H-reflex Conditioning Research Articles

Stimulus Point Distribution in Deep or Superficial Peroneal Nerve for Treatment of Ankle Spasticity

To develop effective electrical stimulation treatment to reduce spasticity, we examined the optimal stimulus point of the common peroneal nerve. The locations of selective stimulus points for the deep peroneal nerve or superficial peroneal nerve fiber were examined in 25 healthy subjects in both legs (50 legs) using the ratio of the tibialis anterior (TA) to the peroneus longus (PL) M-wave amplitude (TA/PL ratio). In addition, we measured reciprocal Ia inhibition in ten healthy subjects. The amount of inhibition was determined from short-latency suppression of the soleus (Sol) H-reflex by conditioning stimuli to the deep or superficial peroneal nerve. The paired t-test was used for statistical analysis. The mean TA/PL ratio during deep peroneal nerve stimulation was significantly different from superficial peroneal nerve stimulation (p < 0.001). The mean stimulus point for the deep peroneal nerve was located 7 ± 5 mm distal and 3 ± 6 mm anterior from the distal edges of the head of fibula and was markedly different from the stimulus point for the superficial peroneal nerve (20 ± 7 mm distal and 12 ± 8 mm posterior). During deep peroneal nerve stimulation, the mean conditioned H-reflex was depressed to 83.8 ± 10.7% of the unconditioned value of the H-reflex. In contrast, during superficial peroneal nerve stimulation, the mean conditioned H-reflex increased to 105.3 ± 5.2%. These values were significantly different (p < 0.001). In the present study, we revealed a stimulus area of the deep peroneal nerve. Also, we observed the inhibitory effects of stimulation upon the deep peroneal nerve at individual stimulus point. Our results appear to indicate that localized stimulation of the deep peroneal nerve is more useful for the reduction of ankle spasticity.

Short-term plasticity of human spinal inhibitory circuits after isometric and isotonic ankle training

The purpose of this study was to determine to what extent one session of isotonic and isometric ankle dorsi and plantar flexion training induces changes in the frequency-dependent depression of the soleus H-reflex. Further, adaptation of reciprocal Ia inhibition exerted from tibialis anterior flexor group I afferents on soleus motoneurons, and presynaptic inhibition of Ia afferent terminals induced by a conditioning afferent volley following stimulation of the antagonist nerve were established with subjects seated before and after training. The soleus H-reflexes evoked at the inter-stimulus intervals of 1, 2, 3, 5, and 8s were normalized to the mean amplitude of the H-reflex evoked every 10s. Conditioned H-reflexes were normalized to the associated control H-reflex evoked with subjects seated before and after training. Twenty-six subjects were randomly assigned to one or more of the 4 exercise groups. Isometric ankle dorsi flexion training decreased the reciprocal and presynaptic inhibition, while isotonic ankle dorsi flexion had no significant effects. Isotonic plantar flexion training decreased only the reciprocal inhibition, whilst isometric plantar flexion had no significant effects on the reciprocal or presynaptic inhibition. None of the training exercise protocols affected the amount of homosynaptic depression of the soleus H-reflex. Our findings support the notion that plastic changes of reciprocal and presynaptic inhibition due to exercise are transferrable to a resting state, and that homosynaptic depression remains unaltered after a single session of ankle training. Further research is needed to outline the time-course of plastic changes of spinal inhibitory mechanisms in humans.

Function of Group IB Inhibition During Assisted Stepping in Human Spinal Cord Injury

The aim of this study was to investigate the function of group Ib inhibition exerted between ankle synergists during assisted stepping in people with chronic motor incomplete spinal cord injury (SCI). During stepping, the soleus (SOL) H-reflex was conditioned by percutaneous stimulation of the medialis gastrocnemius (MG) nerve by a bipolar electrode placed distal and medial to the cathode electrode for the tibial nerve. Stimulation was delivered at 0.95 times MG M-wave motor threshold with a pulse train at short conditioning-test intervals. To counteract movement of recording and stimulating electrodes, a supramaximal stimulus at 80 to 100 milliseconds after the test H-reflex was delivered. Stimuli were randomly dispersed across the step cycle, which was divided into 16 equal bins. The conditioned SOL H-reflex was significantly facilitated throughout the stance phase, while it was depressed during the swing phase when compared with the unconditioned SOL H-reflex. Ankle clonus during the stance phase may have affected the excitability state of spinal motoneuronal and interneuronal circuits, but the presence of group Ib inhibition in the swing phase suggests that in the absence of loading this spinal circuit is modulated in a physiologic manner in human incomplete SCI. Repetitive body loading and unloading promotes functional integration of sensory signals mediating load in human SCI.

Differential control of H-reflex amplitude in different weight-bearing conditions in young and elderly subjects

ObjectiveThis study measured the modulation of conditioned (femoral nerve, paired-stimuli) and unconditioned soleus H-reflexes in young and elderly subjects when changing weight-bearing (WB) requirements and body position. MethodsConditioned and unconditioned H-reflexes were examined in 14 elderly subjects and 11 young subjects during six different WB conditions: (1) lying supine with no WB, (2) supine position inclined by 30° with 50% WB, (3) standing with 50%, (4) 75%, (5) 100% and (6) 125% WB. ResultsThe elderly subjects had consistently higher background soleus EMG activity across the WB conditions compared to the young. Femoral nerve conditioning caused facilitation of the H-reflex that changed across WB conditions in the young subjects, but not in the elderly subjects. Finally, elderly subjects had less depression with paired-stimulation (PRD) across WB conditions, which was not observed in the young subjects. ConclusionsThe elderly may have more direct activation of motoneurons from descending pathways, coupled with less segmental spinal control of inhibitory interneurons, as evidenced by the increased background soleus activity, H/M-max ratios and the lack of modulatory control observed when conditioning the H-reflex. SignificanceThere was an age-specific response from descending and segmental pathways during conditions that involved either different WB requirements or changes in body position.

Pathway‐specific plasticity in the human spinal cord

The aim of the present study was to artificially induce plasticity in the human spinal cord and evaluate whether this plasticity is pathway specific. For this purpose, a technique called paired associative stimulation (PAS) was applied. Volleys evoked by transcranial magnetic stimulation over the primary motor cortex and peripheral nerve stimulation of the nervus tibialis in the popliteal fossa were timed to coincide at the spinal level. The transmission of different corticospinal projections was assessed before and after PAS using conditioned H-reflexes. Different groups of healthy volunteers (28 ± 5 years) were tested; intervention groups 1 (n = 9) and 2 (n = 8) received spinal PAS (360 paired stimuli) and the induced effects were evaluated using cortical (group 1) or cervicomedullary (group 2) conditioning of musculus soleus H-reflexes. After spinal PAS, the conditioned H-reflexes were significantly facilitated when tested with cortical and cervicomedullary stimulation. The effect of the latter technique is independent of changes in the excitability of cortical neurons. Therefore, the finding that conditioned H-reflexes were increased after spinal PAS when tested with both cortical and cervicomedullary stimulation suggests that neural plasticity was induced within the spinal cord. The facilitation could only be observed for specific inter-stimulus intervals between volleys induced by peripheral nerve stimulation and transcranial magnetic stimulation. As the specific inter-stimulus intervals were assumed to relate to transmission within specific motor pathways, it is argued that changes in the corticospinal transmission were pathway-specific. These findings may be helpful in inducing and assessing neural plasticity in pathological conditions like spinal cord injuries.

Effects of homosynaptic depression on spectral properties of H-reflex recordings

The purpose of this study was to determine the effects of homosynaptic depression (HD) on spectral properties of the soleus (SOL) H-reflex. Paired stimulations, separated by 100 ms, were used to elicit an unconditioned and conditioned H-reflex in the SOL muscle of 20 participants during quiet standing. Wavelet and principal component analyses were used to analyze features of the time-varying spectral properties of the unconditioned and conditioned H-reflex. The effects of HD on spectral properties of the H-reflex signal were quantified by comparing extracted principal component scores. The analysis extracted two principal components: one associated with the intensity of the spectra and one associated with its frequency. The scores for both principal components were smaller for the conditioned H-reflex. HD decreases the spectral intensity and changes the spectral frequency of H-reflexes. These results suggest that HD changes the recruitment pattern of the motor units evoked during H-reflex stimulations, in that it not only decreases the intensity, but also changes the types of motor units that contribute to the H-reflex signal.

Spinal inhibition of descending command to soleus motoneurons is removed prior to dorsiflexion

It has recently been demonstrated that soleus motor-evoked potentials (MEPs) are facilitated prior to the onset of dorsiflexion. The purpose of this study was to examine if this could be explained by removal of spinal inhibition of the descending command to soleus motoneurons. To test this, we investigated how afferent inputs from the tibialis anterior muscle modulate the corticospinal activation of soleus spinal motoneurons at rest, during static contraction and prior to movement. MEPs activated by transcranial magnetic stimulation (TMS) and Hoffmann reflexes (H-reflexes), activated by electrical stimulation of the posterior tibial nerve (PTN), were conditioned by prior stimulation of the common peroneal nerve (CPN) at a variety of conditioning-test (CT) intervals. MEPs in the precontracted soleus muscle were inhibited when the TMS pulse was preceded by CPN stimulation with a CT interval of 35 ms, and they were facilitated for CT intervals of 50-55 ms. A similar inhibition of the soleus H-reflex was not observed. To investigate which descending pathways might be responsible for the afferent-evoked inhibition and facilitation, we examined the effect of CPN stimulation on short-latency facilitation (SLF) and long-latency facilitation (LLF) of the soleus H-reflex induced by a subthreshold TMS pulse at different CT intervals. SLF is known to reflect the excitability of the fastest conducting, corticomotoneuronal cells whereas LLF is believed to be caused by more indirect descending pathways. At CT intervals of 40-45 ms, the LLF was significantly more inhibited compared to the SLF when taking the effect on the H-reflex into account. Finally, we investigated how the CPN-induced inhibition and facilitation of the soleus MEP were modulated prior to dorsiflexion. Whereas the late facilitation (CT interval: 55 ms) was similar prior to dorsiflexion and at rest, no inhibition could be evoked at the earlier latency (CT interval: 35 ms) prior to onset of dorsiflexion. The observation that the CPN-induced inhibition of soleus MEPs disappears prior to onset of dorsiflexion may explain why soleus MEPs are facilitated prior to onset of dorsiflexion contraction. A possible mechanism involves the removal of inhibition of the descending command to the motoneurons at a spinal interneuronal level because the inhibition was seen in LLF and not in SLF, and the MEP inhibition was not observed in the H-reflex. The data illustrate that spinal interneuronal pathways modify descending commands to human spinal motoneurons and influence the size of MEPs elicited by TMS.

Operant Conditioning of Rat Soleus H-Reflex Oppositely Affects Another H-Reflex and Changes Locomotor Kinematics

H-reflex conditioning is a model for studying the plasticity associated with a new motor skill. We are exploring its effects on other reflexes and on locomotion. Rats were implanted with EMG electrodes in both solei (SOL(R) and SOL(L)) and right quadriceps (QD(R)), and stimulating cuffs on both posterior tibial (PT) nerves and right posterior femoral nerve. When SOL(R) EMG remained in a defined range, PT(R) stimulation just above M-response threshold elicited the SOL(R) H-reflex. Analogous procedures elicited the QD(R) and SOL(L) H-reflexes. After a control period, each rat was exposed for 50 d to a protocol that rewarded SOL(R) H-reflexes that were above (HRup rats) or below (HRdown rats) a criterion. HRup conditioning increased the SOL(R) H-reflex to 214 ± 37% (mean ± SEM) of control (p = 0.02) and decreased the QD(R) H-reflex to 71 ± 26% (p = 0.06). HRdown conditioning decreased the SOL(R) H-reflex to 69 ± 2% (p < 0.001) and increased the QD(R) H-reflex to 121 ± 7% (p = 0.02). These changes remained during locomotion. The SOL(L) H-reflex did not change. During the stance phase of locomotion, ankle plantarflexion increased in HRup rats and decreased in HRdown rats, hip extension did the opposite, and hip height did not change. The plasticity that changes the QD(R) H-reflex and locomotor kinematics may be inevitable (i.e., reactive) due to the ubiquity of activity-dependent CNS plasticity, and/or necessary (i.e., compensatory) to preserve other behaviors (e.g., locomotion) that would otherwise be disturbed by the change in the SOL(R) H-reflex pathway. The changes in joint angles, coupled with the preservation of hip height, suggest that compensatory plasticity did occur.

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.

Crossed spinal soleus muscle communication demonstrated by H‐reflex conditioning

A conditioning volley to the ipsilateral tibial nerve (iTN) inhibits contralateral soleus (cSOL) electromyographic activity at latencies of 37-41 ms. This is evidence for spinal muscular communication in opposing limbs. The aim of our study was to determine whether the cSOL H-reflex would be inhibited in a similar manner. Thirteen subjects participated in two experiments: (1) stimuli delivered to the iTN at 85% of the maximal peak-to-peak M-wave (85% M-max) with a pre-contracted cSOL; (2) 510 stimuli delivered at 85% M-max to the iTN with a test volley delivered to the contralateral tibial nerve at interstimulus intervals of -6 to 100 ms. Significant inhibition was observed in the cSOL H-reflex when conditioning stimuli were delivered 3-33 ms before the test H-reflex. The activity of this spinal pathway can be quantified using H-reflex conditioning to provide a controlled model for further studies of this response.

Operant down conditioning of the soleus H-reflex in people with spasticity due to chronic incomplete spinal cord injury

Operant down conditioning of the soleus H-reflex in people with spasticity due to chronic incomplete spinal cord injury

Spinal Reflex Adaptation in Dancers

Dancers undergo specific activity-dependent neuromuscular adaptations following long-term training that allow them to develop the refined motor skills required for success in dance. The spinal stretch reflex circuit has demonstrated specific adaptations following prolonged dance training. Adaptations in the spinal stretch reflex can be studied using H-reflex methodology, first described by Paul Hoffmann in 1910. This article discusses H-reflex methodology and presents data that examine the neural mechanisms that contribute to adaptations in the spinal stretch reflex with dance training. Two groups of subjects, modern dancers (N = 5) and untrained controls (N = 5), were tested. On one-half of the trials common peronal nerve (CPN) conditioning of the soleus H-reflex was used to assess one spinal mechanism, pre-synaptic inhibition; the other half tested the soleus H-reflex only (unconditioned). The dependent variables were the Hmax/Mmax ratios, unconditioned and with CPN conditioning, expressed as percent values. The results revealed three main findings: 1. Modern dancers had smaller Hmax/Mmax ratios than control subjects; 2. The Hmax/ Mmax ratio was smaller in standing posture than in prone among both dancers and controls; and 3. Pre-synaptic inhibition was not different between dancers and controls in standing. In conclusion, modern dancers have smaller H-reflexes than untrained controls, but pre-synaptic inhibition does not appear to explain this difference.

Plantar Cutaneous Afferents Normalize the Reflex Modulation Patterns During Stepping in Chronic Human Spinal Cord Injury

Plantar cutaneous afferent transmission is critical for recovery of locomotion in spinalized animals, whereas a phase-dependent reflex modulation is apparent during fictive or real locomotion. In nine people with a chronic spinal cord injury (SCI) the effects of foot sole stimulation on the soleus H-reflex and tibialis anterior (TA) flexion reflex modulation patterns during assisted stepping were established on different days. The soleus H-reflex was elicited by posterior tibial nerve stimulation followed by a supramaximal stimulus 100 ms after the test H-reflex to control for movement of recording electrodes. The flexion reflex was evoked by sural nerve stimulation with a 30-ms pulse train, recorded from the ipsilateral TA muscle, and elicited at 1.2- to twofold the reflex threshold. During assisted stepping, spinal reflexes were conditioned by percutaneous stimulation of the ipsilateral metatarsals at threefold perceptual threshold with a 20-ms pulse train delivered at 9- to 11-ms conditioning-test intervals. Stimuli were randomly dispersed across the step cycle, which was divided into 16 equal bins. The conditioned soleus H-reflex was significantly facilitated at midstance and depressed during midswing when compared with the unconditioned soleus H-reflex recorded during stepping. Foot sole stimulation induced a significant facilitation of the long-latency TA flexion reflex before, during, and after stance-to-swing transition when compared with the unconditioned long-latency TA flexion reflex during stepping. This study provides evidence that plantar cutaneous afferents remarkably influence the soleus H-reflex and TA flexion reflex modulation patterns during stepping and support that actions of plantar cutaneous afferents onto spinal interneuronal circuits engaged in locomotion are manifested in a phase-dependent manner in chronic SCI subjects.

Acquisition of a Simple Motor Skill: Task-Dependent Adaptation Plus Long-Term Change in the Human Soleus H-Reflex

Activity-dependent plasticity occurs throughout the CNS. However, investigations of skill acquisition usually focus on cortex. To expand the focus, we analyzed in humans the development of operantly conditioned H-reflex change, a simple motor skill that develops gradually and involves plasticity in both the brain and the spinal cord. Each person completed 6 baseline and 24 conditioning sessions over 10 weeks. In each conditioning session, the soleus H-reflex was measured while the subject was or was not asked to increase (HRup subjects) or decrease (HRdown subjects) it. When the subject was asked to change H-reflex size, immediate visual feedback indicated whether a size criterion had been satisfied. Over the 24 conditioning sessions, H-reflex size gradually increased in six of eight HRup subjects and decreased in eight of nine HRdown subjects, resulting in final sizes of 140 +/- 12 and 69 +/- 6% of baseline size, respectively. The final H-reflex change was the sum of within-session (i.e., task-dependent) adaptation and across-session (i.e., long-term) change. Task-dependent adaptation appeared within four to six sessions and persisted thereafter, averaging +13% in HRup subjects and -15% in HRdown subjects. In contrast, long-term change began after 10 sessions and increased gradually thereafter, reaching +27% in HRup subjects and -16% in HRdown subjects. Thus, the acquisition of H-reflex conditioning consists of two phenomena, task-dependent adaptation and long-term change, that together constitute the new motor skill. In combination with previous data, this new finding further elucidates the interaction of plasticity in brain and spinal cord that underlies the acquisition and maintenance of motor skills.

H-reflex Modulation During Different Head Movements

Head movement stimulates the vestibulospinal tract, which contributes to the balance system. Head movement increased our postural sway amplitude. The amplitude of the soleus spinal stretch reflex was decreased by contralateral side head movement. However, it has yet to be investigated if presynaptic inhibition to the soleus Ia afferents is modulated by the head movement. PURPOSE: To examine the modulation of the soleus H-reflex and presynaptic inhibition (PI) to the soleus Ia afferents during standing with head movement. METHODS: Seven young active subjects (mean: 23.3 yrs) were examined. The H-wave and M-responses of the right soleus were determined for each subject during a static standing position. Each subject turned the head up and contralaterally or ipsilaterally with or without hands on table for support while the H-reflexes were examined. To examine H-reflex modulation, a PI H-reflex conditioning protocol was administered. The control H-reflex stimulus was set at 10% of M-max. PI was induced via conditioning of the soleus H-reflex with peroneal nerve stimulation at 100 msec prior to the test stimulus on the tibial nerve. The conditioning stimulus was delivered once the EMG activity of sternocleidomastoid muscle occurred as a trigger signal for lateral head movements. For the head up movement, an electrogoniometer was placed on the cervical spine and this was used as a trigger signal. Each mean of PI H-reflexes was normalized by the amplitude of control H-reflex (%). For data analyses, a 2 x 4 repeated measures ANOVA was used to determine each mean of H-reflex and PI-H reflex. RESULTS: The H/M-max ratio during standing without hands on the table was 41%. The ratio of PI H-reflex to the control H-reflex was 68.5% and 85.7% during standing with and without hands on the table, respectively. The mean ratio of PI H-reflex to control H-reflexes was decreased when the head was turned contralaterally by 8.4% and 13.5% with and without hands on the table, respectively. The ratio was decreased by 27.0% when the head was turned ipsilaterally with hands on the table, whereas it was not changed without hands on the table. CONCLUSIONS: The soleus H-reflex was modulated during head movements. PI to the soleus Ia afferents was decreased when the head was turned laterally, yet the modulation was dependant on the head movement and body stability.

Balance training and ballistic strength training are associated with task‐specific corticospinal adaptations

The aim of this study was to investigate the role of presumably direct corticospinal pathways in long-term training of the lower limb in humans. It was hypothesized that corticospinal projections are affected in a training-specific manner. To assess specificity, balance training was compared to training of explosive strength of the shank muscles and to a nontraining group. Both trainings comprised 16 1-h sessions within 4 weeks. Before and after training, the maximum rate of force development was monitored to display changes in motor performance. Neurophysiological assessment was performed during rest and two active tasks, each of which was similar to one type of training. Hence, both training groups were tested in a trained and a nontrained task. H-reflexes in soleus (SOL) muscle were tested in order to detect changes at the spinal level. Corticospinal adaptations were assessed by colliding subthreshold transcranial magnetic stimulation to condition the SOL H-reflex. The short-latency facilitation of the conditioned H-reflex was diminished in the trained task and enhanced in the nontrained task. This was observable in the active state only. On a functional level, training increased the rate of force development suggesting that corticospinal projections play a role in adaptation of leg motor control. In conclusion, long-term training of shank muscles affected fast corticospinal projections. The significant interaction of task and training indicates context specificity of training effects. The findings suggest reduced motor cortical influence during the trained task but involvement of direct corticospinal control for new leg motor tasks in humans.

EXPERIMENTATION AND INNOVATION IN FAMILY MEDICINE RESIDENCY EDUCATION: THE TIME IS NOW

<h3>Abstract</h3> The aim of this study was to investigate whether indirect upper limb vibration (ULV) modulates transmission along spinal and corticospinal pathways that control the human forearm. All measures were assessed under CONTROL (no vibration) and ULV (30 Hz; 0.4 mm displacement) conditions while participants maintained a small contraction of the right flexor carpi radialis (FCR) muscle. To assess spinal pathways, Hoffmann reflexes (H-reflexes) elicited by stimulation of the median nerve were recorded from FCR with motor response (M-wave) amplitudes matched between conditions. An H-reflex conditioning paradigm was also used to assess changes in presynaptic inhibition by stimulating the superficial radial (SR) nerve (5 pulses at 300Hz) 37 ms prior to median nerve stimulation. Cutaneous reflexes in FCR elicited by stimulation of the SR nerve at the wrist were also recorded. To assess corticospinal pathways, motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation of the contralateral motor cortex were recorded from the right FCR and biceps brachii (BB). ULV significantly reduced H-reflex amplitude by 15.7% for both conditioned and unconditioned reflexes (24.0±15.7 vs 18.4±11.2 % M_max; <i>p</i>&lt;0.05). Middle latency cutaneous reflexes were also significantly reduced by 20.0% from CONTROL (−1.50 ± 2.1 % Mmax) to ULV (−1.73 ± 2.2 % Mmax; <i>p</i>&lt;0.05). There was no significant effect of ULV on MEP amplitude (<i>p</i>&gt;0.05). Therefore, ULV inhibits cutaneous and H-reflex transmission without influencing corticospinal excitability of the forearm flexors suggesting increased presynaptic inhibition of afferent transmission as a likely mechanism. A general increase in inhibition of spinal pathways with ULV may have important implications for improving rehabilitation for individuals with spasticity (SCI, stroke, MS, etc).

Operant Conditioning of H-Reflex Can Correct a Locomotor Abnormality after Spinal Cord Injury in Rats

This study asked whether operant conditioning of the H-reflex can modify locomotion in spinal cord-injured rats. Midthoracic transection of the right lateral column of the spinal cord produced a persistent asymmetry in the muscle activity underlying treadmill locomotion. The rats were then either exposed or not exposed to an H-reflex up-conditioning protocol that greatly increased right soleus motoneuron response to primary afferent input, and locomotion was reevaluated. H-reflex up-conditioning increased the right soleus burst and corrected the locomotor asymmetry. In contrast, the locomotor asymmetry persisted in the control rats. These results suggest that appropriately selected reflex conditioning protocols might improve function in people with partial spinal cord injuries. Such protocols might be especially useful when significant regeneration becomes possible and precise methods for reeducating the regenerated spinal cord neurons and synapses are needed for restoring effective function.

Influence of Stance and Support Surface Variations on Soleus Presynaptic Inhibition

Presynaptic inhibition (PI) is instrumental in enabling the nervous system to respond to environmental changes. Subjects who demonstrate little change in PI of the soleus muscle have been shown to have a decreased ability to adapt to environmental changes (i.e. uneven surface conditions). Little research has been done to examine changes in PI during various surface and stance conditions. PURPOSE: this study utilized H-reflex measures to examine extrinsic PI in the soleus muscle during single- and double-legged stance, under stable and unstable surface conditions. Design & settings: A 2 × 2 fully repeated measures design was employed with leg (single vs. double) and support surface (foam, no foam) serving as the independent variables. subjects: Ten healthy subjects (age = 23.±1.7 yr, mass = 75.2±6.4 kg, ht = 172.1±8.5 cm) with no known acute or chronic neurological or lower extremity musculoskeletal injury 6 months prior to the study served as volunteers. Measurements: The tibial nerve was stimulated using an extrinsic PI conditioning protocol. The ratio of the unconditioned H-reflex amplitude to the conditioned H-reflex amplitude represented extrinsic PI. All subjects completed a total of 7 trials for the conditioned and unconditioned soleus H-reflex response for each treatment. RESULTS: No interaction was found between leg and surface condition on soleus PI (p > 0.05). Main effects testing revealed that a significant increase in soleus PI for the single-legged stance compared the double-legged stance (P=0.001). Surface variation did not affect soleus PI (p > 0.05). CONCLUSIONS: The increase in soleus PI during a single-legged stance suggests inhibition of the motoneuron pool may aid in making postural corrections, while maintaining upright stance during less stable positions. Moreover, maintenance of upright stance on an unstable foam surface does not appear to influence extrinsic PI modulation of the soleus muscle.

The Effects of Motor Imagery on the Hoffmann Reflex and Presynaptic Inhibition

Mental imagery of a motor task may influence the amplitude of the Hoffmann reflex (H-reflex) (Hale et al. 2003, Oishi et al. 1995), but these investigations have not examined the potential influence that presynaptic inhibition might have on the H-reflex during mental imagery of a motor task. PURPOSE: The purpose of this project was to investigate the effects of mental imagery of a motor task on the H-reflex and on presynaptic inhibition of the H-reflex. METHODS: In this study 25 adult participants (M=22.8 years, SD=5.4; 15 Females, 10 Males) were examined on one day of testing. The participants mentally imaged a simple plantar flexion movement of the right foot at two intensities: 20 and 60% of maximum voluntary contraction after completing a series of practice trials of actual contractions at these intensities. Five participants served as control subjects who completed the entire testing protocol but did not perform mental imagery. The dependent measures were peak-to-peak amplitude of the H-reflex and the peak to peak amplitude of the conditioned H-reflex. Data analysis was conducted using a 2X2 ANOVA to determine if any changes in H-reflex amplitude occurred from rest and imagery percentage. Additionally, a One-way ANOVA was conducted to determine if any changes in presynaptic inhibition were noted during the imagery trials. RESULTS: Repeated measures ANOVA revealed a significant (P < 0.05) condition effect for H-reflex amplitude during imagery trials. Specifically, H-reflex amplitude increased between 5 and 14% during imagery trials above rest (Mean at Rest = 1.62, Mean at 20% = 1.84, Mean at 60% = 1.71). Additionally, a significant (P < 0.05) difference was also observed between percent of contraction imagined. A significant (P < 0.05) difference was observed in presynaptic inhibition between percent of imagined contraction Mean at 20% = .36, Mean at 60% = .39). Control subjects demonstrated no significant difference for H-reflex or presynaptic inhibition. CONCLUSION: These findings indicate that the amplitude of the H-reflex is increased while performing mental imagery of plantar flexion. Additionally, the measured differences in the conditioned H-reflex indicate that presynaptic inhibition might influence H-reflex response during mental imagery.