Functional relations between primate motor cortex cells and muscles: fixed and flexible

Agonist-Antagonist Coactivation Enhances Corticomotor Excitability of Ankle Muscles

Spinal pathways underlying reciprocal flexion-extension contractions have been well characterized, but the extent to which cortically evoked motor-evoked potentials (MEPs) are influenced by antagonist muscle activation remains unclear. A majority of studies using transcranial magnetic stimulation- (TMS-) evoked MEPs to evaluate the excitability of the corticospinal pathway focus on upper extremity muscles. Due to functional and neural control differences between lower and upper limb muscles, there is a need to evaluate methodological factors influencing TMS-evoked MEPs specifically in lower limb musculature. If and to what extent the activation of the nontargeted muscles, such as antagonists, affects TMS-evoked MEPs is poorly understood, and such gaps in our knowledge may limit the rigor and reproducibility of TMS studies. Here, we evaluated the effect of the activation state of the antagonist muscle on TMS-evoked MEPs obtained from the target (agonist) ankle muscle for both tibialis anterior (TA) and soleus muscles. Fourteen able-bodied participants (11 females, age: 26.1 ± 4.1 years) completed one experimental session; data from 12 individuals were included in the analysis. TMS was delivered during 4 conditions: rest, TA activated, soleus activated, and TA and soleus coactivation. Three pairwise comparisons were made for MEP amplitude and coefficient of variability (CV): rest versus coactivation, rest versus antagonist activation, and agonist activation versus coactivation. We demonstrated that agonist-antagonist coactivation enhanced MEP amplitude and reduced MEP CVs for both TA and soleus muscles. Our results provide methodological considerations for future TMS studies and pave the way for future exploration of coactivation-dependent modulation of corticomotor excitability in pathological cohorts such as stroke or spinal cord injury.

Corticospinal excitability for flexor carpi radialis decreases with baroreceptor unloading during intentional co-contraction with opposing forearm muscles

Concurrent activation of antagonistic muscles (co-contraction) is used for stiffening a joint, whereas its neural control under hemodynamic stress (e.g., posture change, high gravity, and hemorrhage) is unknown. Corticospinal excitability during co-contraction may be altered with baroreceptor unloading due to potential modulations in spinal and/or inhibitory pathways (e.g., disynaptic group I inhibition and GABA-mediated intracortical inhibition). The purpose of this study was to understand the effect of baroreceptor unloading on corticospinal excitability during co-contraction in humans. Motor evoked potential and cortical silent period in a wrist flexor muscle were examined using transcranial magnetic stimulation in two groups of healthy young adults. All subjects performed isometric contraction of the wrist flexors (flexion) and co-contraction of the wrist flexors and extensors (co-contraction). Spinal disynaptic inhibition was also assessed with the ratio of H-reflex responses to unconditioned and conditioned electrical stimulations of the peripheral nerves for the muscles. In one of the groups, baroreflex unloading was induced by applying lower body negative pressure. There was no significant effect of baroreflex unloading on cortical silent period or H-reflex measure of disynaptic inhibition. With baroreflex unloading, motor evoked potential area in the flexor carpi radialis was decreased during co-contraction but not during flexion. The results indicated that baroreceptor unloading decreases corticospinal excitability during co-contraction of antagonistic muscles, apparently by influencing neural pathways that were not probed with cortical silent period or spinal disynaptic inhibition.

Impaired Ability to Suppress Excitability of Antagonist Motoneurons at Onset of Dorsiflexion in Adults with Cerebral Palsy

We recently showed that impaired gait function in adults with cerebral palsy (CP) is associated with reduced rate of force development in ankle dorsiflexors. Here, we explore potential mechanisms. We investigated the suppression of antagonist excitability, calculated as the amount of soleus H-reflex depression at the onset of ankle dorsiflexion compared to rest, in 24 adults with CP (34.3 years, range 18–57; GMFCS 1.95, range 1–3) and 15 healthy, age-matched controls. Furthermore, the central common drive to dorsiflexor motoneurons during a static contraction in the two groups was examined by coherence analyses. The H-reflex was significantly reduced by 37% at the onset of dorsiflexion compared to rest in healthy adults (P < 0.001) but unchanged in adults with CP (P = 0.91). Also, the adults with CP had significantly less coherence. These findings suggest that the ability to suppress antagonist motoneuronal excitability at movement onset is impaired and that the central common drive during static contractions is reduced in adults with CP.

Perspectives on classical controversies about the motor cortex

Primary motor cortex has been studied for more than a century, yet a consensus on its functional contribution to movement control is still out of reach. In particular, there remains controversy as to the level of control produced by motor cortex ("low-level" movement dynamics vs. "high-level" movement kinematics) and the role of sensory feedback. In this review, we present different perspectives on the two following questions: What does activity in motor cortex reflect? and How do planned motor commands interact with incoming sensory feedback during movement? The four authors each present their independent views on how they think the primary motor cortex (M1) controls movement. At the end, we present a dialogue in which the authors synthesize their views and suggest possibilities for moving the field forward. While there is not yet a consensus on the role of M1 or sensory feedback in the control of upper limb movements, such dialogues are essential to take us closer to one.

How plastic are human spinal cord motor circuitries?

Human and animal studies have documented that neural circuitries in the spinal cord show adaptive changes caused by altered supraspinal and/or afferent input to the spinal circuitry in relation to learning, immobilization, injury and neurorehabilitation. Reversible adaptations following, e.g. the acquisition or refinement of a motor skill rely heavily on the functional integration between supraspinal and sensory inputs to the spinal cord networks. Accordingly, what is frequently conceived as a change in the spinal circuitry may be a change in either descending or afferent input or in the relative integration of these, i.e. a change in the neuronal weighting. This is evident from findings documenting only task-specific functional changes after periods of altered inputs whereas resting responses remain unaffected. In fact, the proximity of the spinal circuitry to the outer world may demand a more rigid organization compared to the highly flexible cortical circuits. The understanding of all of this is important for the planning and execution of neurorehabilitation.

Human Spinal Motor Control

Human studies in the past three decades have provided us with an emerging understanding of how cortical and spinal networks collaborate to ensure the vast repertoire of human behaviors. Humans have direct cortical connections to spinal motoneurons, which bypass spinal interneurons and exert a direct (willful) muscle control with the aid of a context-dependent integration of somatosensory and visual information at cortical level. However, spinal networks also play an important role. Sensory feedback through spinal circuitries is integrated with central motor commands and contributes importantly to the muscle activity underlying voluntary movements. Regulation of spinal interneurons is used to switch between motor states such as locomotion (reciprocal innervation) and stance (coactivation pattern). Cortical regulation of presynaptic inhibition of sensory afferents may focus the central motor command by opening or closing sensory feedback pathways. In the future, human studies of spinal motor control, in close collaboration with animal studies on the molecular biology of the spinal cord, will continue to document the neural basis for human behavior.

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.

Subcortical control of precision grip after human spinal cord injury

The motor cortex and the corticospinal system contribute to the control of a precision grip between the thumb and index finger. The involvement of subcortical pathways during human precision grip remains unclear. Using noninvasive cortical and cervicomedullary stimulation, we examined motor evoked potentials (MEPs) and the activity in intracortical and subcortical pathways targeting an intrinsic hand muscle when grasping a small (6 mm) cylinder between the thumb and index finger and during index finger abduction in uninjured humans and in patients with subcortical damage due to incomplete cervical spinal cord injury (SCI). We demonstrate that cortical and cervicomedullary MEP size was reduced during precision grip compared with index finger abduction in uninjured humans, but was unchanged in SCI patients. Regardless of whether cortical and cervicomedullary stimulation was used, suppression of the MEP was only evident 1-3 ms after its onset. Long-term (∼5 years) use of the GABAb receptor agonist baclofen by SCI patients reduced MEP size during precision grip to similar levels as uninjured humans. Index finger sensory function correlated with MEP size during precision grip in SCI patients. Intracortical inhibition decreased during precision grip and spinal motoneuron excitability remained unchanged in all groups. Our results demonstrate that the control of precision grip in humans involves premotoneuronal subcortical mechanisms, likely disynaptic or polysynaptic spinal pathways that are lacking after SCI and restored by long-term use of baclofen. We propose that spinal GABAb-ergic interneuronal circuits, which are sensitive to baclofen, are part of the subcortical premotoneuronal network shaping corticospinal output during human precision grip.

M1 corticospinal mirror neurons and their role in movement suppression during action observation

Evidence is accumulating that neurons in primary motor cortex (M1) respond during action observation, a property first shown for mirror neurons in monkey premotor cortex. We now show for the first time that the discharge of a major class of M1 output neuron, the pyramidal tract neuron (PTN), is modulated during observation of precision grip by a human experimenter. We recorded 132 PTNs in the hand area of two adult macaques, of which 65 (49%) showed mirror-like activity. Many (38 of 65) increased their discharge during observation (facilitation-type mirror neuron), but a substantial number (27 of 65) exhibited reduced discharge or stopped firing (suppression-type). Simultaneous recordings from arm, hand, and digit muscles confirmed the complete absence of detectable muscle activity during observation. We compared the discharge of the same population of neurons during active grasp by the monkeys. We found that facilitation neurons were only half as active for action observation as for action execution, and that suppression neurons reversed their activity pattern and were actually facilitated during execution. Thus, although many M1 output neurons are active during action observation, M1 direct input to spinal circuitry is either reduced or abolished and may not be sufficient to produce overt muscle activity.

The motor cortex drives the muscles during walking in human subjects

Indirect evidence that the motor cortex and the corticospinal tract contribute to the control of walking in human subjects has been provided in previous studies. In the present study we used coherence analysis of the coupling between EEG and EMG from active leg muscles during human walking to address if activity arising in the motor cortex contributes to the muscle activity during gait. Nine healthy human subjects walked on a treadmill at a speed of 3.5–4 km h(-1). Seven of the subjects in addition walked at a speed of 1 km h(-1). Significant coupling between EEG recordings over the leg motor area and EMG from the anterior tibial muscle was found in the frequency band 24–40 Hz prior to heel strike during the swing phase of walking. This signifies that rhythmic cortical activity in the 24–40 Hz frequency band is transmitted via the corticospinal tract to the active muscles during walking. These findings demonstrate that the motor cortex and corticospinal tract contribute directly to the muscle activity observed in steady-state treadmill walking.

Dissociating motor cortex from the motor

During closed-loop control of a brain-computer interface, neurons in the primary motor cortex can be intensely active even though the subject may be making no detectable movement or muscle contraction. How can neural activity in the primary motor cortex become dissociated from the movements and muscles of the native limb that it normally controls? Here we examine circumstances in which motor cortex activity is known to dissociate from movement--including mental imagery, visuo-motor dissociation and instructed delay. Many such motor cortex neurons may be related to muscle activity only indirectly. Furthermore, the integration of thousands of synaptic inputs by individual α-motoneurons means that under certain circumstances even cortico-motoneuronal cells, which make monosynaptic connections to α-motoneurons, can become dissociated from muscle activity. The natural ability of motor cortex neurons under voluntarily control to become dissociated from bodily movement may underlie the utility of this cortical area for controlling brain-computer interfaces.

Effects of anodal transcranial direct current stimulation over the leg motor area on lumbar spinal network excitability in healthy subjects

In recent years, two techniques have become available for the non-invasive stimulation of human motor cortex: transcranial magnetic stimulation (TMS) and transcranial direct current stimulation (tDCS). The effects of TMS and tDCS when applied over motor cortex should be considered with regard not only to cortical circuits but also to spinal motor circuits. The different modes of action and specificity of TMS and tDCS suggest that their effects on spinal network excitability may be different from that in the cortex. Until now, the effects of tDCS on lumbar spinal network excitability have never been studied. In this series of experiments, on healthy subjects, we studied the effects of anodal tDCS over the lower limb motor cortex on (i) reciprocal Ia inhibition projecting from the tibialis anterior muscle (TA) to the soleus (SOL), (ii) presynaptic inhibition of SOL Ia terminals, (iii) homonymous SOL recurrent inhibition, and (iv) SOL H-reflex recruitment curves. The results show that anodal tDCS decreases reciprocal Ia inhibition, increases recurrent inhibition and induces no modification of presynaptic inhibition of SOL Ia terminals and of SOL-H reflex recruitment curves. Our results indicate therefore that the effects of tDCS are the opposite of those previously described for TMS on spinal network excitability. They also indicate that anodal tDCS induces effects on spinal network excitability similar to those observed during co-contraction suggesting that anodal tDCS activates descending corticospinal projections mainly involved in co-contractions.

Voluntary activation of ankle muscles is accompanied by subcortical facilitation of their antagonists

Flexion and extension movements are organized reciprocally, so that extensor motoneurones in the spinal cord are inhibited when flexor muscles are active and vice versa. During and just prior to dorsiflexion of the ankle, soleus motoneurones are thus inhibited as evidenced by a depression of the soleus H-reflex. It is therefore surprising that soleus motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation (TMS) have been found not to be reduced and even facilitated during a voluntary dorsiflexion. The objective of this study was to investigate if MEPs, evoked by TMS, show a similar facilitation prior to and at the onset of contraction of muscles that are antagonists to the muscle in which the MEP is evoked and if so, examine the origin of such a facilitatory motor programme. Eleven seated subjects reacted to an auditory cue by contracting either the tibialis anterior (TA) or soleus muscle of the left ankle. TMS was applied to the hotspot of TA and soleus muscles on separate days. Stimuli were delivered prior to and at the beginning of contraction. Soleus MEPs were significantly facilitated when TMS was applied 50 ms prior to onset of plantar flexion. Surprisingly, soleus MEPs were also facilitated (although to a lesser extent) at a similar time in relation to the onset of dorsiflexion. TA MEPs were facilitated 50 ms prior to onset of dorsiflexion and neither depressed nor facilitated prior to plantar flexion. No difference was found between the facilitation of the soleus MEP and motor evoked responses to cervicomedullary stimulation prior to dorsiflexion, suggesting that the increased soleus MEPs were not caused by changes at a cortical level. This was confirmed by the observation that short-latency facilitation of the soleus H-reflex by subthreshold TMS was increased prior to plantar flexion, but not prior to dorsiflexion. These findings suggest that voluntary contraction at the ankle is accompanied by preceding facilitation of antagonists by a subcortical motor programme. This may help to ensure that the direction of movement may be changed quickly and efficiently during functional motor tasks.

Thumb and finger forces produced by motor units in the long flexor of the human thumb

The uncommonly good proprioceptive performance of the long flexor of the thumb, flexor pollicis longus (FPL), may add significantly to human manual dexterity. We investigated the forces produced by FPL single motor units during a weak static grip involving all digits by spike-triggered averaging from single motor units, and by averaging from twitches produced by intramuscular stimulation. Nine adult subjects were studied. The forces produced at each digit were used to assess how forces produced in FPL are distributed to the fingers. Most FPL motor units produced very low forces on the thumb and were positively correlated with the muscle force at recruitment. Activity in FPL motor units commonly loaded the index finger (42/55 units), but less commonly the other fingers (P < 0.001). On average, these motor units produced small but significant loading forces on the index finger ( approximately 5.3% of their force on the thumb) with the same time-to-peak force as the thumb ( approximately 50 ms), but had no significant effect on other fingers. However, intramuscular stimulation within FPL did not produce significant forces in any finger. Coherence at 2-10 Hz between the thumb and index finger force was twice that for the other finger forces and the coherence to the non-index fingers was not altered when the index finger did not participate in the grasp. These results indicate that, within the long-term coordinated forces of all digits during grasping, FPL motor units generate forces highly focused on the thumb with minimal peripheral transfer to the fingers and that there is a small but inflexible neural coupling to the flexors of the index finger.

Investigating human motor control by transcranial magnetic stimulation

In this review we discuss the contribution of transcranial magnetic stimulation (TMS) to the understanding of human motor control. Compound motor-evoked potentials (MEPs) may provide valuable information about corticospinal transmission, especially in patients with neurological disorders, but generally do not allow conclusions regarding the details of corticospinal function to be made. Techniques such as poststimulus time histograms (PSTHs) of the discharge of single, voluntarily activated motor units and conditioning of H reflexes provide a more optimal way of evaluating transmission in specific excitatory and inhibitory pathways. Through application of such techniques, several important issues have been clarified. TMS has provided the first real evidence that direct monosynaptic connections from the motor cortex to spinal motoneurons exist in man, and it has been revealed that the distribution of these projections roughly follows the same proximal-distal gradient as in other primates. However, pronounced differences also exist. In particular, the tibialis anterior muscle appears to receive as significant a monosynaptic corticospinal drive as muscles in the hand. The reason for this may be the importance of this muscle in controlling the foot trajectory in the swing phase of walking. Conditioning of H reflexes by TMS has provided evidence of changes in cortical excitability prior to and during various movements. These experiments have generally confirmed information obtained from chronic recording of the activity of corticospinal cells in primates, but information about the corticospinal contribution to movements for which information from other primates is sparse or lacking has also been obtained. One example is walking, where TMS experiments have revealed that the corticospinal tract makes an important contribution to the ongoing EMG activity during treadmill walking. TMS experiments have also documented the convergence of descending corticospinal projections and peripheral afferents on spinal interneurons. Current investigations of the functional significance of this convergence also rely on TMS experiments. The general conclusion from this review is that TMS is a powerful technique in the analysis of motor control, but that care is necessary when interpreting the data. Combining TMS with other techniques such as PSTH and H reflex testing amplifies greatly the power of the technique.

Task-dependent modulation of excitatory and inhibitory functions within the human primary motor cortex

We evaluated motor evoked potentials (MEPs) and duration of the cortical silent period (CSP) from the right first dorsal interosseous (FDI) muscle to transcranial magnetic stimulation (TMS) of the left motor cortex in ten healthy subjects performing different manual tasks. They abducted the index finger alone, pressed a strain gauge with the thumb and index finger in a pincer grip, and squeezed a 4-cm brass cylinder with all digits in a power grip. The level of FDI EMG activity across tasks was kept constant by providing subjects with acoustic-visual feedback of their muscle activity. The TMS elicited larger amplitude FDI MEPs during pincer and power grip than during the index finger abduction task, and larger amplitude MEPs during pincer gripping than during power gripping. The CSP was shorter during pincer and power grip than during the index finger abduction task and shorter during power gripping than during pincer gripping. These results suggest excitatory and inhibitory task-dependent changes in the motor cortex. Complex manual tasks (pincer and power gripping) elicit greater motor cortical excitation than a simple task (index finger abduction) presumably because they activate multiple synergistic muscles thus facilitating corticomotoneurons. The finger abduction task probably yielded greater motor cortical inhibition than the pincer and power tasks because muscles uninvolved in the task activated the cortical inhibitory circuit. Increased cortical excitatory and inhibitory functions during precision tasks (pincer gripping) probably explain why MEPs have larger amplitudes and CSPs have longer durations during pincer gripping than during power gripping.

Cerebral functional anatomy of voluntary contractions of ankle muscles in man

1. Cerebral activation elicited by right-sided voluntary ankle muscle contraction was investigated by positron emission tomography measurements of regional cerebral blood flow. Two studies with eight subjects in each were carried out. Tonic isometric plantar and dorsiflexion and co-contraction of the antagonist muscles were investigated in study 1. Tonic contraction was compared with dynamic ramp-and-hold contractions in study 2. 2. All types of contraction elicited activation of the left primary motor cortex (M1). The distance between the M1 peak activation locations for tonic isometric dorsi- and plantar flexion was 17 mm. Co-contraction elicited activation of a larger area of M1 mainly located in between but partially overlapping the M1 areas activated during isolated dorsi-/plantar flexion. 3. A voxel-by-voxel correlation analysis corrected for subject covariance showed for dorsiflexion a significant correlation between tibialis anterior EMG level and cerebral blood flow activation in the cerebellum and the M1 of the medial frontal cortex. For plantar flexion a significant correlation was found between soleus EMG and cerebral activation in the left medial S1 and M1, left thalamus and right cerebellum. 4. The activation during dynamic isotonic and isometric dorsi- and plantar flexion was significantly more extensive than during tonic contractions. In addition to M1, activation was seen in the contralateral supplementary motor area and bilaterally in the premotor and parietal cortices. Isotonic and isometric contractions did not differ except in a small area in the primary somatosensory cortex. 5. One possible explanation of the different cerebral activation during co-contraction compared to that during plantar/dorsiflexion is that slightly different populations of cortical neurones are involved. The more extensive activation during dynamic compared with tonic contractions may reflect a larger cortical drive necessary to initiate and accelerate movements.

Evidence of facilitation of soleus-coupled Renshaw cells during voluntary co-contraction of antagonistic ankle muscles in man

1. The amount of recurrent inhibition onto soleus motoneurones was compared during plantar flexion and co-contraction of antagonistic ankle plantar and dorsiflexors at matched levels of background activity in the soleus muscle. 2. During weak plantar flexion and co-contraction (less than 10% of maximal voluntary plantar flexion effort) a test reflex discharge (H' reflex), which was conditioned by a previous reflex discharge, was found to be significantly more depressed in relation to rest than an unconditioned reference H reflex. During strong plantar flexion (more than 50% of maximal voluntary plantar flexion effort) the H' reflex either increased more or to the same extent as the reference H reflex in relation to rest. In contrast to this, the H' reflex was strongly depressed during co-contraction, whereas the reference H reflex was not significantly different from its resting value. 3. At the end of the ramp phase of a phasic contraction, large variations of the H' reflex were observed during plantar flexion (large increase in relation to rest) and during co-contraction (marked decrease), whereas the reference H reflex was facilitated in the two situations. 4. These observations provide evidence that soleus-coupled Renshaw cells are differently regulated during co-contraction and plantar flexion. It is suggested that the Renshaw cells are inhibited during strong plantar flexion but not during strong co-contraction. The functional significance of the findings is discussed.

Changes in the effect of magnetic brain stimulation accompanying voluntary dynamic contraction in man

1. The soleus (Sol) H reflex was conditioned by magnetic stimulation of the contralateral motor cortex at rest and during voluntary contraction in healthy human subjects. The intensity of the magnetic stimulus was adjusted so as to have no effect on the H reflex at rest. During tonic voluntary contraction the same magnetic stimulus produced a facilitation with a short latency and a long duration, thus reflecting an increased excitation of Sol motoneurones by the magnetic stimulus during voluntary contraction. 2. The amount of reflex facilitation produced by brain stimulation within the initial 0.5-1 ms after its onset was investigated at different times during dynamic ramp-and-hold plantar flexion. The facilitation was largest at the onset of voluntary activity in the Sol muscle. It then decreased abruptly within 100 ms after the onset of the voluntary contraction. Neither the voluntary Sol activity nor the control H reflex decreased at this time. 3. Electrical stimulation of the brain with the anode placed lateral to the vertex produced a facilitation of the H reflex, which preceded the facilitation evoked by magnetic stimulation by 1-2 ms. The facilitation produced by the magnetic stimulus occurred or increased at the onset of contraction in relation to rest in all experiments. However, this was the case in only two out of eight experiments, when the brain was stimulated electrically. 4. The size of the reflex facilitation measured at the onset of contraction was larger the faster the contraction. Positive correlations were found between the size of the facilitation and the peak of the first and second derivative of the torque and the peak Sol EMG activity. 5. It is suggested that the observed changes in the size of the short-latency reflex facilitation produced by magnetic brain stimulation mainly reflects changes in the excitability of corticospinal cells, since similar changes were not observed in the size of the unconditioned Sol H reflex or in the short-latency reflex facilitation produced by electrical brain stimulation. The data support the hypothesis that fast conducting corticospinal fibres with monosynaptic projections to spinal motoneurones are involved in the initiation of voluntary movement in man.

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.

Task-related changes in the effect of magnetic brain stimulation on spinal neurones in man

1. The effect of magnetic stimulation of the human motor cortex on the excitability of soleus, tibialis anterior and flexor carpi radialis motoneurones was investigated by H reflex testing in ten healthy subjects. 2. At rest, an early facilitation of the flexor capri radialis and tibialis anterior H reflexes was always seen, whereas a similar early facilitation of the soleus H reflex was seen in only two out of seven subjects. For all three motoneuronal pools the facilitation was curtailed 1-5 ms later by an inhibition which lasted for another 3-4 ms. In five subjects an inhibition without any evidence of an earlier facilitation was seen for the soleus H reflex. 3. The intensity of the magnetic stimulation was subsequently decreased so that it had no effect on the H reflex at rest. When the subject then performed a voluntary agonist contraction a facilitatory effect with an early onset and a duration of 20-25 ms was observed for all three muscles. When the subject performed a voluntary antagonist contraction an inhibition was seen for the soleus H reflex with an onset 1-3 ms later than the facilitation. This is interpreted as resulting from the excitation by the magnetic stimulus of corticospinal neurones voluntarily activated in relation to the given motor task. 4. The initial part of the facilitation was significantly smaller during co-contraction of both agonists and antagonists than during isolated agonist contraction. 5. Whereas the early part of the facilitation always occurred during plantarflexion when the H reflex was conditioned by magnetic stimulation, this was never the case when it was conditioned by electrical stimulation of the cortex with the stimulus regimes used in these experiments. 6. It is suggested that the early part of the facilitation observed during agonist contraction is caused by activation of cortico-motoneuronal cells projecting to the agonist motoneuronal pool and that the inhibition observed during antagonist contraction is caused by activation of corticospinal cells projecting both to the antagonist motoneuronal pool and Ia inhibitory interneurones to the agonist motoneuronal pool. The smaller size of the earliest part of the facilitation observed during co-contraction in relation to agonist contraction suggests a different cortical control of the two tasks.

The regulation of disynaptic reciprocal Ia inhibition during co-contraction of antagonistic muscles in man

1. The disynaptic reciprocal inhibition from ankle dorsiflexors to ankle plantarflexors was investigated at rest, during tonic plantar- and dorsiflexion and during co-contraction. In relation to rest, it was found to be decreased during plantarflexion and co-contraction, but unchanged during dorsiflexion. 2. When increasing the strength of plantarflexion the amount of inhibition became progressively smaller. Already during weak co-contraction, the amount of inhibition was very small and it did not become smaller during stronger contraction. The decrease of inhibition during co-contraction could not be explained by an addition of the changes of inhibition observed during plantar- and dorsiflexion individually. 3. The disynaptic reciprocal inhibition was also found to be decreased when the peripheral feedback from the muscles was blocked by inducing ischaemia in the leg and at the beginning of a dynamic co-contraction before sensory feedback could interfere. This implies that the observed decrease is caused by a central inhibition of the transmission in the pathway. 4. The amount of disynaptic reciprocal inhibition was also investigated during standing. No significant difference in the amount of inhibition was found when the subjects were standing up at rest as compared to sitting down at rest. When the subjects were forced to make a co-contraction in order to maintain balance, i.e. when they were standing on one leg, leaning backwards or standing on an unstable platform, a decrease of disynaptic reciprocal inhibition was seen. When the subjects leaned forward, thus forcing a contraction of the soleus muscle, a decrease was also seen, but it was smaller than in the co-contraction tasks. Finally, when the subjects lifted the examined leg, thus contracting the tibialis anterior muscle, either no change or a small increase of inhibition was seen. 5. A similar control of the disynaptic reciprocal inhibition as described for the pathway from ankle dorsiflexors to ankle plantarflexors was also observed for the pathway from ankle plantarflexors to dorsiflexors and from wrist extensors to wrist flexors. 6. It is concluded that when co-contraction is used in order to stabilize a joint, i.e. to maintain posture, a specific co-contraction motor programme is activated that depresses the transmission in the disynaptic reciprocal pathway thereby ensuring a high excitability level in the motoneurones of both antagonistic muscles.

Organization of the forelimb area in squirrel monkey motor cortex: representation of digit, wrist, and elbow muscles

The EMG in 8 to 14 hand, forearm, and arm muscles evoked by intracortical electrical stimulation was recorded at 433 sites in layer V in the region of the forelimb area of the primary motor cortex (MI) of three squirrel monkeys during ketamine anesthesia. At each site, the EMG was recorded at movement threshold (T) and at 1.5T and 2T at each site (but less than or equal to 60 microA), and the threshold movement was noted. In the animals examined, the total MI forelimb area identified by movements or EMG occupied about 25 to 35 mm2. At most sites from which a forelimb movement was evoked, EMG activity was evoked in one or more of the recorded muscles. One group of sites located rostrolaterally to the main forelimb area was separated by an intervening zone largely related to the face. The average area from which digit, wrist, elbow, or shoulder movement was evoked at threshold was nearly the same, and their movement thresholds were not significantly different. Average movement thresholds across the anterior-posterior extent of MI were also similar. All muscles recorded could be activated by cortical stimulation. Most commonly more than one muscle was activated from a single site. The highest individual EMG levels were produced at sites from which more than one muscle was activated. These results suggest that small regions of MI influence multiple muscles. Individual muscles were typically activated at multiple, spatially separated locations. For many muscles, increasing the stimulation intensity revealed additional separate areas of activation. Spatial locations of different muscles showed considerable interanimal variation. The size of most muscle representations was relatively large. The smallest representations always included the intrinsic hand muscles and the largest included the proximal muscles. Orderly topographic relationships among forelimb joints or muscles within the MI forelimb area were not apparent. Although distal muscle activation tended to be found posteriorly in the forelimb area and proximal muscles tended to be activated from anterior sites, both could be activated from broadly distributed and overlapping areas. The broad, overlapping nature of the muscle representation supports the concept that a small region of cortex is involved in controlling functional groups of muscles. The intermingling of muscle representations may provide a substrate for local cortical interactions among territories representing various muscle synergies or for changing associations of muscle groups. The representation plan derived from these mappings contains elements of all previously described summaries of MI organization.(ABSTRACT TRUNCATED AT 400 WORDS)

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)