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H reflex [keywords]
- Independent segmental inhibitory modulation of synaptic efficacy of the soleus H-reflex. [Journal Article]
- Somatosens Mot Res 2013 Jun; 30(2):81-9.
Synaptic efficacy associated with muscle spindle feedback is partly regulated via depression at the Ia-motorneuron synapse through paired reflex depression (PRD) and presynaptic inhibition (PI). The purpose of this study was to examine PRD and PI of the soleus H-reflex at rest and with a background voluntary muscle contraction. The experiment was conducted on 10 healthy males with no history of neurological deficits. Soleus H-reflex and M-wave curves were elicited in three conditions: unconditioned, PRD (two consecutive H-reflexes with 100 ms interval), and PI (1.2 × MT to tibialis anterior 100 ms prior to soleus H-reflex). Each condition was tested at rest and with a 10% soleus contraction. PRD and PI both produced a pronounced inhibition to the soleus motor pool at rest, with a significant difference observed between threshold values (78.9, 89.3, and 90.4% for unconditioned, PRD, and PI reflexes, respectively). During the voluntary contraction the threshold for both inhibitory mechanisms was significantly reduced, and were not different from the unconditioned H-reflex (74.5, 78.9, and 77.0% for unconditioned, PRD, and PI reflexes, respectively). The slope of PI and the PI Hmax/Mmax ratio were significantly altered during contraction whereas no differences were observed for PRD. The results suggest these inhibitory mechanisms depend on the interaction between background voluntary activation and stimulus intensity. This behavior of these inhibitory mechanisms underscores the specificity of spinal circuitry in the control of motor behaviors.
- Fatigue modulates synchronous but not asynchronous soleus activation during stimulation of paralyzed muscle. [JOURNAL ARTICLE]
- Clin Neurophysiol 2013 May 11.
OBJECTIVE:Electrical stimulation over a motor nerve yields muscle force via a combination of direct and reflex-mediated activation. We determined the influence of fatigue on reflex-mediated responses induced during supra-maximal electrical stimulation in humans with complete paralysis.
METHODS:We analyzed soleus electromyographic (EMG) activity during repetitive stimulation (15Hz, 125 contractions) in 22 individuals with complete paralysis. The bout of stimulation caused significant soleus muscle fatigue (53.1% torque decline).
RESULTS:Before fatigue, EMG at all latencies after the M-wave was less than 1% of the maximal M-wave amplitude (% MaxM). After fatigue there was a fourfold (p<0.05) increase in EMG at the H-reflex latency; however, the overall magnitude remained low (<2% change in % MaxM). There was no increase in "asynchronous" EMG ∼ 1 s after the stimulus train.
CONCLUSIONS:Fatigue enhanced the activation to the paralyzed soleus muscle, but primarily at the H-reflex latency. The overall influence of this reflex modulation was small. Soleus EMG was not elevated during fatigue at latencies consistent with asynchronous activation.
SIGNIFICANCE:These findings support synchronous reflex responses increase while random asynchronous reflex activation does not change during repetitive supra-maximal stimulation, offering a clinical strategy to consistently dose stress to paralyzed tissues.
- Ipsi- and contralateral H-reflexes and V-waves after unilateral chronic Achilles tendon vibration. [JOURNAL ARTICLE]
- Eur J Appl Physiol 2013 May 8.
Chronic Achilles tendon vibration has previously shown its effectiveness in improving plantar flexor's strength and activation capacities. The present study investigated the related neural mechanisms by analyzing H-reflexes and V-waves of the soleus (SOL) and gastrocnemii (GM gastrocnemius medialis; GL gastrocnemius lateralis) muscles under maximal isometric plantar flexion. Moreover, recordings were conducted bilaterally to address potential crossed effects. 11 subjects were engaged in this study. Maximal voluntary contraction and superimposed H-reflexes and V-waves were quantified in both legs at baseline (PRE) and 2 weeks later to verify repeatability of data (CON). Then, subjects were retested after 14 days of daily unilateral Achilles tendon vibration (VIB; 1 h per day; frequency: 50 Hz). No changes were reported between PRE and CON data. In the VIB condition, there was an increase in MVC for both the vibrated (+9.1 %; p = 0.016) and non-vibrated (+10.2 %; p = 0.009) legs. The H-reflex increased by a mean 25 % in the vibrated SOL (p < 0.001), while it remained unchanged for the contralateral side (p = 0.531). The SOL V-wave also increased in the vibrated limb (+43.3 %; p < 0.001), as well as in the non-vibrated one (+41.9 %; p = 0.006). Furthermore, the GM V-wave increased by 37.8 % (p = 0.081) in the vibrated side and by 39.4 % (p = 0.03) in the non-vibrated side. However, no changes were reported for the GL muscles. While the present study confirmed the strength gains induced by chronic Achilles tendon vibration, the results indicated a cross-education phenomenon with differences in neural adaptations between the vibrated leg and non-vibrated leg.
- Testing the excitability of human motoneurons. [Journal Article]
- Front Hum Neurosci 2013.:152.
The responsiveness of the human central nervous system can change profoundly with exercise, injury, disuse, or disease. Changes occur at both cortical and spinal levels but in most cases excitability of the motoneuron pool must be assessed to localize accurately the site of adaptation. Hence, it is critical to understand, and employ correctly, the methods to test motoneuron excitability in humans. Several techniques exist and each has its advantages and disadvantages. This review examines the most common techniques that use evoked compound muscle action potentials to test the excitability of the motoneuron pool and describes the merits and limitations of each. The techniques discussed are the H-reflex, F-wave, tendon jerk, V-wave, cervicomedullary motor evoked potential (CMEP), and motor evoked potential (MEP). A number of limitations with these techniques are presented.
- The Use of Poly(N-[2-hydroxypropyl] methacrylamide) Hydrogel to Repair a T10 Spinal Cord Hemisection in Rat: A Behavioral, Electrophysiological and Anatomical Examination. [JOURNAL ARTICLE]
- ASN Neuro 2013 Apr 25.
There have been considerable interests in attempting to reverse the deficit due to a spinal cord injury by restoring neural pathways through the lesion and by rebuilding tissue network. In order to provide an appropriate micro-environment for regrowing axotomized neurons and proliferating and migrating cells, we have implanted small block of pHPMA hydrogel into the hemisected T10 rat spinal cord. Locomotor activity was evaluated once a week during 14 weeks with the BBB rating scale in an open field. At the 14th week after spinal cord injury, the reflexivity of the sub-lesional region was measured. We also monitored the ventilatory frequency during an electrically induced muscle fatigue known to elicit the muscle metaboreflex and increase the respiratory rate. Spinal cords were then collected, fixed and stained with anti-ED-1 and anti-NF-H antibodies, and FluoroMyelin. We show in this study that hydrogel implanted animals exhibit 1) an improved locomotor BBB score, 2) an improved breathing adjustments to electrically-evoked isometric contractions, 3) an H-reflex recovery close to Control animals. Qualitative histological results put in evidence higher accumulation of ED-1 positive cells (macrophages/monocytes) at the lesion border, a large number of NF-H positive axons penetrating the applied matrix, and myelin preservion rostrally and caudally to the lesion. Our data confirm that pHPMA hydrogel is a potent biomaterial that can be used for improving neuromuscular adaptive mechanisms and H-reflex responses after spinal cord injury.
- BDNF Val66Met polymorphism alters spinal DC stimulation-induced plasticity in humans. [JOURNAL ARTICLE]
- J Neurophysiol 2013 Apr 10.
The brain-derived neurotrophic factor gene (BDNF) is one of many genes thought to influence neuronal survival, synaptic plasticity and neurogenesis. A common single nucleotide polymorphism (SNP) of the BDNF gene due to valine-to-methionine substitution at codon 66 (BDNF Val66Met) in the normal population has been associated with complex neuronal phenotype including differences in brain morphology, episodic memory or cortical plasticity following brain stimulation and is believed to influence synaptic changes following motor learning task. However, the effect of this polymorphism on spinal plasticity remains largely unknown. Here, we used anodal transcutaneous spinal direct current stimulation (tsDCS), a novel non-invasive technique that induces plasticity of spinal neuronal circuits in healthy subjects. To investigate whether the susceptibility of tsDCS probes of spinal plasticity is significantly influenced by BDNF polymorphism, we collected stimulus-response curves of the soleus (Sol) H reflex before, during, at current offset and 15 minutes after anodal tsDCS delivered at Th11 (2.5mA, 15 min, 0.071mA/cm(2), 64mC/cm(2)) in 17 healthy, Met allele carriers and 17 Val homozygotes who were matched for age and sex. Anodal tsDCS induced a progressive leftward shift of recruitment curve of the H reflex during the stimulation which persisted for at least 15min after current offset in Val/Val individuals. In contrast, this shift was not observed in Met allele carriers. Our findings demonstrate for the first time that BDNF Val66Met genotype impacts spinal plasticity in humans, as assessed by tsDCS, and may be one factor influencing the natural response of the spinal cord to injury or disease.
- Postactivation depression changes after robotic-assisted gait training in hemiplegic stroke patients. [JOURNAL ARTICLE]
- Gait Posture 2013 Apr 6.
Postactivation depression is decreased in patients with spasticity and partially restored by physical exercise in spinal cord injured patients. Up until now, the possibility to modulate postactivation depression with motor training has never been explored in subjects with spasticity following brain lesions. Postactivation depression, assessed as frequency related depression of soleus H-reflex, was investigated before and after robotic-assisted gait training in a group of seven subjects with spastic hemiparesis following hemispheric stroke. Patients received three sessions per week of robotic-assisted gait training for a period of 4 weeks (12 sessions in total). Postactivation depression was measured before the treatment (T0), after the first session (T1) and after the last session (T2). Postactivation depression was quantified as the ratio between H-reflex amplitude at 1Hz and at 0.1Hz. The greater the 1Hz/0.1Hz ratio, the smaller the postactivation depression. Following robotic-assisted gait training, the 1Hz/0.1Hz ratio decreased from 0.79±0.26 at T0 to 0.56±0.18 at T1 and 0.58±0.13 at T2. Post hoc analysis showed a significant difference between T0 and T1 and between T0 and T2, stating an increase of postactivation depression. No significant differences were found between T1 and T2. This study provides the first demonstration that physical exercise can determine a partial normalization of postactivation depression in hemiparetic patients with spasticity following unilateral hemispheric stroke.
- Acute exposure to microgravity does not influence the H-reflex with or without whole body vibration and does not cause vibration-specific changes in muscular activity. [JOURNAL ARTICLE]
- J Electromyogr Kinesiol 2013 Mar 27.
PURPOSE:Many potential countermeasures for muscle and bone loss caused by exposure to microgravity require an uncompromised stretch reflex system. This is especially true for whole body vibration (WBV), as the main source of the neuromuscular activity during WBV has been attributed to stretch reflexes. A priori, it cannot be assumed that reflexes and Ia afferent transmission in particular have the same characteristics in microgravity as in normal gravity (NG). Therefore, the purpose of the study was to compare Ia afferent transmission in microgravity and NG and to assess how microgravity affects muscle activity during WBV.
METHODS:In 14 participants, electromyographic activity of four leg muscles as well as Hoffmann-reflexes were recorded during NG and microgravity induced by parabolic flights.
RESULTS:The size of the Hoffmann-reflex was reduced during WBV, but did not differ during acute exposure to microgravity compared to NG. The influence of the gravity conditions on the electromyographic activity did not change depending on the vibration condition.
CONCLUSIONS:As far as the electromyographic activity of the recorded leg muscles is concerned, the effect of WBV is the same in microgravity as in NG. Moreover, Ia afferent transmission does not seem to be affected by acute exposure to microgravity when subjects are loaded with body weight and postural sway is minimized.
- Serotonin 1A receptors alter expression of movement representations. [Journal Article, Research Support, Non-U.S. Gov't]
- J Neurosci 2013 Mar 13; 33(11):4988-99.
Serotonin has a myriad of central functions involving mood, appetite, sleep, and memory and while its release within the spinal cord is particularly important for generating movement, the corresponding role on cortical movement representations (motor maps) is unknown. Using adult rats we determined that pharmacological depletion of serotonin (5-HT) via intracerebroventricular administration of 5,7 dihydroxytryptamine resulted in altered movements of the forelimb in a skilled reaching task as well as higher movement thresholds and smaller maps derived using high-resolution intracortical microstimulation (ICMS). We ruled out the possibility that reduced spinal cord excitability could account for the serotonin depletion-induced changes as we observed an enhanced Hoffman reflex (H-reflex), indicating a hyperexcitable spinal cord. Motor maps derived in 5-HT1A receptor knock-out mice also showed higher movement thresholds and smaller maps compared with wild-type controls. Direct cortical application of the 5-HT1A/7 agonist 8-OH-DPAT lowered movement thresholds in vivo and increased map size in 5-HT-depleted rats. In rats, electrical stimulation of the dorsal raphe lowered movement thresholds and this effect could be blocked by direct cortical application of the 5-HT1A antagonist WAY-100135, indicating that serotonin is primarily acting through the 5-HT1A receptor. Next we developed a novel in vitro ICMS preparation that allowed us to track layer V pyramidal cell excitability. Bath application of WAY-100135 raised the ICMS current intensity to induce action potential firing whereas the agonist 8-OH-DPAT had the opposite effect. Together our results demonstrate that serotonin, acting through 5-HT1A receptors, plays an excitatory role in forelimb motor map expression.
- Operant Conditioning to Increase Ankle Control or Decrease Reflex Excitability Improves Reflex Modulation and Walking Function in Chronic Spinal Cord Injury. [JOURNAL ARTICLE]
- J Neurophysiol 2013 Mar 6.
Ankle clonus is common after spinal cord injury (SCI) and is attributed to loss of supra-spinally mediated inhibition of soleus stretch reflexes and maladaptive reorganization of spinal reflex pathways. The maladaptive reorganization underlying ankle clonus is associated with other abnormalities such as coactivation and reciprocal facilitation of tibialis anterior (TA) and soleus (SOL), which contribute to impaired walking ability in individuals with motor-incomplete SCI. Operant conditioning can increase muscle activation and decrease stretch reflexes in individuals with SCI. We compared two operant conditioning-based interventions in individuals with ankle clonus and impaired walking ability due to SCI. Training included either TA EMG activation (TA↑) to enhance supraspinal drive or SOL H-reflex suppression (SOL↓) to modulate reflex pathways at the spinal cord level. We measured clonus duration, plantar flexor (PF) reflex threshold angle, timed toe tapping, dorsiflexion (DF) active range of motion (ROM), lower extremity motor scores (LEMS), walking foot clearance, speed and distance, SOL H-reflex amplitude modulation as an index of reciprocal inhibition, presynaptic inhibition, and low-frequency depression, and SOL/TA clonus coactivation. TA↑ decreased PF reflex threshold angle (-4.33°) and DF active ROM angle (-4.32°), and increased LEMS of DF (+0.8 points), training leg (+2.2 points), and non-training leg (+0.8 points), and increased walking foot clearance (+ 4.8 mm) and distance (+12.09 m). SOL↓ decreased SOL/TA coactivation ratio (-0.21) and increased non-training leg LEMS (+1.8 points), walking speed (+0.02 m/s) and distance (+6.25 m). We found increased voluntary control associated with TA↑ outcomes and decreased reflex excitability associated with SOL↓ outcomes.