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

Combining transcutaneous spinal stimulation and functional electrical stimulation increases force generated by lower limbs: When more is more

Background

Transcutaneous Spinal Stimulation (TSS) has been shown to promote activation of the lower limb and trunk muscles and is being actively explored for improving the motor outcomes of people with neurological conditions. However, individual responses to TSS vary, and often the muscle responses are insufficient to produce enough force for self-supported standing. Functional electrical stimulation (FES) can activate individual muscles and assist in closing this functional gap, but it introduces questions regarding timing between modalities.

Methods

To assess the effects of TSS and FES on force generation, ten neurologically intact participants underwent (1) TSS only, (2) FES only, and (3) TSS + FES. TSS was delivered using four electrodes placed at T10-T11 through the L1-L2 intervertebral spaces simultaneously, while FES was delivered to the skin over the right knee extensors and plantarflexors. For all conditions, TSS and FES were delivered using three 0.5 ms biphasic square-wave pulses at 15 Hz. During the TSS + FES condition, timing between the two modalities was adjusted in increments of ¼ time between pulses (16.5 ms).

Results

When TSS preceded FES, a larger force production was observed. We also determined several changes in muscle activation amplitude at different relative stimulus intervals, which help characterize our finding and indicate the facilitating and inhibitory effects of the modalities.

Conclusions

Utilizing a delay ranging from 15 to 30 ms between stimuli resulted in higher mean force generation in both the knee and ankle joints, regardless of the selected FES location (Average; knee: 112.0%, ankle: 103.1%).

Arm cycling increases the short-latency reflex from ankle dorsiflexor afferents to knee extensor muscles

Low-intensity electrical stimulation of the common peroneal nerve (CPN) evokes a short latency reflex in the heteronymous knee extensor muscles (referred to as the CPN reflex). The CPN reflex is facilitated at a heel strike during walking, contributing to body weight support. However, the origin of the CPN reflex increase during walking remains unclear. We speculate that this increase originates from multiple sources due to a body of evidence suggesting the presence of neural coupling between the arms and legs. Therefore, we investigated the extent to which the CPN reflex is modulated during rhythmic arm cycling. Twenty-eight subjects sat in an armchair and were asked to perform arm cycling at a moderate cadence using a stationary ergometer while performing isometric contraction of the knee extensors, such that the CPN reflex was evoked. The CPN reflex was evoked by stimulating the CPN [0.9-2.0× the motor threshold (MT) in the tibialis anterior muscle] at the level of the neck of the fibula. The CPN-reflex amplitude was measured from the vastus lateralis (VL). The biphasic reflex response in the VL was evoked within 27-45 ms following CPN stimulation. The amplitude of the CPN reflex increased during arm cycling compared with that before cycling. The modulation of the CPN reflex during arm cycling was detected only for CPN stimulation intensity around 1.2× MT. Furthermore, CPN-reflex modulation was not observed during the isometric contraction of the arm or passive arm cycling. Our results suggest the presence of neural coupling between the CPN-reflex pathways and neural systems generating locomotive arm movement.NEW & NOTEWORTHY Whether locomotive arm movements contribute to the control of the reflex pathway from ankle dorsiflexor afferents to knee extensor muscles [common peroneal nerve (CPN)-reflex] is an unresolved issue. The CPN reflex in the stationary leg was facilitated only by arm cycling, and not by passive or isometric motor tasks. Our results suggest that the arm locomotor system modulates the reflex pathway from ankle dorsiflexor afferents to the knee extensor muscles.

Music Restores Propriospinal Excitation During Stroke Locomotion

Music-based therapy for rehabilitation induces neuromodulation at the brain level and improves the functional recovery. In line with this, musical rhythmicity improves post-stroke gait. Moreover, an external distractor also helps stroke patients to improve locomotion. We raised the question whether music with irregular tempo (arrhythmic music), and its possible influence on attention would induce neuromodulation and improve the post-stroke gait. We tested music-induced neuromodulation at the level of a propriospinal reflex, known to be particularly involved in the control of stabilized locomotion; after stroke, the reflex is enhanced on the hemiparetic side. The study was conducted in 12 post-stroke patients and 12 controls. Quadriceps EMG was conditioned by electrical stimulation of the common peroneal nerve, which produces a biphasic facilitation on EMG, reflecting the level of activity of the propriospinal reflex between ankle dorsiflexors and quadriceps (CPQ reflex). The CPQ reflex was tested during treadmill locomotion at the preferred speed of each individual, in 3 conditions randomly alternated: without music vs. 2 arrhythmic music tracks, including a pleasant melody and unpleasant aleatory electronic sounds (AES); biomechanical and physiological parameters were also investigated. The CPQ reflex was significantly larger in patients during walking without sound, compared to controls. During walking with music, irrespective of the theme, there was no more difference between groups. In controls, music had no influence on the size of CPQ reflex. In patients, CPQ reflex was significantly larger during walking without sound than when listening to the melody or AES. No significant differences have been revealed concerning the biomechanical and the physiological parameters in both groups. Arrhythmic music listening modulates the spinal excitability during post-stroke walking, restoring the CPQ reflex activity to normality. The plasticity was not accompanied by any clear improvement of gait parameters, but the patients reported to prefer walking with music than without. The role of music as external focus of attention is discussed. This study has shown that music can modulate propriospinal neural network particularly involved in the gait control during the first training session. It is speculated that repetition may help to consolidate plasticity and would contribute to gait recovery after stroke.

The Effects of Ankle Position on Torque and Muscle Activity of the Knee Extensor During Maximal Isometric Contraction

CONTEXT

It is very important to empirically determine the optimal ankle position for the quadriceps femoris (QF) strengthening during isometric exercises.

OBJECTIVE

To examine the effect of different ankle positions on torque and electromyography (EMG) activity of QF during maximal isometric contraction.

STUDY DESIGN

Within-subject repeated measures.

SETTING

University laboratory.

PARTICIPANTS

Thirty-six healthy volunteers (15 males and 21 females).

MAIN OUTCOME MEASURES

The isometric strength of the QF was measured at 3 different ankle positions: active dorsiflexion (AD), active plantar flexion (AP), and neutral position (NP). Simultaneously, 3 different ankle positions were assessed for EMG activity of the vastus medialis, vastus lateralis, and rectus femoris muscles during maximal voluntary isometric contraction.

RESULTS

The peak torque per body weight and average peak torque were significantly higher in AD than in AP and NP (P < .01). The vastus medialis and rectus femoris maximal voluntary isometric contraction EMG activity were significantly higher in AD than in AP and NP (P < .01). The vastus lateralis maximal voluntary isometric contraction EMG activity was significantly higher in AD than in AP and NP (P < .01), and was significantly higher in AP than in NP (P < .05).

CONCLUSIONS

These results indicate that the 3 different ankle positions affect the QF torque and EMG activity. In particular, AD position may be more efficient for improving QF strength than AP and NP position. Future studies should prove whether long-term duration QF isometric exercise effects muscle strength and functional performance in different ankle positions.

Human Spinal Motor Control

Human studies in the past three decades have provided us with an emerging understanding of how cortical and spinal networks collaborate to ensure the vast repertoire of human behaviors. Humans have direct cortical connections to spinal motoneurons, which bypass spinal interneurons and exert a direct (willful) muscle control with the aid of a context-dependent integration of somatosensory and visual information at cortical level. However, spinal networks also play an important role. Sensory feedback through spinal circuitries is integrated with central motor commands and contributes importantly to the muscle activity underlying voluntary movements. Regulation of spinal interneurons is used to switch between motor states such as locomotion (reciprocal innervation) and stance (coactivation pattern). Cortical regulation of presynaptic inhibition of sensory afferents may focus the central motor command by opening or closing sensory feedback pathways. In the future, human studies of spinal motor control, in close collaboration with animal studies on the molecular biology of the spinal cord, will continue to document the neural basis for human behavior.

Reflex control of robotic gait using human walking data

Control of human walking is not thoroughly understood, which has implications in developing suitable strategies for the retraining of a functional gait following neurological injuries such as spinal cord injury (SCI). Bipedal robots allow us to investigate simple elements of the complex nervous system to quantify their contribution to motor control. RunBot is a bipedal robot which operates through reflexes without using central pattern generators or trajectory planning algorithms. Ground contact information from the feet is used to activate motors in the legs, generating a gait cycle visually similar to that of humans. Rather than developing a more complicated biologically realistic neural system to control the robot's stepping, we have instead further simplified our model by measuring the correlation between heel contact and leg muscle activity (EMG) in human subjects during walking and from this data created filter functions transferring the sensory data into motor actions. Adaptive filtering was used to identify the unknown transfer functions which translate the contact information into muscle activation signals. Our results show a causal relationship between ground contact information from the heel and EMG, which allows us to create a minimal, linear, analogue control system for controlling walking. The derived transfer functions were applied to RunBot II as a proof of concept. The gait cycle produced was stable and controlled, which is a positive indication that the transfer functions have potential for use in the control of assistive devices for the retraining of an efficient and effective gait with potential applications in SCI rehabilitation.

Locomotor training improves premotoneuronal control after chronic spinal cord injury

Spinal inhibition is significantly reduced after spinal cord injury (SCI) in humans. In this work, we examined if locomotor training can improve spinal inhibition exerted at a presynaptic level. Sixteen people with chronic SCI received an average of 45 training sessions, 5 days/wk, 1 h/day. The soleus H-reflex depression in response to low-frequency stimulation, presynaptic inhibition of soleus Ia afferent terminals following stimulation of the common peroneal nerve, and bilateral EMG recovery patterns were assessed before and after locomotor training. The soleus H reflexes evoked at 1.0, 0.33, 0.20, 0.14, and 0.11 Hz were normalized to the H reflex evoked at 0.09 Hz. Conditioned H reflexes were normalized to the associated unconditioned H reflex evoked with subjects seated, while during stepping both H reflexes were normalized to the maximal M wave evoked after the test H reflex at each bin of the step cycle. Locomotor training potentiated homosynaptic depression in all participants regardless the type of the SCI. Presynaptic facilitation of soleus Ia afferents remained unaltered in motor complete SCI patients. In motor incomplete SCIs, locomotor training either reduced presynaptic facilitation or replaced presynaptic facilitation with presynaptic inhibition at rest. During stepping, presynaptic inhibition was modulated in a phase-dependent manner. Locomotor training changed the amplitude of locomotor EMG excitability, promoted intralimb and interlimb coordination, and altered cocontraction between knee and ankle antagonistic muscles differently in the more impaired leg compared with the less impaired leg. The results provide strong evidence that locomotor training improves premotoneuronal control after SCI in humans at rest and during walking.

More than at the neuromuscular synapse: actions of botulinum neurotoxin A in the central nervous system

Botulinum neurotoxin type A (BoNT/A) is a metalloprotease that produces a sustained yet transitory blockade of transmitter release from peripheral nerve terminals. Local delivery of this neurotoxin is successfully employed in clinical practice to reduce muscle hyperactivity such as in spasticity and dystonia, and to relieve pain with long-lasting therapeutic effects. However, not all BoNT/A effects can be explained by an action at peripheral nerve terminals. Indeed, it appears that BoNT/A is endowed with trafficking properties similar to the parental tetanus neurotoxin and thus be able to directly affect the CNS. In this review, we present and discuss novel compelling evidence for a direct central effect of BoNT/A in both dorsal and ventral horns of the animal and human spinal cord after peripheral injection of the neurotoxin, with important consequences on pain and motor control. This new knowledge is expected to radically change the approach to the use of BoNT/A in the future. As BoNT/A central action appears to also contribute to functional improvement, for instance in human spastic gait, the challenge will be to develop new subtypes or BoNT derivatives with deliberate, cell-specific central effects in order to fully exploit the spectrum of BoNT/A therapeutic activity.

Task-related modulation of crossed spinal inhibition between human lower limbs

Crossed reflex action mediated by muscle spindle afferent inputs has recently been revealed in humans. This raised the question of whether a complex spinal network involving commissural interneurons receiving inputs from proprioceptors and suprasegmental structures, as described in cats, persists in humans and contributes to the interlimb coordination during movement. First, we investigated the neurophysiological mechanisms underlying crossed reflex action between ankle plantar flexors and its corticospinal control from primary motor cortex. Second, we studied its modulation during motor tasks. We observed crossed inhibition in contralateral soleus motoneurons occurring with about 3 ms central latency, which is consistent with spinal transmission through oligosynaptic pathway. The early phase of inhibition was evoked with lower stimulus intensity than the late phase, suggesting mediation by group I and group II afferents, respectively. The postsynaptic origin of crossed inhibition is confirmed by the finding that both H-reflex and motor-evoked potential were reduced upon conditioning stimulation. Transcranial magnetic stimulation over ipsilateral and contralateral primary motor cortex reduced crossed inhibition, especially its late group II part. Last, late group II crossed inhibition was particularly depressed during motor tasks, especially when soleus was activated during the walking stance phase. Our results suggest that both group I and group II commissural interneurons participate in crossed reflex actions between ankle plantar flexors. Neural transmission at this level is depressed by descending inputs activated by transcranial magnetic stimulation over the primary motor cortex or during movement. The specific modulation of group II crossed inhibition suggests control from monoaminergic midbrain structures and its role for interlimb coordination during locomotion.

Clinical findings and electrodiagnostic testing in 108 consecutive cases of lumbosacral radiculopathy due to herniated disc

STUDY AIM

This prospective study aim to examine whether clinical findings and electrodiagnostic testing (EDX) in patients with lumbosacral monoradiculopathy due to herniated disc (HD) differ as a function of root involvement level (L5 vs. S1) and HD zone (paramedian vs. intraforaminal).

PATIENTS AND METHODS

All patients with L4, L5 or S1 monoradiculopathy were prospectively enrolled at four electromyography (EMG) labs over a 2-year period. The diagnosis was based on a congruence between patient history and MRI evidence of HD. We compared the sensitivities of clinical findings and EDX with respect to both root involvement level and HD zone. Multivariate logistic regression was performed in order to verify the association between abnormal EMG, clinical, and neuroradiological findings.

RESULTS

One hundred and eight patients (mean age 47.7 years, 55% men) were consecutively enrolled. Sensory loss in the painful dermatome was the most frequent finding at physical examination (56% of cases). EMG was abnormal in at least one muscle supplied by femoral and sciatic nerves in 45 cases (42%). Inclusion of paraspinal muscles increased sensitivity to only 49% and that of proximal muscles was useless. Motor and sensory neurography was seldom abnormal. The most frequent motor neurographic abnormalities were a delay of F-wave minimum latency and decrease in the compound muscle action potential amplitude from extensor digitorum brevis and abductor hallucis in L5 and S1 radiculopathies, respectively. Sensory neurography was usually normal, the amplitude of sensory nerve action potential was seldom reduced when HD injured dorsal root ganglion or postganglionic root fibres. Multivariate logistic regression analysis showed that EMG abnormalities could be predicted by myotomal muscular weakness, abnormal deep reflexes, and paraesthesiae. The only clinical and electrophysiological differences with respect to root involvement level concerned deep reflexes and motor neurography of deep peroneal and tibial nerves.

CONCLUSIONS

Only some EDX parameters are helpful for the diagnosis of lumbosacral radiculopathy. EMG was abnormal in less than 50% of cases and its abnormalities could be predicted by some clinical findings. However, neurography is useful as a tool for differential diagnosis between radiculopathy and more diffuse disorders of the peripheral nervous system (polyneuropathy, plexopathy).

Weak motor cortex contribution to the quadriceps activity during human walking

Cortical and sub-cortical contribution to the basic locomotor rhythm is still unclear in humans. While motor cortex is involved in the ankle muscle activity during walking, recent findings suggest lesser contribution to that of knee extensors. This was further tested during treadmill walking (3-4 km/h; end swing and early stance) using transcranial magnetic stimulation (TMS). Sub-threshold TMS successively suppressed and increased Vastus Lateralis (VL) EMG activity during tonic contraction while standing, and both responses were significantly depressed during walking. Paired pulse TMS produced weak intra-cortical inhibition during tonic VL contraction, which did not change during walking. Lastly, sub-threshold TMS did not produce any change in VL H-reflex during walking. It is shown that the excitability of pathways, mediating short intra-cortical inhibition and facilitation in VL motor area, is particularly depressed during walking compared to tonic contraction. The present study thus reveals different modulation in VL than that reported in ankle muscles, suggesting lesser cortical contribution to its activity during walking.

Voluntary ankle flexor activity and adaptive coactivation gain is decreased by spasticity during subacute spinal cord injury

Although spasticity has been defined as an increase in velocity-dependent stretch reflexes and muscle hypertonia during passive movement, the measurement of flexor muscle paresis may better characterize the negative impact of this syndrome on residual motor function following incomplete spinal cord injury (iSCI). In this longitudinal study Tibialis Anterior (TA) muscle paresis produced by a loss in maximal voluntary contraction during dorsiflexion and ankle flexor muscle coactivation during ramp-and-hold controlled plantarflexion was measured in ten patients during subacute iSCI. Tibialis Anterior activity was measured at approximately two-week intervals between 3-5 months following iSCI in subjects with or without spasticity, characterized by lower-limb muscle hypertonia and/or involuntary spasms. Following iSCI, maximal voluntary contraction ankle flexor activity was lower than that recorded from healthy subjects, and was further attenuated by the presence of spasticity. Furthermore the initially high percentage value of TA coactivation increased at 75% but not at 25% maximal voluntary torque (MVT), reflected by an increase in TA coactivation gain (75%/25% MVT) from 2.5+/-0.4 to 7.5+/-1.9, well above the control level of 2.9+/-0.2. In contrast contraction-dependent TA coactivation gain decreased from 2.4+/-0.3 to 1.4+/-0.1 during spasticity. In conclusion the adaptive increase in TA coactivation gain observed in this pilot study during subacute iSCI was also sensitive to the presence of spasticity. The successful early diagnosis and treatment of spasticity would be expected to further preserve and promote adaptive motor function during subacute iSCI neurorehabilitation.

Enhanced spinal excitation from ankle flexors to knee extensors during walking in stroke patients

OBJECTIVES

It is still unclear to what an extent altered reflex activity contributes to gait deficit following stroke. Spinal group I and group II excitations from ankle dorsiflexors to knee extensors were investigated during post-stroke walking.

METHODS

Electrical stimulation was applied to the common peroneal nerve (CPN) in the early stance, and the short-latency biphasic excitation in Quadriceps motoneurones was evaluated from the Vastus Lateralis (VL) rectified and averaged (N=50) EMG activity in 14 stroke patients walking at 0.6-1.6 km/h, and 14 control subjects walking at 3.2-4.8 and at 1 km/h.

RESULTS

The second peak of the CPN-induced biphasic facilitation in VL EMG activity, which is likely mediated by group II excitatory pathways, was larger on the paretic side of the patients, as compared to their nonparetic side or control subjects, whatever their walking speed.

CONCLUSIONS

The spinal, presumed group II, excitation from ankle dorsiflexors to knee extensors is particularly enhanced during post-stroke walking probably due to plastic adaptations in the descending control.

SIGNIFICANCE

This adaptation may help to stabilize the knee in early stance when the patients have recover ankle dorsiflexor functions.

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.

Modulation of recurrent inhibition from knee extensors to ankle motoneurones during human walking

The neural control for muscle coordination during human locomotion involves spinal and supraspinal networks, but little is known about the exact mechanisms implicated. The present study focused on modulation of heteronymous recurrent inhibition from knee extensors to ankle motoneurones at different times in the gait cycle, when quadriceps (Quad) muscle activity overlaps that in tibialis anterior (TA) and soleus (Sol). The effects of femoral nerve stimulation on ankle motoneurones were investigated during treadmill walking and during tonic co-contraction of Quad and TA/Sol while standing. Recurrent inhibition of TA motoneurones depended on the level of background EMG, and was similar during walking and standing for matched background EMG levels. On the other hand, recurrent inhibition in Sol was reduced in early stance, with respect to standing, and enhanced in late stance. Reduced inhibition in Sol was also observed when Quad was coactivated with TA around the time of heel contact, compared to standing at matched background EMG levels in the two muscles. The modulation of recurrent inhibition of Sol during walking might reflect central and/or peripheral control of the Renshaw cells. These modulations could be implicated in the transition phases, from swing to stance to assist Sol activation during the stance phase, and from stance to swing, for its deactivation.

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.

Physiological evaluation of gait disturbances post stroke

A large proportion of stroke survivors have to deal with problems in mobility. Proper evaluations must be undertaken to understand the sensorimotor impairments underlying locomotor disorders post stroke, so that evidence-based interventions can be developed. The current electrophysiological, biomechanical, and imagery evaluations that provide insight into locomotor dysfunction post stroke, as well as their advantages and limitations, are reviewed in this paper. In particular, electrophysiological evaluations focus on the contrast of electromyographic patterns and integrity of spinal reflex pathways during perturbed and unperturbed locomotion between persons with stroke and healthy individuals. At a behavioral level, biomechanical evaluations that include temporal distance factors, kinematic and kinetic analyses, as well as the mechanical energy and metabolic cost, are useful when combined with electrophysiological measures for the interpretation of gait disturbances that are related to the control of the central nervous system or secondary to biomechanical constraints. Finally, current methods in imaging and transcranial magnetic stimulation can provide further insight into cortical control of locomotion and the integrity of the corticospinal pathways.

Spinal control of locomotion--from cat to man

For a large number of vertebrate species it is now indisputable that spinal networks have the capability of generating the basic locomotor rhythm. The aim of this review is to summarize the evidence for spinal pattern generators in cats and primates, including man and its interaction with sensory signals from the limbs. For all species the sensory feed-back from the moving limb is very important to achieve effective locomotor behaviour by adapting to the environment and compensating for unexpected postural disturbances. Sensory regulation of stepping can occur via reflex pathways to motoneurones (by-passing the locomotor rhythm generators) or by acting on the spinal locomotor networks themselves. The sensory feed-back serves to control the timing of the different phases in the step cycle, to shape the pattern of muscle activity, to contribute to the excitatory drive of the motoneurones and to the long-term adaptation of the locomotor activity. In this review we discuss the spinal locomotor circuits and the sensory feed-back in animals (mainly the cat) and human subjects. Special emphasis is given to work that has been of importance for the development of new rehabilitation paradigms following spinal cord injury.

Effects produced in human arm and forearm motoneurones after electrical stimulation of ulnar and median nerves at wrist level

Effects of electrical stimulation of ulnar and median nerves at wrist level were investigated in post-stimulus time histograms (PSTHs) of single motor units from both flexors and extensors in human arm and forearm. Stimulation of ulnar nerve produced late (mean extra time-after monosynaptic group Ia excitation-10.7 +/- 0.1 ms) high-threshold (>1.2 x motor threshold, MT) excitation, which was not reproduced by purely cutaneous stimulation, in all the investigated motor nuclei except in Extensor Carpi Radialis. Stimulation of median nerve, and of the skin of fingers II and III (at palmar side level), produced short latency inhibition (mean extra time 3.8 +/- 0.3 ms), which was most often truncated or followed by late excitation (mean extra time 11.8 +/- 0.3 ms); both effects were of low threshold (0.8 x MT). Short latency inhibition was very strong, and late excitation was rare and weak in almost all the investigated motor units except in those supplying flexors in forearm, in which the main effect was the late facilitation (stronger than in other motoneurones). Since extra time was not more than 13 ms, it is suggested that the late effects may be mediated through spinal pathways, at least during their 3-5 first ms. Based on the electrophysiological results and on the anatomical characteristics of ulnar and median nerves, it is assumed that ulnar-induced late high-threshold peak in PSTHs might reflect group II excitation in spinal motoneurones, and median-induced modifications in motor unit discharge, mainly cutaneous control of motoneurone discharge. Since the central delay of median-induced inhibition is longer the more caudal the motoneurone, inhibitory propriospinal-like interneurones are supposed to mediate cutaneous inhibitory control from hand upon muscles in arm and forearm. Potential roles of proprioceptive and cutaneous control from hand to more proximal musculature, provided by ulnar and median nerve, respectively, during precise hand movements are discussed.

Mediation of late excitation from human hand muscles via parallel group II spinal and group I transcortical pathways

This study addresses the question of the origin of the long-latency responses evoked in flexors in the forearm by afferents from human hand muscles. The effects of electrical stimuli to the ulnar nerve at wrist level were assessed in healthy subjects using post-stimulus time histograms for flexor digitorum superficialis and flexor carpi radialis (FCR) single motor units (eight subjects) and the modulation of the ongoing rectified FCR EMG (19 subjects). Ulnar stimulation evoked four successive peaks of heteronymous excitation that were not produced by purely cutaneous stimuli: a monosynaptic Ia excitation, a second group I excitation attributable to a propriospinally mediated effect, and two late peaks. The first long-latency excitation occurred 8-13 ms after monosynaptic latency and had a high-threshold (1.2-1.5 x motor threshold). When the conditioning stimulation was applied at a more distal site and when the ulnar nerve was cooled, the latency of this late excitation increased more than the latency of monosynaptic Ia excitation. This late response was not evoked in the contralateral FCR of one patient with bilateral corticospinal projections to FCR motoneurones. Finally, oral tizanidine suppressed the long-latency high-threshold excitation but not the early low-threshold group I responses. These results suggest that the late high-threshold response is mediated through a spinal pathway fed by muscle spindle group II afferents. The second long-latency excitation, less frequently observed (but probably underestimated), occurred 16-18 ms after monosynaptic latency, had a low threshold indicating a group I effect, and was not suppressed by tizanidine. It is suggested that this latest excitation involves a transcortical pathway.

Increase in group II excitation from ankle muscles to thigh motoneurones during human standing

In standing subjects, we investigated the excitation of quadriceps (Q) motoneurones by muscle afferents from tibialis anterior (TA) and the excitation of semitendinosus (ST) motoneurones by muscle afferents from gastrocnemius medialis (GM). Standing with a backward lean stretches the anterior muscle pair (TA and Q) and they must be co-contracted to maintain balance. Equally, forward lean stretches the posterior muscle pair (GM and ST) and they must be co-contracted. We used these conditions of enhanced lean to increase the influence of gamma static motoneurones on muscle spindle afferents, which enhances the background input from these afferents to extrafusal motoneurones. The effects of the conditioning volleys on motoneurone excitability was estimated using the modulation of the on-going rectified EMG and of the H reflex. Stimulation of afferents from TA in the deep peroneal nerve at 1.5-2 x MT (motor threshold) evoked early group I and late group II excitation of Q motoneurones. Stimulation of afferents in the GM nerve at 1.3-1.8 MT evoked only late group II excitation of ST motoneurones. The late excitation produced by the group II afferents was significantly greater when subjects were standing and leaning than when they voluntarily co-contracted the same muscle pairs at the same levels of activation. The early effect produced by the group I afferents was unchanged. We propose that this increase in excitation by group II afferents reflects a posture-related withdrawal of a tonic inhibition that is exerted by descending noradrenergic control and is specific to the synaptic actions of group II afferents.

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.

Modulation of the transmission in group II heteronymous pathways by tizanidine in spastic hemiplegic patients

OBJECTIVE

To investigate the effect of tizanidine (an alpha(2) noradrenergic agonist) on transmission in the interneuronal pathway coactivated by group I and group II afferents in post-stroke patients with spastic hemiplegia.

METHODS

Early and late facilitation of the quadriceps H reflex elicited in the common peroneal nerve--attributed to non-monosynaptic group I and group II excitation, respectively--was investigated in 14 spastic hemiplegic patients. All received a single dose of tizanidine (150 microg/kg) or placebo in randomised order at 10 day intervals. Repeated measurements were made at baseline (T0), 45-90 min, and 120 min after drug intake. Spasticity was assessed by modified Ashworth score in the quadriceps muscle and by a leg tone score calculated by the sum of the modified Ashworth score in five muscle groups.

RESULTS

On the spastic side a decrease in late group II and, to a lesser extent, early group I common peroneal nerve induced quadriceps H reflex facilitation occurred with tizanidine (group II, mean (SEM) difference T0-T90: 34.3 (10.2)%, p<0.001; group I, T0-T120: 19.8 (9)%, p<0.05), but not with placebo (group II, difference T0-T90: 12.5 (8)%, NS; group I, T0-T120: 3.2 (7)%, NS). Tizanidine but not placebo decreased the quadriceps muscle and global lower limb Ashworth scores (2.9 (0.2) to 1.9 (0.3), p<0.001; and 12 (0.7) to 9.5 (0.8), p<0.0001, respectively).

CONCLUSIONS

Enhancement of group II-group I facilitation of the quadriceps motor neurones on the spastic side of hemiplegic patients is modulated by alpha(2) noradrenergic agonists. This strengthens the view that late facilitation of quadriceps motor neurones is mediated by group II afferents and suggests that group II pathways may be involved in lower limb spasticity.

Plantar flexor stretch reflex responses to whole body loading/unloading during human walking

Numerous animal and human studies have shown that afferent information from the periphery contributes to the control of walking. In particular, recent studies have consistently shown that load receptor input is an important element of the locomotion control mechanism. The objective of this study was to investigate the contribution of load receptor feedback to the compensatory stretch reflex response. We examined the contribution of load receptor feedback to the magnitude of the short and medium latency components of the ankle plantar flexor stretch reflex responses following an unexpected dorsiflexion perturbation during human walking. Three body load conditions were investigated: normal body load, a 30% increase in body load, and a 30% decrease in body load. Healthy subjects walked on a treadmill at approximately 3.6 km/h with the left ankle attached to a portable stretching device. Dorsiflexion perturbations (8 degrees; 350-425 degrees/s) were generated during the late stance phase of gate (approximately 400 ms following heel contact). Electromyographic activity was recorded from the soleus, tibialis anterior, medial gastrocnemius, rectus femoris, and biceps femoris muscles using bipolar surface electrodes. Stretch reflex responses were observed in the soleus and gastrocnemius muscles for all of the body load conditions; however, increasing or decreasing the body load did not affect the timing and magnitude of the responses. This study provides evidence that load receptor input does not contribute strongly to the corrective response of the stretch reflex in the plantar flexor muscles during walking.

Suppression of the H reflex in humans by disynaptic autogenetic inhibitory pathways activated by the test volley

The present studies were designed to increase an existing limitation on the size of the H reflex by accentuating an inhibitory effect of group I afferents in the test volley. They were precipitated by the observation that, during strong voluntary contractions of quadriceps (Q), the late deep peroneal (DP) facilitation of the Q H reflex was suppressed but the facilitation of the ongoing EMG was not. The effects of conditioning stimuli to DP, superficial peroneal (SP) and articular afferents on the excitation of Q motoneurones (MNs) produced by femoral nerve (FN) stimulation were assessed in 11 healthy human subjects using the H reflex of vastus intermedius or the peak of group I excitation in post-stimulus time histograms (PSTHs) of single motor units (MUs) in vastus lateralis. The suppression of the late H reflex facilitation was observed during strong contractions after stimulation of DP and articular afferents, and at rest when DP and SP volleys were combined. In all single MUs tested, the FN-induced peak of excitation was suppressed by DP stimulation during strong Q contractions and by a combination of conditioning volleys (SP with DP or articular) during weak contractions. By themselves these conditioning volleys did not inhibit the background MU discharge even when delivered together. The suppression did not involve the initial bins of the peak; it began 0.7 ms later than the probable onset of monosynaptic Ia facilitation. It is argued that the suppression is not due to presynaptic inhibition of Ia terminals or to recurrent inhibition, but probably reflects convergence between the conditioning volleys and group I afferents in the test FN volley onto interneurones of the disynaptic non-reciprocal group I inhibition. It is concluded that the size of the H reflex is limited by disynaptic inhibition, and that changes in the excitability of this inhibitory pathway can produce prominent changes in the H reflex.