Article

Spastic Long-Lasting Reflexes in the Awake Rat After Sacral Spinal Cord Injury

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Abstract

Following chronic sacral spinal cord transection in rats the affected tail muscles exhibit marked spasticity, with characteristic long-lasting tail spasms evoked by mild stimulation. The purpose of the present paper was to characterize the long-lasting reflex seen in tail muscles in response to electrical stimulation of the tail nerves in the awake spastic rat, including its development with time and relation to spasticity. Before and after sacral spinal transection, surface electrodes were placed on the tail for electrical stimulation of the caudal nerve trunk (mixed nerve) and for recording EMG from segmental tail muscles. In normal and acute spinal rats caudal nerve trunk stimulation evoked little or no EMG reflex. By 2 wk after injury, the same stimulation evoked long-lasting reflexes that were 1) very low threshold, 2) evoked from rest without prior EMG activity, 3) of polysynaptic latency with >6 ms central delay, 4) about 2 s long, and 5) enhanced by repeated stimulation (windup). These reflexes produced powerful whole tail contractions (spasms) and developed gradually over the weeks after the injury (< or =52 wk tested), in close parallel to the development of spasticity. Pure low-threshold cutaneous stimulation, from electrical stimulation of the tip of the tail, also evoked long-lasting spastic reflexes, not seen in acute spinal or normal rats. In acute spinal rats a strong C-fiber stimulation of the tip of the tail (20 x T) could evoke a weak EMG response lasting about 1 s. Interestingly, when this C-fiber stimulation was used as a conditioning stimulation to depolarize the motoneuron pool in acute spinal rats, a subsequent low-threshold stimulation of the caudal nerve trunk evoked a 300-500 ms long reflex, similar to the onset of the long-lasting reflex in chronic spinal rats. A similar conditioned reflex was not seen in normal rats. Thus there is an unusually long low-threshold polysynaptic input to the motoneurons (pEPSP) that is normally inhibited by descending control. This pEPSP is released from inhibition immediately after injury but does not produce a long-lasting reflex because of a lack of motoneuron excitability. With chronic injury the motoneuron excitability is increased markedly, and the pEPSP then triggers sustained motoneuron discharges associated with long-lasting reflexes and muscle spasms.

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... At the acute stage of SCI, the long EPSPs do not activate voltage-gated persistent inward currents (PICs) in virtually unexcitable motoneurons (Li et al. 2004a). In contrast, at the chronic stage of SCI, these exaggerated long EPSPs, in turn, activate the uncontrolled large PICs in hyperexcitable motoneurons, ultimately driving muscle spasms in rats (Bennett et al. 2004;Li et al. 2004a). Interestingly, before injury the same stimulation for the long EPSPs instead suppresses ongoing muscle activity (Bennett et al. 2004), suggesting the predominance of inhibitory control of sensory transmission in the intact spinal cord. ...
... In contrast, at the chronic stage of SCI, these exaggerated long EPSPs, in turn, activate the uncontrolled large PICs in hyperexcitable motoneurons, ultimately driving muscle spasms in rats (Bennett et al. 2004;Li et al. 2004a). Interestingly, before injury the same stimulation for the long EPSPs instead suppresses ongoing muscle activity (Bennett et al. 2004), suggesting the predominance of inhibitory control of sensory transmission in the intact spinal cord. Unlike 5-HT, glutamate is an intrinsically produced transmitter in the spinal cord and thus still contributes to locomotion mediated by NMDA receptors in the spinal cord after SCI (Chau et al. 2002;Giroux et al. 2003). ...
... The dorsal roots were stimulated with 0.2-ms electrical pulses at four stimulus intensities, i.e., 1ϫ (3-10 A), 2ϫ, 5ϫ, and 10ϫ, to reliably evoke mono-and polysynaptic ventral root reflexes. Reflex threshold is comparable with afferent threshold (Bennett et al. 2004). At a given intensity, the dorsal roots were stimulated five times at intervals of 30 s to avoid reflex depression. ...
Article
Spinal cord injury (SCI) results in a loss of serotonin (5-HT) to the spinal cord and a loss of inhibition to deep dorsal horn (DDH) neurons, which produces an exaggerated excitatory drive to motoneurons. The mechanism of this excitatory drive could involve the DDH neurons triggering long excitatory postsynaptic potentials (EPSPs) in motoneurons, which may ultimately drive muscle spasms. Modifying the activity of DDH neurons with drugs such as NMDA or the 5-HT1B/1D receptor agonist zolmitriptan could have a large effect on motoneuron activity and, therefore, on muscle spasms. In this study, we characterize the firing properties of DDH neurons following acute spinal transection in adult mice during administration of zolmitriptan and NMDA, using the in vitro sacral cord preparation and extracellular electrophysiology. DDH neurons can be categorized into three major types with distinct evoked and spontaneous firing characteristics: burst (bursting), simple (single-spiking), and tonic (spontaneously tonic-firing) neurons. The burst neurons likely contribute to muscle spasm mechanisms due to their bursting behavior. Only the burst neurons show significant changes in their firing characteristics during zolmitriptan and NMDA administration. Zolmitriptan suppresses the burst neurons by reducing their evoked spikes, burst duration, and spontaneous firing rate. Conversely, NMDA facilitates them by enhancing their burst duration and spontaneous firing rate. These results suggest that zolmitriptan may exert its anti-spastic effect on the burst neurons via activation of 5-HT1B/1D receptors, whereas activation of NMDA receptors may facilitate the burst neurons in contributing to muscle spasm mechanisms following SCI.
... Spinal cord injury (SCI) often leads to uncontrolled spastic activity in muscles innervated by motoneurons 54 below the injury, including prolonged muscle spasms triggered by brief sensory inputs (Bennett et To evaluate spasticity after SCI, we employed a sacral spinal cord (S2) transection in mice that specifically 89 affects the axial tail musculature and enables pronounced spasms to be evoked by cutaneous stimulation 90 (Bennett et al. 1999;Bennett et al. 2004). Importantly, the spinal cord caudal to this sacral injury is mostly 91 < 500 µm in radius, allowing good optical access to V3 neurons (light penetration drops sharply beyond 92 300 µm) and also enabling the entire adult sacral spinal cord to remain viable in vitro (Bellardita et The S2 sacral spinal transections were performed using a modified procedure previously described for 127 adult rats (Bennett et al. 1999;Murray et al. 2010). ...
... We next examined whether reductions in V3 activity reduced 392 spasms. Previously we have noted that sensory evoked spasms measured in vivo and in vitro decrease 393 spontaneously over time when tested repeatedly (Bennett et al. 2004;Li et al. 2004b). This also occurred in 394 the present mice, decreasing over a couple hours in vitro (Fig 2 and 3 are from the same mouse, but 395 separated by two hours; tested every 10 mins over a few hours). ...
... Also, the dominant early flexor activity (S3) 340 followed by later extensor activity (S4; Figs 1 -3) corresponds to the dominant flexion spasms that coil the 341 tail in these chronic spinal mice (Bennett et al. 1999; Bennett et al. 2004) (see Fig 8 detailed later). 342 343In contrast, uninjured Sim1//ChR2 mice did not exhibit long-lasting ventral root responses to either light or 344 dorsal root stimulation (responses were < 50 ms long ,Fig 2D), consistent with a lack of spasms in intact 345 animals(Bennett et al. 1999;Bennett et al. 2004). Also, light had no effect on control Sim1-Cre mice not 346 expressing ChR2 (-/-,Fig 2D)with or without SCI.347 ...
Article
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Spinal cord injury leads to a devastating loss of motor function and yet is accompanied by a paradoxical emergence of muscle spasms, which often involve complex muscle activation patterns across multiple joints, reciprocal muscle timing, and rhythmic clonus. We investigated the hypothesis that spasms are a manifestation of partially recovered function in spinal central pattern-generating (CPG) circuits that normally coordinate complex postural and locomotor functions. We focused on the commissural propriospinal V3 neurons that coordinate interlimb movements during locomotion and examined mice with a chronic spinal transection. When the V3 neurons were optogenetically activated with a light pulse, a complex coordinated pattern of motoneuron activity was evoked with reciprocal, crossed, and intersegmental activity. In these same mice, brief sensory stimulation evoked spasms with a complex pattern of activity very similar to that evoked by light, and the timing of these spasms was readily reset by activation of V3 neurons. Given that V3 neurons receive abundant sensory input, these results suggest that sensory activation of V3 neurons is alone sufficient to generate spasms. Indeed, when we silenced V3 neurons optogenetically, sensory evoked spasms were inhibited. Also, inhibiting general CPG activity by blocking N-methyl-d-aspartate (NMDA) receptors inhibited V3 evoked activity and associated spasms, whereas NMDA application did the opposite. Furthermore, overwhelming the V3 neurons with repeated optogenetic stimulation inhibited subsequent sensory evoked spasms, both in vivo and in vitro. Taken together, these results demonstrate that spasms are generated in part by sensory activation of V3 neurons and associated CPG circuits. NEW & NOTEWORTHY We investigated whether locomotor-related excitatory interneurons (V3) play a role in coordinating muscle spasm activity after spinal cord injury (SCI). Unexpectedly, we found that these neurons not only coordinate reciprocal motor activity but are critical for initiating spasms, as well. More generally, these results suggest that V3 neurons are important in initiating and coordinating motor output after SCI and thus provide a promising target for restoring residual motor function.
... Spasticity in tail muscles was measured with percutaneous EMG wires inserted in segmental tail muscles at the midpoint of the tail, as described by Bennett et al. (2004) and adapted to mouse. During EMG recording, muscle spasms were evoked with mechanical stimulation of the tail skin, and the tail was free to move. ...
... To explore this hypothesis, we used an electromyographical protocol adapted from Bennett et al. (2004). This objective and quantitative electromyographical method appears correlated with clinical evaluation of spasticity and facilitates the study of the effect of classical anti-spastic drugs such as baclofen or clonidine (Li et al., 2004;Rank et al., 2011). ...
Thesis
Amyotrophic lateral sclerosis (ALS) is a fatal degenerative disease characterized by loss of upper and lower motor neurons, denervation and skeletal muscle atrophy. ALS is accompanied by metabolic alterations that are early events in mouse models for ALS. The main objective was to identify molecular targets responsible for these alterations. For this, we analyzed several metabolic regulators localized in presymptomatic glycolytic muscle tissue of an ALS mouse model, the SOD1G86R. We identified a pre-symptomatic alteration of metabolic equilibrium, showing an inhibition of glycolysis accompanied by an upregulation of lipid catabolic pathway. This alteration has functional significance, being reflected in a modified capacity of SOD1G86R mice to adapt to different types of exercise. Pharmacological inhibition of PDK4, one of the main inhibitors of glycolysis, delayed disease onset, underpinning the importance of metabolic equilibrium in disease progression. Taking into consideration the metabolic specificity of the different elements on the neuromuscular axis, this work opens towards new therapeutic approaches for ALS.
... This exaggerated monosynaptic reflex is characteristic of SCI and is thought to contribute to the production of clonus and hyperreflexia (Nielsen et al. 2007). In addition to exaggerated phasic responses, long-lasting tonic responses can also be present (Murray et al. 2011b;Li et al. 2004;Bennett et al. 2004Bennett et al. , 1999 and contribute to triggering spasms in SCI patients (Norton et al. 2008). When slow stretches (60 mm/s, 3 mm, 50 ms rising time) were applied to chronic SCI animals, little or no EMG activity was elicited (not shown). ...
... It is unclear, however, whether this mainly involves motoneurons (Fig. 6) or interneurons (Chen et al. 2018). CLP257 not only reduced the exaggerated monosynaptic reflexes that contribute to clonus and hyperreflexia (De Serres et al. 2002;Bennett et al. 1996), but also attenuated the long-lasting polysynaptic responses that contribute to hypertonia and the initiation of spasms (Murray et al. 2011b;Li et al. 2004;Bennett et al. 1999Bennett et al. , 2004. This suggests an effect on multiple spinal pathways, monosynaptic and/or polysynaptic, that play a role in spasticity D'Amico et al. 2013a, b;Murray et al. 2010Murray et al. , 2011a. ...
Article
After spinal cord injury (SCI), the majority of individuals develop spasticity, a debilitating condition involving involuntary movements, co-contraction of antagonistic muscles, and hyperreflexia. By acting on GABAergic and Ca²⁺-dependent signaling, current anti-spastic medications lead to serious side effects, including a drastic decrease in motoneuronal excitability which impairs motor function and rehabilitation efforts. Exercise, in contrast, decreases spastic symptoms without decreasing motoneuron excitability. These functional improvements coincide with an increase in expression of the chloride co-transporter KCC2 in lumbar motoneurons. Thus, we hypothesized that spastic symptoms can be alleviated directly through restoration of chloride homeostasis and endogenous inhibition by increasing KCC2 activity. Here, we used the recently developed KCC2 enhancer, CLP257, to evaluate the effects of acutely increasing KCC2 extrusion capability on spastic symptoms after chronic SCI. Sprague Dawley rats received a spinal cord transection at T12 and were either bike-trained or remained sedentary for 5 weeks. Increasing KCC2 activity in the lumbar enlargement improved the rate-dependent depression of the H-reflex and reduced both phasic and tonic EMG responses to muscle stretch in sedentary animals after chronic SCI. Furthermore, the improvements due to this pharmacological treatment mirror those of exercise. Together, our results suggest that pharmacologically increasing KCC2 activity is a promising approach to decrease spastic symptoms in individuals with SCI. By acting to directly restore endogenous inhibition, this strategy has potential to avoid severe side effects and improve the quality of life of affected individuals.
... Several animal studies have used awake behaving electrophysiological recordings associated with forelimb function [31][32][33][34]. Electrophysiological measurements of hyperreflexia after SCI are usually performed in the anesthetized or restrained state [35,36]. Awake behaving electrophysiological recordings of muscle function across multiple assessments are needed to determine whether these hyperreflexive states contribute to diminished sensorimotor function or are an independent source of sensorimotor impairment. ...
... In spite of reduced volitional EMG activation, triceps EMG increased during reflexive forelimb withdrawal indicating hyperreflexia. Previous work using anesthetized or restrained preparations have shown electrophysiological signatures of hyperreflexia in muscle groups below a SCI in-vivo [35,36]. Additional studies using spinal invitro preparations confirm that the observed hyperreflexia in-vivo is significantly contributed to by increased plateau potentials and monoamine supersensitivity [58,59]. ...
... There are several proposed mechanisms by which hyperreflexia arise such as; changes to chloride homeostasis, imbalance of excitatory and inhibitory signals converging on motor circuitry, alterations in dendritic arbours and innate increases in motor neuron excitability 17,25,26,57,95,96 . NT3 has been shown to modulate a number of these through a mix of in vivo and in vitro studies, which may not have occurred in this study and enabled persistent hyperreflexia. ...
... This is likely caused by technical refinements made over time, such as less repositioning of the recording electrode which would reduce bleeding. Accordingly, we conclude there is no evidence for either SCI or NT3 treatment altering the excitability of motor axons, however additional electrophysiology would need to be performed to confirm this such as threshold tracking 19,101 or measuring of persistent inward currents 96 . ...
Preprint
Traumatic spinal cord injury (SCI) in humans occurs most frequently in the cervical spine where it can cause substantial sensorimotor impairments to upper limb function. The altered input to spinal circuits below the lesion leads to maladaptive reorganisation which often leads to hyperreflexia in proprioceptive circuits. Neurotrophin 3 (NT3) is growth factor essential for the development of proprioceptive neurons. We have previously shown that following bilateral corticospinal tract axotomy, intramuscular delivery of an Adeno-Associated Viral vector encoding NT3 (AAV-NT3) induces proprioceptive circuit reorganisation linked to functional recovery. To assess its therapeutic effects following a clinically relevant bilateral C5-C6 contusion in rats, AAV-NT3 was injected intramuscularly into the dominant limb 24 hours after injury and forelimb function was assessed over 13 weeks. The injury generated hyperreflexia of a distal forelimb proprioceptive circuit. There was also loss of fine motor skills during reach-and-grasp and walking on a horizontal ladder. Ex vivo magnetic resonance imaging (MRI) revealed atrophy of the spinal cord and white matter disruption throughout the lesion site together with extensive loss of grey matter. Unexpectedly, animals treated with AAV-NT3 had a slightly smaller lesion in the regions close to the epicentre compared to PBS treated animals. Rats treated with AAV-NT3 showed subtly better performance on the horizontal ladder and transient benefits on reach-and-grasp. AAV-NT3 did not normalise hyperreflexia in a treated muscle. The treatment increased the amount of NT3 in treated muscles but, unexpectedly, serum levels were only elevated in a small subset of animals. These results show that this dose and delivery of AAV-NT3 may generate subtle improvements in locomotion but additional treatments will be required to overcome the widespread sensorimotor deficits caused by contusion injury.
... Hyperactivity is a pathologic condition found in spinal reflex pathways. Situations that have prolonged periods of depolarization are justified by the alteration of the L-type calcium channels located in the peripheral motoneuron (Bennett et al., 1999;Bennett et al., 2004). In spastic striated musculature, altered L-type calcium channels are also found. ...
... These control mechanisms of excess Ca2+ are also found in modified myoblasts (Jorquera et al., 2013). However, the amount of Ca2+ depends directly on the descendants of motor neuron impulses that SP is changed as result of injury (Bennett et al., 1999;Bennett et al., 2004) besides himself excessive Ca2+ L-type channel this releasing Ca2+ excessively. Continuity of this excessive activity extends to the striated muscle which also presents its Ca2+ L-type channel (Smith et al., 2009). ...
Article
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Essential oil of Alpinia zerumbet (EOAz) demonstrated action to improve the spastic muscle contraction performance in patients with cerebral vascular disease. Stroke, Cerebral Palsy and Spinal Cord Injury part of in the Pyramidal Syndrome (PS) features muscle tone change. Purpose of this study was investigate contraction muscle quality in the treatment with EOAz and its with Kinesiotherapy association in the patients with Syndrome Pyramidal. This study is a Clinical trial type II, monocentric, prospective randomized parallel-group, with volunteers (N = 60) adults with spasticity. Modification of the Modified Ashworth Scale (MMAS), Surface Electromyography (sEMG) and Systemic Arterial Pressure (SAP) were evaluated, before and after application of essential oil (33%) in the dose 05mL/2Kg or 0,05mL/4Kgin cases oftetraparesis, hemiparesis or paraparesis during 10 days of procedures. Results showed significantly decrease spasticity in pathological legs during best muscle contraction (ANOVA or Kruskal-Wallis test, p<0,05). In conclusion, association of kinesiotherapy with EOAz demonstrates efficiency in improving muscle recruitment of people with spasticity who need to perform motor rehabilitation.
... To date, spastic behaviours that have been examined in rat models include tail spasms and clonus (Bennett et al., 2004(Bennett et al., , 1999 and spontaneous hindlimb and tail spasms during swimming (Gonzenbach et al., 2010). One main goal of our study was to model a variety of signs of forelimb spasticity in awake, freely moving rats. ...
... Other groups have modelled spasticity in rodents, which has advanced the understanding of underlying causes of spasticity and the development of novel potential therapies (Gonzenbach et al., 2010;Boulenguez et al., 2010;Corleto et al., 2015). After thoracic cord transection or contusion, rats show lower limb spasticity (Cô té et al., 2011;Thompson et al., 1998;Bose et al., 2002;Corleto et al., 2015;Yates et al., 2008) and after sacral cord transection rodents displayed tail spasms (Bennett et al., 2004;Kapitza et al., 2012;Murray et al., 2010). A midthoracic T-lesion, which essentially damages the dorsomedial, dorsolateral and ventromedial parts of the corticospinal tracts, results in hindlimb spasms during swimming (Gonzenbach et al., 2010). ...
Article
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Brain and spinal injury reduce mobility and often impair sensorimotor processing in the spinal cord leading to spasticity. Here, we establish that complete transection of corticospinal pathways in the pyramids impairs locomotion and leads to increased spasms and excessive mono- and polysynaptic low threshold spinal reflexes in rats. Treatment of affected forelimb muscles with an adeno-associated viral vector (AAV) encoding human Neurotrophin-3 at a clinically-feasible time-point after injury reduced spasticity. Neurotrophin-3 normalized the short latency Hoffmann reflex to a treated hand muscle as well as low threshold polysynaptic spinal reflexes involving afferents from other treated muscles. Neurotrophin-3 also enhanced locomotor recovery. Furthermore, the balance of inhibitory and excitatory boutons in the spinal cord and the level of an ion co-transporter in motor neuron membranes required for normal reflexes were normalized. Our findings pave the way for Neurotrophin-3 as a therapy that treats the underlying causes of spasticity and not only its symptoms.
... Long lasting reflex (LLR) was determined and quantitated as previously described 16 . Briefly, tail muscle spasticity was performed with percutaneous EMG wires inserted in segmental tail muscles as described for rats 22 . Paralyzed mice were kept in a chamber during the recording with the tail free to move. ...
... The major consequence of the rescue of serotonin neurons in G37R ∆Tph2 mice was a complete absence of spasticity. This was obvious from visual inspection of tail spasticity and quantified using electrophysiological characterization of long lasting reflexes in the tail muscles, a well-documented measure of spasticity in rodents 9,22 . We observed that G37R ∆Tph2 mice developed earlier paralysis onset, associated with decreased muscle strength. ...
Article
Objective: Spasticity occurs in a wide range of neurological diseases, including neurodegenerative diseases, after trauma or after stroke and is characterized by increased reflexes leading to muscle hypertonia. Spasticity is a painful symptom and can severely restrict everyday life, but might also participate in maintaining a low level of motor function in severely impaired patients. Constitutive activity of the serotonin receptors 5-HT2B/C is required for the development of spasticity after spinal cord injury or during amyotrophic lateral sclerosis (ALS). We sought here to provide direct evidence for a role of brainstem serotonin neurons in spasticity. Methods: SOD1(G37R) mice expressing a conditional allele of an ALS-linked SOD1 mutation were crossed to Tph2-CRE mice expressing CRE in serotonergic neurons. Measurement of long lasting reflex using EMG, behavioural follow up and histological techniques were used to characterize spasticity and motor phenotype. Results: Deleting mutant SOD1 expression selectively in brainstem serotonin neurons was sufficient to rescue loss of TPH2 immunoreactivity and largely preserve serotonin innervation of motor neurons in the spinal cord. Furthermore, this abrogated constitutive activity of 5-HT2B/C receptors and abolished spasticity in end-stage mice. Consistent with spasticity mitigating motor symptoms, selective deletion worsened motor function and accelerated the onset of paralysis. Interpretation: Degeneration of serotonin neurons is necessary to trigger spasticity through the 5-HT2B/C receptor. The wide range of drugs targeting the serotonergic system could be useful to treat spasticity in neurological diseases. This article is protected by copyright. All rights reserved.
... The pathophysiology of spasticity has been well studied in rodent models mainly using complete spinal cord transection models, wherein supraspinal input is completely disconnected beyond the lesion [8,9]. These studies have provided insights into spasticity mechanisms, including exaggerated spinal reflexes, alternations in synapses, and changes in the expression of motor neuron receptors after SCI [10][11][12][13][14][15][16]. However, inflammatory tissue reactions and the spared neural network, which exerts a critical influence on the spinal circuitry after injury, differ greatly between the complete spinalized model and the contusive model with incomplete paralysis [17][18][19]. ...
Article
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Spasticity is a frequent chronic complication in individuals with spinal cord injury (SCI). However, the severity of spasticity varies in patients with SCI. Therefore, an evaluation method is needed to determine the severity of spasticity. We used a contusive SCI model that is suitable for clinical translation. In this study, we examined the feasibility of the swimming test and an EMG for evaluating spasticity in a contusive SCI rat model. Sprague-Dawley rats received an injury at the 8th thoracic vertebra. Swimming tests were performed 3 to 6 weeks after SCI induction. We placed the SCI rats into spasticity-strong or spasticity-weak groups based on the frequency of spastic behavior during the swimming test. Subsequently, we recorded the Hoffman reflex (H-reflex) and examined the immunoreactivity of serotonin (5-HT) and its receptor (5-HT2A) in the spinal tissues of the SCI rats. The spasticity-strong group had significantly decreased rate-dependent depression of the H-reflex compared to the spasticity-weak group. The area of 5-HT2A receptor immunoreactivity was significantly increased in the spasticity-strong group. Thus, both electrophysiological and histological evaluations indicate that the spasticity-strong group presented with a more severe upper motor neuron syndrome. We also observed the groups in their cages for 20 hours. Our results suggest that the swimming test provides an accurate evaluation of spasticity in this contusive SCI model. We believe that the swimming test is an effective method for evaluating spastic behaviors and developing treatments targeting spasticity after SCI.
... Adaptations in motor neuron properties have been shown to contribute to this reflex hyperexcitability. After complete transection at the sacrolumbar junction, sacral motor neurons become hyperexcitable, resulting in muscle spasms and longlasting reflexes ( Bennett et al. 2004). This hyperexcitability is partly produced by an upregulation in the expression of constitutively active 5-HT 2C receptors, leading to amplification of synaptic inputs and production of muscle spasms from sensory stimuli ( . ...
Article
After spinal cord injury (SCI), reflexes become hyperexcitable, leading to debilitating muscle spasms and compromised motor function. Previous work has described adaptations in spinal systems that might underlie this hyperexcitability, including an increase in constitutively active 5-HT2C receptors in spinal motoneurons. That work, however, examined adaptations following complete transection SCI, whereas SCI in humans is usually anatomically and functionally incomplete. We therefore evaluated whether constitutive activity of 5-HT2C receptors contributes to reflex hyperexcitability in an incomplete compression model of SCI and to spasms in vitro and in vivo. Our results confirm that 5-HT2C receptor constitutive activity contributes to reflex excitability after incomplete SCI. We also evaluated whether constitutive activity could be altered by manipulation of neural activity levels after SCI, testing the hypothesis that it reflects homeostatic processes acting to maintain spinal excitability. We decreased neural activity after SCI by administering baclofen and increased activity by administering the selective serotonin reuptake inhibitor (SSRI) fluoxetine. We found that drug administration produced minimal alterations in in vivo locomotor function or reflex excitability. Similarly, we found that neither baclofen nor fluoxetine altered the contribution of constitutively active 5-HT2C receptors to reflexes after SCI, although the contribution of 5-HT2C receptors to reflex activity was altered after SSRIs. These results confirm the importance of constitutive activity in 5-HT2C receptors to spinal hyperexcitability following SCI in the clinically relevant case of incomplete SCI but suggest that this activity is not driven by homeostatic processes that act to maintain overall levels of spinal excitability. NEW & NOTEWORTHY After spinal cord injury (SCI), most people will develop muscle spasms below their level of injury that can severely impact function. In this work, we examine the adaptations that occur within the spinal cord after SCI that contribute to these motor dysfunctions. We also evaluate one hypothesis about how these adaptations develop, which will potentially lead to intervention strategies to improve functional outcomes in persons with SCI.
... In such a hyperexcitable state, glutamate released from spinal interneurons could ultimately elicit muscle spasms. Indeed, involuntary motor activity and spasticity is a frequent symptom of chronic SCI, and in in vitro models of SCI, dorsal root stimulation evokes unusually long NMDA receptor-dependent excitatory postsynaptic potentials (Bennett et al. 2004). ...
Article
In a recent publication, Thaweerattanasinp and colleagues investigated spinal cord injury and firing properties of deep dorsal horn neurons during NMDA or Zolmitriptan application by employing electrophysiology in an in vitro spinal cord preparation. Deep dorsal horn neurons were classified into bursting, simple or tonic, with bursting neurons showing NMDA and Zolmitriptan sensitivity. Here, we discuss the findings in a methodological framework and propose future experiments of importance for translating the results into physiological settings.
... Êàê áûëî âûÿâëåíî íà ìîäåëè òðàâìû ñïèííîãî ìîçãà ó êðûñ, à ïîçaeå âåðèôèöèðîâàíî ó ëþäåé ñ àíà-ëîãè÷íûìè ïîâðåaeäåíèÿìè, ñïàçì ñêåëåòíîé ìóñêóëàòóðû ðåàëèçóåòñÿ, â áîëüøåé ìåðå, çà ñ÷åò 2 ôàêòîðîâ: ïîâûøåííîé âîçáóäèìîñòè ìîòîíåéðîíîâ, êîòîðàÿ ïàðàäîêñàëüíî ðàçâèâàåòñÿ â òå÷åíèå ìåñÿöåâ ïîñëå ðàçðûâà ñïèííîãî ìîçãà, à òàêaeå èç-çà äåôèöèòà èíãèáèðóþùåãî êîíòðîëÿ íàä ñåíñîðíîé àôôåðåíòíîé ïåðå-äà÷åé, âåäóùåãî ê èçáûòî÷íîé ñòèìóëÿöèè ìîòîíåéðîíîâ [6,46]. Îáíàðóaeåíî, ÷òî ÀÐ ðåãóëèðóþò âîçáóäèìîñòü ìîòîíåéðîíîâ è ñåíñîðíóþ ñèíàïòè÷åñêóþ ïåðåäà÷ó, òàêèì îáðàçîì îòâå÷àÿ è çà âîçíèêíîâåíèå ñïàñòè÷íîñòè ó ëþäåé ñî ñïèííîìîçãîâûìè òðàâìàìè [22]. ...
Article
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Disturbances of the motor function caused by diseases and injuries of the nervous system lead to severe disorders, which dramatically decrease the quality of patient's life and have a high social significance, since the primary source of patient's interaction with the environment, his social life, and labor activity suffers. This shows the importance of developing highly effective pharmacotheurapetic methods of nervous system protection against damages accompanied by locomotor and postural function disturbances. A great interest for theoretical and practical medicine is presented by the investigation of drugs capable of influencing various areas of the brain, descending supraspinal systems, spinal networks, and structures controlling the posture and locomotion. This article provides information about the key points of cholinergic and adrenergic agents for recovering pharmacotherapy after CNS injuries.
... Thirteen female Sprague-Dawley rats, weighing an average of 417 g and average age of 50-weeks (standard deviation 4.8 weeks), were made available for these experiments. These animals were part of an ongoing research program in Dr Bennett's lab examining the pathophysiology and treatment of spasticity following spinal cord injury [9,10,11]. All animals had a complete sacral spinal transection (at S2) at about 2 months of age and were tested in these experiments more than 300 days following the initial injury. ...
Preprint
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Background Nerve excitability tests in sciatic motor axons are sensitive to anaesthetic choice. Results using ketamine/xylazine (KX) are different from those using sodium pentobarbital (SP). It is not clear which results are most similar to the awake condition, though results using SP appear more similar to human results. Methods Nerve excitability in tail motor axons was tested in 8 adult female rats with a chronic sacral spinal cord injury. These animals have no behavioural response to electrical stimulation of the tail and were tested awake and then anaesthetized using SP. Results The nerve excitability test results in the awake condition were indistinguishable from the results when the same rats were anaesthetized with sodium pentobarbital. Summary plots of the test results overlap within the boundaries of the standard error and paired t-tests on the 42 discrete measures generated by nerve excitability testing yielded no significant differences (after Bonferroni correction for multiple comparisons). Conclusions Nerve excitability test results in rat motor axons are the same whether the animals are awake or anesthetized using sodium pentobarbital.
... Because Cav1.3 channels also play an important role in the nervous system, we would like to know whether in some pathophysiological situations, for instance spinal injury, this channel is subject to any changes in levels of expression. It is known that after spinal cord injury PIC properties in motoneurons were lost in the acute phase due to the loss of the brain stem monoaminergic innervation, however the PICs reappeared 2-3 weeks after injury Bennett et al., 1999 Bennett et al., , 2004 Li et al., 2004 a,b]. Because Cav1.3 channels are believed to mediate PICs in spinal cord motoneurons, one may wonder whether these channels in motoneurons undergo a change in the levels of expression after spinal cord injury. ...
... Saline treated rats started to reach pre-injury body weight levels by three weeks, whereas GBP treated rats never returned to pre-injury body weight by termination at 4 weeks post-injury. The gradual return to pre-injury weight beginning approximately 2-weeks after SCI is parallel to the development of fulminant muscle spasticity in rats after SCI (Bennett et al., 2004). Therefore, given the continuous 4 x daily GBP administration in our current study, it is feasible that the failure of GBP treated rats to return to pre-injury weight is due to a lack of spasticity-induced muscle preservation. ...
Article
Spinal cord injury (SCI) can have profound effects on the autonomic and cardiovascular systems, notably with injuries above high-thoracic levels that result in the development of autonomic dysreflexia (AD) characterized by volatile hypertension in response to exaggerated sympathetic reflexes triggered by afferent stimulation below the injury level. Pathophysiological changes associated with the development of AD include sprouting of both nociceptive afferents and ascending propriospinal 'relay' neurons below the injury, as well as dynamic changes in synaptic inputs onto sympathetic preganglionic neurons. However, it remains uncertain whether synapse formation between sprouted c-fibers and propriospinal neurons contributes to the development of exaggerated sympathetic reflexes produced during AD. We previously reported that once daily treatment with the anti-epileptic and neuropathic pain medication, gabapentin (GBP), at low dosage (50 mg/kg) mitigates experimentally induced AD soon after injections, likely by impeding glutamatergic signaling. Since much higher doses of GBP are reported to block the formation of excitatory synapses, we hypothesized that continuous, high dosage GBP treatment after SCI might prevent the formation of aforementioned aberrant synapses and, accordingly, reduce the incidence and severity of AD. Adult female rats implanted with aortic telemetry probes for hemodynamic monitoring underwent T4-transection SCI and immediately received 100 mg/kg (i.p.) of GBP and then every six hours (400 mg/kg/day) for 4-weeks after injury. We assessed daily body weight, mean arterial pressure, heart rate, frequency of spontaneous AD, and hemodynamic changes during colorectal distension (CRD) to establish whether high dose GBP treatment prophylactically mitigates both AD and associated aberrant synaptic plasticity. This regimen significantly reduced both the absolute blood pressure reached during experimentally induced AD and the time required to return to baseline afterwards. Conversely, GBP prevented return to pre-injury body weights and paradoxically increased the frequency of spontaneously occurring AD. While there were significant decreases in the densities of excitatory and inhibitory pre-synaptic markers in the lumbosacral dorsal horn following injury alone, they were unaltered by continuous GBP treatment. This indicates distinct mechanisms of action for acute GBP to mitigate induced AD whereas chronic GBP increases non-induced AD frequencies. While high dose prophylactic GBP is not recommended to treat AD, acute low dose GBP may hold therapeutic value to mitigate evoked AD, notably during iatrogenic procedures under controlled clinical conditions.
... Lorsque l'hyperexcitabilité réflexe se développe à la suite du choc spinal, elle semble être liée à une augmentation des récepteurs 5-HT, à la fois sur des motoneurones et des interneurones, ainsi qu'à la production de substances monoaminergiques à partir de plusieurs populations cellulaires distales à la lésion (Bennett et al, 2004 ;D'Amico et al, 2013D'Amico et al, , 2014Heckman et al, 2005 ;Hultborn et al, 2013 ;Kong et al, 2010 ;Murray et al, 2010 ;Rank et al, 2007 ;Ren et al, 2013 ;Wienecke et al, 2014). L'augmentation de l'excitabilité neuronale par la hausse des courants entrants persistants (PICs), impliquant à la fois des canaux calcium et sodium, pourrait conduire à des épisodes prolongées de décharges continues des motoneurones suite à une stimulation synaptique même brève (D'Amico et al, 2013(D'Amico et al, , 2014Heckman et al, 2005 et al, 2010). ...
Thesis
La parésie spastique est souvent envisagée comme une atteinte de la commande motrice, comportant une parésie de l’agoniste et une hyperactivité de l’antagoniste. Cependant, une seconde affection d’ordre musculaire - la myopathie spastique, apparait rapidement, pendant la phase aigüe après la lésion. L’hypomobilisation en position courte de certains muscles dans le contexte d’une parésie des muscles opposés s’accompagne d’une perte de leur tension longitudinale, première étape d’une cascade de transformations génétiques, structurelles, biomécaniques puis physiologiques des muscles hypomobilisés, incluant entre autres une perte de leur extensibilité et de leur longueur. Aux stades subaigu puis chronique du syndrome, les affections neurologique et musculaire coexistent, et semblent s’entretenir mutuellement.Sur le plan physiopathologique, ce travail de recherche s’est d’abord intéressé à ces intrications entre les affections neurologique et musculaire, et à la part de responsabilité potentielle de la myopathie spastique dans la fonction active. Ce premier travail a montré que le degré d’hypoextensibilité musculaire d’un antagoniste, au-delà d’un certain seuil, est corrélé au degré de perturbation de la commande motrice dirigée sur l’agoniste. La chronologie des différents événements établie dans la littérature, avec des anomalies histologiques musculaires qui apparaissent toujours avant les premières manifestions d’hyperactivité motoneuronale, amène à suggérer une responsabilité causale de la maladie du muscle dans une partie des perturbations ultérieures de la commande neuronale descendante. Ce rôle probablement essentiel de la maladie musculaire au sein de la parésie spastique doit inciter le clinicien à orienter une part significative de son traitement vers une stimulation optimale de la plasticité musculaire pour tenter de prévenir ou d’inverser le processus de myopathie spastique.Si l’étirement est depuis très longtemps une technique courante, sa capacité à augmenter durablement l’extensibilité musculaire est aujourd’hui controversée, même si ses effets à long terme, i.e. au-delà de six mois d’application quotidienne, n’ont pas été explorés.La deuxième partie, thérapeutique, de ce travail a donc été l’étude des effets à long terme d’un programme quotidien d’auto-étirements de haute intensité au sein de la méthode des Contrats d’Autorééducation Guidée, pendant un an ou plus. Il a d’abord été rétrospectivement montré que ce programme avait permis, sur trois années de suivi de sujets parétiques, une amélioration progressive importante de l’extensibilité musculaire mesurée cliniquement. Un essai randomisé contrôlé contre la kinésithérapie conventionnelle avec mesures échographiques de paramètres structuraux des muscles étirés a ensuite établi que l’application de ce programme durant un an chez des sujets atteints d’hémiparésie chronique sur AVC générait une augmentation de la longueur fasciculaire des muscles fléchisseurs plantaires, parallèlement à une amélioration de la fonction active, de façon nettement plus importante que la thérapie conventionnelle.L’ensemble de ces résultats doit participer à une meilleure prise en compte des altérations passives structurelles évolutives du muscle comme une entité nosologique, la myopathie spastique, méritant un traitement spécifique au sein de la parésie spastique, afin de limiter ses interactions avec la maladie neurologique et de réduire les déficits fonctionnels. La pratique quotidienne de postures d’auto-étirement à haute intensité, guidée par un thérapeute et auto-documentée par le patient sur un registre, peut être prescrite et réalisée sur le long terme, afin de traiter la myopathie spastique.
... animais foram separados aleatoriamente e igualmente em cinco grupos da seguinte forma:Protocolo de tratamentoOs animais dos grupos Az, FT e FTAz iniciaram tratamento diário no 15° dia pós-operatório (DPO), quando a espasticidade tornava-se presente(BENNETT et al., 2004), sendo mantidos até o 45° DPO, totalizando 30 dias de tratamento.Realizou-se a massagem abdominal com o objetivo de esvaziar a bexiga e a defecção, quando necessário.Protocolo de avaliaçãoA avaliação abrangeu características comportamentais e histológicas dos animais. Os testes funcionais para avaliação comportamental e neurológica foram realizados em 8 momentos: 4 antes do início dos tratamentos (no 01°, 03°, 07°, 14° DPO) e 4 após este momento (no 21°, 28°, 35° e 45° DPO). ...
... In such a hyper-excitable state, glutamate released from spinal interneurons, could, ultimately, elicit muscle spasms. Indeed, involuntary motor activity and spasticity is a frequent symptom of chronic SCI, and in in vitro models of SCI, dorsal root stimulation evokes unusually long NMDA receptor-dependent excitatory postsynaptic potentials (Bennett et al. 2004). ...
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In a recent publication, Thaweerattanasinp and colleagues employed an in vitro preparation and electrophysiology to investigate firing properties of deep dorsal horn neurons following spinal cord injury during NMDA or zolmitriptan application. Deep dorsal horn neurons were classified into bursting, simple or tonic, with bursting neurons showing NMDA and zolmitriptan sensitivity. Here, we discuss the findings in a methodological framework and propose future experiments of importance for translating the results into a physiological setting.
... The mechanisms underlying spasticity include the decreased inhibition of the spinal network (Delwaide and Oliver, 1988;Mazzocchio and Rossi, 1997), the increased excitatory synaptic inputs associated with Ia afferent fibers (Tan et al., 2012), and the increased motoneuron excitability (Bennett et al., 2004) (Fig. 3A). Plasticity and neural remodeling processes, triggered by the initial damage, contribute to the time course of spasticity (Lang et al., 2013) and the adaptation of spinal inhibitory mechanisms (Ward, 2012). ...
Article
Although spasticity is one of the most common causes of motor disability worldwide, its precise definition and pathophysiology remain elusive, which to date renders its experimental targeting tricky. At least in part, this difficulty is caused by heterogeneous phenotypes of spasticity-causing neurological disorders, all causing spasticity by involving upper motor neurons. The most common clinical symptoms are a series of rapid muscle contractions (clonus), an increased muscle tone (hypertonia), and augmented tendon reflex activity (hyperreflexia). This muscle overactivity is due to disturbed inhibition of spinal reflexes following upper motor neuron dysfunction. Despite a range of physical and pharmacological therapies ameliorating the symptoms, their targeted application remains difficult. Therefore, to date, spasticity impacts rehabilitative therapy, and no therapy exists that reverses the pathology completely. In contrast to the incidence and importance of spasticity, only very little pre-clinical work in animal models exists, and this research is focused on the cat or the rat spastic tail model to decipher altered reflexes and excitability of the motor neurons in the spinal cord. Meanwhile, the characterization of spasticity in clinically more relevant mouse models of neurological disorders, such as stroke, remains understudied. Here, we provide a brief introduction into the clinical knowledge and therapy of spasticity and an in-depth review of pre-clinical studies of spasticity in mice including the current experimental challenges for clinical translation.
... The motor unit potentials within the EMG signal have a duration of about 3 -5 ms, and thus we averaged rectified EMG over 6 -11 ms to quantify the MSR. We have shown that similar considerations hold for the rat where tail nerve conduction velocities are similar, except the distance from the tail stimulation to the spinal cord is larger (150 mm), yielding a peripheral nerve conduction delay of ~10 ms and total MSR delay of ~12 ms (Bennett et al., 2004). In humans the MSR latency is dominated by the nerve conduction latency (50 -60 m/s) over a large distance (~800 mm), yielding MSR latencies of ~30 ms. ...
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GABA is an inhibitory neurotransmitter that produces both postsynaptic and presynaptic inhibition. We describe here an opposing excitatory action of GABA that facilitates spike transmission at nodes of Ranvier in myelinated sensory axons in the spinal cord. This nodal facilitation results from axonal GABAA receptors that depolarize nodes toward threshold, enabling spike propagation past the many branch points that otherwise fail, as observed in spinal cords isolated from mice or rats. Activation of GABAergic neurons, either directly with optogenetics or indirectly with cutaneous stimulation, caused nodal facilitation that increased sensory transmission to motoneurons without postsynaptically exciting motoneurons. This increased transmission with optogenetic or cutaneous stimulation also occurred in awake mice and humans. Optogenetic inhibition of GABAergic neurons decreased sensory transmission, implying that axonal conduction relies on GABA. The concept of nodal facilitation likely generalizes to other large axons in the CNS, enabling recruitment of selective branches and functional pathways.
... These monoamines activate intrinsic membrane properties, such as persistent sodium or calcium inward currents (PICs) that generate sustained depolarizations (Heckman et al., 2003;Hultborn, 2003). Immediately after SCI, the loss of monoaminergic drive markedly decreases spinal reflexes due to the decrease in the excitability of interneurons and motoneurons (Kuhn, 1950;Conway et al., 1987;Bennett et al., 1999Bennett et al., , 2001Bennett et al., , 2004Hultborn, 2003). In humans, stretch reflexes are also absent or depressed during spinal shock, immediately after SCI (Leis et al., 1996;Hultborn, 2003). ...
Article
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Somatosensory feedback from peripheral receptors dynamically interacts with networks located in the spinal cord and brain to control mammalian locomotion. Although somatosensory feedback from the limbs plays a major role in regulating locomotor output, those from other regions, such as lumbar and perineal areas also shape locomotor activity. In mammals with a complete spinal cord injury, inputs from the lumbar region powerfully inhibit hindlimb locomotion, while those from the perineal region facilitate it. Our recent work in cats with a complete spinal cord injury shows that they also have opposite effects on cutaneous reflexes from the foot. Lumbar inputs increase the gain of reflexes while those from the perineal region decrease it. The purpose of this review is to discuss how somatosensory feedback from the lumbar and perineal regions modulate the spinal locomotor central pattern generator and reflex circuits after spinal cord injury and the possible mechanisms involved. We also discuss how spinal cord injury can lead to a loss of functional specificity through the abnormal activation of functions by somatosensory feedback, such as the concurrent activation of locomotion and micturition. Lastly, we discuss the potential functions of somatosensory feedback from the lumbar and perineal regions and their potential for promoting motor recovery after spinal cord injury.
... animais foram separados aleatoriamente e igualmente em cinco grupos da seguinte forma:Protocolo de tratamentoOs animais dos grupos Az, FT e FTAz iniciaram tratamento diário no 15° dia pós-operatório (DPO), quando a espasticidade tornava-se presente(BENNETT et al., 2004), sendo mantidos até o 45° DPO, totalizando 30 dias de tratamento.Realizou-se a massagem abdominal com o objetivo de esvaziar a bexiga e a defecção, quando necessário.Protocolo de avaliaçãoA avaliação abrangeu características comportamentais e histológicas dos animais. Os testes funcionais para avaliação comportamental e neurológica foram realizados em 8 momentos: 4 antes do início dos tratamentos (no 01°, 03°, 07°, 14° DPO) e 4 após este momento (no 21°, 28°, 35° e 45° DPO). ...
... MAS evaluates resistance to passive movements of the hind limbs (e.g., ankle dorsiflexion) and is composed of six grades, where grade 0 means no increase in muscle tone and grade 5 indicates limb rigidity either for flexion or for extension. For the sacral spinal cord injury model, clinical assessment of tail spasticity was developed by Bennett et al. (1999Bennett et al. ( , 2004. Tail manipulations are performed with using a restraint apparatus in which the tail hangs out the back and is free to move over its full length. ...
Article
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Poor validity of preclinical animal models is one of the most commonly discussed explanations for the failures to develop novel drugs in general and in neuroscience in particular. However, there are several areas of neuroscience such as injury-induced spasticity where etiological factor can be adequately recreated and models can focus on specific pathophysiological mechanisms that likely contribute to spasticity syndrome in humans (such as motoneuron hyperexcitability and spinal hyperreflexia). Methods used to study spasticity in preclinical models are expected to have a high translational value (e.g., electromyogram (EMG)-based electrophysiological tools) and can efficiently assist clinical development programs. However, validation of these models is not complete yet. First, true predictive validity of these models is not established as clinically efficacious drugs have been used to reverse validate preclinical models while newly discovered mechanisms effective in preclinical models are yet to be fully explored in humans (e.g., 5-HT2C receptor inverse agonists, fatty acid amid hydrolase inhibitors). Second, further efforts need to be invested into cross-laboratory validation of study protocols and tools, adherence to the highest quality standards (blinding, randomization, pre-specified study endpoints, etc.), and systematic efforts to replicate key sets of data. These appear to be readily achievable tasks that will enable development not only of symptomatic but also of disease-modifying therapy of spasticity, an area that seems to be currently not in focus of research efforts.
... This leads to the activation of 5-HT receptors that facilitate sustained firing of voltage-gated persistent Ca 2+ and Na + currents (also called persistent inward currents, PICs) and cause muscle contractions [280]. PICs and spasms are easily triggered by innocuous stimuli such as touch or muscle stretching [281]. Murray et al. [58,90,97] showed that this effect is due to the activity of 5-HT 2 receptors. ...
Article
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As the nervous system develops, nerve fibers from the brain form descending tracts that regulate the execution of motor behavior within the spinal cord, incoming sensory signals, and capacity to change (plasticity). How these fibers affect function depends upon the transmitter released, the receptor system engaged, and the pattern of neural innervation. The current review focuses upon the neurotransmitter serotonin (5-HT) and its capacity to dampen (inhibit) neural excitation. A brief review of key anatomical details, receptor types, and pharmacology is provided. The paper then considers how damage to descending serotonergic fibers contributes to pathophysiology after spinal cord injury (SCI). The loss of serotonergic fibers removes an inhibitory brake that enables plasticity and neural excitation. In this state, noxious stimulation can induce a form of over-excitation that sensitizes pain (nociceptive) circuits, a modification that can contribute to the development of chronic pain. Over time, the loss of serotonergic fibers allows prolonged motor drive (spasticity) to develop and removes a regulatory brake on autonomic function, which enables bouts of unregulated sympathetic activity (autonomic dysreflexia). Recent research has shown that the loss of descending serotonergic activity is accompanied by a shift in how the neurotransmitter GABA affects neural activity, reducing its inhibitory effect. Treatments that target the loss of inhibition could have therapeutic benefit.
... animais foram separados aleatoriamente e igualmente em cinco grupos da seguinte forma:Protocolo de tratamentoOs animais dos grupos Az, FT e FTAz iniciaram tratamento diário no 15° dia pós-operatório (DPO), quando a espasticidade tornava-se presente(BENNETT et al., 2004), sendo mantidos até o 45° DPO, totalizando 30 dias de tratamento.Realizou-se a massagem abdominal com o objetivo de esvaziar a bexiga e a defecção, quando necessário.Protocolo de avaliaçãoA avaliação abrangeu características comportamentais e histológicas dos animais. Os testes funcionais para avaliação comportamental e neurológica foram realizados em 8 momentos: 4 antes do início dos tratamentos (no 01°, 03°, 07°, 14° DPO) e 4 após este momento (no 21°, 28°, 35° e 45° DPO). ...
... In PLS, abnormal activation of persistent inward currents producing low-voltage stable plateau potentials has been described (Floeter et al., 2005). Activation of plateau potentials at low voltages can lower the threshold for LMN firing (Burke, 1968;Gorassini et al., 2002) leading to the clinical feature of spastic increased muscle tone (Bennett et al., 2004;Pierrot-Deseilligny and Burke, 2005). In ALS, as distinct from PLS and other causes of lower limb spasticity, there is degeneration of the intrinsic wiring of anterior horn cell connexions, as well as degeneration of anterior horn cells themselves and of descending corticospinal and propriospinal projections from central structures, including frontal and motor cortex. ...
Article
Objective: We studied motor unit recruitment to test a new method to identify motor unit firing rate (FR) variability. Methods: We studied 68 ALS patients, with and without upper neuron signs (UMN) in lower limbs, 24 patients with primary lateral sclerosis (PLS), 13 patients with spinal cord lesion and 39 normal subjects. All recordings were made from tibialis anterior muscles of normal strength. Subjects performed a very slight contraction in order to activate 2 motor units in each recording. 5-7 motor unit pairs were recorded in each subject. Mean consecutive differences (MCD) were calculated for each pair of potentials. The mean MCD for each muscle was estimated as the mean from the total number of pairs recorded. Ap value<0.01 was accepted as significant. Results: MCD of FR frequency was less in the subjects with spinal cord lesion and PLS. In addition, the FR frequency of the 1st motor unit in a pair of units was markedly reduced in PLS, and in subjects with spinal cord lesions. Conclusion: These results support a lower threshold and reduced FR fluctuation in spinal motor neurons of spastic patients. Significance: This method can be developed for detection of UMN lesions.
Article
The experimental sacral spine lesion in the rat produce an animal model with low morbidity than those with toracic spinal lesions. The clinical spasticity quantification in the tail of the rat caused by sacral cord lesion was poorly described in the bibliography. Furthermore, there are no investigations about the relantionship between electrophysyological parameters and the grade of spasticity in the rat tail, so our purpose is to evaluate tail spasticity with clinical and electromyographic studies. Experimental spinal surgeries were done in 10 Sprague Dawley rats. Sacral level was choosen to produce spasticity only in the tail. On the postoperatory time the tail was evaluated searching signs of spasticity, such as paresia and hypertonia, and then the grade of spasticity was categorizated using a muscular tone quantitative scale. On the other hand, electromyographic studies were done to obtain the muscular responses of the tail to the caudal nerve stimulation (M, F and waves). Then, the specimens were sacrificed and perfunded. Spinal cord lesions were verified with hystopathology. All the animals with spinal lesion presented paresia of the tail. The spasticity signs began at the second postoperative week. In relantionship with the muscular tone scale five rats presented grade 2 spasticity, three rats grade 4 spasticity, and only one grade 3 spasticity. The electrophysiology showed patterns of response in relation with the clinical evolution and the grade of spasticity. The results obtained showed correspondence between the grade of spasticity and the electrophysiological patterns caused by the grade of the motoneuron excitability. This experimental model allows to make a clinical evaluation and the correspondence with the electrophysiology, with the lowest general morbidity.
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Natural killer (NK) cells critically participate in the immune surveillance against tumors and are pivotal regulators of the adaptive immune through the secretion of cytokines and the cytotoxic function. NKG2D and NKp46 are two key receptors involved in these processes. Howeverr, NK cells are targets of tumor immune escape mechanisms such as ntracellular retention of MICA (MHC class I chain-related protein A) in the endoplasmic reticulum of certain human melanomas. NK cells also become activated in a cooperative manner by cytokines (IL-12 or IFN-alpha) and Toll like receptors, an effect that is potentiated by tumors that promote engagement of NKG2D. Moreover, tumor cell-T cell contact induce secretion of IFN-gamma by NK cells through acquisition of tumor-derived NKG2D and NKp46 ligands by T cells, which subsequently promote NKG2D- and NKp46-dependent NK cell stimulation. Therefore, cells from the adaptive immune response regulate the activity of cells of the innate immune response, which may have relevance during anti-tumor immunity. Also, some anti-neoplastic drugs impair an NK cell function, which reduces their therapeutic efficacy. In summary, NK cells have a great immunotherapeutic potential but to fully exploit it, the cellular and molecular mechanisms that govern their effector function must be elucilated first.
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Motoneurons differ in the behaviours they control and their vulnerability to disease and ageing. For example, brainstem motoneurons such as hypoglossal motoneurons (HMs) are involved in licking, suckling, swallowing, respiration, and vocalization. In contrast spinal motoneurons (SMs) innervating the limbs are involved in postural and locomotor tasks requiring higher loads and lower movement velocities. Surprisingly, the properties of these two motoneuron pools have not been directly compared even though studies on HMs predominate in the literature compared to SMs, especially for adult animals. Here, we used whole cell patch clamp recording to compare the electrophysiological properties of HMs and SMs in age-matched neonatal mice (P7-10). Passive membrane properties were remarkably similar in HMs and SMs and afterhyperpolarization properties did not differ markedly between the two populations. HMs had narrower action potentials (AP), and a faster upstroke on their APs compared to SMs. Furthermore, HMs discharged APs at higher frequencies in response to both step and ramp current injection than SMs. Therefore, while HMs and SMs have similar passive properties, they differ in their response to similar levels of depolarizing current. This suggests each population possess differing suites of ion channels that allow them to discharge at rates matched to the different mechanical properties of the muscle fibers that drive their distinct motor functions.
Chapter
The natural course of disease in spinal cord injury is well known for traumatic etiologies. In the following chapter, this will be illustrated with respect to anatomical and physiological adaptations in the central and peripheral nervous system. Disease course is described for functional recovery in the domains relevant for spinal cord injury (SCI). Underlying mechanisms of adaptation are addressed in the context of neurological and functional recovery. The role and disease course of etiologies, other than traumatic, are discussed together with the effects of treatment, where treatment is available. In a final outlook, recovery mechanisms are discussed in the context of incipient clinical trials to cure traumatic SCI. A key issue in this context is to distinguish treatment effects from natural recovery, in order to deal with the difficulty of determining potential efficacy of an intervention. In this context, understanding of the disease course may require additional knowledge of underlying neurophysiological and pathophysiological adaptations that are not clinically evident but require technical examinations. This may help in the process of trial design and therapy development. Stratification and prediction strategies are crucial in this process.
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Electrophysiological approach: H-reflex frequency-dependent depression Biomechanical approach: windup of the stretch reflex Interventions: passive exercise Interventions: pharmacology (L-Dopa, Modafinil) Conclusions References
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Bradykinin (Bk) is a potent inflammatory mediator that causes hyperalgesia. The action of Bk on the sensory system is well documented but its effects on motoneurons, the final pathway of the motor system, are unknown. By a combination of patch-clamp recordings and two-photon calcium imaging, we found that Bk strongly sensitizes spinal motoneurons. Sensitization was characterized by an increased ability to generate self-sustained spiking in response to excitatory inputs. Our pharmacological study described a dual ionic mechanism to sensitize motoneurons, including inhibition of a barium-sensitive resting K(+) conductance and activation of a nonselective cationic conductance primarily mediated by Na(+). Examination of the upstream signaling pathways provided evidence for postsynaptic activation of B2 receptors, G protein activation of phospholipase C, InsP3 synthesis, and calmodulin activation. This study questions the influence of motoneurons in the assessment of hyperalgesia since the withdrawal motor reflex is commonly used as a surrogate pain model.
Chapter
Spasticity is a highly impairing condition following spinal cord injury (SCI) that develops in more than 60% of patients as early as one-year postinjury. To date, pharmacological therapy shows limited effectiveness and various systemic side effects. Animal models allow carrying out in-depth investigations on pathophysiological mechanisms of spasticity after SCI to identify innovative and efficacious therapeutic strategies. So far, several maladaptive changes in sublesional motoneurons have been reported, causing an increased excitation (hyperexcitability) and a reduced inhibition (disinhibition) in the spinal cord. We demonstrated that calpains, a family of calcium-dependent proteases, have an upstream role in promoting the excitatory/inhibitory imbalance of spinal motoneurons that leads to spasticity after SCI. On the one hand, calpains cleave voltage-gated sodium channels Nav1.6 up-regulating the sodium persistent inward current(INaP), which evokes huge plateau potentials with self-sustained spiking, lastly causing spinal hyperexcitability contributing to spasticity. On the other hand, calpains down-regulate the potassium-chloride cotransporter KCC2 on motoneuron membranes, depolarizing the chloride equilibrium potential (ECl⁻) and inducing spinal disinhibition also involved in the onset of spasticity. Therefore, calpain inhibition might represent an effective antispastic therapy targeting the main upstream molecule accounting for multiple mechanisms of spasticity, instead of singularly acting on downstream players. Furthermore, this therapeutic strategy can be adopted not only to prevent spasticity after SCI but also to counteract neurological disorders where calpains have a critical role.
Article
Following spinal cord injury (SCI), reflexes become hyperexcitable, leading to debilitating muscle spasms and compromised motor function. Previous work has described adaptations in spinal systems that might underlie this hyperexcitability, including an increase in constitutively active 5-HT2C receptors in spinal motoneurons. That work, however, examined adaptations following complete transection SCI, whereas SCI in humans is usually anatomically and functionally incomplete. We therefore evaluated whether constitutive activity of 5-HT2C receptors contributes to reflex hyperexcitability in an incomplete compression model of SCI and to spasms in vitro and in vivo. Our results confirm that 5-HT2C receptor constitutive activity contributes to reflex excitability following incomplete SCI. We also evaluated whether constitutive activity could be altered by manipulating neural activity levels following SCI, testing the hypothesis that it reflects homeostatic processes acting to maintain spinal excitability. We decreased neural activity following SCI by administering baclofen and increased activity by administering the SSRI fluoxetine. We found that drug administration produced minimal alterations in in vivo locomotor function or reflex excitability. Similarly, we found that neither baclofen nor fluoxetine altered the contribution of constitutively active 5-HT2C receptors to reflexes following SCI, although the contribution of 5-HT2C receptors to reflex activity was altered following SSRIs. These results confirm the importance of constitutively activity in 5-HT2C receptors to spinal hyperexcitability following SCI in the clinically relevant case of incomplete SCI, but suggest that this activity is not driven by homeostatic processes that act to maintain overall levels of spinal excitability.
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During development of the spinal cord, a precise interaction occurs between descending projections and sensory afferents with spinal networks that lead to expression of coordinated motor output. In the rodent, during the last embryonic week, motor output first occurs as regular bursts of spontaneous activity progressing to stochastic patterns of episodes that express bouts of coordinated rhythmic activity perinatally. Locomotor activity becomes functionally mature in the second postnatal week and is heralded by the onset of weight-bearing locomotion on the 8th-9th postnatal day. Concomitantly there is a maturation of intrinsic properties and key conductances mediating plateau potentials. In this review, we discuss spinal neuronal excitability, descending modulation, and afferent modulation in the developing rodent spinal cord. In the adult, plastic mechanisms are much more constrained but become more permissive following neurotrauma such as spinal cord injury. We discuss parallel mechanisms that contribute to maturation of network function during development to mechanisms of pathological plasticity that contribute to aberrant motor patterns such as spasticity and clonus that emerge following central injury.
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Abstract: Purpose: The goal of this study was to assessment subacute and chronic treatment with herbal medicine OEAz associated with Kinesiotherapy on patient with muscle spasticity: cases series. Methods: It is a monocentric study for convenience and prospective with different treatment periods in subacute and chronic treatments in two (2), three (3), four (4), five (5), six (6) and seven (7) months by intradermal in the doses of 0.05 mg/ 2Kg in tetraparetic and 0.05 mg/ 4Kg in hemiparetic. Spasticity scores were measured by the Ashworth modify scale. Wilcoxon, Kruskal-Wallis test were used for significant results of p<0.05. Results: In child, teenagers and adults were observed a significant p<0.05 spasticity decrease when treated with herbalmedicine OEAz associated with Kinesiotherapy, in different periods, been the results more expressive in 7 months. Conclusion: herbal medicine OEAz associated with Kinesiotherapy revealed an effective intervention strategy for chronic treatment of spastic muscle.
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Chondroitin sulfate proteoglycans (CSPGs) consist of core proteins and glycosaminoglycan side chains. Tenascins, and hyaluronan and proteoglycan link protein 1 (HAPLN), link CSPGs with a hyaluronan backbone to constitute perineuronal nets (PNNs), which ensheath preferentially highly active neurons to maintain architecture and stabilize synapses, but restrict repair plasticity. Spinal cord injury increases CSPG core protein levels, limiting permissiveness of the extracellular milieu for fiber regrowth within the lesion, however regulation of PNNs structure in the synaptic boutons vicinity of distant α-motoneurons (MNs) in the course of degeneration and reorganization of inputs requires research. Here, we examined relationships between changes in CSPG proteins, HAPLN1, tenascin-R, and glial activation along the spinal cord in male rats with complete spinal cord transection (SCT) at Th10, and their impact on PNNs ensheathing lumbar MNs innervating ankle extensor and flexor muscles, which are in different loading states in paraplegic rats. RT-qPCR, ELISA, immunofluorescence and Wisteria floribunda agglutinin (WFA) labeling revealed that (1) distance from the lesion site and time after injury (2–5 weeks) differentiate degree of changes in transcription rates of PNNs proteins with increased CSPGs and decreased HAPLN1 transcripts, suggesting long-term PNN destabilization in majority of spinal segments, (2) PNN protein expression profiles and composition are not MN-class (extensor vs flexor) specific, both showing postlesion early decrease and late upregulation of WFA(+) PNNs in vicinity of synaptic boutons on MNs, (3) long-term locomotor training reduces WFA(+) PNNs, not reducing neurocan and HAPLN1 levels around MNs. Our results suggest that training-induced regulation may target glycan structures of CSPGs.
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The monosynaptic stretch reflex (MSR) plays an important role in feedback control of movement and posture but can also lead to unstable oscillations associated with tremor and clonus, especially when increased with spinal cord injury (SCI). To control the MSR and clonus after SCI, we examined how serotonin regulates the MSR in the sacrocaudal spinal cord of rats with and without a chronic spinal transection. In chronic spinal rats, numerous 5-HT receptor agonists, including zolmitriptan, methylergonovine, and 5-HT, inhibited the MSR with a potency highly correlated to their binding affinity to 5-HT1D receptors and not other 5-HT receptors. Selective 5-HT1D receptor antagonists blocked this agonist-induced inhibition, although antagonists alone had no action, indicating a lack of endogenous or constitutive receptor activity. In normal uninjured rats, the MSR was likewise inhibited by 5-HT, but at much higher doses, indicating a supersensitivity after SCI. This supersensitivity resulted from the loss of the serotonin transporter SERT with spinal transection, because normal and injured rats were equally sensitive to 5-HT after SERT was blocked or to agonists not transported by SERT (zolmitriptan). Immunolabeling revealed that the 5-HT1D receptor was confined to superficial lamina of the dorsal horn, colocalized with CGRP-positive C-fibers, and eliminated by dorsal rhizotomy. 5-HT1D receptor labeling was not found on large proprioceptive afferents or α-motoneurons of the MSR. Thus serotonergic inhibition of the MSR acts indirectly by modulating C-fiber activity, opening up new possibilities for modulating reflex function and clonus via pain-related pathways. NEW & NOTEWORTHY Brain stem-derived serotonin potently inhibits afferent transmission in the monosynaptic stretch reflex. We show that serotonin produces this inhibition exclusively via 5-HT1D receptors, and yet these receptors are paradoxically mostly confined to C-fibers. This suggests that serotonin acts by gating of C-fiber activity, which in turn modulates afferent transmission to motoneurons. We also show that the classic supersensitivity to 5-HT after spinal cord injury results from a loss of SERT, and not 5-HT1D receptor plasticity.
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The excitability of resting motoneurons increases following spinal cord injury (SCI). The extent to which motoneuron excitability changes during voluntary muscle activity in humans with SCI, however, remains poorly understood. To address this question, we measured F-waves by using supramaximal electrical stimulation of the ulnar nerve at the wrist and cervicomedullary motor evoked potentials (CMEPs) by using high current electrical stimulation over the cervicomedullary junction in the first dorsal interosseous muscle at rest and during 5% and 30% of maximal voluntary contraction into index finger abduction in individuals with chronic cervical incomplete SCI and aged-matched control participants. We found higher persistence (number of F-waves present in each set) and amplitude of F-waves at rest in SCI compared with control participants. With increasing levels of voluntary contraction, the amplitude, but not the persistence, of F-waves increased in both groups but to a lesser extent in SCI compared with control participants. Similarly, the CMEPs amplitude increased in both groups but to a lesser extent in SCI compared with control participants. These results were also found at matched absolutely levels of electromyographic activity, suggesting that these changes were not related to decreases in voluntary motor output in SCI participants. F-waves and CMEPs amplitudes were positively correlated across conditions in both groups. These results support the hypothesis that the responsiveness of the motoneuron pool during voluntary activity decreases following SCI, which could alter the generation and strength of voluntary muscle contractions.
Article
Key points: ALS motoneurons become hypoexcitable with disease progression in experimental models, raising questions about the neural hyperexcitability supported by clinical observations. A variant of the ∆F method, based on motor unit discharge frequency modulations during recruitment and derecruitment, has been developed to investigate the motoneuron capacity to self-sustained discharge in patients. The modulation of motor unit firing rate during ramp contraction and vibration-induced recruitment are modified in ALS, suggesting lower motoneuron capacity to self-sustained discharge, a sign of hypoexcitability. ∆F-D decreases with functional impairment and its reduction is more pronounced in fast progressors. In patients with ALS, motoneurons exhibit hypoexcitability, which increases with disease progression. Abstract: Experimental models have primarily revealed spinal motoneuron hypoexcitability in ALS, which is contentious considering the role of glutamate-induced excitotoxicity in neurodegeneration and clinical features rather supporting hyperexcitability. This phenomenon was evaluated in human patients by investigating changes in motor unit firing during contraction and relaxation. Twenty-two ALS patients with subtle motor deficits and 28 controls performed tonic contractions of extensor carpi radialis, triceps brachialis, tibialis anterior and quadriceps, to isolate a low threshold unit (U1) in EMG. Subsequently, they performed a stronger contraction or tendon vibration was delivered, to recruit higher threshold unit (U2) for 10 s before they relaxed progressively. EMG and motor unit potential analyses suggest altered neuromuscular function in all muscles, including those with normal strength (MRC score at 5). During the preconditioning tonic phase, U1 discharge frequency did not differ significantly between groups. During recruitment, the increase in U1 frequency (∆F-R) was comparable between groups both during contraction and tendon vibration. During derecruitment, the decrease in U1 frequency (∆F-D) was reduced in ALS whatever the recruitment mode, particularly for ∆F-R < 8 Hz in the upper limbs, consistent with the muscle weakness profile of the group. ∆F-D was associated with functional disability (ALSFRS-r) and its reduction was more pronounced in patients with more rapid disease progression rate. This in vivo study has demonstrated reduced motoneuron capacity for self-sustained discharge, and further supports that motoneurons are normo- to hypoexcitable in ALS patients, similarly to the observations in experimental models. This article is protected by copyright. All rights reserved.
Article
The loss of descending serotonin (5-HT) to the spinal cord contributes to muscle spasms in chronic spinal cord injury (SCI). Hyperexcitable motoneurons receive long-lasting excitatory postsynaptic potentials (EPSPs) which, activate their persistent inward currents to drive muscle spasms. Deep dorsal horn (DDH) neurons with bursting behavior could be involved in triggering the EPSPs due to loss of inhibition in the chronically 5-HT-deprived spinal cord. Previously, in an acutely transected preparation, we found that bursting DDH neurons were affected by administration of the 5-HT 1B/1D receptor agonist zolmitriptan, which suppressed their bursts, and by NMDA, which enhanced their bursting behavior. Non-bursting DDH neurons were not influenced by these agents. In the present study, we investigate the firing characteristics of bursting DDH neurons following chronic spinal transection at T10 level in adult mice, and examine the effects of replacing lost endogenous 5-HT with zolmitriptan. Terminal experiments using our in vitro preparation of the sacral cord were carried out ~10 weeks post-transection. Compared with the acute spinal stage of our previous study, DDH neurons in the chronic stage became more responsive to dorsal root stimulation, with burst duration doubling with chronic injury. The suppressive effects of zolmitriptan were stronger overall, but the facilitative effects of NMDA were weaker. In addition, the onset of DDH neuron activity preceded ventral root output and the firing rates of DDH interneurons correlated with the integrated long-lasting ventral root output. These results support a contribution of the bursting DDH neurons to muscle spasms following SCI and inhibition by 5-HT.
Article
Key points: Cutaneous reflexes were tested to examine the neuronal mechanisms contributing to muscle spasms in humans with chronic spinal cord injury (SCI). Specifically, we tested the effect of Achilles and tibialis anterior tendon vibration on the early and late components of the cutaneous reflex and reciprocal Ia inhibition in the soleus and tibialis anterior muscles in humans with chronic SCI. We found that tendon vibration reduced the amplitude of later but not earlier cutaneous reflex activity in the antagonist but not agonist muscle relative to the location of the vibration. In addition, reciprocal Ia inhibition between antagonist ankle muscles increased with tendon vibration and participants with a larger suppression of later cutaneous reflex activity had stronger reciprocal Ia inhibition from the antagonistic muscle. Our study is the first to provide evidence that tendon vibration attenuates late cutaneous spasm-like reflex activity likely via reciprocal inhibitory mechanisms, and may represent a method, when properly targeted, for controlling spasms in humans with SCI. Abstract: The neuronal mechanisms contributing to the generation of involuntary muscle contractions (spasms) in humans with spinal cord injury (SCI) remain poorly understood. To address this question, we examined the effect of Achilles and tibialis anterior tendon vibration at 20, 40, 80, and 120 Hz on the amplitude of the long-polysynaptic (LPR, from reflex onset to 500 ms) and long-lasting (LLR, from 500 ms to reflex offset) cutaneous reflex evoked by medial plantar nerve stimulation in the soleus and tibialis anterior and reciprocal Ia inhibition between these muscles in 25 individuals with chronic SCI. We found that Achilles tendon vibration at 40 and 80 Hz, but not other frequencies, reduced the amplitude of the LLR in the tibialis anterior but not the soleus muscle without affecting the amplitude of the LPR. Vibratory effects were stronger at 80 compared with 40 Hz. Similar results were found in the soleus muscle when the tibialis anterior tendon was vibrated. Notably, tendon vibration at 80 Hz increased reciprocal Ia inhibition between antagonistic ankle muscles and vibratory-induced increases in reciprocal Ia inhibition were correlated with decreases in LLR, suggesting that participants with a larger suppression of later cutaneous reflex activity had stronger reciprocal Ia inhibition from the antagonistic muscle. Our study is the first to provide evidence that tendon vibration suppresses late spasm-like activity in antagonist but not agonist muscles, likely via reciprocal inhibitory mechanisms, in humans with chronic SCI. We argue that targeted vibration of antagonistic tendons might help to control spasms after SCI. This article is protected by copyright. All rights reserved.
Chapter
Chloride homeostasis is critically involved in motor and sensory processing in the spinal cord. After spinal cord injury, spinal networks are hyperexcitable leading to increased transmission in both nociceptive and proprioceptive pathways. It was recently shown that the emergence of chronic pain and spasticity after SCI is not only due to increased excitation resulting from impaired or lost supraspinal inhibitory drive, but also to a lack of endogenous GABAergic inhibition triggered by a shift in chloride homeostasis. This chapter will cover the development of chloride homeostasis in the spinal cord, the effect of spinal cord injury on chloride cotransporters, the functional consequences and, the promising therapeutic strategies.
Article
High spinal cord injuries (SCI) lead to permanent respiratory insufficiency, and the search for new therapeutics to restore this function is essential. To date, the most documented preclinical model for high SCI is the rat cervical C2 hemisection. However, molecular studies with this SCI model are limited due to the poor availability of genetically modified specimens. The aim of this work was to evaluate the pathophysiology of respiratory activity following a cervical C2 injury at different times post-injury in a C57BL/6 mouse model. No significant spontaneous recovery of diaphragmatic activity was observed up to 30 days post-injury in eupneic condition. However, during a respiratory challenge, i.e. mild asphyxia, a partial restoration of the injured diaphragm was observed at 7 days post-injury, corresponding to the crossed phrenic phenomenon. Interestingly, the diaphragmatic recording between 2 respiratory bursts on the injured side showed an amplitude increase between 1–7 days post-injury, reflecting a change in phrenic motoneuronal excitability. This increase in inter-burst excitability returned to pre-injured values when the crossed phrenic phenomenon started to be effective at 7 days post-injury. Taken together, these results demonstrate the ability of the mouse respiratory system to express long-lasting plasticity following a C2 cervical hemisection and genetically modified animals can be used to study the pathophysiological effects on these plasticity phenomena.
Thesis
Axe 1. Impact d’un entraînement moteur sur le développement des réseaux moteurs spinaux chez la souris nouveau-née.L’activité neuronale joue un rôle majeur dans le développement et la mise en forme des réseaux neuronaux. Dans la moelle épinière des vertébrés, les activités spontanées générées provoquent, lorsque les connexions motoneurones-fibres musculaires sont fonctionnelles, des mouvements spontanés des membres visibles chez l’embryon et qui persistent dans les premières semaines postnatales. Chez le fœtus et le nouveau-né, de nombreuses études ont montré que l’altération de ces mouvements spontanés perturbait la mise en place des coordinations appendiculaires indiquant que cette activité spontanée est essentielle pour façonner et calibrer les réseaux moteurs spinaux avant et après la naissance. Dans ce contexte, nous avons cherché à analyser quel pourrait être l’impact de l’augmentation de l’activité motrice sur le développement des réseaux moteurs spinaux. A la naissance les souriceaux ne peuvent pas marcher en raison d’une immaturité posturale, ils sont néanmoins capables, lorsque les contraintes gravitationnelles sont réduites, d’exprimer de la nage. En combinant des approches comportementale, électrophysiologique, moléculaire et génomique, nos données suggèrent qu’un entraînement à la nage réalisé́ très tôt après la naissance (P1-P2), est capable d’induire des modifications comportementales, cellulaires et synaptiques chez la souris nouveau-née.Axe 2. Régulation des réseaux moteurs spinaux par les récepteurs métabotropiques du glutamate au cours du développement chez la souris nouveau-née.Les récepteurs métabotropiques du glutamate (mGluRs) jouent un rôle majeur dans la modulation de la transmission synaptique et des propriétés de membrane des neurones dans le système nerveux central. Cependant, au sein de la moelle épinière et notamment dans les motoneurones lombaires (MNs), le rôle des mGluRs était mal connu. Nous avons dans cette étude analysé l’expression des gènes codant pour les différents sous-types de mGluRs dans la moelle ventrale lombaire de souris âgées de 1 à 3 jours postnataux (P1-P3) et P8-P12 et étudié en détails l’impact de l’activation pharmacologique sélective des trois sous-groupes de mGluRs sur l’excitabilité des MNs, les connections synaptiques qu’ils reçoivent et la plasticité synaptique qu’ils expriment. Afin d’analyser les conséquences fonctionnelles de la modulation exercée par les mGluRs sur la circuiterie motrice, nous avons analysé l’effet des agonistes mGluR sur (1) l’activité de locomotion fictive induite électriquement dans la préparation de moelle épinière isolée et (2) sur le comportement de nage des souriceaux avec des enregistrements électromyographiques. Nous avons pu montrer que l’expression et le rôle des différents sous-types de mGluRs évoluent au cours du développement chez la souris.
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Key points: Spinal motoneurons transmit signals to skeletal muscles to regulate their contraction. Motoneuron firing partly depends on their intrinsic properties, such as the strength of persistent (long-lasting) inward currents (PICs) that make motoneurons more responsive to excitatory input. In this study, we demonstrate that both reciprocal inhibition onto motoneurons and whole-body relaxation reduce the contribution of PICs to human motoneuron firing. This was observed through analysis of the firing of single motor units during voluntary contractions. However, an alternative technique that involves tendon vibration and neuromuscular electrical stimulation to evoke involuntary contractions showed less effect. Thus, it remains unclear whether this alternative technique can be used to estimate PICs under all physiological conditions. These results improve our understanding of the mechanisms of PIC depression in human motoneurons. Potentially, non-pharmacological interventions, such as electrical stimulation or relaxation could attenuate unwanted PIC-induced muscle contractions in conditions characterised by motoneuron hyperexcitability. Abstract: Persistent inward currents (PICs) are crucial for initiation, acceleration, and maintenance of motoneuron firing. As PICs are highly sensitive to synaptic inhibition and facilitated by serotonin and noradrenaline, we hypothesised that both reciprocal inhibition (RI) induced by antagonist nerve stimulation and whole-body relaxation (WBR) would reduce PICs in humans. To test this, we estimated PICs using the well-established paired motor unit (MU) technique. High-density surface electromyograms were recorded from gastrocnemius medialis during voluntary, isometric 20-s ramp, plantarflexor contractions and decomposed into MU discharges to calculate delta frequency (ΔF). Moreover, another technique (VibStim), which evokes involuntary contractions proposed to result from PIC activation, was used. Plantarflexion torque and soleus activity were recorded during 33-s Achilles tendon vibration and simultaneous 20-Hz bouts of neuromuscular electrical stimulation (NMES) of triceps surae. ΔF was decreased by RI (n = 15, 5 females) and WBR (n = 15, 7 females). In VibStim, torque during vibration at the end of NMES and sustained post-vibration torque were reduced by WBR (n = 19, 10 females), while other variables remained unchanged. All VibStim variables remained unaltered in RI (n = 20, 10 females). Analysis of multiple human MUs in this study demonstrates the ability of local, focused inhibition to attenuate the effects of PICs on motoneuron output during voluntary motor control. Moreover, it shows the potential to reduce PICs through non-pharmacological, neuromodulatory interventions such as WBR. The absence of a consistent effect in VibStim might be explained by a floor effect resulting from low-magnitude involuntary torque combined with the negative effects of the interventions. Abstract figure legend Motoneuron firing partly depends on their intrinsic properties, such as the strength of persistent inward currents (PICs) of calcium (Ca2+ ) and sodium (Na+ ) ions. PICs make motoneurons more responsive to excitatory input. The contribution of PICs to human motoneuron firing is reduced during both reciprocal inhibition induced by electrical stimulation of the antagonist nerve (left) and whole-body relaxation (right; participants were asked to relax while listening to a soothing piano song). This article is protected by copyright. All rights reserved.
Conference Paper
Background and aim: Serotonin and noradrenaline release in the spinal cord are critical to motoneuron firing. Without the influence of these neuromodulators, we would not be able to produce more than 40% of our maximum motor output. An important part of this neuromodulation is the activation of persistent (long-lasting) inward currents (PICs) that make motoneurons more responsive to excitatory input. Because arousal state and level of voluntary activity might influence neuromodulation, we examined the effect of whole-body relaxation (WBR) on PIC strength in plantar flexor motoneurons using the well-established paired motor unit (MU) technique. Methods: High-density surface electromyograms (HDsEMG) of the gastrocnemius medialis were recorded from a grid of 32 equally spaced electrodes (GR10MM0804, 10-mm inter-electrode distance; OT Bioelettronica, Italy). 20-s ramp isometric plantar flexor contractions were performed to 20% of maximal voluntary torque by 20 participants (10 women) in the chair of a dynamometer. Three trials were performed in both control and WBR conditions, in the latter of which participants were asked to deliberately relax while listening to relaxing piano music. HDsEMG signals were decomposed into individual MU discharge events and tracked between conditions. Two trials from each condition were selected and PICs estimated through a paired MU analysis, which quantifies MU recruitment/derecruitment hysteresis. The smoothed firing rate of a lower-threshold MU (control unit) was used as a proxy for the level of net excitatory input to a higher-threshold MU (test unit) and the difference in estimated input at the test unit’s recruitment and derecruitment constituted the ΔF (change in frequency) value. A repeated-measures nested linear mixed effects model analysis of the whole sample of test units was conducted to examine the effect of WBR on ∆F. Results: 174 MUs were decomposed, with 391 pairs (101 test units) included in the ΔF analysis and tracked between conditions. The estimates of PIC strength (ΔF) in control and WBR conditions were 3.51 [2.99, 4.03] and 3.03 [2.51, 3.54] Hz, respectively, revealing a significant ~13.7% decrease (p < 0.001). There was no evidence of significant difference in ΔF across MU recruitment thresholds or between men and women (p > 0.05). Conclusion: These findings suggest that WBR reduces the contribution of PICs to gastrocnemius medialis motoneuron firing during a low-intensity contraction. Given that PIC strength is proportional to the level of neuromodulatory drive, this effect is consistent with an attenuation of serotonergic and noradrenergic release associated with decreases in whole-body muscular activity and global stress levels, respectively. This demonstrates the potential to reduce PICs through non-pharmacological, neuromodulatory interventions such as WBR.
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In unanaesthetized spinal cats, injected with L-DOPA, volleys in the flexor reflex afferents (FRA) evoke a long-latency, longlasting discharge in ipsilateral flexor and contralateral extensor motoneurones. It is postulated that this discharge is transmitted by a neuronal pathway which is inhibited in the normal acute spinal cat, presumably from the pathway, which in this state transmits the shortlatency effect from the FRA to motoneurones. The organization of the pathway released by an injection of DOPA has been analyzed by recording the discharges in efferents to flexors and extensors and with intracellular records from motoneurones. Combined stimulation of ipsilateral and contralateral FRA reveals a reciprocal organization in that either flexor or extensor motoneurones are activated. Transmission from the ipsilateral FRA to flexor motoneurones can be inhibited by volleys in the contralateral FRA, and transmissiqn from the contralateral FRA to extensor motoneurones by volleys in the ipsilateral FRA. These inhibitory effects are neither produced postsynaptically in the motoneurones nor presynaptically by depolarization of primary afferents and are hence exerted at an interneuronal level. The organization of reciprocal innervation at an interneuronal level is discussed in relation to the primary afferent depolarization evoked in Ia afferent terminals after DOPA and to rhythmic alternating movements.
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Plasticity at the neuronal level commonly involves use-dependent changes in strength of particular synaptic pathways or regulation of postsynaptic properties by modulatory transmitters. Here we analyze a novel form of short-term plasticity mediated by use-dependent facilitation of postsynaptic responsiveness. Using current- and voltage-clamp recordings, we found that all spinal ventral horn neurons able to generate plateau potentials showed depolarization-induced facilitation of the underlying inward current. Facilitation was noticeable when the neurons were depolarized to more than -50 mV at intervals <4 s. When stimulation with fast triangular voltage ramps was used, the inward current activated at a less depolarized potential during the second ramp. The inward current and facilitation was eliminated by nifedipine, a selective antagonist of L-type calcium channels. Depolarization-induced facilitation of low-voltage-activated L-type calcium channels is suggested to be the underlying mechanism. It is noted that facilitation occurs on a time scale compatible with a role in phasic motor activity.
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Long-lasting facilitations of spinal nociceptive reflexes resulting from temporal summation of nociceptive inputs have been described on many occasions in spinal, nonanesthetized rats. Because noxious inputs also trigger powerful descending inhibitory controls, we investigated this phenomenon in intact, halothane-anesthetized rats and compared our results with those obtained in other preparations. The effects of temporal summation of nociceptive inputs were found to be very much dependent on the type of preparation. Electromyographic responses elicited by single square-wave electrical shocks (2 ms, 0.16 Hz) applied within the territory of the sural nerve were recorded in the rat from the ipsilateral biceps femoris. The excitability of the C-fiber reflex recorded at 1.5 times the threshold (T) was tested after 20 s of electrical conditioning stimuli (2 ms, 1 Hz) within the sural nerve territory. During the conditioning procedure, the C-fiber reflex was facilitated (wind-up) in a stimulus-dependent fashion in intact, anesthetized animals during the application of the first seven conditioning stimuli; thereafter, the magnitude of the responses reached a plateau and then decreased. Such a wind-up phenomenon was seen only when the frequency of stimulation was 0.5 Hz or higher. In spinal, unanesthetized rats, the wind-up phenomenon occurred as a monotonic accelerating function that was obvious during the whole conditioning period. An intermediate picture was observed in the nonanesthetized rat whose brain was transected at the level of the obex, but the effects of conditioning were profoundly attenuated when such a preparation was anesthetized. In intact, anesthetized animals the reflex was inhibited in a stimulus-dependent manner during the postconditioning period. These effects were not dependent on the frequency of the conditioning stimulus. Such inhibitions were blocked completely by transection at the level of the obex, and in nonanesthetized rats were then replaced by a facilitation. A similar long-lasting facilitation was seen in nonanesthetized, spinal rats. It is concluded that, in intact rats, an inhibitory mechanism counteracts the long-lasting increase of excitability of the flexor reflex seen in spinal animals after high-intensity, repetitive stimulation of C-fibers. It is suggested that supraspinally mediated inhibitions also participate in long term changes in spinal cord excitability after noxious stimulation.
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We have investigated sacral spinal cord lesions in rats with the goal of developing a rat model of muscular spasticity that is minimally disruptive, not interfering with bladder, bowel, or hindlimb locomotor function. Spinal transections were made at the S2 sacral level and, thus, only affected the tail musculature. After spinal transection, the muscles of the tail were inactive for 2 weeks. Following this initial period, hypertonia, hyperreflexia, and clonus developed in the tail, and grew more pronounced with time. These changes were assessed in the awake rat, since the tail is readily accessible and easy to manipulate. Muscle stretch or cutaneous stimulation of the tail produced muscle spasms and marked increases in muscle tone, as measured with force and electromyographic recordings. When the tail was unconstrained, spontaneous or reflex induced flexor and extensor spasms coiled the tail. Movement during the spasms often triggered clonus in the end of the tail. The tail hair and skin were extremely hyperreflexive to light touch, withdrawing quickly at contact, and at times clonus could be entrained by repeated contact of the tail on a surface. Segmental tail muscle reflexes, e.g., Hoffman reflexes (H-reflexes), were measured before and after spinalization, and increased significantly 2 weeks after transection. These results suggest that sacral spinal rats develop symptoms of spasticity in tail muscles with similar characteristics to those seen in limb muscles of humans with spinal cord injury, and thus provide a convenient preparation for studying this condition.
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Our intent in this review was to consider the relationship between the biophysical properties of motoneurons and the mechanisms by which they transduce the synaptic inputs they receive into changes in their firing rates. Our emphasis has been on experimental results obtained over the past twenty years, which have shown that motoneurons are just as complex and interesting as other central neurons. This work has shown that motoneurons are endowed with a rich complement of active dendritic conductances, and flexible control of both somatic and dendritic channels by endogenous neuromodulators. Although this new information requires some revision of the simple view of motoneuron input-output properties that was prevalent in the early 1980's (see sections 2.3 and 2.10), the basic aspects of synaptic transduction by motoneurons can still be captured by a relatively simple input-output model (see section 2.3, equations 1-3). It remains valid to describe motoneuron recruitment as a product of the total synaptic current delivered to the soma, the effective input resistance of the motoneuron and the somatic voltage threshold for spike initiation (equations 1 and 2). However, because of the presence of active channels activated in the subthreshold range, both the delivery of synaptic current and the effective input resistance depend upon membrane potential. In addition, activation of metabotropic receptors by achetylcholine, glutamate, noradrenaline, serotonin, substance P and thyrotropin releasing factor (TRH) can alter the properties of various voltage- and calcium-sensitive channels and thereby affect synaptic current delivery and input resistance. Once motoneurons are activated, their steady-state rate of repetitive discharge is linearly related to the amount of injected or synaptic current reaching the soma (equation 3). However, the slope of this relation, the minimum discharge rate and the threshold current for repetitive discharge are all subject to neuromodulatory control. There are still a number of unresolved issues concerning the control of motoneuron discharge by synaptic inputs. Under dynamic conditions, when synaptic input is rapidly changing, time- and activity-dependent changes in the state of ionic channels will alter both synaptic current delivery to the spike-generating conductances and the relation between synaptic current and discharge rate. There is at present no general quantitative expression for motoneuron input-output properties under dynamic conditions. Even under steady-state conditions, the biophysical mechanisms underlying the transfer of synaptic current from the dendrites to the soma are not well understood, due to the paucity of direct recordings from motoneuron dendrites. It seems likely that resolving these important issues will keep motoneuron afficiandoes well occupied during the next twenty years.
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The effects of serotonin (5-HT) on intrinsic properties of spinal motoneurons were investigated with intracellular recordings in a slice preparation from adult turtles. In 55% of the cells that were recorded, addition of 5-HT to the extracellular medium promoted plateau potentials as revealed by the response to depolarizing current pulses applied through the intracellular electrode. In the remaining 45% of cells, 5-HT had an inhibitory effect. However, when tested with an applied electric field that preferentially polarizes distal dendrites, 5-HT facilitated plateau potentials in 100% of the cells. Plateau potentials were also promoted by 5-HT focally applied on a dendrite by iontophoresis. Applied near the soma, 5-HT either promoted plateau potentials or inhibited spike generation. The latter effect was accompanied by a decrease in input resistance. Voltage-clamp recordings showed that the facilitation of plateau potentials mediated by L-type Ca(2+) channels was due to activation of 5-HT(2) receptors. These findings show that 5-HT regulates intrinsic properties of motoneurons in opposite ways: activation of 5-HT receptors in the soma region inhibits spike generation and plateau potentials, while activation of 5-HT(2) receptors in the dendrites and the soma region promotes spiking by facilitation of plateau potentials mediated by L-type Ca(2+) channels.
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The physiological basis of flexion spasms in individuals after spinal cord injury (SCI) may involve alterations in the properties of spinal neurons in the flexion reflex pathways. We hypothesize that these changes would be manifested as progressive increases in reflex response with repetitive stimulus application (i.e., "windup") of the flexion reflexes. We investigated the windup of flexion reflex responses in 12 individuals with complete chronic SCI. Flexion reflexes were triggered using trains of electrical stimulation of plantar skin at variable intensities and inter-stimulus intervals. For threshold and suprathreshold stimulation, windup of both peak ankle and hip flexion torques and of integrated tibialis anterior electromyographic activity was observed consistently in all patients at inter-stimulus intervals < or =3 s. For subthreshold stimuli, facilitation of reflexes occurred only at intervals < or =1 s. Similarly, the latency of flexion reflexes decreased significantly at intervals < or =1 s. Patients that were receiving anti-spasticity medications (e.g., baclofen) had surprisingly larger windup of reflex responses than those who did not take such medications, although this difference may be related to differences of spasm frequency between the groups of subjects. The results indicate that the increase in spinal neuronal excitability following a train of electrical stimuli lasts for < or =3 s, similar to previous studies of nociceptive processing. Such long-lasting increases in flexion reflex responses suggest that cellular mechanisms such as plateau potentials in spinal motoneurons, interneurons, or both, may partially mediate spinal cord hyperexcitability in the absence of descending modulatory input.
Article
Previous studies have indicated that electrical stimulation of the hypothalamus increases erythropoiesis (Seip et al. 1961) and erythropoietin production (Halvorsen 1961). The effects of hypothalamic lesions on the erythropoietic response to hypoxia have been investigated in the present study. The lesioned rabbits were exposed to low oxygen and the erythropoietic response evaluated by means of reticulocyte counts. Eleven out of 26 rabbits showed a normal response to hypoxia while 15 showed a reduced response. Of these 15 rabbits, 6 showed a normal response when ACTH was given prior to the hypoxic test indicating that the reduced response to hypoxia in these rabbits was due to ACTH deficiency following the hypothalamic lesion. The lesions in these 6 rabbits were in the ventral and median part of hypothalamus. Seven out of the 15 rabbits neither responded normally to hypoxia alone nor to hypoxia following ACTH injections. The lesions in these 7 rabbits were in the posterior part of the hypothalamus. It is concluded that an extra-pituitary influence on erythropoiesis may be exerted from this region.
Article
1. The present study investigated regulation of reflex excitability after experimental contusion injury of the spinal cord. 2. Four measures of H-reflex excitability were evaluated in normal rats and at 6, 28, and 60 days after contusion injury at the T8 level: 1) reflex thresholds, 2) slope of the reflex recruitment curves, 3) maximal plantar H-reflex/maximal plantar M-response (Hmax/Mmax) ratios, and 4) rate-sensitive depression (i.e., the decrease in reflex magnitude relative to repetition rate). 3. Tested as a function of the afferent volley magnitude, the thresholds for reflex initiation fell progressively subsequent to contusion injury. No change was observed at 6 days postinjury, and the decrease at 28 days was not significant. However, by 60 days postinjury, the threshold had decreased by 23% of the maximal afferent volley, and this decrease was significant, [analysis of variance (ANOVA, P < or = 0.01)]. 4. Hmax/Mmax ratios elicited in postcontusion animals at 0.3 Hz were not significantly different from those recorded in normal animals. 5. The slopes of the recruitment curves were markedly reduced subsequent to contusion injury. The decrease was greatest at 6 days postinjury. Although some recovery toward normal occurred at 28 and 60 days postinjury, the slopes of recruitment curves in postcontusion animals remained significantly decreased. 6. H-reflexes elicited at 1-5 Hz were less sensitive to rate depression in postcontusion animals than in normal animals at the same respective frequencies. The decrease was progressive in onset, becoming significant by 28 days postinjury, and of an enduring nature, i.e., still significantly different from normal in the reflexes tested 60 days postinjury. 7. Rate sensitivity of the tibial nerve monosynaptic reflex (MSR) was also compared in normal and postcontusion animals. Rate sensitivity of the tibial MSRs was significantly reduced at 28 and 60 days post-contusion, compared with normal animals. 8. These data indicate that significant changes in lumbar reflex excitability result from midthoracic contusion injury of the spinal cord. These changes include reflex threshold, slope of recruitment, and rate-sensitive depression. Although recruitment slope was most altered in the shortest postinjury interval tested, followed by some recovery, the other changes were progressive in onset and enduring in duration.
Article
1. The hypothesis that the exaggerated tendon jerks and stretch reflexes that follow chronic spinal cord lesions in humans result from alterations in transmission from group I muscle afferents to motoneurons was tested by making observations on nine normal subjects and 25 patients with spinal cord lesions. All the patients had increased tendon jerks, one-third of them had both increased tendon jerks and increased, velocity-dependent stretch reflexes (i.e.g spasticity). 2. Changes in the firing probability of single, voluntary-activated soleus or tibialis anterior motor units during stimulation of the muscle nerve below the threshold of the alpha-motoneuron axons were used to derive the characteristics of the postsynaptic potentials produced by group I volleys in single motoneurons. Paired stimuli were used to test how multiple volleys in group I muscle afferents were transmitted to motoneurons. 3. Stimulation of the posterior tibial nerve produced a short-latency period of increased firing probability representing the homonymous composite Ia excitatory postsynaptic potential (EPSP) in all soleus motoneurons tested. There was no detectable alteration in the magnitude, duration, or profile of the short-latency facilitation in the patients with spinal lesions when compared with normal subjects. 4. In patients with traumatic spinal cord lesions less than 8 wk in duration the magnitude of the facilitation representing the composite Ia EPSP was significantly larger than normal, although only one out of the four patients in this group had spasticity. 5. In the patients with the greatest spasticity, group I volleys produced a second period of facilitation 11-15 ms after the facilitation representing the composite Ia EPSP. This is presumed to represent enhanced transmission through polysynaptic pathways from group I afferents to motoneurons. 6. In normal subjects the facilitation of motoneurons produced by the second of two group I volleys is greater 5 and 10 ms after the first volley and less 20, 30, and 50 ms after the first volley. These changes involve at least two factors: 1) changes in excitability of peripheral nerves and 2) changes in transmission at the Ia-motoneuron synapse. 7. In patients with spinal lesions the facilitation produced by the second of two muscle-afferent volleys was less depressed at the 30-ms interstimulus interval. 8. Thus two separate abnormalities have been uncovered in human subjects with chronic spinal lesions: 1) a change in the transmission of multiple volleys from muscle afferents to motoneurons and 2) an increase in transmission through polysynaptic pathways from Ia afferents to motoneurons. Both could contribute to the increased tendon jerks and exaggerated stretch reflexes.
Article
1. The effect of serotonin on the firing properties of motoneurones was studied in transverse sections of the adult turtle spinal cord in vitro with intracellular recording techniques. 2. In normal medium, turtle motoneurones adapt from an initial high frequency to a low steady firing during a depolarizing current pulse. In the presence of serotonin (4-100 microM) motoneurones responded with accelerated firing and a frequency jump during a depolarizing current pulse followed by an after-depolarization outlasting the stimulus. From a depolarized holding potential motoneuronal activity was shifted between two stable states by brief depolarizing and hyperpolarizing current pulses. As an expression of this bistable firing behaviour, the frequency-current relation in response to a triangular current injection was counter-clockwise in serotonin while clockwise in normal medium. 3. The delay to onset of the frequency jump was shortened as the amplitude of the activation pulse was increased. From a positive holding potential the after-depolarization exceeded spike threshold and its duration increased with an increase in steady bias current. The effect of serotonin on turtle motoneurones could be blocked by methysergide (10 microM). 4. When action potentials were depressed by tetrodotoxin, a voltage-dependent, non-inactivating plateau potential, intrinsic to the motoneurone, was revealed. Activation of this voltage plateau provides the motoneurones with two stable states of firing. The apparent input resistance was 2-4-fold lower during the plateau than at rest. 5. The serotonin-induced plateau potential was Ca2+-dependent and was blocked when Ca2+ was replaced by either Co2+ (3 mM) or Mn2+ (3 mM). 6. The Ca2+ plateau was blocked by nifedipine (1-15 microM). 7. Serotonin reduced the slow after-hyperpolarization following action potentials. The change in balance between inward and outward currents seems to be sufficient to reveal the plateau response. 8. Although a small plateau response was induced by Bay K 8644 (1-15 microM), this L-channel agonist could not reproduce the pronounced effect of serotonin. 9. It is concluded that serotonin induces a Ca2+-dependent and nifedipine-sensitive plateau potential in turtle motoneurones primarily by reducing a K+-current responsible for the slow after-hyperpolarization.
Article
Electromyographic (EMG) flexor muscle responses evoked by electrical stimulation of ipsilateral peripheral nerves were studied in 16 patients with clinically complete spinal cord transection. Stimuli were applied either to a cutaneous nerve (sural) or to a mixed nerve (tibial) and muscle responses were recorded from tibialis anterior, biceps femoris and rectus femoris. EMG recordings after both sural and tibial nerve stimulation showed that distinct early and late ipsilateral flexor muscle responses could be elicited. This distinction was more evident for tibialis anterior. The latency of the early responses averaged approximately 100 ms with sural and 75 ms with tibial nerve stimulation. This corresponds to the latency of the flexion withdrawal reflex previously described in normal man. After sural stimulation, the early reflex appeared in biceps femoris at a threshold intensity not significantly different from that in normal man given the same stimulation parameters. Late responses appeared after a longer latency (130 ms) and at a lower threshold than the early flexor reflex. In all patients a striking feature of the late response was that its latency increased with increasing stimulus intensity, the maximum latency being as long as 450 ms. This increase also occurred with increasing duration of high intensity stimulus trains. Neither the appearance of a late response nor its latency increase could be explained by a peripheral loop due to a preceding muscle contraction (from either motor axon stimulation or motoneuronal discharge corresponding to the early flexion reflex). It was therefore concluded that both were directly elicited by the afferent volley set up by electrical stimulation. The low threshold of the late reflex corresponded to the excitation of relatively rapidly conducting afferents and its central spinal delay was more than 100 ms. The late reflexes were compared with those described by Andén et al. (1964) in the acute spinal cat injected with DOPA and were found to have similar characteristics. The mechanism for the increase in latency of the late response is discussed in relation to the interpretation of Lundberg (1979).
Article
1. Intracellular recordings were made from lumbar alpha-motoneurones in unanaesthetized decerebrate acute spinal cats. The response of motoneurones to direct current pulse injection or synaptic excitation was investigated following intravenous injection of L-beta-3,4-dihydroxyphenylalanine (L-DOPA, 20-120 mg/kg) alone, nialamide (10-50 mg/kg) and L-DOPA or clonidine (0.5-1 mg/kg). 2. The response properties of motoneurones were tested with rectangular and triangular current waveforms. Before L-DOPA treatment motoneuronal firing during a rectangular current pulse is characterized by an initial high firing frequency which rapidly decreases to a lower steady-state firing which is maintained only for the duration of the pulse. Following administration of L-DOPA an acceleration in firing frequency is apparent following the initial adaptation seen with rectangular current pulses. A transient after-depolarization or an after-discharge often followed the termination of the pulse. The frequency-current relation in response to a triangular current injection changed from a clockwise to a counter-clockwise hysteresis after L-DOPA treatment (i.e. after L-DOPA the firing frequency was higher for any given current during the descending phase than during the ascending phase of the triangular waveform). 3. Firing acceleration during and self-sustained firing after rectangular current pulses and counter-clockwise hysteresis of firing frequency with triangular current pulses are causally related to the presence of plateau potentials, which can be directly visualized after inactivation of the spikes. Plateau potentials in motoneurones could be generated by short-lasting intracellular depolarizing current pulses or brief excitatory synaptic inputs and terminated by short-lasting hyperpolarizing current pulses or brief inhibitory synaptic inputs. Plateau potentials were demonstrated in flexor and extensor motoneurones. 4. All bistable properties described in the preceding paragraphs following L-DOPA administration could also be seen after administration of the alpha-receptor agonist clonidine. 5. Slow rhythmic oscillations of the membrane potential (7.5-10 Hz) were seen superimposed on plateau potentials in a few cells after administration of L-DOPA and clonidine. The oscillations had an amplitude in the range 10-20 mV and represent the expression of an intrinsic property of the motoneurone. 6. It is demonstrated that plateau potentials in the motoneurones contribute to the late long-lasting reflexes observed in L-DOPA-treated spinal cats. 7. It is concluded that L-DOPA (and clonidine) change the response properties of the motoneurones in an analogous way to 5-hydroxy-DL-tryptophan (5-HTP).(ABSTRACT TRUNCATED AT 400 WORDS)
Article
1. In the preceding paper (Crone, Hultborn, Kiehn, Mazieres & Wigström, 1988) it was shown that a short-lasting synaptic excitation ('on' stimulus) of extensor motoneurones (primarily triceps surae) in the decerebrate cat often resulted in a maintained excitability increase, which could be reset by a short-lasting inhibitory stimulus train ('off' stimulus). In the present experiments intracellular recording from triceps surae motoneurones and the electroneurogram (ENG activity) from triceps surae nerve branches were performed in parallel. 2. Sustained firing of individual triceps surae motoneurones was most often recorded in parallel with the maintained ENG activity following a synaptic 'on' stimulus. When the motoneurone was silenced, by a hyperpolarizing current through the microelectrode, there was no sign of on-going synaptic excitation during the maintained ENG activity following an 'on' stimulus. It was therefore suggested that voltage-dependent intrinsic properties of the motoneurones themselves could be responsible for the maintained firing. 3. In confirmation of this hypothesis it was found that short-lasting depolarizing current pulses through the recording microelectrode could trigger a self-sustained firing in the motoneurone provided that the bias current (i.e. the holding potential) was kept within certain limits. Hyperpolarizing current pulses terminated the firing. When the spike-generating mechanism was inactivated (by long-lasting excessive depolarization) similar depolarizing and hyperpolarizing current pulses could initiate and terminate plateau potentials in the motoneurones. By grading the depolarizing current pulses it was found that the plateau potentials were of all-or-none character, typically around 10 mV in amplitude. The two levels of excitability which can be triggered by short-lasting excitation and inhibition of the motoneurones is referred to as 'bistable' behaviour of the motoneurones. 4. After an acute spinal transection, in the unanaesthetized cat, the bistable behaviour of the motoneurones disappeared. However, it reappears following intravenous injection of the serotonin precursor 5-hydroxytryptophan (50-120 mg/kg). 5. Individual triceps surae motor units were recorded by selective EMG electrodes during tonic stretch reflexes in the decerebrate preparations. Based on an analysis of their firing pattern during lengthening and shortening (or vibration) of the muscle it is suggested that plateau potentials in motoneurones are recruited during the tonic stretch reflex. Furthermore, it is argued that a quantitatively important part of the depolarization of motoneurones during the tonic stretch reflex indeed originates from these plateau potentials.(ABSTRACT TRUNCATED AT 400 WORDS)
Article
The purpose of this study was to characterize the changes in postsynaptic potentials recorded in ankle extensor motoneurons resulting from activation of the sural nerve after spinal cord transection in the adult cat. Eight acute and nine chronic animals were spinalized at T12. Intracellular recordings from motoneurons innervating the triceps surae were performed. Sural nerve stimulation evoked complex synaptic potentials consisting of early and late components in all motoneurons. Early excitatory and inhibitory postsynaptic potentials (PSPs), as well as long latency excitatory postsynaptic potentials were recorded and averaged for assessment of PSP amplitude and duration. Early PSPs, both excitatory and inhibitory, were significantly larger in the motoneurons of cats spinalized 4-6 months earlier. Central latency of excitatory potentials were similar in the two samples of motoneurons, but the central latency associated with the initial inhibitory PSP was significantly shorter in the recordings from motoneurons of chronic spinal cats. In most recordings, an additional inhibitory PSP followed the initial excitatory PSP in motoneurons, and this secondary inhibitory PSP was similar in peak amplitude and duration in both samples of motoneurons. Also, a long latency excitatory PSP was recorded in a large percentage of motoneurons from both samples. This potential was typically of greater amplitude and longer duration in the motoneurons from chronic animals, when compared to recordings from acute animals. Although changes in amplitude and duration of PSP activity could be documented, there was no marked alteration in the frequency of occurrence of each PSP pattern recorded from the two preparations. This suggests that the synaptic pathways mediating the sural nerve reflexes have not qualitatively changed in the chronic spinal animal. The changes in amplitudes and durations of the PSPs in the chronic spinal cat indicate, however, that quantitative changes have occurred. The quantitative changes have probably occurred in the interneuronal networks activated by cutaneous nerve (sural) stimulation, since it was shown that only minor changes in motoneuron membrane properties could be recorded in these same chronic spinal animals.
Article
The human stretch reflex is well known to show 'automatic gain compensation'; in other words, the electromyographic (e.m.g.) response evoked by a given disturbance increases progressively with the level of pre-existing voluntary activity, and so remains an approximately constant proportion of the background. Such behaviour has now been observed using vibration as the stimulus to Ia action and recording the reflexly developed force, in addition to the e.m.g. Inhibition was studied as well as excitation by vibrating the antagonist as well as the agonist, and found to be similarly regulated. The experiments were performed on the elbow flexors while they were contracting isometrically under voluntary drive. The vibration was either square-wave modulated at 5 Hz or delivered in bursts of one to five pulses. The latency of the e.m.g. responses produced by the latter was sufficiently short to show that gain compensation was a feature of spinal reflex action. In the Discussion, it is concluded that in principle 'automatic gain compensation' can be readily attributed to the known organization of the motoneurone pool. As the background force increases so does the number of active motoneurones available to be frequency-modulated by a given input, and the larger and stronger will be those motor units which are on the verge of recruitment or de-recruitment.
Article
Intracellular recordings have been made from twenty antidromically identified posterior biceps femoris/semitendinosus (p.b.s.t.) hamstring flexor alpha-motoneurones in the decerebrate-spinal rat. The hamstring motoneurones had either low or no spontaneous background activity. In nineteen of the twenty cells high-frequency phasic responses could be elicited by stimulation of the ipsilateral hind paw with firm pressure or pinch. There was no response to light touch or brush. Contralateral cutaneous mechanoreceptive fields with higher thresholds and weaker responses were present in 70% of the motoneurones. Noxious heating of the ipsilateral hind paw produced excitatory responses in six of eight cells tested and two of these cells also responded to heating of the contralateral hind paw. Stimulation of the ipsilateral sural nerve at graded strengths that successively activated A beta, A delta and C afferents produced excitatory post-synaptic potentials (e.p.s.p.s) at progressively longer latencies in the motoneurones. The C-fibre induced e.p.s.p. lasted up to 200 ms. Horseradish peroxidase was injected into ten motoneurones and in seven cases full reconstructions of dendritic field, cell body and axon could be made. In agreement with previous reports from studies in the cat, the dendritic fields of rat motoneurones are very extensive in the rostrocaudal, mediolateral and dorsoventral planes. The general pattern of dendritic branching for each motoneurone in this functionally homogeneous population was uniformly organized. Three major spatial orientations were always present: a rostrocaudally restricted series of dendrites emerging from the cell body and directed dorsolaterally towards the dorsolateral funiculus with branches in the lateral dorsal horn, a laterally, and a ventromedially directed series of branches arranged obliquely in the ventral horn, both of which were distributed rostrocaudally for equal distances from the cell body. Many of these dendritic branches terminated within the lateral and ventral white columns. Although the sizes of the rat flexor motoneurones' somas (51 +/- 4.9 micron, S.E., n = 10) were similar to those of cat lumbosacral alpha-motoneurones, the tip-to-tip rostrocaudal extent of their dendritic fields (1130 +/- 34 micron, S.E., n = 7) was half that reported in the cat. These results are discussed in terms of the organization of the cutaneous flexor withdrawal reflex in the rat.
Article
1. The examination consisted of a histological analysis using paraffin sections, gold chloride staining and osmic acid techniques.2. The parallel spindle system found in the rat tail muscles is compared with that found in frog muscles. The rat spindle system occasionally included extrafusal fibres.3. It was evident that there are basic Class differences between the frog and rat tail parallel spindles, but it is suggested that they may serve a similar proprioceptive role in muscles having almost isometric contractions.4. Other short muscles of the rat that might have an analogous function to the tail muscles were examined, but only the capitus and deep masseter muscles showed parallel spindle systems, and none contained extrafusal fibres.5. Of other mammals examined, the muscles of the tails of the cat, rabbit, mouse and guinea-pig contained parallel spindles, while the dog and mole showed none in the small sample taken.6. Nerve trunk fibre size histograms, including de-efferented trunks, indicated a high proportion of sensory nerve fibres from the tail (55%), a low motor unit ratio (1:60) and a beta axon motor supply both to the spindles and to the small muscles of the tail.7. A generalized schema of the rat tail anatomy is presented, and a tabulation of the large sensory endings expected to be found in any particular small tail muscle.8. It was concluded that the rat tail has a spindle density enough to provide a proprioceptive sensitivity equivalent to that of a caudal digit.
Article
The time course of changes in monosynaptic reflex amplitude, after conditioning from both ipsi- and contralateral sural nerves at different stimulus strengths, was studied on two antagonistic motoneuronal pools acting on ankle muscles in spinal cats. Attention was focused on late effects, namely those appearing after a dely of more than 30 ms from the cutaneous stimulus. With low-threshold afferent activation, at conditioning-test intervals to 30 ms, the ipsilateral extensor monosynaptic reflex, recorded from the proximal stump of L7 to S1 ventral roots, showed marked inhibition; at longer intervals, a late facilitation period (LFP) lasting to 100 ms was observed. Increasing stimulus strength did not modify the time course of reflex excitability, but might enhance the amount of the facilitatory effecct. On the flexor monosynaptic reflex, sural conditioning induced, after the expected early facilitation, a second facilitatory period, starting at about 30 ms and recovering at about 130 ms. The excitability of antagonistic contralateral motoneuronal pools was also influenced, showing again a LFP with the same time course. The LFP was present after stimulation of the sural and saphenous nerves and was absent after stimulation of a muscle nerve. These late, long-lasting and nonreciprocal facilitatory effects on flexor and extensor ipsilateral motoneurons were quite distinct from the early reciprocal responses, and were evoked by large cutaneous fibers. An interpretation is put forward in light of the primary afferent hyperpolarization of Ia afferent terminals. A correlation is tentatively proposed with the mechanism subserving the stumbling corrective reaction.
Article
An acute animal model (dorsal hemisection of the spinal cord in the decerebrate cat preparation) has been developed that closely mimics the spasticity in humans that occurs subsequent to partial spinal cord injury and hemiparetic stroke. In this animal model, there are severe disruptions in the pattern of recruitment and rate modulation of motoneurons. The cellular mechanisms of these deficits are being studied with a combined experimental/computer simulation approach. The initial studies indicate that changes in the intrinsic properties of motoneurons are not important, which means the mechanism for changes in recruitment and rate patterns must reside in an alteration in the organization of the synaptic input to motoneurons. Computer simulation studies of the effects of different synaptic inputs on motoneuron outputs show that inhibitory inputs can, under certain conditions, generate substantial disruptions in recruitment and rate modulation. Recent data indicate that the monoamines noradrenalin and serotonin, which are released by fiber tracts originating in the brainstem, may play an important role in maintaining normal levels of inhibition in spinal circuits. Pharmacological interventions based on the monoamines may provide effective means of reducing the deficits in recruitment and rate modulation.
Article
1. Steady-state postsynaptic potentials (PSPs) were generated by prolonged (approximately 1 s) high-frequency (100-200 Hz) electrical stimulation of nerves in the cat hindlimb. The characteristics of these steady-state PSPs were compared for two polysynaptic afferent pathways (ipsilateral cutaneous sural vs. contralateral peroneal nerves), two animal preparations (decerebrate vs. chloralose), and two motoneuron pools (medial gastrocnemius vs. lateral gastrocnemius-soleus). 2. PSPs from both nerves usually (36 of 51 cases) contained a mixture of depolarizing and hyperpolarizing components. In all 36 cases where the PSP contained a hyperpolarizing component, a consistent qualitative pattern emerged during prolonged stimulation: the hyperpolarization reached a peak approximately 20 ms after stimulation onset and then decayed with a biphasic time course that consisted of an initial rapid phase (20-40 ms) and a later slower phase (200-400 ms) before the steady-state value was reached. This pattern occurred regardless of the differences in polysynaptic afferent pathways, animal preparations, and motoneuron pools. 3. The consistency of this overall pattern was remarkable, given the existence of several quantitative differences among the PSPs. These differences include the following: hyperpolarizing components were least common in the sural and peroneal PSPs in the decerebrate preparation. And only these sural and peroneal PSPs tended to have prolonged afterpotentials after stimulus cessation. The steady-state sural PSPs in the decerebrate preparation tended to generate the largest PSPs and, moreover, these PSPs did not follow the overall trend of having a statistically significant relation between the amplitude of the initial hyperpolarization and the amount of its decay. Finally, transient sural PSPs in lateral gastrocnemius-soleus motoneurons in the decerebrate preparation tended to have the largest hyperpolarizations. 4. To determine whether the decay of the hyperpolarization and the subsequent dominance of depolarization was due to a decreased inhibition or an increased excitation, injected current pulses were utilized to measure the changes in the cell's input resistance during the course of the synaptic input. A strong decrease in input resistance accompanied the initial period of maximal hyperpolarization (50% with respect to the resting input resistance). Input resistance then returned toward resting values as hyperpolarization faded and depolarization became dominant. However, there remained a persistent decrease in input resistance during the final phase of the PSP that amounted to < 10% of the initial decrease. These findings indicated that much of the reduction in hyperpolarization reflected a progressive decrease in synaptic efficacy for the inhibition.(ABSTRACT TRUNCATED AT 400 WORDS)
Article
1. In order to study fusimotor control in reduced preparations, soleus muscle spindle afferents were recorded in premammillary decerebrate cats (n = 15) during crossed extensor reflexes and, after spinalization, during locomotion produced by either clonidine or L-beta-3,4-dihydroxyphenylalanine (L-DOPA). The soleus muscle was oscillated sinusoidally (0.25 mm, 4 Hz) and the afferent mean firing rate and modulation were calculated. An increase in firing rate was assumed to arise from activity in dynamic gamma-motoneurones (dynamic gamma-drive) when associated with an increase in modulation to stretching, and in static gamma-motoneurones (static gamma-drive) when modulation decreased. 2. At rest in all preparations the firing rate and modulation in primary muscle spindle afferents were generally much higher than after de-efferentation (ventral root section), suggesting a predominant dynamic gamma-drive. Clonidine decreased and even eliminated this presumed resting gamma-drive in many afferents, both in the decerebrate (7 of 8) and the spinal (6 of 18) state. This effect on gamma-drive may account, at least in part, for its suppressive effect on spasticity in humans. 3. When locomotion commenced in clonidine-treated spinal cats, primary afferents generally fired with much higher mean rates (+121%) and lower sensitivities (-32%), suggesting a large increase in static gamma-drive (possibly accompanied by a small decrease in dynamic gamma-drive). These high rates were usually maintained tonically throughout the step cycle. However, a third of the afferents were silenced during locomotor contractions, and de-efferentation had no significant effect on their firing rates. Thus, for some spindles alpha-activity can occur without significant gamma-drive. 4. During locomotion in L-DOPA-treated spinal cats the inferred static gamma-drive only occurred phasically, coactivated with the EMG, though it could precede the EMG by 100-500 ms. In the flexion phase both the afferent rate and modulation were lower than before locomotion, suggesting a lack of effective gamma-drive. 5. Crossed extensor reflexes in decerebrate cats also produced a substantial increase in primary afferent firing rate (+187%) and decrease in sensitivity (-37%), again suggesting increased static gamma-drive (n = 18). This gamma-drive was largely independent of EMG activity and often occurred without alpha-activity. The mean firing rate of secondary muscle spindle afferents increased significantly during locomotion (with L-DOPA) and crossed extensor reflexes, again indicating increased static gamma-drive. Clonidine reduced or eliminated the gamma-drive in seven of eight afferents during crossed extensor reflexes. 6. In conclusion, although there are some common features, such as a predominant static gamma-drive in all walking preparations, the pattern of static and dynamic gamma-drive is not closely linked to alpha-activity under the conditions studied. As well as gamma-drive without alpha-activity, we have shown for the first time that alpha-motoneurones can be activated without significant gamma-drive to many spindles during behavioural tasks.
Article
1. In decerebrated, non-spinalized rabbits, intrathecal administration of either of the selective 5-HT1A-receptor antagonists (S)WAY-100135 or WAY-100635 resulted in dose-dependent enhancement of the reflex responses of gastrocnemius motoneurones evoked by electrical stimulation of all myelinated afferents of the sural nerve. The approximate ED50 for WAY-100635 was 0.9 nmol and that for (S)WAY-100135 13 nmol. Intrathecal doses of the antagonists which caused maximal facilitation of reflexes in non-spinalized rabbits had no effect in spinalized preparations. 2. In non-spinalized animals, intravenous administration of (S)WAY-100135 was significantly less effective in enhancing reflexes than when it was given by the intrathecal route. 3. When given intrathecally, the selective 5-HT 2A/2C-receptor antagonist, ICI 170,809, produced a bellshaped dose-effect curve, augmenting reflexes at low doses (< or = 44 nmol), but reducing them at higher doses (982 nmol). Idazoxan, the selective alpha 2-adrenoceptor antagonist, was less effective in enhancing reflex responses when given intrathecally after ICI 170,809 compared to when it was given alone. Intravenous ICI 170,809 resulted only in enhancement of reflexes and the facilitatory effects of subsequent intrathecal administration of idazoxan were not compromised. 4. The selective 5-HT3-receptor blocker ondansetron faciliated gastrocnemius medialis reflex responses in a dose-related manner when given by either intrathecal or intravenous routes. This drug was slightly more potent when given i.v. and it did not alter the efficacy of subsequent intrathecal administration of idazoxan. 5. None of the antagonists had any consistent effects on arterial blood pressure or heart rate. 6. These data are consistent with the idea that, in the decrebrated rabbit, 5-HT released from descending axons has multiple roles in controlling transmission through the sural-gastrocnemius medialis reflex pathway. Thus, it appears 5-HT tonically inhibits transmission between sural nerve afferents and gastrocnemius motoneurones by an action at spinal 5-HT1A-receptors. Spinal 5-HT2A/2C-receptors may mediate a weak inhibition of transmission in the spinal cord, but more convincing evidence was obtained for their involvement in descending facilitatory tone. Further, some of the facilitatory consequences of spinal alpha 2-adrenoceptor blockade may be mediated through 5-HT2 type receptors. Spinal 5-HT3 receptors do not appear to have a major role in tonic modulation of the sural-gastrocnemius medialis reflex.
Article
After spinal cord injury, hyper-reflexia can lead to episodic hypertension, muscle spasticity and urinary bladder dyssynergia. This condition may be caused by primary afferent fiber sprouting providing new input to partially denervated spinal interneurons, autonomic neurons and motor neurons. However, conflicting reports concerning afferent neurite sprouting after cord injury do not provide adequate information to associate sprouting with hyper-reflexia. Therefore, we studied the effect of mid-thoracic spinal cord transection on central projections of sensory neurons, quantified by area measurements. The area of myelinated afferent arbors, immunolabeled by cholera toxin B, was greater in laminae I–V in lumbar, but not thoracic cord, by one week after cord transection. Changes in small sensory neurons and their unmyelinated fibers, immunolabeled for calcitonin gene-related peptide, were assessed in the cord and in dorsal root ganglia. The area of calcitonin gene-related peptide-immunoreactive fibers in laminae III–V increased in all cord segments at two weeks after cord transection, but not at one week. Numbers of sensory neurons immunoreactive for calcitonin gene-related peptide were unchanged, suggesting that the increased area of immunoreactivity reflected sprouting rather than peptide up-regulation. Immunoreactive fibers in the lateral horn increased only above the lesion and in lumbar segments at two weeks after cord transection. They were not continuous with dorsal horn fibers, suggesting that they were not primary afferent fibers. Using the fluorescent tracer DiI to label afferent fibers, an increase in area could be seen in Clarke's nucleus caudal to the injury two weeks after transection.
Article
Cat hindlimb motoneurons possess noninactivating voltage-gated inward currents that can, under appropriate conditions, regeneratively produce sustained increments in depolarization and firing of the cell (i.e., plateau potentials). Recent studies in turtle dorsal horn neurons and motoneurons indicate that facilitation of plateaus occurs with repeated plateau activation (decreased threshold and increased duration; this phenomenon is referred to as warm-up). The purpose of the present study was to study warm-up in cat motoneurons. Initially, cells were studied by injecting a slow triangular current ramp intracellularly to determine the threshold for activation of the plateau. In cells where the sodium spikes were blocked with intracellular QX314, plateau activation was readily seen as a sudden jump in membrane potential, which was not directly reversed as the current was decreased (cf. hysteresis). With normal spiking, the plateau activation (the noninactivating inward current) was reflected by a steep and sustained jump in firing rate, which was not directly reversed as the current was decreased (hysteresis). Repetitive plateau activation significantly lowered the plateau activation threshold in 83% of cells (by on average 5 mV and 11 Hz with and without QX314, respectively). This interaction between successive plateaus (warm-up) occurred when tested with 3- to 6-s intervals; no interaction occurred at times >20 s. Plateaus initiated by synaptic activation from muscle stretch were also facilitated by repetition. Repeated slow muscle stretches that produced small phasic responses when a cell was hyperpolarized with intracellular current bias produced a larger and more prolonged responses (plateau) when the bias was removed, and the amplitude and duration of this response grew with repetition. The effects of warm-up seen with intracellular recordings during muscle stretch could also be recorded extracellularly with gross electromyographic (EMG) recordings. That is, the same repetitive stretch as above produced a progressively larger and more prolonged EMG response. Warm-up may be a functionally important form of short-term plasticity in motoneurons that secures efficient motor output once a threshold level is reached for a significant period. Finally, the finding that warm-up can be readily observed with gross EMG recordings will be useful in future studies of plateaus in awake animals and humans.
Article
Measurements of threshold angle and incremental dynamic stiffness (IDS) were derived from triceps surae stretch reflexes, elicited by ramp and hold flexion at the ankle joint of four cats that were tested while awake. Stretch reflexes were assessed from trials that began from different ankle joint start positions or were matched using a post-hoc analysis for initial background force during testing sessions before and following unilateral lesions of the dorsolateral funiculus at levels ranging from T13 to L3. Unilateral lesions of the dorsolateral funiculus (DLF) produced significant ipsilateral decreases in stretch reflex threshold and increases in reflex gain, measured as incremental dynamic stiffness (IDS). ANCOVA testing indicated that the reduction in threshold, but not the increase in IDS, was dependent upon the level of background force. Reflex testing from different start angles demonstrated that DLF lesions diminished the correlation between threshold and IDS. Intravenous infusion of ketamine dose-dependently reduced IDS, compared with testing in the unanaesthetized state. Postoperative reflex testing during infusion of ketamine at 22.2 mg/kg per h, when electromyographic responses were reduced to 24% of control levels, abolished differences in IDS between the ipsilateral and contralateral hindlimbs. These and related observations suggest that the postoperative increase in IDS in awake animals was not due to an increase in passive stiffness.
Article
Enhancement of bistability in spinal motoneurons in vivo by the noradrenergic alpha1 agonist methoxamine. Like many types of motoneurons, spinal motoneurons in the adult mammal can exhibit bistable behavior. This means that short periods of excitatory input can initiate long periods of self-sustained firing and that equally short periods of inhibition can return the cell to the quiescent state. Usually, the presence of one of the monoamines (either serotonin or norepinephrine) is required for spinal motoneurons to express bistable behaviors. Because the decerebrate cat preparation has tonic activity in monoaminergic fibers that originate in the brain stem and project to spinal motoneurons, these cells sometimes exhibit bistable behavior. However, exogenous application of the noradrenergic alpha1 agonist methoxamine greatly enhances bistable behavior in the decerebrate. The goal of this study was to identify the mechanisms of this action of methoxamine. The total persistent inward current (IPIC) in spinal motoneurons in the decerebrate cat was measured from I-V functions generated by triangular voltage commands applied using discontinuous single electrode voltage clamp. The effect of methoxamine on IPIC was assessed by comparing its properties in a control cell sample without methoxamine to its properties in a sample of cells obtained after application of methoxamine. In most experiments, at least one cell was obtained from each sample. Our results showed that methoxamine approximately doubled the amplitude of IPIC without changing its onset voltage, its offset voltage, or its persistence. The reduced amplitude was a consistent finding within experiments and so was unlikely to be caused by interanimal variability. In addition, methoxamine depolarized motoneurons without altering their input conductances, so that a smaller amount of current was required to reach the onset voltage of IPIC. These effects of methoxamine were approximately equal in all cells. As a result of these changes, methoxamine greatly enhanced the tendency for motoneurons to become bistable. It is proposed that the methoxamine-induced increase in the amplitude of IPIC is effective in enhancing the duration of bistable firing because this increase makes IPIC more resistant to the deactivating effects of the afterhyperpolarizations between spikes.
Article
The ability of mammalian spinal cords to generate rhythmic motor behavior in nonlimb moving segments was examined in isolated spinal cords of neonatal rats. Stimulation of sacrocaudal afferents (SCA) induced alternating left-right bursts in lumbosacral efferents and in tail muscles. On each side of the tail, flexors, extensors, and abductors were coactive during each cycle of activity. This rhythm originated mainly in the sacrocaudal region because it persisted in sacrocaudal segments after surgical removal of the thoracolumbar cord. Sacrocaudal commissural pathways were sufficient to maintain the left-right alternation of lumbar efferent bursts, because their timing was unaltered after a complete thoracolumbar hemisection. The lumbar rhythm originated in part from sacrocaudal activity ascending in lateral and ventrolateral funiculi, because efferent bursts in rostral lumbar segments were nearly abolished on a particular side by lesions of the lateral quadrant of the cord at the L(4)-L(5) junction. Intracellular recordings from S(2)-S(3) motoneurons, obtained during the rhythm, revealed the presence of phasic oscillations of membrane potential superimposed on a tonic depolarization. Bursts of spikes occurred on the depolarizing phases of the oscillation. Between these bursts the membrane input conductance increased, and hyperpolarizing drive potentials were revealed. The inhibitory drive and the decreased input resistance coincided with contralateral efferent bursts, suggesting that crossed pathways controlled it. Our studies indicate that pattern generators are not restricted to limb-moving spinal segments and suggest that regional specializations of pattern-generating circuitry and their associated interneurons are responsible for the different motor patterns produced by the mammalian spinal cord.
Article
Effects of locally applied serotonin (5-HT) and noradrenaline (NA) were tested on extracellularly recorded responses of single spinal interneurons in deeply anaesthetized cats. These effects were tested on: (i) interneurons mediating reciprocal inhibition from group Ia afferents; (ii) interneurons mediating non-reciprocal inhibition from group Ia and Ib afferents; (iii) intermediate zone interneurons co-excited by group I and II afferents; and (iv) dorsal horn interneurons excited by group II afferents. Effects of monoamines were tested on responses evoked at latencies compatible with monosynaptic coupling. Responses evoked by group Ia and/or Ib muscle afferents were facilitated in all of the tested interneurons both by NA and 5-HT. Responses evoked by group II muscle afferents were depressed in the majority of the interneurons but were facilitated in some of them. 5-HT depressed these responses in all dorsal horn interneurons and in one subpopulation of intermediate zone interneurons, while it facilitated them in another subpopulation of intermediate zone interneurons. NA depressed them in all intermediate zone interneurons and in one subpopulation of dorsal horn interneurons, while it facilitated them in another subpopulation of dorsal horn interneurons. The results of this study lead to the conclusions that: (i) modulation of synaptic actions of muscle spindle and tendon organ afferents on spinal interneurons by 5-HT and NA is related to both the type of the afferent and the functional type of the interneuron; and that (ii) 5-HT and NA counteract each others' actions on some interneuronal types but mutually enhance them on the others.