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.     

Effect of noradrenaline on the electrical and contractile properties of smooth muscle cells in the pulmonary artery]

Experiments were performed on the smooth muscle cells of rabbit a. pulmonalis using the microelectrode technique. No spontaneous electrical or mechanical activity was recorded in normal Krebs solution. The current-voltage relation in these smooth muscle cells showed marked rectification. No changes in the isometric tension were observed due to the anodal or cathodal stimulating currents. Strong depolarization of the muscle cells produced only local potentials on the cathelectrotone which never developed into a spike. Noradrenaline (10(-8) g/ml) caused depolarization of the 5-7 mV in the muscle cell membrane and a considerable contraction of the muscle strip as well. Under such conditions the contractile apparatus of the muscle cells became sensible to the resting potential level. Anodal stimulation was accompanied by relaxation of the muscle strip, whereas cathodal stimulation--by its contraction. The alpha-adrenoblocking agent (phentolamine) blocked the effect of noradrenaline evidencing the fact that noradrenaline exerted its excitatory action on the smooth muscle cells of the a. pulmonalis through the alpha-adrenoreceptors.     

The relations between prepotential,resting potential,and latent period in frog muscle fibers

1. Prepotentials and action potentials were recorded from amphibian striated muscle fibers. Intracellular electrodes were used for stimulating and recording. The resting potential was varied from 55 to 120 mv. by alterations of the KCl concentration of the Ringer's fluid. The magnitude of the prepotential at the initiation of the spike potential was measured and compared to the resting potential and the latent period (time between stimulus "make" and excitation). The magnitude of this prepotential varied with the resting potential. 2. A large prepotential or cathodal depolarization was required to excite a fiber with a high resting potential. If a fiber with a high resting potential fired late (long latency), the adequate prepotential was larger than if the fiber fired early. Fibers with low resting potentials had smaller adequate prepotentials. Also, the adequate prepotential was independent of the latent period, in these depolarized fibers. 3. If the concentration of Ca⁺⁺ was increased tenfold, the adequate prepotential of depolarized fibers became strongly dependent upon the latency. 4. Fibers with large or normal resting potentials were prone to respond repetitively during the passage of long duration shock, whereas depolarized and Ca⁺⁺-treated fibers were not. 5. The so-called critical membrane potential (which is defined as the transmembrane potential at the point of excitation) was not independent of the resting potential.     

Galvanic Stimulation of Volvox globator II. Mechanism of Galvanic Response, an Effect of Calcium Displacement

Slight increases or decreases in calcium ions in solutions which supported the growth of Volvox globator colonies caused the colonies to fall to the bottoms of their containers. High speed cinematography (600 frames/sec) showed that the flagella beat normally (21/sec) in balanced electrolyte solutions which have calcium concentrations between 0.5 and 1.0 mM. When colonies were placed in 10.0 or 0.0 mM CaCl₂ solutions, flagellar beating disappeared within 1 hr. The cessation of flagellar beating was reversible when colonies were replaced in the balanced solution. The Volvox cell wall has been shown to be a fairly good cation-exchanger with calcium ions acting as the counterion to the fixed negative change. Colonies that were photopositive and gave a cathodal galvanotaxis responded to DC electrical potentials by producing solution patterns that were indicators of colony electronegativity. Colony resistance to electroosmotic flow was compared in potassium and calcium solutions. When colonies were placed in darkness for 24 hr and stimulated by DC electrical potentials, their cation-exchange properties became reduced and the cell walls appeared thinner. Application of a high DC electrical potential to dark-adapted colonies caused the colonies to shrink on their anode sides (anodal contraction). Other workers have found that the flagella on the anodal sides of dark-adapted colonies ceased beating during DC electrical stimulation. It is hypothesized that the electric current caused an increase of calcium ions on the anodal side of the colony that inhibited the flagellar mechanism of beating on that side. It is also hypothesized that the galvanotaxis associated with light-adapted (photopositive) colonies was due to calcium displacements in the colony cell walls that affected the flagellar beating on both sides of the colony.     

Effect of sodium,potassium, and calcium ions on electroreceptor function in catfish

Impulses from single electroreceptors (small pit organs) of catfish (Ictalurus nebulosus) were recorded during stimulation by square pulses. Solutions with different concentrations of potassium, sodium, and calcium ions were applied to the pore of the receptor. Solutions with a low CaCl₂ concentration did not alter the responses of the receptor. Calcium ions in concentrations of over 5 mM increased the threshold of the response to electrical stimulation. The threshold to anodal stimulation was increased in solutions of 2 mM sodium and potassium and no response was given to a cathodal stimulus. The effect of 2 mM solutions of NaCl and KCl was abolished by the addition of 0.4 mM CaCl₂ or by application of a long anodal stimulus of high intensity (10⁻⁸∓10⁻⁷ A/mm²). Increasing the potassium ion concentration to 10–20 mM restored normal receptor function but a further increase led to elevation of the threshold. The action of an electric current is compared with the action of the ions.     

Chloride action potentials and currents in embryonic skeletal muscle of the chick

Chloride-dependent action potentials were elicited from embryonic skeletal muscle fibers of the chick during the last week of in ovo development. The duration of the action potentials was extremely long (greater than 8 sec). The action potentials were reversibly blocked by the stilbene derivative, SITS, a specific blocker of chloride permeability. Using patch clamp pipettes, in which the intracellular chloride concentration was controlled and with other types of ion channels blocked, the membrane potential at the peak of the action potential closely coincided with the chloride equilibrium potential calculated from the Nernst equation. These data indicate that activation of a chloride-selective conductance underlies the long duration action potential. The occurrence of the chloride-dependent action potential was found to increase during embryonic development. The percentage of fibers that displayed the action potential increased from approximately 20% at embryonic day 13 to approximately 70% at hatching. Chloride-dependent action potentials were not found in adult fibers. The voltage and time-dependent currents underlying the action potential were recorded under voltage clamp using the whole-cell version of the patch pipette technique. The reversal potential of the currents was found to shift with the chloride concentration gradient in a manner predicted by the Nernst equation, and the currents were blocked by SITS. These data indicate that chloride ions were the charge carriers. The conductance was activated by depolarization and exhibited very slow activation and deactivation kinetics.     

Propagation of action potentials in squid giant axons. Repetitive firing at regions of membrane inhomogeneities

Effects of reduction in potassium conductance on impulse conduction were studied in squid giant axons. Internal perfusion of axons with tetraethylammonium (TEA) ions reduces G K and causes the duration of action potential to be increased up to 300 ms. This prolongation of action potentials does not change their conduction velocity. The shape of these propagating action potentials is similar to membrane action potentials in TEA. Axons with regions of differing membrane potassium conductances are obtained by perfusing the axon trunk and one of its two main branches with TEA after the second branch has been filled with normal perfusing solution. Although the latter is initially free of TEA, this ion diffuses in slowly. Up until a large amount of TEA has diffused into the second branch, action potentials in the two branches have very different durations. During this period, membrane regions with prolonged action potentials are a source of depolarizing current for the other, and repetitive activity may be initiated at transitional regions. After a single stimulus in either axon region, interactions between action potentials of different durations usually led to rebound, or a short burst, of action potentials. Complex interactions between two axon regions whose action potentials have different durations resembles electric activity recorded during some cardiac arrhythmias.     

Subtle Paranodal Injury Slows Impulse Conduction in a Mathematical Model of Myelinated Axons

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

Calcium transients in asymmetrically activated skeletal muscle fibers.

Skeletal muscle fibers of the frog Rana temporaria were held just taut and stimulated transversely by unidirectional electrical fields. We observed the reversible effects of stimulus duration (0.1-100 ms) and strength on action potentials, intracellular Ca2+ transients (monitored by aequorin), and contractile force during fixed-end contractions. Long duration stimuli (e.g., 10 ms) induced a maintained depolarization on the cathodal side of a cell and a maintained hyperpolarization on its anodal side. The hyperpolarization of the side facing the anode prevented the action potential from reaching mechanical threshold during strong stimuli. Variation of the duration or strength of a stimulus changed the luminescent response from a fiber injected with aequorin. Thus, the intracellular Ca2+ released during excitation-contraction coupling could be changed by the stimulus parameters. Prolongation of a stimulus at field strengths above 1.1 x rheobase decreased the amplitude of aequorin signals and the force of contractions. The decreases in aequorin and force signals from a given fiber paralleled one another and depended on the stimulus strength, but not on the stimulus polarity. These changes were completely reversible for stimulus strengths up to at least 4.2 x rheobase. The graded decreases in membrane depolarization, aequorin signals, and contractile force were correlated with the previously described folding of myofibrils in fibers allowed to shorten in response to the application of a long duration stimulus. The changes in aequorin signals and force suggest an absence of myofilament activation by Ca2+ in the section of the fiber closest to the anode. The results imply that injected aequorin distributes circumferentially in frog muscle with a coefficient of at least 10(-7) cm2/s, which is not remarkably different from the previously measured coefficient of 5 x 10(-8) cm2/s for its diffusion lengthwise.     

The role of passive calcium influx through the cell membrane in galvanotaxis

Passive calcium influx is one of the theories to explain the cathodal galvanotaxis of cells that utilize the electric field to guide their motion. When exposed to an electric field, the intracellular fluid becomes polarized, leading to positive charge accumulation on the cathodal side and negative charge accumulation on the anodal side. The negative charge on the anodal side attracts extracellular calcium ions, increasing the anodal calcium concentration, which is supposed to decrease the mobile properties of this side. Unfortunately, this model does not capture the Ca²⁺ dynamics after its presentation to the intracellular fluid. The ions cannot permanently accumulate on the anodal side because that would build a potential drop across the cytoplasm leading to an ionic current, which would carry positive ions (not only Ca²⁺) from the anodal to the cathodal part through the cytoplasm. If the cytoplasmic conductance for Ca²⁺ is low enough compared to the membrane conductance, the theory could correctly predict the actual behavior. If the ions move through the cytoplasm at a faster rate, compensating for the passive influx, this theory may fail. This paper contains a discussion of the regimes of validity for this theory.     

Postnatal development of electrophysiological properties of pacemaker cells in the rabbit

Using glass microelectrodes, the authors measured basic electrophysiological parameters of the true pacemaker cells of the rabbit in an attempt to elucidate the postnatal drop in the frequency of action potentials produced by the sinoatrial node. The average rate of slow diastolic depolarization falls markedly between birth and adulthood, while the duration of the action potentials of the pacemaker cells become prolonged. These changes explain age-determined chronotropic development: the lower rate of slow diastolic depolarization in adult animals causes later attainment of the threshold; the longer action potential also contributes to prolongation of the cycle of membrane voltage changes in the pacemaker cells. The direct role of the postnatal increase in the maximum diastolic potential value is small, owing to a concomitant increase in the threshold potential value.     

Sodium nitroprusside increases pacemaker rhythm of sinoatrial nodes via nitric oxide-cGMP pathway

Effects of sodium nitroprusside (SNP), a nitric oxide donor, on the action potential in isolated guinea-pig sinoatrial nodes and ventricular papillary muscles were investigated. In the driven ventricular papillary muscle, SNP (10(-10)-10(-3) M) decreased the twitch tension in a concentration-dependent manner without significantly changing the configuration of action potential and the maximal velocity of depolarizing upstroke. In isolated sinoatrial nodes, SNP (10(-8)-10(-3) M) increased the pacemaker rhythm in a concentration-dependent manner. At 10(-5) M SNP, the pacemaker activity increased from 197.2+/-6.1 to 221.4+/-9.7 bpm. Changes of configuration of the action potential included a decrease of the duration of repolarization, i.e., from peak to the maximal diastolic potential (MDP), from 141.4+/-6.4 to 130.0+/-7.0 ms and an increase of the slope of the diastolic membrane potential from 101.6+/-5.3 to 116.5+/-7.3 mV/s (n=6, p<0.05). However, MDP and threshold potential were not significantly changed. Methylene blue (MB, 10(-5) M), a guanylate cyclase inhibitor, significantly decreased the pacemaker activity of the sinoatrial node by increasing the durations of repolarization and diastolic depolarization. After pretreatment with 10(-5) M MB, the effect of SNP was inhibited. The results indicate that nitric oxide, released from SNP, increases the pacemaker activity by enhancing the rates of repolarization and diastolic depolarization. These effects are possibly due to increases in delayed-rectifier K+ and diastolic slow inward currents, which are involved in a mechanism associated with the NO-cGMP pathway.     

Membrane potential responses controlling chemodispersal of Paramecium caudatum from quinine

Membrane potential responses of a ciliate protozoan Paramecium caudatum to the external application of quinine were investigated in relation to its motile activities. Wild-type specimens swimming in the reference solution did not enter into a quinine-containing (0.5 mM) test solution due to avoiding responses exhibited at the border between the two solutions, and therefore stayed in the reference solution (chemodispersal). Squirting of a quinine-containing test solution over a wild-type specimen evoked a train of action potentials superimposed on a depolarizing chemoreceptor potential. Squirting of a quinine-containing test solution over a CNR-mutant specimen defective in voltage-gated Ca²⁺ channel evoked only chemoreceptor potentials, which consisted of an initial transient depolarization, a following transient hyperpolarization and a sustained depolarization. A current-evoked action potential became larger in its amplitude and longer in its duration with the external application of quinine. Under the voltage-clamp condition, the fast inward current did not change whereas the delayed outward current decreased with the external application of quinine. It is concluded that quinine is a potent repellent for Paramecium because it produces a depolarizing chemoreceptor potential which evokes action potentials and prolongs the duration of the action potential.     

Paced monophasic and biphasic waveforms alter transmembrane potentials and metabolism of human fibroblasts

Resting transmembrane potential (TMP) of primary human fibroblast cells was altered in predictable directions by subjecting cell cultures to specific monophasic and biphasic waveforms. Cells electrically stimulated with an anodal pulse resulted in hyperpolarization while a cathodal waveform depolarized the TMP to below that of non-paced control cells. The biphasic waveform, consisting of an anodal pulse followed immediately by an inverse symmetric cathodal pulse, also lessened the TMP similar to that of the cathodal pulse. The effect of short-term pacing on the TMP can last up to 4 h before the potentials equilibrate back to baseline. While subjecting the cells to this electrical field stimulation did not appear to damage the integrity of the cells, the three paced electrical stimulation waves inhibited expansion of the cultures when compared to non-paced control cells. With longer pacing treatments, elongation of the cells and electrotaxis towards the anodal polarity were observed. Pacing the fibroblasts also resulted in modest, yet very statistically significant (and likely underestimated) changes to cellular adenosine-5'-triphosphate (ATP) levels, and cells undergoing anodal and biphasic (anodal/cathodal) stimulation also exhibited altered mitochondrial morphology. These observations indicate an active role of electrical currents, especially with anodal content, in affecting cellular metabolism and function, and help explain accumulating evidence of cellular alterations and clinical outcomes in pacing of the heart and other tissues in general.     

Effects of hirsutine and dihydrocorynantheine on the action potentials of sino-atrial node, atrium and ventricle

The effects of hirsutine, an indole alkaloid from Uncaria rhynchophylla MIQ. JACKSON with antihypertensive, negative chronotropic and antiarrhythmic activity, and its C3 structural epimer, dihydrocorynantheine, on membrane potentials of rabbit sino-atrial node and guinea-pig right ventricle and left atrium were studied with microelectrode techniques. In sino-atrial node preparations, hirsutine and dihydrocorynantheine (0.1 microM to 10 microM) concentration-dependently increased cycle length, decreased slope of the pacemaker depolarization (phase 4 depolarization), decreased maximum rate of rise and prolonged action potential duration. In atrial and ventricular preparations, both compounds (0.1 microM to 30 microM) concentration-dependently decreased maximum rate of rise and prolonged action potential duration. These results indicate that hirsutine and dihydrocorynantheine have direct effects on the action potential of cardiac muscle through inhibition of multiple ion channels, which may explain their negative chronotropic and antiarrhythmic activity.     

Depolarization of electroplax membrane in calcium-free ringer's solution

Summary Electrical stimulation, either cathodal or anodal, of the monocellular electroplax preparation in Ca-free Ringer's solution results in a sustained depolarization which is determined by the amount of current passed through the cell. The membrane potential recovers only when Ca is added again. These changes take place at the innervated side of the electroplax only. This depolarization of the membrane is pH-dependent; it depolarizes more at pH 6.0 than at pH 9.0. The membrane does not depolarize and the action potential is not blocked within an hour in Ca-free solution unless the cell is stimulated. The sustained depolarization is not prevented or reversed by curare, tetracaine, physostigmine, tetrodotoxin, and tetraethylammonium.After stimulation, the outward K current remains unchanged regardless of whether Ca is present. In contrast, the inward current is dependent on Ca in the outside solution on the innervated membrane; in the absence of Ca following stimulation, the inward K current is decreased.The depolarization by carbamylcholine is reduced in Ca-free and increased in Mgfree Ringer's solution. In contrast to the depolarization induced by electrical stimulation, these carbamylcholine depolarizations may be reversed by washing with Ca-free or Ca- and Mg-free Ringer's solution.     

植物性雌激素三羟异黄酮对家兔房室结细胞自发活动的电生理效应

本研究旨在应用经典玻璃微电极方法观察植物性雌激素三羟异黄酮（genistein,GST）对家兔房室结细胞自发活动的电生理效应及其作用机制。GST（10－100μmol/L）不仅以剂量依赖性方式抑制房室结起搏细胞的动作电位0相最大上升速度（Vmax）、4期去极化速率（VDD）、自发起搏频率（RPF）和动作电位幅值（APA）,而且延长复极化90％的时间（APD90）。如提高灌流液中钙离子浓度以及应用L型钙通道开放剂Bay K8644(0.25μmol/L）,则可拮抗GST对起搏细胞的上述电理效应,但NO合酶阻断剂L－NAME（0.5nmol/L）对GST的效应并无影响,以上结果提示,GST可抑制家兔房室结的自发活动,这些效应可能与其抑制钙离子内流有关,但此作用机制中并无NO参与。     

Membrane potential responses of paramecium caudatum to external Na+

The membrane potential responses of Paramecium caudatum to Na+ ions were examined to understand the mechanisms underlying the sensation of external inorganic ions in the ciliate by comparing the responses of the wild type and the behavioral mutant. Wild-type cells exhibited initial continuous backward swimming followed by repeated transient backward swimming in the Na+-containing test solution. A wild-type cell impaled by a microelectrode produced initial action potentials and a sustained depolarization to an application of the test solution. The prolonged depolarization, the depolarizing afterpotential, took place subsequently after stimulation. The ciliary reversal of the cell was closely associated with the depolarizing responses. When the application of the test solution was prolonged, the wild-type cell produced sustained depolarization overlapped by repeated transient depolarization. A behavioral mutant defective in the Ca2+ channel, CNR (caudatum non reversal), produced a sustained depolarization but no action potential or depolarizing afterpotential. The mutant cell responded to prolonged stimulation with sustained depolarization overlapped by transient depolarization, although it did not show backward swimming. The results suggest that Paramecium shows at least two kinds of membrane potential responses to Na+ ions: a depolarizing afterpotential mediating initial backward swimming and repeated transient depolarization responsible for the repeated transient backward swimming.     

Action potential prolongation: an effect of physostigmine (eserine) upon Retzius cells in the leech C.N.S

Physostigmine (PHY; eserine) prolongs the action potentials in the Retzius cells within leech ganglia to about 800 ms. The effect was reversible and occurred at concentrations of 1-10 mM which are several orders of magnitude greater than those required to inhibit cholinesterase. The prolonged action potentials showed an early, spike-like depolarization followed by a plateau. The initial depolarization exhibited a strong dependence on external Na+ while the amplitude of the plateau had somewhat less Na+ dependence: 52 and 24 mV/decade, respectively. The duration of the plateau was increased by elevating Na+ and decreased by elevating Ca2+. Increasing the action potential frequency, by intracellular stimulation, decreased both the duration and amplitude of the plateau. Neostigmine, di-isopropylphosphofluoridate, and acetylcholine did not prolong RZ action potentials. Thus, the membrane effects of physostigmine appear to be independent of any inhibition of cholinesterase or accumulation of acetylcholine.     

Simple models of stimulation of neurones in the brain by electric fields

The excitation of pyramidal cells in the motor cortex, produced by electric fields generated by distant electrodes or by electromagnetic induction, has been modelled. Linear, steady-state models of myelinated axons capture most of the geometrical aspects of neurone activation in electric fields. Some non-linear features can be approximated. Models with a proximal sealed-end and distal infinite axon, or of finite length, are both serviceable. Surface anodal stimulation produces hyperpolarisation of the proximal axon (closest to the anode) and depolarisation in the distal axon. The point of maximum depolarisation can be influenced by the location of the cathode (greater separation of anode and cathode causes more distal depolarisation). Axon bends can produce very localised depolarisation. Cathodal stimulation may be less effective than anodal as a result of anodal block of conduction of action potentials in the distal axon. The latencies of responses to anodal stimulation, recorded in the distal axon, will decrease as the stimulus strength is increased and the point of action potential initiation moves distally node by node. Larger jumps in latency will be produced when the point of action potential initiation moves from one axon bend to another.