Variation Of Interspike Intervals Research Articles

Characterising sex-related differences in lower- and higher-threshold motor unit behaviour through high-density surface electromyography.

Emerging questions in neuromuscular physiology revolve around whether males and females share similar neural control in diverse tasks across a broad range of intensities. In order to explore these features, high-density electromyography was used to record the myoelectrical activity of biceps brachii during trapezoidal isometric contractions at 35% and 70% of maximal voluntary force (MVF) on 11 male and 13 female participants. Identified motor units were then classified as lower-threshold (recruited at ≤30%MVF) and higher-threshold (recruited at>30%MVF). The discharge rate, interspike interval variability, recruitment and derecruitment thresholds, and estimates of neural drive to motor neurons were assessed. Female lower-threshold motor units showed higher neural drive (P<0.001), accompanied by higher discharge rate at recruitment (P=0.006), plateau (P=0.001) and derecruitment (P=0.001). On the other hand, male higher-threshold motor units showed greater neural drive (P=0.04), accompanied by higher discharge rate at recruitment (P=0.005), plateau (P=0.04) and derecruitment (P=0.01). Motor unit discharge rate normalised by the recruitment threshold was significantly higher in female lower-threshold motor units (P<0.001), while no differences were observed in higher-threshold motor units. Recruitment and derecruitment thresholds are higher in males across all intensities (P<0.01). However, males and females have similar activation and deactivation strategies, as evidenced by similar recruitment-to-derecruitment ratios (P>0.05). This study encompasses a broad intensity range to analyse motor unit sex-related differences, highlighting higher neural drive and discharge rates in female lower-threshold motor units, elevated recruitment and derecruitment thresholds in males, and convergences in activation and deactivation strategies. HIGHLIGHTS: What is the central question of the study? Do male and female motor units behave similarly in low- and high-intensity contractions? What is the main finding and its importance? Female motor units show higher discharge rates in low-intensity tasks and lower discharge rates in high-intensity tasks, with no differences in recruitment behaviour. A broader inter-spike interval variability was also observed in females. These findings underline that there are sex-specific differences concern the firing strategies based on task intensity.

P-75 Force production variability coincides with greater motor unit interspike interval variability in autism spectrum disorders

P-75 Force production variability coincides with greater motor unit interspike interval variability in autism spectrum disorders

Noise-induced dynamical regimes in a system of globally coupled excitable units.

We study the interplay of global attractive coupling and individual noise in a system of identical active rotators in the excitable regime. Performing a numerical bifurcation analysis of the nonlocal nonlinear Fokker-Planck equation for the thermodynamic limit, we identify a complex bifurcation scenario with regions of different dynamical regimes, including collective oscillations and coexistence of states with different levels of activity. In systems of finite size, this leads to additional dynamical features, such as collective excitability of different types and noise-induced switching and bursting. Moreover, we show how characteristic quantities such as macroscopic and microscopic variability of interspike intervals can depend in a non-monotonous way on the noise level.

Variability and Randomness of the Instantaneous Firing Rate.

The apparent stochastic nature of neuronal activity significantly affects the reliability of neuronal coding. To quantify the encountered fluctuations, both in neural data and simulations, the notions of variability and randomness of inter-spike intervals have been proposed and studied. In this article we focus on the concept of the instantaneous firing rate, which is also based on the spike timing. We use several classical statistical models of neuronal activity and we study the corresponding probability distributions of the instantaneous firing rate. To characterize the firing rate variability and randomness under different spiking regimes, we use different indices of statistical dispersion. We find that the relationship between the variability of interspike intervals and the instantaneous firing rate is not straightforward in general. Counter-intuitively, an increase in the randomness (based on entropy) of spike times may either decrease or increase the randomness of instantaneous firing rate, in dependence on the neuronal firing model. Finally, we apply our methods to experimental data, establishing that instantaneous rate analysis can indeed provide additional information about the spiking activity.

Resolving molecular contributions of ion channel noise to interspike interval variability through stochastic shielding.

Molecular fluctuations can lead to macroscopically observable effects. The random gating of ion channels in the membrane of a nerve cell provides an important example. The contributions of independent noise sources to the variability of action potential timing have not previously been studied at the level of molecular transitions within a conductance-based model ion-state graph. Here we study a stochastic Langevin model for the Hodgkin-Huxley (HH) system based on a detailed representation of the underlying channel state Markov process, the "[Formula: see text]D model" introduced in (Pu and Thomas in Neural Computation 32(10):1775-1835, 2020). We show how to resolve the individual contributions that each transition in the ion channel graph makes to the variance of the interspike interval (ISI). We extend the mean return time (MRT) phase reduction developed in (Cao et al. in SIAM JApplMath 80(1):422-447, 2020) to the second moment of the return time from an MRT isochron to itself. Because fixed-voltage spike detection triggers do not correspond to MRT isochrons, the inter-phase interval (IPI) variance only approximates the ISI variance. We find the IPI variance and ISI variance agree to within a few percent when both can be computed. Moreover, we prove rigorously, and show numerically, that our expression for the IPI variance is accurate in the small noise (large system size) regime; our theory is exact in the limit of small noise. By selectively including the noise associated with only those few transitions responsible for most of the ISI variance, our analysis extends the stochastic shielding (SS) paradigm (Schmandt and Galán in PhysRevLett 109(11):118101, 2012) from the stationary voltage clamp case to the current clamp case. We show numerically that the SS approximation has a high degree of accuracy even for larger, physiologically relevant noise levels. Finally, we demonstrate that the ISI variance is not an unambiguously defined quantity, but depends on the choice of voltage level set as the spike detection threshold. We find a small but significant increase in ISI variance, the higher the spike detection voltage, both for simulated stochastic HH data and for voltage traces recorded in in vitro experiments. In contrast, the IPI variance is invariant with respect to the choice of isochron used as a trigger for counting "spikes."

Early life maternal deprivation attenuates morphine induced inhibition in lateral paragigantocellularis neurons in adult rats

Early life stress can serve as one of the principle sources leading to individual differences in confronting challenges throughout the lifetime. Maternal deprivation (MD), a model of early life stress, can cause persistent alterations in brain function, and it may constitute a risk factor for later incidence of drug addiction. It is becoming more apparent that early life MD predisposes opiate abuse in adulthood. Although several behavioral and molecular studies have addressed this issue, changes in electrophysiological features of the neurons are yet to be understood. The lateral paragigantocellularis (LPGi) nucleus, which participates in the mediation of opiate dependence and withdrawal, may be susceptible to modifications following MD. This study sought to find whether early life MD can alter the discharge activity of LPGi neurons and their response to acute morphine administration in adult rats.Male Wistar rats experienced MD on postnatal days (PNDs) 1–14 for three h per day. Afterward, they were left undisturbed until PND 70, during which the extracellular activities of LPGi neurons were recorded in anesthetized animals at baseline and in response to acute morphine.In both MD and control groups, acute morphine administration induced heterogeneous (excitatory, inhibitory, and no effect) responses in LPGi neurons. At baseline recording, the interspike interval variability of the LPGi neurons was attenuated in both inhibitory and excitatory responses in animals with the history of MD. The extent of morphine-induced discharge inhibition was also lower in deprived animals compared to the control group.These findings suggest that early life MD induces long-term alterations in LPGi neuronal activity in response to acute administration of morphine. Therefore, the MD may alter the vulnerability to develop opiate abuse in adulthood.

Maternal immune activation in rats attenuates the excitability of monoamine-secreting neurons in adult offspring in a sex-specific way

Higher risk of depression and schizophrenia in descendants of mothers experienced acute infection during the pregnancy has been reported. Since monoamines are fundamental in mentioned psychopathologies, it is possible that maternal immune activation leads to impaired functioning of serotonin (5-HT), noradrenaline, and dopamine neurons in offspring. To test this hypothesis, we examined the effect of maternal immune activation by lipopolysaccharide (LPS) in rats on the excitability of monoamine-secreting neurons in the offspring. LPS was administered during days 15–19 of the gestation in the rising doses of 20–80 µg/kg; control dams received vehicle. During days 53–63 postpartum, rats were anesthetized and electrodes were inserted into the dorsal raphe nucleus, locus coeruleus, and ventral tegmental area for in vivo excitability assessment of 5-HT, noradrenaline, and dopamine neurons. Maternal immune activation suppressed the firing rate of 5-HT neurons in both sexes and stimulated the firing rate of dopamine neurons in males. Decrease in the firing rate of 5-HT neurons was accompanied with an increase, and increase in the firing rate of dopamine neurons with a decrease, in the density of spontaneously active cells. Maternal immune activation also decreased the variability of interspike intervals in 5-HT and dopamine neurons. It is possible that the alteration of excitability of 5-HT and dopamine neurons by maternal immune activation is involved in the psychopathologies induced by infectious disease during the pregnancy. Stimulation of dopamine excitability in males might be a compensatory mechanism secondary to the maternal immune challenge-induced suppression of 5-HT neurons.

Cholinergic Modulation of Membrane Properties of Calyx Terminals in the Vestibular Periphery

Vestibular nerve afferents are divided into regular and irregular groups based on the variability of interspike intervals in their resting discharge. Most afferents receive inputs from bouton terminals that contact type II hair cells as well as from calyx terminals that cover the basolateral walls of type I hair cells. Calyces have an abundance of different subtypes of KCNQ (Kv7) potassium channels and muscarinic acetylcholine receptors (mAChRs) and receive cholinergic efferent inputs from neurons in the brainstem. We investigated whether mAChRs affected membrane properties and firing patterns of calyx terminals through modulation of KCNQ channel activity. Patch clamp recordings were performed from calyx terminals in central regions of the cristae of the horizontal and anterior canals in 13–26 day old Sprague-Dawley rats. KCNQ mediated currents were observed as voltage sensitive currents with slow kinetics (activation and deactivation), resulting in spike frequency adaptation so that calyces at best fired a single action potential at the beginning of a depolarizing step. Activation of mAChRs by application of oxotremorine methiodide or inhibition of KCNQ channels by linopirdine dihydrochloride decreased voltage activated currents by ∼30%, decreased first spike latencies by ∼40%, resulted in action potential generation in response to smaller current injections and at lower (i.e., more hyperpolarized) membrane potentials, and increased the number of spikes fired during depolarizing steps. Interestingly, some of the calyces showed spontaneous discharge in the presence of these drugs. Together, these findings suggest that cholinergic efferents can modulate the response properties and encoding of head movements by afferents.

Neurophysiological correlates of force control improvement induced by sinusoidal vibrotactile stimulation

Objective. An optimal level of vibrotactile stimulation has been shown to improve sensorimotor control in healthy and diseased individuals. However, the underlying neurophysiological mechanisms behind the enhanced motor performance caused by vibrotactile stimulation are yet to be fully understood. Therefore, here we aim to evaluate the effect of a cutaneous vibration on the firing behavior of motor units in a condition of improved force steadiness. Approach. Participants performed a visuomotor task, which consisted of low-intensity isometric contractions of the first dorsal interosseous (FDI) muscle, while sinusoidal (175 Hz) vibrotactile stimuli with different intensities were applied to the index finger. High-density surface electromyogram was recorded from the FDI muscle, and a decomposition algorithm was used to extract the motor unit spike trains. Additionally, computer simulations were performed using a multiscale neuromuscular model to provide a potential explanation for the experimental findings. Main results. Experimental outcomes showed that an optimal level of vibration significantly improved force steadiness (estimated as the coefficient of variation of force). The decreased force variability was accompanied by a reduction in the variability of the smoothed cumulative spike train (as an estimation of the neural drive to the muscle), and the proportion of common inputs to the FDI motor nucleus. However, the interspike interval variability did not change significantly with the vibration. A mathematical approach, together with computer simulation results suggested that vibrotactile stimulation would reduce the variance of the common synaptic input to the motor neuron pool, thereby decreasing the low frequency fluctuations of the neural drive to the muscle and force steadiness. Significance. Our results demonstrate that the decreased variability in common input accounts for the enhancement in force control induced by vibrotactile stimulation.

Noise-induced toroidal excitability in neuron model

Abstract We study the stochastic Hindmarsh–Rose neuron model in the torus canards region of the parameter space. We show that noise can transform the torus canard into the large amplitude torus. This corresponds to the noise-induced transition from amplitude-modulated spiking regime to the bursting one. This phenomenon is confirmed by qualitative changes of both amplitude and frequency characteristics. We investigate this phenomenon with the help of interspike intervals statistics. The onset of noise-induced spiking–bursting transition is accompanied by the increase of the mean values of interspike intervals. Moreover, the anti-coherence resonance reflecting the growth of variability of interspike intervals is observed. On the contrary, the distribution of burst duration values for noise-generated bursting oscillations shows the coherence resonance. We suggest an explanation for the observed stochastic phenomena by the specificity of the deterministic portrait and the high excitability of the system in the zone of torus canards.

Single-motor-unit discharge characteristics in lumbar multifidus muscle of acute low back pain patients

Acute low back pain (ALBP) causes rapid deterioration of paraspinal muscle function. The underlying neurophysiology is poorly understood. We therefore carried out this observational study in patients with ALBP to characterize motor unit (MU) activity in deep lumbar multifidus (LM) muscle and compare with our previous findings from pain-free subjects. Nine subjects (1 woman; age 26-59 yr) with ALBP duration of 1-21 days were recruited from outpatient clinics. Fine wire electromyography (EMG) electrodes were implanted bilaterally at the painful spinal level under computer tomography guidance. EMG was recorded during spontaneous sitting and standing, and during voluntary force production. Linear mixed models were utilized to test or control for the effects of a number of predefined variables. Compared with sitting, standing increased total duration of EMG activity, median MU discharge rate, interspike interval variability, and common drive measured as common drive coefficients (CDC) derived from concurrently active MU pairs. Median discharge rate in 73 MUs was 5.5 and 6.6 pulses per second (pps) during spontaneous sitting and standing, and 7.2 pps during voluntary force production. Interspike interval variability was lower during voluntary tasks than during spontaneous force production. Common drive was less pronounced in bilateral vs. unilateral unit pairs, also in spontaneous standing. This difference was not seen in our previous pain-free subjects, suggesting altered bilateral control of the spine in ALBP. The distribution of CDC values was not a homogeneous continuum but could be seen as two partially overlapping populations of CDC distributions.NEW & NOTEWORTHY We implanted fine-wire electrodes in the deepest part of axial postural muscles in patients with acute low back pain and characterized their motor unit activity. We found less pronounced common drive to the two sides of the spine compared with pain-free subjects, suggesting a different postural control strategy in patients with acute low back pain. An unexpected finding was that common drive coefficient values appeared to consist of two partially overlapping populations of normal distributions.

Temperature dependence of phase and spike synchronization of neural networks

Abstract We simulate a small-world neural network composed of 2000 thermally sensitive identical Hodgkin–Huxley type neurons investigating the synchronization characteristics as a function of the coupling strength and the temperature of the neurons. The Kuramoto order parameter computed over individual neuron membrane potential signals, and recurrence analysis evaluated from the mean field of the network are used to identify the non-monotonous behavior of the synchronization level as a function of the coupling parameter. We show that moderated high temperatures induce a low variability of the inter-burst intervals of neurons leading to phase synchronization and further increases of temperature result in a low variability of inter-spike intervals leading the network to display spike synchronization.

Dynamical Mechanism of Hyperpolarization-Activated Non-specific Cation Current Induced Resonance and Spike-Timing Precision in a Neuronal Model.

Hyperpolarization-activated cyclic nucleotide-gated cation current (Ih) plays important roles in the achievement of many physiological/pathological functions in the nervous system by modulating the electrophysiological activities, such as the rebound (spike) to hyperpolarization stimulations, subthreshold membrane resonance to sinusoidal currents, and spike-timing precision to stochastic factors. In the present paper, with increasing gh (conductance of Ih), the rebound (spike) and subthreshold resonance appear and become stronger, and the variability of the interspike intervals (ISIs) becomes lower, i.e., the enhancement of spike-timing precision, which are simulated in a conductance-based theoretical model and well explained by the nonlinear concept of bifurcation. With increasing gh, the stable node to stable focus, to coexistence behavior, and to firing via the codimension-1 bifurcations (Hopf bifurcation, saddle-node bifurcation, saddle-node bifurcations on an invariant circle, and saddle homoclinic orbit) and codimension-2 bifurcations such as Bogdanov-Takens (BT) point related to the transition between saddle-node and Hopf bifurcations, are acquired with 1- and 2-parameter bifurcation analysis. The decrease of variability of ISIs with increasing gh is induced by the fast decrease of the standard deviation of ISIs, which is related to the increase of the capacity of resisting noisy disturbance due to the firing becomes far away from the bifurcation point. The enhancement of the rebound (spike) with increasing gh builds up a relationship to the decrease of the capacity of resisting disturbance like the hyperpolarization stimulus as the resting state approaches the bifurcation point. The “typical”-resonance and non-resonance appear in the parameter region of the stable focus and node far away from the bifurcation points, respectively. The complex or “strange” dynamics, such as the “weak”-resonance for the stable node near the transition point between the stable node and focus and the non-resonance for the stable focus close to the codimension-1 and −2 bifurcation points, are discussed.

Effects of Voice Therapy on Laryngeal Motor Units During Phonation in Chronic Superior Laryngeal Nerve Paresis Dysphonia

Injury to the superior laryngeal nerve can result in dysphonia, and in particular, loss of vocal range. It can be an especially difficult problem to address with either voice therapy or surgical intervention. Some clinicians and scientists suggest that combining vocal exercises with adjunctive neuromuscular electrical stimulation may enhance the positive effects of voice therapy for superior laryngeal nerve paresis (SLNP). However, the effects of voice therapy without neuromuscular electrical stimulation are unknown. The purpose of this retrospective study was to demonstrate the clinical effectiveness of voice therapy for rehabilitating chronic SLNP dysphonia in two subjects, using interspike interval (ISI) variability of laryngeal motor units by laryngeal electromyography (LEMG). Both patients underwent LEMG and were diagnosed with having 70% recruitment of the cricothyroid muscle, and 70% recruitment of the cricothyroid and thyroarytenoid muscles, respectively. Both patients received voice therapy for 3 months. Grade, roughness, breathiness, asthenia, and strain (GRBAS) scale, stroboscopic examination, aerodynamic assessment, acoustic analysis, and Voice Handicap Index-10 were performed before and after voice therapy. Mean ISI variability during steady phonation was also assessed. After voice therapy, both patients showed improvement in vocal assessments by acoustic, aerodynamic, GRBAS, and Voice Handicap Index-10 analysis. LEMG indicated shortened ISIs in both cases. This study suggests that voice therapy for chronic SLNP dysphonia can be useful for improving SLNP and voice quality.

Rewarding Effects of Operant Dry-Licking Behavior on Neuronal Firing in the Nucleus Accumbens Core.

Certain eating behaviors are characterized by a trend of elevated food consumption. However, neural mechanisms mediating the motivation for food consumption are not fully understood. Food impacts the brain-rewarding-system via both oral-sensory and post-ingestive information. Recent studies have reported an important role of visceral gut information in mediating dopamine (DA) release in the brain rewarding system. This is independent of oral sensation, suggesting a role of the gut-brain-DA-axis in feeding behavior. In this study, we investigated the effects of intra-gastric (IG) self-administration of glucose on neuronal firings in the nucleus accumbens (NA) of water-deprived rats. Rats were trained in an operant-licking paradigm. During training, when the light was on for 2 min (light-period), rats were required to lick a spout to acquire the water oral-intake learning, and either an IG self-infusion of 0.4 M glucose (GLU group) or water (H2O group). Rats rested in the dark-period (3 min) following the light-period. Four cycles of the operant-licking paradigm consisting of the light–dark periods were performed per day, for 4 consecutive days. In the test session, the same rats licked the same spout to acquire the IG self-administration of the corresponding solutions, without oral water ingestion (dry licking). Behavioral results indicated IG self-administration of glucose elicits more dry-licking behavior than that of water. Neurophysiological results indicated in the dark period, coefficient of variance (CV) measuring the inter-spike interval variability of putative medial spiny neurons (pMSNs) in the NA was reduced in the H2O group compared to the GLU group, while there was no significant difference in physical behaviors in the dark period between the two groups. Since previous studies reported that DA release increases CV of MSNs, the present results suggest that greater CV of pMSNs in the GLU group reflects greater DA release in the NA and elevated motivation in the GLU group, which might increase lickings in the test session in the GLU group compared to the H2O group.

Effect of dietary salt intake on epithelial Na+ channels (ENaC) in vasopressin magnocellular neurosecretory neurons in the rat supraoptic nucleus.

Key points A growing body of evidence suggests that epithelial Na+ channels (ENaCs) in the brain play a significant role in the regulation of blood pressure; however, the brain structures that mediate the effect are not well understood.Because vasopressin (VP) neurons play a pivotal role in coordinating neuroendocrine and autonomic responses to maintain cardiovascular homeostasis, a basic understanding of the regulation and activity of ENaC in VP neurons is of great interest.We show that high dietary salt intake caused an increase in the expression and activity of ENaC which resulted in the steady state depolarization of VP neurons.The results help us understand one of the mechanisms underlying how dietary salt intake affects the activity of VP neurons via ENaC activity. All three epithelial Na+ channel (ENaC) subunits (α, β and γ) are located in vasopressin (VP) magnocellular neurons in the hypothalamic supraoptic (SON) and paraventricular nuclei. Our previous study demonstrated that ENaC mediates a Na+ leak current that affects the steady state membrane potential in VP neurons. In the present study, we evaluated the effect of dietary salt intake on ENaC regulation and activity in VP neurons. High dietary salt intake for 7 days caused an increase in expression of β‐ and γENaC subunits in the SON and the translocation of αENaC immunoreactivity towards the plasma membrane. Patch clamp experiments on hypothalamic slices showed that the mean amplitude of the putative ENaC currents was significantly greater in VP neurons from animals that were fed a high salt diet compared with controls. The enhanced ENaC current contributed to the more depolarized basal membrane potential observed in VP neurons in the high salt diet group. These findings indicate that high dietary NaCl intake enhances the expression and activity of ENaCs, which augments synaptic drive by depolarizing the basal membrane potential close to the action potential threshold during hormonal demand. However, ENaCs appear to have only a minor role in the regulation of the firing activity of VP neurons in the absence of synaptic inputs as neither the mean intraburst frequency, burst duration, nor interspike interval variability of phasic bursting activity was affected. Moreover, ENaC activity did not affect the initiation, sustention, or termination of the phasic bursting generated in an intrinsic manner without synaptic inputs.

Fractional-order leaky integrate-and-fire model with long-term memory and power law dynamics

Pyramidal neurons produce different spiking patterns to process information, communicate with each other and transform information. These spiking patterns have complex and multiple time scale dynamics that have been described with the fractional-order leaky integrate-and-Fire (FLIF) model. Models with fractional (non-integer) order differentiation that generalize power law dynamics can be used to describe complex temporal voltage dynamics. The main characteristic of FLIF model is that it depends on all past values of the voltage that causes long-term memory. The model produces spikes with high interspike interval variability and displays several spiking properties such as upward spike-frequency adaptation and long spike latency in response to a constant stimulus. We show that the subthreshold voltage and the firing rate of the fractional-order model make transitions from exponential to power law dynamics when the fractional order α decreases from 1 to smaller values. The firing rate displays different types of spike timing adaptation caused by changes on initial values. We also show that the voltage-memory trace and fractional coefficient are the causes of these different types of spiking properties. The voltage-memory trace that represents the long-term memory has a feedback regulatory mechanism and affects spiking activity. The results suggest that fractional-order models might be appropriate for understanding multiple time scale neuronal dynamics. Overall, a neuron with fractional dynamics displays history dependent activities that might be very useful and powerful for effective information processing.

Correlation between discharge timings of pairs of motor units reveals the presence but not the proportion of common synaptic input to motor neurons.

We investigated whether correlation measures derived from pairs of motor unit (MU) spike trains are reliable indicators of the degree of common synaptic input to motor neurons. Several 50-s isometric contractions of the biceps brachii muscle were performed at different target forces ranging from 10 to 30% of the maximal voluntary contraction relying on force feedback. Forty-eight pairs of MUs were examined at various force levels. Motor unit synchrony was assessed by cross-correlation analysis using three indexes: the output correlation as the peak of the cross-histogram (ρ) and the number of synchronous spikes per second (CIS) and per trigger (E). Individual analysis of MU pairs revealed that ρ, CIS, and E were most often positively associated with discharge rate (87, 85, and 76% of the MU pairs, respectively) and negatively with interspike interval variability (69, 65, and 62% of the MU pairs, respectively). Moreover, the behavior of synchronization indexes with discharge rate (and interspike interval variability) varied greatly among the MU pairs. These results were consistent with theoretical predictions, which showed that the output correlation between pairs of spike trains depends on the statistics of the input current and motor neuron intrinsic properties that differ for different motor neuron pairs. In conclusion, the synchronization between MU firing trains is necessarily caused by the (functional) common input to motor neurons, but it is not possible to infer the degree of shared common input to a pair of motor neurons on the basis of correlation measures of their output spike trains.NEW & NOTEWORTHY The strength of correlation between output spike trains is only poorly associated with the degree of common input to the population of motor neurons. The synchronization between motor unit firing trains is necessarily caused by the (functional) common input to motor neurons, but it is not possible to infer the degree of shared common input to a pair of motor neurons on the basis of correlation measures of their output spike trains.

Stochastic resonance, coherence resonance, and spike timing reliability of Hodgkin–Huxley neurons with ion-channel noise

The random transitions of ion channels between open and closed states are a major source of noise in neurons. In this study, we investigate the stochastic dynamics of a single Hodgkin–Huxley (HH) neuron with realistic, physiological channel noise, which depends on the channel number and the voltage potential of the membrane. Without external input, the stochastic HH model can generate spontaneous spikes induced by ion-channel noise, and the variability of inter-spike intervals attains a minimum for an optimal membrane area, a phenomenon known as coherence resonance. When a subthreshold periodic input current is added, the neuron can optimally detect the input frequency for an intermediate membrane area, corresponding to the phenomenon of stochastic resonance. We also investigate spike timing reliability of neuronal responses to repeated presentations of the same stimulus with different realizations of channel noise. We show that, with increasing membrane area, the reliability of neuronal response decreases for subthreshold periodic inputs, and attains a minimum for suprathreshold inputs. Furthermore, Arnold tongues of high reliability arise in a two-dimensional plot of frequency and amplitude of the sinusoidal input current, resulting from the resonance effect of spike timing reliability.

Motor unit firing rates and synchronisation affect the fractal dimension of simulated surface electromyogram during isometric/isotonic contraction of vastus lateralis muscle

During fatiguing contractions, many adjustments in motor units behaviour occur: decrease in muscle fibre conduction velocity; increase in motor units synchronisation; modulation of motor units firing rate; increase in variability of motor units inter-spike interval. We simulated the influence of all these adjustments on synthetic EMG signals in isometric/isotonic conditions. The fractal dimension of the EMG signal was found mainly influenced by motor units firing behaviour, being affected by both firing rate and synchronisation level, and least affected by muscle fibre conduction velocity. None of the calculated EMG indices was able to discriminate between firing rate and motor units synchronisation.