Interactions between spinal interneurons and ventral spinocerebellar tract neurons

Physiotherapy management focusing on proprioceptive impairment in a patient with gait and balance impairments following stroke: A case report

BACKGROUND

Proprioceptive impairment contributes to gait and balance impairments in patients with stroke. Diagnosis functional impairments and evaluation treatment efficacy require quantitative proprioception assessment. However, proprioception assessment has remained limited to ordinal scale measurement, with a lack of ratio scale measurements.

PURPOSE

This case report describes a physiotherapy management program focusing on proprioceptive impairment in patients with stroke using quantitative tests such as Threshold to Detect Passive Motion (TDPM) and Joint Position Sense (JPS).

CASE DESCRIPTION

A63-year-old male patient with an acute pontine lacunar infarction was admitted to our hospital. His muscle strength, selective movement, and trunk activity were preserved. However, the Berg Balance Scale (BBS) and Gait Assessment andIntervention Tool (GAIT) score were 42 and 9 points, observing balance impairment and the buckling knee pattern with hip ataxia during gait. Based on these, TDPM and JPS using image capture were performed. In physiotherapeuticdiagnosis, proprioceptive impairments in the hip and knee joints were the primary functional impairments related to balance and gait. To address these proprioceptive impairments, a 13-day treatment protocol incorporating transcutaneous electrical nerve stimulation (intensity: sensory threshold, frequency: 100 Hz) targeting the quadriceps femoris was performed.

OUTCOMES

The patient was discharged after achieving independent ambulation and improvement in BBS (56 points) and GAIT (2 points) scores, exceeding the minimum clinically important difference. Recovery of proprioceptive impairment corresponded withimproved balance and gait ability.

CONCLUSION

Quantitatively evaluating proprioceptive impairments may provide novel rehabilitation for patients with stroke who have proprioceptive impairments and contribute to clinical decision-making.

Activity-dependent decline and recovery of synaptic transmission in central parts of surviving primary afferents after their peripheral cut in crayfish

Axons deprived of their nucleus degenerate within a few days in mammals but survive for several months in crustaceans. However, it is not known whether central synapses from sensory axons may preserve their molecular machinery in the absence of spiking activity. To assess this, we used peripheral axotomy, which removes their nuclei combined with electrophysiology techniques and electron microscopy imaging. We report the following. (1) Electron microscopy analysis confirms previous observations that glial cell nuclei present in the sensory nerve proliferate and migrate to axon tubes, where they form close contacts with surviving axons. (2) After peripheral axotomy performed in vivo on the coxo-basipodite chordotonal organ (CBCO), the sensory nerve does not convey any sensory message, but antidromic volleys are observed. (3) Central synaptic transmission from the CBCO to motoneurons (MNs) progressively declines over 200 days (90% of monosynaptic excitatory transmission is lost after 3 weeks, whereas 60% of disynaptic inhibitory transmission persists up to 6 months). After 200 days, no transmission is observed. (4) However, this total loss is apparent only because repetitive electrical stimulation of the sensory nerve in vitro progressively restores first inhibitory post-synaptic potentials and then excitatory post-synaptic potentials. (5) The loss of synaptic transmission can be prevented by in vivo chronic sensory nerve stimulation. (6) Using simulations based on the geometric arrangements of synapses of the monosynaptic excitatory transmission and disynaptic inhibitory pathways, we show that antidromic activity in the CBCO nerve could play a role in the maintenance of synaptic function of inhibitory pathways to MNs, but not monosynaptic excitatory transmission to MNs. Our study confirms the deep changes in glial nuclei observed in axons deprived of their nucleus. We further show that the machinery for spike conduction and synaptic release persists for several months, even if there is no longer any activity. Indeed, we were able to restore, with electrical activity, spike conduction and synaptic function after long silent periods (>6 months).

Synaptic Projections of Motoneurons Within the Spinal Cord

Motoneurons have long been considered as the final common pathway of the nervous system, transmitting the neural impulses that are transduced into action.While many studies have focussed on the inputs that motoneurons receive from local circuits within the spinal cord and from other parts of the CNS, relatively few have investigated the targets of local axonal projections from motoneurons themselves, with the notable exception of those contacting Renshaw cells or other motoneurons.Recent research has not only characterised the detailed features of the excitatory connections between motoneurons and Renshaw cells but has also established that Renshaw cells are not the only target of motoneurons axons within the spinal cord. Motoneurons also form synaptic contacts with other motoneurons as well as with a subset of ventrally located V3 interneurons. These findings indicate that motoneurons cannot be simply viewed as the last relay station delivering the command drive to muscles, but perform an active role in the generation and modulation of motor patterns.

Control of Mammalian Locomotion by Somatosensory Feedback

When animals walk overground, mechanical stimuli activate various receptors located in muscles, joints, and skin. Afferents from these mechanoreceptors project to neuronal networks controlling locomotion in the spinal cord and brain. The dynamic interactions between the control systems at different levels of the neuraxis ensure that locomotion adjusts to its environment and meets task demands. In this article, we describe and discuss the essential contribution of somatosensory feedback to locomotion. We start with a discussion of how biomechanical properties of the body affect somatosensory feedback. We follow with the different types of mechanoreceptors and somatosensory afferents and their activity during locomotion. We then describe central projections to locomotor networks and the modulation of somatosensory feedback during locomotion and its mechanisms. We then discuss experimental approaches and animal models used to investigate the control of locomotion by somatosensory feedback before providing an overview of the different functional roles of somatosensory feedback for locomotion. Lastly, we briefly describe the role of somatosensory feedback in the recovery of locomotion after neurological injury. We highlight the fact that somatosensory feedback is an essential component of a highly integrated system for locomotor control. © 2021 American Physiological Society. Compr Physiol 11:1-71, 2021.

Spinal lumbar dI2 interneurons contribute to stability of bipedal stepping

Peripheral and intraspinal feedback is required to shape and update the output of spinal networks that execute motor behavior. We report that lumbar dI2 spinal interneurons in chicks receive synaptic input from afferents and premotor neurons. These interneurons innervate contralateral premotor networks in the lumbar and brachial spinal cord, and their ascending projections innervate the cerebellum. These findings suggest that dI2 neurons function as interneurons in local lumbar circuits, are involved in lumbo-brachial coupling, and that part of them deliver peripheral and intraspinal feedback to the cerebellum. Silencing of dI2 neurons leads to destabilized stepping in P8 hatchlings, with occasional collapses, variable step profiles and a wide-base walking gait, suggesting that dI2 neurons may contribute to the stabilization of the bipedal gait.

Relative Contribution of Proprioceptive and Vestibular Sensory Systems to Locomotion: Opportunities for Discovery in the Age of Molecular Science

Locomotion is a fundamental animal behavior required for survival and has been the subject of neuroscience research for centuries. In terrestrial mammals, the rhythmic and coordinated leg movements during locomotion are controlled by a combination of interconnected neurons in the spinal cord, referred as to the central pattern generator, and sensory feedback from the segmental somatosensory system and supraspinal centers such as the vestibular system. How segmental somatosensory and the vestibular systems work in parallel to enable terrestrial mammals to locomote in a natural environment is still relatively obscure. In this review, we first briefly describe what is known about how the two sensory systems control locomotion and use this information to formulate a hypothesis that the weight of the role of segmental feedback is less important at slower speeds but increases at higher speeds, whereas the weight of the role of vestibular system has the opposite relation. The new avenues presented by the latest developments in molecular sciences using the mouse as the model system allow the direct testing of the hypothesis.

Feedback to the future: motor neuron contributions to central pattern generator function

Motor behaviors depend on neural signals in the brain. Regardless of where in the brain behavior patterns arise, the central nervous system sends projections to motor neurons, which in turn project to and control temporally appropriate muscle contractions; thus, motor neurons are traditionally considered the last relay from the central nervous system to muscles. However, in an array of species and motor systems, an accumulating body of evidence supports a more complex role of motor neurons in pattern generation. These studies suggest that motor neurons not only relay motor patterns to the periphery, but directly contribute to pattern generation by providing feedback to upstream circuitry. In spinal and hindbrain circuits in a variety of animals - including flies, worms, leeches, crustaceans, rodents, birds, fish, amphibians and mammals - studies have indicated a crucial role for motor neuron feedback in maintaining normal behavior patterns dictated by the activity of a central pattern generator. Hence, in this Review, we discuss literature examining the role of motor neuron feedback across many taxa and behaviors, and set out to determine the prevalence of motor neuron participation in motor circuits.

Evidence for a Supraspinal Contribution to the Human Crossed Reflex Response During Human Walking

In humans, an ipsilateral tibial nerve (iTN) stimulation elicits short-latency-crossed-responses (SLCR) comprised of two bursts in the contralateral gastrocnemius lateralis (cGL) muscle. The average onset latency has been reported to be 57-69 ms with a duration of 30.4 ± 6.6 ms. The aim of this study was to elucidate if a transcortical pathway contributes to the SLCR. In Experiment 1 (n = 9), single pulse supra-threshold transcranial magnetic stimulation (supraTMS) was applied alone or in combination with iTN stimulation (85% of the maximum M-wave) while participants walked on a treadmill (delay between the SLCR and the motor evoked potentials (MEP) varied between -30 and 200 ms). In Experiment 2 (n = 6), single pulse sub-threshold TMS (subTMS) was performed and the interstimulus interval (ISI) varied between 0-30 ms. In Experiment 3, somatosensory evoked potentials (SEPs) were recorded during the iTN stimulation to quantify the latency of the resulting afferent volley at the cortical level. SLCRs and MEPs in cGL occurred at 63 ± 6 ms and 29 ± 2 ms, respectively. The mean SEP latency was 30 ± 3 ms. Thus, a transcortical pathway could contribute no earlier than 62-69 ms (SEP+MEP+central-processing-delay) after iTN stimulation. Combined iTN stimulation and supraTMS resulted in a significant MEP extra-facilitation when supraTMS was timed so that the MEP would coincide with the late component of the SLCR, while subTMS significantly depressed this component. This is the first study that demonstrates the existence of a strong cortical control on spinal pathways mediating the SLCR. This likely serves to enhance flexibility, ensuring that the appropriate output is produced in accord with the functional demand.

Is the cross-over effect of a unilateral high-intensity leg extension influenced by the sex of the participants?

Background

While performing a unilateral muscle contraction, electrical muscle activity also arises in the contralateral homologous muscle, muscle group, or limb. When the muscle contraction induces muscle fatigue, females show not only a greater resistance than males but also a reduced contralateral muscle activation. The study aimed at investigating whether, during a high-intensity 30-s unilateral maximal effort isometric leg extension exercise, the contralateral non-exercising limb (NEL) knee extensor muscle activation would differ between females and males.

Methods

Twenty participants, 11 females (23.80 ± 2.15 years old) and 9 males (26.50 ± 2.45 years old), performed a unilateral 30-s exercise while surface electromyography (sEMG) was measured from the vastus lateralis (VL), vastus medialis (VM), and rectus femoris (RF) on both limbs. The maximal voluntary contraction (MVC) was measured for both the exercising limb (EL) and the NEL before (MVC PRE) and after (MVC POST) the 30-s exercise to assess muscle fatigue.

Results

While both females and males exhibited muscle fatigue in the EL (p = 0.015), females exhibited a lower MVC reduction than males (p = 0.042), suggesting that females were less fatigued than males. Although no muscle fatigue, i.e., no MVC force reduction was found in the NEL for either group before and after the 30-s exercise, the muscle activity of the VL was found to be of greater magnitude during the MVC POST only for females (p = 0.047) while it remained unchanged for males. During the 30-s exercise, the force output of the EL decreased only for males (p = 0.029) while females showed a preservation of the force output (p > 0.05). The sEMG activity of the NEL during the 30-s unilateral exercise increased for both groups in all measured muscles (all p-values < 0.03).

Conclusions

Likely, different underlying muscle fatigue mechanisms occurred in the EL between females and males. Yet, our findings suggest that the cross-over effect to the NEL during the 30-s exercise occurred in a similar fashion in both groups. The current study suggests that the contralateral muscle activation seen with a unilateral exercise is independent of the sex of individuals. Therefore, unilateral training or rehabilitation-based protocols would similarly impact females and males.

Motoneurons regulate the central pattern generator during drug-induced locomotor-like activity in the neonatal mouse

Motoneurons are traditionally viewed as the output of the spinal cord that do not influence locomotor rhythmogenesis. We assessed the role of motoneuron firing during ongoing locomotor-like activity in neonatal mice expressing archaerhopsin-3 (Arch), halorhodopsin (eNpHR), or channelrhodopsin-2 (ChR2) in Choline acetyltransferase neurons (ChAT⁺) or Arch in LIM-homeodomain transcription factor Isl1⁺ neurons. Illumination of the lumbar cord in mice expressing eNpHR or Arch in ChAT⁺ or Isl1⁺ neurons, depressed motoneuron discharge, transiently decreased the frequency, and perturbed the phasing of the locomotor-like rhythm. When the light was turned off motoneuron firing and locomotor frequency both transiently increased. These effects were not due to cholinergic neurotransmission, persisted during partial blockade of gap junctions and were mediated, in part, by AMPAergic transmission. In spinal cords expressing ChR2, illumination increased motoneuron discharge and transiently accelerated the rhythm. We conclude that motoneurons provide feedback to the central pattern generator (CPG) during drug-induced locomotor-like activity.

DOI:http://dx.doi.org/10.7554/eLife.26622.001

Parallel processing of internal and external feedback in the spinocerebellar system of primates

Cerebellar control of voluntary movements is achieved by the integration of external and internal feedback information to adjust and correct properly ongoing actions. In the forelimb of primates, rostral-spinocerebellar tract (RSCT) neurons are thought to integrate segmental, descending, and afferent sources and relay upstream a compound signal that contains both an efference copy of the spinal-level motor command and the state of the periphery. We tested this hypothesis by implanting stimulating electrodes in the superior cerebellar peduncle and recording the activity of cervical spinal neurons in primates. To dissociate motor commands and proprioceptive signals, we used a voluntary wrist task and applied external perturbations to the movement. We identified a large group of antidromically activated RSCT neurons located in deep dorsal sites and a smaller fraction of postsynaptically activated (PSA) cells located in intermediate and ventral laminae. RSCT cells received sensory input from broad, proximally biased receptive fields (RFs) and were not affected by applied wrist perturbations. PSA cells received sensory information from distal RFs and were more strongly related to active and passive movements. The anatomical and functional properties of RSCT and PSA cells suggest that descending signals converging on PSA cells contribute to both motor preparation and motor control. In parallel, RSCT neurons relay upstream an integrated signal that encodes the state of working muscles and can contribute to distal-to-proximal coordination of action. Thus the rostral spinocerebellar system sends upstream an efference copy of the motor command but does not signal abrupt errors in the performed movement.NEW & NOTEWORTHY Cerebellar coordination of voluntary movements relies on integrating feedback information to update motor output. With the use of a novel protocol, we identified spinal neurons constituting the ascending and descending components of the forelimb spinocerebellar system in behaving primates. The data suggest that descending information contributes to both motor preparation and execution, whereas ascending information conveys the spinal level motor command, such that internal and external feedback is relayed through parallel pathways.

Effects of acute spinalization on neurons of postural networks

Postural limb reflexes (PLRs) represent a substantial component of postural corrections. Spinalization results in loss of postural functions, including disappearance of PLRs. The aim of the present study was to characterize the effects of acute spinalization on two populations of spinal neurons (F and E) mediating PLRs, which we characterized previously. For this purpose, in decerebrate rabbits spinalized at T12, responses of interneurons from L5 to stimulation causing PLRs before spinalization, were recorded. The results were compared to control data obtained in our previous study. We found that spinalization affected the distribution of F- and E-neurons across the spinal grey matter, caused a significant decrease in their activity, as well as disturbances in processing of posture-related sensory inputs. A two-fold decrease in the proportion of F-neurons in the intermediate grey matter was observed. Location of populations of F- and E-neurons exhibiting significant decrease in their activity was determined. A dramatic decrease of the efficacy of sensory input from the ipsilateral limb to F-neurons, and from the contralateral limb to E-neurons was found. These changes in operation of postural networks underlie the loss of postural control after spinalization, and represent a starting point for the development of spasticity.

Skilled forelimb movements and internal copy motor circuits

Mammalian skilled forelimb movements are remarkable in their precision, a feature that emerges from the continuous adjustment of motor output. Here we discuss recent progress in bridging the gap between theory and neural implementation in understanding the basis of forelimb motor refinement. One influential theory is that feedback from internal copy motor pathways enables fast prediction, through a forward model of the limb, an idea supported by behavioral studies that have explored how forelimb movements are corrected online and can adapt to changing conditions. In parallel, neural substrates of forelimb internal copy pathways are coming into clearer focus, in part through the use of genetically tractable animal models to isolate spinal and cerebellar circuits and explore their contributions to movement.

Spinal circuits for motor learning

Studies of motor learning have largely focussed on the cerebellum, and have provided key concepts about neural circuits required. However, other parts of the nervous system are involved in learning, as demonstrated by the capacity to 'train' spinal circuits to produce locomotion following spinal cord injury. While somatosensory feedback is necessary for spinal motor learning, feed forward circuits within the spinal cord must also contribute. In fact, motoneurons themselves could act as comparators that integrate feed forward and feedback inputs, and thus contribute to motor learning. Application of cerebellar-derived principles to spinal circuitry leads to testable predictions of spinal organization required for motor learning.

Internal and External Feedback Circuits for Skilled Forelimb Movement

Skilled motor behavior emerges from interactions between efferent neural pathways that induce muscle contraction and feedback systems that report and refine movement. Two broad classes of feedback projections modify motor output, one from the periphery and a second that originates within the central nervous system. The mechanisms through which these pathways influence movement remain poorly understood, however. Here we discuss recent studies that delineate spinal circuitry that binds external and internal feedback pathways to forelimb motor behavior. A spinal presynaptic inhibitory circuit regulates the strength of external feedback, promoting limb stability during goal-directed reaching. A distinct excitatory propriospinal circuit conveys copies of motor commands to the cerebellum, establishing an internal feedback loop that rapidly modulates forelimb motor output. The behavioral consequences of manipulating these two circuits reveal distinct controls on motor performance and provide an initial insight into feedback strategies that underlie skilled forelimb movement.

Putative spinal interneurons mediating postural limb reflexes provide a basis for postural control in different planes

The dorsal-side-up trunk orientation in standing quadrupeds is maintained by the postural system driven mainly by somatosensory inputs from the limbs. Postural limb reflexes (PLRs) represent a substantial component of this system. Earlier we described spinal neurons presumably contributing to the generation of PLRs. The first aim of the present study was to reveal trends in the distribution of neurons with different parameters of PLR-related activity across the gray matter of the spinal cord. The second aim was to estimate the contribution of PLR-related neurons with different patterns of convergence of sensory inputs from the limbs to stabilization of body orientation in different planes. For this purpose, the head and vertebral column of the decerebrate rabbit were fixed and the hindlimbs were positioned on a platform. Activity of individual neurons from L5 to L6 was recorded during PLRs evoked by lateral tilts of the platform. In addition, the neurons were tested by tilts of the platform under only the ipsilateral or only the contralateral limb, as well as during in-phase tilts of the platforms under both limbs. We found that, across the spinal gray matter, strength of PLR-related neuronal activity and sensory input from the ipsilateral limb decreased in the dorsoventral direction, while strength of the input from the contralateral limb increased. A near linear summation of tilt-related sensory inputs from different limbs was found. Functional roles were proposed for individual neurons. The obtained data present the first characterization of posture-related spinal neurons, forming a basis for studies of postural networks impaired by injury.

Identification of interneurons activated at different inclines during treadmill locomotion in adult rats

By using c-fos as an activity-dependent marker, we identified the cholinergic interneurons around the central canal and lumbar interneurons throughout the gray matter that were activated after a 30-min bout of quadrupedal treadmill stepping at a 0° or 25° incline in adult rats. Increased loading (elevated treadmill incline) imposed during treadmill stepping activated more cholinergic interneurons in the proximity of the central canal, i.e., central canal cluster cells and partition neurons. Since cholinergic central canal cells are thought to modulate motoneuron excitability, these data suggest that increased load during stepping may increase motoneuronal activity through activating more cholinergic central canal cells. We identified the muscle-specific motoneurons and afferent terminals in the spinal cord by injecting cholera toxin subunit B in the soleus and tibialis anterior muscles. The number of interneurons in lumbar segments L4 (tibialis anterior) and L5 (soleus) was higher in both groups that stepped on the treadmill compared with control and was highest in rats that stepped at a 25° incline. In a majority of laminae, the distribution of total and muscle-specific activated interneurons was highest in the 25° incline group and lowest in the control group for both muscles. These data could reflect increased peripheral (proprioceptive) input as well as supraspinal drive associated with stepping and demonstrate the differences in 1) the activation of cholinergic interneurons near the central canal and 2) the laminar and segmental location of interneurons throughout the gray matter that play a role in generating stepping under different loading conditions in adult rats.

Motor-circuit communication matrix from spinal cord to brainstem neurons revealed by developmental origin

Accurate motor-task execution relies on continuous comparison of planned and performed actions. Motor-output pathways establish internal circuit collaterals for this purpose. Here we focus on motor collateral organization between spinal cord and upstream neurons in the brainstem. We used a newly developed mouse genetic tool intersectionally with viruses to uncover the connectivity rules of these ascending pathways by capturing the transient expression of neuronal subpopulation determinants. We reveal a widespread and diverse network of spinal dual-axon neurons, with coincident input to forelimb motor neurons and the lateral reticular nucleus (LRN) in the brainstem. Spinal information to the LRN is not segregated by motor pool or neurotransmitter identity. Instead, it is organized according to the developmental domain origin of the progenitor cells. Thus, excerpts of most spinal information destined for action are relayed to supraspinal centers through exquisitely organized ascending connectivity modules, enabling precise communication between command and execution centers of movement.