[Membrane potential and action potential of skeletal muscular fibers of the adrenalectomized rat].
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Action Potentials↗
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[Effects of furosemide on endocochlear potentials, auditory action potentials and summating potentials and the changes of inner ear pathology].
Guinea pigs were injected with furosemide 50 mg/kg (group A) and 25mg/kg (group B). Two minutes after injection, EP of group A decreased to -13.9mv while that of group B decreased to +65 mv. Also, AP of group A disappeared, and recovered at 8.5 mins. while AP amplitude of group B decreased to 78%. The SP value of group A changed from -14.5mv to +23.4mv 1 min after injection and returned to negative polarity in 12 min. Edema of stria vascularis was observed under light microscope. Transmission electron microscope showed edema between marginal cells and intermedia cells, cytoplasm of the marginal cell protruded to the cochlear duct, and cell membrane of outer hair cell folded. The finding of this study illustrates that furosemide inhibits the transportation of the active ions of cochlear duct tissue resulting in decrease of EP and alters the function of hair cells causing the change of AP amplitude. -SP depends on the ion transportation, the polarity can be inversed while large dosage of furosemide was used.
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Dependence of the linear model for the nerve compound action potential on the single fibre action potential waveform.
The linear model of the nerve compound action potential (CAP) depends on the assumed waveform for the single fibre action potential (SFAP). A general method has been developed to investigate the influence of the unknown features of the SFAP on the estimation of nerve fibre conduction velocity (CV) distribution. A SFAP waveform is considered consistent with the model and the experimental data if recorded and reconstructed CAPS fit and the distribution is physically meaningful. Experimental CAPS were monopolarly recorded using surface electrodes over the median nerve at the wrist. To fit the model, SFAP waveforms must satisfy some internal relationship. The most important feature is that the ratio between positive and negative areas of the SFAP is almost one and does not vary in different subjects and recording sites. Many SFAP waveforms fit the model, and the relative conduction velocity distributions may be very different. These must be regarded as conventional distributions. As for inter-subject comparison, the dependence of the method on the recording site has been reduced by choosing the place where stimulus intensity and relative motor response amplitude have given values. In this recording environment CV distributions of normal subjects can be properly compared using the same SFAP and deviations from normality evidenced.
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Far-field potentials due to action potentials traversing curved nerves, reaching cut nerve ends, and crossing boundaries between cylindrical volumes.
A previously published computer simulation was tested in a biological preparation by recording action potentials from frog sciatic nerves within a volume conductor filled with Ringer's solution. Traveling in a straight line, nerve action potentials traversed a constricted cylinder before crossing into a larger, hemicylindrical volume. Recordings from widely spaced electrodes in the larger volume demonstrated a potential associated with the action potential crossing the boundary between the two volumes. Another potential was associated with the action potential reaching the nerve's cut end. These potentials did not diminish in amplitude with increasing distance from the source. In other recordings, a potential associated with a bend in the nerve was found which was dependent upon the angle of the bend. These results indicate that the simple model of a dipole in a bounded sphere in which potentials decrease as a function of distance from the generator does not explain all potentials that can be observed under conditions that approximate human and animal recordings.
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Relation of monophasic action potential recorded with contact electrode to underlying transmembrane action potential properties in isolated cardiac tissues: a systematic microelectrode validation study.
Monophasic action potentials, recorded with contact non-suction electrodes, have been used both clinically and experimentally. However, to date no systematic microelectrode validation studies have been done to underlying myocardial cell populations from different myocardial regions with different transmembrane potential profiles. In the present study transmembrane action potential properties, recorded with standard microelectrodes, were compared with monophasic action potentials recorded with contact electrodes in three different (endocardium, epicardium, and free running Purkinje fibre) isolated canine preparations during pacing and during spontaneous automatic activity. The mean transmembrane durations at 50% and 90% repolarisations (APD50 and APD90) of 19-30 cells at a monophasic action potential recording site was not statistically significant from monophasic action potential duration in all three tissue preparations studied. However, in endocardial preparations, composed of superficial (1-2 cell layers) Purkinje fibres with deeper ventricular muscle cells, the APD50 (139(17) ms) and APD90 (181(26) ms) of monophasic action potentials more closely reflected (but not significantly different) the underlying deeper ventricular muscle cells (APD50 134(14) ms and APD90 167(15) ms) rather than the mean transmembrane action potential durations of the underlying most superficial Purkinje fibres (166(22) ms for APD50 and 210(30) ms for APD90) (p less than 0.025). Tetrodotoxin (TTX) at 1 x 10(-6) mol.litre-1 shortened Purkinje fibre action potential duration and slightly lengthened that of ventricular muscle. Simultaneously recorded monophasic action potential showed an intermediate change in action potential duration. Incremental pacing and applied single premature stimuli resulted in similar degrees of shortening of action potential duration for both monophasic action potential and transmembrane potential in all three preparations. In endocardial preparations, barium chloride (4 mmol.litre-1) superfusion induced early afterdepolarisations, and spontaneous phase 4 depolarisations (n = 6) in both Purkinje and ventricular muscle cells giving rise to spontaneous automatic activity. These abnormal automatic activities were accurately detected by simultaneous monophasic action potential recordings. Suppression of automaticity by verapamil (0.2-0.5 micrograms.ml-1) as confirmed by transmembrane action potential recordings were similarly detected by monophasic action potential recordings (ABSTRACT TRUNCATED AT 250 WORDS)
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Spontaneous subthreshold membrane potential fluctuations and action potential variability of rat corticostriatal and striatal neurons in vivo.
We measured the timing of spontaneous membrane potential fluctuations and action potentials of medial and lateral agranular corticostriatal and striatal neurons with the use of in vivo intracellular recordings in urethan-anesthetized rats. All neurons showed spontaneous subthreshold membrane potential shifts from 7 to 32 mV in amplitude, fluctuating between a hyperpolarized down state and depolarized up state. Action potentials arose only during the up state. The membrane potential state transitions showed a weak periodicity with a peak frequency near 1 Hz. The peak of the frequency spectra was broad in all neurons, indicating that the membrane potential fluctuations were not dominated by a single periodic function. At frequencies >1 Hz, the log of magnitude decreased linearly with the log of frequency in all neurons. No serial dependence was found for up and down state durations, or for the time between successive up or down state transitions, showing that the up and down state transitions are not due to superimposition of noisy inputs onto a single frequency. Monte Carlo simulations of stochastic synaptic inputs to a uniform finite cylinder showed that the Fourier spectra obtained for corticostriatal and striatal neurons are inconsistent with a Poisson-like synaptic input, demonstrating that the up state is not due to an increase in the strength of an unpatterned synaptic input. Frequency components arising from state transitions were separated from those arising from the smaller membrane potential fluctuations within each state. A larger proportion of the total signal was represented by the fluctuations within states, especially in the up state, than was predicted by the simulations. The individual state spectra did not correspond to those of random synaptic inputs, but reproduced the spectra of the up and down state transitions. This suggests that the process causing the state transitions and the process responsible for synaptic input may be the same. A high-frequency periodic component in the up states was found in the majority of the corticostriatal cells in the sample. The average size of the component was not different between neurons injected with QX-314 and control neurons. The high-frequency component was not seen in any of our sample of striatal cells. Corticostriatal and striatal neurons' coefficients of variation of interspike intervals ranged from 1.0 to 1.9. When interspike intervals including a down state were subtracted from the calculation, the coefficient of variation ranged from 0.4 to 1.1, indicating that a substantial proportion of spike interval variance was due to the subthreshold membrane potential fluctuations.
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Instability and triangulation of the action potential predict serious proarrhythmia, but action potential duration prolongation is antiarrhythmic.
BACKGROUND: Prolongation of action potential duration (APD) is considered a major antiarrhythmic mechanism (class 2I), but paradoxically, it frequently is also proarrhythmic (torsade de pointes). METHODS AND RESULTS: The cardiac electrophysiological effects of 702 chemicals (class 2I or HERG channel block) were studied in 1071 rabbit Langendorff-perfused hearts. Temporal instability of APD, triangulation (duration of phase 3 repolarization), reverse use-dependence, and induction of ectopic beats were measured. Instability, triangulation, and reverse use-dependence were found to be important determinants of proarrhythmia. Agents that lengthened the APD by >50 ms, with induction of instability, triangulation, and reverse use-dependence (n=59), induced proarrhythmia (primarily polymorphic ventricular tachycardia); in their absence (n=19), the same prolongation of APD induced no proarrhythmia but significant antiarrhythmia (P<0.001). Shortening of APD, when accompanied by instability and triangulation, was also markedly proarrhythmic (primarily monomorphic ventricular tachycardia). In experiments in which instability and triangulation were present, proarrhythmia declined with prolongation of APD, but this effect was not large enough to become antiarrhythmic. Only with agents without instability did prolongation of APD become antiarrhythmic. For 20 selected compounds, it was shown that instability of APD and triangulation observed in vitro were strong predictors of in vivo proarrhythmia (torsade de pointes). CONCLUSIONS: Lengthening of APD without instability or triangulation is not proarrhythmic but rather antiarrhythmic.
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[Experimental studies on the influence of anterior pituitary hormones on the action potential of the rat optic nerve. I. The action potential of the optic nerve of normal rats. II. The influence of prae-hormone and hypohorin upon the action potential of the optic nerve of the rat].
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Action Potentials↗
Reperfusion arrhythmias in isolated perfused pig hearts. Inhomogeneities in extracellular potassium, ST and TQ potentials, and transmembrane action potentials.
We recorded direct current electrograms and local [K+]o at multiple sites and transmembrane potentials at selected sites during reperfusion after 5 minutes and 10 minutes of regional ischemia in isolated perfused pig hearts. After 10 minutes of ischemia, the incidence of ventricular fibrillation (VF) was 38%. At 80-90 seconds after reperfusion, [K+]o was 0.8 mM less than in normal tissue in half of the reperfused tissue, especially in the border zone. This was associated with TQ elevation of +4.5 mV and large peaked T waves. The latter was caused by an abrupt decrease of action potential duration in reperfused tissue, leading to a difference of up to 165 msec with normal tissue. Reperfusion VF started with a closely coupled ventricular premature beat. Activation block between reperfused and normal tissue permitted reentrant activation, leading to VF. Pretreatment with ryanodine (10(-6) M) and reperfusion with elevated [K+] (both of which prevent delayed afterdepolarizations) did not prevent closely coupled ventricular premature beats or VF. Five minutes of ischemia never caused VF. K+ depletion and TQ elevation in the reperfused zone was less frequent and smaller (-0.4 mM and 1.8 mV, respectively). Peaked T waves did not occur, and shortening of the action potential duration was less. We conclude that extracellular K+ depletion and marked action potential duration shortening in the reperfused tissue play a role in the genesis of reperfusion VF, which is caused by reentry. The closely coupled ventricular premature beat that initiates reentry is not caused by delayed afterdepolarizations but most likely by intramural reentry.
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Motor unit action potential rate and motor unit action potential shape properties in subjects with work-related chronic pain.
The objective of this study was to investigate differences in motor control of the trapezius muscle in cases with work-related chronic pain, compared to healthy controls. Ten cases with chronic pain and 13 controls participated in the study. Electromyographic (EMG) signals were recorded from the upper trapezius during five computer work-related tasks. Motor control was assessed using global root-mean-square value (RMS(G)), motor unit action potential (MUAP) rate (number of MUAPs per second, MR) and two MUAP shape parameters, i.e. root-mean-square (RMS(MUAP)) and median frequency (FMED(MUAP)). MR and FMED(MUAP) were higher for the cases than for the controls (P < 0.05). RMS(MUAP) showed a trend for higher values in the chronic pain group (P < 0.13), whereas RMS(G) did not show a significant difference between the groups. The higher MR, FMED(MUAP) and the trend for higher RMS(MUAP) suggest that more high-threshold MUs contribute to low-level computer work-related tasks in chronic pain cases. Additionally, the results suggest that the input of the central nervous system to the muscle is higher in the cases with chronic pain.
Action Potentials↗
Ionic currents contributing to the action potential in single ventricular myocytes of the guinea pig studied with action potential clamp.
With the action potential clamp procedure we studied the contribution of various ionic currents to the action potential in single ventricular myocytes. Action potentials were elicited by a current pulse through the suction pipette and recorded by a computer. A representative action potential was then repetitively replayed to the same cell under voltage-clamp conditions. Successive pharmacological blocks of ionic currents allowed for the first time the measurement of the contribution of the L-type calcium current (ICa) and the [Ca2+]i-activated currents as well as the potassium current to the action potential. Experiments using caffeine as a tool to increase calcium release from the sarcoplasmic reticulum supported the idea that INaCa contributes to the plateau during the second half of the action potential and even lasts into diastole, whereas strong elevation of the intracellular [Ca]i during the action potential additionally activated the non-specific cation channel.
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Far-field potentials generated by action potentials of isolated frog sciatic nerves in a spherical volume.
Previous results in cylindrical volumes have shown that action potentials generate far-field potentials when experimental conditions are such that quadrupolar components of the action potential are reduced to an equivalent dipole. We now show that the same conclusions are also reached within a spherical volume, again recording far-field potentials from isolated bullfrog nerves. A mathematical proof is given that shows that in a sphere, antipodal electrodes primarily detect far-field potentials from dipole generators and not quadrupole generators. A revised conception of the 'far-field' in evoked responses is discussed which equates far-field recordings with dipole detection.
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Independent fluctuations of the round-window summating potential and compound action potential following the surgical induction of endolymphatic hydrops in the guinea pig.
The diagnosis of Menière's disease is classically based on the triad of symptoms including fluctuating hearing loss, tinnitus and vertigo. Modifications to the electrocochleographic response have been searched as a possible help in the diagnosis. Various authors have reported a tendency for an increased ratio of the summating potential (SP) to action potential (AP) which is generally thought to be due to an enhanced SP. However, the large variability between patients has precluded any clear-cut conclusion. This dispersion of data might represent real individual differences or might be attributed to unavoidable technical variations, such as electrode placement and/or precise control of stimulus levels. As an attempt to answer this issue, we employed an animal model of experimental hydrops in which these difficulties can be overcome by chronic implantation of round-window electrodes and carefully controlling the stimulus level. In the present study, the SP and AP were monitored over several months for different frequencies and different intensities. In the early period of fluctuating thresholds at low and mid frequencies, AP amplitude varied in loose correspondence with the fluctuating audiogram. The SP amplitude also varied apparently not associated with AP or threshold changes and no consistent increase in SP was observed. At a later stage when all thresholds were elevated both SP and AP diminished. In normal ears increases in the stimulus intensity induce an augmentation of SP/AP. In hydropic ears, at the period of fluctuating thresholds, the SP/AP growth curve was at first similar to that of controls but later became very variable for different animals, but in general much larger than normal.(ABSTRACT TRUNCATED AT 250 WORDS)
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In vitro validation of a new cardiac catheter technique for recording monophasic action potentials.
Monophasic action potential (MAP) recording with non-suction, 'contact' electrode catheters has been shown possible and safe during clinical catheterization, but direct validation of this new technique is lacking. We therefore recorded these contact electrode MAPs simultaneously with transmembrane action potentials (TAPs) from closely adjacent sites in perfused and superfused rabbit septum preparations and performed a quantitative comparison between the two signals for duration and area at 30, 60 and 90% repolarization. To obtain a variety of action potential durations and configurations for the comparison, the rate and rhythm of stimulation and the extracellular calcium or potassium ion concentration were changed. With action potential duration at 90% repolarization made to vary from 150 to 513 ms, the mean absolute difference +/- SD between the simultaneous intra- and extracellular recordings was 5.4 +/- 11.3 ms and the linear correlation coefficient was r = 0.96 +/- 0.03. Similar agreement between the two types of recordings was found for measurements for area and at 60 and 30% repolarization levels. These data confirm that MAPs recorded with this clinically safe contact electrode technique can be used to measure accurately the repolarization time course of transmembrane action potentials.
Animals↗
Slow repolarization phase of the intracellular action potential influences the motor unit action potential.
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Action Potentials↗
Endocochlear potentials and compound action potential recovery: functions in the C57BL/6J mouse.
The C57BL/6J mouse suffers from cochlear degeneration beginning at an early age and has been used as a model of age-related hearing loss (presbyacusis). Here, the endocochlear potential (EP) and compound action potential (CAP) responses were determined in one-, four-, 12- and 24-month-old C57BL/6J mice. CAP measures included thresholds to tone pips, input/output (I/O) functions, and recovery functions to conditioning tones. EP values among the four age groups did not differ significantly (P>0.05) in either the basal or apical turns. CAP thresholds were increased significantly by 10 to 30 dB in the four-month group compared to the one-month controls at 11.3, 16, 20, and 22.6 kHz. CAP I/O functions were shallower in the four-month group compared to controls at all frequencies. In the 12- and 24-month-old mice, CAP responses were absent, despite normal EP values in these animals. Recovery functions after conditioning tones were obtained at 8, 16, 20 and 22.6 kHz; the functions had fast and slow components at all frequencies tested in both the one- and four-month-old groups. The corresponding recovery curves were identical for both age groups, even with significant threshold shifts in the older group. The two component recovery curves provide the first physiological evidence that different spontaneous rate (SR) classes of auditory neurons exist in the C57BL/6J mouse. Moreover, the unchanged recovery functions in the older group suggest that there was no loss of activity of the low-SR fiber population with age under conditions where the EP remains stable, in contrast to the gerbil model of presbyacusis where there is a loss of low-SR fiber activity and EP does decline with age.
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Channel currents during spontaneous action potentials in embryonic chick heart cells. The action potential patch clamp.
Single-channel currents were recorded with the cell-attached patch-clamp technique from small clusters (2-20 cells) of spontaneously beating 7-d embryo ventricle cells. Because the preparation was rhythmically active, the trans-patch potential varied with the action potential (AP). The total current through the patch membrane was the patch action current (AC). ACs and APs could be recorded simultaneously, with two electrodes, or sequentially with one electrode. Channel activity, which varied depending on the number and type of channels in the patch, was present during normal cell firing. This method can reveal the kinetics and magnitudes of the specific currents that contributed to the AP, under conditions that reflect not only the time and voltage dependence of the channels, but also environmental factors that may influence channel behavior during the AP.
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Action potentials in chick atria. Ontogenetic changes in the dependence of tetrodotoxin-resistant action potentials on calcium, strontium, barium.
Action potentials were recorded from chick embryo atrial muscle cells bathed in Tyrode's solution. Tetrodotoxin (TTX), 3.1 muM, was added to block the early, transient, Na+-dependent conductance system. Rectangular stimuli were used to evoke action potentials the peak amplitude (Ep) of which depend on the external concentration of divalent cations, [Me2+]0. The relationship between Ep and [Me2+]0 shifted to the right with increasing age. For example, the slope of Ep was 33 +/- 2,22 +/- 1 and 11 +/- 3 mV per 10-fold change in [Ca2+]0 on the 9th, 12th, and 18th incubation days, respectively. In solutions with reduced [Ca2+]0, Ep increased when Ba2+ or Sr2+ was added to the bath. The potency of Me2+ in generating action potentials was Ba2+ greater than Sr2+ greater than Ca2+ and this sequence did not change during development. Action potential amplitude, which was reduced in 18-day preparations, was increased by isoproterenol (increased Ca2+ conductance, gCa2+) and by tetraethylammonium (TEA) ion (decreased K+ conductance, gK). The results show that (1) Me2+-dependent action potentials support membrane excitation in chick atrial cells treated with TTX, and (2) the ability of Me2+ to support action potentials decreases during ontogenesis. We conclude from these experiments that the ontogenetically related decrease in Me2+-induced action potentials is the result of a reduction in gMe2+/gK+ during stimulation.
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