Interactions between focused synaptic inputs and diffuse neuromodulation in the spinal cord

The effects of α-lactalbumin supplementation and handgrip contraction on soleus motoneuron excitability

INTRODUCTION

We tested two strategies that hypothetically increase serotonin availability (α-lactalbumin consumption and a remote submaximal handgrip contraction) on estimates of persistent inward currents (PICs) amplitude of soleus muscle in healthy participants.

METHODS

With a randomised, double-blind, and cross-over design, 13 healthy participants performed triangular-shaped ramp contractions with their plantar flexors (20% of maximal torque), followed by a 30-s handgrip sustained contraction (40% of maximal force) and consecutive repeated triangular-shaped contractions. This was performed before and after the consumption of either 40 g of α-lactalbumin, an isonitrogenous beverage (Zein) or an isocaloric beverage (Corn-starch). Soleus motor units discharge rates were analysed from high-density surface electromyography signals. PICs were estimated by calculating the delta frequency (ΔF) of motor unit train spikes using the paired motor unit technique.

RESULTS

ΔF (0.19 pps; p = 0.001; d = 0.30) and peak discharge rate (0.20 pps; p < 0.001; d = 0.37) increased after the handgrip contraction, irrespective of the consumed supplement. No effects of α-lactalbumin were observed.

CONCLUSIONS

Our results indicate that 40 g of α-lactalbumin was unable to modify intrinsic motoneuron excitability. However, performing a submaximal handgrip contraction before the plantar flexion triangular contraction was capable of increasing ΔF and discharge rates on soleus motor units. These findings highlight the diffused effects of serotonergic input, its effects on motoneuron discharge behaviour, and suggest a cross-effector effect within human motoneurons.

Effects of α-lactalbumin on strength, fatigue and psychological parameters: a randomised double-blind cross-over study

PURPOSE

The neurotransmitter serotonin has a strong effect on behaviour and motor control. Regarding motor control, serotonin contributes to the development of fatigue and is also involved in the ability of motor neurones to operate across a large range of forces (gain control). The consumption of tryptophan-rich supplements (such as α-lactalbumin) is of interest because this amino acid is the only precursor for brain serotonin synthesis. Therefore, the purpose of this study was to determine the effects of α-lactalbumin supplementation on neuromuscular performance.

METHODS

Using a randomised double-blind cross-over design, 16 healthy participants performed plantar flexor and handgrip maximal voluntary contractions, a 30-s submaximal handgrip contraction, and a plantar flexor fatigue protocol before and 90 min after consuming either 40 g of α-lactalbumin, an isonitrogenous beverage (Zein) or an isocaloric beverage (corn-starch). Sleepiness, mood, and cognition were assessed to evaluate any psychological effects.

RESULTS

α-Lactalbumin decreased force steadiness by 25% during the sustained submaximal handgrip contraction (p < 0.01) and induced greater fatigue (15% reduction in total torque-time integral, p = 0.01) during the fatigue protocol. These effects were not observed for the other control beverages. No effects were found for maximal or explosive strength, or psychological measurements.

CONCLUSIONS

40 g of α-lactalbumin increased handgrip force variability and reduced performance during fatiguing muscle contractions but did not influence brief maximal contractions or psychological parameters in healthy individuals. These findings support the hypothesis that the consumption of α-lactalbumin can increase motor neurone input-output gain and exacerbate central fatigue during sustained maximal exercise.

Concurrent Achilles tendon vibration and tibial nerve stimulation to estimate persistent inward current strength in motoneurons

Vibratory (Tvib) and sustained (Tsust) torque responses to concurrent Achilles tendon vibration and neuromuscular electrical stimulation applied over the muscle belly (vib+stim) are used as indicators of motoneuron facilitation and, theoretically, persistent inward current strength. However, neuromuscular electrical stimulation (NMES) applied to the nerve trunk may potentiate motoneuronal excitability more than muscle belly NMES, yet it remains unclear whether NMES applied over the nerve evokes robust Tvib and Tsust responses when used during the vib+stim protocol. This study tested whether a nerve-targeted vib+stim protocol elicits Tvib and Tsust responses in the ankle plantar flexors with acceptable intra- and inter-session reliability. Fifteen men performed the vib+stim protocol with NMES applied over the tibial nerve three times across two sessions; twice in a single session (5-min apart) to test intrasession reliability and then again after 48 h to test intersession reliability. Intraclass correlation coefficients (ICC3,1), within-participant coefficients of variation (CV) and pairwise comparisons were used to verify relative and absolute reliability as well as systematic bias. Thirteen men presented Tvib and Tsust responses (response rate of 87%). Intrasession Tvib and Tsust ICCs were >0.73 but inter-session ICCs were <0.5. Although no systematic bias was detected (p>0.05), both intra- and inter-session CVs were large (>10%) for Tvib and Tsust. The Vib+stim protocol with NMES applied over the nerve evoked Tvib and Tsust in almost all participants, but presented a large intra- and inter-session variability. The method does not appear to be effective for assessing motoneuron facilitation in the plantar flexors.

Neural control of the healthy pectoralis major from low-to-moderate isometric contractions

The pectoralis major critically enables arm movement in several directions. However, its neural control remains unknown. High-density electromyography (HD-sEMG) was acquired from the pectoralis major in two sets of experiments in healthy young adults. Participants performed ramp-and-hold isometric contractions in: adduction, internal rotation, flexion, and horizontal adduction at three force levels: 15%, 25%, and 50% scaled to task-specific maximal voluntary force (MVF). HD-sEMG signals were decomposed into motor unit spike trains using a convolutive blind source separation algorithm and matched across force levels using a motor unit matching algorithm. The mean discharge rate and coefficient of variation were quantified across the hold and compared between 15% and 25% MVF across all tasks, whereas comparisons between 25% and 50% MVF were made where available. Mean motor unit discharge rate was not significantly different between 15% and 25% MVF (all P > 0.05) across all tasks or between 25% and 50% MVF in horizontal adduction (P = 0.11), indicating an apparent saturation across force levels and the absence of rate coding. These findings suggest that the pectoralis major likely relies on motor unit recruitment to increase force, providing first-line evidence of motor unit recruitment in this muscle and paving the way for more deliberate investigations of the pectoralis major involvement in shoulder function.NEW & NOTEWORTHY This work is the first to investigate the relative contribution of rate coding and motor unit recruitment in the pectoralis major muscle in several functionally relevant tasks and across varying force levels in healthy adults. Our results demonstrate the absence of motor unit rate coding with an increase in EMG amplitude with increases in force level in all tasks examined, indicating that the pectoralis major relies on motor unit recruitment to increase force.

Time Course of Alterations in Adult Spinal Motoneuron Properties in the SOD1(G93A) Mouse Model of ALS

Although amyotrophic lateral sclerosis (ALS) is an adult-onset neurodegenerative disease, motoneuron electrical properties are already altered during embryonic development. Motoneurons must therefore exhibit a remarkable capacity for homeostatic regulation to maintain a normal motor output for most of the life of the patient. In the present article, we demonstrate how maintaining homeostasis could come at a very high cost. We studied the excitability of spinal motoneurons from young adult SOD1(G93A) mice to end-stage. Initially, homeostasis is highly successful in maintaining their overall excitability. This initial success, however, is achieved by pushing some cells far above the normal range of passive and active conductances. As the disease progresses, both passive and active conductances shrink below normal values in the surviving cells. This shrinkage may thus promote survival, implying the previously large values contribute to degeneration. These results support the hypothesis that motoneuronal homeostasis may be "hypervigilant" in ALS and a source of accumulating stress.

Differences in estimated persistent inward currents between ankle flexors and extensors in humans

Persistent inward currents (PICs) are responsible for amplifying motoneuronal synaptic inputs and contribute to generating normal motoneuron activation. Delta-F (ΔF) is a well-established method that estimates PICs in humans indirectly from firing patterns of individual motor units. Traditionally, motor unit firing patterns are obtained by manually decomposing electromyography (EMG) signals recorded through intramuscular electrodes (iEMG). A previous iEMG study has shown that in humans the elbow extensors have higher ΔF than the elbow flexors. In this study, EMG signals were collected from the ankle extensors and flexors using high-density surface array electrodes during isometric sitting and standing at 10-30% maximum voluntary contraction. The signals were then decomposed into individual motor unit firings. We hypothesized that comparable to the upper limb, the lower limb extensor muscles (soleus) would have higher ΔF than the lower limb flexor muscles [tibialis anterior (TA)]. Contrary to our expectations, ΔF was higher in the TA than the soleus during sitting and standing despite the difference in cohort of participants and body positions. The TA also had significantly higher maximum discharge rate than the soleus while there was no difference in rate increase. When only the unit pairs with similar maximum discharge rates were compared, ∆F was still higher in the TA than the soleus. Future studies will focus on investigating the functional significance of the findings.NEW & NOTEWORTHY With the use of high-density surface array electrodes and convolutive blind source separation algorithm, thousands of motor units were decomposed from the soleus and tibialis anterior muscles. Persistent inward currents were estimated under seated and standing conditions via delta-F (∆F) calculation, and the results showed that unlike the upper limb, the flexor has higher ∆F than the extensor in the lower limb. Future studies will focus on functional significance of the findings.

Vibration attenuates spasm-like activity in humans with spinal cord injury

KEY POINTS

Cutaneous reflexes were tested to examine the neuronal mechanisms contributing to muscle spasms in humans with chronic spinal cord injury (SCI). Specifically, we tested the effect of Achilles and tibialis anterior tendon vibration on the early and late components of the cutaneous reflex and reciprocal Ia inhibition in the soleus and tibialis anterior muscles in humans with chronic SCI. We found that tendon vibration reduced the amplitude of later but not earlier cutaneous reflex in the antagonist but not in the agonist muscle relative to the location of the vibration. In addition, reciprocal Ia inhibition between antagonist ankle muscles increased with tendon vibration and participants with a larger suppression of the later component of the cutaneous reflex had stronger reciprocal Ia inhibition from the antagonistic muscle. Our study is the first to provide evidence that tendon vibration attenuates late cutaneous spasm-like reflex activity, likely via reciprocal inhibitory mechanisms, and may represent a method, when properly targeted, for controlling spasms in humans with SCI.

ABSTRACT

The neuronal mechanisms contributing to the generation of involuntary muscle contractions (spasms) in humans with spinal cord injury (SCI) remain poorly understood. To address this question, we examined the effect of Achilles and tibialis anterior tendon vibration at 20, 40, 80 and 120 Hz on the amplitude of the long-polysynaptic (LPR, from reflex onset to 500 ms) and long-lasting (LLR, from 500 ms to reflex offset) cutaneous reflex evoked by medial plantar nerve stimulation in the soleus and tibialis anterior, and reciprocal Ia inhibition between these muscles, in 25 individuals with chronic SCI. We found that Achilles tendon vibration at 40 and 80 Hz, but not other frequencies, reduced the amplitude of the LLR in the tibialis anterior, but not the soleus muscle, without affecting the amplitude of the LPR. Vibratory effects were stronger at 80 than 40 Hz. Similar results were found in the soleus muscle when the tibialis anterior tendon was vibrated. Notably, tendon vibration at 80 Hz increased reciprocal Ia inhibition between antagonistic ankle muscles and vibratory-induced increases in reciprocal Ia inhibition were correlated with decreases in the LLR, suggesting that participants with a larger suppression of later cutaneous reflex activity had stronger reciprocal Ia inhibition from the antagonistic muscle. Our study is the first to provide evidence that tendon vibration suppresses late spasm-like activity in antagonist but not agonist muscles, likely via reciprocal inhibitory mechanisms, in humans with chronic SCI. We argue that targeted vibration of antagonistic tendons might help to control spasms after SCI.

Estimates of persistent inward current in human motor neurons during postural sway

Persistent inward current (PIC) plays a critical role in setting the gain of spinal motor neurons. In humans, most estimates of PIC are made from plantarflexor or dorsiflexor motor units in a seated position. This seated and static posture negates the task-dependent nature of the monoaminergic drive and afferent inhibition that modulate PIC activation. Our purpose was to estimate PIC during both the conventional seated posture and in a more functionally relevant anterior postural sway. We hypothesized that paired motor unit estimates of PIC would be greater when during standing compared with sitting. Soleus motor neuron PIC was estimated via the paired motor unit (PMU) technique. For each motor unit pair, difference in reference unit firing frequency (ΔF) estimates of PIC were made during isometric ramps in plantarflexion force during sitting (conventional approach) and during standing anterior postural sway (new approach). Baseline reciprocal inhibition (RI) was also measured in each posture using the poststimulus time histogram technique. ΔF estimates during standing postural sway were not different [2.64 ± 0.95 pulses/s (pps), P = 0.098] from seated PIC estimates (3.15 ± 1.45 pps) measured from the same motor unit pair. Similarly, reciprocal inhibition at the onset of each task was the same in standing (-0.60 ± 0.32, P = 0.301) and seated (-0.86 ± 0.82) postures. PMU recordings made during standing postural sway met all assumptions that underlay the PMU technique, including rate modulation ≥0.5 pps (3.11 ± 1.90 pps), rate-rate correlation r ≥ 0.7 (0.84 ± 0.13), and time between reference and test unit recruitment ≥1 s (1.83 ± 0.81 s). This study presents a novel, functionally relevant standing method for investigating PIC in humans.NEW & NOTEWORTHY Paired motor unit (PMU) estimates of persistent inward current (PIC) in human soleus motor units are typically made in seated posture. Our study demonstrates that these estimates can be made during standing forward sway, a task that more accurately reflects the postural role of human soleus muscle. PMU recordings made during standing postural sway were validated using all previously published criteria used to test the assumptions of the PMU technique. Standing estimates of PIC did not differ from seated estimates made from the same motor unit pairs.

Increased human stretch reflex dynamic sensitivity with height-induced postural threat

KEY POINTS

Threats to standing balance (postural threat) are known to facilitate soleus tendon-tap reflexes, yet the mechanisms driving reflex changes are unknown. Scaling of ramp-and-hold dorsiflexion stretch reflexes to stretch velocity and amplitude were examined as indirect measures of changes to muscle spindle dynamic and static function with height-induced postural threat. Overall, stretch reflexes were larger with threat. Furthermore, the slope (gain) of the stretch-velocity vs. short-latency reflex amplitude relationship was increased with threat. These findings are interpreted as indirect evidence for increased muscle spindle dynamic sensitivity, independent of changes in background muscle activity levels, with a threat to standing balance. We argue that context-dependent scaling of stretch reflexes forms part of a multisensory tuning process where acquisition and/or processing of balance-relevant sensory information is continuously primed to facilitate feedback control of standing balance in challenging balance scenarios.

ABSTRACT

Postural threat increases soleus tendon-tap (t-) reflexes. However, it is not known whether t-reflex changes are a result of central modulation, altered muscle spindle dynamic sensitivity or combined spindle static and dynamic sensitization. Ramp-and-hold dorsiflexion stretches of varying velocities and amplitudes were used to examine velocity- and amplitude-dependent scaling of short- (SLR) and medium-latency (MLR) stretch reflexes as an indirect indicator of spindle sensitivity. t-reflexes were also performed to replicate previous work. In the present study, we examined the effects of postural threat on SLR, MLR and t-reflex amplitude, as well as SLR-stretch velocity scaling. Forty young-healthy adults stood with one foot on a servo-controlled tilting platform and the other on a stable surface. The platform was positioned on a hydraulic lift. Threat was manipulated by having participants stand in low (height 1.1 m; away from edge) then high (height 3.5 m; at the edge) threat conditions. Soleus stretch reflexes were recorded with surface electromyography and SLRs and MLRs were probed with fixed-amplitude variable-velocity stretches. t-reflexes were evoked with Achilles tendon taps using a linear motor. SLR, MLR and t-reflexes were 11%, 9.5% and 16.9% larger, respectively, in the high compared to low threat condition. In 22 out of 40 participants, SLR amplitude was correlated to stretch velocity at both threat levels. In these participants, the gain of the SLR-velocity relationship was increased by 36.1% with high postural threat. These findings provide new supportive evidence for increased muscle spindle dynamic sensitivity with postural threat and provide further support for the context-dependent modulation of human somatosensory pathways.

Modulation of human vestibular reflexes with increased postural threat

Anxiety and arousal have been shown to facilitate human vestibulo-ocular reflexes, presumably through direct neural connections between the vestibular nuclei and emotional processing areas of the brain. However, the effects of anxiety, fear and arousal on balance-relevant vestibular reflexes are currently unknown. The purpose of this study was to manipulate standing height to determine whether anxiety and fear can modulate the direct relationship between vestibular signals and balance reflexes during stance. Stochastic vestibular stimulation (SVS; 2-25 Hz) was used to evoke ground reaction forces (GRF) while subjects stood in both LOW and HIGH surface height conditions. Two separate experiments were conducted to investigate the SVS-GRF relationship, in terms of coupling (coherence and cumulant density) and gain, in the medio-lateral (ML) and antero-posterior (AP) directions. The short- and medium-latency cumulant density peaks were both significantly increased in the ML and AP directions when standing in HIGH, compared to LOW, conditions. Likewise, coherence was statistically greater between 4.3 Hz and 6.7 Hz in the ML, and between 5.5 and 17.7 Hz in the AP direction. When standing in the HIGH condition, the gain of the SVS-GRF relationship was increased 81% in the ML direction, and 231% in the AP direction. The significant increases in coupling and gain observed in both experiments demonstrate that vestibular-evoked balance responses are augmented in states of height-induced postural threat. These data support the possibility that fear or anxiety-mediated changes to balance control are affected by altered central processing of vestibular information.

Dopamine: a parallel pathway for the modulation of spinal locomotor networks

The spinal cord contains networks of neurons that can produce locomotor patterns. To readily respond to environmental conditions, these networks must be flexible yet at the same time robust. Neuromodulators play a key role in contributing to network flexibility in a variety of invertebrate and vertebrate networks. For example, neuromodulators contribute to altering intrinsic properties and synaptic weights that, in extreme cases, can lead to neurons switching between networks. Here we focus on the role of dopamine in the control of stepping networks in the spinal cord. We first review the role of dopamine in modulating rhythmic activity in the stomatogastric ganglion (STG) and the leech, since work from these preparations provides a foundation to understand its role in vertebrate systems. We then move to a discussion of dopamine’s role in modulation of swimming in aquatic species such as the larval xenopus, lamprey and zebrafish. The control of terrestrial walking in vertebrates by dopamine is less studied and we review current evidence in mammals with a focus on rodent species. We discuss data suggesting that the source of dopamine within the spinal cord is mainly from the A11 area of the diencephalon, and then turn to a discussion of dopamine’s role in modulating walking patterns from both in vivo and in vitro preparations. Similar to the descending serotonergic system, the dopaminergic system may serve as a potential target to promote recovery of locomotor function following spinal cord injury (SCI); evidence suggests that dopaminergic agonists can promote recovery of function following SCI. We discuss pharmacogenetic and optogenetic approaches that could be deployed in SCI and their potential tractability. Throughout the review we draw parallels with both noradrenergic and serotonergic modulatory effects on spinal cord networks. In all likelihood, a complementary monoaminergic enhancement strategy should be deployed following SCI.

Nonuniform distribution of contacts from noradrenergic and serotonergic boutons on the dendrites of cat splenius motoneurons

The input-output properties of motoneurons are dynamically regulated. This regulation depends, in part, on the relative location of excitatory and inhibitory synapses, voltage-dependent and -independent channels, and neuromodulatory synapses on the dendritic tree. The goal of the present study was to quantify the number and distribution of synapses from two powerful neuromodulatory systems that originate from noradrenergic (NA) and serotonergic (5-HT) neurons. Here we show that the dendritic trees of motoneurons innervating a dorsal neck extensor muscle, splenius, in the adult cat are densely, but not uniformly innervated by both NA and 5-HT boutons. Identified splenius motoneurons were intracellularly stained with Neurobiotin. Using 3D reconstruction techniques we mapped the distributions of contacts formed by NA and 5-HT boutons on the reconstructed dendritic trees of these motoneurons. Splenius motoneurons received an average of 1,230 NA contacts (range = 647-1,507) and 1,582 5-HT contacts (range = 1,234-2,143). The densities of these contacts were 10 (NA) to 6 (5-HT)-fold higher on small compared to large-diameter dendrites. This relationship largely accounts for the bias of NA and 5-HT contacts on distal dendrites and is partially responsible for the higher density of NA contacts on dendrites located more than 200 μm dorsal to the soma. These results suggest that the neuromodulatory actions of NA and 5-HT are compartmentalized and regulate the input-output properties of motoneurons according to precisely arranged interactions with voltage-dependent and -independent channels that are primarily located on small-diameter dendrites.

Frequency-dependent amplification of stretch-evoked excitatory input in spinal motoneurons

Voltage-dependent calcium and sodium channels mediating persistent inward currents (PICs) amplify the effects of synaptic inputs on the membrane potential and firing rate of motoneurons. CaPIC channels are thought to be relatively slow, whereas the NaPIC channels have fast kinetics. These different characteristics influence how synaptic inputs with different frequency content are amplified; the slow kinetics of Ca channels suggest that they can only contribute to amplification of low frequency inputs (<5 Hz). To characterize frequency-dependent amplification of excitatory postsynaptic potentials (EPSPs), we measured the averaged stretch-evoked EPSPs in cat medial gastrocnemius motoneurons in decerebrate cats at different subthreshold levels of membrane potential. EPSPs were produced by muscle spindle afferents activated by stretching the homonymous and synergist muscles at frequencies of 5-50 Hz. We adjusted the stretch amplitudes at different frequencies to produce approximately the same peak-to-peak EPSP amplitude and quantified the amount of amplification by expressing the EPSP integral at different levels of depolarization as a percentage of that measured with the membrane hyperpolarized. Amplification was observed at all stretch frequencies but generally decreased with increasing stretch frequency. However, in many cells the amount of amplification was greater at 10 Hz than at 5 Hz. Fast amplification was generally reduced or absent when the lidocaine derivative QX-314 was included in the electrode solution, supporting a strong contribution from Na channels. These results suggest that NaPICs can combine with CaPICs to enhance motoneuron responses to modulations of synaptic drive over a physiologically significant range of frequencies.

Voltage-dependent amplification of synaptic inputs in respiratory motoneurones

The role of persistent inward currents (PICs) in cat respiratory motoneurones (phrenic inspiratory and thoracic expiratory) was investigated by studying the voltage-dependent amplification of central respiratory drive potentials (CRDPs), recorded intracellularly, with action potentials blocked with the local anaesthetic derivative, QX-314. Decerebrate unanaesthetized or barbiturate-anaesthetized preparations were used. In expiratory motoneurones, plateau potentials were observed in the decerebrates, but not under anaesthesia. For phrenic motoneurones, no plateau potentials were observed in either state (except in one motoneurone after the abolition of the respiratory drive by means of a medullary lesion), but all motoneurones showed voltage-dependent amplification of the CRDPs, over a wide range of membrane potentials, too wide to result mainly from PIC activation. The measurements of the amplification were restricted to the phase of excitation, thus excluding the inhibitory phase. Amplification was found to be greatest for the smallest CRDPs in the lowest resistance motoneurones and was reduced or abolished following intracellular injection of the NMDA channel blocker, MK-801. Plateau potentials were readily evoked in non-phrenic cervical motoneurones in the same (decerebrate) preparations. We conclude that the voltage-dependent amplification of synaptic excitation in phrenic motoneurones is mainly the result of NMDA channel modulation rather than the activation of Ca2+ channel mediated PICs, despite phrenic motoneurones being strongly immunohistochemically labelled for CaV1.3 channels. The differential PIC activation in different motoneurones, all of which are CaV1.3 positive, leads us to postulate that the descending modulation of PICs is more selective than has hitherto been believed.

Push-pull control of motor output

Inhibition usually decreases input-output excitability of neurons. If, however, inhibition is coupled to excitation in a push-pull fashion, where inhibition decreases as excitation increases, neuron excitability can be increased. Although the presence of push-pull organization has been demonstrated in single cells, its functional impact on neural processing depends on its effect on the system level. We studied push-pull in the motor output stage of the feline spinal cord, a system that allows independent control of inhibitory and excitatory components. Push-pull organization was clearly present in ankle extensor motoneurons, producing increased peak-to-peak modulation of synaptic currents. The effect at the system level was equally strong. Independent control of the inhibitory component showed that the stronger the background of inhibition, the greater the peak force production. This illustrates the paradox at the heart of push-pull organization: increased force output can be achieved by increasing background inhibition to provide greater disinhibition.

Models of passive and active dendrite motoneuron pools and their differences in muscle force control

Motoneuron (MN) dendrites may be changed from a passive to an active state by increasing the levels of spinal cord neuromodulators, which activate persistent inward currents (PICs). These exert a powerful influence on MN behavior and modify the motor control both in normal and pathological conditions. Motoneuronal PICs are believed to induce nonlinear phenomena such as the genesis of extra torque and torque hysteresis in response to percutaneous electrical stimulation or tendon vibration in humans. An existing large-scale neuromuscular simulator was expanded to include MN models that have a capability to change their dynamic behaviors depending on the neuromodulation level. The simulation results indicated that the variability (standard deviation) of a maintained force depended on the level of neuromodulatory activity. A force with lower variability was obtained when the motoneuronal network was under a strong influence of PICs, suggesting a functional role in postural and precision tasks. In an additional set of simulations when PICs were active in the dendrites of the MN models, the results successfully reproduced experimental results reported from humans. Extra torque was evoked by the self-sustained discharge of spinal MNs, whereas differences in recruitment and de-recruitment levels of the MNs were the main reason behind torque and electromyogram (EMG) hysteresis. Finally, simulations were also used to study the influence of inhibitory inputs on a MN pool that was under the effect of PICs. The results showed that inhibition was of great importance in the production of a phasic force, requiring a reduced co-contraction of agonist and antagonist muscles. These results show the richness of functionally relevant behaviors that can arise from a MN pool under the action of PICs.

Contribution of intrinsic properties and synaptic inputs to motoneuron discharge patterns: a simulation study

Motoneuron discharge patterns reflect the interaction of synaptic inputs with intrinsic conductances. Recent work has focused on the contribution of conductances mediating persistent inward currents (PICs), which amplify and prolong the effects of synaptic inputs on motoneuron discharge. Certain features of human motor unit discharge are thought to reflect a relatively stereotyped activation of PICs by excitatory synaptic inputs; these features include rate saturation and de-recruitment at a lower level of net excitation than that required for recruitment. However, PIC activation is also influenced by the pattern and spatial distribution of inhibitory inputs that are activated concurrently with excitatory inputs. To estimate the potential contributions of PIC activation and synaptic input patterns to motor unit discharge patterns, we examined the responses of a set of cable motoneuron models to different patterns of excitatory and inhibitory inputs. The models were first tuned to approximate the current- and voltage-clamp responses of low- and medium-threshold spinal motoneurons studied in decerebrate cats and then driven with different patterns of excitatory and inhibitory inputs. The responses of the models to excitatory inputs reproduced a number of features of human motor unit discharge. However, the pattern of rate modulation was strongly influenced by the temporal and spatial pattern of concurrent inhibitory inputs. Thus, even though PIC activation is likely to exert a strong influence on firing rate modulation, PIC activation in combination with different patterns of excitatory and inhibitory synaptic inputs can produce a wide variety of motor unit discharge patterns.

Altered activation patterns by triceps surae stretch reflex pathways in acute and chronic spinal cord injury

Spinal reflexes are modified by spinal cord injury (SCI) due the loss of excitatory inputs from supraspinal structures and changes within the spinal cord. The stretch reflex is one of the simplest pathways of the central nervous system and was used presently to evaluate how inputs from primary and secondary muscle spindles interact with spinal circuits before and after spinal transection (i.e., spinalization) in 12 adult decerebrate cats. Seven cats were spinalized and allowed to recover for 1 mo (i.e., chronic spinal state), whereas 5 cats were evaluated before (i.e., intact state) and after acute spinalization (i.e., acute spinal state). Stretch reflexes were evoked by stretching the left triceps surae (TS) muscles. The force evoked by TS muscles was recorded along with the activity of several hindlimb muscles. Stretch reflexes were abolished in the acute spinal state due to an inability to activate TS muscles, such as soleus (Sol) and lateral gastrocnemius (LG). In chronic spinal cats, reflex force had partly recovered but Sol and LG activity remained considerably depressed, despite the fact that injecting clonidine could recruit these muscles during locomotor-like activity. In contrast, other muscles not recruited in the intact state, most notably semitendinosus and sartorius, were strongly activated by stretching TS muscles in chronic spinal cats. Therefore, stretch reflex pathways from TS muscles to multiple hindlimb muscles undergo functional reorganization following spinalization, both acute and chronic. Altered activation patterns by stretch reflex pathways could explain some sensorimotor deficits observed during locomotion and postural corrections after SCI.