Initial Firing Rate Research Articles

Motor unit firing rates during slow and fast contractions in boys and men.

Motor unit (MU) activation during maximal contractions is lower in children compared with adults. Among adults, discrete MU activation differs, depending on the rate of contraction. We investigated the effect of contraction rate on discrete MU activation in boys and men. Following a habituation session, 14 boys and 20 men completed two experimental sessions for knee extension and wrist flexion, in random order. Maximal voluntary isometric torque (MVIC) was determined before completing trapezoidal isometric contractions (70%MVIC) at low (10%MVIC/s) and high (35%MVIC/s) contraction rates. Surface electromyography was captured from the vastus lateralis (VL) and flexor carpi radialis (FCR) and decomposed into individual MU action potential (MUAP) trains. In both groups and muscles, the initial MU firing rate (MUFR) was greater (p < 0.05) at high compared with low contraction rates. The increase in initial MUFR at the fast contraction in the VL was greater in men than boys (p < 0.05). Mean MUFR was significantly lower during fast contractions only in the FCR (p < 0.05). In both groups and muscles, the rate of decay of MUFR with increasing MUAP amplitude was less steep (p < 0.05) during fast compared with slow contractions. In both groups and muscles, initial MUFRs, as well as MUFRs of large MUs were higher during fast compared with slow contractions. However, in the VL, the increase in initial MUFR was greater in men compared with boys. This suggests that in large muscles, men may rely more on increasing MUFR to generate torque at faster rates compared with boys.

376 Pathophysiology of voluntary motor commands in patients with multiple sclerosis identified using reverse engineering of motor unit population discharge.

OBJECTIVES/GOALS: Our objective is to characterize excitatory, inhibitory, and neuromodulatory components of the voluntary motor command at the level of the spinal motoneuron in people with multiple sclerosis (MS). This information will provide insight into neural mechanisms of motor dysfunction and their heterogeneity among patients with MS. METHODS/STUDY POPULATION: Due to advances in high-density surface EMG (HDsEMG) decomposition and the recent development of a paradigm for reverse engineering of motor unit population discharge, we can feasibly estimate aspects of excitatory, inhibitory, and neuromodulatory components of the voluntary motor command in humans on a person-specific basis. We tested 11 ambulatory patients with MS and mild-moderate disability. We recorded HDsEMG from tibialis anterior (TA) and soleus (SOL) during isometric plantarflexion and dorsiflexion, performed as slow triangle contractions. EMG was decomposed into motor unit spike trains using blind source separation. We calculated a number of motor unit variables, most notably delta-F, which estimates motoneuron excitability and the balance of neuromodulatory and inhibitory inputs. RESULTS/ANTICIPATED RESULTS: There were consistent differences in MS patients vs. controls. For TA, values were decreased for delta-F (3.9 vs. 5.9 pps), initial firing rate acceleration (5.8 vs. 7.1 pps), firing rate range (9.3 vs. 11.9 pps), and max firing rate (12.3 vs. 15.0 pps). SOL had more modest decreases in delta-F (3.0 vs. 3.8 pps) and firing rate range (4.8 vs. 5.6 pps). Self-sustained firing was longer for MS patients. Within a patient, abnormalities in motor unit variables were not consistent across muscles and legs. Interestingly, there were several abnormalities in the patients with a normal clinical motor exam, indicating that perhaps our measures are sensitive to subclinical changes in processing of voluntary motor commands. DISCUSSION/SIGNIFICANCE: Excitatory, inhibitory, and neuromodulatory components of the voluntary motor command must be appropriately balanced for skilled motor output. This study is the first to characterize how they are disrupted in MS, providing foundational information to inform the development of mechanistically-based rehabilitation interventions.

Intrinsic motoneuron properties in typical human development.

Motoneuron properties and their firing patterns undergo significant changes throughout development and in response to neuromodulators such as serotonin. Here, we examined the age-related development of self-sustained firing and general excitability of tibialis anterior motoneurons in a young development (7-17years), young adult (18-28years) and adult (32-53years) group, as well as in a separate group of participants taking selective serotonin reuptake inhibitors (SSRIs, aged 11-28years). Self-sustained firing, as measured by ΔF, was larger in the young development (∼5.8Hz, n=20) compared to the young adult (∼4.9Hz, n=13) and adult (∼4.8Hz, n=8) groups, consistent with a developmental decrease in self-sustained firing mediated by persistent inward currents (PIC). ΔF was also larger in participants taking SSRIs (∼6.5Hz, n=9) compared to their age-matched controls (∼5.3Hz, n=26), consistent with increased levels of spinal serotonin facilitating the motoneuron PIC. Participants in the young development and SSRI groups also had higher firing rates and a steeper acceleration in initial firing rates (secondary ranges), consistent with the PIC producing a steeper acceleration in membrane depolarization at the onset of motoneuron firing. In summary, both the young development and SSRI groups exhibited increased intrinsic motoneuron excitability compared to the adults, which, in the young development group, was also associated with a larger unsteadiness in the dorsiflexion torque profiles. We propose several intrinsic and extrinsic factors that affect both motoneuron PICs and cell discharge which vary during development, with a time course similar to the changes in motoneuron firing behaviour observed in the present study. KEY POINTS: Neurons in the spinal cord that activate muscles in the limbs (motoneurons) undergo increases in excitability shortly after birth to help animals stand and walk. We examined whether the excitability of human ankle flexor motoneurons also continues to change from child to adulthood by recording the activity of the muscle fibres they innervate. Motoneurons in children and adolescents aged 7-17years (young development group) had higher signatures of excitability that included faster firing rates and more self-sustained activity compared to adults aged ≥18years. Participants aged 11-28years of age taking serotonin reuptake inhibitors had the highest measures of motoneuron excitability compared to their age-matched controls. The young development group also had more unstable contractions, which might partly be related to the high excitability of the motoneurons.

Sex-related differences in motor unit behavior are influenced by myosin heavy chain during high- but not moderate-intensity contractions.

Motor unit recruitment and firing rate patterns of the vastus lateralis (VL) have not been compared between sexes during moderate- and high-intensity contraction intensities. Additionally, the influence of fiber composition on potential sex-related differences remains unquantified. Eleven males and 11 females performed 40% and 70% maximal voluntary contractions (MVCs). Surface electromyographic (EMG) signals recorded from the VL were decomposed. Recruitment thresholds (RTs), MU action potential amplitudes (MUAPAMP ), initial firing rates (IFRs), mean firing rates (MFRs), and normalized EMG amplitude (N-EMGRMS ) at steady torque were analyzed. Y-intercepts and slopes were calculated for MUAPAMP , IFR, and MFR versus RT relationships. Type I myosin heavy chain isoform (MHC) was determined with muscle biopsies. There were no sex-related differences in MU characteristics at 40% MVC. At 70% MVC, males exhibited greater slopes (p = 0.002) for the MUAPAMP , whereas females displayed greater slopes (p = 0.001-0.007) for the IFR and MFR versus RT relationships. N-EMGRMS at 70% MVC was greater for females (p < 0.001). Type I %MHC was greater for females (p = 0.006), and was correlated (p = 0.018-0.031) with the slopes for the MUAPAMP , IFR, and MFR versus RT relationships at 70% MVC (r = -0.599-0.585). Both sexes exhibited an inverse relationship between MU firing rates and recruitment thresholds. However, the sex-related differences in MU recruitment and firing rate patterns and N-EMGRMS at 70% MVC were likely due to greater type I% MHC and smaller twitch forces of the higher threshold MUs for the females. Evidence is provided that muscle fiber composition may explain divergent MU behavior between sexes.

The passive properties of dendrites modulate the propagation of slowly-varying firing rate in feedforward networks

The propagation of slowly-varying firing rates has been proved significant for the development of the central nervous system. Recent reports have shown that the membrane passive properties of dendrites play a key role in the computation of the single neuron, which is of great importance to the function of neural networks. However, it is still unclear how dendritic passive properties affect the ability of cortical networks to propagate slowly-varying spiking activity. Here, we use two-compartment biophysical models to construct multilayered feedforward neural networks (FFNs) to investigate how dendritic passive properties affect the propagation of the slow-varying inputs. In the two-compartment biophysical models, one compartment represents apical dendrites, and the other compartment describes the soma plus the axon initial segment. Area proportion occupied by somatic compartment and coupling conductance between dendritic and somatic compartments are abstracted to capture the dendritic passive properties. A time-varying signal is injected into the first layer of the FFNs and the fidelity of the signal during propagation is used to qualify the ability of the FFN to transmit wave-like signals. Numerical results reveal an optimal value of coupling conductance between dendritic and somatic compartments to maximize the fidelity of the initial spiking activity. An increase of the dendritic area enhances the initial firing rate of neurons in the first layer by increasing the response of neurons to slow-varying wave-like input, resulting in a delay of attenuation of the firing rate, thus promoting the transmission of signals in FFN. Using a mean-field approach, we examine that changes in area proportion occupied by somatic compartment and coupling conductance between dendritic and somatic compartment affect the signal propagation ability of the FFN by adjusting the input–output transform of a single neuron. With the participation of external noise, a wide range of initial firing rates maintains a unique representation during propagation, which ensures the reliable transmission of slow-varying signals in FFNs. These findings are helpful to understand how passive properties of dendrites participate in the propagation of slowly varying signals in the cerebellum.

Firing rate trajectories of human motor units during activity-dependent muscle potentiation.

During activity-dependent potentiation (ADP), motor unit firing rates (MUFRs) are lower; however, the mechanism for this response is not known. During increasing torque isometric contractions at low contraction intensities, MUFR trajectories initially accelerate and saturate demonstrating a nonlinear response due to the activation of persistent inward currents (PICs) at the motoneuron. The purpose was to assess whether PICs are a factor in the reduction of MUFRs during ADP. To assess this, MUFR trajectories were fit with competing functions of linear regression and a rising exponential (i.e., acceleration and saturation). With fine-wire electrodes, discrete MU potential trains were recorded in the tibialis anterior during slowly increasing dorsiflexion contractions to 10% of maximal voluntary contraction following both voluntary [postactivation potentiation (PAP)] and evoked [posttetanic potentiation (PTP)] contractions. In eight participants, 25 MUs were recorded across both ADP conditions and compared with the control with no ADP effect. During PAP and PTP, the average MUFRs were 16.4% and 9.2% lower (both P ≤ 0.001), respectively. More MUFR trajectories were better fit to the rising exponential during control (16/25) compared with PAP (4/25, P < 0.001) and PTP (8/25, P = 0.03). The MU samples that had a rising exponential MUFR trajectory during PAP and PTP displayed an ∼11% lower initial acceleration compared with control (P < 0.05). Thus, presumed synaptic amplification and MUFR saturation due to PIC properties are attenuated during ADP regardless of the type of conditioning contraction. This response may contribute to lower MUFRs and likely occurred because synaptic input is reduced when contractile function is enhanced.NEW & NOTEWORTHY During activity-dependent muscle potentiation (ADP), initial motor unit firing rate (MUFR) acceleration and the occurrence of MUFR trajectory saturation as a function of increasing contraction intensity were assessed. With no ADP (control), trajectories were more likely to accelerate and saturate (16/25 units) compared with voluntary- and stimulated-induced ADP conditions (4/25 and 8/25 units, respectively) that were fit better linearly. Therefore, during ADP, an attenuated intrinsic response to voluntary synaptic inputs occurs.

Firing rate trajectories of human motor units during isometric ramp contractions to 10, 25 and 50% of maximal voluntary contraction

During low torque graded isometric contractions, motor units (MU) exhibit initial firing rate acceleration followed by saturation demonstrating a non-linear response attributed to persistent inward currents (PICs) which contribute to the net excitatory input. Firing rate saturation studies have been done exclusively at recruitment thresholds of low firing threshold MUs below 10% of isometric maximal voluntary contraction(MVC). It remains unclear whether later recruited (i.e. higher-threshold) MUs follow a similar firing rate trajectory as low-threshold units. Thus, MU firing rate trajectories were explored in relation to MU recruitment threshold (RT) at contraction levels between 10 and 50% of MVC. During graded isometric contractions to 10, 25 and 50% of MVC, single MU potentials were recorded from the tibialis anterior from 5 participants using tungsten microelectrodes. To characterize the firing rate trajectory, each MU train was fit by competing functions of torque as an exponential (i.e. saturated) and simple linear regression, using previous analysis methods (Fuglevand et al. 2015). Throughout a RT range of 0.02–41% of MVC, 261 MUs were compared. In 87% of MUs the better fit was by a linear function, whereas the remaining MUs (13%) were fit better with an exponential (saturated) firing rate trajectory. There was no statistical difference in the number of MUs better fit by the exponential function between low (<10% MVC) and relatively higher threshold MUs (>10% MVC; both p < 0.05). Increasing RT and rate of torque development (RTD) of the ramps were correlated with increased firing rate variability (larger error) in both fits (r = 0.3 and r = 0.4, both p < 0.01). Additionally, there was a 4-fold increase in peak antagonist surface electromyography (EMG) from 10 to 50% MVC contraction ramps. When all MUs were plotted with a normalized firing onset (i.e. 0% MVC) the data visually displayed an initial firing rate acceleration followed by a linear response (biphasic trajectory). Increased synaptic drive and greater antagonist surface EMG during moderate torque outputs may dampen PIC activity as compared with MUs during lower torque (<10% MVC) recruitment levels.

Sex-related differences in motor unit firing rates and action potential amplitudes of the first dorsal interosseous during high-, but not low-intensity contractions.

Despite ample evidence that females are weaker and possess smaller muscle cross-sectional areas (CSAs) compared to males, it remains unclear if there are sex-related differences in the properties of motor units (MU). Eleven males (age 22 ± 3years) and 12 females (age 21 ± 1years) performed isometric trapezoid muscle actions at 10% and 70% of maximal voluntary contraction (MVC). Surface electromyography signals were recorded and decomposed into MU action potential (AP) waveforms and firing instances. Average MUAP amplitudes (MUAPAMPS), mean firing rates (MFRs), initial firing rates (IFRs), and recruitment thresholds (RT) were calculated for the 10% MVC, while MUAPAMPS, IFRs, and MFRs were regressed against RT for the 70% MVC. Ultrasonography was used to measure CSA of the first dorsal interosseous (FDI). Males had greater CSAs (p < 0.001; males 2.34 ± 0.28cm2, females 1.82 ± 0.18cm2) and MVC strength (p < 0.001; males 25.9 ± 5.5N, females 16.44 ± 2.5N). No differences existed for MUAPAMPS, IFRs, MFRs, or RTs (p > 0.05) during the 10% MVC. For the 70% MVC, the y-intercepts from the MUAPAMPS vs. RT relationships were greater (p < 0.05) for the males (males - 0.19 ± 0.53mV; females - 0.78 ± 0.75mV), while the inverse was true for the MFR vs. RT relationships (males 31.55 ± 6.92pps, females 38.65 ± 6.71pps) with no differences (p > 0.05) in the slopes. Therefore, smaller CSAs and weaker MVCs are likely the result of smaller higher-threshold MUs for females.

Vastus Lateralis Motor Unit Firing Rate Is Higher in Women With Patellofemoral Pain

ObjectiveTo compare neural drive, determined from motor unit firing rate, in the vastus medialis and lateralis in women with and without patellofemoral pain. DesignCross-sectional study. SettingUniversity research laboratory. ParticipantsWomen (N=56) 19 to 35 years of age, including 36 with patellofemoral pain and 20 controls. InterventionsNot applicable. Main Outcome MeasuresParticipants sustained an isometric knee extension contraction at 10% of their maximal voluntary effort for 70 seconds. Motor units (N=414) were identified using high-density surface electromyography. Average firing rate was calculated between 5 and 35 seconds after recruitment for each motor unit. Initial firing rate was the inverse of the first 3 motor unit interspike intervals. ResultsIn control participants, vastus medialis motor units discharged at higher rates than vastus lateralis motor units (P=.001). This was not observed in women with patellofemoral pain (P=.78) because of a higher discharge rate of vastus lateralis compared with control participants (P=.002). No between-group differences were observed for vastus medialis (P=.93). Similar results were obtained for the initial motor unit firing rate. ConclusionsThese findings suggest that women with patellofemoral pain have a higher neural drive to vastus lateralis but not vastus medialis, which may be a contributor of the altered patellar kinematics observed in some studies. The different neural drive may be an adaptation to patellofemoral pain, possibly to compensate for decreased quadriceps force production, or a precursor of patellofemoral pain.

Hip position and sex differences in motor unit firing patterns of the vastus medialis and vastus medialis oblique in healthy individuals.

Weakness of the vastus medialis oblique (VMO) has been proposed to explain the high prevalence of knee pain in female subjects. Clinicians commonly use exercises in an attempt to preferentially activate the VMO. Recently, our group found evidence to support clinical theory that the VMO is neurologically distinct from the vastus medialis (VM). However, the ability to voluntarily activate these muscle subsections is still disputed. The aim of this study was to determine if VM and VMO activation varies between sexes and if control of the two muscles is different between rehabilitation exercises. Thirteen men and 13 women performed isometric straight leg raises in two hip positions, neutral hip rotation and 30 degrees lateral hip rotation. Bipolar intramuscular fine-wire electrodes were inserted into the VM and VMO to obtain motor unit recruitment thresholds and initial firing rates at recruitment. Linear mixed models and Tukey post hoc tests were used to assess significant differences in 654 motor units. Women demonstrated faster motor unit firing rate at recruitment, 1.18 ± 0.56 Hz higher than men. Motor units fired 0.47 ± 0.19 Hz faster during neutral hip rotation compared with lateral hip rotation. The VMO motor units were recruited 2.92 ± 1.28% earlier than the VM. All motor units were recruited 3.74 ± 1.27% earlier during neutral hip rotation than lateral hip rotation. Thus the VM and the VMO can be activated differentially, and their motor unit recruitment properties are affected by sex and hip position. NEW & NOTEWORTHY This is the first study to reveal differential activation of the vastus medialis oblique from the vastus medialis in clinical exercise protocols. Our research group used fine-wire electrodes to examine EMG signals of the vastus medialis oblique and vastus medialis to avoid possible cross talk. We also consider the effect of sex on motor unit firing patterns because of higher prevalence of knee pain in women, and yet few studies evaluating the sex differences in neuromuscular control.

An input-output linear time invariant model captures neuronal firing responses to external and behavioral events.

Investigating how neurons in different motor regions respond to external stimuli and behavioral events provides insight into motor control. A recent approach to studying neuronal activity is to construct a zero-input linear time invariant (ZI-LTI) state-space model, wherein the state vector consists of firing rate signals for different populations of neurons across motor regions. This approach allows for the populations to influence each other in a dynamical manner given an initial firing rate condition, and the model can accurately reconstruct firing rates within a limited epoch in the motor task during which no event occurs. Here, we generalize this LTI modeling approach to characterize firing responses of neurons to two events (a go cue and movement onset) in a movement task with a non-zero input LTI state-space model, herein referred to as input-output LTI (IO-LTI). Specifically, responses from 196 neurons in the primary motor (M1), ventral premotor (PMv), and dorsal premotor cortex (PMd) were recorded and modeled in two nonhuman primates executing a reach-to-grasp task. We found that a single IO-LTI model can reconstruct neuronal firing rate patterns of six populations of these neurons across the three areas in the presence of multiple events (go cue, movement onset). This is the first step towards constructing generative models of neuronal firing rates in the presence of multiple events, which then can be used to construct better decoders for brain machine interactive control.

High Threshold Motor Units Are Eventually Recruited During Low Force Efforts to Volitional Fatigue

There is disagreement in the literature regarding the magnitude of relative loads necessary to optimize muscle hypertrophy. Most recommend loads of 60-80% of 1 RM, while others demonstrate similar increases with loads as low as 30% 1 RM, as long as volitional fatigue is reached. PURPOSE: To use a deterministic model of individual motor unit and whole muscle fatigue to evaluate MU recruitment during low and high relative force efforts to volitional fatigue. METHODS: Our deterministic model predicts the forces and force capacities of 120 motor units, and the whole muscle, for the full range of excitations. We estimated: relative force, excitation threshold, initial firing rate, maximum firing rate and excitation adaptation of each MU. During sustained, constant force contractions, we calculated the required excitation level to meet the force demand, and monitored the instantaneous levels of each variable listed above, to determine force and force capacity changes in each active MU and the whole muscle over 0.1 s intervals. The model provided very good estimates of endurance times for intensities ranging from 15% to 90% MVC, compared to published values. RESULTS: The model predicted endurance times of 509.0 s and 14.4 s at 20% and 80% MVC, respectively, at which time all MUs were recruited for both intensities. The 20% MVC condition resulted in larger relative decreases in strength (i.e. ‘fatigue’) for all 120 MUs. At the endurance time for the 20% condition, fatigue was 6.4%, 81.8%, 7.4% and 54.3% for the lowest MU (#1), middle MU (#60), and highest threshold MU (#120) and average MU, respectively. In contrast, at the endurance time for the 80% condition, fatigue was 0.2%, 2.3%, 3.8% and 4.3% for MUs 1, 60, and 120, and average MU, respectively. Excitation adaptations resulted in decreases in firing rates for all motor units, relative to initial maximum rested values. CONCLUSION: The 20% MVC endurance trials resulted in the eventual recruitment of all MUs and > 10-fold greater average fatigue for the MU population compared to the 80% MVC condition. Therefore, low intensity contractions, sustained to volitional fatigue, may provide more challenge to the motor unit population, and a more profound overall stimulus for muscle hypertrophy, compared to high intensity contractions.

Recruitment Properties In Vastus Medialis And Vastus Medialis Oblique In Individuals With Patellofemoral Pain Syndrome

Patellofemoral pain syndrome (PFPS) is a commonly diagnosed knee pathology that is twice as prevalent in women. Sports medicine clinicians commonly use exercises to attempt to preferentially activate the vastus medialis oblique (VMO) to enhance medially vectored forces on the patella. Recently, our group confirmed clinical theory that the VMO is neurologically distinct from the vastus medialis (VM). However, the ability to voluntarily activate these muscle sub-sections is still disputed. PURPOSE: To determine how PFPS affects neuromuscular control of VM/VMO and examine if hip rotation during a straight leg raise (SLR) modifies motor patterns. METHODS: Thirteen healthy women and four women with PFPS performed isometric SLR in neutral hip rotation (SLR-NR) and during 30 degrees hip lateral rotation (SLR-LR). Participants performed ramp contractions by tracing a line on a screen with a rate of rise of 7.5% maximal voluntary contraction (MVC) per second up to 75% MVC. Bipolar intramuscular fine-wire electrodes were inserted into the VM and VMO. Initial motor unit firing rates (IFR) and recruitment threshold (RT) forces were measured. Generalized linear mixed models and Tukey post hoc tests were used to assess significant differences. The recruitment thresholds were log-transformed to meet the assumptions of normality of residuals and constant variance within each level. RESULTS: A total of 420 motor units were analyzed. There was a significant interaction effect for Muscle × Group for RT (p=0.02), demonstrating that women with PFPS activated their VMO later (20.4 ± 2.5% MVC) than healthy women (11.2 ± 1.5% MVC; p=0.01). The Group × Hip Position interaction effect was also significant for RT (p=0.02). Healthy women activated their VMO earlier during SLR-NR (9.9 ± 1.5% MVC) than during SLR-LR (14.9 ± 1.5% MVC; p=0.02). Women with PFPS had a delayed VMO onset time (19.1 ± 2.5% MVC) compared to healthy women (9.9 ± 1.5% MVC) in SLR-NR (p=0.02). There was no significant difference in IFR between muscles or hip positions between groups. CONCLUSIONS: Targeted strengthening of the VMO in SLR with a neutral hip rotation is a more effective training position for women with PFPS than using lateral hip rotation.

Characteristics of the Motor Units during Sternocleidomastoid Isometric Flexion among Patients with Mechanical Neck Disorder and Asymptomatic Individuals.

Mechanical neck disorder is a widespread and non-neurological musculoskeletal condition resulting from modern lifestyles. Presently, the fundamental electrophysiological properties of the motor units of the sternocleidomastoid muscles and the characteristics of the short-term synchronization of the motor unit in patients with neck pain are ambiguous. This study therefore aims to clarify the fundamental electrophysiological properties of the motor units of the sternocleidomastoid muscles in patients with mechanical neck disorder and in asymptomatic individuals. We further investigated whether alterations in the degree of motor unit short-term synchronization occur. The surface electrophysiological signals of the bilateral sternal heads of the sternocleidomastoid muscles of twelve patients with mechanical neck disorder and asymptomatic individuals were detected at 25% of the maximum voluntary contraction during cervical isometric flexion and then decomposed into individual motor unit action potential trains. We found that the patients with mechanical neck disorder showed significantly higher initial and mean firing rates of the sternocleidomastoid muscles and displayed substantially lower motor unit short-term synchronization values compared with the asymptomatic subjects. Consequently, these convincing findings support the assertion that patients with mechanical neck disorder display altered neuromuscular control strategies, such as the reinforcement of motor unit recruitment firing rates in the sternocleidomastoid muscles. The motor units of these patients also revealed neural recruitment strategies with relatively poor efficiency when executing the required motor tasks.

The Effects of Nitrate-Rich Supplementation on Neuromuscular Efficiency during Heavy Resistance Exercise

Objective: Nitrate-rich (NR) supplements can enhance exercise performance by improving neuromuscular function and the aerobic cost of exercise. However, little is known about the effects of nitrate on dynamic, multijoint resistance exercise.Methods: Fourteen resistance-trained men (age, 21.1 ± 0.9 years; height, 173.2 ± 2.9 cm: body mass, 77.6 ± 4.3 kg; squat one-repetition maximum [1RM], 127.5 ± 18.8 kg) participated in a randomized, double-blind, crossover experiment. Subjects consumed an NR or nitrate-poor (NP) supplement for 3 days, performed a bout of heavy resistance exercise, completed a washout, and then repeated the procedures with the remaining supplement. Before, during, and after exercise, individual and gross motor unit efficiency was assessed during isometric and dynamic muscle contractions. In addition, we compared physical performance, heart rate, lactate, and oxygen consumption (VO2).Results: Nitrate-rich supplementation resulted in lower initial muscle firing rates at rest and lower mean and maximum firing rates over the course of fatiguing exercise. Nitrate-poor supplementation was accompanied by increased mean and maximum firing rates by the end of exercise and lower initial firing rates. In addition, NR supplementation resulted in higher mean peak electromyography (EMG) amplitudes. Heart rate, lactate, and physical performance did not differ by treatment, but oxygen consumption increased more frequently when the NP supplement was consumed.Conclusion: Supplementation with an NR beetroot extract–based supplement provided neuromuscular advantages during metabolically taxing resistance exercise.

The increase of intrinsic excitability of layer V pyramidal cells in the prelimbic medial prefrontal cortex of adult mice after peripheral inflammation

Symptoms including depression and hypofunction of cognitive and decision-making are commonly associated with chronic pain. Recent studies have shown that the state of the medial prefrontal cortex (mPFC) neurons is important in diverse high-order cognitive and emotional activity in animals and humans. The mPFC layer V neurons mainly integrate information from other brain areas. The abnormal activity and function of pyramidal neurons influence the signal processing in the mPFC. Here we observed the changes of the excitability of the prelimbic mPFC neurons by whole-cell current-clamp recordings in adult mouse with inflammatory pain induced by complete Freund's adjuvant (CFA). Results showed that resting membrane potential and membrane input resistance did not differ between CFA-treated and control animals. Single action potential (AP) did not differ in terms of amplitude, but rheobase, half AP duration, and decay time were significantly decreased, and voltage threshold was more hyperpolarized in CFA group. Although the firing adaptation did not differ in two groups, the repetitive AP firing number and initial firing rate were significantly increased in CFA group. These data suggest that the increase in the intrinsic excitability of prelimbic mPFC layer V pyramidal neurons may be involved, at least in part in peripheral inflammatory pain.

Intrinsic excitability varies by sex in prepubertal striatal medium spiny neurons.

Sex differences in neuron electrophysiological properties were traditionally associated with brain regions directly involved in reproduction in adult, postpubertal animals. There is growing acknowledgement that sex differences can exist in other developmental periods and brain regions as well. This includes the dorsal striatum (caudate/putamen), which shows robust sex differences in gene expression, neuromodulator action (including dopamine and 17β-estradiol), and relevant sensorimotor behaviors and pathologies such as the responsiveness to drugs of abuse. Here we examine whether these sex differences extend to striatal neuron electrophysiology. We test the hypothesis that passive and active medium spiny neuron (MSN) electrophysiological properties in prepubertal rat dorsal striatum differ by sex. We made whole cell recordings from male and females MSNs from acute brain slices. The slope of the evoked firing rate to current injection curve was increased in MSNs recorded from females compared with males. The initial action potential firing rate was increased in MSNs recorded from females compared with males. Action potential after-hyperpolarization peak was decreased, and threshold was hyperpolarized in MSNs recorded from females compared with males. No sex differences in passive electrophysiological properties or miniature excitatory synaptic currents were detected. These findings indicate that MSN excitability is increased in prepubertal females compared with males, providing a new mechanism that potentially contributes to generating sex differences in striatal-mediated processes. Broadly, these findings demonstrate that sex differences in neuron electrophysiological properties can exist prepuberty in brain regions not directly related to reproduction.

Insulin modulates network activity in olfactory bulb slices: impact on odour processing

Odour perception depends closely on nutritional status, in animals as in humans. Insulin, the principal anorectic hormone, appears to be one of the major candidates for ensuring the link between olfactory abilities and nutritional status, by modifying processing in the olfactory bulb (OB), one of its main central targets. The present study investigates whether and how insulin can act in OB, by evaluating its action on the main output neurons activities, mitral cells (MCs), in acute rat OB slices. Insulin was found to act at two OB network levels: (1) on MCs, by increasing their excitability, probably by inhibiting two voltage-gated potassium (K(+)) channels; (2) on interneurons by modifying the GABAergic and on glutamatergic synaptic activity impinging on MCs, mainly reducing them. Insulin also altered the olfactory nerve (ON)-evoked excitatory postsynaptic currents in 60% of MCs. Insulin decreased or increased the ON-evoked responses in equal proportion and the direction of its effect depended on the initial neuron ON-evoked firing rate. Indeed, insulin tended to decrease the high and to increase the low ON-evoked firing rates, thereby reducing inter-MC response firing variability. Therefore, the effects of insulin on the evoked firing rates were not carried out indiscriminately in the MC population. By constructing a mathematical model, the impact of insulin complex effects on OB was assessed at the population activity level. The model shows that the reduction of variability across cells could affect MC detection and discrimination abilities, mainly by decreasing and, less frequently, increasing them, depending on odour quality. Thus, as previously proposed, this differential action of insulin on MCs across odours would allow this hormone to put the olfactory function under feeding signal control, given the discerning valence of an odour as a function of nutritional status.

Self-motion improves head direction cell tuning

Head direction (HD) cells respond when an animal faces a particular direction in the environment and form the basis for the animal's perceived directional heading. When an animal moves through its environment, accurate updating of the HD signal is required to reflect the current heading, but the cells still maintain a representation of HD even when the animal is motionless. This finding suggests that the HD system holds its current state in the absence of input, a view that we tested by rotating a head-restrained rat in the presence of a prominent visual landmark and then stopping it suddenly when facing the cell's preferred firing direction (PFD). Firing rates were unchanged for the first 100 ms, but then progressively decreased over the next 4 s and stabilized at ∼42% of their initial values. When the rat was stopped facing away from the PFD, there was no initial effect of braking, but the firing rate then increased steadily over 4 s and plateaued at ∼14% of its peak firing rate, substantially above initial background firing rates. In experiment 2, the rat was serially placed facing one of eight equidistant directions over 360° and held there for 30 s. Compared with the cell's peak firing rate during a passive rotation session, firing rates were reduced (51%) for in-PFD directions and increased (∼300%) from background levels for off-PFD directions, values comparable to those observed in the braking protocol. These differential HD cell responses demonstrate the importance of self-motion to the HD signal integrity.

Menstrual Cycle Mediates Vastus Medialis and Vastus Medialis Oblique Muscle Activity

Sports medicine professionals commonly describe two functionally different units of the vastus medialis (VM), the VM, and the vastus medialis oblique (VMO), but the anatomical support is equivocal. The functional difference of the VMO is principle to rehabilitation programs designed to alleviate anterior knee pain, a pathology that is known to have a greater occurrence in women. The purpose of this study was to determine whether the motor units of the VM and VMO are differentially recruited and if this recruitment pattern has an effect of sex or menstrual cycle phase. Single motor unit recordings from the VM and VMO were obtained for men and women during an isometric ramp knee extension. Eleven men were tested once. Seven women were tested during five different phases of the menstrual cycle, determined by basal body temperature mapping. The recruitment threshold and the initial firing rate at recruitment were determined from 510 motor unit recordings. The initial firing rate was lower in the VMO than that in the VM in women (P < 0.001) but not in men. There was no difference in recruitment thresholds for the VM and VMO in either sex or across the menstrual cycle. There was a main effect of menstrual phase on initial firing rate, showing increases from the early follicular to late luteal phase (P = 0.003). The initial firing rate in the VMO was lower than that in the VM during ovulatory (P = 0.009) and midluteal (P = 0.009) phases. The relative control of the VM and VMO changes across the menstrual cycle. This could influence patellar pathologies that have a higher incidence in women.