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PubMed1975

Sodium and calcium components of the action potential in a developing skeletal muscle cell line.

1. Developmental changes in action potential properties were studied in a clonal rat skeletal muscle cell line. 2. Small action potentials were evoked in mononucleate myoblasts. No spike was seen in Na-free saline. A similar spike was evoked in a medium where all NaCl was replaced by LiCl. No spike was evoked when NaCl was replaced by CsCl. 3. Action potentials overshot zero membrane potential in multinucleate myotubes. The action potential was composed of two components, an initial fast spike and a hump on the falling phase or in some cases a distinct second peak. 4. Teh overshoot of the initial fast spike decreased when the external Na concentration was decreased. 5. In saline with 10 mM-Ca the second component often formed a distinct peak following the initial fast spike. A slow regenerative potential was evoked in Na-free media with a depolarizing current pulse. 6. In saline containing BaCl-2 instead of CaCl-2 there was always a second peak, the overshoot of which changed with external Ba concentration. A slow regenerative potential was evoked in Na-free, Ba-saline. The membrane conductance at the peak of the Ba-action potential was larger than in the resting state. 7. In adult rat skeletal muscle, the shape of the action potential was not changed when Ca was replaced by Ba. No action potential was evoked in Na-free Ba-saline or Ba-saline with tetrodotoxin (3 times 10-7 M). 8. The significance of the Ca component in the developing muscle is discussed.

Action Potentials↗

PubMed1985

A surface electrode array for detecting action potential trains of single motor units.

Action potentials of single motor units were detected by a linear surface electrode array placed perpendicular to the longitudinal axis of the biceps brachii. Twelve myoelectric signals were derived simultaneously from a voluntarily contracting muscle. Using a visual feedback control, 3 subjects produced spike trains of single motor unit action potentials (MUAPs) at a weak contraction. When the myoelectric signals showed an interference pattern at a moderate contraction, several MUAPs were isolated by a visual analysis. MUAPs occurring at about fixed intervals with constant amplitudes and with identical wave forms were presumed to be the action potential trains of single motor units. Reliable estimates of single MUAP wave forms were obtained by averaging and superimposing the detected signals at a timing of characteristic potential peaks. Then not only the firing rate of the spikes but also the territory and the wave form of single MUAPs were investigated. Most MUAPs had a sharp and symmetrical distribution of potentials on a skin surface along the muscle circumference, while some MUAPs showed complex wave forms with some separate potential peaks. The possible arrangement of muscle fibers belonging to the motor units was estimated from the MUAP wave forms.

Action Potentials↗

PubMed1972

Ionic differences between somatic and axonal action potentials in snail giant neurones.

1. The ionic requirements of the somatic and axonal action potentials of ;H' neurones of the snail Cryptomphallus aspersa were studied using intracellular micro-electrodes.2. The overshoot of the somatic action potential increased by 10 mV for a tenfold increase in [Ca(2+)](o). In calcium-free media the action potential decreased gradually to values of 50 to 90% of the control and they could be completely eliminated with 2 mM-EGTA. The maximum rate of rise also varied with [Ca(2+)](o).3. After 2 hr in sodium-free solution the somatic action potential decreased 6% in overshoot and 24% in rate of rise.4. The somatic action potential was not affected by TTX, 5 x 10(-6) g/ml. Procaine, 18 mM, reduced its rate of rise but did not eliminate it whereas 30 mM-CoCl(2) did.5. The size of the axonal action potential increased with increased [Na(+)](o), but decreased with an increase in [Ca(2+)](o).6. Procaine, 18 mM, abolished the axonal action potential whereas it was not affected by TTX, 5 x 10(-6) g/ml., nor, usually, by 30 mM-CoCl(2).7. The results obtained by studying the compound action potential of the nerves were similar to those from axonal action potentials.8. The possibility that the somatic action potential is mainly calcium dependent while the axonal action potential is mainly produced by sodium is discussed.

Action Potentials↗

PubMed1980

Calcium component to action potentials in rat pars intermedia cells.

1. The ionic dependence of the action potential of rat pars intermedia cells was investigated by using intracellular recording techniques. 2. In the presence of tetrodotoxin (TTX, 5 x 10(-6)M), the action potentials evoked by passing depolarizing current through the recording electrode were abolished, confirming that they are mainly dependent on Na; however, when tetraethylammonium (TEA, 10 mM) was added to the TTX-containing solution the imposed depolarizations triggered all-or-none regenerative potentials indicative of involvement of another ion. 3. There TTX-insensitive regenerative potentials persisted when the cells were perifused with Na-free solution but were severely reduced or abolished by Ca-free solution. This suggests that the ion producing these potentials is Ca. 4. These Ca action potentials were suppressed by Ni, Co and Mn in concentrations that did not suppress the "Na spikes' recorded in the absence of TTX and TEA. 5. Sr and Ba could substitute for Ca in maintaining the action potentials recorded in the presence of TTX. These ions also prolonged the duration of these action potentials. 6. The demonstration of a Ca component to the predominantly Na-dependent action potentials of pars intermedia cells heightens the possibility that these action potentials participate in the regulation of secretion.

Action Potentials↗

PubMed1988

Monitoring of motor action potentials after stimulation of the spinal cord.

We recorded motor action potentials in cats, using surface electrodes placed over the soleus muscle. The action potentials were generated by stimulating the spinal cord with electrodes in the epidural space at the level of the fifth or sixth thoracic vertebra. This also was done in humans, using the same methods of stimulating and recording, but the intensity of the stimulus was adjusted to produce little or no twitch of the paraspinal muscles. In the animal experiment, the motor action potential was abolished after transection of the pyramidal tract and was progressively attenuated with effective doses of a curare-like agent. We also tested the effect of distraction, using the same technique as is used in Harrington instrumentation, and found that the amount of distraction that caused reduction of the amplitude of the motor action potential of more than 50 per cent, when sustained for longer than seven minutes, caused permanent paraplegia in two cats. The evaluation of spinal evoked potentials that were obtained from epidural electrodes placed caudad to the level of distraction, and of motor action potentials that were recorded over the soleus muscle, following the same stimulus, showed a similar pattern of reduction after distraction in five of seven cats. The other two cats had irreversible reduction of motor action potential associated with unchanged spinal evoked potential, and both cats became paraplegic.(ABSTRACT TRUNCATED AT 250 WORDS)

Action Potentials↗

PubMed1993

Anion and cation modulation of the guinea-pig ventricular action potential during beta-adrenoceptor stimulation.

Modulation of the ventricular action potential by beta-adrenergic activation of Ca2+, K+ and cyclic adenosine monophosphate (cAMP)-dependent Cl- channels was assessed in enzymatically isolated guinea-pig ventricular myocytes. The effectiveness and relative selectivity of 9-anthracene carboxylic acid (9-AC), as an antagonist of cAMP-dependent Cl- channels was also tested. Membrane currents and action potentials were recorded using the conventional whole-cell variant of the patch-clamp technique or with the amphotericin B perforated-patch technique. The beta-adrenergic agonist isoproterenol either increased or decreased action potential duration depending on whether the dominant effect was on inward Ca2+ currents or on outward K+ or Cl- currents. When Ca2+ and K+ channel modulation was prevented by nisoldipine and low temperature respectively, beta-adrenergic activation of Cl- channels caused a significant reduction in action potential duration and a slight depolarization of the membrane potential. The beta-adrenergic-mediated effects were reversed by the Cl- channel blocker, 9-AC. In the absence of beta-adrenergic stimulation, 9-AC had no detectable effects on action potentials or Ca2+ currents. These results suggest that beta-adrenergic activation of Cl- channels is a potent mechanism for regulation of action potential duration and that 9-AC may be a useful, relatively specific, pharmacological tool for evaluating the physiological role of cAMP-activated Cl- channels in heart. 9-AC also reversed the ability of isoproterenol to antagonize prolongation of action potential duration by the class III antiarrhythmic agent E-4031.

Action Potentials↗

PubMed2001

Propagation of action potentials in dendrites depends on dendritic morphology.

Action potential propagation links information processing in different regions of the dendritic tree. To examine the contribution of dendritic morphology to the efficacy of propagation, simulations were performed in detailed reconstructions of eight different neuronal types. With identical complements of voltage-gated channels, different dendritic morphologies exhibit distinct patterns of propagation. Remarkably, the range of backpropagation efficacies observed experimentally can be reproduced by the variations in dendritic morphology alone. Dendritic geometry also determines the extent to which modulation of channel densities can affect propagation. Thus in Purkinje cells and dopamine neurons, backpropagation is relatively insensitive to changes in channel densities, whereas in pyramidal cells, backpropagation can be modulated over a wide range. We also demonstrate that forward propagation of dendritically initiated action potentials is influenced by morphology in a similar manner. We show that these functional consequences of the differences in dendritic geometries can be explained quantitatively using simple anatomical measures of dendritic branching patterns, which are captured in a reduced model of dendritic geometry. These findings indicate that differences in dendritic geometry act in concert with differences in voltage-gated channel density and kinetics to generate the diversity in dendritic action potential propagation observed between neurons. They also suggest that changes in dendritic geometry during development and plasticity will critically affect propagation. By determining the spatial pattern of action potential signaling, dendritic morphology thus helps to define the size and interdependence of functional compartments in the neuron.

Action Potentials↗

PubMed1988

Adaptation of sarcolemmal action potential mechanisms to chronic depolarization in denervated skeletal muscle.

Action potential properties were studied in rat extensor digitorum longus fibers, at different times after locally setting the membrane to a holding potential of -90 mV. Whereas in normal muscles holding potential duration had little effect on the action potential, the holding potential duration markedly influenced membrane excitability in the fibers previously depolarized by increasing the K+ concentration of the bathing medium. In this case, when the holding potential was prolonged from 20 to 180 s, action potential overshoot, maximum rate of rise, and maximum rate of fall increased 1.8-, 3.1-, and 1.8-fold, respectively. In the denervated muscle, overshoot and maximum rate of fall were dependent on the duration of holding potential application until denervation day 6, whereas maximum rate of rise was affected throughout the duration of this study (15 days of denervation). However, 180-s application of -90 mV holding potential elicited about a 2-fold increase of maximum rate of rise in the earlier denervation stages, and only a 1.5-fold increase at later times. These observations suggest that ultra-slow processes of Na+ conductance inactivation were less effective after 6 days of denervation. Correspondingly, extensor digitorum longus fibers acquired the ability to generate action potentials at a depolarized holding potential. The partial removal of ultra-slow Na+ inactivation after muscle denervation could substantially contribute to a general process of membrane adaptation, resulting in the capacity of voltage-dependent ion channels to operate in a condition of chronic depolarization.

Action Potentials↗

PubMed1975

The role of the positive dynamic current on the action potential of cardiac Purkinje fibers.

The role of positive dynamic current (chloride current) on action potenitals of cardiac Purkinje fibers was studied. The removal of most extracellular chloride ions brought about a slowing of the repolarization process. The most prominent effect was noted onthe initial rapid repolization (phase 1) of the action potentials. Purkinje action potentials showed marked and consistent slowing of phase 1 with increasing rate of stimulation. The removal of (Cl-)0 caused marked loss of these frequency dependency in phase 1 and this effect was more prominent with slow rate of stimulation. The slopes of phase 2 and phase 3 also changed with varying frequency of stimulations but these changes were not affected much by chloride removal. Chloride conductance at the peak of the positive dynamic current increased linearly with membrane voltages from -20 mV up to +20 mV. Above this voltage, the conductance reached a plateau. A steady-state current voltage relationship was not influenced much by the chloride removal except slight decrease 031% in membrane conductance near the resting membrane potential. The increase in frequency of pulses produced marked decrease in the positive dynamic current in the voltage clamp experiments. These results suggest that the positive dynamic current mainly contribute to the electrogenesis of phase 1 in Purkinje action potentials.

Action Potentials↗

PubMed1975

Propagation of action potentials in inhomogeneous axon regions.

Described are studies of propagation of action potentials through inhomogenous axon regions through experiments performed on squid giant axons and by computer simulations. The initial speed of propagation of the action potential is dependent upon the stimulus waveform. For a rectangular pulse of current, the action potential travel initally at a high speed that declines over the distance, reaching a constant speed of propagation at about 1-5 resting length constants; this distance depends on the stimulus strength. additional experiments studied the effects of changing the axon diameter and of introducing a temperature step. It was found that the propagated action potential suffers profound modification in shape and velocity as it reaches the region of transition. In both cases, it was possible to obtain reflected action potentials. A region of increased effective diameter was produced experimentally in the squid giant axon by insertion of an axial wire as usually employed in voltage clamps. It was found that the action potential, at the axial wire tip region, undergoes shape changes similar to those obtained tn simulations of a region of increased diameter as in a junction with the axon and soma in motor neurons. It is conducluded that the gaint axon can be used to reproduce simple electrical behaviors in other structures.-Ramón, F., R. W. Joyner and J.W. Moore. Propagation of action potentials in inhomogeneous axon regions.

Action Potentials↗

PubMed1993

The bioelectrical source in computing single muscle fiber action potentials.

Generally, single muscle fiber action potentials (SFAPs) are modeled as a convolution of the bioelectrical source (being the transmembrane current) with a weighting or transfer function, representing the electrical volume conduction. In practice, the intracellular action potential (IAP) rather than the transmembrane current is often used as the source, because the IAP is relatively easy to obtain under experimental conditions. Using a core conductor assumption, the transmembrane current equals the second derivative of the IAP. In previous articles, discrepancies were found between experimental and simulated SFAPs. Adaptations in the volume conductor slightly altered the simulation results. Another origin of discrepancy might be an erroneous description of the source. Therefore, in the present article, different sources were studied. First, an analytical description of the IAP was used. Furthermore, an experimental IAP, a special experimental SFAP, and a measured transmembrane current scaled to our experimental situation were applied. The results for the experimental IAP were comparable to those with the analytical IAP. The best agreement between experimental and simulated data was found for a measured transmembrane current as source, but differences are still apparent.

Action Potentials↗

PubMed2003

Slowing of rat diaphragm action potential depolarization by endurance treadmill training.

This study tested the hypothesis that the action potential properties of the diaphragm muscle are altered by endurance exercise treadmill training. Rats underwent treadmill running or sham training for 8 weeks, and intracellular electrophysiological recordings were subsequently performed in vitro. Diaphragm resting membrane potential was not altered by training. The maximal rate of action potential depolarization was reduced significantly by exercise training, from 551+/-16 to 445+/-15 mV/ms (P<0.00002). In contrast the rate of action potential repolarization was not significantly different between the two groups (P=0.25). Action potential height was significantly higher in control compared with trained muscle (84.5+/-1.0 vs. 78.4+/-1.2 mV, P<0.0005). The combination of slowed action depolarization and decreased peak action potential height resulted in no net change in action potential area. Thus treadmill running endurance exercise training slows rat diaphragm action potential depolarization but not repolarization, suggestive of altered Na+ but not K+ channel function.

Action Potentials↗

PubMed1990

[Effects of dauricine on slow action potentials in myocardium].

The effects of dauricine were examined on the slow action potentials induced by both high K+ (24 mmol/L) or TTX (40 mumol/L) in guinea pig papillary muscles and in sinoatrial node cells of rabbit. Dauricine 40 mumol/L decreased the maximal upstroke velocity (Vmax) of slow action potentials and prolonged the action potential duration at 50% repolarization (APD50). Bay K 8644 (50 nmol/L) increased the amplitude and Vmax of slow action potentials, which were antagonized by dauricine (40 mumol/L). An inhibitory influence of dauricine on the action potential amplitude was observed in papillary muscles superfused with Tyrode's solution containing high K+ and in rabbit sinoatrial node cells, but it was not with TTX. The resting potentials of the 3 slow action potentials were not affected by dauricine. The results suggest that dauricine possesses calcium-antagonistic effect.

Action Potentials↗

PubMed1990

Sodium and calcium currents in action potentials of rat somatotrophs: their possible functions in growth hormone secretion.

We report that both Na+ and Ca2+ currents are involved in the action potentials and in the hormone release from rat somatotrophs in primary culture. Single somatotrophs were identified by reverse hemolytic plaque assay (RHPA) and transmembrane voltage and currents were recorded using the whole-cell mode of the patch-clamp technique. Somatotrophs displayed a mean resting potential of -80mV and an average input resistance of 5.7G omega. Most of the cells showed spontaneous or evoked action potentials. Single action potentials or the initial spike in a burst were characterized by their high amplitude and short duration. Tetrodotoxin (TTX, 1 microM) blocked single action potentials and the initial spikes in a burst, whereas action potentials of long duration and low amplitude persisted. Cobalt (2 mM) plus TTX (1 microM) blocked all the action potentials. Voltage-clamp experiments confirmed the presence of both a TTX-sensitive Na+ current and Co2(+)-sensitive Ca2+ currents. TTX or Na(+)-free medium slightly decreased the basal release of GH but did not markedly modify hGRF-stimulated GH release. However, Co2+ (2 mM), which partially decreased the basal release, totally blocked hGRF-stimulated release. We conclude that (1) Na+ currents which initiate rapid action potentials may participate in spontaneous GH release; (2) Ca2+ currents, which give rise to long duration action potentials and membrane voltage fluctuation, are probably involved in both basal and hGRF-stimulated GH releases.

Action Potentials↗

PubMed1994

Canine colonic circular muscle generates action potentials without the pacemaker component.

Two dominant types of action potentials in canine colon are slow wave type action potentials (slow waves) and spike-like action potentials (SLAPs). The slow waves, originating at the submuscular surface where a network of interstitial cells of Cajal (ICCs) is found, possess a pacemaker component. Activation of the pacemaker component is insensitive to voltage changes and L-type calcium channel blockers, and is postulated to involve a metabolic clock sensitive to cyclic AMP. SLAPs are more prominent in the longitudinal muscle. To understand the contribution circular muscle cells make to the generation of these action potentials, a circular muscle preparation (devoid of the submuscular ICC-smooth muscle network, longitudinal muscle, and myenteric plexus) was developed. Circular muscle preparations were spontaneously quiescent, with a resting membrane potential of -62.9 +/- 0.6 mV. Ba2+ (0.5 mM) depolarized the cells to -51.8 +/- 0.6 mV and induced electrical oscillations with a frequency, duration, amplitude, and rate of rise equal to 6.6 +/- 0.4 cpm, 2.2 +/- 0.2 s, 19.4 +/- 0.9 mV, and 21.8 +/- 1.7 mV/s, respectively. In most cases, Ba(2+)-induced oscillations were preceded by a prepotential of 4.4 +/- 0.3 mV, with a rate of rise of 1.1 +/- 0.1 mV/s. Ba(2+)-induced oscillations were abolished by 1 microM D600 as well as by repolarization of 6-12 mV. Addition of 0.1 microM Bay K8644 in the presence of Ba2+ further depolarized circular muscle cells to -42.4 +/- 0.8 mV and increased the oscillation frequency to 16.8 +/- 1.8 cpm. The electrical oscillations induced in circular muscle preparations by Ba2+ and Bay K8644 were similar to the SLAPs exhibited by the isolated longitudinal muscle layer, indicating that generation of SLAPs is an intrinsic property of smooth muscle cells. Forskolin (1 microM), previously shown to dramatically decrease the frequency but not the amplitude of slow waves in preparations including the submuscular ICC network, decreased the amplitude of the Ba(2+)-induced oscillations in circular muscle preparations without changing the frequency. These results provide strong evidence for the hypothesis that the submuscular ICC-smooth muscle network is essential for the initiation of the pacemaker component of the colonic slow waves. The mechanism for regulating the frequency of slow waves is different from that responsible for the Ba(2+)-induced oscillations in circular muscle preparations. Circular muscle cells are shown to be excitable and capable of generating oscillatory activity dominated by L-type calcium channel activity, which is regulated by K+ conductance.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗

PubMed1980

Effects of verapamil on action potentials of Purkinje fibers.

The effects of verapamil (1 and 2 micro M) on automaticity, membrane responsiveness curve and action potential configuration were studied in canine Purkinje fibers using conventional microelectrode techniques. The drug effects were concentration dependent, and more statistically significant changes were obtained during superfusion with 2 micron M verapamil. In spontaneously active fibers, the rate of discharge was decreased due to a decrease in the phase 4 depolarization. Verapamil failed to produce a consistent shift in the membrane responsiveness curve (Vmax versus take-off membrane potentials), although the development of early, slow action potentials was blocked. This blocking resulted in an increased effective refractory period and a shortened membrane responsiveness curve at the lower end. Maximum diastolic potential and action potential amplitude were decreased, and action potential duration was increased by verapamil. The results suggest that verapamil may selectively depress or block the slower action potentials which are thought to be important in the genesis of ventricular arrhythmias.

Action Potentials↗

PubMed1992

[Role of catecholamines in action of nicotine on slow action potentials in guinea pig papillary muscles].

The action potential duration (APD) of histamine-induced slow action potentials (SAP) and force of contraction (FC) were potentiated by nicotine (0.6-1.0 mmol.L-1) on guinea pig papillary muscles in a concentration-dependent manner. In the presence of atropine, nicotine concentration dependently suppressed the action potential amplitude (APA), APD, the maximal upstroke velocity (Vmax), and FC in catecholamine-depleted (reserpine 2.5 mg.kg-1 ip, 15 h prior to the experiment) muscles. Nicotine (0.6 mmol.L-1) itself induced SAP and enhanced FC. These 2 effects were antagonized by verapamil. A linear relationship existed between APA of nicotine-induced SAP and 1g [Ca2+]0 with a slope of 23.2 mV for a 10-fold change in [Ca2+]0. These results suggested that the effects of nicotine on enhancing Isi were mediated by the release of catecholamines in myocardium.

Action Potentials↗

PubMed2003

Control of secretion by temporal patterns of action potentials in adrenal chromaffin cells.

Action potentials (APs) are the principal physiological stimuli for neurotransmitter secretion in neurons. Most studies on stimulus-secretion coupling have been performed under voltage clamp using artificial electrical stimuli. To investigate the modulatory effects of AP codes on neural secretion, we introduce a capacitance method to study AP-induced secretion in single cells. The action potential pattern was defined by a four-parameter "code function:" F(n, m, f, d). With this method, cell secretion evoked by stimulation with an AP code was quantified in real time by membrane capacitance (Cm) in adrenal chromaffin cells. We found, in addition to AP frequency (f), for a given number of APs, another parameter of the AP code, the number of AP bursts (m) in which the set of APs occurs, can effectively modulate cell secretion. Possible mechanisms of the m effect are depletion of the readily releasable pool and inactivation of Ca2+ channels during a burst of APs. The physiological m effect may play a key role in AP-mediated neural information transfer within a single neuron and among the elements of a neural network.

Action Potentials↗