Changes in motoneuron excitability during postnatal life in the mouse

H-reflex recovery, H-amplitude and H/M ratio were recorded in 54 mice aged 3-12 weeks to study the motoneuron excitability and changes in it during various stages of development. The H-reflex recovery curve at 12 wk showed 3 phases: an early, relative facilitation (before 10 ms), an almost total inhibition at 10 ms and a rapid recovery thereafter. At 3 wk, however, there was only slow recovery after 10 ms and the H-reflex recovery was significantly low during the 40-100 ms period as compared to those in the other age groups, indicating that between 3 and 6 wk, there was a significant increase in the motoneuron excitability. The H-reflex amplitude also showed a significant increase during the 3-6 wk period. However, the H/M ratio did not show any significant increase either during the 3-6 wk period or thereafter. It is concluded that the H-reflex recovery at 3 wk suggests hypoexcitability of the motoneurons, possibly due to immaturity. Since there was a significant increase in the H-reflex recovery during the 3-6 wk period without any parallel increase in the H/M ratio, it is concluded that presynaptic and polysynaptic mechanisms acting on the motoneurons develop during this period. The increase in the H-reflex amplitude is possibly due to the increase in the muscle mass. The H-reflex recovery pattern at 12 wk, without the phase of late inhibition observed in man, is suggestive of less supra-spinal control mechanisms acting upon the motoneurons.     

Recovery profile of single motoneurons after electrical stimuli in man

The H-reflex of 120 soleus motoneurons was recorded using fibre EMG. The recovery profile of these motoneurons was studied during monitoring surface H-reflex records in 28 adult subjects. The spectrum of motoneurons tested was homogeneous with two extremes of neurons having different characteristics. A motoneuron population (forming about 69% of our sample) had a high threshold level for electrical stimuli, short recovery time, and short recovery fringe time (called type A). A second population of motoneurons (forming about 20-30% of our sample) had a low threshold level for electrical stimuli, long recovery fringe time (called type B). During an isometric muscle contraction every motoneuron showed an early shift in recovery time (i.e. each had a shorter recovery time) with shortened recovery fringe time. These changes were larger for motoneurons type B than motoneurons type A. With paired identical electrical stimuli of varying interstimulus intervals a motoneuron may fire in response to the conditioning and test stimuli giving an H2, but not in response to both stimuli. This occurred for interstimulus intervals of 4-11 ms. A strong inhibition period was recorded with interstimulus intervals of 12-80 ms in which all motoneurons did not show any recovery. Most motoneurons recovered in orderly fashion between 80 and 300 ms of interstimulus interval, and this recovery coincided with the fast recovery recorded in surface H-reflex. All motoneurons were recovered by 3000 ms of interstimulus intervals. These findings emphasize the importance of eliciting the H-reflex every 3-5 s in H-reflex methodology in order to be assured that all excited motoneurons have been recovered.     

Mechanisms underlying spinal motor neuron excitability during the cutaneous silent period in humans

The transient suppression of muscle contraction during the cutaneous silent period (CSP) could be produced either through postsynaptic inhibition of motoneurons or through presynaptic inhibition of the excitatory inputs to motoneurons that sustain voluntary contraction. We sought to delineate the mechanisms underlying the CSP in hand muscles by measuring changes in H-reflexes and motor-evoked potentials (MEPs) produced by transcranial magnetic stimulation (TMS) during the CSP in 10 healthy volunteers. H-reflexes and MEPs both measure the excitability of the motoneuron pool and activate similar subpopulations of motoneurons through different pathways. Inhibition of H-reflexes and MEPs of similar size was maximal at the midpoint of the CSP and gradually returned to baseline. The similar time course of recovery suggests that the H-reflex and MEP are affected by inhibition at a common site, most likely postsynaptic inhibition of the motoneurons. © 1998 John Wiley & Sons, Inc. Muscle Nerve 21:1256–1264, 1998.     

Effects of H-reflex up-conditioning on GABAergic terminals on rat soleus motoneurons

To explore the role of spinal cord plasticity in motor learning, we evaluated the effects of H-reflex operant conditioning on GABAergic input to rat spinal motoneurons. Previous work indicated that down-conditioning of soleus H-reflex increases GABAergic input to soleus motoneurons. This study explored the effect of H-reflex up-conditioning on GABAergic input. Of nine rats exposed to H-reflex up-conditioning, up-conditioning was successful (H-reflex increase >or= 20%) in seven and failed (change < 20%) in two. These rats and eight naive control (i.e. unconditioned) rats were injected with cholera toxin subunit B-conjugated Alexa fluor 488 into the soleus muscle to retrogradely label soleus motoneurons. Sections containing soleus motoneurons were processed for GAD(67) [one of the two principal forms of the GABA-synthesizing enzyme glutamic acid decarboxylase (GAD)] with an ABC-peroxidase system. Two blinded independent raters counted and measured GABAergic terminals on these motoneurons. Unlike successful down-conditioning, which greatly increased the number of identifiable GABAergic terminals on the motoneurons, up-conditioning did not significantly change GABAergic terminal number. Successful up-conditioning did produce slight but statistically significant increases in GABAergic terminal diameter and soma coverage. These results are consistent with other data indicating that up- and down-conditioning are not mirror images of each other, but rather have different mechanisms. Although the marked changes in GABAergic terminals with down-conditioning probably contribute to H-reflex decrease, the modest changes in GABAergic terminals associated with up-conditioning may be compensatory or reactive plasticity, rather than the plasticity responsible for H-reflex increase. As a variety of spinal and supraspinal GABAergic neurons innervate motoneurons, the changes found with up-conditioning may be in terminals other than those affected in successful down-conditioning.     

Mechanical and focal electrical stimuli applied to the skin of the index fingertip induce both inhibition and excitation in low-threshold flexor carpi radialis motor units

It has been observed that mechanical stimulation of the skin of the index fingertip causes a weak short-latency inhibition followed by a strong long-lasting facilitation of the flexor carpi radialis (FCR) H-reflex. Based on threshold and latency, these cutaneous reflexes are thought to be routed to motoneurons by parallel pathways. As recent studies have shown predominant inhibitory potentials in slow motoneurons and predominant excitatory potentials in faster ones, the question arises as to whether or not the two cutaneous pathways converge onto the same motoneuron. The poststimulus time histogram technique was used to investigate the changes in firing frequency of low-threshold FCR motor units (MUs), induced by passive mechanical or focal electrical stimuli to the index skin. After gently tapping the finger pulp a small sharp inhibition appeared in 20 MUs. On average, inhibition started 10.2 +/- 1.6 ms from the homonymous Ia monosynaptic effect, and its central delay was estimated to be 1.2 +/- 1.6 ms. The subsequent facilitation, more consistent, had a mean latency of 13.5 +/- 1.7 ms. Inhibition and excitation were statistically significant (P < 0.05). A similar biphasic effect was observed in seven other FCR-MUs, also after focal electrical stimulation of the same skin area. Comparison with the time course of the H-reflex, representing the whole population of MUs, showed striking similarities in time course and latency to the present MU effect. It is thus suggested that cutaneous spinal pathways may have a homogeneous distribution within the FCR motoneuron pool, and that the skewed distribution of cutaneous afferents onto motoneurons should be not taken as a rule.     

Single motor unit H-reflex in motor neuron disorders.

The latency fluctuation of single motor unit potentials (MUPH) in the H-reflex is greater than the latency fluctuation of MUPs in the direct (MUPM) and recurrent (MUPF) responses. This has been attributed to the variability in the impulse generation at the site of nerve stimulation, and to the variation in the synaptic delay at the anterior horn cell. We studied the latency fluctuation of single motor unit H-reflex in patients with motor neuron disorders (MND) in comparison with normal subjects. The mean jitter of the H-reflex was 264.3 +/- 17.8 microseconds (mean +/- SEM) in 30 MUPH recorded from 10 patients with ALS, 302.7 +/- 25.2 microseconds in 16 MUPH from 6 patients with chronic motor neuron diseases, as compared with 137.4 +/- 7.3 microseconds in 34 MUPH recorded from 10 normal subjects. This difference, which persisted even after the correction for the latency variation of MUPM, cannot be explained on the basis of an enhanced reciprocal inhibition. Thus, the increased latency fluctuation of the single motor unit H-reflex in patients with MND may reflect changes in the motoneuron pool excitability that may be secondary to altered intrinsic electrophysiological properties of motoneurons, or to an abnormal temporal and spatial summation of synaptic inputs on motoneurons.     

Reduced reciprocal inhibition during assisted stepping in human spinal cord injury

The aim of this study was to establish the modulation pattern of the reciprocal inhibition exerted from tibialis anterior (TA) group I afferents onto soleus motoneurons during body weight support (BWS) assisted stepping in people with spinal cord injury (SCI). During assisted stepping, the soleus H-reflex was conditioned by percutaneous stimulation of the ipsilateral common peroneal nerve at one fold TA M-wave motor threshold with a single pulse delivered at a short conditioning-test interval. To counteract movement of recording and stimulating electrodes, a supramaximal stimulus at 80–100 ms after the test H-reflex was delivered. Stimuli were randomly dispersed across the step cycle which was divided into 16 equal bins. The conditioned soleus H-reflex was significantly facilitated throughout the stance phase, while during swing no significant changes on the conditioned H-reflex were observed when compared to the unconditioned soleus H-reflex recorded during stepping. Spontaneous clonic activity in triceps surae muscle occurred in multiple phases of the step cycle at a mean frequency of 7 Hz for steps with and without stimulation. This suggests that electrical excitation of TA and soleus group Ia afferents did not contribute to manifestation of ankle clonus. Absent reciprocal inhibition is likely responsible for lack of soleus H-reflex depression in swing phase observed in these patients. The pronounced reduced reciprocal inhibition in stance phase may contribute to impaired levels of co-contraction of antagonistic ankle muscles. Based on these findings, we suggest that rehabilitation should selectively target to transform reciprocal facilitation to inhibition through computer controlled reflex conditioning protocols.     

H-reflex depression by propofol and sevoflurane is dependent on stimulus intensity

OBJECTIVE: The H-reflex has been widely used to investigate effects of drugs on motoneuron excitability in humans. However, up to now no systematic investigation has been done to examine the effects at different stimulus intensities. Here, the M. soleus recruitment curves were compared under influence of propofol and sevoflurane with control conditions to investigate these stimulus intensity dependent effects. METHODS: The study was performed in 10 volunteers for propofol and sevoflurane each, aged 23-32 years. The M. soleus H-reflex was evoked by stimulation of the tibial nerve. Recruitment curves were gained by increasing the stimulation current stepwise from below the threshold of a minimal H-reflex up to a maximal (m-response. Measurements were performed under the influence of the respective drug (2mg/l propofol, 0.8 vol% sevoflurane) and compared to control measurements before and after drug administration. RESULTS: The relative amount of depression of the H-reflex at high stimulus intensities is for both drugs significantly (p<0.001, Friedman's test) lower than at low stimulus intensities. CONCLUSIONS: Stimulus dependent effects have to be taken into consideration when experimental settings to investigate the effects of drugs on the H-reflex are being designed. According to the size principle of motoneuron excitation, it can also be assumed that under the influence of propofol and sevoflurane larger motoneurons are not depressed in the same amount as smaller motoneurons. SIGNIFICANCE: Different drug effects on the H-reflex at different stimulus intensities are not only of methodological importance, but also indicate different drug effects on motoneurons of different sizes.     

A close relationship exists between hand skill and the excitability of motor neurons innervating the postural soleus muscle in right-handed male subjects

The claim that there is no consistent inhibition of the H-reflex from the dominant leg was examined and rejected. Hand preference was assessed by the Edinburgh Inventory, and hand skill by the peg moving task. It was re-established that there is an inverse relationship between hand skill and the excitability of motoneurons innervating the postural soleus muscle in right-handed subjects without familial sinistrality. There was no significant difference between the recovery curves from the right and left sides in subjects with familial sinistrality. There was a positive linear correlation between the asymmetry index for hand skill greater than zero (right-hand dominance) and the asymmetry index for the H-reflex recovery curve greater than zero (left dominance in motoneuronal excitability). It was concluded that there is indeed a spinal motor asymmetry in postural leg muscles related to handedness.     

There is a close relationship between hand skill and the excitability of motor neurons innervating the postural soleus muscle in left-handed subjects

The claim that there is not a consistent inhibition of the H-reflex from the dominant leg was examined and rejected. It was re-established that there is an inverse relationship between hand skill and the excitability of motoneurons innervating the postural soleus muscle in left-handed subjects. In left-handers with significantly better left-hand skill, the height of the H-reflex recovery curve was significantly higher on the right leg (nondominant) than the left leg (dominant). There was a positive linear correlation between the asymmetries of hand skill greater than zero (better left-hand skill) and the H-reflex recovery curves from the right and left legs greater than zero (right dominance in reflex excitability). In left-handers with no significant difference between the right- and left-hand skills, there was no significant difference between the mean recovery curves from the right and left legs for the interstimulus intervals from 40 to 100 ms; the height of the left recovery curve was found to be significantly higher than the height of the right recovery curve for the interstimulus intervals from 150 to 1000 ms. It was concluded that there is close relationship between hand skill and motoneuronal excitability from right and left soleus muscle with regard to support and operative functions of legs in left-handers.     

Up-regulation of 5-HT2 receptors is involved in the increased H-reflex amplitude after contusive spinal cord injury

The amplitude of the H-reflex increases chronically after incomplete SCI and is associated with the development of exaggerated hindlimb reflexes. Although the mechanism for this increased H-reflex is not clear, previous studies have shown that pharmacological activation of the 5-HT2 receptors (5-HT2R) can potentiate the monosynaptic reflex. This study tested the hypothesis that increased expression of 5-HT2R on motoneurons is involved in increased H-reflex amplitude after a standardized clinically relevant contusive SCI. Adult female rats were subjected to contusion, complete surgical transection, or a T8 laminectomy only. At 4 weeks after surgery, H-reflex recordings from the hindpaw plantar muscles of contused rats showed twice the amplitude of that in laminectomy controls or transected rats. To probe the role of 5-HT2R in this increased amplitude, dose-response studies were done with the selective antagonists mianserin or LY53857 and the 5-HT2R agonist (+/-)-1-(2,5-Dimethoxy-4-iodophenyl)-2-aminopropane hydrochloride (DOI). The drugs were intrathecally infused into the lumbar cord while recording the H-reflex. Mianserin did not have any significant effects on the H-reflex after transection, consistent with the loss of distal serotonergic innervation. After contusion, both 5-HT2R antagonists reduced the H-reflex reflex amplitude with a significantly higher ID50 compared to the uninjured controls. The 5-HT2R agonist DOI significantly increased reflex amplitude in contused but not control rats. Furthermore, while 5-HT immunoreactivity was similar, contused rats displayed increased 5-HT2AR immunoreactivity in plantar muscle motoneurons compared to uninjured controls. We conclude that increased expression of 5-HT2R is likely to be involved in the enhanced H-reflex that develops after contusive SCI.     

Contralateral influences on triceps surae motoneuron excitability.

In an effort to more fully investigate spinal reflex pathways in humans, we measured the isometric force-time curve of the tibial nerve H-reflex in 12 college age subjects. We also conditioned the reflex with a contralateral H-reflex stimulus or a contralateral tendon-tap, to ascertain the effects of crossed spinal segmental inputs on alpha motoneuron excitability. The conditioning stimulus preceded the test reflex by 10, 25, 40, 55, 70, 85, 100, 115, 130 or 145 msec. The results demonstrate that a conditioning tibial nerve H-reflex produced marked facilitation onto the contralateral triceps surae motoneurons, predominantly at longer-latency intervals. Conversely, a conditioning Achilles tendon-tap produced long-latency inhibition to the triceps surae. These results demonstrate that differential motoneuron excitability changes can be produced by electrical and mechanical conditioning stimuli. Moreover, these excitability changes may be long lasting and only appear after a relatively long latency. Several neurophysiological mechanisms are proposed to contribute to these changes.     

Soleus and vastus medialis H-reflexes: similarities and differences while standing or lying during varied knee flexion angles

The H-reflex may be a useful measure to examine the lower extremity muscles activation and inhibition following an injury. Recording the vastus medialis H-reflex amplitudes in healthy subjects while standing or lying during varied knee flexion angles may establish a reference for comparison for patients with ACL injury. Vastus medialis and soleus H-reflexes were recorded from 14 healthy subjects while lying and standing during 0, 30, 45, and 60 degrees knee flexion. EMG unit was used to electrically stimulate the tibial and femoral nerves (using 0.5 ms pulses at 0.2 pps of H-maximum amplitude) and to record four traces of the soleus and vastus medialis H-wave and one trace of the M-wave peak-to-peak amplitudes. Repeated measures three-way ANOVAs were calculated with the global alpha=0.05. Results showed that (1) the average soleus H-reflex amplitude was significantly less during standing than lying across all knee flexion conditions, (2) the average vastus medialis H-reflex amplitudes showed no measurable significant differences between neutral standing compared with lying, (3) the average vastus medialis H-reflex amplitudes were significantly greater during standing knee flexion conditions (30, 45, and 60 degrees ) than lying or neutral standing, and (4) there were no differences between soleus and vastus medialis H-reflex amplitudes during lying across all knee flexion conditions. Data from H/M ratio follow the same pattern of H-amplitude. Recording the vastus medialis H-reflex amplitude during standing and knee flexion may be a reflective of the knee function. It is more specific than the soleus H-reflex because it reflects the changes in the excitability of the quadriceps motoneurons acting directly around the knee joint.     

A close relationship between hand skill and the excitability of motor neurons innervating the postural soleus muscle in right-handed female subjects

The claim that there is not a consistent inhibition of the H-reflex from the dominant leg was examined and rejected. Hand preference was assessed by the Edinburgh Inventory, and hand skill by the peg moving task. All the subjects were right-handed in preference and skill. It was re-established that there is an inverse relationship between hand skill and the excitability of motoneurons innervating the postural soleus muscle in right-handed female subjects without familial sinistrality. There was no significant difference between the recovery curves from the right and left sides in subjects with familial sinistrality. The H reflex from right leg was different between these right-handers. There was a positive linear correlation between the asymmetry index of hand skill greater than zero (right-hand dominance) and the asymmetry index for H-reflex recovery curve greater than zero (left dominance in reflex activity). It was concluded that there is a spinal motor asymmetry in postural leg muscles to handedness.     

On the effect of chemically activated fine muscle afferents on interneurones mediating group I non-reciprocal inhibition of extensor ankle and knee muscles in humans

In a previous paper it was shown that muscle nociceptive discharge depressed the activity of interneurones mediating group I non-reciprocal inhibition (or Ib interneurones) in humans [A. Rossi, B. Decchi, Changes in Ib heteronymous inhibition to soleus motoneurons during cutaneous and muscle nociceptive stimulation in humans, Brain Res. 774 (1997) 55–61.]. However, since nociceptive discharge depressed the size of the soleus H-reflex (by which Ib inhibition was tested) the question arises as to whether modification of motoneurone membrane conductance per se could depress the size of Ib inhibitory post-synaptic potentials. The results of the present study suggest that the contribution of motoneurone hyperpolarization to Ib disinhibition is negligible and that muscle nociceptive discharge actually depresses the activity of these pathways.     

Clonus: the role of central mechanisms

The peripheral and central components in sustained clonus were investigated. The excitability of the motoneurons responding to maintained stretch by clonus was examined by tendon taps, trains of vibratory stimuli and by H-reflex afferent volleys. Every burst of clonic discharge of the motoneurons was shown to be followed by a refractory period, which was followed by a shorter excitatory period. It was concluded that the motoneurons responding clonically to a continuous stretch cannot respond until their excitability has been regained after the refractory period. Attempts to change the rate of clonus in various ways failed to do so. Whether motoneurons of clonic muscles tend to respond maximally to other Ia volleys at the rate of clonus was examined by applying repeated taps to the tendon at rates from 1 to 15 Hz. There was a maximal response at the rate of clonus. Inputs other than those induced by stretch cause clonus; examples of cutaneous inputs causing it are given.     

The H-reflex as a tool in neurophysiology: its limitations and uses in understanding nervous system function

The Hoffmann reflex (H-reflex) is extensively used as both a research and clinical tool. The ease with which this reflex can be elicited in several muscles throughout the body makes it an attractive tool. This review discusses some of the important limitations in using the H-reflex. In particular, the inaccurate but widely held assumptions that the H-reflex (1). represents the monosynaptic reflex of the Ia afferent onto homonymous motoneurons, and (2). can be used to measure motoneuronal excitability are addressed. The second part of this review explores the utility of the H-reflex as a neural probe in neurophysiology and motor control research. Applications ranging from the investigation of the functional organization of neural circuitry to the study of adaptive plasticity in spinal structures in health and disease suggest that the H-reflex will continue to be an extensively used tool in motor control neurophysiology.     

Amyotrophic lateral sclerosis weakens spinal recurrent inhibition and post-activation depression

ObjectivesAmyotrophic lateral sclerosis (ALS) disrupts motoneurons that control movement and some vital functions, however, exact details of the neuronal circuits involved in ALS have yet to be fully endorsed. To contribute to our understanding of the responsible neuronal circuits, we aimed to investigate the spinal recurrent inhibition (RI) and post-activation depression (P-AD) in ALS patients.MethodsIn two groups of ALS patients, i.e. lumbar-affected (clinical signs in leg muscles) and nonlumbar-affected (clinical signs in arms or bulbar region but not in the legs), RI and P-AD on the soleus muscle were investigated using single motor units and amplitude changes of H-reflex in surface electromyography, respectively. The data were compared with healthy subjects.ResultsCompared to controls, P-AD of H-reflex was reduced severely in lumbar-affected patients and reduced to a certain degree in nonlumbar-affected patients. Similarly, a significant reduction in the duration of RI on firing motoneurons was found in lumbar-affected patients (11.5 ± 2.6 ms) but not in nonlumbar-affected patients (29.7 ± 12.4 ms, P < 0.0001) compared to controls (30.8 ± 7.2 ms, P < 0.0001).ConclusionThe current study revealed that spinal inhibitory circuits are impaired in ALS.SignificanceThese findings may provide insight for proposing new therapeutic approaches and following disease progression in humans.     

Spinal neuronal mechanisms explaining the modulation of soleus H-reflexes during sustained passive rotation of the hip joint

ObjectivesThe aim of this study was to investigate the mechanism of modulation of soleus H-reflexes during sustained passive rotation of the hip joint.MethodsHealthy men participated as subjects in this study. In the supine position, soleus maximum H-reflexes were recorded during rotation of the hip joint. To examine the roles of muscle spindles, we recorded H-reflexes while applying vibration to muscles around the hip joint. Furthermore, to assess the modulation of pre- and postsynaptic inputs to soleus motoneurons, H-reflexes were conditioned by common peroneal nerve stimulation or transcranial magnetic stimulation (TMS).ResultsThe ratio of the maximum H-reflex to the maximum M-response was significantly reduced in externally rotated positions of the hip joint compared to neutral position. In these positions, presynaptic inhibition, assessed by D1 inhibition, was significantly increased. H-reflexes were reduced during vibration, irrespective of muscles involved and hip position. Facilitation of H-reflex induced by conditioning-TMS showed insignificant differences across all hip positions.ConclusionsThese findings suggest that Ia afferents induced by rotation of the hip joint presumably inhibit soleus H-reflexes presynaptically.SignificanceBased on the results, therapists could efficiently modulate the spasticity of the lower legs of hemiplegic patients during rotation of the hip joint.     

F-wave of single firing motor units: correct or misleading criterion of motoneuron excitability in humans?

Motoneuron excitability is a critical property for information processing during motor control. F-wave (a motoneuronal recurrent discharge evoked by a motor antidromic volley) is often used as a criterion of motoneuron pool excitability in normal and neuromuscular diseases. However, such using of F-wave calls in question. The present study was designed to explore excitability of single low-threshold motoneurons during their natural firing in healthy humans and to ascertain whether F-wave is a correct measure of motoneuronal excitability. Single motor units (MUs) were activated by gentle voluntary muscle contractions. MU peri-stimulus time histograms and motoneuron excitability changes within a target interspike interval were analysed during testing by motor antidromic and Ia-afferent volleys. It was found that F-waves could be occasionally recorded in some low-threshold MUs. However, during evoking F-wave, in contrast with the H-reflex, peri-stimulus time histograms revealed no statistically significant increase in MU discharge probability. Moreover, surprisingly, motoneurons appeared commonly incapable to fire a recurrent discharge within the most excitable part of a target interval. Thus, the F-wave, unlike the H-reflex, is the incorrect criterion of motoneuron excitability resulting in misleading conclusions. However, it does not exclude the validity of the F-wave as a clinical tool for other aims. It was concluded that the F-wave was first explored in low-threshold MUs during their natural firing. The findings may be useful at interpretations of changes in the motoneuron pool excitability in neuromuscular diseases.