Plasticity of rat motoneuron rhythmic firing properties with varying levels of afferent and descending inputs

Exercise-induced adaptation of neurons in the vertebrate locomotor system

Vertebrate neurons are highly dynamic cells that undergo several alterations in their functioning and physiologies in adaptation to various external stimuli. In particular, how these neurons respond to physical exercise has long been an area of active research. Studies of the vertebrate locomotor system's adaptability suggest multiple mechanisms are involved in the regulation of neuronal activity and properties during exercise. In this brief review, we highlight recent results and insights from the field with a focus on the following mechanisms: (a) alterations in neuronal excitability during acute exercise; (b) alterations in neuronal excitability after chronic exercise; (c) exercise-induced changes in neuronal membrane properties via modulation of ion channel activity; (d) exercise-enhanced dendritic plasticity; and (e) exercise-induced alterations in neuronal gene expression and protein synthesis. Our hope is to update the community with a cellular and molecular understanding of the recent mechanisms underlying the adaptability of the vertebrate locomotor system in response to both acute and chronic physical exercise.

Signaling pathways regulated by natural active ingredients in the fight against exercise fatigue-a review

Exercise fatigue is a normal protective mechanism of the body. However, long-term fatigue hinders normal metabolism and exercise capacity. The generation and recovery from exercise fatigue involves alterations in multiple signaling pathways, mainly AMPK, PI3K/Akt, Nrf2/ARE, NF-κB, PINK1/Parkin, and BDNF/TrkB, as well as MAPK signaling pathways that mediate energy supply, reduction of metabolites, oxidative stress homeostasis, muscle fiber type switching, and central protective effects. In recent studies, a rich variety of natural active ingredients have been identified in traditional Chinese medicines and plant extracts with anti-fatigue effects, opening up the field of research in new anti-fatigue drugs. In this review we give an overview of the signaling pathways associated with the activity of natural food active ingredients against exercise fatigue. Such a comprehensive review is necessary to understand the potential of these materials as preventive measures and treatments of exercise fatigue. We expect the findings highlighted and discussed here will help guide the development of new health products and provide a theoretical and scientific basis for future research on exercise fatigue.

Endurance-exercise training adaptations in spinal motoneurones: potential functional relevance to locomotor output and assessment in humans

It is clear from non-human animal work that spinal motoneurones undergo endurance training (chronic) and locomotor (acute) related changes in their electrical properties and thus their ability to fire action potentials in response to synaptic input. The functional implications of these changes, however, are speculative. In humans, data suggests that similar chronic and acute changes in motoneurone excitability may occur, though the work is limited due to technical constraints. To examine the potential influence of chronic changes in human motoneurone excitability on the acute changes that occur during locomotor output, we must develop more sophisticated recording techniques or adapt our current methods. In this review, we briefly discuss chronic and acute changes in motoneurone excitability arising from non-human and human work. We then discuss the potential interaction effects of chronic and acute changes in motoneurone excitability and the potential impact on locomotor output. Finally, we discuss the use of high-density surface electromyogram recordings to examine human motor unit firing patterns and thus, indirectly, motoneurone excitability. The assessment of single motor units from high-density recording is mainly limited to tonic motor outputs and minimally dynamic motor output such as postural sway. Adapting this technology for use during locomotor outputs would allow us to gain a better understanding of the potential functional implications of endurance training-induced changes in human motoneurone excitability on motor output.

Health Benefits of Endurance Training: Implications of the Brain-Derived Neurotrophic Factor-A Systematic Review

This article presents a concept that wide expression of brain-derived neurotrophic factor (BDNF) and its receptors (TrkB) in the nervous tissue, evoked by regular endurance training (ET), can cause numerous motor and metabolic adaptations, which are beneficial for human health. The relationships between the training-evoked increase of endogenous BDNF and molecular and/or physiological adaptations in the nervous structures controlling both motor performance and homeostasis of the whole organism have been presented. Due to a very wide range of plastic changes that ET has exerted on various systems of the body, the improvement of motor skills and counteraction of the development of civilization diseases resulting from the posttraining increase of BDNF/TrkB levels have been discussed, as important for people, who undertake ET. Thus, this report presents the influence of endurance exercises on the (1) transformation of motoneuron properties, which are a final element of the motor pathways, (2) reduction of motor deficits evoked by Parkinson disease, and (3) prevention of the metabolic syndrome (MetS). This review suggests that the increase of posttraining levels of BDNF and its TrkB receptors causes simultaneous changes in the activity of the spinal cord, the substantia nigra, and the hypothalamic nuclei neurons, which are responsible for the alteration of the functional properties of motoneurons innervating the skeletal muscles, for the enhancement of dopamine release in the brain, and for the modulation of hormone levels involved in regulating the metabolic processes, responsively. Finally, training-evoked increase of the BDNF/TrkB leads to a change in a manner of regulation of skeletal muscles, causes a reduction of motor deficits observed in the Parkinson disease, and lowers weight, glucose level, and blood pressure, which accompany the MetS. Therefore, BDNF seems to be the molecular factor of pleiotropic activity, important in the modulation processes, underlying adaptations, which result from ET.

Understanding exercise-dependent plasticity of motoneurons using intracellular and intramuscular approaches

Spinal motoneurons (MN) exhibit exercise-dependent adaptations to increased activity, such as exercise and locomotion, as well as decreased activity associated with disuse, spinal cord injury, and aging. The development of several experimental approaches, in both human and animal models, has contributed significantly to our understanding of this plasticity. The purpose of this review is to summarize how intracellular recordings in an animal model and motor unit recordings in a human model have, together, contributed to our current understanding of exercise-dependent MN plasticity. These approaches and techniques will allow neuroscientists to continue to advance our understanding of MN physiology and the plasticity of the "final common path" of the motor system, and to design experiments to answer the critical questions that are emerging in this field.

Motoneuron firing properties are modified by trans-spinal direct current stimulation in rats

Spinal polarization evoked by direct current stimulation [trans-spinal direct current stimulation (tsDCS)] is a novel method for altering spinal network excitability; however, it remains not well understood. The aim of this study was to determine whether tsDCS influences spinal motoneuron activity. Twenty Wistar rats under general pentobarbital anesthesia were subjected to 15 min anodal (n = 10) or cathodal (n = 10) tsDCS of 0.1 mA intensity, and the electrophysiological properties of their motoneurons were intracellularly measured before, during, and after direct current application. The major effects of anodal intervention included increased minimum firing frequency and the slope of the frequency-current (f-I) relationship, as well as decreased rheobase and currents evoking steady-state firing (SSF). The effects of cathodal polarization included decreased maximum SSF frequency, decreased f-I slope, and decreased current evoking the maximum SSF. Notably, the majority of observed effects appeared immediately after the current onset, developed during polarization, and outlasted it for at least 15 min. Moreover, the effects of anodal polarization were generally more pronounced and uniform than those evoked by cathodal polarization. Our study is the first to present polarity-dependent, long-lasting changes in spinal motoneuron firing following tsDCS, which may aid in the development of more safe and accurate application protocols in medicine and sport. NEW & NOTEWORTHY Trans-spinal direct current stimulation induces significant polarity-dependent, long-lasting changes in the threshold and firing properties of spinal motoneurons. Anodal polarization potentiates motoneuron firing whereas cathodal polarization acts mainly toward firing inhibition. The alterations in rheobase and rhythmic firing properties are not restricted to the period of current application and can be observed long after the current offset.

The Alteration of Intrinsic Excitability and Synaptic Transmission in Lumbar Spinal Motor Neurons and Interneurons of Severe Spinal Muscular Atrophy Mice

Spinal muscular atrophy (SMA) is the leading genetic cause of death in infants. Studies with mouse models have demonstrated increased excitability and loss of afferent proprioceptive synapses on motor neurons (MNs). To further understand functional changes in the motor neural network occurring in SMA, we studied the intrinsic excitability and synaptic transmission of both MNs and interneurons (INs) from ventral horn in the lumbar spinal cord in the survival motor neuron (SMN)Δ7 mouse model. We found significant differences in the membrane properties of MNs in SMA mice compared to littermate controls, including hyperpolarized resting membrane potential, increased input resistance and decreased membrane capacitance. Action potential (AP) properties in MNs from SMA mice were also different from controls, including decreased rheobase current, increased amplitude and an increased afterdepolarization (ADP) potential. The relationship between AP firing frequency and injected current was reduced in MNs, as was the threshold current, while the percentage of MNs showing long-lasting potentiation (LLP) in the intrinsic excitability was higher in SMA mice. INs showed a high rate of spontaneous firing, and those from SMA mice fired at higher frequency. INs from SMA mice showed little difference in their input-output relationship, threshold current, and plasticity in intrinsic excitability. The changes observed in both passive membrane and AP properties suggest greater overall excitability in both MNs and INs in SMA mice, with MNs showing more differences. There were also changes of synaptic currents in SMA mice. The average charge transfer per post-synaptic current of spontaneous excitatory and inhibitory synaptic currents (sEPSCs/sIPSCs) were lower in SMA MNs, while in INs sIPSC frequency was higher. Strikingly in light of the known loss of excitatory synapses on MNs, there was no difference in sEPSC frequency in MNs from SMA mice compared to controls. For miniature synaptic currents, mEPSC frequency was higher in SMA MNs, while for SMA INs, both mEPSC and mIPSC frequencies were higher. In SMA-affected mice we observed alterations of intrinsic and synaptic properties in both MNs and INs in the spinal motor network that may contribute to the pathophysiology, or alternatively, may be a compensatory response to preserve network function.

Mechanisms and functional implications of motoneuron adaptations to increased physical activity

Motoneurons demonstrate adaptations in their physiological properties to alterations in chronic activity levels. The most consistent change that appears to result from endurance-type exercise training is the reduced excitatory current required to initiate and maintain rhythmic firing. While the precise mechanisms through which these neurons adapt to activity are currently unknown, evidence exists that adaptation may involve alterations in the expression of genes that code for membrane receptors, which can influence the responses of neurons to transmitters during activation. The influence of these adaptations may also extend to the resting condition, where ambient levels of neuroactive substances may influence ion conductances at rest, and thus result in the activation or inhibition of specific ion conductances that underlie the measurements of increased excitability that have been reported for motoneurons in the anesthetised state. We have applied motoneuron excitability and muscle unit contractile changes with endurance training to a mathematical computerized model of motor unit recruitment (Heckman and Binder 1991; J. Neurophysiol. 65(4):952–967). The results from the modelling exercise demonstrate increased task efficiency at relative levels of effort during a submaximal contraction. The physiological impact that nerve and muscle adaptations have on the neuromuscular system during standardized tasks seem to fit with reported changes in motor unit behaviour in trained human subjects.

Rapid activity-dependent modulation of the intrinsic excitability through up-regulation of KCNQ/Kv7 channel function in neonatal spinal motoneurons

Activity-dependent changes in the properties of the motor system underlie the necessary adjustments in its responsiveness on the basis of the environmental and developmental demands of the organism. Although plastic changes in the properties of the spinal cord have historically been neglected because of the archaic belief that the spinal cord is constituted by a hardwired network that simply relays information to muscles, plenty of evidence has been accumulated showing that synapses impinging on spinal motoneurons undergo short- and long-term plasticity. In the brain, brief changes in the activity level of the network have been shown to be paralleled by changes in the intrinsic excitability of the neurons and are suggested to either reinforce or stabilize the changes at the synaptic level. However, rapid activity-dependent changes in the intrinsic properties of spinal motoneurons have never been reported. In this study, we show that in neonatal mice the intrinsic excitability of spinal motoneurons is depressed after relatively brief but sustained changes in the spinal cord network activity. Using electrophysiological techniques together with specific pharmacological blockers of KCNQ/Kv7 channels, we demonstrate their involvement in the reduction of the intrinsic excitability of spinal motoneurons. This action results from an increased M-current, the product of the activation of KCNQ/Kv7 channels, which leads to a hyperpolarization of the resting membrane potential and a decrease in the input resistance of spinal motoneurons. Computer simulations showed that specific up-regulations in KCNQ/Kv7 channels functions lead to a modulation of the intrinsic excitability of spinal motoneurons as observed experimentally. These results indicate that KCNQ/Kv7 channels play a fundamental role in the activity-dependent modulation of the excitability of spinal motoneurons.

Retracing your footsteps: developmental insights to spinal network plasticity following injury

During development of the spinal cord, a precise interaction occurs between descending projections and sensory afferents, with spinal networks that lead to expression of coordinated motor output. In the rodent, during the last embryonic week, motor output first occurs as regular bursts of spontaneous activity, progressing to stochastic patterns of episodes that express bouts of coordinated rhythmic activity perinatally. Locomotor activity becomes functionally mature in the 2nd postnatal wk and is heralded by the onset of weight-bearing locomotion on the 8th and 9th postnatal day. Concomitantly, there is a maturation of intrinsic properties and key conductances mediating plateau potentials. In this review, we discuss spinal neuronal excitability, descending modulation, and afferent modulation in the developing rodent spinal cord. In the adult, plastic mechanisms are much more constrained but become more permissive following neurotrauma, such as spinal cord injury. We discuss parallel mechanisms that contribute to maturation of network function during development to mechanisms of pathological plasticity that contribute to aberrant motor patterns, such as spasticity and clonus, which emerge following central injury.

α-Motoneurons maintain biophysical heterogeneity in obesity and diabetes in Zucker rats

Small-diameter sensory dysfunction resulting from diabetes has received much attention in the literature, whereas the impact of diabetes on α-motoneurons (MN) has not. In addition, the chance of developing insulin resistance and diabetes is increased in obesity. No study has examined the impact of obesity or diabetes on the biophysical properties of MN. Lean Zucker rats and Zucker diabetic fatty (ZDF) rats were separated into lean, obese (ZDF fed standard chow), and diabetic (ZDF fed high-fat diet that led to diabetes) groups. Glass micropipettes recorded hindlimb MN properties from identified flexor and extensor MN. MN were separated within their groups on the basis of input conductance, which created high- and low-input conductance subpopulations for each. A significant shorter (20%) afterhyperpolarization half-decay (AHP_1/2) was found in low-conductance MN for the diabetic group only, whereas AHP½ tended to be shorter in the obese group (19%). Significant positive correlations were found among rheobase and input conductance for both lean and obese animals. No differences were found between the groups for afterhyperpolarization amplitude (AHP_amp), input conductance, rheobase, or any of the rhythmic firing properties (frequency-current slope and spike-frequency adaptation index). MN properties continue to be heterogeneous in obese and diabetic animals. Obesity does not seem to influence lumbar MN. Despite the resistance of MN to the impact of diabetes, the reduced AHP_1/2 decay and the tendency for a reduction in AHP_amp may be the first sign of change to MN function.NEW & NOTEWORTHY Knowledge about the impact of obesity and diabetes on the biophysical properties of motoneurons is lacking. We found that diabetes reduces the duration of the afterhyperpolarization and that motoneuron function is unchanged by obesity. A reduced afterhyperpolarization may impact discharge characteristics and may be the first sign of change to motoneuron function.

Adaptations of motoneuron properties after weight-lifting training in rats

Resistance training, with repeated short-term and high-intensity exercises, is responsible for an increase in muscle mass and force. The aim of this study was to determine whether such training induces adaptations in the electrophysiological properties of motoneurons innervating the trained muscles and to relate these adaptive changes to previous observations made on motor unit contractile properties. The study was performed on adult male Wistar rats. Animals from the training group were subjected to a 5-wk voluntary progressive weight-lifting program, whereas control rats were restricted to standard cage activity. Intracellular recordings from lumbar spinal motoneurons were made under pentobarbital anesthesia. Membrane properties were measured, and rhythmic firing of motoneurons was analyzed. Strength training evoked adaptive changes in both slow- and fast-type motoneurons, indicating their increased excitability. A shorter spike duration, a higher input resistance, a lower rheobase, a decrease in the minimum current required to evoke rhythmic firing, an increase in the maximum frequencies of the early-state firing (ESF) and the steady-state firing (SSF), and an increase in the respective slopes of the frequency-current ( f/ I) relationship were observed in fast motoneurons of the trained group. The increase in the maximum ESF and SSF frequencies and an increase in the SSF f/ I slope were also present in slow motoneurons. Higher maximum firing rates of motoneurons as well as higher discharge frequencies evoked at the same level of intracellular depolarization current imply higher levels of tetanic forces developed by motor units over the operating range of force production after strength training.

NEW & NOTEWORTHY Neuronal responses to weight-lifting training can be observed in altered properties of both slow and fast motoneurons. Motoneurons of trained animals are more excitable, require lower intracellular currents to evoke rhythmic firing, and have the ability to evoke higher maximum discharge frequencies during repetitive firing.

Midbrain stimulation-evoked lumbar spinal activity in the adult decerebrate mouse

BACKGROUND

Genetic techniques rendering murine models a popular choice for neuroscience research has led to important insights on neural networks controlling locomotor function. Using genetically altered mouse models for in vivo, electrophysiological studies in the adult state could validate key principles of locomotor network organization that have been described in neonatal, in vitro preparations.

NEW METHOD

The experimental model presented here describes a decerebrate, in vivo adult mouse preparation in which focal, electrical midbrain stimulation was combined with monitoring lumbar neural activity and motor output after pre-collicular decerebration and neuromuscular blockade.

RESULTS

Lumbar cord dorsum potentials (in 9/10 animals) and motoneuron output (in 3/5 animals) including fictive locomotion, was achieved by focal midbrain stimulation. The stimulation electrode locations could be reconstructed (in 6/7 animals) thereby allowing anatomical identification of the stimulated supraspinal regions.

COMPARISON WITH EXISTING METHODS

This preparation allows for concomitant recording or stimulation in the spinal cord and in the mid/hindbrain of adult mice. It differs from other methods used in the past with adult mice as it does not require pharmacological manipulation of neural excitability in order to generate motor output.

CONCLUSIONS

Midbrain stimulation can consistently be used for inducing lumbar neural activity in adult mice under neuromuscular blockade. This model is suited for examination of brain-spinal connectivity and it may benefit a wide range of fields depending on the features of the genetically modified mouse models used in combination with the presented methods.

Corticospinal excitability to the biceps brachii and its relationship to postactivation potentiation of the elbow flexors

We examined the effects of a submaximal voluntary elbow flexor contraction protocol on measures of corticospinal excitability and postactivation potentiation of evoked muscle forces and if these measures were state‐dependent (rest vs. voluntary muscle contraction). Participants completed four experimental sessions where they rested or performed a 5% maximum voluntary contraction (MVC) of the elbow flexors prior to, immediately, and 5 min following a submaximal contraction protocol. During rest or 5% MVC, transcranial magnetic stimulation, transmastoid electrical stimulation, electrical stimulation of biceps brachii motor point and Erb's point were elicited to induce motor‐evoked potentials (MEPs), cervicomedullary MEPs (CMEPs), potentiated twitch (PT) force, and maximal muscle compound action potential (M_max), respectively prior to, immediately, and 5 min postcontraction protocol. MEP amplitudes increased (215 and 165%M_max, P ≤ 0.03) only at 1 and 6s postcontraction protocol, respectively during rest but not 5% MVC. CMEP amplitudes decreased during rest and 5% MVC (range:21–58%M_max, P ≤ 0.04) for up to 81 sec postcontraction protocol. Peak twitch force increased immediately postcontraction protocol and remained elevated for 90 sec (range:122–147% increase, P < 0.05). There was a significant positive correlation between MEP and PT force during rest (r = 0.88, P = 0.01) and a negative correlation between CMEP and PT force during rest (r = −0.85, P < 0.02 and 5% MVC (r = −0.96, P < 0.01) immediately postcontraction protocol. In conclusion, the change in corticospinal and spinal excitability was state‐ and time‐dependent whereas spinal excitability and postactivation potentiation were time‐dependent following the contraction protocol. Changes in corticospinal excitability and postactivation potentiation correlated and were also state‐dependent.

Extensor motoneurone properties are altered immediately before and during fictive locomotion in the adult decerebrate rat

This study examined motoneurone properties during fictive locomotion in the adult rat for the first time. Fictive locomotion was induced via electrical stimulation of the mesencephalic locomotor region in decerebrate adult rats under neuromuscular blockade to compare basic and rhythmic motoneurone properties in antidromically identified extensor motoneurones during: (1) quiescence, before and after fictive locomotion; (2) the 'tonic' period immediately preceding locomotor-like activity, whereby the amplitude of peripheral flexor (peroneal) and extensor (tibial) nerves are increased but alternation has not yet occurred; and (3) locomotor-like episodes. Locomotion was identified by alternating flexor-extensor nerve activity, where the motoneurone either produced membrane oscillations consistent with a locomotor drive potential (LDP) or did not display membrane oscillation during alternating nerve activity. Cells producing LDPs were referred to as such, while those that did not were referred to as 'idle' motoneurones. LDP and idle motoneurones during locomotion had hyperpolarized spike threshold (Vth ; LDP: 3.8 mV; idle: 5.8 mV), decreased rheobase and an increased discharge rate (LDP: 64%; idle: 41%) during triangular ramp current injection even though the frequency-current slope was reduced by 70% and 55%, respectively. Modulation began in the tonic period immediately preceding locomotion, with a hyperpolarized Vth and reduced rheobase. Spike frequency adaptation did not occur in spiking LDPs or firing generated from sinusoidal current injection, but occurred during a sustained current pulse during locomotion. Input conductance showed no change. Results suggest motoneurone modulation occurs across the pool and is not restricted to motoneurones engaged in locomotion.

Differences in supraspinal and spinal excitability during various force outputs of the biceps brachii in chronic- and non-resistance trained individuals

Motor evoked potentials (MEP) and cervicomedullary evoked potentials (CMEP) may help determine the corticospinal adaptations underlying chronic resistance training-induced increases in voluntary force production. The purpose of the study was to determine the effect of chronic resistance training on corticospinal excitability (CE) of the biceps brachii during elbow flexion contractions at various intensities and the CNS site (i.e. supraspinal or spinal) predominantly responsible for any training-induced differences in CE. Fifteen male subjects were divided into two groups: 1) chronic resistance-trained (RT), (n = 8) and 2) non-RT, (n = 7). Each group performed four sets of ∼5 s elbow flexion contractions of the dominant arm at 10 target forces (from 10%-100% MVC). During each contraction, subjects received 1) transcranial magnetic stimulation, 2) transmastoid electrical stimulation and 3) brachial plexus electrical stimulation, to determine MEP, CMEP and compound muscle action potential (Mmax) amplitudes, respectively, of the biceps brachii. All MEP and CMEP amplitudes were normalized to Mmax. MEP amplitudes were similar in both groups up to 50% MVC, however, beyond 50% MVC, MEP amplitudes were lower in the chronic RT group (p<0.05). CMEP amplitudes recorded from 10-100% MVC were similar for both groups. The ratio of MEP amplitude/absolute force and CMEP amplitude/absolute force were reduced (p<0.012) at all contraction intensities from 10-100% MVC in the chronic-RT compared to the non-RT group. In conclusion, chronic resistance training alters supraspinal and spinal excitability. However, adaptations in the spinal cord (i.e. motoneurone) seem to have a greater influence on the altered CE.

Motoneurone afterhyperpolarisation time-course following stroke

OBJECTIVE

Our aim was to investigate any changes in the estimated time-course of the afterhyperpolarisation (AHP) in motoneurones innervating the tibialis anterior following stroke, with a secondary objective to compare the results from two different AHP estimation techniques.

METHODS

Motor units from tibialis anterior on the paretic and non-paretic sides of 15 subjects with chronic stroke were recorded using intramuscular electrodes during voluntary isometric contraction. Participants varied the motor unit firing rate from its lowest rate to approximately 10 Hz. The AHP duration was estimated using the interval death rate (IDR) and transition point methods.

RESULTS

The AHP decay time-constant was significantly different between sides (paretic: 41.7 ± 8.5 ms, non-paretic: 36.2 ± 6.4 ms). Additionally, the paretic AHP time-constant was significantly longer in participants with low motor recovery (45.9 ± 9.1 ms) than with high motor recovery (39.3 ± 10.0 ms) as measured by CMSA score. The AHP estimates from the two techniques were correlated (r=0.78).

CONCLUSIONS

The AHP time-course prolongation on the paretic side of people with chronic stroke is more pronounced in people with low motor recovery.

SIGNIFICANCE

Changes in the motoneurone AHP time course post-stroke were related to muscle function and may play a role in the commonly-observed reduction of motor unit discharge rate during voluntary contractions following stroke.