Stochastic resonance in sparse neuronal network: functional role of ongoing activity to detect weak sensory input in awake auditory cortex of rat.

The awake cortex is characterized by a higher level of ongoing spontaneous activity, but it has a better detectability of weak sensory inputs than the anesthetized cortex. However, the computational mechanism underlying this paradoxical nature of awake neuronal activity remains to be elucidated. Here, we propose a hypothetical stochastic resonance, which improves the signal-to-noise ratio (SNR) of weak sensory inputs through nonlinear relations between ongoing spontaneous activities and sensory-evoked activities. Prestimulus and tone-evoked activities were investigated via in vivo extracellular recording with a dense microelectrode array covering the entire auditory cortex in rats in both awake and anesthetized states. We found that tone-evoked activities increased supralinearly with the prestimulus activity level in the awake state and that the SNR of weak stimulus representation was optimized at an intermediate level of prestimulus ongoing activity. Furthermore, the temporally intermittent firing pattern, but not the trial-by-trial reliability or the fluctuation of local field potential, was identified as a relevant factor for SNR improvement. Since ongoing activity differs among neurons, hypothetical stochastic resonance or "sparse network stochastic resonance" might offer beneficial SNR improvement at the single-neuron level, which is compatible with the sparse representation in the sensory cortex.

Role of HCN channels in the functions of basal ganglia and Parkinson's disease.

Parkinson's disease (PD) is a motor disorder resulting from dopaminergic neuron degeneration in the substantia nigra caused by age, genetics, and environment. The disease severely impacts a patient's quality of life and can even be life-threatening. The hyperpolarization-activated cyclic nucleotide-gated (HCN) channel is a member of the HCN1-4 gene family and is widely expressed in basal ganglia nuclei. The hyperpolarization-activated current mediated by the HCN channel has a distinct impact on neuronal excitability and rhythmic activity associated with PD pathogenesis, as it affects the firing activity, including both firing rate and firing pattern, of neurons in the basal ganglia nuclei. This review aims to comprehensively understand the characteristics of HCN channels by summarizing their regulatory role in neuronal firing activity of the basal ganglia nuclei. Furthermore, the distribution and characteristics of HCN channels in each nucleus of the basal ganglia group and their effect on PD symptoms through modulating neuronal electrical activity are discussed. Since the roles of the substantia nigra pars compacta and reticulata, as well as globus pallidus externus and internus, are distinct in the basal ganglia circuit, they are individually described. Lastly, this investigation briefly highlights that the HCN channel expressed on microglia plays a role in the pathological process of PD by affecting the neuroinflammatory response.

IGF-1 receptor regulates upward firing rate homeostasis via the mitochondrial calcium uniporter.

Regulation of firing rate homeostasis constitutes a fundamental property of central neural circuits. While intracellular Ca2+ has long been hypothesized to be a feedback control signal, the molecular machinery enabling a network-wide homeostatic response remains largely unknown. We show that deletion of insulin-like growth factor-1 receptor (IGF-1R) limits firing rate homeostasis in response to inactivity, without altering the distribution of baseline firing rates. The deficient firing rate homeostatic response was due to disruption of both postsynaptic and intrinsic plasticity. At the cellular level, we detected a fraction of IGF-1Rs in mitochondria, colocalized with the mitochondrial calcium uniporter complex (MCUc). IGF-1R deletion suppressed transcription of the MCUc members and burst-evoked mitochondrial Ca2+ (mitoCa2+) by weakening mitochondria-to-cytosol Ca2+ coupling. Overexpression of either mitochondria-targeted IGF-1R or MCUc in IGF-1R-deficient neurons was sufficient to rescue the deficits in burst-to-mitoCa2+ coupling and firing rate homeostasis. Our findings indicate that mitochondrial IGF-1R is a key regulator of the integrated homeostatic response by tuning the reliability of burst transfer by MCUc. Based on these results, we propose that MCUc acts as a homeostatic Ca2+ sensor. Faulty activation of MCUc may drive dysregulation of firing rate homeostasis in aging and in brain disorders associated with aberrant IGF-1R/MCUc signaling.

Inducing post-activation potentiation alters human motor unit behavior of the elbow extensors during unconstrained velocity contractions.

The purpose was to test whether inducing post-activation potentiation (PAP) altered motor unit (MU) activity during dynamic isotonic contractions. From 12 participants (3 females), 39 MUs were recorded from the anconeus (n=31) and lateral triceps brachii (n=8) with fine-wire electrodes during elbow extensions at 50 and 75% of peak power with, and without PAP. To induce PAP participants produced a 2s ramp conditioning contraction (CC) up to maximal isometric elbow extension with a 3s hold. Following the CC (~2s), independent electrical stimulation to the triceps and anconeus showed twitch torques were potentiated by 84 and 66%, respectively, (both p<0.001). Compared to baseline (i.e., without PAP), at both intensities (50 and 75%) PAP increased MU recruitment thresholds (40% and 80%, p<0.001) with lowered mean MU rates (-20 and -26%), and instantaneous rates at recruitment threshold (-26 and -25%) (all p<0.001). Firing rates increased 20% (p<0.001) from 50 to 75% power, but rates during potentiated contractions targeting 75% were lower than baseline at 50% (-10%, p<0.001). Dynamic contractions provide a more functional paradigm to assess MU activity with PAP and showed larger effects across a wider range of contractile intensities compared to previously described isometric tasks. Findings indicate that peripheral feedback from the potentiated muscle is likely not the primary mechanism in modifying MU behaviors as changes occurred at recruitment which is relatively insensitive to afferent feedback. Therefore, MU activity during dynamic contractions is responsive to activation history force potentiation and can make compensatory adjustments to optimize contractile output.

Firing rate models for gamma oscillations in I-I and E-I networks.

Firing rate models for describing the mean-field activities of neuronal ensembles can be used effectively to study network function and dynamics, including synchronization and rhythmicity of excitatory-inhibitory populations. However, traditional Wilson-Cowan-like models, even when extended to include an explicit dynamic synaptic activation variable, are found unable to capture some dynamics such as Interneuronal Network Gamma oscillations (ING). Use of an explicit delay is helpful in simulations at the expense of complicating mathematical analysis. We resolve this issue by introducing a dynamic variable, u, that acts as an effective delay in the negative feedback loop between firing rate (r) and synaptic gating of inhibition (s). In effect, u endows synaptic activation with second order dynamics. With linear stability analysis, numerical branch-tracking and simulations, we show that our r-u-s rate model captures some key qualitative features of spiking network models for ING. We also propose an alternative formulation, a v-u-s model, in which mean membrane potential v satisfies an averaged current-balance equation. Furthermore, we extend the framework to E-I networks. With our six-variable v-u-s model, we demonstrate in firing rate models the transition from Pyramidal-Interneuronal Network Gamma (PING) to ING by increasing the external drive to the inhibitory population without adjusting synaptic weights. Having PING and ING available in a single network, without invoking synaptic blockers, is plausible and natural for explaining the emergence and transition of two different types of gamma oscillations.

Neuromuscular behaviour in the first dorsal interosseus following mental fatigue.

We examined sex-specific changes to neuromuscular function in response to mental fatigue. Twenty-five young, healthy adults (13 F, 12 M) performed a mentally fatiguing task and control condition for 30 min on two separate days. Neuromuscular function was assessed in the first dorsal interosseous before and after each condition. Reaction time decreased after the mentally fatiguing task (P < 0.001, η2  = 0.47). Males and females reported higher levels of subjective fatigue after the mentally fatiguing task (P < 0.02, η2  = 0.07). Motor unit firing rate increased over time at 10% maximal voluntary contraction (MVC; P < 0.04, η2  = 0.16), and decreased over time at 50% MVC (P < 0.01, η2  = 0.14); however, this was not unique to either sex. During a variable force contraction, error decreased in females over time and increased in males (P < 0.05, η2  = 0.13), although changes were not unique to mental fatigue. Physiological function of the neuromuscular system was not specifically affected by mental fatigue in males or females.

Logarithmically scaled, gamma distributed neuronal spiking.

Naturally log-scaled quantities abound in the nervous system. Distributions of these quantities have non-intuitive properties, which have implications for data analysis and the understanding of neural circuits. Here, we review the log-scaled statistics of neuronal spiking and the relevant analytical probability distributions. Recent work using log-scaling revealed that interspike intervals of forebrain neurons segregate into discrete modes reflecting spiking at different timescales and are each well-approximated by a gamma distribution. Each neuron spends most of the time in an irregular spiking 'ground state' with the longest intervals, which determines the mean firing rate of the neuron. Across the entire neuronal population, firing rates are log-scaled and well approximated by the gamma distribution, with a small number of highly active neurons and an overabundance of low rate neurons (the 'dark matter'). These results are intricately linked to a heterogeneous balanced operating regime, which confers upon neuronal circuits multiple computational advantages and has evolutionarily ancient origins.

Odor modality is transmitted to cortical brain regions from the olfactory bulb.

Odor perception is the impetus for important animal behaviors with two predominate modes of processing: odors pass through the front of the nose (orthonasal) while inhaling and sniffing, or through the rear (retronasal) during exhalation and while eating. Despite the importance of olfaction for an animal's well-being and that ortho and retro naturally occur, it is unknown how the modality (ortho vs. retro) is even transmitted to cortical brain regions, which could significantly affect how odors are processed and perceived. Using multielectrode array recordings in tracheotomized anesthetized rats, which decouples ortho-retro modality from breathing, we show that mitral cells in rat olfactory bulb can reliably and directly transmit orthonasal versus retronasal modality with ethyl butyrate, a common food odor. Drug manipulations affecting synaptic inhibition via GABAA lead to worse decoding of ortho versus retro, independent of whether overall inhibition increases or decreases, suggesting that the olfactory bulb circuit may naturally favor encoding this important aspect of odors. Detailed data analysis paired with a firing rate model that captures population trends in spiking statistics shows how this circuit can encode odor modality. We have not only demonstrated that ortho/retro information is encoded to downstream brain regions but also used modeling to demonstrate a plausible mechanism for this encoding; due to synaptic adaptation, it is the slower time course of the retronasal stimulation that causes retronasal responses to be stronger and less sensitive to inhibitory drug manipulations than orthonasal responses.NEW & NOTEWORTHY Whether ortho (sniffing odors) versus retro (exhalation and eating) is encoded from the olfactory bulb to other brain areas is not completely known. Using multielectrode array recordings in anesthetized rats, we show that the olfactory bulb transmits this information downstream via spikes. Altering inhibition degrades ortho/retro information on average. We use theory and computation to explain our results, which should have implications on cortical processing considering that only food odors occur retronasally.

Responses in fast-spiking interneuron firing rates to parameter variations associated with degradation of perineuronal nets.

The perineuronal nets (PNNs) are sugar coated protein structures that encapsulate certain neurons in the brain, such as parvalbumin positive (PV) inhibitory neurons. As PNNs are theorized to act as a barrier to ion transport, they may effectively increase the membrane charge-separation distance, thereby affecting the membrane capacitance. Tewari et al. (2018) found that degradation of PNNs induced a 25%-50% increase in membrane capacitance [Formula: see text] and a reduction in the firing rates of PV-cells. In the current work, we explore how changes in [Formula: see text] affects the firing rate in a selection of computational neuron models, ranging in complexity from a single compartment Hodgkin-Huxley model to morphologically detailed PV-neuron models. In all models, an increased [Formula: see text] lead to reduced firing, but the experimentally reported increase in [Formula: see text] was not alone sufficient to explain the experimentally reported reduction in firing rate. We therefore hypothesized that PNN degradation in the experiments affected not only [Formula: see text], but also ionic reversal potentials and ion channel conductances. In simulations, we explored how various model parameters affected the firing rate of the model neurons, and identified which parameter variations in addition to [Formula: see text] that are most likely candidates for explaining the experimentally reported reduction in firing rate.

Assessment of motor unit firing by high-density surface electromyography detects motor neuronal hyperexcitable state in amyotrophic lateral sclerosis.

INTRODUCTION/AIMS: In amyotrophic lateral sclerosis (ALS), the impact of motor neuron dysfunction on the motor unit (MU) firing pattern remains to be elucidated. The aim of this study was to clarify the characteristics of the MU firing rate and its association with clinical factors in ALS patients using high-density surface electromyography (HDSEMG) and MU decomposition analysis. METHODS: Nineteen ALS patients and 20 controls prospectively underwent HDSEMG recording of the vastus lateralis muscle during ramp-up (30% of maximum voluntary contraction) and sustained (10% of maximal voluntary contraction for 60 seconds) contractions on performing isometric knee extension. After decomposition analysis, instantaneous firing rates (IFRs) of individually identified MUs were calculated. Comparison of IFRs and clinical variables between ALS patients and controls and analysis of the correlation between individual mean IFR and clinical variables in ALS patients were performed. RESULTS: The number of identified MUs was lower in ALS patients than in controls (P = .017). Mean IFRs of MUs (i.e., mean MU firing rates) were higher in ALS patients than in controls at some force levels on ramp-up contraction (P < .05) and at 50 to 60 seconds during sustained contraction (9.1 [ALS] vs 8.3 [controls] pulses per second; P = .036). There was no correlation between the clinical parameters and mean IFR of each patient. DISCUSSION: ALS patients had a higher MU firing rate during muscle contraction at a low force level. Noninvasive assessment of the MU firing rate by HDSEMG can detect a motor neuronal hyperexcitable state in ALS patients.

A Fundamental Inequality Governing the Rate Coding Response of Sensory Neurons.

A fundamental inequality governing the spike activity of peripheral neurons is derived and tested against auditory data. This inequality states that the steady-state firing rate must lie between the arithmetic and geometric means of the spontaneous and peak activities during adaptation. Implications towards the development of auditory mechanistic models are explored.

Increases in muscle temperature by hot water improve muscle contractile function and reduce motor unit discharge rates.

PURPOSE: Examine the effects of 42°C hot-water immersion on muscle contraction function and motor unit discharge rates. Voluntary and evoked contraction assessments were examined first with a concomitant increase in the core and muscle temperature, and thereafter with increased muscle temperature but cooled core temperature. METHODS: Fifteen participants (24.9 ± 5.6 years) performed neuromuscular assessments before, after, and ~15-min after either 90-min of 42°C (hot) or 36°C (control) water immersion. Maximal voluntary contraction (MVC) assessment of knee extension was performed along with surface electromyography (sEMG) (vastus lateralis and medialis [VL, VM]) and voluntary activation level (VAL). Resting evoked twitch was elicited for peak torque and time to peak torque analysis. In addition, the VL and VM motor unit discharge rates (MUDR) were measured. RESULTS: After hot-water immersion (core temperature ↑1°C; muscle temperature ↑2.4°C), MVC torque and VAL decreased (p < 0.05). The sEMG (VL and VM) and peak twitch torque did not change (p > 0.05), while time to peak torque decreased (p = 0.007). The VL and VM MUDR decreased, showing a time effect, after both water immersion conditions (36 and 42°C) (p > 0.001). Fifteen minutes after the hot-water immersion (core temperature at baseline; muscle temperature ↑1.4°C), MVC torque returned to baseline, but VAL remained lower. The sEMG (VL and VM) remained unchanged. Peak twitch torque increased (p < 0.002) and time to peak torque remained lower (p = 0.028). The MUDR remained lower after both water immersion conditions (p < 0.05). CONCLUSION: Increased core temperature evoked by 42°C hot-water immersion decreases MVC torque and VAL. However, a passive increase in muscle temperature improved evoked muscle contractile function (i.e., time to peak torque [after] and peak twitch torque [~15 min after]). Moreover, a passive increase in muscle temperature reduced the required MUDR to attain the same torque.

Lower motor unit discharge rates in gastrocnemius lateralis, but not in gastrocnemius medialis or soleus, in runners with Achilles tendinopathy: a pilot study.

OBJECTIVES: Deficits in muscle performance could be a consequence of a reduced ability of a motor neuron to increase the rate in which it discharges. This study aimed to investigate motor unit (MU) discharge properties of each triceps surae muscle (TS) and TS torque steadiness during submaximal intensities in runners with Achilles tendinopathy (AT). METHODS: We recruited runners with (n = 12) and without (n = 13) mid-portion AT. MU discharge rate was analysed for each of the TS muscles, using high-density surface electromyography during 10 and 20% isometric plantar flexor contractions. RESULTS: MU mean discharge rate was lower in the gastrocnemius lateralis (GL) in AT compared to controls. In AT, GL MU mean discharge rate did not increase as torque increased from 10% peak torque, 8.24 pps (95% CI 7.08 to 9.41) to 20%, 8.52 pps (7.41 to 9.63, p = 0.540); however, in controls, MU discharge rate increased as torque increased from 10%, 8.39 pps (7.25-9.53) to 20%, 10.07 pps (8.89-11.25, p < 0.001). There were no between-group difference in gastrocnemius medialis (GM) or soleus (SOL) MU discharge rates. We found no between-group differences in coefficient of variation of MU discharge rate in any of the TS muscles nor in TS torque steadiness. CONCLUSION: Our data demonstrate that runners with AT may have a lower neural drive to GL, failing to increase MU discharge rate to adjust for the increase in torque demand. Further research is needed to understand how interventions focussing on increasing neural drive to GL would affect muscle function in runners with AT.

Behavior and electrophysiological effects on striatum-nigra circuit after high frequency stimulation. Relevance to Parkinson and epilepsy.

The phenomenon of plasticity in the striatum, and its relation with the striatum-nigra neuronal circuit has clinical and neurophysiological relevance to Parkinson and epilepsy. High frequency stimulation (HFS) can induce neural plasticity. Furthermore, it is possible to induce plasticity in the dorsal striatum and this can be modulated by substantia nigra activity. But it has not been shown yet what would be the effects in the striatum-nigra circuit after plasticity induction in striatum with HSF. Literature also misses a detailed description of the way back loop of the circuit: the striatal firing rate after substantia nigrás inhibition. We here conducted: First Experiment, application of HFS in dorsomedial striatum and measure of spontaneous and longlasting behavior expression in the open field three days later; Second, application of single pulses on dorsomedial striatum and measure of the evoked potentials in substantia nigra before and after HFS; Third Experiment: inhibition of substantia nigra and recording of the firing rate of dorsomedial striatum. HFS in dorsomedial striatum caused increased locomotion behaviors, but not classical stereotypy. However, rats had either an increase or decrease in substantia nigrás evoked potentials. Also, substantia nigrás inhibition caused an increase in dorsomedial striatum firing rate. Present data are suggestive of a potential application of HFS in striatum, as an attempt to modulate behavior rigidity and hypokinesia of diseases involving the basal ganglia, especially Parkinson´s Disease.

Correlations among Firing Rates of Tactile, Thermal, Gustatory, Olfactory, and Auditory Sensations Mimicked by Artificial Hybrid Fluid (HF) Rubber Mechanoreceptors.

In order to advance the development of sensors fabricated with monofunctional sensation systems capable of a versatile response to tactile, thermal, gustatory, olfactory, and auditory sensations, mechanoreceptors fabricated as a single platform with an electric circuit require investigation. In addition, it is essential to resolve the complicated structure of the sensor. In order to realize the single platform, our proposed hybrid fluid (HF) rubber mechanoreceptors of free nerve endings, Merkel cells, Krause end bulbs, Meissner corpuscles, Ruffini endings, and Pacinian corpuscles mimicking the bio-inspired five senses are useful enough to facilitate the fabrication process for the resolution of the complicated structure. This study used electrochemical impedance spectroscopy (EIS) to elucidate the intrinsic structure of the single platform and the physical mechanisms of the firing rate such as slow adaption (SA) and fast adaption (FA), which were induced from the structure and involved the capacitance, inductance, reactance, etc. of the HF rubber mechanoreceptors. In addition, the relations among the firing rates of the various sensations were clarified. The adaption of the firing rate in the thermal sensation is the opposite of that in the tactile sensation. The firing rates in the gustation, olfaction, and auditory sensations at frequencies of less than 1 kHz have the same adaption as in the tactile sensation. The present findings are useful not only in the field of neurophysiology, to research the biochemical reactions of neurons and brain perceptions of stimuli, but also in the field of sensors, to advance salient developments in sensors mimicking bio-inspired sensations.

Estimation of Fast and Slow Adaptions in the Tactile Sensation of Mechanoreceptors Mimicked by Hybrid Fluid (HF) Rubber with Equivalent Electric Circuits and Properties.

In order to advance engineering applications of robotics such as wearable health-monitoring devices, humanoid robots, etc., it is essential to investigate the tactile sensations of artificial haptic sensors mimicking bioinspired human cutaneous mechanoreceptors such as free nerve endings, Merkel's cells, Krause end bulbs, Meissner corpuscles, Ruffini endings, and Pacinian corpuscles. The generated receptor's potential response to extraneous stimuli, categorized as slow adaption (SA) or fast adaption (FA), is particularly significant as a typical property. The present study addressed the estimation of SA and FA by utilizing morphologically fabricated mechanoreceptors made of our proposed magnetically responsive intelligent fluid, hybrid fluid (HF), and by applying our proposed electrolytic polymerization. Electric circuit models of the mechanoreceptors were generated using experimental data on capacitance and inductance on the basis of the electric characteristics of impedance. The present results regarding equivalent firing rates based on FA and SA are consistent with the FA and SA findings of vital mechanoreceptors by biomedical analysis. The present investigative process is useful to clarify the time of response to a force on the fabricated artificial mechanoreceptor.

Diverse risk-avoidance behaviors in DISC1 mice are associated with different neuronal firing patterns in BLA neurons.

It is very important to maintain normal levels of risk avoidance in daily life. We found that DISC1-NTM mice, which are a model for mental disorders, had a phenotype marked by a risk-avoidance impairment as measured in an open-field test (OFT). We used optogenetic methods to modulate glutamatergic neurons in the basolateral amygdala (BLA) in an attempt to rescue this risk-avoidance impairment. We found that photostimulation of BLA neurons at 20 Hz modified DISC1-NTM mouse behavior from low risk avoidance to high risk avoidance. We observed following photostimulation that, compared to controls, the number of entries to the center of the open field was lower and less time was spent in the central area. We also found that the time spent immobile was higher during photostimulation compared with WT mice. We also used a lower photostimulation frequency of 5 Hz, which activated BLA glutamatergic neurons and rescued the risk-avoidance impairment in DISC1-NTM mice. Our findings confirm that the BLA participates in diverse risk-avoidance behavior. Our results are also a reminder that differences in neuronal firing patterns within the same pathway may lead to different physiological functions.

Training-induced improvements in knee extensor force accuracy are associated with reduced vastus lateralis motor unit firing variability.

NEW FINDINGS: What is the central question of this study? Can bilateral knee extensor force accuracy be improved following 4 weeks of unilateral force accuracy training and are there any subsequent alterations to central and/or peripheral motor unit features? What is the main finding and its importance? In the trained limb only, knee extensor force tracking accuracy improved with reduced motor unit firing rate variability in the vastus lateralis, and there was no change to neuromuscular junction transmission instability. Interventional strategies to improve force accuracy may be directed to older/clinical populations where such improvements may aid performance of daily living activities. ABSTRACT: Muscle force output during sustained submaximal isometric contractions fluctuates around an average value and is partly influenced by variation in motor unit (MU) firing rates. MU firing rate (FR) variability seemingly reduces following exercise training interventions; however, much less is known with respect to peripheral MU properties. We therefore investigated whether targeted force accuracy training could lead to improved muscle functional capacity and control, in addition to determining any alterations of individual MU features. Ten healthy participants (seven females, three males, 27 ± 6 years, 170 ± 8 cm, 69 ± 16 kg) underwent a 4-week supervised, unilateral knee extensor force accuracy training intervention. The coefficient of variation for force (FORCECoV ) and sinusoidal wave force tracking accuracy (FORCESinu ) were determined at 25% maximal voluntary contraction (MVC) pre- and post-training. Intramuscular electromyography was utilised to record individual MU potentials from the vastus lateralis (VL) muscles at 25% MVC during sustained contractions, pre- and post-training. Knee extensor muscle strength remained unchanged following training, with no improvements in unilateral leg-balance. FORCECoV and FORCESinu significantly improved in only the trained knee extensors by ∼13% (P = 0.01) and ∼30% (P < 0.0001), respectively. MU FR variability significantly reduced in the trained VL by ∼16% (n = 8; P = 0.001), with no further alterations to MU FR or neuromuscular junction transmission instability. Our results suggest muscle force control and tracking accuracy is a trainable characteristic in the knee extensors, which is likely explained by the reduction in MU FR variability which was apparent in the trained limb only.

Motor unit recruitment and firing rate at low force of contraction.

INTRODUCTION/AIMS: A prevailing concept of motor unit (MU) recruitment used for calculating recruitment ratio (RR) suggests a progressive linear increase in firing rate (FR). The objective of this study is to assess its validity. METHODS: Concentric needle electromyography (EMG) recordings were made in normal muscle and abnormal muscle of patients with neurogenic findings. Signals recorded at low force were visually decomposed to study MU FR at onset, recruitment of a second MU, and recruitment of more MUs with further increases in force. RESULTS: We observed one to six MUs discharging at a rate < 15 Hz in normal muscles at low force. The MU FR was 5-8 Hz at onset. With increasing force, FR increased by 3-5 Hz and then idled at <15 Hz while other MUs were recruited. The recruitment frequency (RF) and RR had low sensitivity and were abnormal mainly in moderately to severely weak muscles. DISCUSSION: Our data are consistent with FR analysis results described by other investigators. It does not support a progressive linear increase in MU FR with recruitment. A revised model for MU recruitment at low effort during gradual increase in force is presented. On subjective assessment, the FR of the fastest firing MU can help detect MU loss in neurogenic processes.

A simple one-dimensional map-based model of spiking neurons with wide ranges of firing rates and complexities.

This paper introduces a simple 1-dimensional map-based model of spiking neurons. During the past decades, dynamical models of neurons have been used to investigate the biology of human nervous systems. The models simulate experimental records of neurons' voltages using difference or differential equations. Difference neuronal models have some advantages besides the differential ones. They are usually simpler, and considering the cost of needed computations, they are more efficient. In this paper, a simple 1-dimensional map-based model of spiking neurons is introduced. Sample entropy is applied to analyze the complexity of the model's dynamics. The model can generate a wide range of time series with different firing rates and different levels of complexities. Besides, using some tools like bifurcation diagrams and cobwebs, the introduced model is analyzed.