Distribution of effective synaptic currents in cat triceps surae motoneurons. VI. Contralateral pyramidal tract

Adaptive Neural Decoder for Prosthetic Hand Control

The overarching goal was to resolve a major barrier to real-life prosthesis usability-the rapid degradation of prosthesis control systems, which require frequent recalibrations. Specifically, we sought to develop and test a motor decoder that provides (1) highly accurate, real-time movement response, and (2) unprecedented adaptability to dynamic changes in the amputee's biological state, thereby supporting long-term integrity of control performance with few recalibrations. To achieve that, an adaptive motor decoder was designed to auto-switch between algorithms in real-time. The decoder detects the initial aggregate motoneuron spiking activity from the motor pool, then engages the optimal parameter settings for decoding the motoneuron spiking activity in that particular state. "Clear-box" testing of decoder performance under varied physiological conditions and post-amputation complications was conducted by comparing the movement output of a simulated prosthetic hand as driven by the decoded signal vs. as driven by the actual signal. Pearson's correlation coefficient and Normalized Root Mean Square Error were used to quantify the accuracy of the decoder's output. Our results show that the decoder algorithm extracted the features of the intended movement and drove the simulated prosthetic hand accurately with real-time performance (<10 ms) (Pearson's correlation coefficient >0.98 to >0.99 and Normalized Root Mean Square Error <13-5%). Further, the decoder robustly decoded the spiking activity of multi-speed inputs, inputs generated from reversed motoneuron recruitment, and inputs reflecting substantial biological heterogeneity of motoneuron properties, also in real-time. As the amputee's neuromodulatory state changes throughout the day and the electrical properties and ratio of slower vs. faster motoneurons shift over time post-amputation, the motor decoder presented here adapts to such changes in real-time and is thus expected to greatly enhance and extend the usability of prostheses.

Recruitment gain of spinal motor neuron pools in cat and human

The output from a motor nucleus is determined by the synaptic input to the motor neurons and their intrinsic properties. Here, we explore whether the source of synaptic inputs to the motor neurons (cats) and the age or post-stroke conditions (humans) may change the recruitment gain of the motor neuron pool. In cats, the size of Ia EPSPs in triceps surae motor neurons (input) and monosynaptic reflexes (MSRs; output) was recorded in the soleus and medial gastrocnemius motor nerves following graded stimulation of dorsal roots. The MSR was plotted against the EPSP thereby obtaining a measure of the recruitment gain. Conditioning stimulation of sural and peroneal cutaneous afferents caused significant increase in the recruitment gain of the medial gastrocnemius, but not the soleus motor neuron pool. In humans, the discharge probability of individual soleus motor units (input) and soleus H-reflexes (output) was performed. With graded stimulation of the tibial nerve, the gain of the motor neuron pool was assessed as the slope of the relation between probability of firing and the reflex size. The gain in young subjects was higher than in elderly subjects. The gain in post-stroke survivors was higher than in age-matched neurologically intact subjects. These findings provide experimental evidence that recruitment gain of a motor neuron pool contributes to the regulation of movement at the final output stage from the spinal cord and should be considered when interpreting changes in reflex excitability in relation to movement or injuries of the nervous system.

Glutamatergic input varies with phrenic motor neuron size

Like all skeletal muscles, the diaphragm muscle accomplishes a range of motor behaviors by recruiting different motor unit types in an orderly fashion. Recruitment of phrenic motor neurons (PhMNs) is generally assumed to be based primarily on the intrinsic properties of PhMNs with an equal distribution of descending excitatory inputs to all PhMNs. However, differences in presynaptic excitatory input across PhMNs of varying sizes could also contribute to the orderly recruitment pattern. In the spinal cord of Sprague-Dawley rats, we retrogradely labeled PhMNs using cholera toxin B (CTB) and validated a robust confocal imaging-based technique that utilizes semiautomated processing to identify presynaptic glutamatergic (Glu) terminals within a defined distance around the somal membrane of PhMNs of varying size. Our results revealed an ~10% higher density of Glu terminals at PhMNs in the lower tertile of somal surface area. These smaller PhMNs are likely recruited first to accomplish lower force ventilatory behaviors of the diaphragm as compared with larger PhMNs in the upper tertile that are recruited to accomplish higher force expulsive behaviors. These results suggest that differences in excitatory synaptic input to PhMNs may also contribute to the orderly recruitment of diaphragm motor units.NEW & NOTEWORTHY The distribution of excitatory glutamatergic synaptic input to phrenic motor neurons differs across motor neurons of varying size. These findings support the size principle of motor unit recruitment that underlies graded force generation in a muscle, which is based on intrinsic electrophysiological properties of motor neurons resulting from differences in somal surface area. A higher density of glutamatergic inputs at smaller, more excitable motor neurons substantiates the earlier and more frequent recruitment of these units.

Beta-band motor unit coherence and nonlinear surface EMG features of the first dorsal interosseous muscle vary with force

Motor unit firing times are weakly coupled across a range of frequencies during voluntary contractions. Coherent activity within the beta-band (15-35 Hz) has been linked to oscillatory cortical processes, providing evidence of functional connectivity between the motoneuron pool and motor cortex. The aim of this study was to investigate whether beta-band motor unit coherence is altered with increasing abduction force in the first dorsal interosseous muscle. Coherence between motor unit firing times, extracted from decomposed surface electromyography (EMG) signals, was investigated in 17 subjects at 10, 20, 30, and 40% of maximum voluntary contraction. Corresponding changes in nonlinear surface EMG features (specifically sample entropy and determinism, which are sensitive to motor unit synchronization) were also examined. A reduction in beta-band and alpha-band coherence was observed as force increased [F(3, 151) = 32, P < 0.001 and F(3, 151) = 27, P < 0.001, respectively], accompanied by corresponding changes in nonlinear surface EMG features. A significant relationship between the nonlinear features and motor unit coherence was also detected (r = -0.43 ± 0.1 and r = 0.45 ± 0.1 for sample entropy and determinism, respectively; both P < 0.001). The reduction in beta-band coherence suggests a change in the relative contribution of correlated and uncorrelated presynaptic inputs to the motoneuron pool, and/or a decrease in the responsiveness of the motoneuron pool to synchronous inputs at higher forces. The study highlights the importance of considering muscle activation when investigating changes in motor unit coherence or nonlinear EMG features and examines other factors that can influence coherence estimation.NEW & NOTEWORTHY Intramuscular alpha- and beta-band coherence decreased as muscle contraction force increased. Beta-band coherence was higher in groups of high-threshold motor units than in simultaneously active lower threshold units. Alterations in motor unit coherence with increases or decreases in force and with the onset of fatigue were accompanied by corresponding changes in surface electromyography sample entropy and determinism. Mixed-model analysis indicated mean firing rate and number of motor units also influenced the coherence estimate.

The effects of model composition design choices on high-fidelity simulations of motoneuron recruitment and firing behaviors

OBJECTIVE

Computational models often require tradeoffs, such as balancing detail with efficiency; yet optimal balance should incorporate sound design features that do not bias the results of the specific scientific question under investigation. The present study examines how model design choices impact simulation results.

APPROACH

We developed a rigorously-validated high-fidelity computational model of the spinal motoneuron pool to study three long-standing model design practices which have yet to be examined for their impact on motoneuron recruitment, firing rate, and force simulations. The practices examined were the use of: (1) generic cell models to simulate different motoneuron types, (2) discrete property ranges for different motoneuron types, and (3) biological homogeneity of cell properties within motoneuron types.

MAIN RESULTS

Our results show that each of these practices accentuates conditions of motoneuron recruitment based on the size principle, and minimizes conditions of mixed and reversed recruitment orders, which have been observed in animal and human recordings. Specifically, strict motoneuron orderly size recruitment occurs, but in a compressed range, after which mixed and reverse motoneuron recruitment occurs due to the overlap in electrical properties of different motoneuron types. Additionally, these practices underestimate the motoneuron firing rates and force data simulated by existing models.

SIGNIFICANCE

Our results indicate that current modeling practices increase conditions of motoneuron recruitment based on the size principle, and decrease conditions of mixed and reversed recruitment order, which, in turn, impacts the predictions made by existing models on motoneuron recruitment, firing rate, and force. Additionally, mixed and reverse motoneuron recruitment generated higher muscle force than orderly size motoneuron recruitment in these simulations and represents one potential scheme to increase muscle efficiency. The examined model design practices, as well as the present results, are applicable to neuronal modeling throughout the nervous system.

The potential for understanding the synaptic organization of human motor commands via the firing patterns of motoneurons

Motoneurons are unique in being the only neurons in the CNS whose firing patterns can be easily recorded in human subjects. This is because of the one-to-one relationship between the motoneuron and muscle cell behavior. It has long been appreciated that the connection of motoneurons to their muscle fibers allows their action potentials to be amplified and recorded, but only recently has it become possible to simultaneously record the firing pattern of many motoneurons via array electrodes placed on the skin. These firing patterns contain detailed information about the synaptic organization of motor commands to the motoneurons. This review focuses on parameters in these firing patterns that are directly linked to specific features of this organization. It is now well established that motor commands consist of three components, excitation, inhibition, and neuromodulation; the importance of the third component has become increasingly evident. Firing parameters linked to each of the three components are discussed, along with consideration of potential limitations in their utility for understanding the underlying organization of motor commands. Future work based on realistic computer simulations of motoneurons may allow quantitative "reverse engineering" of human motoneuron firing patterns to provide good estimates of the relative amplitudes and temporal patterns of all three components of motor commands.

Principles of Motor Unit Physiology Evolve With Advances in Technology

Movements are generated by the coordinated activation of motor units. Recent technological advances have made it possible to identify the concurrent activity of several tens of motor units, in contrast with much smaller samples available in classic studies. We discuss how these advances in technology have enabled the development of a population perspective of how the central nervous system controls motor unit activity and thereby the forces exerted by muscles.

Distinguishing intrinsic from extrinsic factors underlying firing rate saturation in human motor units

During voluntary contraction, firing rates of individual motor units (MUs) increase modestly over a narrow force range beyond which little additional increase in firing rate is seen. Such saturation of MU discharge may be a consequence of extrinsic factors that limit net synaptic excitation acting on motor neurons (MNs) or may be due to intrinsic properties of the MNs. Two sets of experiments involving recording of human biceps brachii MUs were carried out to evaluate saturation. In the first set, the extent of saturation was quantified for 136 low-threshold MUs during isometric ramp contractions. Firing rate-force data were best fit by a saturating function for 90% of MUs recorded with a maximum rate of 14.8 ± 2.0 impulses/s. In the second set of experiments, to distinguish extrinsic from intrinsic factors underlying saturation, we artificially augmented descending excitatory drive to biceps MNs by activation of muscle spindle afferents through tendon vibration. We examined the change in firing rate caused by tendon vibration in 96 MUs that were voluntarily activated at rates below and at saturation. Vibration had little effect on the discharge of MUs that were firing at saturation frequencies but strongly increased firing rates of the same units when active at lower frequencies. These results indicate that saturation is likely caused by intrinsic mechanisms that prevent further increases in firing rate in the presence of increasing synaptic excitation. Possible intrinsic cellular mechanisms that limit firing rates of motor units during voluntary effort are discussed.

Modulation of the input-output function by GABAA receptor-mediated currents in rat oculomotor nucleus motoneurons

The neuronal input-output function depends on recruitment threshold and gain of the firing frequency-current (f-I) relationship. These two parameters are positively correlated in ocular motoneurons (MNs) recorded in alert preparation and inhibitory inputs could contribute to this correlation. Phasic inhibition mediated by γ-amino butyric acid (GABA) occurs when a high concentration of GABA at the synaptic cleft activates postsynaptic GABAA receptors, allowing neuronal information transfer. In some neuronal populations, low concentrations of GABA activate non-synaptic GABAA receptors and generate a tonic inhibition, which modulates cell excitability. This study determined how ambient GABA concentrations modulate the input-output relationship of rat oculomotor nucleus MNs. Superfusion of brain slices with GABA (100 μm) produced a GABAA receptor-mediated current that reduced the input resistance, increased the recruitment threshold and shifted the f-I relationship rightward without any change in gain. These modifications did not depend on MN size. In absence of exogenous GABA, gabazine (20 μm; antagonist of GABAA receptors) abolished spontaneous inhibitory postsynaptic currents and revealed a tonic current in MNs. Gabazine increased input resistance and decreased recruitment threshold mainly in larger MNs. The f-I relationship shifted to the left, without any change in gain. Gabazine effects were chiefly due to MN tonic inhibition because tonic current amplitude was five-fold greater than phasic. This study demonstrates a tonic inhibition in ocular MNs that modulates cell excitability depending on cell size. We suggest that GABAA tonic inhibition acting concurrently with glutamate receptors activation could reproduce the positive covariation between threshold and gain reported in alert preparation.

Selective responses to tonic descending commands by temporal summation in a spinal motor pool

Motor responses of varying intensities rely on descending commands to heterogeneous pools of motoneurons. In vertebrates, numerous sources of descending excitatory input provide systematically more drive to progressively less excitable spinal motoneurons. While this presumably facilitates simultaneous activation of motor pools, it is unclear how selective patterns of recruitment could emerge from inputs weighted this way. Here, using in vivo electrophysiological and imaging approaches in larval zebrafish, we find that, despite weighted excitation, more excitable motoneurons are preferentially activated by a midbrain reticulospinal nucleus by virtue of longer membrane time constants that facilitate temporal summation of tonic drive. We confirm the utility of this phenomenon by assessing the activity of the midbrain and motoneuron populations during a light-driven behavior. Our findings demonstrate that weighted descending commands can generate selective motor responses by exploiting systematic differences in the biophysical properties of target motoneurons and their relative sensitivity to tonic input.

Motor unit

Movement is accomplished by the controlled activation of motor unit populations. Our understanding of motor unit physiology has been derived from experimental work on the properties of single motor units and from computational studies that have integrated the experimental observations into the function of motor unit populations. The article provides brief descriptions of motor unit anatomy and muscle unit properties, with more substantial reviews of motoneuron properties, motor unit recruitment and rate modulation when humans perform voluntary contractions, and the function of an entire motor unit pool. The article emphasizes the advances in knowledge on the cellular and molecular mechanisms underlying the neuromodulation of motoneuron activity and attempts to explain the discharge characteristics of human motor units in terms of these principles. A major finding from this work has been the critical role of descending pathways from the brainstem in modulating the properties and activity of spinal motoneurons. Progress has been substantial, but significant gaps in knowledge remain.

Cortical output to fast and slow muscles of the ankle in the rhesus macaque

The cortical control of fast and slow muscles of the ankle has been the subject of numerous reports yielding conflicting results. Although it is generally agreed that cortical stimulation yields short latency facilitation of fast muscles, the effects on the slow muscle, soleus, remain controversial. Some studies have shown predominant facilitation of soleus from the cortex while others have provided evidence of differential control in which soleus is predominantly inhibited from the cortex. The objective of this study was to investigate the cortical control of fast and slow muscles of the ankle using stimulus triggered averaging (StTA) of EMG activity, which is a sensitive method of detecting output effects on muscle activity. This method also has relatively high spatial resolution and can be applied in awake, behaving subjects. Two rhesus macaques were trained to perform a hindlimb push-pull task. Stimulus triggered averages (StTAs) of EMG activity (15, 30, and 60 μA at 15 Hz) were computed for four muscles of the ankle [tibialis anterior (TA), medial gastrocnemius (MG), lateral gastrocnemius (LG), and soleus] as the monkeys performed the task. Poststimulus facilitation (PStF) was observed in both the fast muscles (TA, MG, and LG) as well as the slow muscle (soleus) and was as common and as strong in soleus as in the fast muscles. However, while poststimulus suppression (PStS) was observed in all muscles, it was more common in the slow muscle compared to the fast muscles and was as common as facilitation at low stimulus intensities. Overall, our results demonstrate that cortical facilitation of soleus has an organization that is very similar to that of the fast ankle muscles. However, cortical inhibition is organized differently allowing for more prominent suppression of soleus motoneurons.

Do additional inputs change maximal voluntary motor unit firing rates after spinal cord injury?

BACKGROUND

Motor unit firing frequencies are low during maximal voluntary contractions (MVCs) of human thenar muscles impaired by cervical spinal cord injury (SCI).

OBJECTIVE

This study aimed to examine whether thenar motor unit firing frequencies increase when driven by both maximal voluntary drive and other concurrent inputs compared with an MVC alone.

METHODS

Motor unit firing rates, force, and surface electromyographic activity (EMG) were compared across 2 contractions: (a) MVC alone and (b) MVC combined with another input (combination contraction). Other inputs (conditions) included vibration, heat, or cold applied to the anterior surface of the forearm, electrical stimulation delivered to the anterior surface of the middle finger, a muscle spasm, or a voluntary contraction of the contralateral thenar muscles against resistance.

RESULTS

The maximal firing frequency (n = 68 units), force, and electromyographic activity (n = 92 contraction pairs) were all significantly higher during the combined contractions compared with MVCs alone. There was a 3-way interaction between contraction, condition, and subject for maximal motor unit firing rates, force, and EMG. Thus, combined contraction responses were different for conditions across subjects. Some conditions (eg, a muscle spasm) resulted in more effective and more frequent responses (increases in unit firing frequency, force, EMG in >50% contractions) than others. Recruitment of new units also occurred in combined contractions.

CONCLUSIONS

Motoneurons are still responsive to additional afferent inputs from various sources when rate modulation from voluntary drive is limited by SCI. Individuals with SCI may be able to combine inputs to control functional tasks they cannot perform with voluntary drive alone.

Changes in presumed motor cortical activity during fatiguing muscle contraction in humans

AIM

Changes in sensory information from active muscles accompany fatiguing exercise and the force-generating capacity deteriorates. The central motor commands therefore must adjust depending on the task performed. Muscle potentials evoked by transcranial magnetic stimulation (TMS) change during the course of fatiguing muscle activity, which demonstrates activity changes in cortical or spinal networks during fatiguing exercise. Here, we investigate cortical mechanisms that are actively involved in driving the contracting muscles.

METHODS

During a sustained submaximal contraction (30% of maximal voluntary contraction) of the elbow flexor muscles we applied TMS over the motor cortex. At an intensity below motor threshold, TMS reduced the ongoing muscle activity in biceps brachii. This reduction appears as a suppression at short latency of the stimulus-triggered average of rectified electromyographic (EMG) activity. The magnitude of the suppression was evaluated relative to the mean EMG activity during the 50 ms prior to the cortical stimulus.

RESULTS

During the first 2 min of the fatiguing muscle contraction the suppression was 10 +/- 0.9% of the ongoing EMG activity. At 2 min prior to task failure the suppression had reached 16 +/- 2.1%. In control experiments without fatigue we did not find a similar increase in suppression with increasing levels of ongoing EMG activity.

CONCLUSION

Using a form of TMS which reduces cortical output to motor neurones (and disfacilitates them), this study suggests that neuromuscular fatigue increases this disfacilitatory effect. This finding is consistent with an increase in the excitability of inhibitory circuits controlling corticospinal output.

Movement-related receptive fields of spinal motoneurones with active dendrites

The primary control of spinal motoneurone excitability is mediated by descending monoaminergic systems, which have diffuse effects on multiple motor pools. Much of the sensory input evoked by movement is also distributed broadly to multiple joints. The muscle spindle Ia afferent system, however, is sharply focused, with Ia excitation restricted to close synergists and Ia reciprocal inhibition only shared between antagonists acting at a single joint. We studied the interaction of neuromodulatory and sensory inputs in determining the movement-related receptive field (MRRF) of motoneurones during passive joint movements of the cat hindlimb. In a decerebrate preparation with tonic monoaminergic input to the cord, the MRRFs tended to be focused for the ankle and knee extensor motor pools studied. Ankle rotation produced larger synaptic currents in ankle extensors than knee or hip rotations and knee rotation dominated input to the knee extensors. The persistent inward current (PIC) in motoneurone dendrites, which is facilitated by monoaminergic input, amplified the MRRF about 2-fold, consistent with its effects on other inputs. Acute spinal transaction markedly broadened MRRFs, with hip rotation generating large currents in both ankle and knee extensors. Spinalization also eliminated amplification of MRRFs, as expected from elimination of descending monoaminergic input. Ia reciprocal inhibition is very effective in suppressing dendritic PICs and thus provides a local and specific PIC control system to oppose the diffuse PIC facilitation from descending monoaminergic systems. The focused MRRF seen in the intact cord state would allow reciprocal inhibition to fulfil this role without undue interference from multijoint input from other afferent systems.

Intrinsic electrical properties of spinal motoneurons vary with joint angle

The dendrites of spinal motoneurons amplify synaptic inputs to a marked degree through persistent inward currents (PICs). Dendritic amplification is subject to neuromodulatory control from the brainstem by axons releasing the monoamines serotonin and norepinephrine; however, the monoaminergic projection to the cord is diffusely organized and does not allow independent adjustment of amplification in different motor pools. Using in vivo voltage-clamp techniques, here we show that dendritic PICs in ankle extensor motoneurons in the cat are reduced about 50% by small rotations (+/-10 degrees ) of the ankle joint. This reduction is primarily due to reciprocal inhibition, a tightly focused input shared only among strict muscle antagonists. These results demonstrate how a specific change in limb position can regulate intrinsic cellular properties set by a background of diffuse descending neuromodulation.

Persistent inward currents in motoneuron dendrites: implications for motor output

The dendrites of motoneurons are not, as once thought, passive conduits for synaptic inputs. Instead they have voltage-dependent channels that provide the capacity to generate a very strong persistent inward current (PIC). The amplitude of the PIC is proportional to the level of neuromodulatory input from the brainstem, which is mediated primarily by the monoamines serotonin and norepinephrine. During normal motor behavior, monoaminergic drive is likely to be moderately strong and the dendritic PIC generates many of the characteristic features of motor unit firing patterns. Most of the PIC activates at or below recruitment threshold and thus motor unit firing patterns exhibit a linear increase just above recruitment. The dendritic PIC allows motor unit derecruitment to occur at a lower input level than recruitment, thus providing sustained tonic firing with little or no synaptic input, especially in low-threshold units. However the dendritic PIC can be readily deactivated by synaptic inhibition. The overall amplification due to the dendritic PIC and other effects of monoamines on motoneurons greatly increases the input-output gain of the motor pool. Thus the brainstem neuromodulatory input provides a mechanism by which the excitability of motoneurons can be varied for different motor behaviors. This control system is lost in spinal cord injury but PICs nonetheless recover near-normal amplitudes in the months following the initial injury. The relationship of these findings to the cause of the spasticity syndrome developing after spinal cord injury is discussed.

Recruitment order of cat abducens motoneurons and internuclear neurons

Abducens neurons undergo a dose-dependent synaptic blockade (either disinhibition or complete blockade) when tetanus neurotoxin (TeNT) is injected into the lateral rectus muscle at either a low (0.5) or a high dose (5 ng/kg). We studied the firing pattern and recruitment order in abducens neurons both in control and after TeNT injection. The eye position threshold for recruitment of control abducens neurons was exponentially related to the eye position and velocity sensitivities. We also found a constancy of recruitment threshold for different eye movement modalities (spontaneous, optokinetic, and vestibular). Exponential relationships were found, as well, for eye velocity sensitivity during saccades and for position and velocity sensitivities during the vestibulo-ocular reflex. Likewise, inverse relationships were found between recruitment threshold or position sensitivity with the antidromic latency in control abducens neurons. These relationships, however, did not apply following TeNT treatment. Neuronal firing after TeNT appeared either disinhibited (low dose) or depressed (high dose), but the relationships between neuronal sensitivities and recruitment still applied. However, the pattern of recruitment shifted toward the treated side as more inputs were blocked by the low- and high-dose treatments, respectively. Nonetheless, although the recruitment-to-sensitivity relationships persisted under the TeNT synaptic blockade, we conclude that synaptic inputs are determinant for establishing the recruitment threshold and recruitment spacing of abducens motoneurons and internuclear neurons.

Relative strengths and distributions of different sources of synaptic input to the motoneurone pool: implications for motor unit recruitment

Understanding how synaptic inputs from segmental and descending systems shape motor output from the spinal cord requires information on the relative magnitudes of the synaptic currents produced by the different systems and their patterns of distribution within a motoneurone pool. Equally important are quantitative descriptions of how different synaptic inputs are integrated when they are concurrently active and of how voltage- and ligand-gated conductances on the dendrites of motoneurones affect the transfer of synaptic currents to the soma. We have carried out a number of experimental studies of these inter-related problems on motoneurones in the cat spinal cord and have explored the implications of our findings with computer simulations utilizing a synthetic model of the cat medial gastrocnemius motoneurone pool. This paper provides a brief review of the principal results of our studies.

The effect of electrical stimulation of the corticospinal tract on motor units of the human biceps brachii

In healthy human subjects, descending motor pathways including the corticospinal tract were stimulated electrically at the level of the cervicomedullary junction to determine the effects on the discharge of motoneurones innervating the biceps brachii. Post-stimulus time histograms (PSTHs) were constructed for 15 single motor units following electrical stimulation of the corticospinal tract and for 11 units following electrical stimulation of large diameter afferents at the brachial plexus. Responses were assessed during weak voluntary contraction. Both types of stimulation produced a single peak at short latency in the PSTH (mean 8.5 and 8.7 ms, respectively) and of short duration (< 1.4 ms). In separate studies, we compared the latency of the responses to electrical stimulation of the corticospinal tract in the relaxed muscle with that in the contracting muscle. The latency was the same in the two conditions when the intensity of the stimulation was adjusted so that responses of the same size could be compared. Estimates of the descending conduction velocity and measurements of presumed peripheral conduction time suggest that there is less than 0.5 ms for spinal events (including synaptic delays). We propose that in response to electrical stimulation of the descending tract fibres, biceps motoneurones receive a large excitatory input with minimal dispersion and it presumably contains a dominant monosynaptic component.

Integration of synaptic and intrinsic dendritic currents in cat spinal motoneurons

The results of recent studies designed to reveal some of the 'rules' governing the integration of synaptic and intrinsic dendritic currents in spinal motoneurons are reviewed. When two or more sources of synaptic input are activated concurrently, their combined postsynaptic effects on cat spinal motoneurons with 'passive dendrites' are generally equal to or slightly less than those predicted from the linear sum of their individual effects. However, for experimental preparations in which active conductances on motoneuron dendrites are enabled, instances of greater-than- or less-than linear summation can occur. Further, these studies demonstrate that the persistent inward currents that are generated by motoneuron dendrites provide an intrinsic source of excitatory drive that is larger than those associated with any of the individual synaptic input systems studied to date. Since these intrinsic depolarizing currents can be rapidly inactivated by a hyperpolarizing input, they are ideally suited to providing a major source of the alternating 'drive' to motoneurons during locomotion.

Relationship between simulated common synaptic input and discharge synchrony in cat spinal motoneurons

Synchronized discharge of individual motor units is commonly observed in the muscles of human subjects performing voluntary contractions. The amount of this synchronization is thought to reflect the extent to which motoneurons in the same and related pools share common synaptic input. However, the relationship between the proportion of shared synaptic input and the strength of synchronization has never been measured directly. In this study, we simulated common shared synaptic input to cat spinal motoneurons by driving their discharge with noisy, injected current waveforms. Each motoneuron was stimulated with a number of different injected current waveforms, and a given pair of waveforms were either completely different or else shared a variable percentage of common elements. Cross-correlation histograms were then compiled between the discharge of motoneurons stimulated with noise waveforms with variable degrees of similarity. The strength of synchronization increased with the amount of simulated "common" input in a nonlinear fashion. Moreover, even when motoneurons had >90% of their simulated synaptic inputs in common, only approximately 25-45% of their spikes were synchronized. We used a simple neuron model to explore how variations in neuron properties during repetitive discharge may lead to the low levels of synchronization we observed experimentally. We found that small variations in spike threshold and firing rate during repetitive discharge lead to large decreases in synchrony, particularly when neurons have a high degree of common input. Our results may aid in the interpretation of studies of motor unit synchrony in human hand muscles during voluntary contractions.

Spinal and supraspinal factors in human muscle fatigue

Muscle fatigue is an exercise-induced reduction in maximal voluntary muscle force. It may arise not only because of peripheral changes at the level of the muscle, but also because the central nervous system fails to drive the motoneurons adequately. Evidence for "central" fatigue and the neural mechanisms underlying it are reviewed, together with its terminology and the methods used to reveal it. Much data suggest that voluntary activation of human motoneurons and muscle fibers is suboptimal and thus maximal voluntary force is commonly less than true maximal force. Hence, maximal voluntary strength can often be below true maximal muscle force. The technique of twitch interpolation has helped to reveal the changes in drive to motoneurons during fatigue. Voluntary activation usually diminishes during maximal voluntary isometric tasks, that is central fatigue develops, and motor unit firing rates decline. Transcranial magnetic stimulation over the motor cortex during fatiguing exercise has revealed focal changes in cortical excitability and inhibitability based on electromyographic (EMG) recordings, and a decline in supraspinal "drive" based on force recordings. Some of the changes in motor cortical behavior can be dissociated from the development of this "supraspinal" fatigue. Central changes also occur at a spinal level due to the altered input from muscle spindle, tendon organ, and group III and IV muscle afferents innervating the fatiguing muscle. Some intrinsic adaptive properties of the motoneurons help to minimize fatigue. A number of other central changes occur during fatigue and affect, for example, proprioception, tremor, and postural control. Human muscle fatigue does not simply reside in the muscle.

Input-output functions of mammalian motoneurons

Our intent in this review was to consider the relationship between the biophysical properties of motoneurons and the mechanisms by which they transduce the synaptic inputs they receive into changes in their firing rates. Our emphasis has been on experimental results obtained over the past twenty years, which have shown that motoneurons are just as complex and interesting as other central neurons. This work has shown that motoneurons are endowed with a rich complement of active dendritic conductances, and flexible control of both somatic and dendritic channels by endogenous neuromodulators. Although this new information requires some revision of the simple view of motoneuron input-output properties that was prevalent in the early 1980's (see sections 2.3 and 2.10), the basic aspects of synaptic transduction by motoneurons can still be captured by a relatively simple input-output model (see section 2.3, equations 1-3). It remains valid to describe motoneuron recruitment as a product of the total synaptic current delivered to the soma, the effective input resistance of the motoneuron and the somatic voltage threshold for spike initiation (equations 1 and 2). However, because of the presence of active channels activated in the subthreshold range, both the delivery of synaptic current and the effective input resistance depend upon membrane potential. In addition, activation of metabotropic receptors by achetylcholine, glutamate, noradrenaline, serotonin, substance P and thyrotropin releasing factor (TRH) can alter the properties of various voltage- and calcium-sensitive channels and thereby affect synaptic current delivery and input resistance. Once motoneurons are activated, their steady-state rate of repetitive discharge is linearly related to the amount of injected or synaptic current reaching the soma (equation 3). However, the slope of this relation, the minimum discharge rate and the threshold current for repetitive discharge are all subject to neuromodulatory control. There are still a number of unresolved issues concerning the control of motoneuron discharge by synaptic inputs. Under dynamic conditions, when synaptic input is rapidly changing, time- and activity-dependent changes in the state of ionic channels will alter both synaptic current delivery to the spike-generating conductances and the relation between synaptic current and discharge rate. There is at present no general quantitative expression for motoneuron input-output properties under dynamic conditions. Even under steady-state conditions, the biophysical mechanisms underlying the transfer of synaptic current from the dendrites to the soma are not well understood, due to the paucity of direct recordings from motoneuron dendrites. It seems likely that resolving these important issues will keep motoneuron afficiandoes well occupied during the next twenty years.

Relationship between the time course of the afterhyperpolarization and discharge variability in cat spinal motoneurones

1. We elicited repetitive discharges in cat spinal motoneurones by injecting noisy current waveforms through a microelectrode to study the relationship between the time course of the motoneurone's afterhyperpolarization (AHP) and the variability in its spike discharge. Interspike interval histograms were used to estimate the interval death rate, which is a measure of the instantaneous probability of spike occurrence as a function of the time since the preceding spike. It had been previously proposed that the death rate can be used to estimate the AHP trajectory. We tested the accuracy of this estimate by comparing the AHP trajectory predicted from discharge statistics to the measured AHP trajectory of the motoneurone. 2. The discharge statistics of noise-driven cat motoneurones shared a number of features with those previously reported for voluntarily activated human motoneurones. At low discharge rates, the interspike interval histograms were often positively skewed with an exponential tail. The standard deviation of the interspike intervals increased with the mean interval, and the plots of standard deviation versus the mean interspike interval generally showed an upward bend, the onset of which was related to the motoneurone's AHP duration. 3. The AHP trajectories predicted from the interval death rates were generally smaller in amplitude (i.e. less hyperpolarized) than the measured AHP trajectories. This discrepancy may result from the fact that spike threshold varies during the interspike interval, so that the distance to threshold at a given time depends upon both the membrane trajectory and the spike threshold trajectory. Nonetheless, since the interval death rate is likely to reflect the instantaneous distance to threshold during the interspike interval, it provides a functionally relevant measure of fluctuations in motoneurone excitability during repetitive discharge.

Influence of temporal correlation of synaptic input on the rate and variability of firing in neurons

The spike trains that transmit information between neurons are stochastic. We used the theory of random point processes and simulation methods to investigate the influence of temporal correlation of synaptic input current on firing statistics. The theory accounts for two sources for temporal correlation: synchrony between spikes in presynaptic input trains and the unitary synaptic current time course. Simulations show that slow temporal correlation of synaptic input leads to high variability in firing. In a leaky integrate-and-fire neuron model with spike afterhyperpolarization the theory accurately predicts the firing rate when the spike threshold is higher than two standard deviations of the membrane potential fluctuations. For lower thresholds the spike afterhyperpolarization reduces the firing rate below the theory's predicted level when the synaptic correlation decays rapidly. If the synaptic correlation decays slower than the spike afterhyperpolarization, spike bursts can occur during single broad peaks of input fluctuations, increasing the firing rate over the prediction. Spike bursts lead to a coefficient of variation for the interspike intervals that can exceed one, suggesting an explanation of high coefficient of variation for interspike intervals observed in vivo.

Summation of effective synaptic currents and firing rate modulation in cat spinal motoneurons

The aim of this study was to examine how cat spinal motoneurons integrate the synaptic currents generated by the concurrent activation of large groups of presynaptic neurons. We obtained intracellular recordings from cat triceps surae motoneurons and measured the effects of repetitive activity in different sets of presynaptic neurons produced by electrical stimulation of descending fibers or peripheral nerves and by longitudinal vibration of the triceps surae muscles (to activate primary muscle spindle Ia afferent fibers). We combined synaptic activation with subthreshold injected currents to obtain estimates of effective synaptic currents at the resting potential (I(Nrest)) and at the threshold for repetitive discharge (I(Nthresh)). We then superimposed synaptic activation on suprathreshold injected current steps to measure the synaptically evoked change in firing rate. We studied eight different pairs of synaptic inputs. When any two synaptic inputs were activated concurrently, both the effective synaptic currents (I(Nrest)) and the synaptically evoked changes in firing rate generally were equal to or slightly less than the linear sum of the effects produced by activating each input alone. However, there were several instances in which the summation was substantially less than linear. In some motoneurons, we induced a partial blockade of potassium channels by adding tetraethylammonium (TEA) or cesium to the electrolyte solution in the intracellular pipette. In these cells, persistent inward currents were evoked by depolarization that led to instances of substantially greater-than linear summation of injected and synaptic currents. Overall our results indicate that the spatial distribution of synaptic boutons on motoneurons acts to minimize electrical interactions between synaptic sites permitting near linear summation of synaptic currents. However, modulation of voltage-gated conductances on the soma and dendrites of the motoneuron can lead to marked nonlinearities in synaptic integration.

Twitch interpolation in human muscles: mechanisms and implications for measurement of voluntary activation

An electrical stimulus delivered to a muscle nerve during a maximal voluntary contraction usually produces a twitchlike increment in force. The amplitude of this "interpolated twitch" is widely used to measure voluntary "activation" of muscles. In the present study, a computer model of the human adductor pollicis motoneuron pool was used to investigate factors that affect the interpolated twitch. Antidromic occlusion of naturally occurring orthodromic potentials was modeled, but reflex effects of the stimulus were not. In simulations, antidromic collisions occurred with probabilities of between approximately 16% (in early recruited motoneurons) and nearly 100% (in late recruited motoneurons). The model closely predicted experimental data on the amplitude and time course of the rising phase of interpolated twitches over the full range of voluntary forces, except that the amplitude of interpolated twitches was slightly overestimated at intermediate contraction intensities. Small interpolated twitches (4.7% of the resting twitch) were evident in simulated maximal voluntary contractions, but were nearly completely occluded when mean peak firing rate was increased to approximately 60 Hz. Simulated interpolated twitches did not show the marked force drop that follows the peak of the twitch, and when antidromic collisions were excluded from the model interpolated twitch amplitude was slightly increased and time-to-peak force was prolonged. These findings suggest that both antidromic and reflex effects reduce the amplitude of the interpolated twitch and contribute to the force drop that follows the twitch. The amplitude of the interpolated twitch was related to "excitation" of the motoneuron pool in a nonlinear way, so that at near-maximal contraction intensities (>90% maximal voluntary force) increases in excitation produced only small changes in interpolated twitch amplitude. Thus twitch interpolation may not provide a sensitive measure of motoneuronal excitation at near-maximal forces. Increases in the amplitude of interpolated twitches such as have been observed in fatigue and various pathologies may reflect large reductions in excitation of the motoneuron pool.

Synaptic integration in spinal motoneurones

Spinal motoneurones receive thousands of presynaptic excitatory and inhibitory synaptic contacts distributed throughout their dendritic trees. Despite this extensive convergence, there have been very few studies of how synaptic inputs interact in mammalian motoneurones when they are activated concurrently. In the experiments reported here, we measured the effective synaptic currents and the changes in firing rate evoked in cat spinal motoneurones by concurrent repetitive activation of two separate sets of presynaptic neurons. We compared these effects to those predicted by a linear sum of the effects produced by activating each set of presynaptic neurons separately. We generally found that when two inputs were activated concurrently, both the effective synaptic currents and the synaptically-evoked changes in firing rate they produced in motoneurones were generally linear, or slightly less than the linear sum of the effects produced by activating each input alone. The results suggest that the spatial distribution synaptic terminals on the dendritic trees of motoneurones may help isolate synapses from one another, minimizing non-linear interactions.