Differential projection of the sural nerve to early and late recruited human tibialis anterior motor units: change of recruitment gain

Evaluation of spasticity: IFCN Handbook Chapter

There is no generally accepted definition of spasticity, but hyperexcitable stretch reflexes, exaggerated tendon jerks, clonus, spasms, cramps, increased resistance to passive joint movement, sustained involuntary muscle activity and aberrant muscle activation, including co-contraction of antagonist muscles are all signs and symptoms which are usually associated clinically to the term spasticity. This review describes how biomechanical and electrophysiological techniques may be used to provide quantitative and objective measures of each of these signs and symptoms. The review further describes how neurophysiological techniques may be used to evaluate pathophysiological changes in spinal motor control mechanisms. It is emphasized that understanding the pathophysiology and distinguishing the specific signs and symptoms associated with spasticity, using objective, valid, and reproducible measurements, is essential for providing optimal therapy.

Electrical cutaneous stimulation of the foot sole does not enhance rate of torque development during maximal effort isometric plantarflexion in females

Rate of torque development (RTD) measures how rapidly one can generate torque and is crucial for balance and athletic performance. Fast RTD depends on the rapid recruitment of high threshold motor units (MUs). Cutaneous electrical stimulation has been shown to alter MU excitability, favoring high threshold MUs via reduced recruitment thresholds. A strong coupling exists between foot sole cutaneous mechanoreceptors and motor neurons of lower-limb muscles, yet it remains unknown if cutaneous input can impact RTD via modulation of MU excitability. This study aimed to investigate whether electrical stimulation across the heel could alter plantarflexion RTD. 11 young and healthy females underwent eight sets of five explosive isometric plantarflexion contractions on a dynamometer while sitting with hip, knee and ankle angles of 80°, 110°, and 90°, respectively. All participants achieved > 95 % voluntary activation of their plantar flexors. Four sets of contractions were performed with heel cutaneous electrical stimulation (1.0 ms pulses delivered at 300 Hz, at 2 × perceptual threshold) and four sets with no stimulation. Instantaneous RTD values were analyzed in 25 ms epochs from onset to 250 ms. No significant differences were observed between stimulation conditions within each epoch, thus our results suggest that electrical cutaneous stimulation does not alter RTD in this population.

Facilitation of sensory transmission to motoneurons during cortical or sensory-evoked primary afferent depolarization (PAD) in humans

Sensory and corticospinal tract (CST) pathways activate spinal GABAergic interneurons that have axoaxonic connections onto proprioceptive (Ia) afferents that cause long-lasting depolarizations (termed primary afferent depolarization, PAD). In rodents, sensory-evoked PAD is produced by GABAA receptors at nodes of Ranvier in Ia afferents, rather than at presynaptic terminals, and facilitates spike propagation to motoneurons by preventing branch-point failures, rather than causing presynaptic inhibition. We examined in 40 human participants whether putative activation of Ia-PAD by sensory or CST pathways can also facilitate Ia afferent activation of motoneurons via the H-reflex. H-reflexes in several leg muscles were facilitated by prior conditioning from low-threshold proprioceptive, cutaneous or CST pathways, with a similar long-lasting time course (∼200 ms) to phasic PAD measured in rodent Ia afferents. Long trains of cutaneous or proprioceptive afferent conditioning produced longer-lasting facilitation of the H-reflex for up to 2 min, consistent with tonic PAD in rodent Ia afferents mediated by nodal α5-GABAA receptors for similar stimulation trains. Facilitation of H-reflexes by this conditioning was likely not mediated by direct facilitation of the motoneurons because isolated stimulation of sensory or CST pathways did not alone facilitate the tonic firing rate of motor units. Furthermore, cutaneous conditioning increased the firing probability of single motor units (motoneurons) during the H-reflex without increasing their firing rate at this time, indicating that the underlying excitatory postsynaptic potential was more probable, but not larger. These results are consistent with sensory and CST pathways activating nodal GABAA receptors that reduce intermittent failure of action potentials propagating into Ia afferent branches. KEY POINTS: Controlled execution of posture and movement requires continually adjusted feedback from peripheral sensory pathways, especially those that carry proprioceptive information about body position, movement and effort. It was previously thought that the flow of proprioceptive feedback from Ia afferents was only reduced by GABAergic neurons in the spinal cord that sent axoaxonic projections to the terminal endings of sensory axons (termed GABAaxo neurons). Based on new findings in rodents, we provide complementary evidence in humans to suggest that sensory and corticospinal pathways known to activate GABAaxo neurons that project to dorsal parts of the Ia afferent also increase the flow of proprioceptive feedback to motoneurons in the spinal cord. These findings support a new role for spinal GABAaxo neurons in facilitating afferent feedback to the spinal cord during voluntary or reflexive movements.

Randomized Clinical Study of Temporary Transvenous Phrenic Nerve Stimulation in Difficult-to-Wean Patients

Rationale: Diaphragm dysfunction is frequently observed in critically ill patients with difficult weaning from mechanical ventilation. Objectives: To evaluate the effects of temporary transvenous diaphragm neurostimulation on weaning outcome and maximal inspiratory pressure. Methods: Multicenter, open-label, randomized, controlled study. Patients aged ⩾18 years on invasive mechanical ventilation for ⩾4 days and having failed at least two weaning attempts received temporary transvenous diaphragm neurostimulation using a multielectrode stimulating central venous catheter (bilateral phrenic stimulation) and standard of care (treatment) (n = 57) or standard of care (control) (n = 55). In seven patients, the catheter could not be inserted, and in seven others, pacing therapy could not be delivered; consequently, data were available for 43 patients. The primary outcome was the proportion of patients successfully weaned. Other endpoints were mechanical ventilation duration, 30-day survival, maximal inspiratory pressure, diaphragm-thickening fraction, adverse events, and stimulation-related pain. Measurements and Main Results: The incidences of successful weaning were 82% (treatment) and 74% (control) (absolute difference [95% confidence interval (CI)], 7% [-10 to 25]), P = 0.59. Mechanical ventilation duration (mean ± SD) was 12.7 ± 9.9 days and 14.1 ± 10.8 days, respectively, P = 0.50; maximal inspiratory pressure increased by 16.6 cm H₂O and 4.8 cm H₂O, respectively (difference [95% CI], 11.8 [5 to 19]), P = 0.001; and right hemidiaphragm thickening fraction during unassisted spontaneous breathing was +17% and -14%, respectively, P = 0.006, without correlation with changes in maximal inspiratory pressure. Serious adverse event frequency was similar in both groups. Median stimulation-related pain in the treatment group was 0 (no pain). Conclusions: Temporary transvenous diaphragm neurostimulation did not increase the proportion of successful weaning from mechanical ventilation. It was associated with a significant increase in maximal inspiratory pressure, suggesting reversal of the course of diaphragm dysfunction. Clinical trial registered with www.clinicaltrials.gov (NCT03096639) and the European Database on Medical Devices (CIV-17-06-020004).

Influence of an Acute Exposure to a Moderate Real Altitude on Motoneuron Pool Excitability and Jumping Performance

The aim of the study was to test whether ascending to a moderate real altitude affects motoneuron pool excitability at rest, as expressed by a change in the H-reflex amplitude, and also to elucidate whether a possible alteration in the motoneuron pool excitability could be reflected in the execution of lower-body concentric explosive (squat jump; SJ) and fast eccentric-concentric (drop jump; DJ) muscle actions. Fifteen participants performed four experimental sessions that consisted of the combination of two real altitude conditions [low altitude (low altitude, 690 m), high altitude (higher altitude, 2,320 m)] and two testing procedures (H-reflex and vertical jumps). Participants were tested on each testing day at 8, 11, 14 and 17 h. The only significant difference (p < 0.05) detected for the H-reflex was the higher H-reflex response (25.6%) obtained 15 min after arrival at altitude compared to baseline measurement. In terms of motor behavior, DJ height was the only variable that showed a significant interaction between altitude conditions (LA and HA) and time of measurement (8, 11, 14 and 17 h) as DJ height increased more during successive measurements at HA compared to LA. The only significant difference between the LA and HA conditions was observed for DJ height at 17 h which was higher for the HA condition (p = 0.04, ES = 0.41). Although an increased H-reflex response was detected after a brief (15–20 min) exposure to real altitude, the effect on motorneuron pool excitability could not be confirmed since no significant changes in the H-reflex were detected when comparing LA and HA. On the other hand, the positive effect of altitude on DJ performance was accentuated after 6 h of exposure.

TRPM8-mediated cutaneous stimulation modulates motor neuron activity during treadmill stepping in mice

Motor units are generally recruited from the smallest to the largest following the size principle, while cutaneous stimulation has the potential to affect spinal motor control. We aimed to examine the effects of stimulating transient receptor potential channel sub-family M8 (TRPM8) combined with exercise on the modulation of spinal motor neuron (MN) excitability. Mice were topically administrated 1.5% icilin on the hindlimbs, followed by treadmill stepping. Spinal cord sections were immunostained with antibodies against c-fos and choline acetyltransferase. Icilin stimulation did not change the number of c-fos+ MNs, but increased the average soma size of the c-fos+ MNs during low-speed treadmill stepping. Furthermore, icilin stimulation combined with stepping increased c-fos+ cholinergic interneurons near the central canal, which are thought to modulate MN excitability. These findings suggest that TRPM8-mediated cutaneous stimulation with low-load exercise promotes preferential recruitment of large MNs and is potentially useful as a new training method for rehabilitation.

Recruitment gain of spinal motor neuron pools in cat and human

The output from a motor nucleus is determined by the synaptic input to the motor neurons and their intrinsic properties. Here, we explore whether the source of synaptic inputs to the motor neurons (cats) and the age or post-stroke conditions (humans) may change the recruitment gain of the motor neuron pool. In cats, the size of Ia EPSPs in triceps surae motor neurons (input) and monosynaptic reflexes (MSRs; output) was recorded in the soleus and medial gastrocnemius motor nerves following graded stimulation of dorsal roots. The MSR was plotted against the EPSP thereby obtaining a measure of the recruitment gain. Conditioning stimulation of sural and peroneal cutaneous afferents caused significant increase in the recruitment gain of the medial gastrocnemius, but not the soleus motor neuron pool. In humans, the discharge probability of individual soleus motor units (input) and soleus H-reflexes (output) was performed. With graded stimulation of the tibial nerve, the gain of the motor neuron pool was assessed as the slope of the relation between probability of firing and the reflex size. The gain in young subjects was higher than in elderly subjects. The gain in post-stroke survivors was higher than in age-matched neurologically intact subjects. These findings provide experimental evidence that recruitment gain of a motor neuron pool contributes to the regulation of movement at the final output stage from the spinal cord and should be considered when interpreting changes in reflex excitability in relation to movement or injuries of the nervous system.

Modulation of corticospinal input to the legs by arm and leg cycling in people with incomplete spinal cord injury

The spinal cervico-lumbar interaction during rhythmic movements in humans has recently been studied; however, the role of arm movements in modulating the corticospinal drive to the legs is not well understood. The goals of this study were to investigate the effect of active rhythmic arm movements on the corticospinal drive to the legs (study 1) and assess the effect of simultaneous arm and leg training on the corticospinal pathway after incomplete spinal cord injury (iSCI) (study 2). In study 1, neurologically intact (NI) participants or participants with iSCI performed combinations of stationary and rhythmic cycling of the arms and legs while motor evoked potentials (MEPs) were recorded from the vastus lateralis (VL) muscle. In the NI group, arm cycling alone could facilitate the VL MEP amplitude, suggesting that dynamic arm movements strongly modulate the corticospinal pathway to the legs. No significant difference in VL MEP between conditions was found in participants with iSCI. In study 2, participants with iSCI underwent 12 wk of electrical stimulation-assisted cycling training: one group performed simultaneous arm and leg (A&L) cycling and the other legs-only cycling. MEPs in the tibialis anterior (TA) muscle were compared before and after training. After training, only the A&L group had a significantly larger TA MEP, suggesting increased excitability in the corticospinal pathway. The findings demonstrate the importance of arm movements in modulating the corticospinal drive to the legs and suggest that active engagement of the arms in lower limb rehabilitation may produce better neural regulation and restoration of function.NEW & NOTEWORTHY This study aimed to demonstrate the importance of arm movements in modulating the corticospinal drive to the legs. It provides direct evidence in humans that active movement of the arms could facilitate corticospinal transmission to the legs and, for the first time, shows that facilitation is absent after spinal cord injury. Active engagement of the arms in lower limb rehabilitation increased the excitability of the corticospinal pathway and may produce more effective improvement in leg function.

Vestibular stimulation-induced facilitation of cervical premotoneuronal systems in humans

It is unclear how descending inputs from the vestibular system affect the excitability of cervical interneurons in humans. To elucidate this, we investigated the effects of galvanic vestibular stimulation (GVS) on the spatial facilitation of motor-evoked potentials (MEPs) induced by combined pyramidal tract and peripheral nerve stimulation. To assess the spatial facilitation, electromyograms were recorded from the biceps brachii muscles (BB) of healthy subjects. Transcranial magnetic stimulation (TMS) over the contralateral primary motor cortex and electrical stimulation of the ipsilateral ulnar nerve at the wrist were delivered either separately or together, with interstimulus intervals of 10 ms (TMS behind). Anodal/cathodal GVS was randomly delivered with TMS and/or ulnar nerve stimulation. The combination of TMS and ulnar nerve stimulation facilitated BB MEPs significantly more than the algebraic summation of responses induced separately by TMS and ulnar nerve stimulation (i.e., spatial facilitation). MEP facilitation significantly increased when combined stimulation was delivered with GVS (p < 0.01). No significant differences were found between anodal and cathodal GVS. Furthermore, single motor unit recordings showed that the short-latency excitatory peak in peri-stimulus time histograms during combined stimulation increased significantly with GVS. The spatial facilitatory effects of combined stimulation with short interstimulus intervals (i.e., 10 ms) indicate that facilitation occurred at the premotoneuronal level in the cervical cord. The present findings therefore suggest that GVS facilitates the cervical interneuron system that integrates inputs from the pyramidal tract and peripheral nerves and excites motoneurons innervating the arm muscles.

Testing the excitability of human motoneurons

The responsiveness of the human central nervous system can change profoundly with exercise, injury, disuse, or disease. Changes occur at both cortical and spinal levels but in most cases excitability of the motoneuron pool must be assessed to localize accurately the site of adaptation. Hence, it is critical to understand, and employ correctly, the methods to test motoneuron excitability in humans. Several techniques exist and each has its advantages and disadvantages. This review examines the most common techniques that use evoked compound muscle action potentials to test the excitability of the motoneuron pool and describes the merits and limitations of each. The techniques discussed are the H-reflex, F-wave, tendon jerk, V-wave, cervicomedullary motor evoked potential (CMEP), and motor evoked potential (MEP). A number of limitations with these techniques are presented.

Similar alteration of motor unit recruitment strategies during the anticipation and experience of pain

A motor unit consists of a motoneurone and the multiple muscle fibres that it innervates, and forms the final neural pathway that influences movement. Discharge of motor units is altered (decreased discharge rate and/or cessation of firing; and increased discharge rate and/or recruitment of new units) during matched-force contractions with pain. This is thought to be mediated by nociceptive (pain) input on motoneurones, as demonstrated in animal studies. It is also possible that motoneurone excitability is altered by pain related descending inputs, that these changes persist after noxious stimuli cease, and that direct nociceptive input is not necessary to induce pain related changes in movement. We aimed to determine whether anticipation of pain (descending pain related inputs without nociceptor discharge) alters motor unit discharge, and to observe motor unit discharge recovery after pain has ceased. Motor unit discharge was recorded with fine-wire electrodes in the quadriceps of 9 volunteers. Subjects matched isometric knee-extension force during anticipation of pain (anticipation: electrical shocks randomly applied over the infrapatellar fat-pad); pain (hypertonic saline injected into the fat-pad); and 3 intervening control conditions. Discharge rate of motor units decreased during pain (P<.001) and anticipation (P<.01) compared with control contractions. De-recruitment of 1 population of units and new recruitment of another population were observed during both anticipation and pain; some changes in motor unit recruitment persisted after pain ceased. This challenges the fundamental theory that pain-related changes in muscle activity result from direct nociceptor discharge, and provides a mechanism that may underlie long-term changes in movement/chronicity in some musculoskeletal conditions.

Involvement of the corticospinal tract in the control of human gait

Given the inherent mechanical complexity of human bipedal locomotion, and that complete spinal cord lesions in human leads to paralysis with no recovery of gait, it is often suggested that the corticospinal tract (CST) has a more predominant role in the control of walking in humans than in other animals. However, what do we actually know about the contribution of the CST to the control of gait? This chapter will provide an overview of this topic based on the premise that a better understanding of the role of the CST in gait will be essential for the design of evidence-based approaches to rehabilitation therapy, which will enhance gait ability and recovery in patients with lesions to the central nervous system (CNS). We review evidence for the involvement of the primary motor cortex and the CST during normal and perturbed walking and during gait adaptation. We will also discuss knowledge on the CST that has been gained from studies involving CNS lesions, with a particular focus on recent data acquired in people with spinal cord injury.

Load rather than length sensitive feedback contributes to soleus muscle activity during human treadmill walking

Walking requires a constant adaptation of locomotor output from sensory afferent feedback mechanisms to ensure efficient and stable gait. We investigated the nature of the sensory afferent feedback contribution to the soleus motoneuronal drive and to the corrective stretch reflex by manipulating body load and ankle joint angle. The volunteers walked on a treadmill ( approximately 3.6 km/h) connected to a body weight support (BWS) system. To manipulate the load sensitive afferents the level of BWS was switched between 5 and 30% of body weight. The effect of transient changes in BWS on the soleus stretch reflex was measured by presenting dorsiflexion perturbations ( approximately 5 degrees, 360-400 degrees/s) in mid and late stances. Short (SLRs) and medium latency reflexes (MLRs) were quantified in a 15 ms analysis window. The MLR decreased with decreased loading (P = 0.045), but no significant difference was observed for the SLR (P = 0.13). Similarly, the effect of the BWS was measured on the unload response, i.e., the depression in soleus activity following a plantar-flexion perturbation ( approximately 5.6 degrees, 203-247 degrees/s), quantified over a 50 ms analysis window. The unload response decreased with decreased load (P > 0.001), but was not significantly affected (P = 0.45) by tizanidine induced depression of the MLR (P = 0.039, n = 6). Since tizanidine is believed to depress the group II afferent pathway, these results are consistent with the idea that force-related afferent feedback contributes both to the background locomotor activity and to the medium latency stretch reflex. In contrast, length-related afferent feedback may contribute to only the medium latency stretch reflex.

Corticospinal contribution to arm muscle activity during human walking

When we walk, our arm muscles show rhythmic activity suggesting that the central nervous system contributes to the swing of the arms. The purpose of the present study was to investigate whether corticospinal drive plays a role in the control of arm muscle activity during human walking. Motor evoked potentials (MEPs) elicited in the posterior deltoid muscle (PD) by transcranial magnetic stimulation (TMS) were modulated during the gait cycle in parallel with changes in the background EMG activity. There was no significant difference in the size of the MEPs at a comparable level of background EMG during walking and during static PD contraction. Short latency intracortical inhibition (SICI; 2 ms interval) studied by paired-pulse TMS was diminished during bursts of PD EMG activity. This could not be explained only by changes in background EMG activity and/or control MEP size, since SICI showed no correlation to the level of background EMG activity during static PD contraction. Finally, TMS at intensity below the threshold for activation of corticospinal tract fibres elicited a suppression of the PD EMG activity during walking. Since TMS at this intensity is likely to only activate intracortical inhibitory interneurones, the suppression is in all likelihood caused by removal of a corticospinal contribution to the ongoing EMG activity. The data thus suggest that the motor cortex makes an active contribution, through the corticospinal tract, to the ongoing EMG activity in arm muscles during walking.

Effects of prolonged walking on neural and mechanical components of stretch responses in the human soleus muscle

After repeated passive stretching, tendinous tissue compliance increases in the human soleus (SOL) muscle-tendon unit. During movement, such changes would have important consequences for neural and mechanical stretch responses. This study examined the existence of such effects in response to a 75 min walking intervention. Eleven healthy subjects walked on a treadmill at 4 km h(1) with a robotic stretch device attached to the left leg. Ultrasonography was used to measure SOL fascicle lengths, and surface EMG activity was recorded in the SOL and tibialis anterior (TA) muscles. Perturbations of 6 deg were imposed at three different measurement intervals: Pre (immediately before the walking intervention), Mid (after approximately 30 min of walking) and Post (immediately after the intervention). Between the Pre-Mid and Mid-Post intervals, subjects walked for 30 min at a gradient of 3%. After the intervention, the amplitude and velocity of fascicle stretch both decreased (by 46 and 59%, respectively; P < 0.001) in response to a constant external perturbation, as did short (33%; P < 0.01) and medium (25%; P < 0.01) latency stretch reflex amplitudes. A faster perturbation elicited at the end of the protocol resulted in a recovery of fascicle stretch velocities and short latency reflex amplitudes to the pre-exercise values. These findings suggest that repeated stretching and shortening of a muscle-tendon unit can induce short-term structural changes in the tendinous tissues during human walking. The data also highlight the effect of these changes on neural feedback from muscle sensory afferents.

Location-specific modulations of plantar cutaneous reflexes in human (peroneus longus muscle) are dependent on co-activation of ankle muscles

Cutaneous reflexes induced in lower leg muscles by non-noxious electrical stimulation to the foot sole are strongly modified depending on the stimulated location. Little is known, however, about the functional importance of this location-specificity. We examined modulation of cutaneous reflexes in the peroneus longus muscle during co-activation of the peroneus longus (PL), soleus, and tibialis anterior muscles in ten healthy volunteers. We successfully recorded 121 intramuscular single motor units (MU) of cutaneous reflexes in PL elicited by stimulating either fore-medial, fore-lateral, or heel regions of the plantar foot while performing plantarflexion and eversion (PF + EV), dorsiflexion and eversion (DF + EV), or isolated eversion (EV). Firing probability increased following fore-lateral stimulation during the PF + EV and EV tasks, but not during the DF + EV. Fore-medial stimulation, irrespective of the task, suppressed the reflex. Heel stimulation facilitated the reflex only during the PF + EV and DF + EV tasks. In general, cutaneous reflex magnitudes were larger during the PF + EV task than during the others, irrespective of whether the effects were facilitatory or suppressive. These results suggest that the magnitude of the reflex effects on the PL motoneurons strongly depends on activation of plantarflexors and dorsiflexors.

Corticospinal inhibition of transmission in propriospinal-like neurones during human walking

It is crucial for human walking that muscles acting at different joints are optimally coordinated in relation to each other. This is ensured by interaction between spinal neuronal networks, sensory feedback and supraspinal control. Here we investigated the cortical control of spinal excitation from ankle dorsiflexor afferents to quadriceps motoneurones mediated by propriospinal-like interneurones. During walking and tonic contraction of ankle dorsiflexors and knee extensors while standing [at matched electromyography (EMG) levels], the effect of common peroneal nerve (CPN) stimulation on quadriceps motoneurones was tested by assessing averaged and rectified EMG activity, H-reflexes [evoked by femoral nerve (FN) stimulation] and motor evoked potentials (MEPs) produced by transcranial magnetic stimulation (TMS). The biphasic EMG facilitation (CPQ-reflex) produced by isolated CPN stimulation was enhanced during walking, and when CPN stimulation was combined with FN or TMS, the resulting H-reflexes and MEPs were inhibited. The CPQ-reflex was also depressed when CPN stimulation was combined with subthreshold TMS. The peripheral (in CPN and FN) and corticospinal volleys may activate inhibitory non-reciprocal group I interneurones, masking spinal excitations to quadriceps motoneurones mediated by propriospinal-like interneurones. It is proposed that the enhanced CPQ-reflex produced by isolated CPN stimulation during walking cannot be fully explained by an increase in corticospinal and peripheral inputs, but is more likely caused by central facilitation of the propriospinal-like interneurones from other sources. The corticospinal control of non-reciprocal group I interneurones may be of importance for reducing reflex activity between ankle dorsiflexors and quadriceps during walking when not functionally relevant.

Postactivation Depression of the Soleus H Reflex Measured Using Threshold Tracking

The interpretation of changes in the soleus H reflex is problematic in the face of reflex gain changes, a nonlinear input/output relationship for the motoneuron pool, and a nonhomogeneous response of different motoneurons to afferent inputs. By altering the stimulus intensity to maintain a constant reflex output, threshold tracking allows a relatively constant population of α-motoneurons to be studied. This approach was used to examine postactivation (“homosynaptic”) depression of the H reflex (HD) in 23 neurologically healthy subjects. The H reflex was elicited by tibial nerve stimulation at 0.05, 0.1, 0.3, 1, and 2 Hz at rest and during voluntary plantar flexion at 2.5, 5, and 10% of maximum. A computerized threshold tracking procedure was used to set the current needed to generate a target H reflex 10% of Mmax. The current needed to produce the target reflex increased with stimulus rate but not significantly beyond 1 Hz. In three subjects, the current needed to produce H reflexes of 5, 10, 15, and 20% Mmax at 0.3, 1, and 2 Hz increased with rate and with the size of the test H reflex. HD was significantly reduced during voluntary contractions. Using threshold tracking, HD was maximal at lower frequencies than previously emphasized, probably because HD is greater the larger the test H reflex. This would reinforce the greater sensitivity of small motoneurons to reflex inputs.

Immobilization induces changes in presynaptic control of group Ia afferents in healthy humans

Neural plasticity occurs throughout adult life in response to maturation, use and disuse. Recent studies have documented that H-reflex amplitudes increase following a period of immobilization. To elucidate the mechanisms contributing to the increase in H-reflex size following immobilization we immobilized the left foot and ankle joint for 2 weeks in 12 able-bodied subjects. Disynaptic reciprocal inhibition of soleus (SOL) motoneurons and presynaptic control of SOL group Ia afferents was measured before and after the immobilization as well as following 2 weeks of recovery. Following immobilization, maximal voluntary plantar- and dorsiflexion torque (MVC) was significantly reduced and the maximal SOL H-reflex amplitude increased with no changes in the maximal compound motor response (M(max)). Decreased presynaptic inhibition of the Ia afferents probably contributed to the increase of the H-reflex size, since we observed a significant decrease in the long-latency depression of the SOL H-reflex evoked by peroneal nerve stimulation (D2 inhibition) and an increase in the size of the monosynaptic Ia facilitation of the SOL H-reflex evoked by femoral nerve stimulation. These two measures provide independent evidence of changes in presynaptic inhibition of SOL Ia afferents and taken together suggest that GABAergic presynaptic inhibition of the SOL Ia afferents is decreased following 2 weeks of immobilization. The depression of the SOL H-reflex when evoked at intervals shorter than 10 s (homosynaptic post-activation depression) also decreased following immobilization, suggesting that the activity-dependent regulation of transmitter release from the afferents was also affected by immobilization. We observed no significant changes in disynaptic reciprocal Ia inhibition. Two weeks after cast removal measurements returned to pre-immobilization levels. Together, these observations suggest that disuse causes plastic changes in spinal interneuronal circuitries responsible for presynaptic control of sensory input to the spinal cord. This may be of significance for the motor disabilities seen following immobilization as well as the development of spasticity following central motor lesions.

Speed-related spinal excitation from ankle dorsiflexors to knee extensors during human walking

Automatic adjustments of muscle activity throughout the body are required for the maintenance of balance during human walking. One mechanism that is likely to contribute to this control is the heteronymous spinal excitation between human ankle dorsiflexors and knee extensors (CPQ-reflex). Here, we investigated the CPQ-reflex at different walking speeds (1-6 km/h) and stride frequencies (0.6-1.3 Hz) in healthy human subjects to provide further evidence of its modulation, and its role in ensuring postural stability during walking. The CPQ-reflex was small or absent at walking speeds below 2-3 km/h, then increased with walking speeds about 3-4 km/h, and reached a plateau without any further change at walking speeds from 4 to 6 km/h. The reflex showed no modulation when the stride cycle was varied at constant speed (4 km/h; short steps versus long steps). These changes were unlikely to be only caused by changes in the background EMG activity and modifications in peripheral input, and likely reflected central modulation of transmission in the involved reflex pathways as well. It is suggested that the purpose of the reflex is to ensure knee stability at moderate-to-high walking speeds.

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.

Group III and IV muscle afferents differentially affect the motor cortex and motoneurones in humans

The influence of group III and IV muscle afferents on human motor pathways is poorly understood. We used experimental muscle pain to investigate their effects at cortical and spinal levels. In two studies, electromyographic (EMG) responses in elbow flexors and extensors to stimulation of the motor cortex (MEPs) and corticospinal tract (CMEPs) were evoked before, during, and after infusion of hypertonic saline into biceps brachii to evoke deep pain. In study 1, MEPs and CMEPs were evoked in relaxed muscles and during contractions to a constant elbow flexion force. In study 2, responses were evoked during elbow flexion and extension to a constant level of biceps or triceps brachii EMG, respectively. During pain, the size of CMEPs in relaxed biceps and triceps increased (by approximately 47% and approximately 56%, respectively; P < 0.05). MEPs did not change with pain, but relative to CMEPs, they decreased in biceps (by approximately 34%) and triceps (by approximately 43%; P < 0.05). During flexion with constant force, ongoing background EMG and MEPs decreased for biceps during pain (by approximately 14% and 15%; P < 0.05). During flexion with a constant EMG level, CMEPs in biceps and triceps increased during pain (by approximately 30% and approximately 26%, respectively; P < 0.05) and relative to CMEPs, MEPs decreased for both muscles (by approximately 20% and approximately 17%; P < 0.05). For extension, CMEPs in triceps increased during pain (by approximately 22%) whereas MEPs decreased (by approximately 15%; P < 0.05). Activity in group III and IV muscle afferents produced by hypertonic saline facilitates motoneurones innervating elbow flexor and extensor muscles but depresses motor cortical cells projecting to these muscles.

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.

Task-specific modulation of cutaneous reflexes expressed at functionally relevant gait cycle phases during level and incline walking and stair climbing

Reflexes are exquisitely sensitive to the motor task that is being performed at the time they are evoked; in other words, they are "task-dependent". The purpose of this study was to investigate the extent to which the pattern of reflex modulation is conserved across three locomotor tasks that differ in muscle activity, joint kinematics, and stability demands. Subjects performed continuous level and incline walking on a treadmill and stair climbing on a stepping mill. Cutaneous reflexes were evoked by delivering trains of electrical stimulation to the sural nerve at the ankle at an intensity of two times the radiating threshold. Electromyographic (EMG) recordings were collected continuously from muscles in the arms, legs and trunk. Results showed that middle-latency reflex modulation patterns were generally conserved across the three locomotor tasks with a few notable exceptions related to specific functional requirements. For example, a reflex reversal was observed for tibialis anterior during stair climbing, which may be indicative of a specific adaptation to the task constraints. Overall our data suggest that the underlying neural mechanisms involved in coordinating level walking can be modified to also coordinate other locomotor tasks such as incline walking and stair climbing. Therefore, there may be considerable overlap in the neural control of different forms of locomotion.

Presynaptic control of group Ia afferents in relation to acquisition of a visuo-motor skill in healthy humans

Sensory information continuously converges on the spinal cord during a variety of motor behaviours. Here, we examined presynaptic control of group Ia afferents in relation to acquisition of a novel motor skill. We tested whether repetition of two motor tasks with different degrees of difficulty, a novel visuo-motor task involving the ankle muscles, and a control task involving simple voluntary ankle movements, would induce changes in the size of the soleus H-reflex. The slope of the H-reflex recruitment curve and the H-max/M-max ratio were depressed after repetition of the visuo-motor skill task and returned to baseline after 10 min. No changes were observed after the control task. To elucidate the mechanisms contributing to the H-reflex depression, we measured the size of the long-latency depression of the soleus H-reflex evoked by peroneal nerve stimulation (D1 inhibition) and the size of the monosynaptic Ia facilitation of the soleus H-reflex evoked by femoral nerve stimulation. The D1 inhibition was increased and the femoral nerve facilitation was decreased following the visuo-motor skill task, suggesting an increase in presynaptic inhibition of Ia afferents. No changes were observed in the disynaptic reciprocal Ia inhibition. Somatosensory evoked potentials (SEPs) evoked by stimulation of the tibial nerve (TN) were also unchanged, suggesting that transmission in ascending pathways was unaltered following the visuo-motor skill task. Together these observations suggest that a selective presynaptic control of Ia afferents contributes to the modulation of sensory inputs during acquisition of a novel visuo-motor skill in healthy humans.

Depression of h-reflex following carbonic anhydrase inhibition appears unrelated to changes in synaptic effectiveness

Presynaptic inhibition (PI) of Ia afferents was examined as a possible contributor to the depression of the soleus H-reflex following carbonic anhydrase (CA) inhibition with Acetazolamide (ACZ). Ten males (aged 22-32) were studied in two randomized conditions, control and ACZ administration (250 mg 14, 8, and 2 h before testing) separated by at least one week. PI of soleus Ia afferents was indirectly assessed two ways: a conditioning stimulus of Ia afferents in the common peroneal nerve (N = 6), and heteronymous Ia facilitation from the quadriceps to soleus muscle (N = 4). Conditioning (C) of the soleus H-reflex (common peroneal nerve stimulation protocol) resulted in depression of the H-reflex in the supine and standing position compared to the test (T, unconditioned) H-reflex in the same position. This result was unaltered following ACZ treatment. C (heteronymous facilitation protocol) resulted in facilitation of the H-reflex in the supine, but not the standing position. This result was unaltered following ACZ treatment. It was concluded that the depression of the H-reflex following CA inhibition (present study; Brechue et al., 1997) appears to be unrelated to changes in the tonic level of PI of Ia afferents. The best hypothesis for the reduction in the H-reflex appears to be conduction block of the primary afferent fibers secondary to local increases in PCO2.

Pathophysiology of spastic paresis. II: Emergence of muscle overactivity

In the subacute and chronic stages of spastic paresis, stretch-sensitive (spastic) muscle overactivity emerges as a third fundamental mechanism of motor impairment, along with paresis and soft tissue contracture. Part II of this review primarily addresses the pathophysiology of the various forms of spastic overactivity. It is argued that muscle contracture is one of the factors that cause excessive responsiveness to stretch, which in turn aggravates contracture. Excessive responsiveness to stretch also impedes voluntary motor neuron recruitment, a concept termed stretch-sensitive paresis. None of the three mechanisms of impairment (paresis, contracture, and spastic overactivity) is symmetrically distributed between agonists and antagonists, which generates torque imbalance around joints and limb deformities. Thus, each may be best treated focally on an individual muscle-by-muscle basis. Intensive motor training of the less overactive muscles should disrupt the cycle of paresis-disuse-paresis, and concomitant use of aggressive stretch and focal weakening agents in their more overactive and shortened antagonists should break the cycle of overactivity-contracture-overactivity.

Short-term adaptations in spinal cord circuits evoked by repetitive transcranial magnetic stimulation: possible underlying mechanisms

Repetitive transcranial magnetic stimulation (rTMS) has been shown to induce adaptations in cortical neuronal circuitries. In the present study we investigated whether rTMS, through its effect on corticospinal pathways, also produces adaptations at the spinal level, and what the neuronal mechanisms involved in such changes are. rTMS (15 trains of 20 pulses at 5 Hz) was applied over the leg motor cortical area in ten healthy human subjects. At rest motor evoked potentials (MEPs) in the soleus and tibialis anterior muscles were facilitated by rTMS (at 1.2xMEP threshold). In contrast, the soleus H-reflex was depressed for 1 s at stimulus intensities from 0.92 to 1.2xMEP threshold. rTMS increased the size of the long-latency depression of the soleus H-reflex evoked by common peroneal nerve stimulation and decreased the femoral nerve facilitation of the soleus H-reflex. These observations suggest that the depression of the H-reflex by rTMS can be explained, at least partly, by an increased presynaptic inhibition of soleus Ia afferents. In contrast, rTMS had no effect on disynaptic reciprocal Ia inhibition from ankle dorsiflexors to plantarflexors. We conclude that a train of rTMS may modulate transmission in specific spinal circuitries through changes in corticospinal drive. This may be of relevance for future therapeutic strategies in patients with spasticity.

Group II excitations from plantar foot muscles to human leg and thigh motoneurones

Projections of group II afferents from intrinsic foot muscles to lower limb motoneurones were investigated in humans after electrical stimuli were applied to the tibial nerve (TN) at ankle level, using modulation of the quadriceps H reflex, on-going EMG of the quadriceps and peroneus brevis, and PSTHs of single quadriceps, biceps, semitendinosus, tibialis anterior, and peroneus brevis motor units. TN stimulation evoked late and high-threshold excitation in all leg and thigh muscles investigated. The mean latency of the late excitation was 13.5+/-0.4 ms longer than that of the heteronymous monosynaptic Ia excitation, and the more caudal the motor nucleus the longer the central delay of the late effect, suggesting mediation through interneurones located rostral to motoneurones. The electrical threshold and conduction velocity of the largest diameter fibres evoking the late excitation were estimated to be approximately 2 and 0.67 times, respectively, those of the fastest Ia afferents, i.e. consistent with a mediation by group II afferents. Stimulation of the skin areas innervated by TN did not evoke late excitations. Further support for mediation through group II afferents was provided by the findings that: 1. the latency of the TN-induced late and high-threshold excitation in Per brev units was more delayed by cooling the nerve than that of the excitation evoked by group I afferents, and 2. tizanidine intake (known to depress selectively transmission of group II effects) suppressed the TN-induced late and high-threshold excitation whereas the group I facilitation was not modified.

Sensorimotor integration at spinal level as a basis for muscle coordination during voluntary movement in humans

Spinal reflexes have traditionally been treated as separate from voluntary movements. However, animal experiments since the 1950s and human experiments since the 1970s have documented that sensory activities in afferents from muscles, skin, and joints are integrated with descending motor commands at the level of common spinal interneurons. Two different roles of this sensorimotor integration at the spinal level may be discerned. First, sensory feedback evoked by the active muscles may help to drive the motoneurons. Second, external stimuli, such as sudden perturbations of a limb, may give rise to “error signals,” which are integrated into the ongoing motor activity and form the basis of corrective responses. When interpreting experimental data, it is important to consider these two different roles. Application of external stimuli may provide little information about how the spinal cord integrates sensory feedback evoked as part of ongoing movements. The complexity of the spinal machinery that is activated by external stimuli also makes the interpretation of data obtained from experiments dealing with artificial external stimuli, such as electrical stimuli, difficult. Nevertheless, such experiments have provided and will continue to provide very valuable information about how the brain and spinal cord ensure coordination of muscle activity during voluntary movement. So far, spinal control mechanisms have only been investigated to a limited extent in relation to sports and occupational activities. Provided that researchers consider the methodological problems of the techniques and that they seek independent validation of the findings, this may be a very fruitful research field in the future.

Key mechanisms for setting the input-output gain across the motoneuron pool

This chapter summarizes a number of factors that control the "input-output" function across the motoneurons (MNs) comprising a single spinal motor nucleus. The main focus is on intrinsic properties of individual MNs that can be controlled by neuromodulators. These include: (1) amplification of the synaptic input at the cell's dendritic level by voltage-gated, persistent inward currents (plateau potentials); and (2) transduction of the net synaptic excitation into a frequency code (the MN's stimulus current-spike frequency relation) at the cell's soma/initial segment. Two other aspects of the synaptic control of MNs, which may affect their input-output gain, are also discussed. They include the hypotheses that: (1) a non-uniform distribution of synaptic effects to low- and high-threshold motor units causes a change in recruitment gain; and (2) recurrent inhibition, via motor axon collaterals and Renshaw cells, functions as a variable gain regulator of MN discharge.

Reassessment of parameters for applying motor-unit triggered stimuli in peri-stimulus time histograms

We reassessed the response properties of peri-stimulus time histograms (PSTHs) in cases when a test stimulus was triggered by a motor-unit discharge with a constant delay time. In this experiment, single motor unit action potentials were recorded from the right tibialis anterior (TA) muscle of five healthy persons. A test stimulus of the common peroneal nerve with low intensity to activate only Ia afferents of the TA was applied through a bipolar stimulating electrode placed distal to the neck of the fibula. We obtained several PSTHs with various delay times and stimulus intensities in the same recording session for maintaining the background property as the same among the test situations. As a result, we confirmed three characteristics of PSTHs from observed data: (1) given the same delay time (the same background firing properties), a weaker stimulus intensity evokes a lessened effect on PSTHs, naturally; (2) delay time alters the induction balance of direct and indirect effects on PSTHs even if the stimulus intensity is the same because the background firing properties are different; and (3) response probabilities do not correspond directly to stimulus intensities when background firing properties are different; it is possible for a relatively strong intensity stimulus to produce a weaker effect than a weak stimulus. We concluded that comparisons of effects taken at different phases in the control distribution (and also effects taken from different control distributions or different motor units) can be misleading. Therefore, such comparisons should only be made within data obtained from the same phase in the same control distribution of the same motor unit.

Modulation of transmission in the corticospinal and group Ia afferent pathways to soleus motoneurons during bicycling

Transmission in the corticospinal and Ia pathways to soleus motoneurons was investigated in healthy human subjects during bicycling. Soleus H reflexes and motor evoked potentials (MEPs) after transcranial magnetic stimulation (TMS) were modulated similarly during the crank cycle being large during downstroke [concomitant with soleus background electromyographic (EMG) activity] and small during upstroke. Tibialis anterior MEPs were in contrast large during upstroke and small during downstroke. The soleus H reflexes and MEPs were also recorded during tonic plantarflexion at a similar ankle joint position, corresponding ankle angle, and matched background EMG activity as during the different phases of bicycling. Relative to their size during tonic plantarflexion, the MEPs were found to be facilitated in the early part of downstroke during bicycling, whereas the H reflexes were depressed in the late part of downstroke. The intensity of TMS was decreased below MEP threshold and used to condition the soleus H reflex. At short intervals (conditioning-test intervals of -3 to -1 ms), TMS produced a facilitation of the H reflex that is in all likelihood caused by activation of the fast monosynaptic corticospinal pathway. This facilitation was significantly larger in the early part of downstroke during bicycling than during tonic plantarflexion. This suggests that the increased MEP during downstroke was caused by changes in transmission in the fast monosynaptic corticospinal pathway. To investigate whether the depression of H reflexes in the late part of downstroke was caused by increased presynaptic inhibition of Ia afferents, the soleus H reflex was conditioned by stimulation of the femoral nerve. At a short interval (conditioning-test interval: -7 to -5 ms), the femoral nerve stimulation produced a facilitation of the H reflex that is mediated by the heteronymous monosynaptic Ia pathway from the femoral nerve to soleus motoneurons. Within the initial 0.5 ms after its onset, the size of this facilitation depends on the level of presynaptic inhibition of the Ia afferents, which mediate the facilitation. The size of the facilitation was strongly depressed in the late part of downstroke, compared with the early part of downstroke, suggesting that increased presynaptic inhibition was indeed responsible for the depression of the H reflex. These findings suggest that there is a selectively increased transmission in the fast monosynaptic corticospinal pathway to soleus motoneurons in early downstroke during bicycling. It would seem likely that one cause of this is increased excitability of the involved cortical neurons. The increased presynaptic inhibition of Ia afferents in late downstroke may be of importance for depression of stretch reflex activity before and during upstroke.

Firing rate modulation of motoneurons activated by cutaneous and muscle receptor afferents in the decerebrate cat

The aim of this study was to investigate whether activation of spinal motoneurons by sensory afferents of the caudal cutaneous sural (CCS) nerve evokes an atypical motor control scheme. In this scheme, motor units that contract fast and forcefully are driven by CCS afferents to fire faster than motor units that contract more slowly and weakly. This is the opposite of the scheme described by the size principle. Earlier studies from this lab do not support the atypical scheme and instead demonstrate that both CCS and muscle stretch recruit motor units according to the size principle. The latter finding may indicate that CCS and muscle-stretch inputs have similar functional organizations or that comparison of recruitment sequence was simply unable to resolve a difference. In the present experiments, we examine this issue using rate modulation as a more sensitive index of motoneuron activation than recruitment. Quantification of the firing output generated by these two inputs in the same pairs of motoneurons enabled direct comparison of the functional arrangements of CCS versus muscle-stretch inputs across the pool of medial gastrocnemius (MG) motoneurons. No systematic difference was observed in the rate modulation produced by CCS versus muscle-stretch inputs for 35 pairs of MG motoneurons. For the subset of 24 motoneuron pairs exhibiting linear co-modulation of firing rate (r > 0.5) in response to both CCS and muscle inputs, the slopes of the regression lines were statistically indistinguishable between the two inputs. For individual motoneuron pairs, small differences in slope between inputs were not related to differences in conduction velocity (CV), recruitment order, or, for a small sample, differences in motor unit force. We conclude that an atypical motor control scheme involving selective activation of typically less excitable motoneurons, if it does occur during normal movement, is not an obligatory consequence of activation by sural nerve afferents. On average and for both muscle-stretch and skin-pinch inputs, the motoneuron with the faster CV in the pair tended to be driven to fire at slightly but significantly faster firing rates. Computer simulations based in part on frequency-current relations measured directly from motoneurons revealed that properties intrinsic to motoneurons are sufficient to account for the higher firing rates of the faster CV motoneuron in a pair.

A cumulative sum test for a peri-stimulus time histogram using the Monte Carlo method

We have established a cumulative sum (CUSUM) test for a peri-stimulus time histogram (PSTH) for the case where a conditioning stimulus is delivered at a fixed interval after previous discharge of a motor unit. (We refer to this kind of PSTH as an 'arranged PSTH'). Expectations of the firing probability after the conditioning stimulus vary among the bins in this arranged PSTH, while the expectations among the bins are all the same in the original PSTH; thus, we could not apply conventional tests for statistical analysis. We, therefore, propose a novel CUSUM test that uses the Monte Carlo method. With this method, the range of the statistical scattering noise on a CUSUM is computationally found by simulating the statistical process in order to calculate the confidence interval. We verified this CUSUM test using both simulated and actual experiments. This paper presents the procedure for performing this new method, along with an example of its application.

Statistical test for peri-stimulus time histograms in assessing motor neuron activity

The peri-stimulus time histogram is a valuable tool for evaluating neural connections in humans. To detect the degree to which a conditioning stimulus to a sensory nerve modulates motor neuron activity, a histogram of motor unit spike intervals after a conditioning stimulus is measured. This histogram allows the effect of the conditioning stimulus to be visualised. By comparison with a reference histogram of motor unit spike intervals after a sham stimulus, the noise caused by spontaneous firing sway can be removed. However, no valid statistical test has yet been developed to separate the physiological effect from the spontaneous sway and statistical noise. A computational method has been proposed to detect modulation caused by a conditioning stimulus. To clarify the effect of a conditioning stimulus, this new method used reference histograms to calculate a confidence interval. A simulated experiment demonstrated that about 2000 re-samplings were sufficient to estimate a confidence interval for a histogram with 1 ms bin width constructed from 300 triggers. Testing of the experimental data, measured from the tibialis anterior muscles during the elicitation of the excitatory spinal reflex, confirmed that significant peaks were produced at 30, 34, 35 and 38ms after the conditioning stimulus. These correspond appropriately to the delay of the spinal reflex.

Modulation of non-monosynaptic excitation from ankle dorsiflexor afferents to quadriceps motoneurones during human walking

Modulation of non-monosynaptic excitation from ankle dorsiflexors to quadriceps (Q) motoneurones during human treadmill walking was investigated in 25 healthy human subjects. Stimulation of the common peroneal nerve (CPN) evoked a biphasic facilitation in the rectified and averaged (n = 50) Q electromyographic (EMG) activity between 0 and 100 ms after heel strike. Prior to heel strike, the stimulation had no effect on the Q EMG. The latency of both peaks in the response was too long to be explained by a monosynaptic pathway to Q motoneurones. During voluntary tonic co-contraction of Q and tibialis anterior (TA) while standing, only the first of the two peaks was evoked by the CPN stimulation despite a background EMG activity level in the Q and TA muscles corresponding to that observed 30-60 ms after heel strike during walking. Stimulation of cutaneous nerves did not evoke a similar biphasic facilitation in the Q motoneurones, which suggests that muscular afferents mediate the response. The second peak had a higher threshold than the earlier peak. During cooling of the CPN, the latency of the second peak was more prolonged than the latency of the earlier peak. This suggests that afferents of different diameters contributed to the two peaks. It is proposed that afferents from TA assist the contraction of Q during walking via spinal interneurones to stabilize the knee joint and maintain upright posture during walking.

Recruitment of cat motoneurons in the absence of homonymous afferent feedback

This study provides the first test in vivo of the hypothesis that group Ia muscle-stretch afferents aid in preventing reversals in the orderly recruitment of motoneurons. This hypothesis was tested by studying recruitment of motoneurons deprived of homonymous afferent input. Recruitment order was measured in decerebrate, paralyzed cats from dual intra-axonal records obtained simultaneously from pairs of medial gastrocnemius (MG) motoneurons. Pairs of MG motor axons were recruited in eight separate trials of the reflex discharge evoked by stimulation of the caudal cutaneous sural (CCS) nerve. Some reports suggest that reflex recruitment by this cutaneous input should bias recruitment against order by the size principle in which the axon with the slower conduction velocity (CV) in a pair is recruited to fire before the faster CV axon. Recruitment was studied in three groups of cats: ones with the MG nerve intact and untreated (UNTREATED); ones with the MG nerve cut (CUT); and ones with the MG nerve cut and bathed at its proximal end in lidocaine solution (CUT+). The failure of electrical stimulation to initiate a dorsal root volley and the absence of action potentials in MG afferents demonstrated the effective elimination of afferent feedback in the CUT+ group. Recruitment order by the size principle predominated and was not statistically distinguishable among the three groups. The percentage of pairs recruited in reverse order of the size principle was actually smaller in the CUT+ group (6%) than in CUT (15%) or UNTREATED (19%) groups. Thus homonymous afferent feedback is not necessary to prevent recruitment reversal. However, removing homonymous afferent input did result in the expression of inconsistency in order, i.e., switches in recruitment sequence from one trial to the next, for more axon pairs in the CUT+ group (33%) than for the other groups combined (13%). Increased inconsistency in the absence of increased reversal of recruitment order was approximated in computer simulations by increasing time-varying fluctuations in synaptic drive to motoneurons and could not be reproduced simply by deleting synaptic current from group Ia homonymous afferents, regardless of how that current was distributed to the motoneurons. These findings reject the hypothesis that synaptic input from homonymous group Ia afferents is necessary to prevent recruitment reversals, and they are consistent with the assertion that recruitment order is established predominantly by properties intrinsic to motoneurons.

Corticospinal excitation of presumed cervical propriospinal neurones and its reversal to inhibition in humans

1. This study addresses whether in human subjects indirect corticospinal excitation of upper limb motoneurones (MNs) relayed through presumed cervical propriospinal neurones (PNs) is paralleled by corticospinal activation of inhibitory projections to these premotoneurones. 2. The responses to transcranial magnetic stimulation (TMS), whether assessed as the compound motor-evoked potential (MEP) or the peak of corticospinal excitation elicited in the post-stimulus time histograms (PSTHs) of single motor units, were conditioned by weak volleys to musculo-cutaneous, ulnar and superficial radial nerves. 3. Afferent volleys, which hardly modified the H reflex, significantly facilitated the corticospinal response produced by weak TMS. In PSTHs, the central delay of the peripheral facilitation of the peak of corticospinal excitation in MNs located at either end of the cervical enlargement was longer the more caudal the MN pool, suggesting an interaction in premotoneurones located rostral to the tested MNs. 4. Small increases in the strength of TMS (approximately 2--5 % of the maximal stimulator output) caused the facilitation to disappear and then to be reversed to inhibition. The facilitatory and inhibitory effects had the same latencies and spared the initial 0.5--1 ms of the corticospinal excitatory response. Both effects were more readily demonstrable when there was a co-contraction of the target muscle and the muscle innervated by nerve used for the conditioning stimulus. 5. The above features suggest that the inhibition resulted from disfacilitation due to suppression of corticospinal excitation passing through the presumed premotoneuronal relay. The reversal of the facilitation to inhibition by stronger corticospinal volleys is consistent with a well-developed system of 'feedback inhibitory interneurones' activated by corticospinal and afferent inputs inhibiting the presumed propriospinal excitatory premotoneurones. 6. It is argued that these findings might explain why simply stimulating the pyramidal tract or the motor cortex would fail to demonstrate this indirect corticospinal projection in the macaque monkey and in humans.

Evidence for transcortical reflex pathways in the lower limb of man

The existence of transcortical reflex pathways in the control of distal arm and hand muscles in man is now widely accepted. Much more controversy exists regarding a possible contribution of such reflexes to the control of leg muscles. It is often assumed that transcortical reflex pathways play no, or only a minor, role in the control of leg muscles. Transcortical reflex pathways according to this view are reserved for the control of the distal upper limb and are seen in close relation to the evolution of the primate hand. Here we review data, which provide evidence that transcortical reflexes do exist for lower limb muscles and may play a significant role in the control of at least some of these muscles. This evidence is based on animal research, recent experiments combining transcranial magnetic stimulation with peripheral electrical and mechanical stimulation in healthy subjects and neurological patients. We propose that afferent activity from muscle and skin may play a role in the regulation of bipedal gait through transcortical pathways.

The monosynaptic reflex: a tool to investigate motor control in humans. Interest and limits

The principle of the monosynaptic reflex used as a tool to explore the excitability of the motoneurones (MNs) is explained and the general methodology of the H reflex is described. The different drawbacks inherent in the technique are then considered: mechanisms other than the monosynaptic la excitation of MNs contributing to the H reflex size (limitation of the H reflex size by disynaptic IPSPs, presynaptic inhibition of la terminals, post-activation depression); non-linearity and changes in the 'recruitment gain' in the MN pool; and poor time resolution of the method. Despite these drawbacks, it is emphasized that the H reflex is the only available technique enabling one to investigate changes in transmission in spinal pathways during motor tasks.

Mechanical cutaneous stimulation alters Ia presynaptic inhibition in human wrist extensor muscles: a single motor unit study

Reflex responses were evoked by radial nerve stimulation in 25 single motor units in the extensor carpi radialis muscles of seven subjects during voluntary isometric wrist extension. The responses consisted of narrow peaks in the post-stimulus time histograms with latencies compatible with monosynaptic activation. When the skin of the palm and finger tips was continuously swept using a soft rotating brush, the purely monosynaptic components of the motor unit responses, as assessed from the contents of the first two 0.25 ms bins of the peak, were found to increase. This increase did not affect the motoneurone net excitatory drive, as assessed by measuring the mean duration of the inter-spike intervals. The cutaneous inputs activated by the brush may have reduced the tonic presynaptic inhibition exerted on the Ia afferents homonymous to the extensor motor units tested. To further investigate whether Ia presynaptic inhibition was involved, the responses of the extensor motor units were conditioned by stimulating the median nerve 20 ms earlier, using a protocol which is known to induce Ia extensor presynaptic inhibition originating from flexor Ia afferents. The median nerve stimulation did not affect the motoneurone excitatory drive, but led to a decrease in the responses of the extensor motor units to the radial nerve stimulation, especially in the purely monosynaptic components. This decrease was consistent with the Ia presynaptic inhibition known to occur under these stimulation conditions. The cutaneous inputs activated by the brush were found to reduce the Ia presynaptic inhibition generated by the median nerve stimulation, without affecting the distribution of the Ia presynaptic inhibition among the various types of motor units tested. The present data suggest that cutaneous inputs from the palm and finger tips may relieve the Ia presynaptic inhibition exerted on the wrist extensor motor nuclei, and thus enhance the proprioceptive assistance to fit the specific requirements of the ongoing motor task.

What functions do reflexes serve during human locomotion?

Studies on the reflex modulation of vertebrate locomotion have been conducted in many different laboratories and with many different preparations: for example, lamprey swimming, bird flight, quadrupedal walking in cats and bipedal walking in humans. Emerging concepts are that reflexes are task-, phase- and context-dependent. To function usefully in a behaviour such as locomotion wherein initial conditions change from step to step, reflexes would have to show modulation. Papers are reviewed in which the study of different reflexes have been conducted during different behaviours, with an emphasis on experiments in humans. A framework is developed in which the modulation and flexibility of reflexes are demonstrated. Alterations in cutaneous, and muscle (stretch and load receptor) reflexes between sitting, standing and walking are discussed. Studies in which both electrical, mechanical and 'natural' receptor activation have been conducted during walking are reviewed. Reflexes are shown to have important regulatory functions during human locomotion. A framework for discussion of reflex function throughout the step cycle is developed. The function of a given reflex pathway changes dynamically throughout the locomotor cycle. While all reflexes act in concert to a certain extent, generally cutaneous reflexes act to alter swing limb trajectory to avoid stumbling and falling. Stretch reflexes act to stabilize limb trajectory and assist force production during stance. Load receptor reflexes are shown to have an effect on both stance phase body weight support and step cycle timing. After neurotrauma or in disease, reflexes no longer function as during normal locomotion, but still have the potential to be clinically exploited in gait modification regimens.

Cortical control of spinal pathways mediating group II excitation to human thigh motoneurones

1. The possibility was investigated that cortical excitation to human thigh motoneurones is relayed via lumbar premotoneurones. 2. Test responses were evoked by transcranial magnetic stimulation (TMS) in voluntarily contracting quadriceps (Q) and semitendinosus (ST) muscles: either a motor evoked potential (MEP) in surface recordings or a peak of cortical excitation in the post-stimulus time histogram (PSTH) of single motor units was used. These test responses were conditioned by stimuli to the common peroneal (CP) or gastrocnemius medialis (GM) nerves. 3. CP stimulation evoked a large biphasic facilitation of the Q MEP, with early, short-lasting, low-threshold (0.6-0.8 x motor threshold (MT)) and late, longer lasting and higher threshold (1.2-1.5 x MT) peaks separated by a period of depression. GM nerve stimulation evoked a similar early depression and late facilitation in the ST MEP. 4. CP-induced effects in the Q H reflex were different (smaller late facilitation not preceded by any depression), suggesting that CP and cortical volleys interact at a premotoneuronal level to modify the Q MEP. 5. Peaks of cortical excitation evoked by TMS in single motor unit PSTHs were modulated by the conditioning volley like the MEPs with, in Q motor units, early and late CP-induced facilitations separated by a depression, and in ST motor units a late GM-induced facilitation. Facilitations on combined stimulation (i) were greater than the sum of effects by separate stimuli and (ii) never affected the initial part of the cortical peak. 6. It is concluded that the features of the reported facilitatory interactions between cortical and peripheral volleys are consistent with interactions in a population of lumbar excitatory premotoneurones co-activated by group I and group II afferents. The potency of the effects suggests that a significant part of the cortical excitation to motoneurones of thigh muscles is relayed via these interneurones. 7. It is argued that the early depression in ST motoneurones and the separation of the two peaks of facilitation in Q motoneurones reflect a cortical facilitation of spinal inhibitory interneurones projecting on excitatory premotoneurones.

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.

Assessing changes in presynaptic inhibition of Ia afferents during movement in humans

Different methods, based on different principles, have been proposed to estimate changes in presynaptic inhibition of Ia terminals (accompanied by primary afferent depolarization, (PAD)) during voluntary contraction in humans. (i) A discrepancy between the H-reflex amplitude, at an equal level of EMG activity, in two situations (e.g., walking and standing) may be taken as suggesting a different control of PAD interneurones in the two cases. (ii) A conditioning stimulation (vibration or electrical stimulation) is used to activate PAD interneurones and to evoke presynaptic inhibition of the afferent volley of the test reflex. The resulting long-lasting depression of the reflex depends on the excitability of PAD interneurones, but can be contaminated by long-lasting post-synaptic effects. (iii) The amount of reflex facilitation evoked by a purely monosynaptic Ia volley varies inversely with the on-going presynaptic inhibition of Ia afferents mediating the conditioning volley, and can be used to assess this on-going presynaptic inhibition. None of these methods can provide by itself unequivocal evidence for a change in presynaptic inhibition of Ia terminals, but reasonably reliable interpretations may be proposed when congruent results are obtained with different methods. Thus it has been shown that, during selective voluntary contraction, presynaptic inhibition is decreased on Ia afferents projecting on motoneurones of the contracting muscle and increased on Ia afferents projecting on motor nuclei not involved in the contraction.