Modulation by corticospinal volleys of presynaptic inhibition to Ia afferents in man

Characterizing the effect of low intensity transcranial magnetic stimulation on the soleus H-reflex at rest

Modulation of a Hoffmann (H)-reflex following transcranial magnetic stimulation (TMS) has been used to assess the nature of signals transmitted from cortical centers to lower motor neurons. Further characterizing the recruitment and time-course of the TMS-induced effect onto the soleus H-reflex adds to the discussion of these pathways and may improve its utility in clinical studies. In 10 healthy controls, TMS was used to condition the soleus H-reflex using TMS intensities from 65 to 110% of the resting motor threshold (RMT). Early facilitation [- 5 to - 3 ms condition-test (C-T) interval] was evident when TMS was 110% of RMT (P < 0.05). By comparison, late facilitation (+ 10 to + 20 ms C-T interval) was several times larger and observed over a wider range of TMS intensities, including 65-110% of RMT. The early inhibition (- 3 to - 1 ms C-T interval) had a low TMS threshold and was elicited over a wide range of intensity from 65% to 95% of RMT (all P < 0.05). A second inhibitory phase was seen ~ 4 ms later (+ 1 to + 4 ms C-T intervals) and was only observed for a TMS intensity of 95% of RMT (P < 0.05). The present findings reaffirm that subthreshold TMS strongly modulates soleus motor neurons and demonstrates that distinct pathways can be selectively probed at discrete C-T intervals when using specific TMS intensities.

Vestibulospinal and Corticospinal Modulation of Lumbosacral Network Excitability in Human Subjects

As part of a project aimed to develop a novel, non-invasive techniques for comprehensive assessment of supraspinal-spinal connectivity in humans, the present study sought to explore the convergence of descending vestibulospinal and corticospinal pathways onto lumbosacral motor pools. Transcutaneous electrical spinal stimulation-evoked motor potentials were recorded from knee and ankle flexors and extensors in resting neurologically intact participants. Descending influences on lumbosacral motor neurons were studied using galvanic vestibular (GVS) or transcranial magnetic stimulation (TMS) to elicit descending vestibulospinal or corticospinal volleys, respectively. Facilitatory conditioning effects of descending corticospinal volleys were manifested by a significant increase of spinally evoked motor potentials in recorded knee and ankle muscles bilaterally, and were observed at the 10–30 ms conditioning-test intervals (CTIs); whereas, facilitatory conditioning effects of vestibulospinal volleys manifested at longer latencies (CTIs of 90 and 110 ms), and lasted up to 250 ms. TMS mediated volleys revealed the conditioning effects at both short and long latencies, suggestive of both direct and indirect influence. In contrast, vestibulospinally mediated conditioning effects occurred at longer latencies, consistent with this pathway’s known anatomical and functional interfaces with other descending systems including the reticulospinal pathway and, suggestively, propriospinal interneurons. Our work demonstrates the utility and sensitivity of transcutaneous spinal stimulation in human neurophysiological studies as a technique for quantitative characterization of excitatory conditioning effects in multiple lumbosacral motor pools, obtained through descending pathways. This characterization becomes critical in understanding the neuroplasticity in the central nervous system during motor learning and neurological recovery.

Balance highly influences flexibility measured by the toe-touch test

Flexibility evaluation is a standard assessment in clinical and in sports settings. The Toe-touch test (TTT) is a common assessment tool to evaluate posterior muscular chain flexibility, but the test procedure implies a balance demand. The objective of the study was to verify the hypothesis that the balance demand during the TTT may affect the flexibility measured by the test. Twenty healthy, active young adults participated in the present study. Toe-touch test outcome (the linear finger to toe distance), Center of Pressure (CP) and sagittal plane joint angles were compared under two balance conditions: (1) standard TTT (ST) and (2) minimised postural demand (MPD) during TTT (using a device that restrained the participant against a forward fall). Then, ST was re-tested to verify a possible effect of motor learning on TTT outcome. Compared to ST, MPD showed an improvement of 73% in test outcome, greater flexion of the ankle, greater total body flexion, and a forward displacement of the CP. Re-test of ST showed indications of motor learning with a different balance strategy compared to the first trial in the same condition. The test outcome showed significant negative correlations with CP position in ST (weak correlation), in the re-test (strong correlation) and when the conditions were combined (moderate correlation). In conclusion, TTT outcome was highly affected by balance performance. Maximum range of motion during ST was a fraction of the range obtained when balance demand was minimized.

Evidence for a subcortical contribution to intracortical facilitation

Intracortical facilitation (ICF) describes the facilitation of an EMG response (motor evoked potential) to a suprathreshold pulse (S2) of transcranial magnetic stimulation (TMS) by a preceding subthreshold pulse (S1) given 10-15 ms earlier. ICF is widely assumed to originate from intracortical mechanisms. In this study, we used spinal H-reflexes to test whether subcortical mechanisms can also contribute to the facilitation. Measurements were performed in the upper limb muscle flexor carpi radialis in 17 healthy volunteers, and in the lower limb muscle soleus in 16 healthy volunteers. S2 given alone facilitated the H-reflex. When S1 preceded S2 by 10 ms, the amount of facilitation increased, compatible with ICF. However, S1 given alone also facilitated the H-reflex, suggesting that it had evoked descending activity even though its intensity was well below resting motor threshold. Across participants, the amount of H-reflex facilitation from S1 alone was proportional to the degree of H-reflex facilitation with combined S1-S2. These results indicate that subcortical mechanisms can contribute to ICF and potentially add to the variability of the ICF measure reported in previous studies.

Descending Inputs to Spinal Circuits Facilitating and Inhibiting Human Wrist Flexors

Recently we reported in humans that electrical stimulation of the wrist extensor muscle extensor carpi radialis (ECR) could facilitate or suppress the H reflex elicited in flexor carpi radialis (FCR), for inter-stimulus intervals (ISIs) of 30 ms or 70 ms, respectively. The facilitation at 30 ms may be produced by both flexor afferents and extensor Ib afferents acting on a spinal circuit; the origin of the suppression at 70 ms is less certain. In this study, we investigated possible descending inputs to these systems. We used magnetic stimulation of the contralateral primary motor cortex, and click sound stimulation, to activate the corticospinal and the reticulospinal tracts respectively, and measured the effects on the H reflex conditioned by ECR stimulation. Corticospinal inputs reduced both the 30 ms facilitation and 70 ms suppression, indicating corticospinal inhibition of both circuits. By contrast, we failed to show any effect of clicks, either on the H reflex or on its modulation by ECR stimulation. This suggests that click-activated reticulospinal inputs to these circuits may be weak or absent.

Assessing TMS-induced D and I waves with spinal H-reflexes

Transcranial magnetic stimulation (TMS) of motor cortex produces a series of descending volleys known as D (direct) and I (indirect) waves. In the present study, we questioned whether spinal H-reflexes can be used to dissect D waves and early and late I waves from TMS. We therefore probed H-reflex facilitation at arrival times of D and I waves at the spinal level and thereby changed TMS parameters that have previously been shown to have selective effects on evoked D and different I waves. We changed TMS intensity and current direction and applied a double-pulse paradigm known as short-interval intracortical inhibition (SICI). Experiments were conducted in flexor carpi radialis (FCR) in the arm and soleus (SOL) in the leg. There were two major findings: 1) in FCR, H-reflex facilitation showed characteristic modulations with altered TMS parameters that correspond to the changes of evoked D and I waves; and 2) H-reflexes in SOL did not, possibly because of increased interference from other spinal circuits. Therefore, the most significant outcome of this study is that in FCR, H-reflexes combined with TMS seem to be a useful technique to dissect TMS-induced D and I waves.

NEW & NOTEWORTHY Questions that relate to corticospinal function in pathophysiology and movement control demand sophisticated techniques to provide information about corticospinal mechanisms. We introduce a noninvasive electrophysiological technique that may be useful in describing such mechanisms in more detail by dissecting D and I waves from transcranial magnetic stimulation (TMS). Based on the combination of spinal H-reflexes and TMS in the flexor carpi radialis muscle, the technique was shown to measure selective effects on D and I waves from changing TMS parameters.

Spinal Excitability Changes after Transspinal and Transcortical Paired Associative Stimulation in Humans

Paired associative stimulation (PAS) produces enduring neuroplasticity based on Hebbian associative plasticity. This study established the changes in spinal motoneuronal excitability by pairing transcortical and transspinal stimulation. Transcortical stimulation was delivered after (transspinal-transcortical PAS) or before (transcortical-transspinal PAS) transspinal stimulation. Before and after 40 minutes of each PAS protocol, spinal neural excitability was assessed based on the amplitude of the transspinal-evoked potentials (TEPs) recorded from ankle muscles of both legs at different stimulation intensities (recruitment input-output curve). Changes in TEPs amplitude in response to low-frequency stimulation and paired transspinal stimuli were also established before and after each PAS protocol. TEP recruitment input-output curves revealed a generalized depression of TEPs in most ankle muscles of both legs after both PAS protocols that coincided with an increased gain only after transcortical-transspinal PAS. Transcortical-transspinal PAS increased and transspinal-transcortical PAS decreased the low-frequency-dependent TEP depression, whereas neither PAS protocol affected the TEP depression observed upon paired transspinal stimuli. These findings support the notion that transspinal and transcortical PAS has the ability to alter concomitantly cortical and spinal synaptic activity. Transspinal and transcortical PAS may contribute to the development of rehabilitation strategies in people with bilateral increased motoneuronal excitability due to cortical or spinal lesions.

The optimal neural strategy for a stable motor task requires a compromise between level of muscle cocontraction and synaptic gain of afferent feedback

Increasing joint stiffness by cocontraction of antagonist muscles and compensatory reflexes are neural strategies to minimize the impact of unexpected perturbations on movement. Combining these strategies, however, may compromise steadiness, as elements of the afferent input to motor pools innervating antagonist muscles are inherently negatively correlated. Consequently, a high afferent gain and active contractions of both muscles may imply negatively correlated neural drives to the muscles and thus an unstable limb position. This hypothesis was systematically explored with a novel computational model of the peripheral nervous system and the mechanics of one limb. Two populations of motor neurons received synaptic input from descending drive, spinal interneurons, and afferent feedback. Muscle force, simulated based on motor unit activity, determined limb movement that gave rise to afferent feedback from muscle spindles and Golgi tendon organs. The results indicated that optimal steadiness was achieved with low synaptic gain of the afferent feedback. High afferent gains during cocontraction implied increased levels of common drive in the motor neuron outputs, which were negatively correlated across the two populations, constraining instability of the limb. Increasing the force acting on the joint and the afferent gain both effectively minimized the impact of an external perturbation, and suboptimal adjustment of the afferent gain could be compensated by muscle cocontraction. These observations show that selection of the strategy for a given contraction implies a compromise between steadiness and effectiveness of compensations to perturbations. This indicates that a task-dependent selection of neural strategy for steadiness is necessary when acting in different environments.

Reduced postactivation depression of soleus H reflex and root evoked potential after transcranial magnetic stimulation

Postactivation depression of the Hoffmann (H) reflex is associated with a transient period of suppression following activation of the reflex pathway. In soleus, the depression lasts for 100-200 ms during voluntary contraction and up to 10 s at rest. A reflex root evoked potential (REP), elicited after a single pulse of transcutaneous stimulation to the thoracolumbar spine, has been shown to exhibit similar suppression. The present study systematically characterized the effect of transcranial magnetic stimulation (TMS) on postactivation depression using double-pulse H reflexes and REPs. A TMS pulse reduced the period of depression to 10-15 ms for both reflexes. TMS could even produce postactivation facilitation of the H reflex, as the second reflex response was increased to 243 ± 51% of control values at the 75-ms interval. The time course was qualitatively similar for the REP, yet the overall increase was less. While recovery of the H reflex was slower in the relaxed muscle, the profile exhibited a distinct bimodal shape characterized by an early peak at the 25-ms interval, reaching 72 ± 23% of control values, followed by a trough at 50 ms, and then a gradual recovery at intervals > 50 ms. The rapid recovery of two successively depressed H reflexes, ∼ 25 ms apart, was also possible with double-pulse TMS. The effect of the TMS-induced corticospinal excitation on postactivation depression may be explained by a combination of pre- and postsynaptic mechanisms, although further investigation is required to distinguish between them.

Upper-extremity spinal reflex inhibition is reproducible and strongly related to grip force poststroke

PURPOSE

Impaired reflex regulation is assumed to contribute to upper-extremity motor impairment poststroke; however, the relationship between reflex inhibition and motor function remains unclear. To address this question, it is first necessary to determine the reproducibility of reflex responses. The objective of this study was to establish the test-retest reliability of flexor carpi radialis H-reflex inhibition in healthy control and stroke participants and investigate the correlation between H-reflex inhibition and grip strength.

MATERIALS AND METHODS

Eighteen persons poststroke (mean ± SD: age 63 ± 13 years; 6 ± 5 years poststroke; 13 males) and 16 healthy controls (age: 62 ± 12 years) participated. Reflex inhibition was tested on 2 separate days by conditioning the H-reflex with radial nerve stimulation at two different interstimulus intervals: 13 ms (presynaptic Ia inhibition-PSI) and 0 ms (disynaptic inhibition). Pearson's and intraclass correlation coefficients [two-way mixed model-ICC (1, 2)], and standard error of measurement (SEM) were calculated.

RESULTS

Relative reliability (ICCs) ranged from good to excellent (0.61-0.78). SEM was low (range 10-19%, stroke; 15-20%, healthy controls). Paretic grip strength and paretic limb PSI revealed a positive correlation (r = 0.70; p < 0.0125). Disynaptic inhibition and paretic grip strength were not correlated.

CONCLUSIONS

To our knowledge, this is the first study to demonstrate reproducibility of reflex inhibition in individuals poststroke. Furthermore, we quantify smallest real differences, which provide an estimate of the magnitude of effect required to determine a meaningful change, exceeding measurement error. The correlation between PSI and grip strength suggests the potential contribution of PSI to grip force production and upper-extremity motor function.

Interaction of transcutaneous spinal stimulation and transcranial magnetic stimulation in human leg muscles

Transcutaneous spinal stimulation is a noninvasive method that can activate dorsal and/or ventral roots depending on the location and intensity of stimulation. Reflex root-evoked potentials (REPs) were studied in muscles that traditionally evoke large (soleus) and small H-reflexes (tibialis anterior), as well as muscles where H-reflexes are difficult to study (hamstrings). This study characterizes the interaction of the REP and the motor-evoked potential (MEP). Transcranial magnetic stimulation (TMS) delivered 11-25 ms before spinal stimulation resulted in more than linear summation of the two responses. Because of overlap, the modulation was quantified after subtracting the contribution of the conditioning MEP or REP. At rest, the mean-rectified soleus response was facilitated by up to ~250 μV (21-times the MEP or 161% of the REP). The increases were more reliable during a voluntary contraction (up to ~300 μV, 517% of the MEP or 181% of the REP). At the 13-ms interval, the mean-rectified response in the pre-contracted hamstrings was increased by 227% of the MEP or 300% of the REP. In some subjects, TMS could also eliminate the post-activation depression produced using two spinal stimuli, confirming that the interaction can extend to presynaptic spinal neurons. The spatiotemporal facilitation in tibialis anterior was not significant. However, the large MEP was facilitated when the spinal stimulus preceded TMS by 100-150 ms, presumably because of rebound excitation. These strong interactions may be important for inducing motor plasticity and improved training procedures for recovery after neurological damage.

Modulation of reciprocal and presynaptic inhibition during robotic-assisted stepping in humans

OBJECTIVE

To establish the modulation pattern of reciprocal inhibition and presynaptic inhibition of soleus Ia afferents during robot-assisted stepping in healthy subjects.

METHODS

During stepping, the soleus H-reflex was conditioned by percutaneous stimulation of the ipsilateral common peroneal nerve with a single pulse at stimulation intensities that ranged from 0.9 to 1.2 TA M-wave motor thresholds across subjects. To control for movement of recording and stimulating electrodes, a supramaximal stimulus 80ms after the conditioned and/or unconditioned H-reflexes was delivered to the posterior tibial nerve. The short (2, 3, 4ms) and long (60-80ms) conditioning-test intervals at which the largest amount of reflex depression was observed with the subjects seated were utilized during stepping. Stimuli were randomly dispersed across the step cycle which was divided into 16 equal bins.

RESULTS

Reciprocal inhibition exerted from flexor group I afferents onto soleus motoneurons was decreased at mid-stance and increased and late-stance and throughout the swing phase. Presynaptic inhibition of soleus Ia afferents was increased at heel strike and decreased at late-stance and early swing phases.

CONCLUSION

Reciprocal inhibition between ankle antagonistic muscles and presynaptic inhibition of soleus Ia afferents are modulated in a similar pattern to that reported during walking on a treadmill with full weight bearing and without robot-assisted leg movement.

SIGNIFICANCE

The activity of spinal interneuronal circuits engaged in patterned locomotor activity supports a reciprocal gait pattern during robot-assisted stepping in healthy humans.

Plasticity of corticospinal neural control after locomotor training in human spinal cord injury

Spinal lesions substantially impair ambulation, occur generally in young and otherwise healthy individuals, and result in devastating effects on quality of life. Restoration of locomotion after damage to the spinal cord is challenging because axons of the damaged neurons do not regenerate spontaneously. Body-weight-supported treadmill training (BWSTT) is a therapeutic approach in which a person with a spinal cord injury (SCI) steps on a motorized treadmill while some body weight is removed through an upper body harness. BWSTT improves temporal gait parameters, muscle activation patterns, and clinical outcome measures in persons with SCI. These changes are likely the result of reorganization that occurs simultaneously in supraspinal and spinal cord neural circuits. This paper will focus on the cortical control of human locomotion and motor output, spinal reflex circuits, and spinal interneuronal circuits and how corticospinal control is reorganized after locomotor training in people with SCI. Based on neurophysiological studies, it is apparent that corticospinal plasticity is involved in restoration of locomotion after training. However, the neural mechanisms underlying restoration of lost voluntary motor function are not well understood and translational neuroscience research is needed so patient-orientated rehabilitation protocols to be developed.

Spinal associative stimulation: a non-invasive stimulation paradigm to modulate spinal excitability

OBJECTIVE

Repetitive, paired peripheral and transcranial stimulation targeting the cerebral cortex can increase cortical excitability, outlasting the stimulation period. It is unknown whether paired stimulation specifically targeting the spinal cord can modulate spinal excitability. We tested whether the H-reflex facilitation from a sub-threshold conditioning TMS pulse could modulate spinal excitability if delivered repetitively.

METHOD

In 13 healthy subjects, we delivered single-pulse TMS (80% RMT) for the right soleus muscle, 20 ms prior to an electrical peripheral nerve stimulus delivered over the posterior tibial nerve on the same side at 0.1 Hz during 15 min.

RESULTS

PNS alone evoked an H-reflex of 0.25 mV ± 0.06 SEM, while pairing of TMS and PNS facilitated the H-reflex to 0.7 ± 0.11 mV. TMS-PNS pairs delivered at 0.1 Hz for 15 min progressively increased in the evoked response to ∼130% (r(2) = 0.97) of the starting amplitude (normalized to 1st min). Post-intervention, H-reflex threshold decreased (pre = 12.9 ± 1.7 mA; post =11.6 ± 1.6 mA; p = 0.04), as did the stimulus intensity at maximum H-reflex amplitude (pre = 23.5 ± 02.8 mA; post = 21.6 ± 2.6 mA; p = 0.03), and recruitment curve width (pre = 11.6 ± 1.5 mA; post = 10.93 ± 1.4 mA; p = 0.03). No such changes were observed with intervention of PNS or TMS alone.

CONCLUSION

Paired stimulation targeting spinal facilitatory interactions, when applied repetitively, can increase spinal excitability during and after the intervention.

SIGNIFICANCE

Spinal associative stimulation may have potential for neuromodulation in spinal cord injury patients.

Reflex responsiveness of a human hand muscle when controlling isometric force and joint position

OBJECTIVE

This study compared reflex responsiveness of the first dorsal interosseus muscle during two tasks that employ different strategies to stabilize the finger while exerting the same net muscle torque.

METHODS

Healthy human subjects performed two motor tasks that involved either pushing up against a rigid restraint to exert a constant isometric force equal to 20% of maximum or maintaining a constant angle at the metacarpophalangeal joint while supporting an equivalent inertial load. Each task consisted of six 40-s contractions during which electrical and mechanical stimuli were delivered.

RESULTS

The amplitude of short and long latency reflex responses to mechanical stretch did not differ significantly between tasks. In contrast, reflexes evoked by electrical stimulation were significantly greater when supporting the inertial load.

CONCLUSIONS

Agonist motor neurons exhibited heightened reflex responsiveness to synaptic input from heteronymous afferents when controlling the position of an inertial load. Task differences in the reflex response to electrical stimulation were not reflected in the response to mechanical perturbation, indicating a difference in the efficacy of the pathways that mediate these effects.

SIGNIFICANCE

Results from this study suggest that modulation of spinal reflex pathways may contribute to differences in the control of force and position during isometric contractions of the first dorsal interosseus muscle.

Changes in Spinal Excitability After PAS

Repetitive pairing of a peripheral stimulation with a magnetic transcortical stimulation (PAS) is widely used to induce plastic changes in the human motor cortex noninvasively. Based on the contrast between PAS-induced increase of corticospinal excitability and absence of PAS-induced increase of the spinal F wave size, it has been generally accepted that PAS-induced plasticity is cortical in origin. Here, instead of F waves, we used H reflex recruitment curves to assess spinal excitability, and we demonstrate that PAS induces parallel changes in cortical and spinal excitability.

Proprioception and myoclonus

This review focuses on sensory information originating from muscle spindles and its role in proprioception and motor control. The first part reminds of the structural and functional properties of these muscle mechanoreceptors, with arguments for an independent fusimotor command, i.e. the gamma-motoneurons, that would regulate spindle mechanical sensitivity in keeping with the requirements of ongoing motor action. The possibility that dysfunction of the fusimotor system might be responsible for clinical signs is discussed with respect to the hyperexcitability of the sensorimotor cortex that is observed in myoclonus of cortical origin. What is known about the spindle afferents projections into the spinal cord and about the dysfunction of the spinal sensorimotor networks in patients with neurological disorders, is put together in the second part. It is stressed on the significant complexity of the monosynaptic reflex in spite of its "simple" organization. The monosynaptic reflex constitutes the only possible way for testing the excitability of motoneurons and spinal networks. This method is extensively used clinically to examine changes in the nervous system with diseases. When studying changes from the norm, it is important to understand how the reflex functions in neurologically normal conditions. Different mechanisms such as pre-synaptic inhibition, post-activation depression and motoneuronal intrinsic properties are reviewed as they may induce changes in reflex amplitude and have therefore consequences for interpretation of spinal excitability.

Stretch reflex modulation during imposed static and dynamic hip movements in standing humans

The purpose of this study was to investigate the effects of hip proprioceptors on soleus stretch reflex excitability in standing humans. A custom-made device to stretch the ankle extensors was mounted on the lower leg portion of a gait orthosis and was used to elicit stretch reflex responses while standing. Six subjects with motor complete spinal cord injury (SCI) and six spinal intact subjects were placed in the orthosis, and stretch reflex responses were elicited when static and/or dynamic hip joint angle changes were imposed. We found that static hip extension significantly enhanced the stretch reflex responses as compared to the neutral position and the hip flexion position only in the SCI group. The EMG magnitude induced by hip extension was 142 +/- 16.6% greater than that induced by the neutral position. When the leg was dynamically swung, the reflex responses also changed with the phase of the hip angle in the SCI group; in particular, the reflex amplitude was enhanced with hip extension and in the transition phase from flexion to extension. Although the magnitude of the changes was less than that in the SCI group, a similar type of modulation was found in the normal group. Given the fact that the persons with SCI had lost the neural connection between higher nervous center and the paralyzed lower limb muscles, the mechanism underlying the present results can be attributed to the peripheral afferent input due to the hip angle changes. We concluded that hip mediated afferent input has a significant influence on the excitability modulation of the soleus stretch reflex pathway. Such neural modulation may play a role in the mechanism responsible for the phase-dependent modulation of the stretch reflex while walking.

Diurnal H-reflex variation in mice

Mice exhibit diurnal variation in complex motor behaviors, but little is known about diurnal variation in simple spinally mediated functions. This study describes diurnal variation in the H-reflex (HR), a wholly spinal and largely monosynaptic reflex. Six mice were implanted with tibial nerve cuff electrodes and electrodes in the soleus and gastrocnemius muscles, for recording of ongoing and nerve-evoked electromyographic activity (EMG). Stimulation and recording were under computer control 24 h/day. During a 10-day recording period, HR amplitude varied throughout the day, usually being larger in the dark than in the light. This diurnal HR variation could not be attributed solely to differences in the net ongoing level of descending and segmental excitation to the spinal cord or stimulus intensity. HRs were larger in the dark than in the light even after restricting the evoked responses to subsets of trials having similar ongoing EMG and M-responses. The diurnal variation in the HR was out of phase with that reported previously for rats, but was in phase with that observed in monkeys. These data, supported by those in other species, suggest that the supraspinal control of the excitability of the HR pathway varies throughout the day in a species-specific pattern. This variation should be taken into account in experimental and clinical studies of spinal reflexes recorded at different times of day.

Interaction between peripheral afferent activity and presynaptic inhibition of ia afferents in the cat

It has been demonstrated in man that the H-reflex is more depressed by presynaptic inhibition than the stretch reflex. Here we investigated this finding further in the alpha-chloralose-anesthetized cat. Soleus monosynaptic reflexes were evoked by electrical stimulation of the tibial nerve or by stretch of the triceps surae muscle. Conditioning stimulation of the posterior biceps and semitendinosus nerve (PBSt) produced a significantly stronger depression of the electrically than the mechanically evoked reflexes. The depression of the reflexes has been shown to be caused by presynaptic inhibition of triceps surae Ia afferents. We investigated the hypothesis that repetitive activation of peripheral afferents may reduce their sensitivity to presynaptic inhibition. In triceps surae motoneurones, we measured the effect of presynaptic inhibition on excitatory postsynaptic potentials (EPSPs) produced by repetitive activation of the peripheral afferents or by fast and slow muscle stretch. EPSPs evoked by single electrical stimulation of the tibial nerve or by fast muscle stretch were significantly depressed by PBSt stimulation. However, the last EPSP in a series of EPSPs evoked by a train of electrical stimuli (5-6 shocks, 150-200 Hz) was significantly less depressed by the conditioning stimulation than the first EPSP. In addition, the last part of the long-lasting EPSPs evoked by a slow muscle stretch was also less depressed than the first part. A single EPSP evoked by stimulation of the medial gastrocnemius nerve was less depressed when preceded by a train of stimuli applied to the same nerve than when the same train of stimuli was applied to a synergistic nerve. The decreased sensitivity of the test EPSP to presynaptic inhibition was maximal when it was evoked within 20 ms after the train of EPSPs. It was not observed at intervals longer than 30 ms. These findings suggest that afferent activity may decrease the efficiency of presynaptic inhibition. We propose that the described interaction between afferent nerve activity and presynaptic inhibition may partly explain why electrically and mechanically evoked reflexes are differently sensitive to presynaptic inhibition.

Presynaptic inhibition in humans: a comparison between normal and spastic patients

During the last 40 years, several studies in man have been devoted to the pathophysiological mechanisms underlying spasticity. Spasticity is characterised by a velocity dependent increase in muscle tone. Many spinal pathways control stretch reflex excitability and a malfunction in any one of them could theoretically produce the exaggeration of the stretch reflex. Delwaide showed that the vibration-induced inhibition of Ia fibres is reduced in spastic patients. However, the relation between a decrease in presynaptic Ia inhibition and the pathophysiology of spasticity has been recently questioned since it was argued that homosynaptic depression (or post-activation depression) also contributes to the vibratory-induced depression of monosynaptic reflexes. This paper is thus devoted to a review of the methods recently developed to study selectively presynaptic Ia inhibition in man and to a reevaluation of the relations between modifications in presynaptic Ia inhibition and spasticity in hemiplegic and spinal spastic patients.