Properties of primary motor cortex output to hindlimb muscles in the macaque monkey

Haptic Human-Human Interaction During an Ankle Tracking Task: Effects of Virtual Connection Stiffness

While treating sensorimotor impairments, a therapist may provide physical assistance by guiding their patient's limb to teach a desired movement. In this scenario, a key aspect is the compliance of the interaction, as the therapist can provide subtle cues or impose a movement as demonstration. One approach to studying these interactions involves haptically connecting two individuals through robotic interfaces. Upper-limb studies have shown that pairs of connected individuals estimate one another's goals during tracking tasks by exchanging haptic information, resulting in improved performance dependent on the ability of one's partner and the stiffness of the virtual connection. In this study, our goal was to investigate whether these findings generalize to the lower limb during an ankle tracking task. Pairs of healthy participants (i.e., dyads) independently tracked target trajectories with and without connections rendered between two ankle robots. We tested the effects of connection stiffness as well as visual noise to manipulate the correlation of tracking errors between partners. In our analysis, we compared changes in task performance across conditions while tracking with and without the connection. We found that tracking improvements while connected increased with connection stiffness, favoring the worse partner in the dyad during hard connections. We modeled the interaction as three springs in series, considering the stiffness of the connection and each partners' ankle, to show that improvements were likely due to a cancellation of random tracking errors between partners. These results suggest a simplified mechanism of improvements compared to what has been reported during upper-limb dyadic tracking.

Corticospinal excitability across lower limb muscles in humans

Electrophysiological studies in nonhuman primates reported the existence of strong corticospinal output from the primary motor cortex to distal compared with proximal hindlimb muscles. The extent to which corticospinal output differs across muscles in the leg in humans remains poorly understood. Using transcranial magnetic stimulation over the leg representation of the primary motor cortex, we constructed motor evoked potential (MEP) recruitment curves to measure the resting motor threshold (RMT), maximum MEP amplitude (MEP-max), and slope in the biceps femoris, rectus femoris, tibialis anterior, soleus, and a foot muscle (i.e., abductor hallucis) in intact humans. We found that the RMT was lower and the MEP-max and slope were larger in the abductor hallucis compared with most other muscles tested. In contrast, the RMT was higher and the MEP-max and slope were lower in the biceps femoris compared to all other muscles tested. Corticospinal responses in the rectus femoris, tibialis anterior, and soleus were in between those obtained from other leg muscles, with the soleus having a higher RMT and lower MEP-max and slope than the rectus femoris and tibialis anterior. To examine the origin of increases in corticospinal excitability in the abductor hallucis, we compared short-interval intracortical inhibition (SICI) and F-waves between the abductor hallucis and tibialis anterior. SICI was similar across muscles while the F-wave amplitude was larger in the abductor hallucis compared with the tibialis anterior. These results support a nonuniform distribution of corticospinal output to leg muscles, highlighting that increases in corticospinal excitability in a foot muscle could be related to a spinal origin.NEW & NOTEWORTHY We provide evidence on how corticospinal output differs across muscles in the leg in intact humans. We found that corticospinal responses were larger in a distal intrinsic foot muscle and were smaller in the biceps femoris compared to all other muscles in the leg. Increases in corticospinal excitability to an intrinsic foot muscle could have a spinal origin.

Activity dependent stimulation increases synaptic efficacy in spared pathways in an anesthetized rat model of spinal cord contusion injury

BACKGROUND

Closed-loop neuromodulation systems have received increased attention in recent years as potential therapeutic approaches for treating neurological injury and disease.

OBJECTIVE

The purpose of this study was to assess the ability of intraspinal microstimulation (ISMS), triggered by action potentials (spikes) recorded in motor cortex, to alter synaptic efficacy in descending motor pathways in an anesthetized rat model of spinal cord injury (SCI).

METHODS

Experiments were carried out in adult, male, Sprague Dawley rats with a moderate contusion injury at T8. For activity-dependent stimulation (ADS) sessions, a recording microelectrode was used to detect neuronal spikes in motor cortex that triggered ISMS in the spinal cord grey matter. SCI rats were randomly assigned to one of four experimental groups differing by: a) cortical spike-ISMS stimulus delay (10 or 25 ms) and b) number of ISMS pulses (1 or 3). Four weeks after SCI, ADS sessions were conducted in three consecutive 1-hour conditioning bouts for a total of 3 hours. At the end of each conditioning bout, changes in synaptic efficacy were assessed using intracortical microstimulation (ICMS) to examine the number of spikes evoked in spinal cord neurons during 5-minute test bouts. A multichannel microelectrode recording array was used to record cortically-evoked spike activity from multiple layers of the spinal cord.

RESULTS

The results showed that ADS resulted in an increase in cortically-evoked spikes in spinal cord neurons at specific combinations of spike-ISMS delays and numbers of pulses. Efficacy in descending motor pathways was increased throughout all dorsoventral depths of the hindlimb spinal cord.

CONCLUSIONS

These results show that after an SCI, ADS can increase synaptic efficacy in spared pathways between motor cortex and spinal cord. This study provides further support for the potential of ADS therapy as an effective method for enhancing descending motor control after SCI.

An intracortical neuroprosthesis immediately alleviates walking deficits and improves recovery of leg control after spinal cord injury

Most rehabilitation interventions after spinal cord injury (SCI) only target the sublesional spinal networks, peripheral nerves, and muscles. However, mammalian locomotion is not a mere act of rhythmic pattern generation. Recovery of cortical control is essential for voluntary movement and modulation of gait. We developed an intracortical neuroprosthetic intervention to SCI, with the goal to condition cortical locomotor control. Neurostimulation delivered in phase coherence with ongoing locomotion immediately alleviated primary SCI deficits, such as leg dragging, in rats with incomplete SCI. Cortical neurostimulation achieved high fidelity and markedly proportional online control of leg trajectories in both healthy and SCI rats. Long-term neuroprosthetic training lastingly improved cortical control of locomotion, whereas short training held transient improvements. We performed longitudinal awake cortical motor mapping, unveiling that recovery of cortico-spinal transmission tightly parallels return of locomotor function in rats. These results advocate directly targeting the motor cortex in clinical neuroprosthetic approaches.

Distinct patterns of spasticity and corticospinal connectivity following complete spinal cord injury

KEY POINTS

Damage to corticospinal axons have implications for the development of spasticity following spinal cord injury (SCI). Here, we examined to which extent residual corticospinal connections and spasticity are present in muscles below the injury (quadriceps femoris and soleus) in humans with motor complete thoracic SCI. We found three distinct sub-groups of people: participants with spasticity and corticospinal responses in the quadriceps femoris and soleus, participants with spasticity and corticospinal responses in the quadriceps femoris only, and participants with no spasticity or corticospinal responses in either muscle. Spasticity and corticospinal responses were present in the quadriceps but never only in the soleus muscle, suggesting a proximal to distal gradient of symptoms of hyperreflexia. These results suggest that concomitant patterns of residual corticospinal connectivity and spasticity exist in humans with motor complete SCI and that a clinical exam of spasticity might be a good predictor of residual corticospinal connectivity.

ABSTRACT

The loss of corticospinal axons has implications for the development of spasticity following spinal cord injury (SCI). However, the extent to which residual corticospinal connections and spasticity are present across muscles below the injury remains unknown. To address this question, we tested spasticity using the Modified Ashworth Scale and transmission in the corticospinal pathway by examining motor evoked potentials elicited by transcranial magnetic stimulation over the leg motor cortex (cortical MEPs) and by direct activation of corticospinal axons by electrical stimulation over the thoracic spine (thoracic MEPs), in the quadriceps femoris and soleus muscles, in 30 individuals with motor complete thoracic SCI. Cortical MEPs were also conditioned by thoracic electrical stimulation at intervals allowing their summation or collision. We found three distinct sub-groups of participants: 47% showed spasticity in the quadriceps femoris and soleus muscle, 30% showed spasticity in the quadriceps femoris muscle only, and 23% showed no spasticity in either muscle. While cortical MEPs were present only in the quadriceps in participants with spasticity, thoracic MEPs were present in both muscles when spasticity was present. Thoracic electrical stimulation facilitated and suppressed cortical MEPs, showing that both forms of stimulation activated similar corticospinal axons. Cortical and thoracic MEPs correlated with the degree of spasticity in both muscles. These results provide the first evidence that related patterns of residual corticospinal connectivity and spasticity exist in muscles below the injury after motor complete thoracic SCI and highlight that a clinical exam of spasticity can predict residual corticospinal connectivity after severe paralysis. This article is protected by copyright. All rights reserved.

Effects of a contusive spinal cord injury on cortically-evoked spinal spiking activity in rats

Objective.The purpose of this study was to determine the effects of spinal cord injury (SCI) on spike activity evoked in the hindlimb spinal cord of the rat from cortical electrical stimulation.Approach.Adult, male, Sprague Dawley rats were randomly assigned to a Healthy or SCI group. SCI rats were given a 175 kDyn dorsal midline contusion injury at the level of the T8 vertebrae. At 4 weeks post-SCI, intracortical microstimulation (ICMS) was delivered at several sites in the hindlimb motor cortex of anesthetized rats, and evoked neural activity was recorded from corresponding sites throughout the dorsoventral depths of the spinal cord and EMG activity from hindlimb muscles.Main results.In healthy rats, post-ICMS spike histograms showed reliable, evoked spike activity during a short-latency epoch 10-12 ms after the initiation of the ICMS pulse train (short). Longer latency spikes occurred between ∼20 and 60 ms, generally following a Gaussian distribution, rising above baseline at timeLON, followed by a peak response (Lp), and then falling below baseline at timeLOFF. EMG responses occurred betweenLONandLp( 25-27 ms). In SCI rats, short-latency responses were still present, long-latency responses were disrupted or eliminated, and EMG responses were never evoked. The retention of the short-latency responses indicates that spared descending spinal fibers, most likely via the cortico-reticulospinal pathway, can still depolarize spinal cord neurons after a dorsal midline contusion injury.Significance.This study provides novel insights into the role of alternate pathways for voluntary control of hindlimb movements after SCI that disrupts the corticospinal tract in the rat.

Dissociated leg muscle atrophy in amyotrophic lateral sclerosis/motor neuron disease: the 'split-leg' sign

Disproportionate muscle atrophy is a distinct phenomenon in amyotrophic lateral sclerosis (ALS); however, preferentially affected leg muscles remain unknown. We aimed to identify this split-leg phenomenon in ALS and determine its pathophysiology. Patients with ALS (n = 143), progressive muscular atrophy (PMA, n = 36), and age-matched healthy controls (HC, n = 53) were retrospectively identified from our motor neuron disease registry. We analyzed their disease duration, onset region, ALS Functional Rating Scale-Revised Scores, and results of neurological examination. Compound muscle action potential (CMAP) of the extensor digitorum brevis (EDB), abductor hallucis (AH), and tibialis anterior (TA) were reviewed. Defined by CMAP_EDB/CMAP_AH (SI_EDB) and CMAP_TA/CMAP_AH (SI_TA), respectively, the values of split-leg indices (SI) were compared between these groups. SI_EDB was significantly reduced in ALS (p < 0.0001) and PMA (p < 0.0001) compared to the healthy controls (HCs). SI_TA reduction was more prominent in PMA (p < 0.05 vs. ALS, p < 0.01 vs. HC), but was not significant in ALS compared to the HCs. SI was found to be significantly decreased with clinical lower motor neuron signs (SI_EDB), while was rather increased with clinical upper motor neuron signs (SI_TA). Compared to the AH, TA and EDB are more severely affected in ALS and PMA patients. Our findings help to elucidate the pathophysiology of split-leg phenomenon.

Locomotor kinematics and EMG activity during quadrupedal versus bipedal gait in the Japanese macaque

Several qualitative features distinguish bipedal from quadrupedal locomotion in mammals. In this study we show quantitative differences between quadrupedal and bipedal gait in the Japanese monkey in terms of gait patterns, trunk/hindlimb kinematics, and electromyographic (EMG) activity, obtained from 3 macaques during treadmill walking. We predicted that as a consequence of an almost upright body axis, bipedal gait would show properties consistent with temporal and spatial optimization countering higher trunk/hindlimb loads and a less stable center of mass (CoM). A comparatively larger step width, an ~9% longer duty cycle, and ~20% increased relative duration of the double-support phase were all in line with such a strategy. Bipedal joint kinematics showed the strongest differences in proximal, and least in distal, hindlimb joint excursions compared with quadrupedal gait. Hindlimb joint coordination (cyclograms) revealed more periods of single-joint rotations during bipedal gait and predominance of proximal joints during single support. The CoM described a symmetrical, quasi-sinusoidal left/right path during bipedal gait, with an alternating shift toward the weight-supporting limb during stance. Trunk/hindlimb EMG activity was nonuniformally increased during bipedal gait, most prominently in proximal antigravity muscles during stance (up to 10-fold). Non-antigravity hindlimb EMG showed altered temporal profiles during liftoff or touchdown. Muscle coactivation was more, but muscle synergies less, frequent during bipedal gait. Together, these results show that behavioral and EMG properties of bipedal vs. quadrupedal gait are quantitatively distinct and suggest that the neural control of bipedal primate locomotion underwent specific adaptations to generate these particular behavioral features to counteract increased load and instability.

NEW & NOTEWORTHY Bipedal locomotion imposes particular biomechanical constraints on motor control. In a within-species comparative study, we investigated joint kinematics and electromyographic characteristics of bipedal vs. quadrupedal treadmill locomotion in Japanese macaques. Because these features represent (to a large extent) emergent properties of the underlying neural control, they provide a comparative, behavioral, and neurophysiological framework for understanding the neural system dedicated to bipedal locomotion in this nonhuman primate, which constitutes a critical animal model for human bipedalism.

Properties of primary motor cortical local field potentials in the leg and trunk representations during arm movements

Large, spatially-distributed populations of motor cortical neurons are recruited during upper limb movements. Here, we examined how beta attenuation, a mesoscopic reflection of unit engagement, varies across a spatially expansive sampling of primary motor cortex in a non-human primate (macaca mulatta). We found that electrodes in both the trunk and leg representation of motor cortex exhibit qualitatively similar behavior to electrodes in the arm representation during a planar reaching task, despite the fact that there were no overt movements of the trunk or leg. These findings are interpreted in the context of a state-based brain machine interface.

Representation of individual forelimb muscles in primary motor cortex

Stimulus-triggered averaging (StTA) of forelimb muscle electromyographic (EMG) activity was used to investigate individual forelimb muscle representation within the primary motor cortex (M1) of rhesus macaques with the objective of determining the extent of intra-areal somatotopic organization. Two monkeys were trained to perform a reach-to-grasp task requiring multijoint coordination of the forelimb. EMG activity was simultaneously recorded from 24 forelimb muscles including 5 shoulder, 7 elbow, 5 wrist, 5 digit, and 2 intrinsic hand muscles. Microstimulation (15 µA at 15 Hz) was delivered throughout the movement task and individual stimuli were used as triggers for generating StTAs of EMG activity. StTAs were used to map the cortical representations of individual forelimb muscles. As reported previously (Park et al. 2001), cortical maps revealed a central core of distal muscle (wrist, digit, and intrinsic hand) representation surrounded by a horseshoe-shaped proximal (shoulder and elbow) muscle representation. In the present study, we found that shoulder and elbow flexor muscles were predominantly represented in the lateral branch of the horseshoe whereas extensors were predominantly represented in the medial branch. Distal muscles were represented within the core distal forelimb representation and showed extensive overlap. For the first time, we also show maps of inhibitory output from motor cortex, which follow many of the same organizational features as the maps of excitatory output.NEW & NOTEWORTHY While the orderly representation of major body parts along the precentral gyrus has been known for decades, questions have been raised about the possible existence of additional more detailed aspects of somatotopy. In this study, we have investigated this question with respect to muscles of the arm and show consistent features of within-arm (intra-areal) somatotopic organization. For the first time we also show maps of how inhibitory output from motor cortex is organized.

A 3D map of the hindlimb motor representation in the lumbar spinal cord in Sprague Dawley rats

OBJECTIVE

Spinal cord injury (SCI) is a devastating neurological trauma with a prevalence of about 282 000 people living with an SCI in the United States in 2016. Advances in neuromodulatory devices hold promise for restoring function by incorporating the delivery of electrical current directly into the spinal cord grey matter via intraspinal microstimulation (ISMS). In such designs, detailed topographic maps of spinal cord outputs are needed to determine ISMS locations for eliciting hindlimb movements. The primary goal of the present study was to derive a topographic map of functional motor outputs in the lumbar spinal cord to hindlimb skeletal muscles as defined by ISMS in a rat model.

APPROACH

Experiments were carried out in nine healthy, adult, male, Sprague Dawley rats. After a laminectomy of the T13-L1 vertebrae and removal of the dura mater, a four-shank, 16-channel microelectrode array was inserted along a 3D (200 µm) stimulation grid. Trains of three biphasic current pulses were used to determine evoked movements and electromyographic (EMG) activity. Via fine wire EMG electrodes, stimulus-triggered averaging (StTA) was used on rectified EMG data to determine response latency.

MAIN RESULTS

Hindlimb movements were elicited at a median current intensity of 6 µA, and thresholds were significantly lower in ventrolateral sites. Movements typically consisted of whole leg, hip, knee, ankle, toe, and trunk movements. Hip movements dominated rostral to the T13 vertebral segment, knee movements were evoked at the T13-L1 vertebral junction, while ankle and digit movements were found near the rostral L1 vertebra. Whole leg movements spanned the entire rostrocaudal region explored, while trunk movements dominated medially. StTAs of EMG activity demonstrated a latency of ~4 ms.

SIGNIFICANCE

The derived motor map provides insight into the parameters needed for future neuromodulatory devices.

Sensorimotor Integration by Corticospinal System

The corticospinal (CS) tract is a complex system which targets several areas of the spinal cord. In particular, the CS descending projection plays a major role in motor command, which results from direct and indirect control of spinal cord pre-motor interneurons as well as motoneurons. But in addition, this system is also involved in a selective and complex modulation of sensory feedback. Despite recent evidence confirms that CS projections drive distinct segmental neural circuits that are part of the sensory and pre-motor pathways, little is known about the spinal networks engaged by the corticospinal tract (CST), the organization of CS projections, the intracortical microcircuitry, and the synaptic interactions in the sensorimotor cortex (SMC) that may encode different cortical outputs to the spinal cord. Here is stressed the importance of integrated approaches for the study of sensorimotor function of CS system, in order to understand the functional compartmentalization and hierarchical organization of layer 5 output neurons, who are key elements for motor control and hence, of behavior.

Downregulating Aberrant Motor Evoked Potential Synergies of the Lower Extremity Post Stroke During TMS of the Contralesional Hemisphere

BACKGROUND

Growing evidence demonstrates unique synergistic signatures in the lower limb (LL) post-stroke, with specific across-plane and across-joint representations. While the inhibitory role of the ipsilateral hemisphere in the upper limb (UL) has been widely reported, examination of the contralesional hemisphere (CON-H) in modulating LL expressions of synergies following stroke is lacking.

OBJECTIVE

We hypothesize that stimulation of lesioned and contralesional motor cortices will differentially regulate paretic LL motor outflow. We propose a novel TMS paradigm to identify synergistic motor evoked potential (MEP) patterns across multiple muscles.

METHODS

Amplitude and background activation matched adductor MEPs were elicited using single pulse TMS of L-H and CON-H (control ipsilateral) during an adductor torque matching task from 11 stroke and 10 control participants. Associated MEPs of key synergistic muscles were simultaneously observed.

RESULTS

By quantifying CON-H/L-H MEP ratios, we characterized a significant targeted inhibition of aberrant MEP coupling between ADD and VM (p = 0.0078) and VL (p = 0.047) exclusive to the stroke group (p = 0.028) that was muscle dependent (p = 0.039). We find TA inhibition in both groups following ipsilateral hemisphere stimulation (p = 0.0014; p = 0.015).

CONCLUSION

We argue that ipsilaterally mediated attenuation of abnormal synergistic activations post stroke may reflect an adaptive intracortical inhibition. The predominance of sub 3ms interhemispheric MEP latency differences implicates LL ipsilateral corticomotor projections. These findings provide insight into the association between CON-H reorganization and post-stroke LL recovery. While a prevailing view of driving L-H disinhibition for UL recovery seems expedient, presuming analogous LL neuromodulation may require further examination for rehabilitation. This study provides a step toward this goal.

Cortical Effects on Ipsilateral Hindlimb Muscles Revealed with Stimulus-Triggered Averaging of EMG Activity

While a large body of evidence supports the view that ipsilateral motor cortex may make an important contribution to normal movements and to recovery of function following cortical injury (Chollet et al. 1991; Fisher 1992; Caramia et al. 2000; Feydy et al. 2002), relatively little is known about the properties of output from motor cortex to ipsilateral muscles. Our aim in this study was to characterize the organization of output effects on hindlimb muscles from ipsilateral motor cortex using stimulus-triggered averaging of EMG activity. Stimulus-triggered averages of EMG activity were computed from microstimuli applied at 60-120 μA to sites in both contralateral and ipsilateral M1 of macaque monkeys during the performance of a hindlimb push-pull task. Although the poststimulus effects (PStEs) from ipsilateral M1 were fewer in number and substantially weaker, clear and consistent effects were obtained at an intensity of 120 μA. The mean onset latency of ipsilateral poststimulus facilitation was longer than contralateral effects by an average of 0.7 ms. However, the shortest latency effects in ipsilateral muscles were as short as the shortest latency effects in the corresponding contralateral muscles suggesting a minimal synaptic linkage that is equally direct in both cases.