Both dorsal horn and lamina VIII interneurones contribute to crossed reflexes from feline group II muscle afferents

Multimodal sensory control of motor performance by glycinergic interneurons of the mouse spinal cord deep dorsal horn

Sensory feedback is integral for contextually appropriate motor output, yet the neural circuits responsible remain elusive. Here, we pinpoint the medial deep dorsal horn of the mouse spinal cord as a convergence point for proprioceptive and cutaneous input. Within this region, we identify a population of tonically active glycinergic inhibitory neurons expressing parvalbumin. Using anatomy and electrophysiology, we demonstrate that deep dorsal horn parvalbumin-expressing interneuron (dPV) activity is shaped by convergent proprioceptive, cutaneous, and descending input. Selectively targeting spinal dPVs, we reveal their widespread ipsilateral inhibition onto pre-motor and motor networks and demonstrate their role in gating sensory-evoked muscle activity using electromyography (EMG) recordings. dPV ablation altered limb kinematics and step-cycle timing during treadmill locomotion and reduced the transitions between sub-movements during spontaneous behavior. These findings reveal a circuit basis by which sensory convergence onto dorsal horn inhibitory neurons modulates motor output to facilitate smooth movement and context-appropriate transitions.

Bilateral Sensorimotor Impairments in Individuals with Unilateral Chronic Ankle Instability: A Systematic Review and Meta-Analysis

BACKGROUND

Chronic ankle instability (CAI) is manifested by sensorimotor impairments in the sprained ankle, including deficits in sensation, motor function, and central integration or processing. These impairments have a significant impact on physical activities and daily life. Recently, some studies have suggested that bilateral deficits were observed in unilateral CAI, but contradictory evidence disputes this finding. Therefore, the objective of this study was to investigate whether bilateral sensorimotor deficits presented in individuals with unilateral CAI.

METHODS

Without language restriction, the following databases were retrieved from database inception up until 3 November 2023, including PubMed, WOS, EMBASE, Cochrane, SPORTDiscus and CINAHL. Case-control and cross-sectional studies that investigated bilateral sensorimotor functions in individuals with unilateral CAI were included. Sensorimotor functions contained static and dynamic balance, functional performance, muscle strength and activation, as well as sensation. Outcome measures contained centre-of-pressure parameters, normalised reach distance, activation time and magnitude of muscle, sensory errors and threshold. The risk of bias and quality assessment of included studies were evaluated using a standardised tool recommended by the Cochrane Collaboration and the Epidemiological Appraisal Instrument, respectively. To explore the potential bilateral deficits associated with unilateral CAI, a comprehensive meta-analysis was conducted using Review Manager version 5.4. The analysis compared the injured limb of unilateral CAI with healthy controls and the uninjured limb with healthy controls. The main focus of this study was to investigate the differences between the uninjured limb and healthy controls. A random-effects model was employed and effect sizes were estimated using the standardised mean difference (SMD) with 95% confidence intervals (CIs). Effect sizes were deemed as weak (0.2-0.5), moderate (0.5-0.8), or large (> 0.8).

RESULTS

A total of 11,442 studies were found; 30 studies were contained in the systematic review and 20 studies were included in the meta-analysis. Compared with healthy controls, those with unilateral CAI presented weak to moderate impairments in their uninjured limbs in static balance with eyes open (SMD = 0.32, 95% CI: 0.08 to 0.56), functional performance (SMD = 0.37; 95% CI: 0.08 to 0.67), kinesthesia (SMD = 0.52; 95% CI: 0.09 to 0.95) and tibialis anterior activation (SMD = 0.60, 95% CI: 0.19 to 1.01). There were no significant differences in other comparisons between the uninjured limb and healthy controls.

CONCLUSIONS

Patients with unilateral CAI may present bilateral deficits in static balance with eyes open, functional performance and kinaesthesia. However, further evidence is required to confirm this point due to limited studies included in some analyses and small effect size.

REGISTRATION

The protocol was registered in the International Prospective Register of Systematic Reviews platform (CRD: 42,022,375,855).

Distinct roles of spinal commissural interneurons in transmission of contralateral sensory information

Crossed reflexes are mediated by commissural pathways transmitting sensory information to the contralateral side of the body, but the underlying network is not fully understood. Commissural pathways coordinating the activities of spinal locomotor circuits during locomotion have been characterized in mice, but their relationship to crossed reflexes is unknown. We show the involvement of two genetically distinct groups of commissural interneurons (CINs) described in mice, V0 and V3 CINs, in the crossed reflex pathways. Our data suggest that the exclusively excitatory V3 CINs are directly involved in the excitatory crossed reflexes and show that they are essential for the inhibitory crossed reflexes. In contrast, the V0 CINs, a population that includes excitatory and inhibitory CINs, are not directly involved in excitatory or inhibitory crossed reflexes but downregulate the inhibitory crossed reflexes. Our data provide insights into the spinal circuitry underlying crossed reflexes in mice, describing the roles of V0 and V3 CINs in crossed reflexes.

Distinct roles of spinal commissural interneurons in transmission of contralateral sensory information

Crossed reflexes (CR) are mediated by commissural pathways transmitting sensory information to the contralateral side of the body, but the underlying network is not fully understood. Commissural pathways coordinating the activities of spinal locomotor circuits during locomotion have been characterized in mice, but their relationship to CR is unknown. We show the involvement of two genetically distinct groups of commissural interneurons (CINs) described in mice, V0 and V3 CINs, in the CR pathways. Our data suggest that the exclusively excitatory V3 CINs are directly involved in the excitatory CR, and show that they are essential for the inhibitory CR. In contrast, the V0 CINs, a population that includes excitatory and inhibitory CINs, are not directly involved in excitatory or inhibitory CRs but down-regulate the inhibitory CR. Our data provide insights into the spinal circuitry underlying CR in mice, describing the roles of V0 and V3 CINs in CR.

Crossed reflex responses to flexor nerve stimulation in mice

Motor responses in one leg to sensory stimulation of the contralateral leg have been named "crossed reflexes" and are extensively investigated in cats and humans. Despite this effort, a circuit-level understanding of the crossed reflexes has remained missing. In mice, advances in molecular genetics enabled insights into the "commissural spinal circuitry" that ensures coordinated leg movements during locomotion. Despite some common features between the commissural spinal circuitry and the circuit for the crossed reflexes, the degree to which they overlap has remained obscure. Here, we describe excitatory crossed reflex responses elicited by electrically stimulating the common peroneal nerve that mainly innervates ankle flexor muscles and the skin on anterolateral aspect of the hind leg. Stimulation of the peroneal nerve with low current intensity evoked low-amplitude motor responses in the contralateral flexor and extensor muscles. At higher current strengths, stimulation of the same nerve evoked stronger and more synchronous responses in the same contralateral muscles. In addition to the excitatory crossed reflex pathway indicated by muscle activation, we demonstrate the presence of an inhibitory crossed reflex pathway, which was modulated when the motor pools were active during walking. The results are compared with the crossed reflex responses initiated by stimulating proprioceptors from extensor muscles and cutaneous afferents from the posterior part of the leg. We anticipate that these findings will be essential for future research combining the in vivo experiments presented here with mouse genetics to understand crossed reflex pathways at the network level in vivo.NEW & NOTEWORTHY Insights into the mechanisms of crossed reflexes are essential for understanding coordinated leg movements that maintain stable locomotion. Advances in mouse genetics allow for the selective manipulation of spinal interneurons and provide opportunities to understand crossed reflexes. Crossed reflexes in mice, however, are poorly described. Here, we describe crossed reflex responses in mice initiated by stimulation of the common peroneal nerve, which serves as a starting point for investigating crossed reflexes at the cellular level.

Rapid crossed responses in an intrinsic hand muscle during perturbed bimanual movements

Mechanical perturbations in one upper limb often elicit corrective responses in both the perturbed as well as its contralateral and unperturbed counterpart. These crossed corrective responses have been shown to be sensitive to the bimanual requirements of the perturbation, but crossed responses (CRs) in hand muscles are far less well studied. Here, we investigate corrective CRs in an intrinsic hand muscle, the first dorsal interosseous (1DI), to clockwise and anticlockwise mechanical perturbations to the contralateral index finger while participants performed a bimanual finger abduction task. We found that the CRs in the unperturbed 1DI were sensitive to the direction of the perturbation of the contralateral index finger. However, the size of the CRs was not sensitive to the amplitude of the contralateral perturbation nor its context within the bimanual task. The onset latency of the CRs was too fast to be purely transcortical (<70 ms) in 12/12 participants. This confirms that during isolated bimanual finger movements, sensory feedback from one hand can influence the other, but the pathways mediating the earliest components of this interaction are likely to involve subcortical systems such as the brainstem or spinal cord, which may afford less flexibility to the task demands.NEW & NOTEWORTHY An intrinsic hand muscle shows a crossed response to a perturbation of the contralateral index finger. The crossed response is dependent on the direction of the contralateral perturbation but not on the amplitude or the bimanual requirements of the movement, suggesting a far less flexible control policy than those governing crossed responses in more proximal muscles. The crossed response is too fast to be purely mediated by transcortical pathways, suggesting subcortical contributions.

The mammalian spinal commissural system: properties and functions

Commissural systems are essential components of motor circuits that coordinate left-right activity of the skeletomuscular system. Commissural systems are found at many levels of the neuraxis including the cortex, brainstem, and spinal cord. In this review we will discuss aspects of the mammalian spinal commissural system. We will focus on commissural interneurons, which project from one side of the cord to the other and form axonal terminations that are confined to the cord itself. Commissural interneurons form heterogeneous populations and influence a variety of spinal circuits. They can be defined according to a variety of criteria including, location in the spinal gray matter, axonal projections and targets, neurotransmitter phenotype, activation properties, and embryological origin. At present, we do not have a comprehensive classification of these cells, but it is clear that cells located within different areas of the gray matter have characteristic properties and make particular contributions to motor circuits. The contribution of commissural interneurons to locomotor function and posture is well established and briefly discussed. However, their role in other goal-orientated behaviors such as grasping, reaching, and bimanual tasks is less clear. This is partly because we only have limited information about the organization and functional properties of commissural interneurons in the cervical spinal cord of primates, including humans. In this review we shall discuss these various issues. First, we will consider the properties of commissural interneurons and subsequently examine what is known about their functions. We then discuss how they may contribute to restoration of function following spinal injury and stroke.

Excitatory and inhibitory crossed reflex pathways in mice

Sensory information from one leg has been known to elicit reflex responses in the contralateral leg, known as "crossed reflexes," and these have been investigated extensively in cats and humans. Furthermore, experiments with mice have shown commissural pathways in detail by using in vitro and in vivo physiological approaches combined with genetics. However, the relationship between these commissural pathways discovered in mice and crossed reflex pathways described in cats and humans is not known. In this study, we analyzed the crossed reflex in mice by using in vivo electromyographic recording techniques combined with peripheral nerve stimulation protocols to provide a detailed description of the crossed reflex pathways. We show that excitatory crossed reflexes are mediated by both proprioceptive and cutaneous afferent activation. In addition, we provide evidence for a short-latency inhibitory crossed reflex pathway likely mediated by cutaneous feedback. Furthermore, the short-latency crossed inhibition is downregulated in the knee extensor muscle and the ankle flexor muscle during locomotion. In conclusion, this article provides an analysis of excitatory and inhibitory crossed reflex pathways during resting and locomoting mice in vivo. The data presented in this article pave the way for future research aimed at understanding crossed reflexes using genetics in mice. NEW & NOTEWORTHY We describe for the first time excitatory and inhibitory crossed reflex pathways in mouse spinal cord in vivo and show that the inhibitory pathways are modulated during walking. This is a first step toward an understanding of crossed reflexes and their function during walking using in vivo recording techniques combined with mouse genetics.

Muscle proprioceptors in adult rat: mechanosensory signaling and synapse distribution in spinal cord

The characteristic signaling and intraspinal projections of muscle proprioceptors best described in the cat are often generalized across mammalian species. However, species-dependent adaptations within this system seem necessary to accommodate asymmetric scaling of length, velocity, and force information required by the physics of movement. In the present study we report mechanosensory responses and intraspinal destinations of three classes of muscle proprioceptors. Proprioceptors from triceps surae muscles in adult female Wistar rats anesthetized with isoflurane were physiologically classified as muscle spindle group Ia or II or as tendon organ group Ib afferents, studied for their firing responses to passive-muscle stretch, and in some cases labeled and imaged for axon projections and varicosities in spinal segments. Afferent projections and the laminar distributions of provisional synapses in rats closely resembled those found in the cat. Afferent signaling of muscle kinematics was also similar to reports in the cat, but rat Ib afferents fired robustly during passive-muscle stretch and Ia afferents displayed an exaggerated dynamic response, even after locomotor scaling was accounted for. These differences in mechanosensory signaling by muscle proprioceptors may represent adaptations for movement control in different animal species.NEW & NOTEWORTHY Muscle sensory neurons signal information necessary for controlling limb movements. The information encoded and transmitted by muscle proprioceptors to networks in the spinal cord is known in detail only for the cat, but differences in size and behavior of other species challenge the presumed generalizability. This report presents the first findings detailing specializations in mechanosensory signaling and intraspinal targets for functionally identified subtypes of muscle proprioceptors in the rat.

Bilateral Proprioceptive Evaluation in Individuals With Unilateral Chronic Ankle Instability

CONTEXT

  Despite extensive research on chronic ankle instability, the findings regarding proprioception have been conflicting and focused only on the injured limb. Also, the different components of proprioception have been evaluated in isolation.

OBJECTIVE

  To evaluate bilateral ankle proprioception in individuals with unilateral ankle instability.

DESIGN

  Cohort study.

SETTING

  Research laboratory center in a university.

PATIENTS OR OTHER PARTICIPANTS

  Twenty-four individuals with a history of unilateral ankle sprain and chronic ankle instability (mechanical ankle instability group, n = 10; functional ankle instability [FAI] group, n = 14) and 20 controls.

MAIN OUTCOME MEASURE(S)

  Ankle active and passive joint position sense, kinesthesia, and force sense.

RESULTS

  We observed a significant interaction between the effects of limb and group for kinesthesia (F = 3.27, P = .049). Increased error values were observed in the injured limb of the FAI group compared with the control group (P = .031, Cohen d = 0.47). Differences were also evident for force sense (F = 9.31, P < .001): the FAI group demonstrated increased error versus the control group (injured limb: P < .001, Cohen d = 1.28; uninjured limb: P = .009, Cohen d = 0.89) and the mechanical ankle instability group (uninjured limb: P = .023, Cohen d = 0.76).

CONCLUSIONS

  Individuals with unilateral FAI had increased error ipsilaterally (injured limb) for inversion movement detection (kinesthesia) and evertor force sense and increased error contralaterally (uninjured limb) for evertor force sense.

Sensory Feedback in Interlimb Coordination: Contralateral Afferent Contribution to the Short-Latency Crossed Response during Human Walking

A constant coordination between the left and right leg is required to maintain stability during human locomotion, especially in a variable environment. The neural mechanisms underlying this interlimb coordination are not yet known. In animals, interneurons located within the spinal cord allow direct communication between the two sides without the need for the involvement of higher centers. These may also exist in humans since sensory feedback elicited by tibial nerve stimulation on one side (ipsilateral) can affect the muscles activation in the opposite side (contralateral), provoking short-latency crossed responses (SLCRs). The current study investigated whether contralateral afferent feedback contributes to the mechanism controlling the SLCR in human gastrocnemius muscle. Surface electromyogram, kinematic and kinetic data were recorded from subjects during normal walking and hybrid walking (with the legs moving in opposite directions). An inverse dynamics model was applied to estimate the gastrocnemius muscle proprioceptors’ firing rate. During normal walking, a significant correlation was observed between the magnitude of SLCRs and the estimated muscle spindle secondary afferent activity (P = 0.04). Moreover, estimated spindle secondary afferent and Golgi tendon organ activity were significantly different (P ≤ 0.01) when opposite responses have been observed, that is during normal (facilitation) and hybrid walking (inhibition) conditions. Contralateral sensory feedback, specifically spindle secondary afferents, likely plays a significant role in generating the SLCR. This observation has important implications for our understanding of what future research should be focusing on to optimize locomotor recovery in patient populations.

Computational modeling of spinal circuits controlling limb coordination and gaits in quadrupeds

Interactions between cervical and lumbar spinal circuits are mediated by long propriospinal neurons (LPNs). Ablation of descending LPNs in mice disturbs left-right coordination at high speeds without affecting fore-hind alternation. We developed a computational model of spinal circuits consisting of four rhythm generators coupled by commissural interneurons (CINs), providing left-right interactions, and LPNs, mediating homolateral and diagonal interactions. The proposed CIN and diagonal LPN connections contribute to speed-dependent gait transition from walk, to trot, and then to gallop and bound; the homolateral LPN connections ensure fore-hind alternation in all gaits. The model reproduces speed-dependent gait expression in intact and genetically transformed mice and the disruption of hindlimb coordination following ablation of descending LPNs. Inputs to CINs and LPNs can affect interlimb coordination and change gait independent of speed. We suggest that these interneurons represent the main targets for supraspinal and sensory afferent signals adjusting gait.

Interlimb Coordination During Step-to-Step Transition and Gait Performance

Most energy spent in walking is due to step-to-step transitions. During this phase, the interlimb coordination assumes a crucial role to meet the demands of postural and movement control. The authors review studies that have been carried out regarding the interlimb coordination during gait, as well as the basic biomechanical and neurophysiological principles of interlimb coordination. The knowledge gathered from these studies is useful for understanding step-to-step transition during gait from a motor control perspective and for interpreting walking impairments and inefficiency related to pathologies, such as stroke. This review shows that unimpaired walking is characterized by a consistent and reciprocal interlimb influence that is supported by biomechanical models, and spinal and supraspinal mechanisms. This interlimb coordination is perturbed in subjects with stroke.

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.

Spinal commissural connections to motoneurons controlling the primate hand and wrist

Left-right coordination is essential for locomotor movements and is partly mediated by spinal commissural systems. Such coordination is also essential for reaching and manipulation in primates, but the role of spinal commissural systems here has not been studied. We investigated commissural connectivity to motoneurons innervating forelimb muscles using intracellular recordings in acutely anesthetized macaque monkeys. In 57 of 81 motoneurons, synaptic responses (52 of 57 excitatory) were evoked after contralateral intraspinal microstimulation in the gray matter (cISMS; 300 μA maximum current intensity). Some responses (15 of 57) occurred at latencies compatible with a monosynaptic linkage, including in motoneurons projecting to intrinsic hand muscles (9 cells). Three pieces of evidence suggest that these effects reflected the action of commissural interneurons. In two cells, preceding cISMS with stimulation of the contralateral medial brainstem descending pathways facilitated the motoneuron responses, suggesting that cISMS acted on cell bodies whose excitability was increased by descending inputs. Pairing cISMS with stimulation of the contralateral corticospinal tract yielded no evidence of response occlusion in 16 cells tested, suggesting that the effects were not merely axon reflexes generated by stimulation of corticospinal axon branches, which cross the midline. Finally, stimulation of contralateral peripheral nerves evoked responses in 28 of 52 motoneurons (7 of 9 projecting to the hand). Our results demonstrate the existence of commissural neurons with access to spinal motoneurons in primate cervical spinal cord that receive inputs from the periphery as well as descending pathways. Most importantly, commissural neurons also innervate motoneurons of intrinsic hand muscles.

Crossed reflex reversal during human locomotion

During human walking, precise coordination between the two legs is required in order to react promptly to any sudden hazard that could threaten stability. The networks involved in this coordination are not yet completely known, but a direct spinal connection between soleus (SOL) muscles has recently been revealed. For this response to be functional, as previously suggested, we hypothesize that it will be accompanied by a reaction in synergistic muscles, such as gastrocnemius lateralis (GL), and that a reversal of the response would occur when an opposite reaction is required. In the present study, surface EMGs of contralateral SOL and GL were analyzed after tibial nerve (TN), sural nerve (SuN), and medial plantar nerve (MpN) stimulation during two tasks in which opposite reactions are functionally expected: normal walking (NW), just before ipsilateral heel strike, and hybrid walking (HW) (legs walking in opposite directions), at ipsilateral push off and contralateral touchdown. Early crossed facilitations were observed in the contralateral GL after TN stimulation during NW, and a reversal of such responses occurred during HW. These results underline the functional significance of short-latency crossed responses and represent the first evidence for short-latency reflex reversal in the contralateral limb for humans. Muscle afferents seem to mediate the response during NW, while during HW cutaneous afferents are likely involved. It is thus possible that different afferents mediate the crossed response during different tasks.

Absence of effects of contralateral group I muscle afferents on presynaptic inhibition of Ia terminals in humans and cats

Crossed effects from group I afferents on reflex excitability and their mechanisms of action are not yet well understood. The current view is that the influence is weak and takes place indirectly via oligosynaptic pathways. We examined possible contralateral effects from group I afferents on presynaptic inhibition of Ia terminals in humans and cats. In resting and seated human subjects the soleus (SO) H-reflex was conditioned by an electrical stimulus to the ipsilateral common peroneal nerve (CPN) to assess the level of presynaptic inhibition (PSI_control). A brief conditioning vibratory stimulus was applied to the triceps surae tendon at the contralateral side (to activate preferentially Ia muscle afferents). The amplitude of the resulting H-reflex response (PSI_conditioned) was compared to the H-reflex under PSI_control, i.e., without the vibration. The interstimulus interval between the brief vibratory stimulus and the electrical shock to the CPN was -60 to 60 ms. The H-reflex conditioned by both stimuli did not differ from that conditioned exclusively by the ipsilateral CPN stimulation. In anesthetized cats, bilateral monosynaptic reflexes (MSRs) in the left and right L(7) ventral roots were recorded simultaneously. Conditioning stimulation applied to the contralateral group I posterior biceps and semitendinosus (PBSt) afferents at different time intervals (0-120 ms) did not have an effect on the ipsilateral gastrocnemius/soleus (GS) MSR. An additional experimental paradigm in the cat using contralateral tendon vibration, similar to that conducted in humans, was also performed. No significant differences between GS-MSRs conditioned by ipsilateral PBSt stimulus alone and those conditioned by both ipsilateral PBSt stimulus and contralateral tendon vibration were detected. The present results strongly suggest an absence of effects from contralateral group I fibers on the presynaptic mechanism of MSR modulation in relaxed humans and anesthetized cats.

Changes in descending motor pathway connectivity after corticospinal tract lesion in macaque monkey

Damage to the corticospinal tract is a leading cause of motor disability, for example in stroke or spinal cord injury. Some function usually recovers, but whether plasticity of undamaged ipsilaterally descending corticospinal axons and/or brainstem pathways such as the reticulospinal tract contributes to recovery is unknown. Here, we examined the connectivity in these pathways to motor neurons after recovery from corticospinal lesions. Extensive unilateral lesions of the medullary corticospinal fibres in the pyramidal tract were made in three adult macaque monkeys. After an initial contralateral flaccid paralysis, motor function rapidly recovered, after which all animals were capable of climbing and supporting their weight by gripping the cage bars with the contralesional hand. In one animal where experimental testing was carried out, there was (as expected) no recovery of fine independent finger movements. Around 6 months post-lesion, intracellular recordings were made from 167 motor neurons innervating hand and forearm muscles. Synaptic responses evoked by stimulating the unlesioned ipsilateral pyramidal tract and the medial longitudinal fasciculus were recorded and compared with control responses in 207 motor neurons from six unlesioned animals. Input from the ipsilateral pyramidal tract was rare and weak in both lesioned and control animals, suggesting a limited role for this pathway in functional recovery. In contrast, mono- and disynaptic excitatory post-synaptic potentials elicited from the medial longitudinal fasciculus significantly increased in average size after recovery, but only in motor neurons innervating forearm flexor and intrinsic hand muscles, not in forearm extensor motor neurons. We conclude that reticulospinal systems sub-serve some of the functional recovery after corticospinal lesions. The imbalanced strengthening of connections to flexor, but not extensor, motor neurons mirrors the extensor weakness and flexor spasm which in neurological experience is a common limitation to recovery in stroke survivors.

Differential modulation of crossed and uncrossed reflex pathways by clonidine in adult cats following complete spinal cord injury

Clonidine, an α-noradrenergic agonist, facilitates hindlimb locomotor recovery after complete spinal transection (i.e. spinalization) in adult cats. However, the mechanisms involved in clonidine-induced functional recovery are poorly understood. Sensory feedback from the legs is critical for hindlimb locomotor recovery in spinalized mammals and clonidine could alter how spinal neurons respond to peripheral inputs in adult spinalized cats. To test this hypothesis we evaluated the effect of clonidine on the responses of hindlimb muscles, primarily in the left hindlimb, evoked by stretching the left triceps surae muscles and by stimulating the right tibial and superficial peroneal nerves in eight adult decerebrate cats that were spinalized 1 month before the terminal experiment. Cats were not trained following spinalization. Clonidine had no consistent effect on responses of ipsilateral muscles evoked by triceps surae muscle stretch. However, clonidine consistently potentiated the amplitude and duration of crossed extensor responses. Moreover, following clonidine injection, stretch and tibial nerve stimulation triggered episodes of locomotor-like activity in approximately one-third of trials. Differential effects of clonidine on crossed reflexes and on ipsilateral responses to muscle stretch indicate an action at a pre-motoneuronal site. We conclude that clonidine facilitates hindlimb locomotor recovery following spinalization in untrained cats by enhancing the excitability of central pattern generating spinal neurons that also participate in crossed extensor reflex transmission.

Short-latency crossed spinal responses are impaired differently in sub-acute and chronic stroke patients

OBJECTIVE

Investigate if patients with supraspinal lesions have impaired interlimb spinal reflex pathways. The short-latency crossed spinal response will be investigated during sitting from the non-paretic to paretic and paretic to non-paretic extremities at different stimulation intensities in chronic and sub-acute stroke patients.

METHODS

The ipsilateral tibial nerve of the paretic and non-paretic extremities were stimulated at motor threshold, 35% M-max and 85% M-max of the ipsilateral soleus while the contralateral soleus was contracted from 5% to 15% of the maximum voluntary contraction of the paretic soleus.

RESULTS

Chronic patients (from both extremities) had significantly less prominent inhibitory responses than healthy controls (post hoc tests: P<.01-P<.05). The responses were significantly modulated by stimulus intensity in healthy controls and chronic patients (P<.001-P<.05) but not sub-acute patients (P>.05). Some sub-acute patients had significantly more variable responses than chronic patients and healthy controls (P<.001-P⩽.05).

CONCLUSIONS

Short-latency interlimb reflexes are impaired differently in sub-acute vs. chronic patients, are impaired from the non-paretic and paretic extremity, and abnormal when compared to healthy controls.

SIGNIFICANCE

The inappropriate coordination could result in an inability to quickly avoid obstacles following a mechanical disturbance to the ipsilateral extremity. It also indicates that bilateral descending projections affect the response.

Crossed spinal soleus muscle communication demonstrated by H-reflex conditioning

INTRODUCTION

A conditioning volley to the ipsilateral tibial nerve (iTN) inhibits contralateral soleus (cSOL) electromyographic activity at latencies of 37-41 ms. This is evidence for spinal muscular communication in opposing limbs. The aim of our study was to determine whether the cSOL H-reflex would be inhibited in a similar manner.

METHODS

Thirteen subjects participated in two experiments: (1) stimuli delivered to the iTN at 85% of the maximal peak-to-peak M-wave (85% M-max) with a pre-contracted cSOL; (2) 510 stimuli delivered at 85% M-max to the iTN with a test volley delivered to the contralateral tibial nerve at interstimulus intervals of -6 to 100 ms.

RESULTS

Significant inhibition was observed in the cSOL H-reflex when conditioning stimuli were delivered 3-33 ms before the test H-reflex.

CONCLUSIONS

The activity of this spinal pathway can be quantified using H-reflex conditioning to provide a controlled model for further studies of this response.

Phase Modulation of the Short-Latency Crossed Spinal Response in the Human Soleus Muscle

Short-latency spinally mediated interlimb reflex pathways were recently reported between the left and right soleus muscles in the human lower-limb during sitting. The aim of the current study was to establish if these pathways were observed during a functional motor task such as human gait and modulated by the gait cycle phase and/or electrical stimulation intensity. The second aim was to elucidate on the afferents involved. Two interventions were investigated. First was ipsilateral tibial nerve (iTN) stimulation at motor threshold (MT), 35% of the maximal peak-to-peak M-wave (M-Max) and 85% M-Max (85M-Max) with stimuli applied at 60×, 70×, 80×, 90×, and 100% of the gait cycle of the ipsilateral leg. Second was ipsilateral sural nerve (SuN) and medial plantar nerve (MpN) stimulation at 1, 2, and 3 perceptual threshold at 90% of the gait cycle. The root mean squared (RMS) of the contralateral soleus (cSOL) responses were analyzed in a time window, 40–55 ms (or 45–60 ms for subjects >50 y/o) following iTN stimulation. The most consistent responses occurred at 90 and 100% of the gait cycle at higher stimulation intensities of the iTN. Significantly inhibitory responses ( P = 0.006) were reported at 60 versus 80% ( P = 0.03), 90% ( P = 0.006), and 100% ( P = 0.002) and 70 versus 90% ( P = 0.02) and 100% ( P = 0.009) of the gait cycle at 85M-Max. The responses became more inhibitory with increasing stimulation intensities at 80% ( P = 0.01), 90% ( P = 0.001), and 100% ( P = 0.004) of the gait cycle. Stimulation of the MpN and SuN at all stimulation intensities demonstrated no short-latency responses. Therefore, it is unlikely that afferents within these nerves contribute to the response. This is the first study to show short-latency spinally mediated responses in the cSOL following iTN stimulation, during walking. It provides evidence for a new spinal pathway contributing to motor control and demonstrates that the response likely has functional relevance.

Short-latency crossed inhibitory responses in the human soleus muscle

Even though interlimb coordination is critical in bipedal locomotion, the role of muscle afferent mediated feedback is unknown. The aim of this study was to establish if ipsilateral muscle generated afferent feedback can influence contralateral muscle activation patterns in the human lower limb and to elucidate the mechanisms involved. The effect of ipsilateral tibial nerve stimulation on contralateral soleus (cSOL) responses were quantified. Three interventions were investigated, 1) electrical stimulation applied to the tibial nerve at stimulation intensities from 0 to 100% of maximal M-wave (M-max) with the cSOL contracted from 5 to 15% of maximal voluntary contraction (MVC) and 15 to 30% MVC, 2) ispsilateral tibial nerve stimulation at 75% M-max prior to, during, and following the application of ischemia to the ipsilateral thigh. 3) Electrical stimulation applied to the ipsilateral sural (SuN) and medial plantar nerves at stimulation intensities from 1 to 3 times perceptual threshold. A short-latency depression in the cSOL electromyogram (EMG; onset: 37-41 ms) was observed following ipsilateral tibial nerve stimulation. The magnitude of this depression increased (P = 0.0005 and P = 0.000001) with increasing stimulus intensities. Ischemia delayed the time of the minimum of the cSOL depression (P = 0.04). SuN and medial plantar nerve stimulation evoked a longer latency depression [average; 91.2 ms (SuN); 142 ms (medial plantar nerve)] and therefore do not contribute to the response. This is the first study to demonstrate a short-latency depression in the cSOL following ipsilateral tibial nerve stimulation. Due to its short latency, the response is spinally mediated. The involvement of crossed spinal interneurons receiving input from low-threshold muscle afferents is discussed.

Commissural interneurons with input from group I and II muscle afferents in feline lumbar segments: neurotransmitters, projections and target cells

The aim of this study was to analyse neurotransmitter content, projection areas and target cells of commissural interneurons with input from group I and/or II muscle afferents in lumbar segments in the cat. Axonal projections of 15 intracellularly labelled commissural interneurons were reconstructed. Ten interneurons (nine located in laminae VI-VII, one in lamina VIII) were glutamatergic; only one interneuron (located in lamina VIII) was glycinergic. Contralateral terminal projections were found both in motor nuclei and within laminae VI-VIII. In order to identify target cells of commissural interneurons, effects of stimulation of contralateral group I and II muscle afferents were investigated on interneurons within these laminae. Three tests were used: intracellular records from individual interneurons, modulation of probability of activation of extracellularly recorded interneurons and modulation of their actions on motoneurons using disynaptic PSPs evoked in motoneurons as a measure. All these tests revealed much more frequent and/or stronger excitatory actions of contralateral afferents. The results indicate that commissural interneurons with input from contralateral group I and II afferents target premotor interneurons in disynaptic pathways from ipsilateral group I and II afferents and that excitatory disynaptic actions of contralateral afferents on these interneurons are mediated primarily by intermediate zone commissural interneurons. A second group of commissural interneurons activated by reticulospinal neurons, previously described, frequently had similar, but occasionally opposing, actions to the cells described here, thus indicating that these two subpopulations may act on the same premotor interneurons and either mutually enhance or counteract each other's actions.

Genetic ablation of V2a ipsilateral interneurons disrupts left-right locomotor coordination in mammalian spinal cord

The initiation and coordination of activity in limb muscles are the main functions of neural circuits that control locomotion. Commissural neurons connect locomotor circuits on the two sides of the spinal cord, and represent the known neural substrate for left-right coordination. Here we demonstrate that a group of ipsilateral interneurons, V2a interneurons, plays an essential role in the control of left-right alternation. In the absence of V2a interneurons, the spinal cord fails to exhibit consistent left-right alternation. Locomotor burst activity shows increased variability, but flexor-extensor coordination is unaffected. Anatomical tracing studies reveal a direct excitatory input of V2a interneurons onto commissural interneurons, including a set of molecularly defined V0 neurons that drive left-right alternation. Our findings imply that the neural substrate for left-right coordination consists of at least two components; commissural neurons and a class of ipsilateral interneurons that activate commissural pathways.

Presynaptic control of transmission through group II muscle afferents in the midlumbar and sacral segments of the spinal cord is independent of corticospinal control

Transmission of information from the terminals group II muscle afferents is subject to potent presynaptic modulation by both segmental group II and cutaneous afferents and by descending monoaminergic systems. Currently it is unknown whether descending corticospinal fibres affect this transmission. Here we have examined whether corticospinal tract activation modulates the size of monosynaptic focal synaptic potentials (FSPs) evoked by group II muscle afferents, and the excitability of intraspinal terminals of group II afferents, both of which are indices used to show presynaptic control. Conditioning stimulation of corticospinal pathways had no effects on the sizes of group II evoked FSPs in the midlumbar or sacral segments at either dorsal horn or intermediate zone locations. These stimuli also had no effect on the excitability of single group II afferent terminals in the dorsal horn of the midlumbar segments. As positive controls, we verified that the corticospinal conditioning stimuli used did effectively depress FSPs evoked from cutaneous afferents recorded at the same spinal locations as the group II field potentials in all experiments. Corticospinal tract conditioning stimuli did not consistently enhance or reduce the depression of group II FSPs that was evoked by stimulation of ipsilateral segmental group II or cutaneous afferents; in the large majority of cases there was no effect. The results reveal that the control of transmission of information from group II afferents in these regions of the spinal cord is independent of direct corticospinal control.

Excitatory components of the mammalian locomotor CPG

Locomotion in mammals is to a large degree controlled directly by intrinsic spinal networks, called central pattern generators (CPGs). The overall function of these networks is governed by interaction between inhibitory and excitatory neurons. In the present review, we will discuss recent findings addressing the role of excitatory synaptic transmission for network function including the role of specific excitatory neuronal populations in coordinating muscle activity and in generating rhythmic activity.

Spinal interneuronal networks in the cat: elementary components

This review summarises features of networks of commissural interneurones co-ordinating muscle activity on both sides of the body as an example of feline elementary spinal interneuronal networks. The main feature of these elementary networks is that they are interconnected and incorporated into more complex networks as their building blocks. Links between networks of commissural interneurones and other networks are quite direct, with mono- and disynaptic input from the reticulospinal and vestibulospinal neurones, disynaptic from the contralateral and ipsilateral corticospinal neurones and fastigial neurones, di- or oligosynaptic from the mesencephalic locomotor region and mono-, di- or oligosynaptic from muscle afferents. The most direct links between commissural interneurones and motoneurones are likewise simple: monosynaptic and disynaptic via premotor interneurones with input from muscle afferents. By such connections, a particular elementary interneuronal network may subserve a wide range of movements, from simple reflex and postural adjustments to complex centrally initiated phasic and rhythmic movements, including voluntary movements and locomotion. Other common features of the commissural and other interneuronal networks investigated so far is that input from several sources is distributed to their constituent neurones in a semi-random fashion and that there are several possibilities of interactions between neurones both within and between various populations. Neurones of a particular elementary network are located at well-defined sites but intermixed with neurones of other networks and distributed over considerable lengths of the spinal cord, which precludes the topography to be used as their distinguishing feature.

Nonlocomotor and locomotor hindlimb responses evoked by electrical microstimulation of the lumbar cord in spinalized cats

As a preliminary step to using intraspinal microstimulation (ISMS) for rehabilitation purposes, the distribution of various types of hindlimb responses evoked by ISMS in spinal cats (T(13)) is described. The responses to ISMS applied through a single electrode was assessed, before and after an intravenous injection of clonidine (noradrenergic agonist), using kinematics and electromyographic recordings in subacute (5-7 days, untrained) or chronic (3-5 wk trained on a treadmill) spinal cats. ISMS was applied in the dorsal, intermediate and ventral areas of segments L(3)-L(7), from midline to 3 mm laterally. Uni- and bilateral non-locomotor responses as well as rhythmical locomotor responses were evoked. In the subacute cats, ipsilateral flexion was elicited in the dorsal region of L(3)-L(7), whereas ipsilateral extension was evoked more ventrally and mainly in the caudal segments. Dorsal stimuli could induce ipsilateral flexion followed by ipsilateral extension. Sites inducing bilateral flexion and bilateral extension were similarly distributed to those evoking ipsilateral flexion and extension in the rostrocaudal axis but were evoked from more medial sites. Ipsilateral flexion with crossed extension was evoked from intermediate and ventral zones of all segments and lateralities. Unilateral ipsilateral locomotion was rarely observed. Contralateral locomotion was more frequent and mainly evoked medially, whereas bilateral locomotion was evoked exclusively from dorsal regions. With some exceptions, those distribution gradients were similar in the four conditions (subacute, chronic, pre- and postclonidine), but the proportion of each response could vary. The distribution of ISMS-evoked responses is discussed as a function of known localization of interneurons and motoneurons.

LOCOMOTOR CIRCUITS IN THE MAMMALIAN SPINAL CORD

Intrinsic spinal networks, known as central pattern generators (CPGs), control the timing and pattern of the muscle activity underlying locomotion in mammals. This review discusses new advances in understanding the mammalian CPGs with a focus on experiments that address the overall network structure as well as the identification of CPG neurons. I address the identification of excitatory CPG neurons and their role in rhythm generation, the organization of flexor-extensor networks, and the diverse role of commissural interneurons in coordinating left-right movements. Molecular and genetic approaches that have the potential to elucidate the function of populations of CPG interneurons are also discussed.

Differential projections of excitatory and inhibitory dorsal horn interneurons relaying information from group II muscle afferents in the cat spinal cord

Dorsal horn interneurons with input from group II muscle spindle afferents are components of networks involved in motor control. Thirteen dorsal horn interneurons with monosynaptic group II input were characterized electrophysiologically and labeled intracellularly with Neurobiotin. Their axonal projections were traced, and neurotransmitter content was established by using immunocytochemistry. Two subpopulations were identified: five interneurons had axons that contained vesicular glutamate transporter 2 and hence were glutamatergic and excitatory. Terminals of the remaining eight interneurons were immunoreactive for the glycine transporter 2 or were apposed to gephyrin but did not contain the GABA-synthesizing enzyme glutamic acid decarboxylase and were therefore glycinergic and inhibitory. Excitatory cells were located mainly in the central region of lamina IV and had relatively small somata and restricted dendritic trees. In contrast, inhibitory interneurons were located more ventrally, in lamina V and had relatively larger somata and more extensive dendritic trees. Axonal projections of the two subpopulations differed considerably. Excitatory interneurons predominantly projected ipsilaterally, whereas most inhibitory interneurons projected both ipsilaterally and contralaterally. Three inhibitory axons formed contacts with large cholinergic cells in motor nuclei, thus revealing a novel direct coupling between inhibitory dorsal horn interneurons and motoneurons. The organization of the excitatory interneurons is consistent with current knowledge of reflex pathways to motoneurons, but the existence and connections of the inhibitory subpopulation could not be predicted from previous data. Our results indicate that these latter interneurons exercise widespread inhibitory control over a variety of cell types located on both sides of the spinal cord.

How can corticospinal tract neurons contribute to ipsilateral movements? A question with implications for recovery of motor functions

In this review, the authors discuss some recent findings that bear on the issue of recovery of function after corticospinal tract lesions. Conventionally the corticospinal tract is considered to be a crossed pathway, in keeping with the clinical findings that damage to one hemisphere, for example, in stroke, leads to a contralateral paresis and, if the lesion is large, a paralysis. However, there has been great interest in the possibility of compensatory recovery of function using the undamaged hemisphere. There are several substrates for this including ipsilaterally descending corticospinal fibers and bilaterally operating neuronal networks. Recent studies provide important evidence bearing on both of these issues. In particular, they reveal networks of neurons interconnecting two sides of the gray matter at both brainstem and spinal levels, as well as intrahemispheric transcallosal connections. These may form "detour circuits" for recovery of function, and here the authors will consider some possibilities for exploiting these networks for motor control, even though their analysis is still at an early stage.

Projection patterns of lamina VIII commissural neurons in the lumbar spinal cord of the adult cat: an anterograde neural tracing study

This study was designed to characterize the morphology of commissural axons, with the goal of revealing some of the organizing principles of their projections in the lumbosacral cord. Axons were labeled anterogradely with biotinylated-dextran amine which was injected in the left lamina VIII and the adjoining parts of lamina VII in the lumbar segments L5-L6 in seven cats. After 3-4 weeks, commissural axons were well labeled throughout lumbosacral segments L1-S2. After crossing the midline at the injection level, labeled axons traveled rostrally and/or caudally in the contralateral ventral and lateral funiculi giving off multiple axon collaterals. The trajectories of 34 single axons were traced in their entirety from their points of origin to their distal ends. Most of these axons were thin (proximal diameter <3.5 microm) and short (<30 mm), and gave off 6 to 32 axon collaterals at short intercollateral distances (mean <2 mm) in the lumbosacral enlargement. Some thicker axons (diameter >3.5 microm) ascended as far as the thoracic level; these supplied only four to six collaterals at long intercollateral intervals ( approximately 6.5 mm). All of the axons except one projected unilaterally. The axons as a whole terminated throughout the contralateral ventral horn. However, axons that traveled in different parts of the white matter had different characteristic terminal arborizations. The collaterals of axons that traveled in the ventral funiculus terminated preferentially in laminae VII-VIII, while those in the lateral funiculus terminated in lamina IX. Although the collateral branching patterns differed from one axon to another, collaterals arising from a particular axon usually exhibited similar patterns at different rostrocaudal levels. These uniform collateral termination patterns indicate that the morphology of each neuron might be specifically related to its function. This may allow future studies to identify different functional types of commissural neurons on the basis of much less extensive reconstructions.

Short latency crossed inhibitory reflex actions evoked from cutaneous afferents

Although stimulation of cutaneous limb afferents has been shown to evoke crossed extension reflexes in unanaesthetised decerebrate or spinalised animals, here we show that stimulation of cutaneous nerves evokes crossed inhibition rather than excitation of contralateral extensor motoneurones in anaesthetised, spinal cord intact cats. Single pulse stimuli delivered to the saphenous, sural or superficial peroneal nerves evoked IPSPs in a high proportion of contralateral motoneurones including those of knee and ankle extensors. These IPSPs had thresholds of less than 1.5 times the threshold of the most excitable fibres and so large myelinated afferents contributed to them. The relative latencies of IPSPs evoked by stimulation of the contralateral superficial peroneal and sural nerves were longer than those evoked via ipsilateral pathways by approximately 1 ms, suggesting that there are at least three synaptic relays in the crossed reflexes. The IPSPs evoked by stimulation of both ipsilateral and contralateral saphenous nerves had minimal latencies suggesting at least three synaptic delays. Like IPSPs evoked by group II afferents, the frequencies of occurrence of crossed IPSPs evoked by stimulation of cutaneous afferents were significantly reduced after spinal transection and the IPSPs recorded after spinalisation were significantly smaller. These findings are consistent with the recent proposal that dorsal horn neurones, which receive input from cutaneous afferents and contact premotor commissural interneurones may mediate the crossed inhibition.

Spinal Source for the Synchronous Fluctuations of Bilateral Monosynaptic Reflexes in Cats

Successive stimuli of constant intensity applied to Ia afferents produce spinal monosynaptic reflexes (MSRs) of variable amplitude. We recorded simultaneous MSRs in the left and right L7 (or L6) ventral roots of anesthetized cats. We analyzed the cross-covariance (CCV) between the amplitudes of bilateral MSRs. Long-time series (5 to 8 h) of these bilateral MSRs exhibited transitory changes in their covariations (as measured by the zero-lag peak of their CCV), thus suggesting the existence of certain neural sources contributing to produce these changes. The aim of the present study was to show that spinal centers producing negative spontaneous cord dorsum potentials (nSCDPs) contribute to maintain correlations in the amplitude of bilateral MSRs. After spinal cord transection at the L1 segment, no significant changes were observed in the correlation between the amplitude of bilateral nSCDPs versus the amplitude of bilateral MSRs. However, this correlation, as well as the peak at zero lag in the CCV between bilateral MSRs and the CCV between bilateral nSCDPs, respectively, were abolished after a subsequent longitudinal bisection at the L1–S2 spinal segments. These results suggest that lumbar spinal neurons (bilaterally interconnected) contribute to maintain the synchronous fluctuations of bilateral MSRs.

Membrane receptors involved in modulation of responses of spinal dorsal horn interneurons evoked by feline group II muscle afferents

Modulatory actions of a metabotropic 5-HT1A&7 membrane receptor agonist and antagonist [(+/-)-8-hydroxy-2-(di-n-propylamino)-tetralin; N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclohexane-carboxamide] and an ionotropic 5-HT3 membrane receptor agonist and antagonist [2-methyl-serotonin (2-Me 5-HT); N-(1-azabicyclo[2.2.2]oct-3-yl)-6-chloro-4-methyl-3-oxo-3,4-dihydro-2H-1,4-benzoxazine-8-carboxamide hydrochloride] were investigated on dorsal horn interneurons mediating reflex actions of group II muscle afferents. All drugs were applied ionophoretically in deeply anesthetized cats. Effects of agonists were tested on extracellularly recorded responses of individual interneurons evoked by electrical stimulation of group II afferents in a muscle nerve. Effects of antagonists were tested against the depression of these responses after stimulation of raphe nuclei. The results show that both 5-HT1A&7 and 5-HT3 membrane receptors are involved in counteracting the activation of dorsal horn interneurons by group II afferents. Because only quantitative differences were found within the sample of the tested neurons, these results suggest that modulatory actions of 5-HT on excitatory and inhibitory interneurons might be similar. The relationship between 5-HT axons and axons immunoreactive for the 5-HT3A receptor subunit, which contact dorsal horn interneurons, was analyzed using immunofluorescence and confocal microscopy. Contacts from both types of axons were found on all interneurons, but their distribution and density varied, and there was no obvious relationship between them. In two of six interneurons, 5-HT3A-immunoreactive axons formed ring-like arrangements around the cell bodies. In previous studies, axons possessing 5-HT3 receptors were found to be excitatory, and as 2-Me 5-HT depressed transmission to dorsal horn interneurons, the results indicate that 5-HT operates at 5-HT3 receptors presynaptic to these neurons to depress excitatory transmission.

Relative contribution of Ia inhibitory interneurones to inhibition of feline contralateral motoneurones evoked via commissural interneurones

The aim of the study was to examine to what extent the crossed inhibition of hindlimb lumbar alpha-motoneurones is evoked via interneurones that mediate reciprocal inhibition between flexors and extensors (Ia inhibitory interneurones), and to what extent via other spinal interneurones. The crossed inhibition was evoked by reticulospinal and vestibulospinal tract fibres, stimulated in the contralateral medullary longitudinal fascicle and the lateral vestibular nucleus, respectively, or by group II muscle afferents in the contralateral quadriceps nerve. The components of the IPSPs recorded in motoneurones that were mediated by Ia inhibitory interneurones were identified by their depression following activation of Renshaw cells. Trisynaptic components of IPSPs of reticulospinal and vestibulospinal origin, and polysynaptic (but not trisynaptic) components of IPSPs from group II afferents were found to be depressed in the majority of the motoneurones, while disynaptic components, those due to direct actions of inhibitory commissural interneurones, were not depressed. These results indicate that the coordination of left and right hindlimb movements based on crossed inhibition from reticulospinal and vestibulospinal neurones, depends on the degree of activation of Ia inhibitory interneurones by muscle spindle afferents and on their inhibition by Renshaw cells. Our results also indicate that Ia inhibitory interneurones do not operate as last-order inhibitory interneurones in crossed trisynaptic pathways from group II afferents, even though they mediate inhibition evoked by interneurones in shared polysynaptic pathways of crossed flexor and extensor reflexes coactivated by group II and other high-threshold muscle, skin and joint afferents.

Functional differentiation and organization of feline midlumbar commissural interneurones

Interneurones interconnecting the two sides of the spinal cord (commissural interneurones) are critically important for interlimb coordination, but little is known about their organization. We have examined the inputs to commissural interneurones located in the midlumbar segments with projections to contralateral motor nuclei, aiming to determine whether they form distinct subpopulations. Based on intracellular records from 78 interneurones, two major non-overlapping subpopulations were identified: one monosynaptically excited by group II muscle afferents (n=10), the other monosynaptically excited by reticulospinal neurones (n=52). Monosynaptic input from group I muscle afferents and/or from vestibulospinal tract neurones was found in those with monosynaptic reticulospinal, but not group II input, and in a few other neurones (n=6). Only disynaptic input from these sources was found in the remaining 10 interneurones. Disynaptic excitatory input from ipsilateral and contralateral muscle afferents and from descending tracts was distributed less selectively and might mediate coexcitation of interneurones with monosynaptic afferent or descending input. The dominant disynaptic and polysynaptic input was, however, inhibitory. IPSPs were evoked from the descending tracts in a high proportion of the commissural interneurones that were monosynaptically excited by group II afferents (55%) and from group II afferents in a high proportion of the commissural interneurones that were monosynaptically excited by reticulospinal fibres (78%). This distribution suggests that the two subpopulations are activated differentially, rather than being coactivated, in either centrally initiated movements or reflex adjustments. This would be consistent with the previous demonstration that noradrenaline differentially affects commissural neurones of the two subpopulations.

Ipsilateral actions of feline corticospinal tract neurons on limb motoneurons

Contralateral pyramidal tract (PT) neurons arising in the primary motor cortex are the major route through which volitional limb movements are controlled. However, the contralateral hemiparesis that follows PT neuron injury on one side may be counteracted by ipsilateral of actions of PT neurons from the undamaged side. To investigate the spinal relays through which PT neurons may influence ipsilateral motoneurons, we analyzed the synaptic actions evoked by stimulation of the ipsilateral pyramid on hindlimb motoneurons after transecting the descending fibers of the contralateral PT at a low thoracic level. The results show that ipsilateral PT neurons can affect limb motoneurons trisynaptically by activating contralaterally descending reticulospinal neurons, which in turn activate spinal commissural interneurons that project back across to motoneurons ipsilateral to the stimulated pyramidal tract. Stimulation of the pyramids alone did not evoke synaptic actions in motoneurons but potently facilitated disynaptic EPSPs and IPSPs evoked by stimulation of reticulospinal tract fibers in the medial longitudinal fascicle. In parallel with this double-crossed pathway, corticospinal neurons could also evoke ipsilateral actions via ipsilateral descending reticulospinal tract fibers, acting through ipsilaterally located spinal interneurons. Because the actions mediated by commissural interneurons were found to be stronger than those of ipsilateral premotor interneurons, the study leads to the conclusion that ipsilateral actions of corticospinal neurons via commissural interneurons may provide a better opportunity for recovery of function in hemiparesis produced by corticospinal tract injury.

Effects of spinal and peripheral nerve lesions on the intersegmental synchronization of the spontaneous activity of dorsal horn neurons in the cat lumbosacral spinal cord

In the anesthetized and paralyzed cat, spontaneous negative cord dorsum potentials (nCDPs) appeared synchronously in the L3 to S1 segments, both ipsi- and contralaterally. The acute section of both the intact sural and the superficial peroneal nerve increased the variability of the spontaneous nCDPs without affecting their intersegmental coupling. On the other hand, the synchronization between the spontaneous nCDPs recorded in segments L5-L6 was strongly reduced following an interposed lesion of the left (ipsilateral) dorsolateral spinal quadrant and it was almost completely abolished by an additional lesion of the contralateral dorsolateral quadrant at the same level. Our observations support the existence of a system of spontaneously active dorsal horn neurons that is bilaterally distributed along the lumbosacral segments and affects, in a synchronized and organized manner, impulse transmission along many reflex pathways, including those mediating presynaptic inhibition.

The actions of monoamines and distribution of noradrenergic and serotoninergic contacts on different subpopulations of commissural interneurons in the cat spinal cord

Modulatory actions of monoamines were investigated on spinal commissural interneurons which coordinate left-right hindlimb muscle activity through direct projections to the contralateral motor nuclei. Commissural interneurons located in Rexed lamina VIII, with identified projections to the contralateral gastrocnemius-soleus motor nuclei, were investigated in deeply anaesthetized cats. Most interneurons had dominant input from either the reticular formation or from group II muscle afferents; a small proportion of neurons had input from both. Actions of ionophoretically applied serotonin and noradrenaline were examined on extracellularly recorded spikes evoked monosynaptically by group II muscle afferents or reticulospinal tract fibres. Activation by reticulospinal fibres was facilitated by both serotonin and noradrenaline. Activation by group II afferents was also facilitated by serotonin but was strongly depressed by noradrenaline. To investigate the possible morphological substrates of this differential modulation, seven representative commissural interneurons were labelled intracellularly with tetramethylrhodamine-dextran and neurobiotin. Contacts from noradrenergic and serotoninergic fibres were revealed by immunohistochemistry and analysed with confocal microscopy. There were no major differences in the numbers and distributions of contacts among the interneurons studied. The findings suggest that differences in modulatory actions of monoamines, and subsequent changes in the recruitment of subpopulations of commissural interneurons in various behavioural situations, depend on intrinsic interneuron properties rather than on the patterns of innervation by monoaminergic fibres. The different actions of noradrenaline on different populations of interneurons might permit reconfiguration of the actions of the commissural neurons according to behavioural context.