar length and time constants were found using a conventional model of the axon, based on measurements from isolated peripheral fibers, but this did not reproduce the afterpotentials. Action-potential conduction velocity is enhanced by reducing the time constant and increasing the length constant. The problem of minimizing the internodal time constant was met in the conventional model through the low parallel resistance of the node, while in the new model it was met by reducing the resistance of the myelin sheath. The latter strategy required the nodal leakage resistance to be higher than values from single fiber measurements (ca 250 M omega rather than ca 50 M omega) in order to maintain the length constant similar to the conventional model. Simulation of the recorded potentials required the resistance of the myelin lamellae to be approx. 100 omega cm2. The model quantitatively reproduced the voltage response of the axon to injected current pulses and to propagated action potentials, using Frankenhaeuser-Huxley kinetics. [Frankenhaeuser and Huxley (1964) J. Physiol., Lond. 171, 302-315; Frankenhaeuser and Moore (1963) J. Physiol., Lond. 169, 431-437]. The short duration components of the afterpotential, observed in mammalian recordings were reproduced by assuming a leakage pathway in the myelin sheath, at the impalement site. The calculated lower resistance of the myelin sheath was such that it minimized the effective internodal time constant for a given nodal resistance. This appears to free the myelinated fiber from the alternative requirement for a high nodal leakage conductance.(ABSTRACT TRUNCATED AT 400 WORDS)

Three functions of sodium channels in the toad node of Ranvier are altered by trimethyloxonium ions

Voltage-clamped nodes of Ranvier of the toad Xenopus laevis were treated with trimethyloxonium ions (TMO) which are known to methylate carboxyl groups. TMO did not affect potassium channels but the sodium system was modified in three ways: a) the current was reduced, b) channels were rendered insensitive to tetrodotoxin (TTX) and c) the inactivation of all channels, TTX-resistant or not, was slowed in a potential range between 50 and 110 mV. Steady-state inactivation, however, was not changed. Presence of 100 nM TTX during TMO treatment prevented the production of TTX-resistant channels but did not hinder current reduction and slowing of inactivation. The TTX-resistant sodium channels were blocked by protons and had normal relative permeabilities to alkali metal ions. Repeated application of TMO further decreased the current and increased the relative amount of TTX-resistant channels; the slowing of inactivation, however, was quantitatively terminated after the first TMO treatment. It is concluded that the sodium channel contains at least three TMO-modifiable groups, which probably are carboxyl groups.

Discharge patterns of cat primary auditory fibers with electrical stimulation of the cochlea

Intact and destroyed cat cochleae were electrically stimulated through round window electrodes. Intact cochleae provided information about fiber properties with acoustic stimuli. With sinusoidal currents thresholds for synchronization were 4-68 microA rms. Thresholds were independent of the fiber's characteristic frequencies and thus of their places of origin in the intact cochleae. This shows large current spread. Phase-locking of the responses to electric stimulation was much stronger than it was to acoustic stimulation. Destroyed cochleae had no spontaneous activity and showed even stronger phase-locking. Thresholds obtained using 0.2 ms per phase biphasic pulse stimuli were 60-350 microApp. Action potentials were found to be released with as little as 0.3 ms latency. The neuronal responses to any electric stimulus differed considerably from the responses to corresponding acoustic stimuli. Vestibular fibers were easily activated by electric stimulation.

Electron microscopic serial section analysis of nodes of Ranvier in lumbar spinal roots of the cat: a morphometric study of nodal compartments in fibres of different sizes

Serially sectioned nodes of Ranvier from nerve fibres 2-20 micron in diameter of feline ventral and dorsal spinal roots were examined electron microscopically, reconstructed to scale and analysed morphometrically. The assumed 'fresh-state' value of several structural variables, considered to be of functional significance, were calculated by the use of compensation factors. The compensated data were plotted against fibre and axon diameters. It was calculated that the membranous area of the 'fresh-state' nodal axon segment increased more or less exponentially from less than 5 micron2 to 30 micron2 with increasing fibre diameter (D). Most variables associated with the nodal gap and the Schwann cell initially increased rapidly with D and then levelled out or even decreased in fibres with a D value greater than 8-12 micron. The area open for communication between the nodal axolemma and the endoneurial space was 30-100 times smaller than the membrane area of the nodal axolemma. The volume of the extracellular space in the nodal gap, outside the nodal axolemma, increased linearly from less than 0.1 micron3 to about 0.6 micron3 with increasing fibre size. The Schwann cell membrane area facing the nodal gap outnumbered the membrane area of the nodal axon by 10-15 times in nerve fibres with a D value between 5 and 15 microns. Some functional implications of the 'fresh-state' nodal model are discussed.

Changes of ultraviolet sensitivity of voltage-clamped sodium channels during their potential-induced conductance cycle

The effects of ultraviolet (UV) radiation at various intermediate states of excitation of the frog nerve membrane were measured under voltage-clamp conditions. By synchronizing a UV flash source (flash duration: 10 microseconds) with the stimulating voltage-clamp pulses it was possible to irradiate the membrane at any state of excitation. The resulting UV sensitivities gamma Na and gamma h of gNa and h infinity (V = 0) were compared with those found at the end of a hyperpolarizing prepulse (m = 0, h = 1). The irradiation always induced a decrease of both parameters, gNa and h infinity (V = 0), and the more "h-gates" there were in the closed position at the moment of irradiation, the more pronounced were the effects. The normalized sensitivity of both parameters can be described fairly well by linear relations of the form: gamma x(m,h)/ gamma x(0, 1) = 1 + Px(1 - h) + Qxm2h (x = Na or h). The coefficients P and Q depend on wavelength. The results suggest a conformational change of the nodal membrane during excitation and an interdependence of m and h to some extent.

Electrical properties of isolated demyelinated rat nerve fibres

Isolated large rat nerve fibres with diphtheria toxin induced paranodal demyelination were investigated. These fibres had increased membrane time constant of the demyelinated nodal segment, related to a large increase in capacitance (5 to 50 times). The resting conductance was less increased (2 to 3 times), meaning that the internodal axolemma has considerably higher resting resistance than the nodal membrane. A large variation in action potential amplitude was found which was unrelated to the size of the nodal widening and the passive electrical properties.

Ultrastructural observations on nodes of Ranvier from isolated single frog peripheral nerve fibres

A preparative procedure is described by which well-preserved nodes of Ranvier from isolated frog peripheral nerve fibres may be obtained. The following steps are crucial: careful and gentle dissection and isolation of a single nerve fibre, mounting in a chamber limiting lateral movements of the fibre during the fluid exchange, simultaneous glutaraldehyde-OsO4 fixation and embedding in the same chamber used for fixation. Serial sectioning of individual nodes from both motor and sensory fibres made it possible to reconstitute three-dimensional models of several nodes and to study their morphology extensively. In addition to well-known ultrastructural features of the nodal and paranodal architecture, evaginations of a nodal membrane containing mitochondria and outpouchings of the paranodal axoplasm containing vesicles are described.

On the evaluation of photoreceptor properties by micro-fluorimetric measurements of fluorochrome diffusion

By use of the microfluorimetric technique it is possible to study the diffusion of the fluorochrome di-dansylcystine (DDC) within isolated frog rod outer segments (ros) which are immobilysed in agarose gel. For this purpose, by a short hypotonic shock a leak is applied to one end of the ros. By this open end the DDC enters the rod and migrates through the whole outer segment. Following the propagation of the fluorescence boundary with time the cytoplasmatic diffusion constant can be determined if a chromatographic model is used to allow for the considerable binding of DDC to the inner membrane surface. With a binding constant K = 5 . 10(-4) cm the cytoplasmatic diffusion constant was found to be D = 1.3 . 10(-6) cm2/s whereas Dg = 2 . 10(-6 cm2/s and Dr = 3.5 . 10(-6) cm2/s were found in agarose gel or ringer solution, respectively. Using the mobility reduction factor given by D/Dr approximately equal to 0.4 to calculate the cytoplasmatic conductivity an inner resistance per length of 1.7 M omega/mu could be calculated for a frog rod which is in good agreement with corresponding data obtained from electrophysiological measurements.

A comparison of the stimulus-response curves of aortic and carotid sinus baroreceptors in decerebrated cats

Recordings of total nerve activity suggested differences in the sensitivities and working ranges between aortic and carotid sinus baroreceptors. This result however, conflicts with single fibre studies from isolated receptor zones. Thus it appeared of some interest to compare the function curves of aortic and carotid sinus baroreceptors in the intact animal. This was achieved by comparing the response characteristics of two groups of aortic and carotid sinus baroreceptors in decerebrated cats. One smaller group consisted of 11 receptor pairs, each member of the pair being studied simultaneneously in the same cat, and a larger group consisting of 98 aortic and 49 carotid sinus baroreceptors studied independently of each other. The response of each receptor to wide pressure variations was recorded by inflating and deflating an intraaortic catheter tip balloon. Function curves were derived by plotting receptor discharge in terms of spikes per second against mean aortic pressure. No significant differences were found either in the slope of the function curves or their mean pressures at minimum activity, the latter appearing to be set by the working blood pressure level. Thus it was concluded that aortic and carotid sinus baroreceptors differ neither in their sensitivities nor in their working ranges when in their physiological environment.

Temperature experiments on nerve and muscle membranes of frogs. Indications for a phase transition

The influence of temperature changes in the range of 25 degrees C to -6 degrees C on the time constants of Na activation (tau m) and inactivation (tau h) was studied in twitch muscle fibers and the node of Ranvier under voltage-clamp conditions. Arrhenius plots of tau m and tau h exhibit a change in activation enthalpy at temperatures below 10 degrees C. Cooling and subsequent heating induce a hysteresis in the temperature dependence of tau m and tau h; Ni2+ and UO22+ increase the hysteresis width. With fast temperature changes the gating kinetics relax to their new values more slowly than the temperature change. Hence, temperature must be changed more slowly than 5 degrees C/min if an additional apparent hysteresis due simply to this relaxation is to be avoided. The data are explained by the hypothesis of a phase transition in the membrane lipids. This conception is favoured over a temperature-induced change in protein conformation, since the neutral local anaesthetic benzocaine shows use-dependent block as if low temperature restricted the access of the drug through the lipid phase to its receptor.

Asymmetrical displacement currents in the membrane of frog myelinated nerve: early time course and effects of membrane potential

1. Asymmetrical displacement currents were studied in myelinated nerve fibres from Rana esculenta with a voltage clamp technique. 2. For brief pulses symmetrical with respect to a holding potential of--97mV, the asymmetry current flowing during pulses (on-response) exhibited a rising phase to a peak followed by an approximately exponential decline. After the pulses the rising phase in the off-response could not be resolved; the time constant varied about 2-fold with either size or duration of the pulse. 3. For longer pulses a second slower component could be detected both in on- and off-responses. 4. The rapidly declining on- and off-responses associated with brief pulses carried about the same charges Qon and Qoff. Increasing the duration of the pulse reduced Qoff. For all pulses tested Qoff approached about one fifth of Qmax. The reduction of Qoff was roughly characterised by time constants ranging between 1.5 and 0.5 ms for potentials between--25 and + 23 mV. Analysis of individual membrane currents confirmed that the capacity current after depolarizing pulses decreased with pulse length. 5. The effects of membrane potential on asymmetry current were studied by varying the level from which pulses were applied during 46.9ms prepulses in the range from--97 to--29mV. The fast and slow components of asymmetry current were affected differently. For potentials more positive than--90mV the fast on-response was reduced and reversed its sign at a potential 25mV more negative than the potential estimated from the steady-state charge distribution measured from--97mV.

Displacement currents in the node of Ranvier. Voltage and time dependence

The time course of the membrane currents in the node of Ranvier in which the sodium and potassium conductances have been blocked reveals asymmetries during and after the application of depolarizing and hyperpolarizing voltage-clamp pulses of identical size. Since 1. the integrals of the "on" and "off" current transients were found to be equal and opposite, 2. the change displaced reached saturation (about 140-10-minus 15 C/node) when the internal potential was taken to a sufficiently positive value during the depolarizing pulses and, 3. the size of the charge transferred was unaffected by temperature although its time constant had a large temperature coefficient (Q10 = 2.4), these currents to our opinion must result form charge movements confined to the membrane and, therefore, can be considered as non-liner displacement currents. The steady-state rearrangement of the charges is consistent with a Boltzmann distribution of charges (effective valence z = 1.65) between two configurations characterized by different energy levels. The midpoint potential of the distribution curve is -33.7mV and its maximum slope, kT/ZE, is 14.9 mV. Following changes in membrane potential the charges undergo a first order transition between these states. We propose that these displacement currents arise from a redistribution of the charges involved in the sodium gating system.