Effects of synchrony between primate corticomotoneuronal cells on post-spike facilitation of muscles and motor units

Cerebellar Shaping of Motor Cortical Firing Is Correlated with Timing of Motor Actions

In higher mammals, motor timing is considered to be dictated by cerebellar control of motor cortical activity, relayed through the cerebellar-thalamo-cortical (CTC) system. Nonetheless, the way cerebellar information is integrated with motor cortical commands and affects their temporal properties remains unclear. To address this issue, we activated the CTC system in primates and found that it efficiently recruits motor cortical cells; however, the cortical response was dominated by prolonged inhibition that imposed a directional activation across the motor cortex. During task performance, cortical cells that integrated CTC information fired synchronous bursts at movement onset. These cells expressed a stronger correlation with reaction time than non-CTC cells. Thus, the excitation-inhibition interplay triggered by the CTC system facilitates transient recruitment of a cortical subnetwork at movement onset. The CTC system may shape neural firing to produce the required profile to initiate movements and thus plays a pivotal role in timing motor actions.

Joint cross-correlation analysis reveals complex, time-dependent functional relationship between cortical neurons and arm electromyograms

Correlation between cortical activity and electromyographic (EMG) activity of limb muscles has long been a subject of neurophysiological studies, especially in terms of corticospinal connectivity. Interest in this issue has recently increased due to the development of brain-machine interfaces with output signals that mimic muscle force. For this study, three monkeys were implanted with multielectrode arrays in multiple cortical areas. One monkey performed self-timed touch pad presses, whereas the other two executed arm reaching movements. We analyzed the dynamic relationship between cortical neuronal activity and arm EMGs using a joint cross-correlation (JCC) analysis that evaluated trial-by-trial correlation as a function of time intervals within a trial. JCCs revealed transient correlations between the EMGs of multiple muscles and neural activity in motor, premotor and somatosensory cortical areas. Matching results were obtained using spike-triggered averages corrected by subtracting trial-shuffled data. Compared with spike-triggered averages, JCCs more readily revealed dynamic changes in cortico-EMG correlations. JCCs showed that correlation peaks often sharpened around movement times and broadened during delay intervals. Furthermore, JCC patterns were directionally selective for the arm-reaching task. We propose that such highly dynamic, task-dependent and distributed relationships between cortical activity and EMGs should be taken into consideration for future brain-machine interfaces that generate EMG-like signals.

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.

The representation of egocentric space in the posterior parietal cortex

The posterior parietal cortex (PPC) is the most likely site where egocentric spatial relationships are represented in the brain. PPC cells receive visual, auditory, somaesthetic, and vestibular sensory inputs; oculomotor, head, limb, and body motor signals; and strong motivational projections from the limbic system. Their discharge increases not only when an animal moves towards a sensory target, but also when it directs its attention to it. PPC lesions have the opposite effect: sensory inattention and neglect. The PPC does not seem to contain a "map" of the location of objects in space but a distributed neural network for transforming one set of sensory vectors into other sensory reference frames or into various motor coordinate systems. Which set of transformation rules is used probably depends on attention, which selectively enhances the synapses needed for making a particular sensory comparison or aiming a particular movement.

Synthesizing complex movement fragment representations from motor cortical ensembles

We have previously shown that the responses of primary motor cortical neurons are more accurately predicted if one assumes that individual neurons encode temporally-extensive movement fragments or preferred trajectories instead of static movement parameters (Hatsopoulos et al., 2007). Building on these findings, we examine here how these preferred trajectories can be combined to generate a rich variety of preferred movement trajectories when neurons fire simultaneously. Specifically, we used a generalized linear model to fit each neuron's spike rate to an exponential function of the inner product between the actual movement trajectory and the preferred trajectory; then, assuming conditional independence, when two neurons fire simultaneously their spiking probabilities multiply implying that their preferred trajectories add. We used a similar exponential model to fit the probability of simultaneous firing and found that the majority of neuron pairs did combine their preferred trajectories using a simple additive rule. Moreover, a minority of neuron pairs that engaged in significant synchronization combined their preferred trajectories through a small scaling adjustment to the additive rule in the exponent, while preserving the shape of the predicted trajectory representation from the additive rule. These results suggest that complex movement representations can be synthesized in simultaneously firing neuronal ensembles by adding the trajectory representations of the constituents in the ensemble.

Does the nervous system depend on kinesthetic information to control natural limb movements?

This target article draws together two groups of experimental studies on the control of human movement through peripheral feedback and centrally generated signals of motor commands. First, during natural movement, feedback from muscle, joint, and cutaneous afferents changes; in human subjects these changes have reflex and kinesthetic consequences. Recent psychophysical and microneurographic evidence suggests that joint and even cutaneous afferents may have a proprioceptive role. Second, the role of centrally generated motor commands in the control of normal movements and movements following acute and chronic deafferentation is reviewed. There is increasing evidence that subjects can perceive their motor commands under various conditions, but that this is inadequate for normal movement; deficits in motor performance arise when the reliance on proprioceptive feedback is abolished either experimentally or because of pathology. During natural movement, the CNS appears to have access to functionally useful input from a range of peripheral receptors as well as from internally generated command signals. The unanswered questions that remain suggest a number of avenues for further research.

Implications of neural networks for how we think about brain function

Engineers use neural networks to control systems too complex for conventional engineering solutions. To examine the behavior of individual hidden units would defeat the purpose of this approach because it would be largely uninterpretable. Yet neurophysiologists spend their careers doing just that! Hidden units contain bits and scraps of signals that yield only arcane hints about network function and no information about how its individual units process signals. Most literature on single-unit recordings attests to this grim fact. On the other hand, knowing a system's function and describing it with elegant mathematics tell one very little about what to expect of interneuronal behavior. Examples of simple networks based on neurophysiology are taken from the oculomotor literature to suggest how single-unit interpretability might decrease with increasing task complexity. It is argued that trying to explain how any real neural network works on a cell-by-cell, reductionist basis is futile and we may have to be content with trying to understand the brain at higher levels of organization.

Differential involvement of excitatory and inhibitory neurons of cat motor cortex in coincident spike activity related to behavioral context

To assess temporal associations in spike activity between pairs of neurons in the primary motor cortex (MI) related to different behaviors, we compared the incidence of coincident spiking activity of task-related (TR) and non-task-related (NTR) neurons during a skilled motor task and sitting quietly in adult cats (Felis domestica). Chronically implanted microwires were used to record spike activity of MI neurons in four animals (two male and two female) trained to perform a skilled reaching task or sit quietly. Neurons were identified as TR if spike activity was modulated during the task (and NTR if not). Based on spike characteristics, they were also classified as either regular-spiking (RS, putatively excitatory) or fast-spiking (FS, putatively inhibitory) neurons. Temporal associations in the activities of simultaneously recorded neurons were evaluated using shuffle-corrected cross-correlograms. Pairs of NTR and TR neurons showed associations in their firing patterns over wide areas of MI (representing forelimb and hindlimb movements) during quiet sitting, more commonly involving RS neurons. During skilled task performance, however, significantly coincident firing was seen almost exclusively between TR neurons in a smaller part of MI (representing forelimb movements), involving mainly FS neurons. The findings of this study show evidence for widespread interactions in MI when the animal sits quietly, which changes to a more specific and restricted pattern of interactions during task performance. Different populations of excitatory and inhibitory neurons appear to be synchronized during skilled movement and quiet sitting.

Neural integration of reaching and posture: interhemispheric spike correlations in cat motor cortex

To study the interlimb coordination of reaching and postural movements, chronically implanted microelectrodes were used to record single unit activity from the primary motor cortex (MI) of cats during performance of a trained reaching task. Recordings were made from both cerebral hemispheres to record neurons that modulated their activity during reaching (reach-related neurons) and supportive (posture-related neurons) movements of either forelimb. Evidence of temporal associations in the activities of simultaneously recorded reach- and posture-related neurons was evaluated using shuffle-corrected cross correlograms. The spike activity of approximately 34% of reach-related neurons was temporally correlated with the spike activity of simultaneously recorded posture-related neurons in the opposite motor cortex. Significant associations in the spike activity of neurons recorded from homotopic representational areas of the motor cortex in opposite hemispheres have not previously been reported. These interactions may have an important role in the coordination of opposite forelimbs during reaching movements and postural actions.

Patterns of spatio-temporal correlations in the neural activity of the cat motor cortex during trained forelimb movements

In order to study how neurons in the primary motor cortex (MI) are dynamically linked together during skilled movement, we recorded simultaneously from many cortical neurons in cats trained to perform a reaching and retrieval task using their forelimbs. Analysis of task-related spike activity in the MI of the hemisphere contralateral to the reaching forelimb (in identified forelimb or hindlimb representations) recorded through chronically implanted microwires, was followed by pairwise evaluation of temporally correlated activity in these neurons during task performance using shuffle corrected cross-correlograms. Over many months of recording, a variety of task-related modulations of neural activities were observed in individual efferent zones. Positively correlated activity (mainly narrow peaks at zero or short latencies) was seen during task performance frequently between neurons recorded within the forelimb representation of MI, rarely within the hindlimb area of MI, and never between forelimb and hindlimb areas. Correlated activity was frequently observed between neurons with different patterns of task-related activity or preferential activity during different task elements (reaching, feeding, etc.), and located in efferent zones with dissimilar representation as defined by intracortical microstimulation. The observed synchronization of action potentials among selected but functionally varied groups of MI neurons possibly reflects dynamic recruitment of network connections between efferent zones during skilled movement.

Effect of cancellation on triggered averaging used to determine synchronization between motor unit discharge in separate muscles

Synchronization between single motor unit (SMU) discharges in separate muscles has been estimated from peaks in averaged electromyographic (EMG) recordings from one muscle triggered from SMU discharge in another. This study evaluated the effect of EMG signal cancellation on this measure of synchronization. SMU activity was recorded with 8 fine-wire electrodes in vastus medialis obliquus (VMO) and vastus lateralis (VL) during gentle isometric knee extension in 7 subjects. Data from 5 VL recordings were summed then rectified, or rectified then summed, to produce multi-unit recordings with and without cancellation, respectively. Averages of summed VL data were triggered from VMO SMUs. Synchronization, defined as a peak >3 SD above the triggered average mean, occurred in 73.68% and 78.95% of recordings with and without cancellation, respectively. To further investigate the effect of cancellation on synchronization, 250 "virtual" EMG recordings were created from VL data. VL SMUs were sorted and modified with respect to discharge rate, amplitude and polarity to create a collection of possible SMU discharge patterns. Virtual recordings were added one-by-one to VL recordings that showed synchronization. Virtual channels were rectified then added or added then rectified, to create data with and without cancellation. Identification of synchronization decreased similarly in both conditions with addition of virtual data. Our data show estimation of synchronization from triggered averages is more likely to detect synchronization in recordings with fewer SMUs, but cancellation has little effect. Synchronization must be interpreted with caution if number of SMUs changes between conditions.

Synaptic linkages between corticomotoneuronal cells affecting forelimb muscles in behaving primates

To elucidate the cortical circuitry controlling primate forelimb muscles we investigated the synaptic interactions between neighboring motor cortex cells that had postspike output effects in target muscles. In monkeys generating isometric ramp-and-hold wrist torques, pairs of cortical cells were recorded simultaneously with independent electrodes and corticomotoneuronal ("CM") cells were identified by their postspike effects on target forelimb muscles in spike-triggered averages (SpTAs) of electromyographs (EMGs). The response patterns of the cells were determined in response-aligned averages and their synaptic interactions were identified by cross-correlograms of action potentials. The possibility that synchronized firing between cortical cells could mediate spike-correlated effects in the SpTA of EMG was examined in several ways. Sixty-two pairs consisted of one CM cell and a non-CM cell; 15 of these had correlogram peaks of the same magnitude as that of other pairs, but the synchrony peaks did not mediate any postspike effect from the non-CM cell. Twelve pairs of simultaneously recorded CM cells were cross-correlated. Half had features (usually synchrony peaks) in their cross-correlograms and the cells of these pairs also shared some target muscles in common. The other half had flat correlograms and, in most of these pairs, the CM cells affected different muscles. The latter group included pairs of CM cells that facilitated synergistic muscles. These results indicate that common synaptic input specifically affects CM cells that have overlapping muscle fields. Reconstruction of the cortical locations of CM cells affecting 12 different muscles showed a wide and overlapping distribution of cortical colonies of forelimb muscles.

Discharge rate during low-force isometric contractions influences motor unit coherence below 15 Hz but not motor unit synchronization

The purpose of the study was to determine whether pairs of motor units that discharge action potentials at different rates during isometric contractions exhibit different levels of motor unit synchronization or coherence. Twelve subjects (28.6 +/- 6.1 years) performed isometric contractions at target forces slightly above the recruitment threshold (1.02-20.9%) of an isolated motor unit. Based on audio feedback, subjects maintained a relatively constant discharge rate of the isolated unit for about 80 s. Intramuscular electrodes were used to record the discharge of 47 pairs of motor units at rates that ranged from 8.07 to 13.6 pps. Correlated discharge between pairs of motor units was quantified with the common input strength (CIS) index, k' index, and coherence spectrum. Greater discharge rates across pairs of motor units were predicted (R2 = 0.36, P < 0.001) by higher coherence from 8 to 13 Hz (r = -0.52) and lower coherence from 0 to 4 Hz (r = 0.37). Indexes of motor unit synchronization (CIS and k') were strongly associated with motor unit coherence from 16 to 32 Hz (CIS: R2 = 0.63; k': R2 = 0.4; P = 0.001). The CIS index of motor unit synchronization and the motor unit coherence from 16 to 32 Hz did not vary with discharge rate. In contrast, the k' index of motor unit synchronization declined with discharge rate (r2 = 0.20, P = 0.001). Furthermore, greater discharge rates across pairs of motor units were accompanied by higher motor unit coherence in the 8-13 Hz band and lower motor unit coherence in the 0-4 Hz band. These results demonstrate that differences in discharge rate between pairs of motor units in first dorsal interosseus during low-force, isometric contractions were associated with modulation of the correlation in the discharge times of the two motor units at frequencies less than 15 Hz.

Descending signals from the pontomedullary reticular formation are bilateral, asymmetric, and gated during reaching movements in the cat

We examined the contribution of neurons within the pontomedullary reticular formation (PMRF) to the control of reaching movements in the cat. We recorded the activity of 127 reticular neurons, including 56 reticulospinal neurons, during movements of each forelimb; 67/127 of these neurons discharged prior to the onset of activity in the prime flexor muscles during the reach of the ipsilateral limb and form the focus of this report. Most neurons (63/67) showed similar patterns and levels of discharge activity during reaches of either limb, although activity was slightly greater during reach of the ipsilateral limb. In 26/67 cells, the initial change in discharge activity was time-locked to the go signal during reaches of either limb; we have argued that this early discharge contributes to the anticipatory postural adjustments that precede movement. In 11/26 cells, the initial change in activity was reciprocal for reaches with the left and right limbs, although activity during the movement was nonreciprocal. Spike-triggered averaging produced postspike facilitation or depression (PSD) in 12/50 cells during reaches of the limb ipsilateral to the recording site and in 17/49 cells during reach of the contralateral limb. Some cells produced PSD in ipsilateral extensor muscles before the start of the reach and during reaches made with the contralateral, but not the ipsilateral limb; this suggests the signal must be differentially gated. Overall, the results suggest a strong bilateral, albeit asymmetric, contribution from the PMRF to the control of posture and movement during voluntary movement.

A spectrum from pure post-spike effects to synchrony effects in spike-triggered averages of electromyographic activity during skilled finger movements

During individuated finger movements, a high proportion of synchrony effects was found in spike-triggered averages (SpikeTAs) of rectified electromyographic activity aligned on the spikes discharged by primary motor cortex (M1) neurons. Because synchrony effects can be produced even if the trigger neuron itself provides no direct synaptic connections to motoneurons, such nonoscillatory synchrony effects often are discounted when considering control of motoneuron pools. We therefore examined the distinctions between pure postspike effects and synchrony effects. The criteria usually applied to distinguish pure and synchrony effects-onset latency and peak width-failed to separate the present SpikeTA effects objectively into distinct subpopulations. Synchrony effects generally were larger than pure effects. Many M1 neurons produced pure effects in some muscles while producing synchrony effects in others. M1 neurons producing no effects, only pure effects, only synchrony effects, or both pure and synchrony effects did not fall into different groups based on discharge characteristics during finger movements. Nor were neurons producing different types of SpikeTA effects segregated spatially in M1. These observations suggest that neurons producing pure and synchrony SpikeTA effects come from similar M1 populations. We discuss potential mechanisms that might have produced a continuous spectrum of variation from pure to synchrony effects in the present monkeys. Although synchrony effects cannot be taken as evidence of mono- or disynaptic connections from the recorded neuron to the motoneuron pool, the functional linkages indicated by synchrony effects represent a substantial fraction of M1 input to motoneuron pools during skilled, individuated finger movements.

Coherence at 16-32 Hz Can Be Caused by Short-Term Synchrony of Motor Units

Time- and frequency-domain measures of discharge times for pairs of motor units are used to infer the proportion of common synaptic input received by motor neurons. The physiological mechanisms that can produce the experimentally observed peaks in the cross-correlation histogram and the coherence spectrum are uncertain. The present study used a computational model to impose synchronization on the discharge times of motor units. Randomly selected discharge times of a unit that was being synchronized to a reference unit were aligned with some of the discharge times of the reference unit, provided the original discharge time was within 30 ms of the discharge by the reference unit. All time-domain measures (indexes CIS, E, and k′) were sensitive to changes in the level of imposed motor-unit synchronization ( P < 0.01). In addition, synchronization caused a peak between 16 and 32 Hz in the coherence spectrum. The shape of the cross-correlogram determined the frequency at which the peak occurred in the coherence spectrum. Further, the magnitude of the coherence peak was highly correlated with the time-domain measures of motor-unit synchronization ( r2 > 0.80), with the highest correlation occurring for index E ( r2 = 0.98). Thus the peak in the 16- to 32-Hz band of the coherence spectrum can be caused by the time that individual discharges are advanced or delayed to produce synchrony. Although the in vivo processes that adjust the timing of motor-unit discharges are not fully understood, these results suggest that they may not depend entirely on an oscillatory drive by the CNS.

Superlinear population encoding of dynamic hand trajectory in primary motor cortex

Neural activity in primary motor cortex (MI) is known to correlate with hand position and velocity. Previous descriptions of this tuning have (1) been linear in position or velocity, (2) depended only instantaneously on these signals, and/or (3) not incorporated the effects of interneuronal dependencies on firing rate. We show here that many MI cells encode a superlinear function of the full time-varying hand trajectory. Approximately 20% of MI cells carry information in the hand trajectory beyond just the position, velocity, and acceleration at a single time lag. Moreover, approximately one-third of MI cells encode the trajectory in a significantly superlinear manner; as one consequence, even small position changes can dramatically modulate the gain of the velocity tuning of MI cells, in agreement with recent psychophysical evidence. We introduce a compact nonlinear "preferred trajectory" model that predicts the complex structure of the spatiotemporal tuning functions described in previous work. Finally, observing the activity of neighboring cells in the MI network significantly increases the predictability of the firing rate of a single MI cell; however, we find interneuronal dependencies in MI to be much more locked to external kinematic parameters than those described recently in the hippocampus. Nevertheless, this neighbor activity is approximately as informative as the hand velocity, supporting the view that neural encoding in MI is best understood at a population level.

Temporal Processing in Primate Motor Control: Relation Between Cortical and EMG Activity

We investigated spatio-temporal information processing in the primate motor system. Corticomotoneuronal (CM) cells provide monosynaptic excitatory connections from motor cortex to spinal motoneurons and contribute causally to the time-varying electromyogram (EMG) of their target muscle. A multilayer perceptron (MLP) was used to evaluate the transfer function between neural activity of single CM cells and their target muscle EMG, using data from in-vivo recordings in primate motor cortex. For an optimal MLP performance, i.e., minimal error between recorded target EMG and MLP-derived EMG, the CM cell input period had to span the latency observed between CM cell peak activity and EMG peak activity. We argue that the same spike train may code two types of information: 1) rate coding within the input window accounted for large-amplitude variations in the EMG signal and 2) temporal coding within a window of 40 ms just prior to the EMG output signal accounted for EMG variations of small amplitude. The transfer function of the MLP, thus, combines rate and temporal coding and suggests that CM cell output may also combine these two forms of coding. We predict that mutual constraints of rate and temporal coding would, however, would limit the CM output to code for particular temporal profiles of EMG, possibly adapted to bio-mechanical constraints.

Firing properties of spinal interneurons during voluntary movement. II. Interactions between spinal neurons

The relationship between the activity of pairs of simultaneously recorded spinal interneurons (INs) in the cervical enlargement was studied in five monkeys performing voluntary wrist movements. The tendency for INs to exhibit similar response properties and synchronized firing was tested as a function of physical distance between the cells and their correlational linkages with forearm muscles. Nearby INs tended to have more similar torque and direction turning (signal correlation) and more similar response profiles (e.g., tonic vs phasic firing) than INs that were far apart. This suggests that nearby cells receive common synaptic input. In contrast, the trial-to-trial covariation of rate around the mean rate for all trials (noise correlation) was independent of the distance between the neurons. Furthermore, signal and noise correlation were independent, suggesting different underlying mechanisms. Surprisingly, spike-to-spike correlation between INs was relatively infrequent and weak, as measured by cross-correlation histograms. In contrast, single motor units (SMUs) in forearm muscles fired more synchronously, particularly for SMUs in single extensor muscles. Either common drive to INs is too weak to induce synchronized firing, or there is an active decorrelation mechanism within IN networks.

Synchrony between neurons with similar muscle fields in monkey motor cortex

Synchronous firing of motor cortex cells exhibiting postspike facilitation (PSF) or suppression (PSS) of hand muscle EMG was examined to investigate the relationship between synchrony and output connectivity. Recordings were made in macaque monkeys performing a precision grip task. Synchronization was assessed with cross-correlation histograms of the activity from 144 pairs of simultaneously recorded neurons, while spike-triggered averages of EMG defined the muscle field for each cell. Cell pairs with similar muscle fields showed greater synchronization than pairs with nonoverlapping fields. Furthermore, cells with opposing effects in the same muscles exhibited negative synchronization. We conclude that synchrony in motor cortex engages networks of neurons directly controlling the same muscle set, while inhibitory connections exist between neuronal populations with opposing output effects.

Synchronization in monkey motor cortex during a precision grip task. II. effect of oscillatory activity on corticospinal output

Recordings from primary motor cortex (M1) during periods of steady contraction show oscillatory activity; these oscillations are coherent with the activity of contralateral muscles. We investigated synchronization of corticospinal output neurons with the oscillations, which could provide the pathway for their transmission to the spinal motoneurons. One hundred seventy-six antidromically identified pyramidal tract neurons (PTNs) were recorded from M1 in three macaque monkeys trained to perform a precision grip task. Local field potentials (LFP) were simultaneously recorded. All analysis was confined to the hold period of the task, where our previous work has shown that there is the strongest oscillatory activity. Coherence was calculated between LFP and PTN discharge. Significant coherence was seen in three bands, with frequencies of 10-14, 17-31, and 34-44 Hz. Coherence values were low, with the majority of PTN-LFP coherences having a peak lower than 0.05. The phase of coherence was approximately -pi/2 radians for each band (with LFP polarity defined as negative upward), although there was some dispersion of phase across the population of PTNs. Coherence was also calculated between pairs of PTNs that had been simultaneously recorded. Where there was significant coherence, it was also generally smaller than 0.05. The phase of PTN-PTN coherence clustered around zero radians. A computer model was constructed to assist the interpretation of the experimental results. It simulated an integrate-and-fire neuron responding to synaptic inputs. A fraction of the synaptic inputs was synchronized with a simulated LFP; the remainder were uncorrelated with it. The model showed that coherence between the LFP and the output spike train considerably underestimated the fraction of synchronized inputs. Additionally, for a given fraction of synchronized inputs, coherence was smaller for high- compared with low-frequency bins. Cell discharge rate also influenced the spike-LFP coherence: coherence was higher for simulations in which the cell discharged at a faster rate. Thus although levels of PTN-LFP coherence seen experimentally were low, a considerable proportion of the input to the PTN must be synchronized with the global oscillatory activity recorded by the LFP. The low LFP-PTN coherences do however indicate that cortical oscillations are transmitted with only low fidelity in the discharge of a single PTN. Using further computer simulations, it was demonstrated that a small population of PTNs could encode the cortical oscillatory signal effectively, since the action of averaging across the population improves the signal:noise ratio. The oscillations will therefore be effectively transmitted to spinal motoneurons, and this has important consequences for the possible role of oscillations in motor control of the hand.

Neocortical mechanisms in motor learning

The ability to learn novel motor skills has fundamental importance for adaptive behavior. Neocortical mechanisms support human motor skill learning, from simple practice to adaptation and arbitrary sensory-motor associations. Behavioral and neural manifestations of motor learning evolve in time and involve multiple structures across the neocortex. Modifications of neural properties, synchrony and synaptic efficacy are all related to the development and maintenance of motor skill.

Training and synchrony in the motor system

Two monkeys trained for >5 years to perform 12 finger and wrist movements had both a greater prevalence of motor cortex neurons with significant effects in spike-triggered averages and a greater ratio of synchrony effects to pure postspike effects than a monkey trained <1 year to perform six movements. By comparison, stimulus-triggered averages were generally similar in all three monkeys, indicating that the increased prevalence of synchrony in spike-triggered averages was a feature of voluntary motor system activity in the monkeys trained for a longer period of time. Synchronization among neurons with relatively direct connections to spinal alpha-motoneuron pools, including motor cortex neurons, may increase as a repertoire of skilled movements is acquired and practiced during long-term training.

A novel algorithm to remove electrical cross-talk between surface EMG recordings and its application to the measurement of short-term synchronisation in humans

Pairs of discharges of single motor units recorded in the same or different muscles often show synchronisation above chance levels. If large numbers of units are synchronous within and between muscles then the synchrony will be measurable in population recordings such as surface EMG. Measuring synchrony between surface EMG recordings has a number of practical and scientific advantages compared with single motor units recorded from intramuscular electrodes. However, the measurement of such synchrony in the time domain between surface EMGs is complicated because the recordings are contaminated by electrical cross-talk. In this study we recorded surface EMG simultaneously from five hand and forearm muscles during a precision grip task. Using a novel 'blind signal separation' algorithm, we were able to remove electrical cross-talk. The cross-talk-corrected EMGs could then be used to assess task-dependent modulation in both oscillatory (15-30 Hz) and non-oscillatory synchrony (all other frequencies). In agreement with previous studies, the oscillatory component was maximal during steady holding but abolished during movement. By contrast, the non-oscillatory component of the EMG synchrony appeared remarkably constant throughout all phases of the task. We conclude that surface EMG recordings can be of considerable use in the assessment of population synchrony changes, providing that electrical cross-talk between nearby channels is removed using a statistical signal processing technique. Our results show a striking difference in the task-dependent modulation of oscillatory and non-oscillatory synchrony between muscles during a dynamic precision grip task.

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.

Constraints on somatotopic organization in the primary motor cortex

Since the 1870s, the primary motor cortex (M1) has been known to have a somatotopic organization, with different regions of cortex participating in control of face, arm, and leg movements. Through the middle of the 20th century, it seemed possible that the principle of somatotopic organization extended to the detailed representation of different body parts within each of the three major representations. The arm region of M1, for example, was thought to contain a well-ordered, point-to-point representation of the movements or muscles of the thumb, index, middle, ring, and little fingers, the wrist, elbow, and shoulder, as conveyed by the iconic homunculus and simiusculus. In the last quarter of the 20th century, however, experimental evidence has accumulated indicating that within-limb somatotopy in M1 is not spatially discrete nor sequentially ordered. Rather, beneath gradual somatotopic gradients of representation, the representations of different smaller body parts or muscles each are distributed widely within the face, arm, or leg representation, such that the representations of any two smaller parts overlap extensively. Appreciation of this underlying organization will be essential to further understanding of the contribution to control of movement made by M1. Because no single experiment disproves a well-ordered within-limb somatotopic organization in M1, here I review the accumulated evidence, using a framework of six major features that constrain the somatotopic organization of M1: convergence of output, divergence of output, horizontal interconnections, distributed activation, effects of lesions, and ability to reorganize. Review of the classic experiments that led to development of the homunculus and simiusculus shows that these data too were consistent with distributed within-limb somatotopy. I conclude with speculations on what the constrained somatotopy of M1 might tell us about its contribution to control of movement.

Distributed processing in the motor system: spinal cord perspective

Recordings of spinal INs during a flexion/extension wrist task with an instructed delay period have shown directly that many spinal neurons modulate their rate during the preparatory period soon after a visual cue. The onset time and the relation between the delay period activity of spinal INs and the ensuing movement response suggest that this type of activity is not simply related to the forthcoming motor action, but rather reflects a correct match between the visual cue and the motor response. The existence of such activity further supports the notion that the motor system operates in a parallel mode of processing, so that even during early stages of motor processing multiple centers are activated regardless of their anatomical distance from muscles. The firing properties of spinal INs during the performance of the task seem to differ from the comparable properties of motor cortical cells. Spinal INs fire in a highly regular manner--their CV is substantially lower than the observed CV of cortical cells. Also, although neighboring cells tend to have similar response properties, the frequency of significant correlation is lower than for cortical cells and the anatomical extent of the correlation seems to be narrower. The similarity and differences between cortical and spinal cells in terms of response and firing properties suggests that while both type of cells are active in parallel throughout the behavioral phases of the motor task, each may operate in a different mode of information processing.

Synchronization in monkey motor cortex during a precision grip task. I. Task-dependent modulation in single-unit synchrony

Neural synchronization in the cortex, and its potential role in information coding, has attracted much recent attention. In this study, we have recorded long spike trains (mean, 33,000 spikes) simultaneously from multiple single neurons in the primary motor cortex (M1) of two conscious macaque monkeys performing a precision grip task. The task required the monkey to use its index finger and thumb to move two spring-loaded levers into a target, hold them there for 1 s, and release for a food reward. Synchrony was analyzed using a time-resolved cross-correlation method, normalized using an estimate of the instantaneous firing rate of the cell. This was shown to be more reliable than methods using trial-averaged firing rate. A total of 375 neurons was recorded from the M1 hand area; 235 were identified as pyramidal tract neurons. Synchrony was weak [mean k' = 1.05 +/- 0.04 (SD)] but widespread among pairs of M1 neurons (218/1359 pairs with above-chance synchrony), including output neurons. Synchrony usually took the form of a broad central peak [average width, 18.7 +/- 8.7 (SD) ms]. There were marked changes during different phases of the task. As a population, synchrony was greatest during the steady hold period in striking contrast to the averaged cell firing rate, which was maximal when the animal was moving the levers into target. However, the modulation of synchrony during task performance showed considerable variation across individual cell pairs. Two types of synchrony were identified: oscillatory (with periodic side lobes in the cross-correlation) and nonoscillatory. Their relative contributions were quantified by filtering the cross-correlations to exclude either frequencies from 18 to 37 Hz or all higher and lower frequencies. At the peak of population synchrony during the hold period, about half (51.7% in one monkey, 56.2% in the other) of the synchronization was within this oscillatory bandwidth. This study provides strong support for assemblies of neurons being synchronized during specific phases of a complex task with potentially important consequences for both information processing within M1 and for the impact of M1 commands on target motoneurons.

Improvements to the sensitivity of gravitational clustering for multiple neuron recordings

We outline two improvements to the technique of gravitational clustering for detection of neuronal synchrony, which are capable of improving the method's detection of weak synchrony with limited data. The advantages of the enhancements are illustrated using data with known levels of synchrony and different interspike interval distributions. The novel simulation method described can easily generate such test data. An important dependence of the sensitivity of gravitational clustering to the interspike interval distribution of the analysed spike trains is described.

The importance of the cortico-motoneuronal system for control of grasp

Our recent work has revealed new evidence of the importance of direct cortico-motoneuronal (CM) connections for voluntary control of the hand. Most of these connections are derived from corticospinal neurons located in the M1 hand area, although there are some much smaller contributions from other secondary motor areas, such as the supplementary motor area (SMA). Intracellular recordings show that 75% of upper limb motoneurons in the chloralose-anaesthetized macaque monkey receive a monosynaptic projection from the corticospinal tract; evidence for non-monosynaptic, propriospinal excitatory influences from the corticospinal tract was conspicuously lacking in these anaesthetized preparations. Moreover, in the conscious monkey, hand and arm muscle motor unit responses to corticospinal tract input are dominated by single, brief peaks compatible with monosynaptic excitation. CM excitatory post-synaptic potentials, recorded from a comparable sample of hand and arm motoneurons in anaesthetized macaque and squirrel monkeys, were found to be larger and faster rising in the macaque, which is by far the more dexterous of the two species. CM cells facilitating a given muscle in the conscious macaque are distributed over a wide region of M1 cortex, and each contributes a particular pattern of discharge during a skilled task. In addition to their direct effects on target muscles there may be weaker but potentially important effects that derive from the synchronous binding of assemblies of output neurons. Synchronous oscillations between these neurons are particularly prevalent during steady grip, but disappear during digit movement.

Corticomotoneuronal postspike effects in shoulder, elbow, wrist, digit, and intrinsic hand muscles during a reach and prehension task

We used spike-triggered averaging of rectified electromyographic activity to determine whether corticomotoneuronal (CM) cells produce postspike effects in muscles of both proximal and distal forelimb joints in monkeys performing a reach and prehension task. Two monkeys were trained to perform a self-paced task in which they reached forward from a starting position to retrieve a food reward from a small cylindrical well. We compiled spike-triggered averages from 22 to 24 separate forelimb muscles at both proximal (shoulder, elbow) and distal (wrist, digits, intrinsic hand) joints. Of 174 cells examined, 112 produced postspike effects in at least one of the target muscles. Of those cells, 45.5% produced postspike effects in both proximal and distal forelimb muscles. A nearly equal number (44.7%) produced postspike effects in distal muscles only, whereas a clear minority (9.8%) produced postspike effects in only proximal muscles. The majority of CM cells (71.4%) produced effects in two or more muscles, with an average muscle field of 3.1 +/- 2.1 (mean +/- SD) for facilitation plus suppression. Of 345 postspike effects identified, 70.7% were facilitation effects and 29.3% were suppression effects. The large majority of effects (72.2%) were in distal muscles. When averaged by joint, the latency and peak magnitude of postspike facilitation showed a stepwise increase from proximal to distal joints. The results of this study show that the majority of CM cells engaged in coordinated forelimb reaching movements facilitate and/or suppress muscles at multiple joints, including muscles at both proximal and distal joints. The results also show that CM cells make more frequent and more potent terminations in motoneuron pools of distal compared with proximal muscles.

Computer simulation of post-spike facilitation in spike-triggered averages of rectified EMG

When the spikes of a motor cortical cell are used to compile a spike-triggered average (STA) of rectified electromyographic (EMG) activity, a post-spike facilitation (PSF) is sometimes seen. This is generally thought to be indicative of direct corticomotoneuronal (CM) connections. However, it has been claimed that a PSF could be caused by synchronization between CM and non-CM cells. This study investigates the generation of PSF using a computer model. A population of cortical cells was simulated, some of which made CM connections to a pool of 103 motoneurons. Motoneurons were simulated using a biophysically realistic model. A subpopulation of the cortical cells was synchronized together. After a motoneuron discharge, a motor unit action potential was generated; these were summed to produce an EMG output. Realistic values were used for the corticospinal and peripheral nerve conduction velocity distribution, for slowing of impulse conduction in CM terminal axons, and for the amount of cortical synchrony. STA of the rectified EMG from all cortical neurons showed PSF; however, these were qualitatively different for CM versus non-CM cells. Using an epoch analysis to determine reliability in a quantitative manner, it was shown that the onset latency of PSF did not distinguish the two classes of cells after 10,000 spikes because of high noise in the averages. The time of the PSF peak and the peak width at half-maximum (PWHM) could separate CM from synchrony effects. However, only PWHM was robust against changes in motor unit action-potential shape and duration and against changes in the width of cortical synchrony. The amplitude of PSF from a CM cell could be doubled by the presence of synchrony. It is proposed that, if a PSF has PWHM < 7 ms, this reliably indicates that the trigger is a CM cell projecting to the muscle whose EMG is averaged. In an analysis of experimental data where macaque motor cortical cells facilitated hand and forearm muscle EMG, 74% of PSFs fulfilled this criterion. The PWHM criterion could be applied to other STA studies in which it is important to exclude the effects of synchrony.

Evidence from motoneurone synchronization for disynaptic pathways in the control of inspiratory motoneurones in the cat

1. Motoneurone synchronization was measured by cross-correlation between paired inspiratory discharges in external and internal intercostal nerves or their intramuscular branches (T3 to T8) or in the phrenic nerve (C5 root or both C5 and C6 roots independently) in anaesthetized, paralysed cats. 2. All cross-correlation histograms showed central peaks, for which the durations at half-amplitude (half-widths) from internal nerve pairs in adjacent segments were all less than for external nerve pairs in adjacent segments or within a segment (means, 1.6 ms vs. 3.4 ms for adjacent segments). Values for external-internal pairs covered the ranges for both these two. Lowest values came from two phrenic pairs (1.2 and 1.4 ms). 3. The peaks from ipsisegmental external-internal pairs were usually asymmetric and the maximum of the peak was often displaced to a lag of about -1 ms (external nerve providing the reference spikes), whereas peaks from external-external pairs were always symmetrical and centred on zero. Phrenic-internal peaks gave maxima with lags about 1 ms less than for phrenic-external peaks from the same segments. 4. Two explanations were considered possible for the differences in duration and timing: an extra synapse on the pathway to the external nerve motoneurones, or a correlation kernel for a monosynaptic connection to the external nerve motoneurones that had a slower time course than that for the internal or phrenic nerve motoneurones. Computer simulations, assuming the extra synapse, gave a good fit to the observed time courses of the correlation peaks for all categories of nerve pairs using single values of parameters (e.g. EPSP rise time) consistent with those in the literature. This could not be achieved with the different correlation kernel model. The timing of high-frequency oscillation (HFO), which was sometimes present in the correlations, was also better predicted with the extra synapse model. 5. It is concluded that most of the synchronization between external nerve motoneurones is derived from disynaptic common inputs and that any motoneurone synchronization peak with a half-width greater than about 2.2 ms should be assumed to be likely to contain di- or oligosynaptically derived components.

Relationship between motor unit short-term synchronization and common drive in human first dorsal interosseous muscle

We assessed the strength of motor unit (MU) short-term synchronization and common fluctuations in mean firing rate (common drive) in the same pairs of MUs in order to evaluate whether these features of voluntary MU discharge arise from a common mechanism. Shared, branched-axon inputs, with the most important being widely divergent monosynaptic projections to motoneurons from motor cortical cells, are regarded as the principal determinants of MU short-term synchronization. It is not known to what extent these synaptic inputs are responsible for common drive behaviour of MUs. MU spike trains from 77 pairs of concurrently active MUs in first dorsal interosseous muscle of 17 subjects were discriminated with the high reliability needed for common drive analysis. For each MU pair, the data used for comparison of the two analyses of correlated MU discharge came from a single trial (1-5 min duration) of isometric abduction of the index finger. Linear regression revealed a weak, significant positive correlation between the strength of MU short-term synchronization and the strength of common drive in the MU pairs (r2 = 0.06, P < 0.05, n = 77), which was slightly stronger when MU pairs with broad synchronous peaks (> 20 ms) were excluded (r2 = 0.09, P < 0.05, n = 63). These data suggest that less than 10% of the variation in the strength of common drive exhibited by pairs of MUs could be accounted for by differences in the strength of MU short-term synchronization. These two phenomena are therefore likely to arise predominantly from separate mechanisms. At least under these task conditions, the widely divergent, branched-axon inputs from single corticospinal neurons which are important in the generation of MU short-term synchronization play only a minor role in the production of common drive of MU discharge rates.

Corticospinal input onto motor neurons projecting to ankle muscles in individuals with cerebral palsy

Cross-correlograms between voluntarily active soleus (SOL) and tibialis anterior (TA) motor units were generated from seven control subjects and six subjects with spastic cerebral palsy (CP). Short-duration central peaks were observed in three subjects with spastic diplegia only. All subjects demonstrated reciprocal inhibition in TA following electrical stimulation of group I afferents to SOL, and all subjects with CP demonstrated strong activation of both TA and SOL in response to transcranial magnetic stimulation. Responses in SOL were stronger than those observed from controls. These data support the existence of abnormal corticospinal projections to soleus motor neurons in individuals with spastic CP. In spastic diplegia, short-term discharge synchrony between SOL and TA motor units may reflect abnormal interneuronal modulation at the spinal level. Abnormal corticospinal projections and/or modulation of spinal interneurons may contribute to the disordered movement patterns and co-activation observed in this population.

Organization of inputs to motoneurone pools in man

1. Surface EMGs were recorded from pairs of muscles involved in movements of the wrist and/or digits in the upper limb and from pairs of intrinsic foot muscles in the lower limb during voluntary isometric contractions. 2. EMGs were also recorded from lower limb and trunk muscles during three different tasks: lying, standing and balancing. 3. To investigate if the co-contraction of muscles was due to the presence of a common drive to each of the two motoneurone pools, cross-correlation analysis of the two multiunit EMG signals was used. 4. Evidence for a common drive was seen between pairs of muscles that share a common joint or joint complex (such as the metacarpophalangeal joints); no evidence was found for a common drive to co-contracting muscles that did not share a common joint. 5. When considering analogous hand and foot muscle pairs, the degree of synchrony was significantly greater for lower limb pairs. 6. Where a common drive was detected with lower limb muscle pairs, the degree of synchrony was significantly larger during balancing than during either lying or standing. 7. The origin of the common drive is discussed. It is concluded that activity in both last-order branched presynaptic fibers and presynaptic synchronization is involved.

The influence of single monkey cortico-motoneuronal cells at different levels of activity in target muscles

1. This study assessed the facilitation by cortico-motoneuronal (CM) cells of hand and forearm muscles at different levels of EMG activity. 2. Twenty-three CM cells were recorded in six hemispheres of four trained monkeys. CM cells were identified by the presence of post-spike facilitation (PSF) in spike-triggered averages (STAs) of their target muscles. Cell and muscle activity was recorded during performance of a low force (0.2-1.5 N) precision grip task between the index finger and thumb. The hold periods of this task lasted 1-1.5 s and provided segments of steady EMG activity. 3. The discharge activity of each CM cell, and the amplitude of the PSF produced in one or two target muscles, were compared across two to six different levels of EMG activity during the hold periods. 4. Of the forty-two CM cell-muscle combinations tested, twenty (48%) showed a significant increase in CM cell discharge rate with increased target muscle EMG activity (P < 0.001); three (7%) showed significant negative correlation; and no correlation was found for nineteen combinations (45%). 5. From a low to a high level of EMG activity (0.3-8.65% of the maximum EMG activity recorded), the absolute amount of facilitation produced by each CM cell increased by a factor of 1.2-32 (median value 3.7). This increase in facilitation occurred irrespective of the presence or absence of correlation between CM cell discharge rate and target muscle activity. 6. For thirty cell-muscle combinations in which a significant PSF could be measured at more than one level of EMG activity, the relative degree of facilitation remained constant in nine, increased in thirteen and decreased in seven combinations. In some cases saturation effects were evident. For ten combinations PSF was observed at high but not at low levels of EMG activity. 7. The changes in PSF amplitude with level of EMG activity were also present in STAs compiled from only those spikes with long interspike intervals (20-25 ms or greater). The results suggested that spikes with short interspike intervals did not make a significant contribution to the increase in PSF amplitude observed at the higher levels of EMG activity. 8. The changes in PSF amplitude with target muscle activity are probably explained best by changes at the spinal motoneuronal level, which set the response to the CM input. These changes may also reflect differences in the strength of synaptic connectivity made by a CM cell within the motoneurone pool of the target muscle.

Evidence for bilateral innervation of certain homologous motoneurone pools in man

1. Surface EMG recordings were made from left and right homologous muscle pairs in healthy adults. During each recording session subjects were requested to maintain a weak isometric contraction of both the left and right muscle. 2. Cross-correlation analysis of the two multiunit EMG recordings from each pair of muscles was performed. Central peaks of short duration (mean durations, 11.3-13.0 ms) were seen in correlograms constructed from multiunit EMG recordings obtained from left and right diaphragm, rectus abdominis and masseter muscles. No central peaks were seen in correlograms constructed from the multiunit EMG recordings from left and right upper limb muscles. 3. To investigate descending pathways to the homologous muscle pairs, the dominant motor cortex was stimulated using a focal magnetic brain stimulator whilst recording from homologous muscle pairs. 4. Following magnetic stimulation of the dominant motor cortex, a response was recorded from both right and left diaphragm, rectus abdominis and masseter muscles. In contrast, when recording from homologous upper limb muscles, a response was only seen contralateral to the side of stimulation. 5. The finding of short duration central peaks in the cross-correlograms constructed from multiunit recordings from left and right diaphragm, rectus abdominis and masseter, suggests that muscles such as these, that are normally co-activated, share a common drive. The mechanism is discussed and it is argued that the time course of the central correlogram peaks is consistent with the hypothesis that they could be produced by a common drive that arises from activity in last-order branched presynaptic fibres although presynaptic synchronization of last-order inputs is also likely to be involved. 6. The results of the magnetic stimulation experiments suggest that this common drive may involve the corticospinal tract. 7. We saw no evidence for a common drive to left and right homologous muscle pairs that may be voluntarily co-activated but often act independently.

Firing rate interactions among human orbicularis oris motor units

Groups of human motor units from the same muscle exhibit joint fluctuations in firing rate during voluntary muscle contraction. In an effort to determine whether similar behavior would be observed in a muscle lacking muscle spindles, motor unit firing behavior was examined in the human orbicularis oris inferior (OOI) during mild voluntary effort. Motor unit activity was recorded with a quadrifilar needle inserted in the OOI. Firing occurrences were identified using a motor unit decomposition procedure. Cross-correlation of motor unit firing rates revealed a tendency for motor unit firing rates to covary, although the effect was somewhat more variable than that observed previously in other skeletal muscles. There was also a statistically significant tendency for pairs of motor units to fire at simultaneous or near-simultaneous (+/- 5 ms) intervals (synchronization). Firing rate variability in OOI motor units was not significantly different (p > .05) from that observed in the FDI. Thus, the present results suggest that the common drive of human motor unit activity may not depend on the presence of muscle spindles.

Mirror movements studied in a patient with Klippel-Feil syndrome

1. Electromyographic (EMG) recordings have been made from upper limb muscles in a patient with well-defined congenital mirror movements occurring in association with Klippel-Feil syndrome and the results compared to those obtained in normal control subjects. 2. In the patient, liminal percutaneous electrical or magnetic brain stimulation applied over either hemisphere elicited bilateral and symmetrical short-latency muscle responses in relaxed intrinsic hand muscles. In the normal subjects unilateral brain stimulation only elicited contralateral muscle responses. 3. F response and H reflex studies for the patient's ulnar-supplied intrinsic hand muscles were normal. No crossed responses were recorded in the homologous muscles of the contralateral hand. 4. Scalp-recorded somatosensory-evoked responses following ulnar or median nerve stimulation were of normal latency and distribution in the patient. 5. In the patient, cross-correlation analysis of on-going single and multiunit needle EMGs recorded between muscles of left and right hands revealed a central peak in the cross-correlogram. No cross-correlogram peaks were found between left- and right-hand muscles in normal subjects. The magnitude and time course of the central peaks in the cross-correlograms constructed between the firing of motor units on opposite sides of the body in the patient were similar to those found in cross-correlograms constructed between the firing of motor units from muscles on the same side of the body in the patient and in normal subjects. 6. The magnitude of cross-correlogram peaks detected within a muscle and those detected between left and right homologous muscles showed a gradient in which the largest peaks were found in the intrinsic hand and forearm extensor muscles. The smallest peaks were observed in the forearm flexor muscles. No peaks were detected between left and right biceps brachii muscles. In intrinsic hand muscles, the size of the cross-correlogram peak detected between the EMGs of homologous muscle pairs was greater than that found for non-homologous muscle pairs. 7. Cutaneous reflex responses were recorded from first dorsal interosseous muscle following unilateral electrical stimulation of the digital nerves of the index finger. In the patient, this produced an early excitatory (E1) response on the stimulated side. Later excitatory (E2 and E3) responses, of approximately equal size and latency, were distributed bilaterally. In the normal subjects, reflex responses were confined to the stimulated side.(ABSTRACT TRUNCATED AT 400 WORDS)

The influence of changes in discharge frequency of corticospinal neurones on hand muscles in the monkey

1. The possibility that the discharge pattern of monkey corticomotoneuronal cells influences the degree to which they facilitate their target hand muscles was tested by compiling spike-triggered averages of EMG recorded from these muscles. 2. Records were made from area 4 corticomotoneuronal cells in three conscious macaque monkeys while they performed a precision grip between index finger and thumb. Simultaneous EMG recordings were made from up to six different intrinsic hand muscles. Twenty cells which produced clear post-spike facilitation of one or more muscles were selected for further analysis. 3. Spikes recorded from these cells were grouped according to the occurrence of a previous spike in the periods 0-10 ms, 10-20 ms, and so on up to 60-70 ms before the trigger spike. The post-spike period in which no additional spikes were allowed to fall was kept at either 12.5 or 25 ms. 4. Spikes selected in this way produced a transient facilitation of their target muscle EMG activity. The peak amplitude of this facilitation was normalized as a percentage of modulation of the background EMG level. The background level was determined from a period in the average to which the cell could not have contributed, because of the post-trigger spike interval. We verified that the percentage of modulation was not influenced by the overall level of EMG activity, since, for a given interval, the modulation was the same whether the relevant spikes were selected during periods of high- or low-level EMG activity. 5. The relative amplitude of the post-spike facilitation (i.e. the percentage of modulation) showed marked variation with interspike interval. A full analysis was completed for seventeen neurones. Spikes with the shortest intervals (less than 10 ms) usually produced the strongest effects, and evidence is presented that this was due to temporal summation and facilitation at the corticomotoneuronal synapse. Mid-range intervals (10-40 ms) were generally far less effective, although they constituted the highest proportion of cell activity. 6. A striking finding was the strong facilitation generated by the longer interspike intervals (40-70 ms). Although the absolute size of this post-spike effect was much smaller than that of the shortest intervals, its percentage of modulation was similar. It is suggested that this enhanced facilitation results from a combination of lower frequency discharge among the active motoneurones, and increased synchrony in the corticomotoneuronal input to them. 7. All of the above results were confirmed by examining cross-correlations between single corticomotoneuronal cells and single motor units in their target muscle.(ABSTRACT TRUNCATED AT 400 WORDS)