Pairs Of Action Potentials Research Articles

O-36 Listening the sound of neuromuscular junction during voluntary contraction

Background Sound is an indispensable component of electromyography. Although neurophysiologists are familiar with the end-plate noise and spikes, it is preferred to measure the neuromuscular transmission during voluntary contraction in time domain that is expressed in microseconds (jitter). Present study is aimed to create an audible voice of jitter and enable the physician to notice the difference between a healthy and a diseased neuromuscular transmission by hearing. Material and methods During voluntary contraction of frontalis muscle, single-fiber-like action potential pairs were recorded by using smallest concentric needle with 1 kHz high-pass filtering. Traces harboring potential pairs acquired from healthy participants and myasthenia patients were transferred. Signals were de-noised, locations of the two highest peak’s amplitude and the difference between them were determined. The average of these differences was calculated (jitter). “The sound of jitter” can be composed from the difference values of the locations of two highest peak’s amplitude. These values are duplicated in same amount proportionally up to 48,000 samples for achieving a 5 s length of sound. Then, they were sampled with 8 kHz. “The sound of jitter” was heard by using a function in “Matlab”. Results Sound of a healthy junction is soft; but in patients, it’s louder and contains more high frequency components. Conclusions Increased jitter reflects high variability of neuromuscular transmission. Converting it to an audible sound serves a different perspective. By doing so, the only irregularity that is heard would be the failure in junction transmission rather than the irregular firing of motor unit.

Jitter Values on Voluntary Active Periocular Muscles of Healthy Subjects with Conventional (37 mm) Concentric Needle Electrode.

The aim of this study was to re-evaluate jitter values of healthy subjects in whom pairs of single-fiber-like potentials were recorded from voluntary activated periocular muscles using a disposable 37-mm concentric needle electrode (CNE) with 2-kHz low-cut filtering. We reviewed the recordings of 129 subjects (85 women; 44 men; mean age, 43.8±15.3 years). The m. frontalis group included 116 subjects, and the m. orbicularis oculi group included 18 subjects. Jitter values were expressed as the mean consecutive difference (MCD) of 20 different pairs. The mean MCD (n=2680) was 22.5±9.7 μs (range, 5-121 μs), and the upper 95% confidence limit (CL) was 39 μs. The mean of 134 MCD values for each subject was 22.5±3.7 μs (range, 15-33 μs), and the upper 95% CL was 30 μs. The outer limit of the 18th highest MCD values out of 20 recordings for each subject was 31.3±6.5 μs (range, 18-53 μs), with an upper 95% CL of 43.3 μs. Using a conventional 37-mm CNE with 2-kHz low-cut filtering may be a cost effective alternative to a single-fiber electrode in periocular muscles if strict criteria are used for acceptable signals. Jitter values of >44 μs that were calculated from single-fiber-like action potential pairs should alert the physician regarding the possibility of neuromuscular junction disorders and constitute an indication for a further diagnostic investigation.

Analysis of motor unit firing characteristics in patients with motor neuron diseases

This study was designed to evaluate firing rate variability in patients with upper/lower motor neuron disorders. Twenty healthy subjects and 19 patients with motor neuron disorders participated in the study. Consecutive motor unit action potential pairs from extensor digitorum communis (EDC) muscle were recorded from each subject with trigger-delay line mode. Patients with motor neuron disorders (17.7 ± 10.8 ms) showed significantly higher mean time variability of interpotential interval value than healthy volunteers (10.3 ± 0.1 ms) (p < 0.001).

Jitter analysis with concentric needle electrodes in the extensor digitorum communis for the diagnosis of myasthenia gravis: a pilot study

Single-fiber electromyography is the most sensitive neurophysiological test for the diagnosis of myasthenia gravis (MG), but its use is limited by the potential risk of transmission of infections. Jitter analysis with disposable concentric needle electrodes (CNEs) is therefore being investigated. This pilot study aimed to evaluate jitter analysis with CNEs for the diagnosis of MG. Forty-two healthy Chinese volunteers and 44 MG patients were prospectively enrolled. MG patients were classified according to the Osserman classification, and acetylcholine receptor antibody titer was measured. Jitter analysis with CNEs in the extensor digitorum communis and repetitive nerve stimulation (RNS) testing were performed. Jitter was expressed as the mean consecutive difference (MCD), and 20 action potential pairs were analyzed in each subject. The mean MCD in each subject and the mean individual MCD of all action potential pairs were compared between groups. The mean MCD and mean individual MCD were higher in MG patients ((42.3 ± 20.0) µs and (42.2 ± 26.0) µs) than in healthy volunteers ((23.0 ± 3.1) µs and (22.8 ± 7.5) µs). The area under the receiver operating characteristic curve for the mean MCD of MG patients and healthy volunteers combined was 0.85. The mean MCD and mean individual MCD were higher in generalized MG patients ((64.1 ± 18.5) µs and (63.6 ± 30.0) µs) than in ocular MG patients ((33.1 ± 12.0) µs and (33.2 ± 17.6) µs), and were higher in MG patients with abnormal RNS results ((57.2 ± 18.3) µs and (57.3 ± 29.2) µs) than in MG patients with normal RNS results ((32.9 ± 14.8) µs and (32.7 ± 18.3) µs). Abnormal RNS results were observed in 38.60% (17/44) of MG patients and abnormal jitter were observed in 72.70% (32/44) of MG patients. Jitter analysis with CNEs is feasible for the diagnosis of MG.

Gephyrin Regulates GABAergic and Glutamatergic Synaptic Transmission in Hippocampal Cell Cultures

Gephyrin is a scaffold protein essential for stabilizing glycine and GABA(A) receptors at inhibitory synapses. Here, recombinant intrabodies against gephyrin (scFv-gephyrin) were used to assess whether this protein exerts a transynaptic action on GABA and glutamate release. Pair recordings from interconnected hippocampal cells in culture revealed a reduced probability of GABA release in scFv-gephyrin-transfected neurons compared with controls. This effect was associated with a significant decrease in VGAT, the vesicular GABA transporter, and in neuroligin 2 (NLG2), a protein that, interacting with neurexins, ensures the cross-talk between the post- and presynaptic sites. Interestingly, hampering gephyrin function also produced a significant reduction in VGLUT, the vesicular glutamate transporter, an effect accompanied by a significant decrease in frequency of miniature excitatory postsynaptic currents. Overexpressing NLG2 in gephyrin-deprived neurons rescued GABAergic but not glutamatergic innervation, suggesting that the observed changes in the latter were not due to a homeostatic compensatory mechanism. Pulldown experiments demonstrated that gephyrin interacts not only with NLG2 but also with NLG1, the isoform enriched at excitatory synapses. These results suggest a key role of gephyrin in regulating transynaptic signaling at both inhibitory and excitatory synapses.

Synaptic Output of Individual Layer 4 Neurons in Guinea Pig Visual Cortex

More than 90% of geniculocortical axons from the dorsal lateral geniculate nucleus of the thalamus innervate layer 4 (L4) of V1 (primary visual cortex). Excitatory neurons, which comprise >80% of the neuronal population in L4, synapse mainly onto adjacent L4 neurons and layer 2/3 (L2/3) neurons. It has been suggested that intralaminar L4-L4 connections contribute to amplifying and refining thalamocortical signals before routing to L2/3. To unambiguously probe the properties of the synaptic outputs from these L4 excitatory neurons, we used multiple simultaneous whole-cell patch-clamp recording and stimulation from two to four neighboring L4 neurons. We recorded uEPSCs (evoked unitary synaptic currents) in response to pairs of action potentials elicited in single presynaptic L4 neurons for 102 L4 cell pairs and found that their properties are more diverse than previously described. Averaged unitary synaptic response peak amplitudes spanned almost three orders of magnitude, from 0.42 to 192.92 pA. Although connections were, on average, reliable (average failure rate, 25%), we recorded a previously unknown subset of unreliable (failure rates from 30 to 100%) and weak (averaged response amplitudes, <5 pA) connections. Reliable connections with high probability of neurotransmitter release responded to paired presynaptic pulses with depression, whereas unreliable connections underwent paired-pulse facilitation. Recordings from interconnected sets of L4 triplets revealed that synaptic response amplitudes and reliability were equally variable between independent cell pairs and those that shared a common presynaptic or postsynaptic cell, suggesting local perisynaptic influences on the development and/or state of synaptic function.

Reliability and Heterogeneity of Calcium Signaling at Single Presynaptic Boutons of Cerebellar Granule Cells

Activity-dependent elevation of calcium within presynaptic boutons regulates many aspects of synaptic transmission. Here, we examine presynaptic residual calcium (Ca(res)) transients in individual presynaptic boutons of cerebellar granule cells at near-physiological temperatures using two-photon microscopy. Properties of Ca(res) under conditions of zero-added buffer were determined by measuring Ca(res) transients while loading boutons to a steady-state indicator concentration. These experiments revealed that, in the absence of exogenous calcium buffers, a single action potential evokes transients of Ca(res) that vary widely in different boutons both in amplitude (400-900 nM) and time course (25-55 ms). Variation in calcium influx density, endogenous buffer capacity, and calcium extrusion density contribute to differences in Ca(res) among boutons. Heterogeneity in Ca(res) within different boutons suggests that plasticity can be regulated independently at different synapses arising from an individual granule cell. In a given bouton, Ca(res) signals were highly reproducible from trial to trial and failures of calcium influx were not observed. We find that a factor contributing to this reliability is that an action potential opens a large number of calcium channels (20-125) in a bouton. Presynaptic calcium signals were also used to assess the ability of granule cell axons to convey somatically generated action potentials to distant synapses. In response to pairs of action potentials or trains, granule cell boutons showed a remarkable ability to respond reliably at frequencies up to 500 Hz. Thus, individual boutons appear specialized for reliable calcium signaling during bursts of high-frequency activation such as those that are observed in vivo.

Activity-Dependent Modulation of Axonal Excitability in Unmyelinated Peripheral Rat Nerve Fibers by the 5-HT(3) Serotonin Receptor

Activity-dependent fluctuations in axonal excitability and changes in interspike intervals modify the conduction of trains of action potentials in unmyelinated peripheral nerve fibers. During inflammation of a nerve trunk, long stretches of axons are exposed to inflammatory mediators such as 5-hydroxytryptamine [5-HT]. In the present study, we have tested the effects of m-chlorophenylbiguanide (mCPBG), an agonist at the 5-HT(3) serotonin receptor, on activity- and potential-dependent variations in membrane threshold and conduction velocity of unmyelinated C-fiber axons of isolated rat sural nerve segments. The increase in axonal excitability during application of mCPBG was much stronger at higher frequencies of action potentials and/or during axonal membrane hyperpolarization. The effects on the postspike recovery cycle also depended on the rate of stimulation. At an action potential frequency of 1 Hz or in hyperpolarized axons, mCPBG produced a loss of superexcitability. In contrast, at 0.33 Hz, a small increase in the postspike subexcitability was observed. Similar effects on excitability changes were found when latency instead of threshold was recorded, but only at higher action potential frequencies: at 1.8 Hz, mCPBG increased conduction velocity and reduced postspike supernormality. The latter effect would increase the interspike interval if pairs of action potentials were conducted along several cm in an inflamed nerve trunk. These data indicate that activation of axonal 5-HT(3) receptors not only enhances membrane excitability but also modulates action potential trains in unmyelinated, including nociceptive, nerve fibers at high impulse rates.

Developmental regulation of the a-type potassium-channel current in hippocampal neurons: role of the kvβ1.1 subunit

The rapidly inactivating A-type K + current (I A) is prominent in hippocampal neurons; and the speed of its inactivation may regulate electrical excitability. The auxiliary K + channel subunit Kvβ1.1 confers fast inactivation to Shaker-related channels and is postulated to affect I A. Whole-cell patch clamp recordings of rat hippocampal pyramidal neurons in primary culture showed a developmental decrease in the time constant of inactivation (τ in) of voltage-gated K + currents: 17.9±1.5 ms in young neurons (5–7 days in vitro; n=53, mean±S.E.M.); 9.9±1.0 ms in mature neurons (12–15 days in vitro; n=72, mean±S.E.M., P<0.01). During the same developmental time, the level of Kvβ1.1 transcript increased more than two-fold in vitro and in vivo, determined by semi-quantitative reverse transcriptase-polymerase chain reaction for hippocampus. The hypothesis that up-regulation of Kvβ1.1 led to the changes in τ in was tested in vitro, using antisense knockdown. Kvβ1.1-specific antisense DNA was introduced with a modified herpes virus co-expressing enhanced green fluorescent protein and knockdown of Kvβ1.1 was verified by immunocytochemistry. Following transduction with the antisense virus, mature neurons reverted to τ in values characteristic of young neurons: 18.3±2.4 ms ( n=20). The effect of antisense knockdown on electrical excitability was tested using current-clamp protocols to induce repetitive firing. Treatment with the antisense virus increased the interspike interval over a range of membrane depolarization (baseline membrane potential=−40 to +20 mV). This effect was most pronounced at −40 mV, where the ISI of the first pair of action potentials was nearly doubled. These data indicate that Kvβ1.1 contributes to the developmental control of I A in hippocampal neurons and that the magnitude of effect is sufficient to regulate electrical excitability. Viral-mediated antisense knockdown of Kvβ1.1 is capable of decreasing the electrical excitability of post-mitotic hippocampal neurons, suggesting this approach has applicability to gene therapy of neurological diseases associated with hyperexcitability.

Spike-triggered dendritic calcium transients depend on synaptic activity in the cricket giant interneurons.

The relationship between electrical activity and spike-induced Ca2+ increases in dendrites was investigated in the identified wind-sensitive giant interneurons in the cricket. We applied a high-speed Ca2+ imaging technique to the giant interneurons, and succeeded in recording the transient Ca2+ increases (Ca2+ transients) induced by a single action potential, which was evoked by presynaptic stimulus to the sensory neurons. The dendritic Ca2+ transients evoked by a pair of action potentials accumulated when spike intervals were shorter than 100 ms. The amplitude of the Ca2+ transients induced by a train of spikes depended on the number of action potentials. When stimulation pulses evoking the same numbers of action potentials were separately applied to the ipsi- or contra-lateral cercal sensory nerves, the dendritic Ca2+ transients induced by these presynaptic stimuli were different in their amplitude. Furthermore, the side of presynaptic stimulation that evoked larger Ca2+ transients depended on the location of the recorded dendritic regions. This result means that the spike-triggered Ca2+ transients in dendrites depend on postsynaptic activity. It is proposed that Ca2+ entry through voltage-dependent Ca2+ channels activated by the action potentials will be enhanced by excitatory synaptic inputs at the dendrites in the cricket giant interneurons.

The return of the spike: postsynaptic action potentials and the induction of LTP and LTD.

. Formulated to ex- renewed interest in postsynaptic spiking and synapticplain how memories could be stored as alterations in plasticity and have refocused attention on the literalsynaptic strength, it reads: interpretation of Hebb’s postulate. The basic obser-vation that sparked this new era was the conclusiveWhen an axon of cell A is near enough to excite a cell demonstration (with simultaneous whole-cell patch re-B and repeatedly or persistently takes part in firing it, cordingataxonal, somatic,anddendriticloci onasinglesome growth process or metabolic change takes neuron) that Na

Electrophysiologic analysis of the actions of valproate on pyramidal neurons in the rat hippocampal slice.

Studies in invertebrates and cultured mammalian neurons suggested that valproate (VPA) mediates its main antiepileptic effect by slowing the recovery from inactivation of voltage-dependent sodium channels. This predicts an effect on the refractory period of the action potential and, consequently, on the bursting behavior of neurons. We investigated this prediction using intracellular and extracellular recording techniques in hippocampal slices prepared from adult rats. The refractory period (RFP) and the ratio of the slopes (SR) of a pair of action potentials were used as indices of the recovery from inactivation of sodium channels. They were measured by injecting a series of paired depolarizing current pulses into CA1 pyramidal neurons. No significant changes were observed in the RFP or SR measured during a 1-h recording period when VPA was bath-applied (1 mM), or when it was present in the recording electrode (10-50 mM). Lowering the temperature from 34.5 degrees C to 26.4 degrees C resulted in an increase of the RFP by 100% and a decrease of the SR by 40%. However, VPA did not affect any of the measured action potential parameters at this lower temperature. VPA was also without effect on the presynaptic fiber volley of axons recorded extracellularly in the stratum radiatum. The antidromic population spike was unaffected by VPA (2 mM), whereas phenytoin (50 microM) clearly affected this spike in the same slices. The absence of effect of VPA on each of the measured parameters could not be attributed to poor penetration through the slice because bath-applied VPA reduced the frequency of extracellularly recorded spontaneous interictal bursts, induced by bicuculline and elevated K+, within 10 min. These findings suggest that at least in the hippocampal slice the drug's principal antiepileptic effect cannot be explained by its action on voltage-dependent sodium channels.

Single fiber EMG and repetitive stimulation of the same muscle in myasthenia gravis.

We performed RNS and SFEMG studies of the same muscle in 46 patients with myasthenia gravis. Maximum decrement to 3-5-Hz stimulation before and after maximum voluntary exercise, percentage of action potential pairs with increased jitter and blocking, and mean MCD in each study were compared. A significant decrement (> 10% decrease in CMAP area or amplitude between the first and fourth response) was never found without increased jitter and impulse blocking on SFEMG. Increased jitter, blocking, and mean MCD were each correlated with maximum decrement (r > 0.61, P < 0.0001). We conclude that decrement to RNS and impulse blocking on SFEMG result from the same physiologic phenomenon, and that SFEMG is more sensitive at detecting disordered neuromuscular transmission given its ability to detect impulse blocking at levels below the resolution of RNS and increased neuromuscular jitter when there is not blocking.

Simulation of ECG from two pairs of action potentials

A simple model for simulation of ECG is presented with the purpose to mimic some common ECG configurations and to generate a hypothesis regarding their electrophysiological background. Action potentials (AP) were simulated on a personal computer from ion currents as described previously (Wohlfart & Arlock, 1993). The difference between two APs of a pair was used to create an electrogram (EG). The subendocardial AP of the pair was triggered by means of a simulated current injection and the subepicardial Ap was triggered from the first AP due to electric coupling within the pair. The subendocardial APs were of longer duration than the epicardial AP because of differences in background currents. A second pair of APs representing another ventricular site was simulated in an analogous way and this pair was activated somewhat later in time. ECG was calculated as the difference between the two EGs. Right-bundle branch block could be imitated by reducing the coupling between the APs representing the right ventricle. Left-bundle branch block was generated in an analogous way. ECG in acute myocardial infarction was created after making one of the epicardial APs ischaemic in appearance (reduced amplitude, short duration). Status-post infarction ECG (Q-wave and negative T) was produced by reducing the influence from the EG of the infarcted area. Negative T and increased R-wave as in hypertrophy could also be reproduced. Down sloping ST-segment and negative T as in subendocardial ischaemia was also possible to imitate. The simulations showed that biphasic T-waves or T and U-waves follows when the two EGs are separated properly in time.

Single-fiber EMG and repetitive stimulation of the same muscle in myasthenia gravis.

We performed RNS and SFEMG studies of the same muscle in 46 patients with myasthenia gravis. Maximum decrement to 3-5-Hz stimulation before and after maximum voluntary exercise, percentage of action potential pairs with increased jitter and blocking, and mean MCD in each study were compared. A significant decrement (> 10% decrease in CMAP area or amplitude between the first and fourth response) was never found without increased jitter and impulse blocking on SFEMG. Increased jitter, blocking, and mean MCD were each correlated with maximum decrement (r > 0.61, P < 0.0001). We conclude that decrement to RNS and impulse blocking on SFEMG result from the same physiologic phenomenon, and that SFEMG is more sensitive at detecting disordered neuromuscular transmission given its ability to detect impulse blocking at levels below the resolution of RNS and increased neuromuscular jitter when there is not blocking.

Single axon excitatory postsynaptic potentials in neocortical interneurons exhibit pronounced paired pulse facilitation

In slices of adult rat somatosensory/motor cortex, paired recordings were made from pyramidal and non-pyramidal neurons. Single axon excitatory postsynaptic potentials evoked in the non-pyramidal neuron by action potentials in the pyramidal neuron were large and fast and demonstrated large fluctuations in amplitude, with coefficients of variation between 0.1 and 1.25. Excitatory postsynaptic potential amplitude distributions included a large number of apparent failures of transmission as well as some extremely large events. This contrasted dramatically with the relatively narrow distribution of amplitudes for pyramid-pyramid connections in neocortex. Excitatory postsynaptic potentials increased in amplitude with postsynaptic membrane hyperpolarization. Very small changes in the coefficient of variation when mean amplitudes increased substantially were consistent with the increase being due to a change in quantal amplitude. These excitatory postsynaptic potentials displayed profound paired pulse facilitation. Moreover, third and fourth spikes in a presynaptic burst also evoked large responses. This facilitation was associated with a decrease in the proportion of apparent failures in transmission and a change in the shape of the excitatory postsynaptic potential amplitude distribution, both indicative of an increase in the probability of transmitter release. However a large change in the mean amplitude was not associated with a similar change in the inverse square of the coefficient of variation. The result of this third test, taken in isolation, might therefore suggest that quantal amplitude had increased with paired-pulse facilitation. However, of the three tests applied, this last is the most heavily model-dependent and produced a result inconsistent with the results of the other two tests. The possibility is therefore discussed that both the shape of the excitatory postsynaptic potential amplitude distribution and the failure of coefficient of variation analysis to detect an apparently presynaptic change might result from the release at these synapses being poorly fit by a simple model. Based on a more complex model of synaptic release proposed by Faber and Korn [ Faber and Korn (1991) Biophys. J. 60, 1288–1294] and a hypothesis proposed by Scharfman et al. [ Scharfman et al. (1990) Neuroscience 37, 693–707], two hypotheses arising from the present study are discussed: (i) that branch point failure contributes to the pattern of synaptic activation at these connections; (ii) that both presynaptic pyramidal firing pattern and axonal geometry contribute to the selection of the type of postsynaptic neurone preferentially activated. Although the underlying mechanisms remain to be determined, the present observations indicate that other pyramidal neurons are best recruited by tonic presynaptic pyramidal firing, or pairs of action potentials, while non-pyramidal neurons are preferentially recruited by bursts of action potentials in presynaptic pyramidal neurons. These results indicate that the type of postsynaptic neuron preferentially activated by a presynaptic neocortical pyramid is selected by the pattern of firing in that pyramidal neuron.

The effect of a change in shape of the cardiac action potential on the ionic conductances.

SUMMARYRabbit ventricular muscle was stimulated at a low constant rate and an extra stimulus was interposed shortly after every fourth action potential. Pairs of differently shaped action potentials were obtained in this way.The magnitude and time course of polarisation resistance have been determined throughout these action potential pairs. The increase in polarisation resistance during the repolarisation phase was more prolonged and somewhat greater in the action potential of longer duration and area.Changes in membrane conductance to sodium and potassium ions throughout the action potentials have been computed. Throughout the whole of the repolarisation phase the computed potassium conductance is lower and the computed sodium conductance is higher than in the resting state.