Hand Motor Recovery Following Extensive Frontoparietal Cortical Injury Is Accompanied by Upregulated Corticoreticular Projections in Monkey

Adaptation in the spinal cord after stroke: Implications for restoring cortical control over the final common pathway

Control of voluntary movement is predicated on integration between circuits in the brain and spinal cord. Although damage is often restricted to supraspinal or spinal circuits in cases of neurological injury, both spinal motor neurons and axons linking these cells to the cortical origins of descending motor commands begin showing changes soon after the brain is injured by stroke. The concept of 'transneuronal degeneration' is not new and has been documented in histological, imaging and electrophysiological studies dating back over a century. Taken together, evidence from these studies agrees more with a system attempting to survive rather than one passively surrendering to degeneration. There tends to be at least some preservation of fibres at the brainstem origin and along the spinal course of the descending white matter tracts, even in severe cases. Myelin-associated proteins are observed in the spinal cord years after stroke onset. Spinal motor neurons remain morphometrically unaltered. Skeletal muscle fibres once innervated by neurons that lose their source of trophic input receive collaterals from adjacent neurons, causing spinal motor units to consolidate and increase in size. Although some level of excitability within the distributed brain network mediating voluntary movement is needed to facilitate recovery, minimal structural connectivity between cortical and spinal motor neurons can support meaningful distal limb function. Restoring access to the final common pathway via the descending input that remains in the spinal cord therefore represents a viable target for directed plasticity, particularly in light of recent advances in rehabilitation medicine.

Targeting thalamocortical circuits for closed-loop stimulation in Lennox-Gastaut syndrome

This paper outlines the therapeutic rationale and neurosurgical targeting technique for bilateral, closed-loop, thalamocortical stimulation in Lennox-Gastaut syndrome, a severe form of childhood-onset epilepsy. Thalamic stimulation can be an effective treatment for Lennox-Gastaut syndrome, but complete seizure control is rarely achieved. Outcomes may be improved by stimulating areas beyond the thalamus, including cortex, but the optimal targets are unknown. We aimed to identify a cortical target by synthesizing prior neuroimaging studies, and to use this knowledge to advance a dual thalamic (centromedian) and cortical (frontal) approach for closed-loop stimulation. Multi-modal brain network maps from three group-level studies of Lennox-Gastaut syndrome were averaged to define the area of peak overlap: simultaneous EEG-functional MRI of generalized paroxysmal fast activity, [18F]fluorodeoxyglucose PET of cortical hypometabolism and diffusion MRI structural connectivity associated with clinical efficacy in a previous trial of thalamic deep brain stimulation. The resulting 'hotspot' was used as a seed in a normative functional MRI connectivity analysis to identify connected networks. Intracranial electrophysiology was reviewed in the first two trial patients undergoing bilateral implantations guided by this hotspot. Simultaneous recordings from cortex and thalamus were analysed for presence and synchrony of epileptiform activity. The peak overlap was in bilateral premotor cortex/caudal middle frontal gyrus. Functional connectivity of this hotspot revealed a distributed network of frontoparietal cortex resembling the diffuse abnormalities seen on EEG-functional MRI and PET. Intracranial electrophysiology showed characteristic epileptiform activity of Lennox-Gastaut syndrome in both the cortical hotspot and thalamus; most detected events occurred first in the cortex before appearing in the thalamus. Premotor frontal cortex shows peak involvement in Lennox-Gastaut syndrome and functional connectivity of this region resembles the wider epileptic brain network. Thus, it may be an optimal target for a range of neuromodulation therapies, including thalamocortical stimulation and emerging non-invasive treatments like focused ultrasound or transcranial magnetic stimulation. Compared to thalamus-only approaches, the addition of this cortical target may allow more rapid detections of seizures, more diverse stimulation paradigms and broader modulation of the epileptic network. A prospective, multi-centre trial of closed-loop thalamocortical stimulation for Lennox-Gastaut syndrome is currently underway.

Corticospinal and corticoreticulospinal projections benefit motor behaviors in chronic stroke

After corticospinal tract (CST) stroke, several motor deficits in the upper extremity (UE) emerge, including diminished muscle strength, motor control, and muscle individuation. Both the ipsilesional CST and contralesional corticoreticulospinal tract (CReST) innervate the paretic UE and may have different innervation patterns for the proximal and distal UE segments. These patterns may underpin distinct pathway relationships to separable motor behaviors. In this cross-sectional study of 15 chronic stroke patients and 28 healthy subjects, we examined two key questions: (1) whether segmental motor behaviors differentially relate to ipsilesional CST and contralesional CReST projection strengths, and (2) whether motor behaviors segmentally differ in the paretic UE. We measured strength, motor control, and muscle individuation in a proximal (biceps, BIC) and distal muscle (first dorsal interosseous, FDI) of the paretic UE. We measured the projection strengths of the ipsilesional CST and contralesional CReST to these muscles using transcranial magnetic stimulation (TMS). Stroke subjects had abnormal motor control and muscle individuation despite strength comparable to healthy subjects. In stroke subjects, stronger ipsilesional CST projections were linked to superior motor control in both UE segments, whereas stronger contralesional CReST projections were linked to superior muscle strength and individuation in both UE segments. Notably, both pathways also shared associations with behaviors in the proximal segment. Motor control deficits were segmentally comparable, but muscle individuation was worse for distal motor performance. These results suggest that each pathway has specialized contributions to chronic motor behaviors but also work together, with varying levels of success in supporting chronic deficits.

Key points summary

Individuals with chronic stroke typically have deficits in strength, motor control, and muscle individuation in their paretic upper extremity (UE). It remains unclear how these altered behaviors relate to descending motor pathways and whether they differ by proximal and distal UE segment.In this study, we used transcranial magnetic stimulation (TMS) to examine projection strengths of the ipsilesional corticospinal tract (CST) and contralesional corticoreticulospinal tract (CReST) with respect to quantitated motor behaviors in chronic stroke.We found that stronger ipsilesional CST projections were associated with better motor control in both UE segments, whereas stronger contralesional CReST projections were associated with better strength and individuation in both UE segments. In addition, projections of both pathways shared associations with motor behaviors in the proximal UE segment.We also found that deficits in strength and motor control were comparable across UE segments, but muscle individuation was worse with controlled movement in the distal UE segment.These results suggest that the CST and CReST have specialized contributions to chronic motor behaviors and also work together, although with different degrees of efficacy.

The Corticospinal System and Amyotrophic Lateral Sclerosis: IFCN handbook chapter

Corticospinal neurons located in motor areas of the cerebral neocortex project corticospinal axons which synapse with the spinal network; a parallel corticobulbar system projects to the cranial motor network and to brainstem motor pathways. The primate corticospinal system has a widespread cortical origin and an extensive range of different fibre diameters, including thick, fast-conducting axons. Direct cortico-motoneuronal (CM) projections from the motor cortex to arm and hand alpha motoneurons are a recent evolutionary feature, that is well developed in dexterous primates and particularly in humans. Many of these projections originate from the caudal subdivision of area 4 ('new' M1: primary motor cortex). They arise from corticospinal neurons of varied soma size, including those with fast- and relatively slow-conducting axons. This CM system has been shown to be involved in the control of skilled movements, carried out with fractionation of the distal extremities and at low force levels. During movement, corticospinal neurons are activated quite differently from 'lower' motoneurons, and there is no simple or fixed functional relationship between a so-called 'upper' motoneuron and its target lower motoneuron. There are key differences in the organisation and function of the corticospinal and CM system in primates versus non-primates, such as rodents. These differences need to be recognized when making the choice of animal model for understanding disorders such as amyotrophic lateral sclerosis (ALS). In this neurodegenerative brain disease there is a selective loss of fast-conducting corticospinal axons, and their synaptic connections, and this is reflected in responses to non-invasive cortical stimuli and measures of cortico-muscular coherence. The loss of CM connections influencing distal limb muscles results in a differential loss of muscle strength or 'split-hand' phenotype. Importantly, there is also a unique impairment in the coordination of skilled hand tasks that require fractionation of digit movement. Scores on validated tests of skilled hand function could be used to assess disease progression.

Moving toward elucidating alternative motor pathway structures post-stroke: the value of spinal cord neuroimaging

Stroke results in varying levels of motor and sensory disability that have been linked to the neurodegeneration and neuroinflammation that occur in the infarct and peri-infarct regions within the brain. Specifically, previous research has identified a key role of the corticospinal tract in motor dysfunction and motor recovery post-stroke. Of note, neuroimaging studies have utilized magnetic resonance imaging (MRI) of the brain to describe the timeline of neurodegeneration of the corticospinal tract in tandem with motor function following a stroke. However, research has suggested that alternate motor pathways may also underlie disease progression and the degree of functional recovery post-stroke. Here, we assert that expanding neuroimaging techniques beyond the brain could expand our knowledge of alternate motor pathway structure post-stroke. In the present work, we will highlight findings that suggest that alternate motor pathways contribute to post-stroke motor dysfunction and recovery, such as the reticulospinal and rubrospinal tract. Then we review imaging and electrophysiological techniques that evaluate alternate motor pathways in populations of stroke and other neurodegenerative disorders. We will then outline and describe spinal cord neuroimaging techniques being used in other neurodegenerative disorders that may provide insight into alternate motor pathways post-stroke.

Adaptation of the layer V supraspinal motor corticofugal projections from the primary (M1) and premotor (PM) cortices after CNS motor disorders in non-human primates: A survey

Motor commands are transmitted from the motor cortical areas to effectors mostly via the corticospinal (CS) projection. Several subcortical motor nuclei also play an important role in motor control, the subthalamic nucleus, the red nucleus, the reticular nucleus and the superior colliculus. These nuclei are influenced by motor cortical areas via respective corticofugal projections, which undergo complex adaptations after motor trauma (spinal cord/motor cortex injury) or motor disease (Parkinson), both in the absence or presence of putative treatments, as observed in adult macaque monkeys. A dominant effect was a nearly complete suppression of the corticorubral projection density and a strong downregulation of the corticoreticular projection density, with the noticeable exception in the latter case of a considerable increase of projection density following spinal cord injury, even enhanced when an anti-NogoA antibody treatment was administered. The effects were diverse and less prominent on the corticotectal and corticosubthalamic projections. The CS projection may still be the major efferent pathway through which motor adaptations can take place after motor trauma or disease. However, the parallel supraspinal motor corticofugal projections may also participate in connectional adaptations supporting the functional recovery of motor abilities, representing potential targets for future clinical strategies, such as selective electrical neurostimulations.

Motor unit firing rate modulation is more impaired during flexion synergy-driven contractions of the biceps brachii in chronic stroke

Following a hemiparetic stroke, individuals exhibit altered motor unit firing patterns during voluntary muscle contractions, including impairments in firing rate modulation and recruitment. These individuals also exhibit abnormal muscle coactivation through multi-joint synergies (e.g., flexion synergy). Here, we investigate whether motor unit firing activity during flexion synergy-driven contractions of the paretic biceps brachii differs from that of voluntary contractions and use these differences to predict changes in descending motor commands. To accomplish this, we characterized motor unit firing patterns of the biceps brachii in individuals with chronic hemiparetic stroke during voluntary isometric elbow flexion contractions in the paretic and non-paretic limbs, as well as during contractions driven by voluntary effort and by flexion synergy expression in the paretic limb. We observed significant reductions in motor unit firing rate modulation from the non-paretic to paretic limb (non-paretic - paretic: 0.14 pps/%MVT, 95% CI: [0.09 0.19]) that were further reduced during synergy-driven contractions (voluntary paretic - synergy driven: 0.19 pps/%MVT, 95% CI: [0.14 0.25]). Moreover, using recently developed metrics, we evaluated how a stroke-induced reliance on indirect motor pathways alters the inputs that motor units receive and revealed progressive increases in neuromodulatory and inhibitory drive to the motor pool in the paretic limb, with the changes greatest during synergy-driven contractions. These findings suggest that an interplay between heightened neuromodulatory drive and alterations in inhibitory command structure may account for the observed motor unit impairments, further illuminating underlying neural mechanisms involved in the flexion synergy and its impact on motor unit firing patterns post-stroke.

Express Visuomotor Responses Reflect Knowledge of Both Target Locations and Contextual Rules during Reaches of Different Amplitudes

When humans reach to visual targets, extremely rapid (∼90 ms) target-directed responses can be observed in task-relevant proximal muscles. Such express visuomotor responses are inflexibly locked in time and space to the target and have been proposed to reflect rapid visuomotor transformations conveyed subcortically via the tecto-reticulo-spinal pathway. Previously, we showed that express visuomotor responses are sensitive to explicit cue-driven information about the target, suggesting that the express pathway can be modulated by cortical signals affording contextual prestimulus expectations. Here, we show that the express visuomotor system incorporates information about the physical hand-to-target distance and contextual rules during visuospatial tasks requiring different movement amplitudes. In one experiment, we recorded the activity from two shoulder muscles as 14 participants (6 females) reached toward targets that appeared at different distances from the reaching hand. Increasing the reaching distance facilitated the generation of frequent and large express visuomotor responses. This suggests that both the direction and amplitude of veridical hand-to-target reaches are encoded along the putative subcortical express pathway. In a second experiment, we modulated the movement amplitude by asking 12 participants (4 females) to deliberately undershoot, overshoot, or stop (control) at the target. The overshoot and undershoot tasks impaired the generation of large and frequent express visuomotor responses, consistent with the inability of the express pathway to generate responses directed toward nonveridical targets as in the anti-reach task. Our findings appear to reflect strategic, cortically driven modulation of the express visuomotor circuit to facilitate rapid and effective response initiation during target-directed actions.SIGNIFICANCE STATEMENT Express (∼90 ms) arm muscle responses that are consistently tuned toward the location of visual stimuli suggest a subcortical contribution to target-directed visuomotor behavior in humans, potentially via the tecto-reticulo-spinal pathway. Here, we show that express muscle responses are modulated appropriately to reach targets at different distances, but generally suppressed when the task required nonveridical responses to overshoot/undershoot the real target. This suggests that the tecto-reticulo-spinal pathway can be exploited strategically by the cerebral cortex to facilitate rapid initiation of effective responses during a visuospatial task.

Identifying the role of the reticulospinal tract for strength and motor recovery: A scoping review of nonhuman and human studies

In addition to the established postural control role of the reticulospinal tract (RST), there has been an increasing interest on its involvement in strength, motor recovery, and other gross motor functions. However, there are no reviews that have systematically assessed the overall motor function of the RST. Therefore, we aimed to determine the role of the RST underpinning motor function and recovery. We performed a literature search using Ovid Medline, Embase, CINAHL Plus, and Scopus to retrieve papers using key words for RST, strength, and motor recovery. Human and animal studies which assessed the role of RST were included. Studies were screened and 32 eligible studies were included for the final analysis. Of these, 21 of them were human studies while the remaining were on monkeys and rats. Seven experimental animal studies and four human studies provided evidence for the involvement of the RST in motor recovery, while two experimental animal studies and eight human studies provided evidence for strength gain. The RST influenced gross motor function in two experimental animal studies and five human studies. Overall, the RST has an important role for motor recovery, gross motor function and at least in part, underpins strength gain. The role of RST for strength gain in healthy people and its involvement in spasticity in a clinical population has been limitedly described. Further studies are required to ascertain the role of the RST's role in enhancing strength and its contribution to the development of spasticity.

Finger movement functions remain in the ipsilesional hemisphere and compensation by the contralesional hemisphere might not be expected after hemispherotomy -pre- and post-hemispherotomy evaluations in 8 cases

BACKGROUND

We hypothesized that fine finger motor functions are controlled by the ipsilesional hemisphere, and that gross motor functions are compensated for by the contralesional hemisphere after brain injury in humans. The purpose of this study was to compare finger movements before and after hemispherotomy that defunctionated the ipsilesional hemisphere for patients with hemispherical lesions.

METHODS

We statistically compared Brunnstrom stage of the fingers, arm (upper extremity), and leg (lower extremity) before and after hemispherotomy. Inclusion criteria for this study were: 1) hemispherotomy for hemispherical epilepsy; 2) a ≥ 6-month history of hemiparesis; 3) post-operative follow-up ≥ 6 months; 4) complete freedom from seizures without aura; and 5) application of our protocol for hemispherotomy.

RESULTS

Among 36 patients who underwent multi-lobe disconnection surgeries, 8 patients (2 girls, 6 boys) met the study criteria. Mean age at surgery was 6.38 years (range, 2-12 years; median, 6 years; standard deviation, 3.5 years). Paresis of the fingers was significantly exacerbated (p = 0.011) compared to pre-operatively, whereas that of the upper limbs (p = 0.07) and lower limbs (p = 0.103) was not.

CONCLUSION

Finger movement functions tend to remain in the ipsilesional hemisphere after brain injury, whereas gross motor movement functions such as those of the arms and legs are compensated for by the contralesional hemisphere in humans.

Pairing Transcranial Magnetic Stimulation and Loud Sounds Produces Plastic Changes in Motor Output

Most current methods for neuromodulation target the cortex. Approaches for inducing plasticity in subcortical motor pathways, such as the reticulospinal tract, could help to boost recovery after damage (e.g., stroke). In this study, we paired loud acoustic stimulation (LAS) with transcranial magnetic stimulation (TMS) over the motor cortex in male and female healthy humans. LAS activates the reticular formation; TMS activates descending systems, including corticoreticular fibers. Two hundred paired stimuli were used, with 50 ms interstimulus interval at which LAS suppresses TMS responses. Before and after stimulus pairing, responses in the contralateral biceps muscle to TMS alone were measured. Ten, 20, and 30 min after stimulus pairing ended, TMS responses were enhanced, indicating the induction of LTP. No long-term changes were seen in control experiments which used 200 unpaired TMS or LAS, indicating the importance of associative stimulation. Following paired stimulation, no changes were seen in responses to direct corticospinal stimulation at the level of the medulla, or in the extent of reaction time shortening by a loud sound (StartReact effect), suggesting that plasticity did not occur in corticospinal or reticulospinal synapses. Direct measurements in female monkeys undergoing a similar paired protocol revealed no enhancement of corticospinal volleys after paired stimulation, suggesting no changes occurred in intracortical connections. The most likely substrate for the plastic changes, consistent with all our measurements, is an increase in the efficacy of corticoreticular connections. This new protocol may find utility, as it seems to target different motor circuits compared with other available paradigms.SIGNIFICANCE STATEMENT Induction of plasticity by neurostimulation protocols may be promising to enhance functional recovery after damage such as following stroke, but current protocols mainly target cortical circuits. In this study, we developed a novel paradigm which may generate long-term changes in connections between cortex and brainstem. This could provide an additional tool to modulate and improve recovery.

Reorganization in the macaque interoceptive-allostatic network following anterior cingulate cortex damage

Accumulating evidence indicates that the adult brain is capable of significant structural change following damage-a capacity once thought to be largely limited to developing brains. To date, most existing research on adult plasticity has focused on how exteroceptive sensorimotor networks compensate for damage to preserve function. Interoceptive networks-those that represent and process sensory information about the body's internal state-are now recognized to be critical for a wide range of physiological and psychological functions from basic energy regulation to maintaining a sense of self, but the extent to which these networks remain plastic in adulthood has not been established. In this report, we used detailed histological analyses to pinpoint precise changes to gray matter volume in the interoceptive-allostatic network in adult rhesus monkeys (Macaca mulatta) who received neurotoxic lesions of the anterior cingulate cortex (ACC) and neurologically intact control monkeys. Relative to controls, monkeys with ACC lesions had significant and selective unilateral expansion of the ventral anterior insula and significant relative bilateral expansion of the lateral nucleus of the amygdala. This work demonstrates the capacity for neuroplasticity in the interoceptive-allostatic network which, given that changes included expansion rather than atrophy, is likely to represent an adaptive response following damage.

Loss of Motor Cortical Inputs to the Red Nucleus after CNS Disorders in Nonhuman Primates

The premotor (PM) and primary motor (M1) cortical areas broadcast voluntary motor commands through multiple neuronal pathways, including the corticorubral projection that reaches the red nucleus (RN). However, the respective contribution of M1 and PM to corticorubral projections as well as changes induced by motor disorders or injuries are not known in nonhuman primates. Here, we quantified the density and topography of axonal endings of the corticorubral pathway in RN in intact monkeys, as well as in monkeys subjected to either cervical spinal cord injury (SCI), Parkinson's disease (PD)-like symptoms or primary motor cortex injury (MCI). Twenty adult macaque monkeys of either sex were injected with the biotinylated dextran amine anterograde tracer either in PM or in M1. We developed a semiautomated algorithm to reliably detect and count axonal boutons within the magnocellular and parvocellular (pRN) subdivisions of RN. In intact monkeys, PM and M1 preferentially target the medial part of the ipsilateral pRN, reflecting its somatotopic organization. Projection of PM to the ipsilateral pRN is denser than that of M1, matching previous observations for the corticotectal, corticoreticular, and corticosubthalamic projections (Fregosi et al., 2018, 2019; Borgognon et al., 2020). In all three types of motor disorders, there was a uniform and strong decrease (near loss) of the corticorubral projections from PM and M1. The RN may contribute to functional recovery after SCI, PD, and MCI, by reducing direct cortical influence. This reduction possibly privileges direct access to the final output motor system, via emphasis on the direct corticospinal projection.SIGNIFICANCE STATEMENT We measured the corticorubral projection density arising from the PM or the M1 cortices in adult macaques. The premotor cortex sent denser corticorubral projections than the primary motor cortex, as previously observed for the corticotectal, corticoreticular, and corticosubthalamic projections. The premotor cortex may thus exert more influence than primary motor cortex onto subcortical structures. We next asked whether the corticorubral motor projections undergo lesion-dependent plasticity after either cervical spinal cord injury, Parkinson's disease-like symptoms, or primary motor cortex lesion. In all three types of pathology, there was a strong decrease of the corticorubral motor projection density, suggesting that the red nucleus may contribute to functional recovery after such motor system disorders based on a reduced direct cortical influence.

The brainstem connections of the supplementary motor area and its relations to the corticospinal tract: Experimental rat and human 3-tesla tractography study

Although the supplementary motor area (SMA) is a large region on the medial surface of the frontal lobe of the brain, little is known about its function. The current study uses 3-tesla high-resolution diffusion tensor tractography (DTI) in healthy individuals and biotinylated dextran amine (BDA) and fluoro-gold (FG) tracer in rats to demonstrate the afferent and efferent connections of the SMA with brainstem structures. It also aims to clarify how SMA fibers relate to the corticospinal tract (CST). The BDA (n = 6) and FG (n = 8) tracers were pressure-injected into the SMA of 14 Wistar albino rats. Light and fluorescence microscopy was used to capture images of the FG and BDA-labeled cells and axons. High-resolution 3-tesla DTI data were acquired from the Human Connectome Project database. Tracts between the SMA and brainstem structures were analyzed using diffusion spectrum imaging (DSI) studio software. The FG injections into the SMA showed afferent projections from mesencephalic (periaqueductal gray matter, substantia nigra pars reticulata, ventral tegmental area, inferior colliculus, mesencephalic reticular, tegmental, and raphe nuclei), pontine (locus coeruleus, pontine reticular and vestibular nuclei), and medullary (area postrema, parabrachial, and medullary reticular nuclei) structures. The anterograde tracer BDA injections into the SMA showed efferent connections with mesencephalic (periaqueductal gray, substantia nigra pars compacta, dorsal raphe, trigeminal motor mesencephalic, and mesencephalic reticular nuclei), pontine (locus coeruleus, nucleus of the lateral lemniscus, vestibular, cochlear, and pontine reticular nuclei), and medullary (area postrema, medullary reticular, olivary, and parabrachial nuclei) structures. The SMA had efferent but no afferent connections with the cerebellar nuclei. The DTI results in healthy human subjects highly corresponded with the experimental results. Further, the DTI results showed a distinct bundle that descended to spinal levels closely related to the CST. Understanding SMA's afferent and efferent connections will enrich our knowledge of its contribution to various brainstem networks and may provide new perspectives for understanding its motor and non-motor functions.

Gait control by the frontal lobe

The frontal lobe is crucial and contributes to controlling truncal motion, postural responses, and maintaining equilibrium and locomotion. The rich repertoire of frontal gait disorders gives some indication of this complexity. For human walking, it is necessary to simultaneously achieve at least two tasks, such as maintaining a bipedal upright posture and locomotion. Particularly, postural control plays an extremely significant role in enabling the subject to maintain stable gait behaviors to adapt to the environment. To achieve these requirements, the frontal cortex (1) uses cognitive information from the parietal, temporal, and occipital cortices, (2) creates plans and programs of gait behaviors, and (3) acts on the brainstem and spinal cord, where the core posture-gait mechanisms exist. Moreover, the frontal cortex enables one to achieve a variety of gait patterns in response to environmental changes by switching gait patterns from automatic routine to intentionally controlled and learning the new paradigms of gait strategy via networks with the basal ganglia, cerebellum, and limbic structures. This chapter discusses the role of each area of the frontal cortex in behavioral control and attempts to explain how frontal lobe controls walking with special reference to postural control.

Phylogenetic view of the compensatory mechanisms in motor and sensory systems after neuronal injury

Through phylogeny, novel neural circuits are added on top of ancient circuits. Upon injury of a novel circuit which enabled fine control, the ancient circuits can sometimes take over its function for recovery; however, the recovered function is limited according to the capacity of the ancient circuits. In this review, we discuss two examples of functional recovery after neural injury in nonhuman primate models. The first is the recovery of dexterous hand movements following damage to the corticospinal tract. The second is the recovery of visual function after injury to the primary visual cortex (V1). In the former case, the functions of the direct cortico-motoneuronal pathway, which specifically developed in higher primates for the control of fractionated digit movements, can be partly compensated for by other descending motor pathways mediated by rubrospinal, reticulospinal, and propriospinal neurons. However, the extent of recovery depends on the location of the damage and which motor systems take over its function. In the latter case, after damage to V1, which is highly developed in primates, either the direct pathway from the lateral geniculate nucleus to extrastriate visual cortices or that from the midbrain superior colliculus-pulvinar-extrastriate/parietal cortices partly takes over the function of V1. However, the state of visual awareness is no longer the same as in the intact state, which might reflect the limited capacity of the compensatory pathways in visual recognition. Such information is valuable for determining the targets of neuromodulatory therapies and setting treatment goals after brain and spinal cord injuries.

The Existence of the StartReact Effect Implies Reticulospinal, Not Corticospinal, Inputs Dominate Drive to Motoneurons during Voluntary Movement

Reaction time is accelerated if a loud (startling) sound accompanies the cue-the "StartReact" effect. Animal studies revealed a reticulospinal substrate for the startle reflex; StartReact may similarly involve the reticulospinal tract, but this is currently uncertain. Here we trained two female macaque monkeys to perform elbow flexion/extension movements following a visual cue. The cue was sometimes accompanied by a loud sound, generating a StartReact effect in electromyogram response latency, as seen in humans. Extracellular recordings were made from antidromically identified corticospinal neurons in primary motor cortex (M1), from the reticular formation (RF), and from the spinal cord (SC; C5-C8 segments). After loud sound, task-related activity was suppressed in M1 (latency, 70-200 ms after cue), but was initially enhanced (70-80 ms) and then suppressed (140-210 ms) in RF. SC activity was unchanged. In a computational model, we simulated a motoneuron pool receiving input from different proportions of the average M1 and RF activity recorded experimentally. Motoneuron firing generated simulated electromyogram, allowing reaction time measurements. Only if ≥60% of motoneuron drive came from RF (≤40% from M1) did loud sound shorten reaction time. The extent of shortening increased as more drive came from RF. If RF provided <60% of drive, loud sound lengthened the reaction time-the opposite of experimental findings. The majority of the drive for voluntary movements is thus likely to originate from the brainstem, not the cortex; changes in the magnitude of the StartReact effect can measure a shift in the relative importance of descending systems.SIGNIFICANCE STATEMENT Our results reveal that a loud sound has opposite effects on neural spiking in corticospinal cells from primary motor cortex, and in the reticular formation. We show that this fortuitously allows changes in reaction time produced by a loud sound to be used to assess the relative importance of reticulospinal versus corticospinal control of movement, validating previous noninvasive measurements in humans. Our findings suggest that the majority of the descending drive to motoneurons producing voluntary movement in primates comes from the reticulospinal tract, not the corticospinal tract.

Modulation of neural co-firing to enhance network transmission and improve motor function after stroke

Stroke is a leading cause of disability. While neurotechnology has shown promise for improving upper limb recovery after stroke, efficacy in clinical trials has been variable. Our central thesis is that to improve clinical translation, we need to develop a common neurophysiological framework for understanding how neurotechnology alters network activity. Our perspective discusses principles for how motor networks, both healthy and those recovering from stroke, subserve reach-to-grasp movements. We focus on neural processing at the resolution of single movements, the timescale at which neurotechnologies are applied, and discuss how this activity might drive long-term plasticity. We propose that future studies should focus on cross-area communication and bridging our understanding of timescales ranging from single trials within a session to across multiple sessions. We hope that this perspective establishes a combined path forward for preclinical and clinical research with the goal of more robust clinical translation of neurotechnology.

Symbolic cues enhance express visuomotor responses in human arm muscles at the motor planning rather than the visuospatial processing stage

Humans can produce "express" (~100ms) arm muscle responses that are inflexibly locked in time and space to visual target presentations, consistent with subcortical visuomotor transformations via the tecto-reticulo-spinal pathway. These express visuomotor responses are sensitive to explicit cue-driven expectations, but it is unclear at what stage of sensory-to-motor transformation such modulation occurs. Here, we recorded electromyographic activity from shoulder muscles as participants reached toward one of four virtual targets whose physical location was partially predictable from a symbolic cue. In an experiment in which targets could be veridically reached, express responses were inclusive of the biomechanical requirements for reaching the cued locations and not systematically modulated by cue validity. In a second experiment, movements were restricted to the horizontal plane so that the participants could perform only rightward or leftward reaches, irrespective of target position on the vertical axis. Express muscle responses were almost identical for targets that were validly cued in the horizontal direction, regardless of cue validity in the vertical dimension. Together, these findings suggest that the cue-induced enhancements of express responses are dominated by effects at the level of motor plans and not solely via facilitation of early visuospatial target processing. Notably, direct cortico-tectal and cortico-reticular projections exist that are well-placed to modulate pre-stimulus motor preparation state in subcortical circuits. Our results could reflect a neural mechanism by which contextually relevant motor responses to compatible visual inputs are rapidly released via subcortical circuits that are sufficiently along the sensory- to-motor continuum.

Startling stimuli increase maximal motor unit discharge rate and rate of force development in humans

Maximal rate of force development in adult humans is determined by the maximal motor unit discharge rate, however the origin of the underlying synaptic inputs remains unclear. Here, we tested a hypothesis that the maximal motor unit discharge rate will increase in response to a startling cue, a stimulus that purportedly activates the pontomedullary reticular formation neurons that make mono- and disynaptic connections to motoneurons via fast-conducting axons. Twenty-two men were required to produce isometric knee extensor forces "as fast and as hard" as possible from rest to 75% of maximal voluntary force, in response to visual (VC), visual-auditory (VAC; 80 dB), or visual-startling cue (VSC; 110 dB). Motoneuron activity was estimated via decomposition of high-density surface electromyogram recordings over the vastus lateralis and medialis muscles. Reaction time was significantly shorter in response to VSC compared to VAC and VC. The VSC further elicited faster neuromechanical responses including a greater number of discharges per motor unit per second and greater maximal rate of force development, with no differences between VAC and VC. We provide evidence, for the first time, that the synaptic input to motoneurons increases in response to a startling cue, suggesting a contribution of subcortical pathways to maximal motoneuron output in humans.

Muscle synergy analysis yields an efficient and physiologically relevant method of assessing stroke

The Fugl-Meyer Assessment is widely used to test motor function in stroke survivors. In the Fugl-Meyer Assessment, stroke survivors perform several movement tasks and clinicians subjectively rate the performance of each task item. The individual task items in the Fugl-Meyer Assessment are selected on the basis of clinical experience, and their physiological relevance has not yet been evaluated. In the present study, we aimed to objectively rate the performance of task items by measuring the muscle activity of 41 muscles from the upper body while stroke survivors and healthy participants performed 37 Fugl-Meyer Assessment upper extremity task items. We used muscle synergy analysis to compare muscle activity between subjects and found that 13 muscle synergies in the healthy participants (which we defined as standard synergies) were able to reconstruct all of the muscle activity in the Fugl-Meyer Assessment. Among the standard synergies, synergies involving the upper arms, forearms and fingers were activated to varying degrees during different task items. In contrast, synergies involving posterior trunk muscles were activated during all tasks, which suggests the importance of posterior trunk muscle synergies throughout all sequences. Furthermore, we noted the inactivation of posterior trunk muscle synergies in stroke survivors with severe but not mild impairments, suggesting that lower trunk stability and the underlying activity of posterior trunk muscle synergies may have a strong influence on stroke severity and recovery. By comparing the synergies of stroke survivors with standard synergies, we also revealed that some synergies in stroke survivors corresponded to merged standard synergies; the merging rate increased with the impairment of stroke survivors. Moreover, the degrees of severity-dependent changes in the merging rate (the merging rate–severity relationship) were different among different task items. This relationship was significant for 26 task items only and not for the other 11 task items. Because muscle synergy analysis evaluates coordinated muscle activities, this different dependency suggests that these 26 task items are appropriate for evaluating muscle coordination and the extent of its impairment in stroke survivors. Overall, we conclude that the Fugl-Meyer Assessment reflects physiological function and muscle coordination impairment and suggest that it could be performed using a subset of the 37 task items.

Lack of somatotopy among corticospinal tract fibers passing through the primate craniovertebral junction and cervical spinal cord: pathoanatomical substrate of central cord syndrome and cruciate paralysis

OBJECTIVE

In some cases of incomplete cervical spinal cord injury (iSCI) there is marked paresis and dysfunction of upper-extremity movement but not lower-extremity movement. A continued explanation of such symptoms is a somatotopic organization of corticospinal tract (CST) fibers passing through the decussation at the craniovertebral junction (CVJ) and lateral CST (LCST). In central cord syndrome, it has been suggested that injury to the core of the cervical cord may include selective damage to medially located arm/hand LCST fibers, without compromising laterally located leg fibers. Because such somatotopic organization in the primate CST might contribute to the disproportionate motor deficits after some forms of iSCI, the authors made a systematic investigation of CST organization in the CVJ and LCST using modern neuroanatomical techniques.

METHODS

High-resolution anterograde tracers were used in 11 rhesus macaque monkeys to define the course of the corticospinal projection (CSP) through the CVJ and LCST from the arm/hand, shoulder, and leg areas of the primary motor cortex (M1). This approach labels CST fibers of all sizes, large and small, arising in these areas. The CSP from the dorsolateral and ventrolateral premotor cortex and supplementary motor area were also studied. A stereological approach was adapted to quantify labeled fiber distribution in 8 cases.

RESULTS

There was no evidence for somatotopic organization of CST fibers passing through the CVJ or contralateral LCST. Fiber labeling from each cortical representation was widespread throughout the CST at the CVJ and LCST and overlapped extensively with fibers from other representations. This study demonstrated no significant difference between medial versus lateral subsectors of the LCST in terms of number of fibers labeled from the M1 arm/hand area.

CONCLUSIONS

This investigation firmly rejects the concept of somatotopy among CST fibers passing through the CVJ and LCST, in contrast with the somatotopy in the cortex, corona radiata, and internal capsule. All CST fibers in the CVJ and LCST would thus appear to be equally susceptible to focal or diffuse injury, regardless of their cortical origin. The disproportionate impairment of arm/hand movement after iSCI must therefore be due to other factors, including greater dependence of hand/arm movements on the CST compared with the lower limb. The dispersed and intermingled nature of frontomotor fibers may be important in motor recovery after cervical iSCI.

Mapping the human corticoreticular pathway with multimodal delineation of the gigantocellular reticular nucleus and high-resolution diffusion tractography

The corticoreticular pathway (CRP) is a major motor tract that transmits cortical input to the reticular formation motor nuclei and may be an important mediator of motor recovery after central nervous system damage. However, its cortical origins, trajectory and laterality are incompletely understood in humans. This study aimed to map the human CRP and generate an average CRP template in standard MRI space. Following recently established guidelines, we manually delineated the primary reticular formation motor nucleus (gigantocellular reticular nucleus [GRN]) using several group-mean MRI contrasts from the Human Connectome Project (HCP). CRP tractography was then performed with HCP diffusion-weighted MRI data (N = 1065) by selecting diffusion streamlines that reached both the cortex and GRN. Corticospinal tract (CST) tractography was also performed for comparison. Results suggest that the human CRP has widespread origins, which overlap with the CST across most of the motor cortex and include additional exclusive inputs from the medial and anterior prefrontal cortices. The estimated CRP projected through the anterior and posterior limbs of the internal capsule before partially decussating in the midbrain tegmentum and converging bilaterally on the pontomedullary reticular formation. Thus, the CRP trajectory appears to partially overlap the CST, while being more distributed and anteromedial to the CST in the cerebrum before moving posterior to the CST in the brainstem. These findings have important implications for neurophysiologic testing, cortical stimulation and movement recovery after brain lesions. We expect that our GRN and tract maps will also facilitate future CRP research.

The Cortical Motor System in the Domestic Pig: Origin and Termination of the Corticospinal Tract and Cortico-Brainstem Projections

The anatomy of the cortical motor system and its relationship to motor repertoire in artiodactyls is for the most part unknown. We studied the origin and termination of the corticospinal tract (CST) and cortico-brainstem projections in domestic pigs. Pyramidal neurons were retrogradely labeled by injecting aminostilbamidine in the spinal segment C1. After identifying the dual origin of the porcine CST in the primary motor cortex (M1) and premotor cortex (PM), the axons descending from those regions to the spinal cord and brainstem were anterogradely labeled by unilateral injections of dextran alexa-594 in M1 and dextran alexa-488 in PM. Numerous corticospinal projections from M1 and PM were detected up to T6 spinal segment and showed a similar pattern of decussation and distribution in the white matter funiculi and the gray matter laminae. They terminated mostly on dendrites of the lateral intermediate laminae and the internal basilar nucleus, and some innervated the ventromedial laminae, but were essentially absent in lateral laminae IX. Corticofugal axons terminated predominantly ipsilaterally in the midbrain and bilaterally in the medulla oblongata. Most corticorubral projections arose from M1, whereas the mesencephalic reticular formation, superior colliculus, lateral reticular nucleus, gigantocellular reticular nucleus, and raphe received abundant axonal contacts from both M1 and PM. Our data suggest that the porcine cortical motor system has some common features with that of primates and humans and may control posture and movement through parallel motor descending pathways. However, less cortical regions project to the spinal cord in pigs, and the CST neither seems to reach the lumbar enlargement nor to have a significant direct innervation of cervical, foreleg motoneurons.

Mapping the corticoreticular pathway from cortex-wide anterograde axonal tracing in the mouse

The corticoreticular pathway (CRP) has been implicated as an important mediator of motor recovery and rehabilitation after central nervous system damage. However, its origins, trajectory and laterality are not well understood. This study mapped the mouse CRP in comparison with the corticospinal tract (CST). We systematically searched the Allen Mouse Brain Connectivity Atlas (© 2011 Allen Institute for Brain Science) for experiments that used anterograde tracer injections into the right isocortex in mice. For each eligible experiment (N = 607), CRP and CST projection strength were quantified by the tracer volume reaching the reticular formation motor nuclei (RF_motor ) and pyramids, respectively. Tracer density in each brain voxel was also correlated with RF_motor versus pyramids projection strength to explore the relative trajectories of the CRP and CST. We found significant CRP projections originating from the primary and secondary motor cortices, anterior cingulate, primary somatosensory cortex, and medial prefrontal cortex. Compared with the CST, the CRP had stronger projections from each region except the primary somatosensory cortex. Ipsilateral projections were stronger than contralateral for both tracts (above the pyramidal decussation), but the CRP projected more bilaterally than the CST. The estimated CRP trajectory was anteromedial to the CST in the internal capsule and dorsal to the CST in the brainstem. Our findings reveal a widespread distribution of CRP origins and confirm strong bilateral CRP projections, theoretically increasing the potential for partial sparing after brain lesions and contralesional compensation after unilateral injury.

Lesion Area in the Cerebral Cortex Determines the Patterns of Axon Rewiring of Motor and Sensory Corticospinal Tracts After Stroke

The corticospinal tract (CST) is an essential neural pathway for reorganization that recovers motor functions after brain injuries such as stroke. CST comprises multiple pathways derived from different sensorimotor areas of the cerebral cortex; however, the patterns of reorganization in such complex pathways postinjury are largely unknown. Here we comprehensively examined the rewiring patterns of the CST pathways of multiple cerebral origins in a mouse stroke model that varied in size and location in the sensorimotor cortex. We found that spared contralesional motor and sensory CST axons crossed the midline and sprouted into the denervated side of the cervical spinal cord after stroke in a large cortical area. In contrast, the contralesional CST fibers did not sprout in a small stroke, whereas the ipsilesional axons from the spared motor area grew on the denervated side. We further showed that motor and sensory CST axons did not innervate the projecting areas mutually when either one was injured. The present results reveal the basic principles that generate the patterns of CST rewiring, which depend on stroke location and CST subtype. Our data indicate the importance of targeting different neural substrates to restore function among the types of injury.

A cortical injury model in a non-human primate to assess execution of reach and grasp actions: implications for recovery after traumatic brain injury

BACKGROUND

Technological advances in developing experimentally controlled models of traumatic brain injury (TBI) are prevalent in rodent models and these models have proven invaluable in characterizing temporal changes in brain and behavior after trauma. To date no long-term studies in non-human primates (NHPs) have been published using an experimentally controlled impact device to follow behavioral performance over time.

NEW METHOD

We have employed a controlled cortical impact (CCI) device to create a focal contusion to the hand area in primary motor cortex (M1) of three New World monkeys to characterize changes in reach and grasp function assessed for 3 months after the injury.

RESULTS

The CCI destroyed most of M1 hand representation reducing grey matter by 9.6 mm³, 12.9 mm³, and 15.5 mm³ and underlying corona radiata by 7.4 mm³, 6.9 mm³, and 5.6 mm³ respectively. Impaired motor function was confined to the hand contralateral to the injury. Gross hand-use was only mildly affected during the first few days of observation after injury while activity requiring skilled use of the hand was impaired over three months.

COMPARISON WITH EXISTING METHOD(S)

This study is unique in establishing a CCI model of TBI in an NHP resulting in persistent impairments in motor function evident in volitional use of the hand.

CONCLUSIONS

Establishing an NHP model of TBI is essential to extend current rodent models to the complex neural architecture of the primate brain. Moving forward this model can be used to investigate novel therapeutic interventions to improve or restore impaired motor function after trauma.

Coordinated increase of reliable cortical and striatal ensemble activations during recovery after stroke

Skilled movements rely on a coordinated cortical and subcortical network, but how this network supports motor recovery after stroke is unknown. Previous studies focused on the perilesional cortex (PLC), but precisely how connected subcortical areas reorganize and coordinate with PLC is unclear. The dorsolateral striatum (DLS) is of interest because it receives monosynaptic inputs from motor cortex and is important for learning and generation of fast reliable actions. Using a rat focal stroke model, we perform chronic electrophysiological recordings in motor PLC and DLS during long-term recovery of a dexterous skill. We find that recovery is associated with the simultaneous emergence of reliable movement-related single-trial ensemble spiking in both structures along with increased cross-area alignment of spiking. Our study highlights the importance of consistent neural activity patterns across brain structures during recovery and suggests that modulation of cross-area coordination can be a therapeutic target for enhancing motor function post-stroke.

Corticospinal vs Rubrospinal Revisited: An Evolutionary Perspective for Sensorimotor Integration

The knowledge about how different subsystems participate and interplay in sensorimotor control is fundamental to understand motor deficits associated with CNS injury and movement recovery. The role of corticospinal (CS) and rubrospinal (RS) projections in motor control has been extensively studied and compared, and it is clear that both systems are important for skilled movement. However, during phylogeny, the emerging cerebral cortex took a higher hierarchical role controlling rubro-cerebellar circuits. Here, we present anatomical, neurophysiological, and behavioral evidence suggesting that both systems modulate complex segmental neuronal networks in a parallel way, which is important for sensorimotor integration at spinal cord level. We also highlight that, although specializations exist, both systems could be complementary and potentially subserve motor recovery associated with CNS damage.

A New Definition of Poststroke Spasticity and the Interference of Spasticity With Motor Recovery From Acute to Chronic Stages

The relationship of poststroke spasticity and motor recovery can be confusing. "True" motor recovery refers to return of motor behaviors to prestroke state with the same end-effectors and temporo-spatial pattern. This requires neural recovery and repair, and presumably occurs mainly in the acute and subacute stages. However, according to the International Classification of Functioning, Disability and Health, motor recovery after stroke is also defined as "improvement in performance of functional tasks," i.e., functional recovery, which is mainly mediated by compensatory mechanisms. Therefore, stroke survivors can execute motor tasks in spite of disordered motor control and the presence of spasticity. Spasticity interferes with execution of normal motor behaviors ("true" motor recovery), throughout the evolution of stroke from acute to chronic stages. Spasticity reduction does not affect functional recovery in the acute and subacute stages; however, appropriate management of spasticity could lead to improvement of motor function, that is, functional recovery, during the chronic stage of stroke. We assert that spasticity results from upregulation of medial cortico-reticulo-spinal pathways that are disinhibited due to damage of the motor cortex or corticobulbar pathways. Spasticity emerges as a manifestation of maladaptive plasticity in the early stages of recovery and can persist into the chronic stage. It coexists and shares similar pathophysiological processes with related motor impairments, such as abnormal force control, muscle coactivation and motor synergies, and diffuse interlimb muscle activation. Accordingly, we propose a new definition of spasticity to better account for its pathophysiology and the complex nuances of different definitions of motor recovery.

Greater Reduction in Contralesional Hand Use After Frontoparietal Than Frontal Motor Cortex Lesions in Macaca mulatta

We previously reported that rhesus monkeys recover spontaneous use of the more impaired (contralesional) hand following neurosurgical lesions to the arm/hand representations of primary motor cortex (M1) and lateral premotor cortex (LPMC) (F2 lesion) when tested for reduced use (RU) in a fine motor task allowing use of either hand. Recovery occurred without constraint of the less impaired hand and with occasional forced use of the more impaired hand, which was the preferred hand for use in fine motor tasks before the lesion. Here, we compared recovery of five F2 lesion cases in the same RU test to recovery after unilateral lesions of M1, LPMC, S1 and anterior portion of parietal cortex (F2P2 lesion - four cases). Average and highest %use of the contralesional hand in the RU task in F2 cases were twice that in F2P2 cases (p < 0.05). Recovery in the RU task was closely associated with volume and percentage of lesion to caudal (new) M1 (M1c) in both F2 and F2P2 lesion cases. One F2P2 case, with the largest M1c lesion and a large rostral somatosensory cortex (S1r) lesion developed severe contralesional hand non-use despite exhibiting some recovery of fine motor function initially. We conclude that the degree of reduced use of the contralesional hand is primarily related to the volume of M1c injury and that severe non-use requires extensive injury to M1c and S1r. Thus, assessing peri-Rolandic injury extent in stroke patients may have prognostic value for predicting susceptibility to RU and non-use in rehabilitation.

Structure and function of corticospinal projection originating from supplementary motor area

PURPOSE

The importance of supplementary motor area (SMA) for motor function and compensation for primary motor area (M1) has received increased attention.

METHODS

We used diffusion tensor imaging (DTI) and transcranial magnetic stimulation (TMS) to evaluate structure and function of corticospinal projection originating from SMA. Fibers of corticospinal projection originating from M1 (CST) and SMA (ACST) were analyzed. ACST originating from mesial SMA area formed separate white matter bundles leaving the anterior part of M1 area, which then entered the posterior limb of the internal capsule. Projection and overlap of both CST and ACST were detected on medulla.

RESULTS

Fibers of contralesional ACST were more than that of ipsilesional ACST in patients with SMA tumors (p<0.05). In patients with SMA tumor, all patients experienced temporary akinesia postoperatively. Seven hundred forty-one fibers of ipsilateral ACST and no fibers of ipsilateral CST were detected in the patient with M1 glioma, while most of contralateral limb movement was preserved. MEP could be evoked by stimulating SMA area as well as M1 area. ACST originated from SMA area and projected to the medial medulla.

CONCLUSION

SMA area and ACST integrity contributed to contralateral motor function and were a compensation for M1 lesion and damaged CST.

Evidence for Subcortical Plasticity after Paired Stimulation from a Wearable Device

Existing non-invasive stimulation protocols can generate plasticity in the motor cortex and its corticospinal projections; techniques for inducing plasticity in subcortical circuits and alternative descending pathways such as the reticulospinal tract (RST) are less well developed. One possible approach developed by this laboratory pairs electrical muscle stimulation with auditory clicks, using a wearable device to deliver stimuli during normal daily activities. In this study, we applied a variety of electrophysiological assessments to male and female healthy human volunteers during a morning and evening laboratory visit. In the intervening time (∼6 h), subjects wore the stimulation device, receiving three different protocols, in which clicks and stimulation of the biceps muscle were paired at either low or high rate, or delivered at random. Paired stimulation: (1) increased the extent of reaction time shortening by a loud sound (the StartReact effect); (2) decreased the suppression of responses to transcranial magnetic brain stimulation (TMS) following a loud sound; (3) enhanced muscle responses elicited by a TMS coil oriented to induce anterior-posterior (AP) current, but not posterior-anterior (PA) current, in the brain. These measurements have all been suggested to be sensitive to subcortical, possibly reticulospinal, activity. Changes were similar for either of the two paired stimulus rates tested, but absent after unpaired (control) stimulation. Taken together, these results suggest that pairing clicks and muscle stimulation for long periods does indeed induce plasticity in subcortical systems such as the RST.SIGNIFICANCE STATEMENT Subcortical systems such as the reticulospinal tract (RST) are important motor pathways, which can make a significant contribution to functional recovery after cortical damage such as stroke. Here, we measure changes produced after a novel non-invasive stimulation protocol, which uses a wearable device to stimulate for extended periods. We observed changes in electrophysiological measurements consistent with the induction of subcortical plasticity. This protocol may prove an important tool for enhancing motor rehabilitation, in situations where insufficient cortical tissue survives to be a plausible substrate for recovery of function.

Low-frequency stimulation enhances ensemble co-firing and dexterity after stroke

Electrical stimulation is a promising tool for modulating brain networks. However, it is unclear how stimulation interacts with neural patterns underlying behavior. Specifically, how might external stimulation that is not sensitive to the state of ongoing neural dynamics reliably augment neural processing and improve function? Here, we tested how low-frequency epidural alternating current stimulation (ACS) in non-human primates recovering from stroke interacted with task-related activity in perilesional cortex and affected grasping. We found that ACS increased co-firing within task-related ensembles and improved dexterity. Using a neural network model, we found that simulated ACS drove ensemble co-firing and enhanced propagation of neural activity through parts of the network with impaired connectivity, suggesting a mechanism to link increased co-firing to enhanced dexterity. Together, our results demonstrate that ACS restores neural processing in impaired networks and improves dexterity following stroke. More broadly, these results demonstrate approaches to optimize stimulation to target neural dynamics.

Extensive Cortical Convergence to Primate Reticulospinal Pathways

Early evolution of the motor cortex included development of connections to brainstem reticulospinal neurons; these projections persist in primates. In this study, we examined the organization of corticoreticular connections in five macaque monkeys (one male) using both intracellular and extracellular recordings from reticular formation neurons, including identified reticulospinal cells. Synaptic responses to stimulation of different parts of primary motor cortex (M1) and supplementary motor area (SMA) bilaterally were assessed. Widespread short latency excitation, compatible with monosynaptic transmission over fast-conducting pathways, was observed, as well as longer latency responses likely reflecting a mixture of slower monosynaptic and oligosynaptic pathways. There was a high degree of convergence: 56% of reticulospinal cells with input from M1 received projections from M1 in both hemispheres; for SMA, the equivalent figure was even higher (70%). Of reticulospinal neurons with input from the cortex, 78% received projections from both M1 and SMA (regardless of hemisphere); 83% of reticulospinal cells with input from M1 received projections from more than one of the tested M1 sites. This convergence at the single cell level allows reticulospinal neurons to integrate information from across the motor areas of the cortex, taking account of the bilateral motor context. Reticulospinal connections are known to strengthen following damage to the corticospinal tract, such as after stroke, partially contributing to functional recovery. Extensive corticoreticular convergence provides redundancy of control, which may allow the cortex to continue to exploit this descending pathway even after damage to one area.SIGNIFICANCE STATEMENT The reticulospinal tract (RST) provides a parallel pathway for motor control in primates, alongside the more sophisticated corticospinal system. We found extensive convergent inputs to primate reticulospinal cells from primary and supplementary motor cortex bilaterally. These redundant connections could maintain transmission of voluntary commands to the spinal cord after damage (e.g., after stroke or spinal cord injury), possibly assisting recovery of function.

Time- and area-dependent macrophage/microglial responses after focal infarction of the macaque internal capsule

We quantitatively investigated temporal changes of macrophages and microglia (MΦ/MG) after focal infarction of the internal capsule using a macaque model we recently established. Immunoreactivity for Iba1, a general marker for MΦ/MG, in the periinfarct core gradually increased from 0 days to 2-3 weeks after infarction, and the increased immunoreactivity continued at least until 6 months; no study in rodents has reported increased Iba1-immunoreactive cells for so long. Retrograde atrophy or degeneration of neurons in layer V of the primary motor cortex, where the descending motor tract originates, was seen as secondary damage. Here we found that Iba1-positive MΦ/MG transiently increased in layer V during several weeks after the infarction. Therefore, the time course of MΦ/MG activation differs between the perilesional area and the remote brain area where secondary damage occurs to tissue initially preserved after the infarct. Detailed analyses using the functional phenotype markers CD68, CD86, and CD206, as well as cytokines released by cells with each phenotype, suggest an anti-inflammatory role for activated MΦ/MG both in the periinfarct core during the chronic phase and in the primary motor cortex.

Functional muscle strength recovery from nail gun injury to the primary motor cortex

Aim: Functional recovery following injury to the primary motor cortex is an uncommon phenomenon, given the limited ability of neurons of the adult central nervous system to regenerate. Case description: We report on a patient with near complete functional muscle strength recovery from a marked monoparesis due to nail gun injury to the medial primary motor cortex. Besides surgical decision-making, we discuss possible related mechanisms and current challenges in the regenerative processes responsible for the functional recovery. Discussion: To achieve a favorable outcome, surgical decision-making to prevent secondary damage is of upmost importance. Lesion-induced inflammatory response may potentiate endogenous neurogenesis and neuronal plasticity and potentially contribute to the regenerative process involved.

Limited capacity for ipsilateral secondary motor areas to support hand function post-stroke

KEY POINTS

Ipsilateral-projecting corticobulbar pathways, originating primarily from secondary motor areas, innervate the proximal and even distal portions, although they branch more extensively at the spinal cord. It is currently unclear to what extent these ipsilateral secondary motor areas and subsequent cortical projections may contribute to hand function following stroke-induced damage to one hemisphere. In the present study, we provide both structural and functional evidence indicating that individuals increasingly rely on ipsilateral secondary motor areas, although at the detriment of hand function. Increased activity in ipsilateral secondary motor areas was associated with increased involuntary coupling between shoulder abduction and finger flexion, most probably as a result of the low resolution of these pathways, making it increasingly difficult to open the hand. These findings suggest that, although ipsilateral secondary motor areas may support proximal movements, they do not have the capacity to support distal hand function, particularly for hand opening.

ABSTRACT

Recent findings have shown connections of ipsilateral cortico-reticulospinal tract (CRST), predominantly originating from secondary motor areas to not only proximal, but also distal muscles of the arm. Following a unilateral stroke, CRST from the ipsilateral side remains intact and thus has been proposed as a possible backup system for post-stroke rehabilitation even for the hand. We argue that, although CRST from ipsilateral secondary motor areas can provide control for proximal joints, it is insufficient to control either hand or coordinated shoulder and hand movements as a result of its extensive spinal branching compared to contralateral corticospinal tract. To address this issue, we combined magnetic resonance imaging, high-density EEG, and robotics in 17 individuals with severe chronic hemiparetic stroke and 12 age-matched controls. We tested for changes in structural morphometry of the sensorimotor cortex and found that individuals with stroke demonstrated higher grey matter density in secondary motor areas ipsilateral to the paretic arm compared to controls. We then measured cortical activity when participants were attempting to generate hand opening either supported on a table or when lifting against a shoulder abduction load. The addition of shoulder abduction during hand opening increased reliance on ipsilateral secondary motor areas in stroke, but not controls. Crucially, the increased use of ipsilateral secondary motor areas was associated with decreased hand opening ability when lifting the arm as a result of involuntary coupling between the shoulder and wrist/finger flexors. Taken together, this evidence implicates a compensatory role for ipsilateral (i.e. contralesional) secondary motor areas post-stroke, although with no apparent capacity to support hand function.

Functional near-infrared-spectroscopy-based measurement of changes in cortical activity in macaques during post-infarct recovery of manual dexterity

Because compensatory changes in brain activity underlie functional recovery after brain damage, monitoring of these changes will help to improve rehabilitation effectiveness. Functional near-infrared spectroscopy (fNIRS) has the potential to measure brain activity in freely moving subjects. We recently established a macaque model of internal capsule infarcts and an fNIRS system for use in the monkey brain. Here, we used these systems to study motor recovery in two macaques, for which focal infarcts of different sizes were induced in the posterior limb of the internal capsule. Immediately after the injection, flaccid paralysis was observed in the hand contralateral to the injected hemisphere. Thereafter, dexterous hand movements gradually recovered over months. After movement recovery, task-evoked hemodynamic responses increased in the ventral premotor cortex (PMv). The response in the PMv of the infarcted (i.e., ipsilesional) hemisphere increased in the monkey that had received less damage. In contrast, the PMv of the non-infarcted (contralesional) hemisphere was recruited in the monkey with more damage. A pharmacological inactivation experiment with muscimol suggested the involvement of these areas in dexterous hand movements during recovery. These results indicate that fNIRS can be used to evaluate brain activity changes crucial for functional recovery after brain damage.

Experts, but not novices, exhibit StartReact indicating experts use the reticulospinal system more than novices

Motor skill acquisition utilizes a wide array of neural structures; however, few articles evaluate how the relative contributions of these structures shift over the course of learning. Recent evidence from rodents and songbirds suggests there is a transfer from cortical to subcortical structures following intense, repetitive training. Evidence from humans indicate that the reticulospinal system is modulated over the course of skill acquisition and may be a subcortical facilitator of learning. The objective of this study was to evaluate how reticulospinal contributions are modulated by task expertise. Reticulospinal contributions were assessed using StartReact (SR). We hypothesized that expert typists would show SR during an individuated, keystroke task but SR would be absent in novices. Expert (75.2 ± 9.8 WPM) and novice typists (41.6 ± 8.2 WPM) were evaluated during an individuated, keystroke movements. In experts, SR was present but was absent in novices. Together, these results suggest that experts use reticulospinal contributions more for movement than novices indicating that the reticular formation becomes increasingly important for movement execution in highly trained, skilled tasks even those that require individuated movement of the fingers.

Alternative routes for recovery of hand functions after corticospinal tract injury in primates and rodents

PURPOSE OF REVIEW

Recent studies on various corticospinal tract (CST) lesions have shown the plastic changes at a variety of motor systems after the lesion. This review provides the alternative routes associated with the motor functional recovery after the CST lesions at various levels in nonhuman primates and rodents.

RECENT FINDINGS

In the case of the motor cortical lesions, the perilesional area compensates for the lesion. In contrast, sprouting of the corticoreticular tracts was observed after the lesions involving sensorimotor cortical areas. After the internal capsule lesion, sprouting in the cortico-rubral pathway contributes to the recovery. In case of the pyramidal lesion, rubrospinal and reticulospinal tracts play a role of the functional recovery. After the dorsolateral funiculus (DLF) lesion at C4/C5, the indirect pathway via propriospinal tract contributes to the recovery. In case of the hemisection at lower cervical cord, the CST fibers sprouted from the bilateral motor cortex and descended to the contralesional DLF and crossed below the lesion area.

SUMMARY

The central pathways can change their structure and activity dynamically depending on the lesion sites and size. Revealing the difference of the alternative pathways should be crucial to understand the whole recovery mechanism and develop the further neurorehabilitative treatment.

Dynamic Interaction between Cortico-Brainstem Pathways during Training-Induced Recovery in Stroke Model Rats

Reorganization of residual descending motor circuits underlies poststroke recovery. We previously clarified a causal relationship between the cortico-rubral tract and intensive limb use-induced functional recovery after internal capsule hemorrhage (ICH). However, other descending tracts, such as the cortico-reticular tract, might also be involved in rehabilitation-induced compensation. To investigate whether rehabilitation-induced recovery after ICH involves a shift in the compensatory circuit from the cortico-rubral tract to the cortico-reticular tract, we established loss of function of the cortico-rubral tract or/and cortico-reticular tract using two sets of viral vectors comprising the Tet-on system and designer receptors exclusively activated by the designer drug system. We used an ICH model that destroyed almost 60% of the corticofugal fibers. Anterograde tracing in rehabilitated rats revealed abundant sprouting of axons from the motor cortex in the red nucleus but not in the medullary reticular formation during the early phase of recovery. This primary contribution of the cortico-rubral tract was demonstrated by its selective blockade, whereas selective cortico-reticular tract silencing had little effect. Interestingly, cortico-rubral tract blockade from the start of rehabilitation induced an obvious increase of axon sprouting in the reticular formation with substantial functional recovery. Additional cortico-reticular tract silencing under the cortico-rubral tract blockade significantly worsened the recovered forelimb function. Furthermore, the alternative recruitment of the cortico-reticular tract was gradually induced by intensive limb use under cortico-rubral tract blockade, in which cortico-reticular tract silencing caused an apparent motor deficit. These findings indicate that individual cortico-brainstem pathways have dynamic compensatory potency to support rehabilitative functional recovery after ICH.SIGNIFICANCE STATEMENT This study aimed to clarify the interaction between the cortico-rubral and the cortico-reticular tract during intensive rehabilitation and functional recovery after capsular stroke. Pathway-selective disturbance by two sets of viral vectors revealed that the cortico-rubral tract was involved in rehabilitation-induced recovery of forelimb function from an early phase after internal capsule hemorrhage, but that the cortico-reticular tract was not. The sequential disturbance of both tracts revealed that the cortico-reticular tract was recruited and involved in rehabilitation-induced recovery when the cortico-rubral tract failed to function. Our data demonstrate a dynamic compensatory action of individual cortico-brainstem pathways for recovery through poststroke rehabilitation.

Differential Poststroke Motor Recovery in an Arm Versus Hand Muscle in the Absence of Motor Evoked Potentials

Background. After stroke, recovery of movement in proximal and distal upper extremity (UE) muscles appears to follow different time courses, suggesting differences in their neural substrates. Objective. We sought to determine if presence or absence of motor evoked potentials (MEPs) differentially influences recovery of volitional contraction and strength in an arm muscle versus an intrinsic hand muscle. We also related MEP status to recovery of proximal and distal interjoint coordination and movement fractionation, as measured by the Fugl-Meyer Assessment (FMA). Methods. In 45 subjects in the year following ischemic stroke, we tracked the relationship between corticospinal tract (CST) integrity and behavioral recovery in the biceps (BIC) and first dorsal interosseous (FDI) muscle. We used transcranial magnetic stimulation to probe CST integrity, indicated by MEPs, in BIC and FDI. We used electromyography, dynamometry, and UE FMA subscores to assess muscle-specific contraction, strength, and inter-joint coordination, respectively. Results. Presence of MEPs resulted in higher likelihood of muscle contraction, greater strength, and higher FMA scores. Without MEPs, BICs could more often volitionally contract, were less weak, and had steeper strength recovery curves than FDIs; in contrast, FMA recovery curves plateaued below normal levels for both the arm and hand. Conclusions. There are shared and separate substrates for paretic UE recovery. CST integrity is necessary for interjoint coordination in both segments and for overall recovery. In its absence, alternative pathways may assist recovery of volitional contraction and strength, particularly in BIC. These findings suggest that more targeted approaches might be needed to optimize UE recovery.

Terminal organization of the corticospinal projection from the lateral premotor cortex to the cervical enlargement (C5-T1) in rhesus monkey

High-resolution tract tracing and stereology were used to study the terminal organization of the corticospinal projection (CSP) from the ventral (v) and dorsal (d) regions of the lateral premotor cortex (LPMC) to spinal levels C5-T1. The LPMCv CSP originated from the postarcuate sulcus region, was bilateral, sparse, and primarily targeted the dorsolateral and ventromedial sectors of contralateral lamina VII. The convexity/lateral part of LPMCv did not project below C2. Thus, very little LPMCv corticospinal output reaches the cervical enlargement. In contrast, the LPMCd CSP was 5× more prominent in terminal density. Bilateral terminal labeling occurred in the medial sectors of lamina VII and adjacent lamina VIII, where propriospinal neurons with long-range bilateral axon projections reside. Notably, lamina VIII also harbors axial motoneurons. Contralateral labeling occurred in the lateral sectors of lamina VII and the dorsomedial quadrant of lamina IX, noted for harboring proximal upper limb flexor motoneurons. Segmentally, the CSP to contralateral laminae VII and IX preferentially innervated C5-C7, which supplies shoulder, elbow, and wrist musculature. In contrast, terminations in axial-related lamina VIII were distributed bilaterally throughout all cervical enlargement levels, including C8 and T1. These findings demonstrate the LPMCd CSP is structured to influence axial and proximal upper limb movements, supporting Kuypers conceptual view of the LPMCd CSP being a major component of the medial motor control system. Thus, distal upper extremity control influenced by LPMC, including grasping and manipulation, must occur through indirect neural network connections such as corticocortical, subcortical, or intrinsic spinal circuits.

A Unifying Pathophysiological Account for Post-stroke Spasticity and Disordered Motor Control

Cortical and subcortical plastic reorganization occurs in the course of motor recovery after stroke. It is largely accepted that plasticity of ipsilesional motor cortex primarily contributes to recovery of motor function, while the contributions of contralesional motor cortex are not completely understood. As a result of damages to motor cortex and its descending pathways and subsequent unmasking of inhibition, there is evidence of upregulation of reticulospinal tract (RST) excitability in the contralesional side. Both animal studies and human studies with stroke survivors suggest and support the role of RST hyperexcitability in post-stroke spasticity. Findings from animal studies demonstrate the compensatory role of RST hyperexcitability in recovery of motor function. In contrast, RST hyperexcitability appears to be related more to abnormal motor synergy and disordered motor control in stroke survivors. It does not contribute to recovery of normal motor function. Recent animal studies highlight laterality dominance of corticoreticular projections. In particular, there exists upregulation of ipsilateral corticoreticular projections from contralesional premotor cortex (PM) and supplementary motor area (SMA) to medial reticular nuclei. We revisit and revise the previous theoretical framework and propose a unifying account. This account highlights the importance of ipsilateral PM/SMA-cortico-reticulospinal tract hyperexcitability from the contralesional motor cortex as a result of disinhibition after stroke. This account provides a pathophysiological basis for post-stroke spasticity and related movement impairments, such as abnormal motor synergy and disordered motor control. However, further research is needed to examine this pathway in stroke survivors to better understand its potential roles, especially in muscle strength and motor recovery. This account could provide a pathophysiological target for developing neuromodulatory interventions to manage spasticity and thus possibly to facilitate motor recovery.

Dexterous Hand Movements and Their Recovery After Central Nervous System Injury

Hand dexterity has uniquely developed in higher primates and is thought to rely on the direct corticomotoneuronal (CM) pathway. Recent studies have shown that rodents and carnivores lack the direct CM pathway but can control certain levels of dexterous hand movements through various indirect CM pathways. Some homologous pathways also exist in higher primates, and among them, propriospinal (PrS) neurons in the mid-cervical segments (C3-C4) are significantly involved in hand dexterity. When the direct CM pathway was lesioned caudal to the PrS and transmission of cortical commands to hand motoneurons via the PrS neurons remained intact, dexterous hand movements could be significantly recovered. This recovery model was intensively studied, and it was found that, in addition to the compensation by the PrS neurons, a large-scale reorganization in the bilateral cortical motor-related areas and mesolimbic structures contributed to recovery. Future therapeutic strategies should target these multihierarchical areas.

Diversity of reticulospinal systems in mammals

Reticulospinal (RS) neurons provide the spinal cord with the executive signals for a large repertoire of motor and autonomic functions, ensuring at the same time that these functions are adapted to the different behavioral contexts. This requires the coordinated action of many RS neurons. In this mini-review, we examine how the RS neurons that carry out specific functions distribute across the three parts of the brain stem. Extensive overlap between populations suggests a need to explore multi-functionality at the single cell-level. We next contrast functional diversity and homogeneity in transmitter phenotype. Then, we examine the molecular genetic mechanisms that specify brain stem development and likely contribute to RS neurons identities. We advocate that a better knowledge of the developmental lineage of the RS neurons and a better knowledge of RS neuron activity across multiple behaviors will help uncover the fundamental principles behind the diversity of RS systems in mammals.

A startling acoustic stimulation (SAS)-TMS approach to assess the reticulospinal system in healthy and stroke subjects

Reticulospinal (RS) hyperexcitability is observed in stroke survivors with spastic hemiparesis. Habituated startle acoustic stimuli (SAS) can be used to stimulate the RS pathways non-reflexively. However, the role of RS pathways in motor function and its interactions with the corticospinal system after stroke still remain unclear. Therefore, the purpose of this study was to investigate the effects of conditioning SAS on the corticospinal system in healthy subjects and in stroke subjects with spastic hemiparesis. An established conditioning SAS- transcranial magnetic stimulation (TMS) paradigm was used to test the interactions between the RS pathways and the corticospinal system. TMS was delivered to the right hemisphere of eleven healthy subjects and the contralesional hemisphere of eleven stroke subjects during isometric elbow flexor contraction on the non-impaired (or left) side. Conditioning SAS had similar effects on the corticospinal motor system in both healthy and stroke subjects, including similar SAS-induced motor evoked potential (MEP) reduction at rest, but not during voluntary contraction tasks; similar magnitudes of TMS-induced MEP and force increment and shortening of the silent period during voluntary elbow flexor contraction. This study provides evidence that RS excitability on the contralesional side in stroke subjects with spastic hemiparesis is not abnormal, and suggests that RS projections are likely to be primarily unilateral in humans.

New Corticopontine Connections in the Primate Brain: Contralateral Projections From the Arm/Hand Area of the Precentral Motor Region

The ipsilateral corticopontine projection (iCPP) represents a massive descending axon system terminating in the pontine nuclei (PN). In the primate, this projection is well known for its dominant influence on contralateral upper limb movements through the classical cerebrocerebellar circuity system. Although a much weaker contralateral corticopontine projection (cCPP) from motor cortex to the paramedian region has been reported in the non-human primate brain, we provide the first comprehensive description of the cCPP from the lateral motor cortex using high resolution anterograde tract tracing in Macaca mulatta. We found a relatively light cCPP from the hand/arm area of the primary motor cortex (M1), comparatively moderate cCPP from ventrolateral premotor cortex (LPMCv) and a more robust and widespread cCPP from the dorsolateral premotor cortex (LPMCd) that involved all nine contralateral PN. The M1 projection primarily targeted the dorsal pontine region, the LPMCv projection targeted the medial pontine region and LPMCd targeted both regions. These results show the first stage of the primate frontomotor cerebrocerebellar projection is bilateral, and may affect both ipsilateral and contralateral limbs. Clinically, the cCPP originating in the non-injured hemisphere may influence the recovery process of the more affected upper extremity following subtotal unilateral damage to the lateral cortical region. The cCPP may also contribute to the mild impairment of the upper limb contralateral to a unilateral cerebellar injury.