Reticular formation responses to magnetic brain stimulation of primary motor cortex

Research progress and prospects of benefit-risk assessment methods for umbilical cord mesenchymal stem cell transplantation in the clinical treatment of spinal cord injury

Over the past decade, we have witnessed the development of cell transplantation as a new strategy for repairing spinal cord injury (SCI). However, due to the complexity of the central nervous system (CNS), achieving successful clinical translation remains a significant challenge. Human umbilical cord mesenchymal stem cells (hUMSCs) possess distinct advantages, such as easy collection, lack of ethical concerns, high self-renewal ability, multilineage differentiation potential, and immunomodulatory properties. hUMSCs are promising for regenerating the injured spinal cord to a significant extent. At the same time, for advancing SCI treatment, the appropriate benefit and risk evaluation methods play a pivotal role in determining the clinical applicability of treatment plans. Hence, this study discusses the advantages and risks of hUMSCs in SCI treatment across four dimensions-comprehensive evaluation of motor and sensory function, imaging, electrophysiology, and autonomic nervous system (ANS) function-aiming to improve the rationality of relevant clinical research and the feasibility of clinical translation.

Different descending pathways mediate early and late portions of lower limb responses to transcranial magnetic stimulation

Although recent studies in nonhuman primates have provided evidence that transcranial magnetic stimulation (TMS) activates cells within the reticular formation, it remains unclear whether descending brain stem projections contribute to the generation of TMS-induced motor evoked potentials (MEPs) in skeletal muscles. We compared MEPs in muscles with extensive direct corticomotoneuronal input (first dorsal interosseous) versus a prominent role in postural control (gastrocnemius) to determine whether the amplitudes of early and late MEPs were differentially modulated by cortical suppression. Suprathreshold TMS was applied with and without a preceding suprathreshold TMS pulse at two interstimulus intervals (50 and 80 ms). H reflexes in target muscles were also tested with and without TMS conditioning. Early and late gastrocnemius MEPs were differentially modulated by cortical inhibition, the amplitude of the early MEP being significantly reduced by cortical suppression and the late MEP facilitated. The amplitude of H reflexes in the gastrocnemius was reduced within the cortical silent period. Early MEPs in the first dorsal interosseous were also reduced during the silent period, but late MEPs were unaffected. Independent modulation of early and late MEPs in the gastrocnemius muscle supports the idea that the MEP is generated by multiple descending pathways. Suppression of the early MEP is consistent with transmission along the fast-conducting corticospinal tract, whereas facilitation of the late MEP suggests transmission along a corticofugal, potentially cortico-reticulospinal, pathway. Accordingly, differences in late MEP modulation between the first dorsal interosseous and gastrocnemius reflect an increased role of corticofugal pathways in the control of postural muscles.NEW & NOTEWORTHY Early and late portions of the response to transcranial magnetic stimulation (TMS) in a lower limb postural muscle are modulated independently by cortical suppression, late motor evoked potentials (MEPs) being facilitated during cortical inhibition. These results suggest a cortico-brain stem transmission pathway for late portions of the TMS-induced MEP.

The effects of music combined to paired associative stimulation on motor-evoked potentials and alertness in spinal cord injury patients and healthy subjects

Paired associative stimulation (PAS) consisting of high-intensity transcranial magnetic stimulation (TMS) and high-frequency peripheral nerve stimulation (known as high-PAS) induces plastic changes and improves motor performance in patients with incomplete spinal cord injury (SCI). Listening to music during PAS may potentially improve mood and arousal and facilitate PAS-induced neuroplasticity via auditory-motor coupling, but the effects have not been explored. This pilot study aimed to determine if the effect of high-PAS on motor-evoked potentials (MEPs) and subjective alertness can be augmented with music. Ten healthy subjects and nine SCI patients received three high-PAS sessions in randomized order (PAS only, PAS with music synchronized to TMS, PAS with self-selected music). MEPs were measured before (PRE), after (POST), 30 min (POST30), and 60 min (POST60) after stimulation. Alertness was evaluated with a questionnaire. In healthy subjects, MEPs increased at POST in all sessions and remained higher at POST60 in PAS with synchronized music compared with the other sessions. There was no difference in alertness. In SCI patients, MEPs increased at POST and POST30 in PAS only but not in other sessions, whereas alertness was higher in PAS with self-selected music. More research is needed to determine the potential clinical effects of using music during high-PAS.

Strength-trained adults demonstrate greater corticoreticular activation versus untrained controls

The rapid increase in strength following strength-training involves neural adaptations, however, their specific localisation remains elusive. Prior focus on corticospinal responses prompts this study to explore the understudied cortical/subcortical adaptations, particularly cortico-reticulospinal tract responses, comparing healthy strength-trained adults to untrained peers. Fifteen chronically strength-trained individuals (≥2 years of training, mean age: 24 ± 7 years) were compared with 11 age-matched untrained participants (mean age: 26 ± 8 years). Assessments included maximal voluntary force (MVF), corticospinal excitability using transcranial magnetic stimulation (TMS), spinal excitability (cervicomedullary stimulation), voluntary activation (VA) and reticulospinal tract (RST) excitability, utilizing StartReact responses and ipsilateral motor-evoked potentials (iMEPs) for the flexor carpi radialis muscle. Trained participants had higher normalized MVF (6.4 ± 1.1 N/kg) than the untrained participants (4.8 ± 1.3 N/kg) (p = .003). Intracortical facilitation was higher in the strength-trained group (156 ± 49%) (p = .02), along with greater VA (98 ± 3.2%) (p = .002). The strength-trained group displayed reduced short-interval-intracortical inhibition (88 ± 8.0%) compared with the untrained group (69 ± 17.5%) (p < .001). Strength-trained individuals exhibited a greater normalized rate of force development (38.8 ± 10.1 N·s-1/kg) (p < .009), greater reticulospinal gain (2.5 ± 1.4) (p = .02) and higher ipsilateral-to-contralateral MEP ratios compared with the untrained group (p = .03). Strength-trained individuals displayed greater excitability within the intrinsic connections of the primary motor cortex and the RST. These results suggest greater synaptic input from the descending cortico-reticulospinal tract to α-motoneurons in strength-trained individuals, thereby contributing to the observed increase in VA and MVF.

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.

Neural coordination of bilateral hand movements: evidence for an involvement of brainstem motor centres

Bilateral hand movements are assumed to be coordinated by a neural coupling mechanism. Neural coupling is experimentally reflected in complex electromyographic (EMG) responses in the forearm muscles of both sides to unilateral electrical arm nerve stimulation (ES). The aim of this study was to examine a potential involvement of the reticulospinal system in neural coupling by the application of loud acoustic stimuli (LAS) known to activate neurons of this system. LAS, ES and combined LAS/ES were applied to healthy subjects during visually guided bilateral hand flexion-extension movements. Muscle responses to the different stimuli were evaluated by electrophysiological recordings. Unilateral electrical ulnar nerve stimulation resulted in neural coupling responses in the forearm extensors (FE) of both sides. Interestingly, LAS evoked bilateral EMG responses that were similar in their configuration to those induced by ES. The presence of startles was associated with a shift of the onset and enhanced amplitude of LAS-induced coupling-like responses. Upon combined LAS/ES application, ES facilitated ipsilateral startles and coupling-like responses. Modulation of coupling-like responses by startles, the similarity of the responses to ES and LAS, and their interaction following combined stimulation suggests that both responses are mediated by the reticulospinal system. Our findings provide novel indirect evidence that the reticulospinal system is involved in the neural coupling of hand movements. This becomes clinically relevant in subjects with a damaged corticospinal system where a dominant reticulospinal system leads to involuntary limb coupling, referred to as associated movements. KEY POINTS: Automatic coordination of hand movements is assumed to be mediated by a neural coupling mechanism reflected by bilateral reflex responses in forearm muscles to unilateral electrical arm nerve stimulation (ES). Loud acoustic stimuli (LAS) were applied to assess a potential involvement of the reticulospinal system in the neural coupling mechanism. LAS evoked a bilateral reflex response in the forearm extensors that was similar to the neural coupling response to ES, and which could be separated from the acoustic startle response. Combined application of LAS and ES resulted in a facilitation of startle and coupling-like responses ipsilateral to ES, thus indicating an interaction of afferences from both stimuli. These novel findings provide indirect evidence that the reticulospinal system is a key motor structure for the coupling of bilateral hand movements.

Mapping subcortical motor pathways in humans with startle-conditioned TMS

Subcortical motor pathways, such as the reticulospinal tract, are critical for producing and modulating voluntary movements and have been implicated in neurological conditions. Previous research has described the presence of ipsilateral motor evoked potentials (iMEPs) in the arm to transcranial magentic stimulation (TMS), and suggested they could be mediated by the uncrossed corticospinal tract or by ipsilateral cortico-reticulospinal connections. Here, we sought to elucidate the role of the reticulospinal tract in mediating iMEPs by assessing their modulation by a startling acoustic stimulus and mapping these responses across multiple upper limb effectors. In a first experiment, we delivered TMS at various intervals (1, 5, 10 and 15 ms) after a startling acoustic stimulus, known to excite the reticular formation, to elicit iMEPs in the arm. We observed robust facilitation of iMEP area when startle conditioning preceded TMS at the 10 ms interval. In a second experiment, we replicated our findings showing that both the area and number of iMEPs in the arm increases with startle conditioning. Using this technique, we observed that iMEPs are more prominent in the arm compared with the hand. In a third experiment, we also observed greater presence of iMEPs in flexor compared with extensor muscles. Together, these findings are consistent with properties of the reticulospinal tract observed in animals, suggesting that iMEPs primarily reflect reticulospinal activity. Our findings imply that we can use this approach to track modulation of cortico-reticulospinal excitability following interventions or neurological conditions where the reticulospinal tract may be involved in motor recovery.

Transcranial magnetic stimulation in non-human primates: A systematic review

Transcranial magnetic stimulation (TMS) is widely employed as a tool to investigate and treat brain diseases. However, little is known about the direct effects of TMS on the brain. Non-human primates (NHPs) are a valuable translational model to investigate how TMS affects brain circuits given their neurophysiological similarity with humans and their capacity to perform complex tasks that approach human behavior. This systematic review aimed to identify studies using TMS in NHPs as well as to assess their methodological quality through a modified reference checklist. The results show high heterogeneity and superficiality in the studies regarding the report of the TMS parameters, which have not improved over the years. This checklist can be used for future TMS studies with NHPs to ensure transparency and critical appraisal. The use of the checklist would improve methodological soundness and interpretation of the studies, facilitating the translation of the findings to humans. The review also discusses how advancements in the field can elucidate the effects of TMS in the brain.

Motor potentials evoked by transcranial magnetic stimulation: interpreting a simple measure of a complex system

Transcranial magnetic stimulation (TMS) is a non-invasive technique that is increasingly used to study the human brain. One of the principal outcome measures is the motor-evoked potential (MEP) elicited in a muscle following TMS over the primary motor cortex (M1), where it is used to estimate changes in corticospinal excitability. However, multiple elements play a role in MEP generation, so even apparently simple measures such as peak-to-peak amplitude have a complex interpretation. Here, we summarize what is currently known regarding the neural pathways and circuits that contribute to the MEP and discuss the factors that should be considered when interpreting MEP amplitude measured at rest in the context of motor processing and patients with neurological conditions. In the last part of this work, we also discuss how emerging technological approaches can be combined with TMS to improve our understanding of neural substrates that can influence MEPs. Overall, this review aims to highlight the capabilities and limitations of TMS that are important to recognize when attempting to disentangle sources that contribute to the physiological state-related changes in corticomotor excitability.

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.

Testing a Novel Wearable Device for Motor Recovery of the Elbow Extensor Triceps Brachii in Chronic Spinal Cord Injury

After corticospinal tract damage, reticulospinal connections to motoneurons strengthen preferentially to flexor muscles. This could contribute to the disproportionately poor recovery of extensors often seen after spinal cord injury (SCI) and stroke. In this study, we paired electrical stimulation over the triceps muscle with auditory clicks, using a wearable device to deliver stimuli over a prolonged period of time. Healthy human volunteers wore the stimulation device for ∼6 h and a variety of electrophysiological assessments were used to measure changes in triceps motor output. In contrast to previous results in the biceps muscle, paired stimulation: (1) did not increase the StartReact effect; (2) did not decrease the suppression of responses to transcranial magnetic brain stimulation (TMS) following a loud sound; (3) did not enhance muscle responses elicited by a TMS coil oriented to induce anterior-posterior current. In a second study, chronic cervical SCI survivors wore the stimulation device for ∼4 h every day for four weeks; this was compared with a four-week period without wearing the device. Functional and electrophysiological assessments were repeated at week 0, week 4, and week 8. No significant changes were observed in electrophysiological assessments after paired stimulation. Functional measurements such as maximal force and variability and speed of trajectories made during a planar reaching task also remained unchanged. Our results suggest that the triceps muscle shows less potential for plasticity than biceps; pairing clicks with muscle stimulation does not seem beneficial in enhancing triceps recovery after SCI.

Contribution of neural circuits tested by transcranial magnetic stimulation in corticomotor control of low back muscle: a systematic review

Introduction

Transcranial magnetic stimulation (TMS) is widely used to investigate central nervous system mechanisms underlying motor control. Despite thousands of TMS studies on neurophysiological underpinnings of corticomotor control, a large majority of studies have focused on distal muscles, and little is known about axial muscles (e.g., low back muscles). Yet, differences between corticomotor control of low back and distal muscles (e.g., gross vs. fine motor control) suggest differences in the neural circuits involved. This systematic review of the literature aims at detailing the organisation and neural circuitry underlying corticomotor control of low back muscles tested with TMS in healthy humans.

Methods

The literature search was performed in four databases (CINAHL, Embase, Medline (Ovid) and Web of science) up to May 2022. Included studies had to use TMS in combination with EMG recording of paraspinal muscles (between T12 and L5) in healthy participants. Weighted average was used to synthesise quantitative study results.

Results

Forty-four articles met the selection criteria. TMS studies of low back muscles provided consistent evidence of contralateral and ipsilateral motor evoked potentials (with longer ipsilateral latencies) as well as of short intracortical inhibition/facilitation. However, few or no studies using other paired pulse protocols were found (e.g., long intracortical inhibition, interhemispheric inhibition). In addition, no study explored the interaction between different cortical areas using dual TMS coil protocol (e.g., between primary motor cortex and supplementary motor area).

Discussion

Corticomotor control of low back muscles are distinct from hand muscles. Our main findings suggest: (i) bilateral projections from each single primary motor cortex, for which contralateral and ipsilateral tracts are probably of different nature (contra: monosynaptic; ipsi: oligo/polysynaptic) and (ii) the presence of intracortical inhibitory and excitatory circuits in M1 influencing the excitability of the contralateral corticospinal cells projecting to low back muscles. Understanding of these mechanisms are important for improving the understanding of neuromuscular function of low back muscles and to improve the management of clinical populations (e.g., low back pain, stroke).

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.

Multipulse transcranial magnetic stimulation of human motor cortex produces short-latency corticomotor facilitation via two distinct mechanisms

Single-pulse transcranial magnetic stimulation (TMS) of the precentral hand representation (M1_HAND) can elicit indirect waves in the corticospinal tract at a periodicity of ∼660 Hz, called I-waves. These descending volleys are produced by transsynaptic excitation of fast-conducting corticospinal axons in M1_HAND. Paired-pulse TMS can induce short-interval intracortical facilitation (SICF) of motor evoked potentials (MEPs) at interpulse intervals that match I-wave periodicity. This study examined whether short-latency corticospinal facilitation engages additional mechanisms independently of I-wave periodicity. In 19 volunteers, one to four biphasic TMS pulses were applied to left M1_HAND with interpulse intervals adjusted to the first peak or trough of the individual SICF curve at different intensities to probe the intensity-response relationship. Multipulse TMS_HAND at individual peak latency facilitated MEP amplitudes and reduced resting motor threshold (RMT) compared with single pulses. Multipulse TMS_HAND at individual trough latency also produced a consistent facilitation of MEPs and a reduction of RMT. Short-latency facilitation at trough latency was less pronounced, but the relative difference in facilitation decreased with increasing stimulus intensity. Increasing the pulse number had only a modest effect. Two mechanisms underlie short-latency facilitation caused by biphasic multipulse TMS_HAND. One intracortical mechanism is related to I-wave periodicity and engages fast-conducting direct projections to spinal motoneurons. A second corticospinal mechanism does not rely on I-wave rhythmicity and may be mediated by slower-conducting indirect pyramidal tract projections from M1_HAND to spinal interneurons. The latter mechanism deserves more attention in studies of the corticomotor system and its link to manual motor control using the MEP.NEW & NOTEWORTHY TMS pairs evoke SICF at interpulse intervals (IPIs) that match I-wave periodicity. Biphasic bursts with IPIs at the latency of the first peak facilitate MEPs and reduce corticomotor threshold. Bursts at the latency of the first trough facilitate MEPs and reduce corticomotor threshold to a lesser extent. TMS bursts facilitate corticomotor excitability via two mechanisms: SICF-dependently via fast-conducting direct projections from M1_HAND to spinal motoneurons and SICF-independently, probably through slower-conducting indirect pyramidal tract projections.

Altered frontoparietal activity in acoustic startle priming tasks during reticulospinal tract facilitation: An fNIRS study

Background

Because it is one of the important pathways for promoting motor recovery after cortical injury, the function of the reticulospinal tract (RST) has received increasing attention in recent years. However, the central regulatory mechanism of RST facilitation and reduction of apparent response time is not well understood.

Objectives

To explore the potential role of RST facilitation in the acoustic startle priming (ASP) paradigm and observe the cortical changes induced by ASP reaching tasks.

Methods

Twenty healthy participants were included in this study. The reaching tasks were performed with their left and right hands. Participants were instructed to get ready after the warning cue and complete the reach as soon as they heard the Go cue. Half of the testing trials were set as control trials with an 80-dB Go cue. The other half of the trials had the Go cue replaced with 114-dB white noise to evoke the StartleReact effect, inducing reticulospinal tract facilitation. The response of the bilateral sternocleidomastoid muscle (SCM) and the anterior deltoid was recorded via surface electromyography. Startle trials were labeled as exhibiting a positive or negative StartleReact effect, according to whether the SCM was activated early (30-130 ms after the Go cue) or late, respectively. Functional near-infrared spectroscopy was used to synchronously record the oxyhemoglobin and deoxyhemoglobin fluctuations in bilateral motor-related cortical regions. The β values representing cortical responses were estimated via the statistical parametric mapping technique and included in the final analyses.

Results

Separate analyses of data from movements of the left or right side revealed significant activation of the right dorsolateral prefrontal cortex during RST facilitation. Moreover, left frontopolar cortex activation was greater in positive startle trials than in control or negative startle trials during left-side movements. Furthermore, decreased activity of the ipsilateral primary motor cortex in positive startle trials during ASP reaching tasks was observed.

Conclusion

The right dorsolateral prefrontal cortex and the frontoparietal network to which it belongs may be the regulatory center for the StartleReact effect and RST facilitation. In addition, the ascending reticular activating system may be involved. The decreased activity of the ipsilateral primary motor cortex suggests enhanced inhibition of the non-moving side during the ASP reaching task. These findings provide further insight into the SE and into RST facilitation.

Postural support requirements preferentially modulate late components of the gastrocnemius response to transcranial magnetic stimulation

Mounting evidence suggests that motor evoked potentials (MEPs) recorded in upper limb muscles with postural support roles following transcranial magnetic stimulation receive contributions from both corticospinal and non-corticospinal descending pathways. We tested the hypothesis that neural structures responsible for regulating upright balance are involved in transmitting late portions of TMS-induced MEPs in a lower limb muscle. MEPs were recorded in the medial gastrocnemius muscles of each leg, while participants supported their upright posture in five postural conditions that required different levels of support from the target muscles. We observed that early and late portions of the MEP were modulated independently, with early MEP amplitude being reduced when high levels of postural support were required from a target muscle. Independent modulation of early and late MEPs by altered postural demand suggests largely separable transmission of each part of the MEP. The early component of the MEP is likely generated by fast-conducting corticospinal pathways, whereas the later component may be primarily transmitted along a polysynaptic cortico-reticulospinal pathway.

Motor overflow in the lower limb after stroke: insights into mechanisms

Motor overflow (involuntary muscle activation) is common after stroke, particularly in the non-paretic upper limb. Two potential cortical mechanisms are: 1) the contralesional hemisphere controls both limbs, and 2) inhibition from the ipsilesional to the contralesional hemisphere is diminished. Few studies have differentiated between these hypotheses or investigated motor overflow in the lower limb after stroke. To investigate these potential mechanisms, individuals with chronic stroke performed unilateral isometric and dynamic dorsiflexion. Motor overflow was quantified in the contralateral, resting (non-target) ankle. Transcranial magnetic stimulation was applied, and responses were measured in both legs. Relations between motor overflow, excitability of ipsilateral motor pathways, and interhemispheric inhibition were assessed. Non-target muscle activity (motor overflow) was greater during isometric and dynamic conditions than rest in both legs (p≤0.001) and was higher in the non-paretic than the paretic leg (p=0.03). Some participants (25%) had motor overflow >4SD above the group mean in the non-paretic leg. Greater motor overflow in the non-paretic leg was associated with lesser inhibition from the ipsilesional to the contralesional hemisphere (p=0.04). In both legs, non-target TMS responses were greater during the isometric and dynamic than the rest condition (p≤0.01), but not when normalized to background muscle activity. Overall, motor overflow occurred in both legs after stroke, suggesting a common bilateral mechanism. Our correlational results suggest that alterations in interhemispheric inhibition may contribute to motor overflow. Furthermore, the lack of differences in non-target MEPs between rest, isometric, and dynamic conditions, suggests that subcortical and/or spinal pathways may contribute to motor overflow.

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.

Impact of Voluntary Muscle Activation on Stretch Reflex Excitability in Individuals With Hemiparetic Stroke

Objective

To characterize how, following a stretch-induced attenuation, volitional muscle activation impacts stretch reflex activity in individuals with stroke.

Methods

A robotic device rotated the paretic elbow of individuals with hemiparetic stroke from 70° to 150°, and then back to 70° elbow flexion at an angular speed of 120°/s. This stretching sequence was repeated 20 times. Subsequently, participants volitionally activated their elbow musculature or rested. Finally, the stretching sequence was repeated another 20 times. The flexors' stretch reflex activity was quantified as the net torque measured at 135°.

Results

Data from 15 participants indicated that the stretching sequence attenuated the flexion torque (p < 0.001) and resting sustained the attenuation (p = 1.000). Contrastingly, based on data from 14 participants, voluntary muscle activation increased the flexion torque (p < 0.001) to an initial pre-stretch torque magnitude (p = 1.000).

Conclusions

Stretch reflex attenuation induced by repeated fast stretches may be nullified when individuals post-stroke volitionally activate their muscles. In contrast, resting may enable a sustained reflex attenuation if the individual remains relaxed.

Significance

Stretching is commonly implemented to reduce hyperactive stretch reflexes following a stroke. These findings suggest that stretch reflex accommodation arising from repeated fast stretching may be reversed once an individual volitionally moves their paretic arm.

Understanding them to understand ourselves: The importance of NHP research for translational neuroscience

Studying higher brain function presents fundamental scientific challenges but has great potential for impactful translation to the clinic, supporting the needs of many patients suffering from conditions that relate to neuronal dysfunction. For many key questions relevant to human neurological conditions and clinical interventions, non-human primates (NHPs) remain the only suitable model organism and the only effective way to study the relationship between brain structure and function with the knowledge and tools currently available. Here we present three exemplary studies of current research yielding important findings that are directly translational to human clinical patients but which would be impossible without NHP studies. Our first example shows how studies of the NHP prefrontal cortex are leading to clinically relevant advances and potential new treatments for human neuropsychiatric disorders such as depression and anxiety. Our second example looks at the relevance of NHP research to our understanding of visual pathways and the visual cortex, leading to visual prostheses that offer treatments for otherwise blind patients. Finally, we consider recent advances in treatments leading to improved recovery of movement and motor control in stroke patients, resulting from our improved understanding of brain stem parallel pathways involved in movement in NHPs. The case for using NHPs in neuroscience research, and the direct benefits to human patients, is strong but has rarely been set out directly. This paper reviews three very different areas of neuroscience research, expressly highlighting the unique insights offered to each by NHP studies and their direct applicability to human clinical conditions.

Neural mechanisms mediating cross education: With additional considerations for the ageing brain

CALVERT, G.H.M., and CARSON, R.G. Neural mechanisms mediating cross education: With additional considerations for the ageing brain. NEUROSCI BIOBEHAV REV 21(1) XXX-XXX, 2021. - Cross education (CE) is the process whereby a regimen of unilateral limb training engenders bilateral improvements in motor function. The contralateral gains thus derived may impart therapeutic benefits for patients with unilateral deficits arising from orthopaedic injury or stroke. Despite this prospective therapeutic utility, there is little consensus concerning its mechanistic basis. The precise means through which the neuroanatomical structures and cellular processes that mediate CE may be influenced by age-related neurodegeneration are also almost entirely unknown. Notwithstanding the increased incidence of unilateral impairment in later life, age-related variations in the expression of CE have been examined only infrequently. In this narrative review, we consider several mechanisms which may mediate the expression of CE with specific reference to the ageing CNS. We focus on the adaptive potential of cellular processes that are subserved by a specific set of neuroanatomical pathways including: the corticospinal tract, corticoreticulospinal projections, transcallosal fibres, and thalamocortical radiations. This analysis may inform the development of interventions that exploit the therapeutic utility of CE training in older persons.

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.

The Effect of Sound and Stimulus Expectation on Transcranial Magnetic Stimulation-Elicited Motor Evoked Potentials

The amplitude of motor-evoked potentials (MEPs) elicited by transcranial magnetic stimulation (TMS) over the motor cortex is influenced by multiple factors. TMS delivery is accompanied by an abrupt clicking noise which can induce a startle response. This study investigated how masking/attenuating the sound produced by the TMS system discharging influences MEP amplitudes. In addition, the effects of increasing the time between consecutive stimuli and of making participants aware of the time at which they would be stimulated were studied. MEPs were recorded from the Flexor Carpi Radialis (FCR) muscle at rest by stimulation at motor threshold (MT), 120% MT and 140% MT intensity. Participants (N = 23) received stimulation under normal (NORMAL) conditions and while: wearing sound-attenuating earmuffs (EAR); listening to white noise (NOISE); the interval between stimuli were prolonged (LONG); stimulation timing was presented on a screen (READY). The results showed that masking (p = 0.020) and attenuating (p = 0.004) the incoming sound significantly reduced the amplitude of MEPs recorded across the intensities of stimulation. Increasing the interval between pulses had no effect on the recorded traces if a jitter was introduced (p = 1), but making participants aware of stimulation timing decreased MEP amplitudes (p = 0.049). These findings suggest that the sound produced by TMS at discharging increases MEP amplitudes and that MEP amplitudes are influenced by stimulus expectation. These confounding factors need to be considered when using TMS to assess corticospinal excitability.

Corticoreticulospinal tract neurophysiology in an arm and hand muscle in healthy and stroke subjects

KEY POINTS

The corticoreticulospinal tract (CReST) is a descending motor pathway that reorganizes after corticospinal tract (CST) injury in animals. In humans, the pattern of CReST innervation to upper limb muscles has not been carefully examined in healthy individuals or individuals with CST injury. In the present study, we assessed CReST projections to an arm and hand muscle on the same side of the body in healthy and chronic stoke subjects using transcranial magnetic stimulation. We show that CReST connection strength to the muscles differs between healthy and stroke subjects, with stronger connections to the hand than arm in healthy subjects, and stronger connections to the arm than hand in stroke subjects. These results help us better understand CReST innervation patterns in the upper limb, and may point to its role in normal motor function and motor recovery in humans.

ABSTRACT

The corticoreticulospinal tract (CReST) is a major descending motor pathway in many animals, but little is known about its innervation patterns in proximal and distal upper extremity muscles in humans. The contralesional CReST furthermore reorganizes after corticospinal tract (CST) injury in animals, but it is less clear whether CReST innervation changes after stroke in humans. We thus examined CReST functional connectivity, connection strength, and modulation in an arm and hand muscle of healthy (n = 15) and chronic stroke (n = 16) subjects. We delivered transcranial magnetic stimulation to the contralesional hemisphere (assigned in healthy subjects) to elicit ipsilateral motor evoked potentials (iMEPs) from the paretic biceps (BIC) and first dorsal interosseous (FDI) muscle. We operationalized CReST functional connectivity as iMEP presence/absence, CReST projection strength as iMEP size and CReST modulation as change in iMEP size by head rotation. We found comparable CReST functional connectivity to the BICs and FDIs in both subject groups. However, the pattern of CReST connection strength to the muscles diverged between groups, with stronger connections to FDIs than BICs in healthy subjects and stronger connections to BICs than FDIs in stroke subjects. Head rotation modulated only FDI iMEPs of healthy subjects. Our findings indicate that the healthy CReST does not have a proximal innervation bias, and its strong FDI connections may have functional relevance to finger individuation. The reversed CReST innervation pattern in stroke subjects confirms its reorganization after CST injury, and its strong BIC connections may indicate upregulation for particular upper extremity muscles or their functional actions.

Premovement inhibition can protect motor actions from interference by response-irrelevant sensory stimulation

KEY POINTS

Suppression of corticospinal excitability is reliably observed during preparation for a range of motor actions, leading to the belief that this preparatory inhibition is a physiologically obligatory component of motor preparation. The neurophysiological function of this suppression is uncertain. We restricted the time available for participants to engage in preparation and found no evidence for preparatory inhibition. The function of preparatory inhibition can be inferred from our findings that sensory stimulation can disrupt motor output in the absence of preparatory inhibition, but enhance motor output when inhibition is present. These findings suggest preparatory inhibition may be a strategic process which acts to protect prepared actions from external interference. Our findings have significant theoretical implications for preparatory processes. Findings may also have a pragmatic benefit in that acoustic stimulation could be used therapeutically to facilitate movement, but only if the action can be prepared well in advance.

ABSTRACT

Shortly before movement initiation, the corticospinal system undergoes a transient suppression. This phenomenon has been observed across a range of motor tasks, suggesting that it may be an obligatory component of movement preparation. We probed whether this was also the case when the urgency to perform a motor action was high, in a situation where little time was available to engage in preparatory processes. We controlled the urgency of an impending motor action by increasing or decreasing the foreperiod duration in an anticipatory timing task. Transcranial magnetic stimulation (TMS; experiment 1) or a loud acoustic stimulus (LAS; experiment 2) were used to examine how corticospinal and subcortical excitability were modulated during motor preparation. Preparatory inhibition of the corticospinal tract was absent when movement urgency was high, though motor actions were initiated on time. In contrast, subcortical circuits were progressively inhibited as the time to prepare increased. Interestingly, movement force and vigour were reduced by both TMS and the LAS when movement urgency was high, and enhanced when movement urgency was low. These findings indicate that preparatory inhibition may not be an obligatory component of motor preparation. The behavioural effects we observed in the absence of preparatory inhibition were induced by both TMS and the LAS, suggesting that accessory sensory stimulation may disrupt motor output when such stimulation is presented in the absence of preparatory inhibition. We conclude that preparatory inhibition may be an adaptive strategy which can serve to protect the prepared motor action from external interference.

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.

A Re-evaluation of Whether Non-monosynaptic Homonymous H Reflex Facilitation Tests Propriospinal Circuits

The C3-C4 propriospinal system is an important pathway mediating movement in cats; it contributes to movements in primates (including humans), and may have a role in recovery after lesion. Validated clinical tests of this system would find many applications, therefore we sought to test whether non-monosynaptic homonymous facilitation of the forearm flexor H reflex is mediated solely via a C3-C4 propriospinal pathway. In one anesthetized macaque monkey, median nerve stimulation elicited an H reflex in the flexor carpi radialis (FCR). Median nerve conditioning stimuli at sub-threshold intensities facilitated the H reflex, for inter-stimulus intervals up to 30 ms. Successive spinal surgical hemisections were then made. C2 lesion left the homonymous facilitation intact, suggesting mediation by spinal, not supraspinal pathways. Facilitation also remained after a second lesion at C5, indicating a major role for segmental (C7-C8) rather than propriospinal (C3-C4) interneurons. In separate experiments in five healthy human subjects, a threshold tracking approach assessed changes in peripheral axon excitability after conditioning stimulation. This was found to be enhanced up to 20 ms after the conditioning stimulus, and could partly, although not completely, underlie the H reflex facilitation seen. We conclude that homonymous facilitation of the H reflex in FCR can be produced by segmental spinal mechanisms, as well as by a supranormal period of nerve excitability. Unfortunately, this straightforward test cannot therefore be used for selective assessment of propriospinal circuits.

Ipsilateral motor pathways to the lower limb after stroke: Insights and opportunities

Stroke-related damage to the crossed lateral corticospinal tract causes motor deficits in the contralateral (paretic) limb. To restore functional movement in the paretic limb, the nervous system may increase its reliance on ipsilaterally descending motor pathways, including the uncrossed lateral corticospinal tract, the reticulospinal tract, the rubrospinal tract, and the vestibulospinal tract. Our knowledge about the role of these pathways for upper limb motor recovery is incomplete, and even less is known about the role of these pathways for lower limb motor recovery. Understanding the role of ipsilateral motor pathways to paretic lower limb movement and recovery after stroke may help improve our rehabilitative efforts and provide alternate solutions to address stroke-related impairments. These advances are important because walking and mobility impairments are major contributors to long-term disability after stroke, and improving walking is a high priority for individuals with stroke. This perspective highlights evidence regarding the contributions of ipsilateral motor pathways from the contralesional hemisphere and spinal interneuronal pathways for paretic lower limb movement and recovery. This perspective also identifies opportunities for future research to expand our knowledge about ipsilateral motor pathways and provides insights into how this information may be used to guide rehabilitation.

Ipsilateral Motor Evoked Potentials as a Measure of the Reticulospinal Tract in Age-Related Strength Changes

Background: The reticulospinal tract (RST) is essential for balance, posture, and strength, all functions which falter with age. We hypothesized that age-related strength reductions might relate to differential changes in corticospinal and reticulospinal connectivity. Methods: We divided 83 participants (age 20-84) into age groups <50 (n = 29) and ≥50 (n = 54) years; five of which had probable sarcopenia. Transcranial Magnetic Stimulation (TMS) was applied to the left cortex, inducing motor evoked potentials (MEPs) in the biceps muscles bilaterally. Contralateral (right, cMEPs) and ipsilateral (left, iMEPs) MEPs are carried by mainly corticospinal and reticulospinal pathways respectively; the iMEP/cMEP amplitude ratio (ICAR) therefore measured the relative importance of the two descending tracts. Grip strength was measured with a dynamometer and normalized for age and sex. Results: We found valid iMEPs in 74 individuals (n = 44 aged ≥50, n = 29 < 50). Younger adults had a significant negative correlation between normalized grip strength and ICAR (r = -0.37, p = 0.045); surprisingly, in older adults, the correlation was also significant, but positive (r = 0.43, p = 0.0037). Discussion: Older individuals who maintain or strengthen their RST are stronger than their peers. We speculate that reduced RST connectivity could predict those at risk of age-related muscle weakness; interventions that reinforce the RST could be a candidate for treatment or prevention of sarcopenia.

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.

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.

Changes in corticospinal excitability during the preparation phase of ballistic and ramp contractions

KEY POINTS

Changes in corticospinal excitability prior to a contraction may depend on its characteristics, including the rate of torque development. This study compared the specific modulation of cortical and spinal excitability during the preparation phase (last 500 ms before contraction) of fast (ballistic) and ramp contractions of ankle dorsiflexors, using transcranial magnetic stimulation and peripheral nerve stimulation. The results indicate earlier changes at the cortical than at the spinal level during the preparation phase of both contraction types. However, these adjustments are delayed prior to ballistic relative to ramp contractions. This study suggests that the time course of change in cortical and spinal excitability during the preparation phase of a voluntary action is specific to the intended rate of torque development of the upcoming contraction.

ABSTRACT

The present study investigated cortical and spinal excitability during the preparation phase of ballistic (BAL) and ramp (RAMP) isometric contractions. To this end, young adults performed BAL and RAMP (1500 ms torque rise time) contractions, reaching a similar torque level, with the ankle dorsiflexor muscles. Transcranial magnetic stimulation of the motor cortex was randomly applied to record motor evoked potentials (MEP) in the tibialis anterior during the last 500 ms preceding the contraction (n = 16). Short-interval intracortical inhibition (SICI; n = 10) and spinal motor neurone excitability (F-wave occurrence; n = 8) were also assessed during this period. Data were averaged over 100 ms time windows beginning 500 ms prior to the onset of contractions. An increase in MEP amplitude and a decrease in SICI were observed from the 200-100 ms and 300-200 ms time windows prior to BAL and RAMP contractions (P < 0.05), respectively, with greater changes prior to RAMP than to BAL within the 300-200 ms time window (P < 0.05). F-wave occurrence, used to assess spinal motor neurone excitability, increased prior to RAMP (200-100 ms, P < 0.05) but not BAL contractions. Data obtained in a few participants during the last 100 ms confirmed a delayed and steeper rise in corticospinal excitability prior to BAL contractions. These results indicate earlier changes at the cortical than at the spinal level, with delayed changes prior to BAL contractions. This study suggests that the time course of change in cortical and spinal excitability during the preparation phase of a voluntary action is specific to the intended rate of torque development of the upcoming contraction.

Plastic changes in primate motor cortex following paired peripheral nerve stimulation

Repeated paired stimulation of two peripheral nerves can produce lasting changes in motor cortical excitability, but little is known of the underlying neuronal basis. Here, we trained two macaque monkeys to perform selective thumb and index finger abduction movements. Neural activity was recorded from the contralateral primary motor cortex during task performance, and following stimulation of the ulnar and median nerves, and the nerve supplying the extensor digitorum communis (EDC) muscle. Responses were compared before and after 1 h of synchronous or asynchronous paired ulnar/median nerve stimulation. Task performance was significantly enhanced after asynchronous and impaired after synchronous stimulation. The amplitude of short latency neural responses to median and ulnar nerve stimulation was increased after asynchronous stimulation; later components were reduced after synchronous stimulation. Synchronous stimulation increased neural activity during thumb movement and decreased it during index finger movement; asynchronous stimulation decreased activity during both movements. To assess how well neural activity could separate behavioral or sensory conditions, linear discriminant analysis was used to decode which nerve was stimulated, or which digit moved. Decoding accuracy for nerve stimulation was decreased after synchronous and increased after asynchronous paired stimulation. Decoding accuracy for task performance was decreased after synchronous but was unchanged after asynchronous paired stimulation. Paired stimulation produces changes in motor cortical circuits that outlast the stimulation. Some of these changes depend on precise stimulus timing.

NEW & NOTEWORTHY Paired stimulation of peripheral nerves for 1 h induced lasting changes in neural responses within the motor cortex to nerve stimulation and to performance of a behavioral task. These changes were sufficient to alter the efficiency with which activity could encode stimulus type. Stimuli that can be easily applied noninvasively in human subjects can alter central motor circuits.

Comparison of Muscle MEPs From Transcranial Magnetic and Electrical Stimulation and Appearance of Reflexes in Horses

Introduction

Transcranial electrical (TES) and magnetic stimulation (TMS) are both used for assessment of the motor function of the spinal cord in horses. Muscular motor evoked potentials (mMEP) were compared intra-individually for both techniques in five healthy horses. mMEPs were measured twice at increasing stimulation intensity steps over the extensor carpi radialis (ECR), tibialis cranialis (TC), and caninus muscles. Significance was set at p < 0.05. To support the hypothesis that both techniques induce extracranially elicited mMEPs, literature was also reviewed.

Results

Both techniques show the presence of late mMEPs below the transcranial threshold appearing as extracranially elicited startle responses. The occurrence of these late mMEPs is especially important for interpretation of TMS tracings when coil misalignment can have an additional influence. Mean transcranial motor latency times (MLT; synaptic delays included) and conduction velocities (CV) of the ECR and TC were significantly different between both techniques: respectively, 4.2 and 5.5 ms (MLT _{TMS -}-MLT _TES ), and -7.7 and -9.9 m/s (CV _TMS -CV _TES ). TMS and TES show intensity-dependent latency decreases of, respectively, -2.6 (ECR) and -2.7 ms (TC)/30% magnetic intensity and -2.6 (ECR) and -3.2 (TC) ms/30V. When compared to TMS, TES shows the lowest coefficients of variation and highest reproducibility and accuracy for MLTs. This is ascribed to the fact that TES activates a lower number of cascaded interneurons, allows for multipulse stimulation, has an absence of coil repositioning errors, and has less sensitivity for varying degrees of background muscle tonus. Real axonal conduction times and conduction velocities are most closely approximated by TES.

Conclusion

Both intracranial and extracranial mMEPs inevitably carry characteristics of brainstem reflexes. To avoid false interpretations, transcranial mMEPs can be identified by a stepwise latency shortening of 15-20 ms when exceeding the transcranial motor threshold at increasing stimulation intensities. A ring block around the vertex is advised to reduce interference by extracranial mMEPs. mMEPs reflect the functional integrity of the route along the brainstem nuclei, extrapyramidal motor tracts, propriospinal neurons, and motoneurons. The corticospinal tract appears subordinate in horses. TMS and TES are interchangeable for assessing the functional integrity of motor functions of the spinal cord. However, TES reveals significantly shorter MLTs, higher conduction velocities, and better reproducibility.

A Novel Wearable Device for Motor Recovery of Hand Function in Chronic Stroke Survivors

Background. In monkey, reticulospinal connections to hand and forearm muscles are spontaneously strengthened following corticospinal lesions, likely contributing to recovery of function. In healthy humans, pairing auditory clicks with electrical stimulation of a muscle induces plastic changes in motor pathways (probably including the reticulospinal tract), with features reminiscent of spike-timing dependent plasticity. In this study, we tested whether pairing clicks with muscle stimulation could improve hand function in chronic stroke survivors. Methods. Clicks were delivered via a miniature earpiece; transcutaneous electrical stimuli at motor threshold targeted forearm extensor muscles. A wearable electronic device (WD) allowed patients to receive stimulation at home while performing normal daily activities. A total of 95 patients >6 months poststroke were randomized to 3 groups: WD with shock paired 12 ms before click; WD with clicks and shocks delivered independently; standard care. Those allocated to the device used it for at least 4 h/d, every day for 4 weeks. Upper-limb function was assessed at baseline and weeks 2, 4, and 8 using the Action Research Arm Test (ARAT), which has 4 subdomains (Grasp, Grip, Pinch, and Gross). Results. Severity across the 3 groups was comparable at baseline. Only the paired stimulation group showed significant improvement in total ARAT (median baseline: 7.5; week 8: 11.5; P = .019) and the Grasp subscore (median baseline: 1; week 8: 4; P = .004). Conclusion. A wearable device delivering paired clicks and shocks over 4 weeks can produce a small but significant improvement in upper-limb function in stroke survivors.

Multimodal stimuli modulate rapid visual responses during reaching

The reticulospinal tract plays an important role in primate upper limb function, but methods for assessing its activity are limited. One promising approach is to measure rapid visual responses (RVRs) in arm muscle activity during a visually cued reaching task; these may arise from a tecto-reticulospinal pathway. We investigated whether changes in reticulospinal excitability can be assessed noninvasively using RVRs, by pairing the visual stimuli of the reaching task with electrical stimulation of the median nerve, galvanic vestibular stimulation, or loud sounds, all of which are known to activate the reticular formation. Surface electromyogram (EMG) recordings were made from the right deltoid of healthy human subjects as they performed fast reaching movements toward visual targets. Stimuli were delivered up to 200 ms before target appearance, and RVR was quantified as the EMG amplitude in a window 75–125 ms after visual target onset. Median nerve, vestibular, and auditory stimuli all consistently facilitated the RVRs, as well as reducing the latency of responses. We propose that this facilitation reflects modulation of tecto-reticulospinal excitability, which is consistent with the idea that the amplitude of RVRs can be used to assess changes in brain stem excitability noninvasively in humans.

NEW & NOTEWORTHY Short-latency responses in arm muscles evoked during a visually driven reaching task have previously been proposed to be tecto-reticulospinal in origin. We demonstrate that these responses can be facilitated by pairing the appearance of a visual target with stimuli that activate the reticular formation: median nerve, vestibular, and auditory stimuli. We propose that this reflects noninvasive measurement and modulation of reticulospinal excitability.

The Cortical Physiology of Ipsilateral Limb Movements

Whereas voluntary movements have long been understood to derive primarily from the cortical hemisphere contralateral to a moving limb, substantial cortical activations also occur in the same-sided, or ipsilateral, cortical hemisphere. These ipsilateral motor activations have recently been shown to be useful to decode specific movement features. Furthermore, in contrast to the classical understanding that unilateral limb movements are solely driven by the contralateral hemisphere, it appears that the ipsilateral hemisphere plays an active and specific role in the planning and execution of voluntary movements. Here we review the movement-related activations observed in the ipsilateral cortical hemisphere, interpret this evidence in light of the potential roles of the ipsilateral hemisphere in the planning and execution of movements, and describe the implications for clinical populations.

Application of electrophysiological measures in spinal cord injury clinical trials: a narrative review

STUDY DESIGN

Narrative review.

OBJECTIVES

To discuss how electrophysiology may contribute to future clinical trials in spinal cord injury (SCI) in terms of: (1) improvement of SCI diagnosis, patient stratification and determination of exclusion criteria; (2) the assessment of adverse events; and (3) detection of therapeutic effects following an intervention.

METHODS

An international expert panel for electrophysiological measures in SCI searched and discussed the literature focused on the topic.

RESULTS

Electrophysiology represents a valid method to detect, track, and quantify readouts of nerve functions including signal conduction, e.g., evoked potentials testing long spinal tracts, and neural processing, e.g., reflex testing. Furthermore, electrophysiological measures can predict functional outcomes and thereby guide rehabilitation programs and therapeutic interventions for clinical studies.

CONCLUSION

Objective and quantitative measures of sensory, motor, and autonomic function based on electrophysiological techniques are promising tools to inform and improve future SCI trials. Complementing clinical outcome measures, electrophysiological recordings can improve the SCI diagnosis and patient stratification, as well as the detection of both beneficial and adverse events. Specifically composed electrophysiological measures can be used to characterize the topography and completeness of SCI and reveal neuronal integrity below the lesion, a prerequisite for the success of any interventional trial. Further validation of electrophysiological tools with regard to their validity, reliability, and sensitivity are needed in order to become routinely applied in clinical SCI trials.

High-intensity transcranial magnetic stimulation reveals differential cortical contributions to prepared responses

Corticospinal output pathways have typically been considered to be the primary driver for voluntary movements of the hand/forearm; however, more recently, reticulospinal drive has also been implicated in the production of these movements. Although both pathways may play a role, the reticulospinal tract is thought to have stronger connections to flexor muscles than to extensors. Similarly, movements involuntarily triggered via a startling acoustic stimulus (SAS) are believed to receive greater reticular input than voluntary movements. To investigate a differential role of reticulospinal drive depending on movement type or acoustic stimulus, corticospinal drive was transiently interrupted using high-intensity transcranial magnetic stimulation (TMS) applied during the reaction time (RT) interval. This TMS-induced suppression of cortical drive leads to RT delays that can be used to assess cortical contributions to movement. Participants completed targeted flexion and extension movements of the wrist in a simple RT paradigm in response to a control auditory go signal or SAS. Occasionally, suprathreshold TMS was applied over the motor cortical representation for the prime mover. Results revealed that TMS significantly increased RT in all conditions. There was a significantly longer TMS-induced RT delay seen in extension movements than in flexion movements and a greater RT delay in movements initiated in response to control stimuli compared with SAS. These results suggest that the contribution of reticulospinal drive is larger for wrist flexion than for extension. Similarly, movements triggered involuntarily by an SAS appear to involve greater reticulospinal drive, and relatively less corticospinal drive, than those that are voluntarily initiated. NEW & NOTEWORTHY Through the use of the transcranial magnetic stimulation-induced silent period, we provide novel evidence for a greater contribution of reticulospinal drive, and a relative decrease in corticospinal drive, to movements involuntarily triggered by a startle compared with voluntary movements. These results also provide support for the notion that both cortical and reticular structures are involved in the neural pathway underlying startle-triggered movements. Furthermore, our results indicate greater reticulospinal contribution to wrist flexion than extension movements.

The Relationship Between Enhanced Reticulospinal Outflow and Upper Limb Function in Chronic Stroke Patients

Background. Recent evidence from both monkey and human studies suggests that the reticulospinal tract may contribute to recovery of arm and hand function after stroke. In this study, we evaluated a marker of reticulospinal output in stroke survivors with varying degrees of motor recovery. Methods. We recruited 95 consecutive stroke patients presenting 6 months to 12 years after their index stroke, and 19 heathy control subjects. Subjects were asked to respond to a light flash with a rapid wrist flexion; at random, the flash was paired with either a quiet or loud (startling) sound. The mean difference in electromyogram response time after flash with quiet sound compared with flash with loud sound measured the StartReact effect. Upper limb function was assessed by the Action Research Arm Test (ARAT), spasticity was graded using the Modified Ashworth Scale (MAS) and active wrist angular movement using an electrogoniometer. Results. StartReact was significantly larger in stroke patients than healthy participants (78.4 vs 45.0 ms, P < .005). StartReact showed a significant negative correlation with the ARAT score and degree of active wrist movement. The StartReact effect was significantly larger in patients with higher spasticity scores. Conclusion. We speculate that in some patients with severe damage to their corticospinal tract, recovery led to strengthening of reticulospinal connections and an enhanced StartReact effect, but this did not occur for patients with milder impairment who could use surviving corticospinal connections to mediate recovery.

Full Activation Profiles and Integrity of Corticospinal Pathways in Adults With Bilateral Spastic Cerebral Palsy

Background. Dysfunction of corticospinal pathways has been implicated in motor impairments in people with bilateral spastic cerebral palsy (CP). While structural damage to corticospinal pathways in people with CP is known, its impact on the activation of these pathways is not. Objective. To provide the first, complete activation profile of corticospinal pathways in adults with CP using a full range of transcranial magnetic stimulation (TMS) intensities and voluntary contractions. Methods. TMS targeted the soleus muscle of 16 adults with bilateral spastic CP and 15 neurologically intact (NI) control participants. Activation profiles were generated using motor-evoked potentials (MEPs) produced by varying both stimulation intensity and degree of voluntary muscle activity. Anatomical integrity of corticospinal pathways was also measured with diffusion tractography. Results. Participants with CP had smaller MEPs produced by TMS at 1.2× active motor threshold during submaximal (20%) muscle activity and smaller maximal MEPs produced under any combination of stimulation intensity and voluntary muscle activity. At a fixed stimulation intensity, increasing voluntary muscle activity facilitated MEP amplitudes to a lesser degree in the participants with CP. Consistent differences in diffusion tractography suggested structural abnormalities in the corticospinal pathways of participants with CP that correlated with maximal MEPs. Conclusion. People with bilateral spastic CP have impaired activation of low and high-threshold corticospinal pathways to soleus motoneurons by TMS and reduced facilitation by voluntary activity that may be associated with structural damage to these pathways. These impairments likely contribute to impaired voluntary movement.

Neuromodulation in the restoration of function after spinal cord injury

Neuromodulation, the use of electrical interfaces to alter neuronal activity, has been successful as a treatment approach in several neurological disorders, including deep brain stimulation for Parkinson's disease and epidural spinal stimulation for chronic pain. Neuromodulation can also be beneficial for spinal cord injury, from assisting basic functions such as respiratory pacing and bladder control, through to restoring volitional movements and skilled hand function. Approaches range from electrical stimulation of peripheral muscles, either directly or via brain-controlled bypass devices, to stimulation of the spinal cord and brain. Limitations to widespread clinical application include durability of neuromodulation devices, affordability and accessibility of some approaches, and poor understanding of the underlying mechanisms. Efforts to overcome these challenges through advances in technology, together with pragmatic knowledge gained from clinical trials and basic research, could lead to personalised neuromodulatory interventions to meet the specific needs of individuals with spinal cord injury.

Hyperexcitability of brain stem pathways in cerebral palsy

Individuals with cerebral palsy (CP) experience impairments in the control of head and neck movements, suggesting dysfunction in brain stem circuitry. To examine if brain stem circuitry is altered in CP, we compared reflexes evoked in the sternocleidomastoid (SCM) muscle by trigeminal nerve stimulation in adults with CP and in age/sex-matched controls. Increasing the intensity of trigeminal nerve stimulation produced progressive increases in the long-latency suppression of ongoing SCM electromyography in controls. In contrast, participants with CP showed progressively increased facilitation around the same reflex window, suggesting heightened excitability of brain stem pathways. We also examined if there was altered activation of cortico-brain stem pathways in response to prenatal injury of the brain. Motor-evoked potentials (MEPs) in the SCM that were conditioned by a prior trigeminal afferent stimulation were more facilitated in CP compared with controls, especially in ipsilateral MEPs that are likely mediated by corticoreticulospinal pathways. In some participants with CP, but not in controls, a combined trigeminal nerve and cortical stimulation near threshold intensities produced large, long-lasting responses in both the SCM and biceps brachii muscles. We propose that the enhanced excitatory responses evoked from trigeminal and cortical inputs in CP are produced by heightened excitability of brain stem circuits, resulting in the augmented activation of reticulospinal pathways. Enhanced activation of reticulospinal pathways in response to early injury of the corticospinal tract may provide a compensated activation of the spinal cord or, alternatively, contribute to impairments in the precise control of head and neck functions.

NEW & NOTEWORTHY This is the first study to show that in adults with spastic cerebral palsy, activation of brain stem circuits by cortical and/or trigeminal afferents produces excitatory responses in anterior neck muscles compared with inhibitory responses in age/sex-matched controls. This may reflect a more excitable reticulospinal tract in response to early brain injury to provide a compensated activation of postural muscles. On the other hand, a hyperexcitable brain stem may contribute to impairments in the precise control of head and neck functions.

Classification of Neurons in the Primate Reticular Formation and Changes after Recovery from Pyramidal Tract Lesion

The reticular formation is important in primate motor control, both in health and during recovery after brain damage. Little is known about the different neurons present in the reticular nuclei. Here we recorded extracellular spikes from the reticular formation in five healthy female awake behaving monkeys (193 cells), and in two female monkeys 1 year after recovery from a unilateral pyramidal tract lesion (125 cells). Analysis of spike shape and four measures derived from the interspike interval distribution identified four clusters of neurons in control animals. Cluster 1 cells had a slow firing rate. Cluster 2 cells had narrow spikes and irregular firing, which often included high-frequency bursts. Cluster 3 cells were highly rhythmic and fast firing. Cluster 4 cells showed negative spikes. A separate population of 42 cells was antidromically identified as reticulospinal neurons in five anesthetized female monkeys. The distribution of spike width in these cells closely overlaid the distribution for cluster 2, leading us tentatively to suggest that cluster 2 included neurons with reticulospinal projections. In animals after corticospinal lesion, cells could be identified in all four clusters. The firing rate of cells in clusters 1 and 2 was increased in lesioned animals relative to control animals (by 52% and 60%, respectively); cells in cluster 2 were also more regular and more bursting in the lesioned animals. We suggest that changes in both membrane properties and local circuits within the reticular formation occur following lesioning, potentially increasing reticulospinal output to help compensate for lost corticospinal descending drive.

SIGNIFICANCE STATEMENT This work is the first to subclassify neurons in the reticular formation, providing insights into the local circuitry of this important but little understood structure. The approach developed can be applied to any extracellular recording from this region, allowing future studies to place their data within our current framework of four neural types. Changes in reticular neurons may be important to subserve functional recovery after damage in human patients, such as after stroke or spinal cord injury.

Descending Inputs to Spinal Circuits Facilitating and Inhibiting Human Wrist Flexors

Recently we reported in humans that electrical stimulation of the wrist extensor muscle extensor carpi radialis (ECR) could facilitate or suppress the H reflex elicited in flexor carpi radialis (FCR), for inter-stimulus intervals (ISIs) of 30 ms or 70 ms, respectively. The facilitation at 30 ms may be produced by both flexor afferents and extensor Ib afferents acting on a spinal circuit; the origin of the suppression at 70 ms is less certain. In this study, we investigated possible descending inputs to these systems. We used magnetic stimulation of the contralateral primary motor cortex, and click sound stimulation, to activate the corticospinal and the reticulospinal tracts respectively, and measured the effects on the H reflex conditioned by ECR stimulation. Corticospinal inputs reduced both the 30 ms facilitation and 70 ms suppression, indicating corticospinal inhibition of both circuits. By contrast, we failed to show any effect of clicks, either on the H reflex or on its modulation by ECR stimulation. This suggests that click-activated reticulospinal inputs to these circuits may be weak or absent.