Neurons in red nucleus and primary motor cortex exhibit similar responses to mechanical perturbations applied to the upper-limb during posture

Red nucleus mGluR1 and mGluR5 facilitate the development of neuropathic pain through stimulating the expressions of TNF-α and IL-1β

Our previous study has identified that glutamate in the red nucleus (RN) facilitates the development of neuropathic pain through metabotropic glutamate receptors (mGluR). Here, we further explored the actions and possible molecular mechanisms of red nucleus mGluR Ⅰ (mGluR1 and mGluR5) in the development of neuropathic pain induced by spared nerve injury (SNI). Our data indicated that both mGluR1 and mGluR5 were constitutively expressed in the RN of normal rats. Two weeks after SNI, the expressions of mGluR1 and mGluR5 were significantly boosted in the RN contralateral to the nerve injury. Administration of mGluR1 antagonist LY367385 or mGluR5 antagonist MTEP to the RN contralateral to the nerve injury at 2 weeks post-SNI significantly ameliorated SNI-induced neuropathic pain. However, unilateral administration of mGluRⅠ agonist DHPG to the RN of normal rats provoked a significant mechanical allodynia, this effect could be blocked by LY367385 or MTEP. Further studies indicated that the expressions of TNF-α and IL-1β in the RN were also elevated at 2 weeks post-SNI. Administration of mGluR1 antagonist LY367385 or mGluR5 antagonist MTEP to the RN at 2 weeks post-SNI significantly inhibited the elevations of TNF-α and IL-1β. However, administration of mGluR Ⅰ agonist DHPG to the RN of normal rats significantly enhanced the expressions of TNF-α and IL-1β, these effects were blocked by LY367385 or MTEP. These results suggest that activation of red nucleus mGluR1 and mGluR5 facilitate the development of neuropathic pain by stimulating the expressions of TNF-α and IL-1β. mGluR Ⅰ maybe potential targets for drug development and clinical treatment of neuropathic pain.

Rapid Online Corrections for Proprioceptive and Visual Perturbations Recruit Similar Circuits in Primary Motor Cortex

An important aspect of motor function is our ability to rapidly generate goal-directed corrections for disturbances to the limb or behavioral goal. The primary motor cortex (M1) is a key region involved in processing feedback for rapid motor corrections, yet we know little about how M1 circuits are recruited by different sources of sensory feedback to make rapid corrections. We trained two male monkeys (Macaca mulatta) to make goal-directed reaches and on random trials introduced different sensory errors by either jumping the visual location of the goal (goal jump), jumping the visual location of the hand (cursor jump), or applying a mechanical load to displace the hand (proprioceptive feedback). Sensory perturbations evoked a broad response in M1 with ∼73% of neurons (n = 257) responding to at least one of the sensory perturbations. Feedback responses were also similar as response ranges between the goal and cursor jumps were highly correlated (range of r = [0.91, 0.97]) as were the response ranges between the mechanical loads and the visual perturbations (range of r = [0.68, 0.86]). Lastly, we identified the neural subspace each perturbation response resided in and found a strong overlap between the two visual perturbations (range of overlap index, 0.73-0.89) and between the mechanical loads and visual perturbations (range of overlap index, 0.36-0.47) indicating each perturbation evoked similar structure of activity at the population level. Collectively, our results indicate rapid responses to errors from different sensory sources target similar overlapping circuits in M1.

Synergic control in asymptomatic welders during multi-finger force exertion and load releasing while standing

Motor synergies, i.e., neural mechanisms that organize multiple motor elements to ensure stability of actions, are affected by several neurological condition. Asymptomatic welders showed impaired synergy controlling the stability of multi-finger action compared to non-welders and this impairment was associated with microstructural damage in the globus pallidus. We further explored the effect of welding-related metal exposure on multi-finger synergy and extended our investigation to posture-stabilizing synergy during a standing task. Occupational, MRI, and performance-stabilizing synergies during multi-finger accurate force production and load releasing while standing were obtained from 29 welders and 19 age- and sex-matched controls. R2* and R1 relaxation rate values were used to estimate brain iron and manganese content, respectively, and diffusion tensor imaging was used to reflect brain microstructural integrity. Associations of brain MRI (caudate, putamen, globus pallidus, and red nucleus), and motor synergy were explored by group status. The results revealed that welders had higher R2* values in the caudate (p = 0.03), putamen (p = 0.01), and red nucleus (p = 0.08, trend) than controls. No group effect was revealed on multi-finger synergy index during steady-state phase of action (ΔVZss). Compared to controls, welders exhibited lower ΔVZss (-0.106 ± 0.084 vs. 0.160 ± 0.092, p = 0.04) and variance that did not affect the performance variable (VUCM, 0.022 ± 0.003 vs. 0.038 ± 0.007, p = 0.03) in the load releasing, postural task. The postural synergy index, ΔVZss, was associated negatively with higher R2* in the red nucleus in welders (r = -0.44, p = 0.03), but not in controls. These results suggest that the synergy index in the load releasing during a standing task may reflect welding-related neurotoxicity in workers with chronic metals exposure. This finding may have important clinical and occupational health implications.

Celsr3 Inactivation in the Brainstem Impairs Rubrospinal Tract Development and Mouse Behaviors in Motor Coordination and Mechanic-Induced Response

Inactivation of Celsr3 in the forebrain results in defects of longitudinal axonal tracts such as the corticospinal tract. In this study, we inactivated Celsr3 in the brainstem using En1-Cre mice (Celsr3 cKO) and analyzed axonal and behavioral phenotypes. Celsr3 cKO animals showed an 83% reduction of rubrospinal axons and 30% decrease of corticospinal axons in spinal segments, associated with increased branching of dopaminergic fibers in the ventral horn. Decreases of spinal motoneurons, neuromuscular junctions, and electromyographic signal amplitude of the biceps were also found in mutant animals. Mutant mice had impaired motor coordination and defective response to heavy mechanical stimulation, but no disability in walking and food pellet handling. Transsynaptic tracing demonstrated that rubrospinal axons synapse on spinal neurons in the deep layer of the dorsal horn, and mechanical stimulation of hindpaws induced strong calcium signal of red nuclei in control mice, which was less prominent in mutant mice. In conclusion, Celsr3 regulates development of spinal descending axons and the motor network in cell and non-cell autonomous manners, and the maturation of the rubrospinal system is required for motor coordination and response to mechanical stimulation.

Clinical neurophysiology of Parkinson's disease and parkinsonism

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This review is part of the series on the clinical neurophysiology of movement disorders and focuses on Parkinson’s disease and parkinsonism.

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The pathophysiology of cardinal parkinsonian motor symptoms and myoclonus are reviewed.

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The recordings from microelectrode and deep brain stimulation electrodes are reported in detail.

This review is part of the series on the clinical neurophysiology of movement disorders. It focuses on Parkinson’s disease and parkinsonism. The topics covered include the pathophysiology of tremor, rigidity and bradykinesia, balance and gait disturbance and myoclonus in Parkinson’s disease. The use of electroencephalography, electromyography, long latency reflexes, cutaneous silent period, studies of cortical excitability with single and paired transcranial magnetic stimulation, studies of plasticity, intraoperative microelectrode recordings and recording of local field potentials from deep brain stimulation, and electrocorticography are also reviewed. In addition to advancing knowledge of pathophysiology, neurophysiological studies can be useful in refining the diagnosis, localization of surgical targets, and help to develop novel therapies for Parkinson’s disease.

Rapid Feedback Responses Parallel the Urgency of Voluntary Reaching Movements

Optimal feedback control is a prominent theory used to interpret human motor behaviour. The theory posits that skilled actions emerge from control policies that link voluntary motor control (feedforward) with flexible feedback corrections (feedback control). It is clear the nervous system can generate flexible motor corrections (reflexes) when performing actions with different goals. We know little, however, about shared features of voluntary actions and feedback control in human movement. Here we reveal a link between the timing demands of voluntary actions and flexible responses to mechanical perturbations. In two experiments, 40 human participants (21 females) made reaching movements with different timing demands. We disturbed the arm with mechanical perturbations at movement onset (Experiment 1) and at locations ranging from movement onset to completion (Experiment 2). We used the resulting muscle responses and limb displacements as a proxy for the control policies that support voluntary reaching movements. We observed an increase in the sensitivity of elbow and shoulder muscle responses and a reduction in limb motion when the task imposed greater urgency to respond to the same perturbations. The results reveal a relationship between voluntary actions and feedback control as the limb was displaced less when moving faster in perturbation trials. Muscle responses scaled with changes in the displacement of the limb in perturbation trials within each timing condition. Across both experiments, human behaviour was captured by simulations based on stochastic optimal feedback control. Taken together, the results highlight flexible control that links sensory processing with features of human reaching movements.

Reactive and Proactive Adaptation of Cognitive and Motor Neural Signals during Performance of a Stop-Change Task

The ability to inhibit or suppress unwanted or inappropriate actions, is an essential component of executive function and cognitive health. The immense selective pressure placed on maintaining inhibitory control processes is exemplified by the relatively small number of instances in which these systems completely fail in the average person's daily life. Although mistakes and errors do inevitably occur, inhibitory control systems not only ensure that this number is low, but have also adapted behavioral strategies to minimize future failures. The ability of our brains to adapt our behavior and appropriately engage proper motor responses is traditionally depicted as the primary domain of frontal brain areas, despite evidence to the fact that numerous other brain areas contribute. Using the stop-signal task as a common ground for comparison, we review a large body of literature investigating inhibitory control processes across frontal, temporal, and midbrain structures, focusing on our recent work in rodents, in an effort to understand how the brain biases action selection and adapts to the experience of conflict.

Interjoint coupling of position sense reflects sensory contributions of biarticular muscles

Perception of limb position and motion combines sensory information from spindles in muscles that span one joint (monoarticulars) and two joints (biarticulars). This anatomical organization should create interactions in estimating limb position. We developed two models, one with only monoarticulars and one with both monoarticulars and biarticulars, to explore how biarticulars influence estimates of arm position in hand (x, y) and joint (shoulder, elbow) coordinates. In hand coordinates, both models predicted larger medial-lateral than proximal-distal errors, although the model with both muscle groups predicted that biarticulars would reduce this bias. In contrast, the two models made significantly different predictions in joint coordinates. The model with only monoarticulars predicted that errors would be uniformly distributed because estimates of angles at each joint would be independent. In contrast, the model that included biarticulars predicted that errors would be coupled between the two joints, resulting in smaller errors for combinations of flexion or extension at both joints and larger errors for combinations of flexion at one joint and extension at the other joint. We also carried out two experiments to examine errors made by human subjects during an arm position matching task in which a robot passively moved one arm to different positions and the subjects moved their other arm to mirror-match each position. Errors in hand coordinates were similar to those predicted by both models. Critically, however, errors in joint coordinates were only similar to those predicted by the model with monoarticulars and biarticulars. These results highlight how biarticulars influence perceptual estimates of limb position by helping to minimize medial-lateral errors.NEW & NOTEWORTHY It is unclear how sensory information from muscle spindles located within muscles spanning multiple joints influences perception of body position and motion. We address this issue by comparing errors in estimating limb position made by human subjects with predicted errors made by two musculoskeletal models, one with only monoarticulars and one with both monoarticulars and biarticulars. We provide evidence that biarticulars produce coupling of errors between joints, which help to reduce errors.

Utility of somatosensory and motor-evoked potentials in reflecting gross and fine motor functions after unilateral cervical spinal cord contusion injury

Fine motor skills are thought to rely on the integrity of ascending sensory pathways in the spinal dorsal column as well as descending motor pathways that have a neocortical origin. However, the neurophysiological processes underlying communication between the somatosensory and motor pathways that regulate fine motor skills during spontaneous recovery after spinal cord contusion injury remain unclear. Here, we established a rat model of cervical hemicontusive injury using C5 laminectomy followed by contusional displacement of 1.2 mm (mild injury) or 2.0 mm (severe injury) to the C5 spinal cord. Electrophysiological recordings were performed on the brachial muscles up to 12 weeks after injury to investigate the mechanisms by which spinal cord pathways participate in motor function. After spinal cord contusion injury, the amplitudes of somatosensory and motor-evoked potentials were reduced, and the latencies were increased. The forelimb open field locomotion test, grooming test, rearing test and Montoya staircase test revealed improvement in functions. With increasing time after injury, the amplitudes of somatosensory and motor-evoked potentials in rats with mild spinal cord injury increased gradually, and the latencies gradually shortened. In comparison, the recovery times of somatosensory and motor-evoked potential amplitudes and latencies were longer, and the recovery of motor function was delayed in rats with severe spinal cord injury. Correlation analysis revealed that somatosensory-evoked potential and motor-evoked potential parameters were correlated with gross and fine motor function in rats with mild spinal cord contusion injury. In contrast, only somatosensory-evoked potential amplitude was correlated with fine motor skills in rats with severe spinal cord injury. Our results show that changes in both somatosensory and motor-evoked potentials can reflect the changes in gross and fine motor functions after mild spinal cord contusion injury, and that the change in somatosensory-evoked potential amplitude can also reflect the change in fine motor function after severe spinal cord contusion injury. This study was approved by the Animal Ethics Committee of Nanfang Hospital, Southern Medical University, China (approval No. NFYY-2017-67) on June 11, 2017.

Red nucleus structure and function: from anatomy to clinical neurosciences

The red nucleus (RN) is a large subcortical structure located in the ventral midbrain. Although it originated as a primitive relay between the cerebellum and the spinal cord, during its phylogenesis the RN shows a progressive segregation between a magnocellular part, involved in the rubrospinal system, and a parvocellular part, involved in the olivocerebellar system. Despite exhibiting distinct evolutionary trajectories, these two regions are strictly tied together and play a prominent role in motor and non-motor behavior in different animal species. However, little is known about their function in the human brain. This lack of knowledge may have been conditioned both by the notable differences between human and non-human RN and by inherent difficulties in studying this structure directly in the human brain, leading to a general decrease of interest in the last decades. In the present review, we identify the crucial issues in the current knowledge and summarize the results of several decades of research about the RN, ranging from animal models to human diseases. Connecting the dots between morphology, experimental physiology and neuroimaging, we try to draw a comprehensive overview on RN functional anatomy and bridge the gap between basic and translational research.

Neural Signals in Red Nucleus during Reactive and Proactive Adjustments in Behavior

The ability to adjust behavior is an essential component of cognitive control. Much is known about frontal and striatal processes that support cognitive control, but few studies have investigated how motor signals change during reactive and proactive adjustments in motor output. To address this, we characterized neural signals in red nucleus (RN), a brain region linked to motor control, as male and female rats performed a novel variant of the stop-signal task. We found that activity in RN represented the direction of movement and was strongly correlated with movement speed. Additionally, we found that directional movement signals were amplified on STOP trials before completion of the response and that the strength of RN signals was modulated when rats exhibited cognitive control. These results provide the first evidence that neural signals in RN integrate cognitive control signals to reshape motor outcomes reactively within trials and proactivity across them.SIGNIFICANCE STATEMENT Healthy human behavior requires the suppression or inhibition of errant or maladaptive motor responses, often called cognitive control. While much is known about how frontal brain regions facilitate cognitive control, less is known about how motor regions respond to rapid and unexpected changes in action selection. To address this, we recorded from neurons in the red nucleus, a motor region thought to be important for initiating movement in rats performing a cognitive control task. We show that red nucleus tracks motor plans and that selectivity was modulated on trials that required shifting from one motor response to another. Collectively, these findings suggest that red nucleus contributes to modulating motor behavior during cognitive control.

Long-latency Responses to a Mechanical Perturbation of the Index Finger Have a Spinal Component

In an uncertain external environment, the motor system may need to respond rapidly to an unexpected stimulus. Limb displacement causes muscle stretch; the corrective response has multiple activity bursts, which are suggested to originate from different parts of the neuraxis. The earliest response is so fast, it can only be produced by spinal circuits; this is followed by slower components thought to arise from primary motor cortex (M1) and other supraspinal areas.

In an uncertain external environment, the motor system may need to respond rapidly to an unexpected stimulus. Limb displacement causes muscle stretch; the corrective response has multiple activity bursts, which are suggested to originate from different parts of the neuraxis. The earliest response is so fast, it can only be produced by spinal circuits; this is followed by slower components thought to arise from primary motor cortex (M1) and other supraspinal areas. Spinal cord (SC) contributions to the slower components are rarely considered. To address this, we recorded neural activity in M1 and the cervical SC during a visuomotor tracking task, in which 2 female macaque monkeys moved their index finger against a resisting motor to track an on-screen target. Following the behavioral trial, an increase in motor torque rapidly returned the finger to its starting position (lever velocity >200°/s). Many cells responded to this passive mechanical perturbation (M1: 148 of 211 cells, 70%; SC: 67 of 119 cells, 56%). The neural onset latency was faster for SC compared with M1 cells (21.7 ± 11.2 ms vs 25.5 ± 10.7 ms, respectively, mean ± SD). Using spike-triggered averaging, some cells in both regions were identified as likely premotor cells, with monosynaptic connections to motoneurons. Response latencies for these cells were compatible with a contribution to the muscle responses following the perturbation. Comparable fractions of responding neurons in both areas were active up to 100 ms after the perturbation, suggesting that both SC circuits and supraspinal centers could contribute to later response components.

SIGNIFICANCE STATEMENT Following a limb perturbation, multiple reflexes help to restore limb position. Given conduction delays, the earliest part of these reflexes can only arise from spinal circuits. By contrast, long-latency reflex components are typically assumed to originate from supraspinal centers. We recorded from both spinal and motor cortical cells in monkeys responding to index finger perturbations. Many spinal interneurons, including those identified as projecting to motoneurons, responded to the perturbation; the timing of responses was compatible with a contribution to both short- and long-latency reflexes. We conclude that spinal circuits also contribute to long-latency reflexes in distal and forearm muscles, alongside supraspinal regions, such as the motor cortex and brainstem.

Degradation of Praxis Brain Networks and Impaired Comprehension of Manipulable Nouns in Stroke

Distributed brain systems contribute to representation of semantic knowledge. Whether sensory and motor systems of the brain are causally involved in representing conceptual knowledge is an especially controversial question. Here, we tested 57 chronic left-hemisphere stroke patients using a semantic similarity judgment task consisting of manipulable and nonmanipulable nouns. Three complementary methods were used to assess the neuroanatomical correlates of semantic processing: voxel-based lesion-symptom mapping, resting-state functional connectivity, and gray matter fractional anisotropy. The three measures provided converging evidence that injury to the brain networks required for action observation, execution, planning, and visuomotor coordination are associated with specific deficits in manipulable noun comprehension relative to nonmanipulable items. Damage or disrupted connectivity of areas such as the middle posterior temporal gyrus, anterior inferior parietal lobe, and premotor cortex was related specifically to the impairment of manipulable noun comprehension. These results suggest that praxis brain networks contribute especially to the comprehension of manipulable object nouns.

Corticospinal excitability of tibialis anterior and soleus differs during passive ankle movement

The purpose of this study was to assess corticospinal excitability of soleus (SOL) and tibialis anterior (TA) at a segmental level during passive ankle movement. Four experimental components were performed to assess the effects of passive ankle movement and muscle length on corticospinal excitability (MEP/M_max) at different muscle lengths, subcortical excitability at the level of lumbar spinal segments (LEP/M_max), intracortical inhibition (SICI) and facilitation (ICF), and H-reflex in SOL and TA. In addition, the degree of fascicle length changes between SOL and TA was assessed in a subpopulation during passive ankle movement. Fascicles shortened and lengthened with joint movement during passive shortening and lengthening of SOL and TA to a similar degree (p < 0.001). Resting motor threshold was greater in SOL compared to TA (p ≤ 0.014). MEP/M_max was facilitated in TA during passive shortening relative to the static position (p ≤ 0.023) and passive lengthening (p ≤ 0.001), but remained similar during passive ankle movement in SOL (p ≥ 0.497), regardless of muscle length at the point of stimulus (p = 0.922). LEP/M_max (SOL: p = 0.075, TA: p = 0.071), SICI (SOL: p = 0.427, TA: p = 0.540), and ICF (SOL: p = 0.177, TA: p = 0.777) remained similar during passive ankle movement. H-reflex was not different across conditions in TA (p = 0.258), but was reduced during passive lengthening compared to shortening in SOL (p = 0.048). These results suggest a differential modulation of corticospinal excitability between plantar and dorsiflexors during passive movement. The corticospinal behaviour observed might be mediated by an increase in corticospinal drive as a result of reduced afferent input during muscle shortening and appears to be flexor-biased.

Shoulder reflexes integrate elbow information at "long-latency" delay throughout a corrective action

Previous studies have demonstrated a progression of function when healthy subjects counter a sudden mechanical load. Short-latency reflexes are linked to local stretch of the particular muscle and its antagonist. Long-latency reflexes integrate stretch information from both local sources and muscles crossing remote joints appropriate for a limb's mechanical interactions. Unresolved is how sensory information is processed throughout the corrective response, since capabilities at some time can be produced by circuits acting at that delay and at briefer delays. One possibility is that local abilities are always expressed at a short-latency delay and integrative abilities are always expressed at a long-latency delay. Alternatively, the neural circuits may be altered over time, leading to a temporal shift in expressing certain abilities; a refractory period could retard integrative responses to a second perturbation, whereas priming could enable integrative responses at short latency. We tested between these three hypotheses in a shoulder muscle by intermixing trials of step torque with either torque pulses ( experiment 1) or double steps of torque ( experiment 2). The second perturbation occurred at 35, 60, and 110 ms after the first perturbation to probe processing throughout the corrective action. The second perturbation reliably evoked short-latency responses in the shoulder muscle linked to only shoulder motion and long-latency responses linked to both shoulder and elbow motion. This pattern is best accounted by the continuous action of controllers with fixed functions. NEW & NOTEWORTHY Sudden displacement of the limb evokes a short-latency reflex, 20-50 ms, based on local muscle stretch and a long-latency reflex based on integrating muscle stretch at different joints. A novel double-perturbation paradigm tested if these abilities are temporally conserved throughout the corrective response or are shifted (retarded or delayed) due to functional changes in the responsible circuits. Multi-joint integration was reliably expressed at a long-latency delay consistent with the continuous operation of circuits with fixed abilities.

Long-latency reflexes for inter-effector coordination reflect a continuous state feedback controller

Successful performance in many everyday tasks requires compensating unexpected mechanical disturbance to our limbs and body. The long-latency reflex plays an important role in this process because it is the fastest response to integrate sensory information across several effectors, like when linking the elbow and shoulder or the arm and body. Despite the dozens of studies on inter-effector long-latency reflexes, there has not been a comprehensive treatment of how these reveal the basic control organization that sets constraints on any candidate model of neural feedback control such as optimal feedback control. We considered three contrasting ways that controllers can be organized: multiple independent controllers vs. a multiple-input multiple-output (MIMO) controller, a continuous feedback controller vs. an intermittent feedback controller, and a direct MIMO controller vs. a state feedback controller. Following a primer on control theory and review of the relevant evidence, we conclude that continuous state feedback control best describes the organization of inter-effector coordination by the long-latency reflex.

Mechanical perturbations can elicit triggered reactions in the absence of a startle response

Perturbations delivered to the upper limbs elicit reflexive responses in stretched muscle at short- (M1: 25-50 ms) and long- (M2: 50-100 ms) latencies. When presented in a simple reaction time (RT) task, the perturbation can also elicit a preprogrammed voluntary response at a latency (< 100 ms) that overlaps the M2 response. This early appearance of the voluntary response following a proprioceptive stimulus causing muscle stretch is called a triggered reaction. Recent work has demonstrated that a perturbation also elicits activity in sternocleidomastoid (SCM) over a time-course consistent with the startle response and it was, therefore, proposed that the StartReact effect underlies triggered reactions (Ravichandran et al., Exp Brain Res 230:59-69, 2013). The present work investigated whether perturbation-evoked SCM activity results from startle or postural control and whether triggered reactions can also occur in the absence of startle. In Experiment 1, participants "compensated" against a wrist extension perturbation. A prepulse inhibition (PPI) stimulus (known to attenuate startle) was randomly presented before the perturbation. Rather than attenuating SCM activity, the responses in SCM were advanced by the PPI stimulus. In Experiment 2, participants "assisted" a wrist extension perturbation. The perturbation did not reliably elicit startle but despite this, two-thirds of trials had RTs of less than 100 ms and the earliest responses began at ~ 70 ms. These findings suggest that SCM activity following a perturbation is the result of postural control and is not related to startle. Moreover, an overt startle response is not a prerequisite for the elicitation of a triggered reaction.

Immunohistochemical localization of ionotropic glutamate receptors in the rat red nucleus

In this study, we aimed to determine the presence as well as the diverse distribution of N-methyl-D-aspartate (NMDA) and non-NMDA glutamate receptor subunits in the rat red nucleus. Using adult Sprague-Dawley rats as the experimental animals, immunohistochemistry was performed on 30 µm thick coronal brain sections with antibodies against α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (GluA1-4), kainate (GluK1, GluK2/3, and GluK5), and NMDA (GluN1 and GluN2A) receptor subunits. The results showed that all ionotropic glutamate receptor subunits are expressed in the red nucleus. Specific staining was localized in the neuron bodies and processes. However, the pattern of immunoreactivity and the number of labeled neurons changed depending on the type of ionotropic glutamate receptor subunits and the localization of neurons in the red nucleus. The neurons localized in the magnocellular part of the red nucleus were particularly immunopositive for GluA2, GluA4, GluK2/3, GluK5, GluN1, and GluN2A receptor proteins. In the parvocellular part of the red nucleus, ionotropic glutamate receptor subunit immunoreactivity of variable intensity (lightly to moderately stained) was detected in the neurons. These results suggest that red nucleus neurons in rat heterogeneously express ionotropic glutamate receptor subunits to form functional receptor channels. In addition, the likelihood of the coexpression of different subunits in the same subgroup of neurons suggests the formation of receptor channels with diverse structure by way of different subunit combination, and the possibility of various neuronal functions through these channels in the red nucleus.

Long-latency reflexes of elbow and shoulder muscles suggest reciprocal excitation of flexors, reciprocal excitation of extensors, and reciprocal inhibition between flexors and extensors

Postural corrections of the upper limb are required in tasks ranging from handling an umbrella in the changing wind to securing a wriggling baby. One complication in this process is the mechanical interaction between the different segments of the arm where torque applied at one joint induces motion at multiple joints. Previous studies have shown the long-latency reflexes of shoulder muscles (50-100 ms after a limb perturbation) account for these mechanical interactions by integrating information about motion of both the shoulder and elbow. It is less clear whether long-latency reflexes of elbow muscles exhibit a similar capability and what is the relation between the responses of shoulder and elbow muscles. The present study utilized joint-based loads tailored to the subjects' arm dynamics to induce well-controlled displacements of their shoulder and elbow. Our results demonstrate that the long-latency reflexes of shoulder and elbow muscles integrate motion from both joints: the shoulder and elbow flexors respond to extension at both joints, whereas the shoulder and elbow extensors respond to flexion at both joints. This general pattern accounts for the inherent flexion-extension coupling of the two joints arising from the arm's intersegmental dynamics and is consistent with spindle-based reciprocal excitation of shoulder and elbow flexors, reciprocal excitation of shoulder and elbow extensors, and across-joint inhibition between the flexors and extensors.