Specific modulation of corticomuscular coherence during submaximal voluntary isometric, shortening and lengthening contractions.

During voluntary contractions, corticomuscular coherence (CMC) is thought to reflect a mutual interaction between cortical and muscle oscillatory activities, respectively measured by electroencephalography (EEG) and electromyography (EMG). However, it remains unclear whether CMC modulation would depend on the contribution of neural mechanisms acting at the spinal level. To this purpose, modulations of CMC were compared during submaximal isometric, shortening and lengthening contractions of the soleus (SOL) and the medial gastrocnemius (MG) with a concurrent analysis of changes in spinal excitability that may be reduced during lengthening contractions. Submaximal contractions intensity was set at 50% of the maximal SOL EMG activity. CMC was computed in the time-frequency domain between the Cz EEG electrode signal and the unrectified SOL or MG EMG signal. Spinal excitability was quantified through normalized Hoffmann (H) reflex amplitude. The results indicate that beta-band CMC and normalized H-reflex were significantly lower in SOL during lengthening compared with isometric contractions, but were similar in MG for all three muscle contraction types. Collectively, these results highlight an effect of contraction type on beta-band CMC, although it may differ between agonist synergist muscles. These novel findings also provide new evidence that beta-band CMC modulation may involve spinal regulatory mechanisms.

Spinal mechanisms contribute to differences in the time to failure of submaximal fatiguing contractions performed with different loads.

This study compared the mechanisms that limit the time to failure of a sustained submaximal contraction at 20% of maximum when the elbow flexors either supported an inertial load (position task) or exerted an equivalent constant torque against a rigid restraint (force task). The surface electromyogram (EMG), the motor-evoked potential (MEP) in response to transcranial magnetic stimulation (TMS) of the motor cortex, and the Hoffmann reflex (H-reflex) and maximal M-wave (Mmax) elicited by electrical stimulation of the brachial plexus were recorded in biceps brachii during the two tasks. Although the time to failure for the position task was only 44% of that for the force task, the rate of increase of the average EMG (aEMG; % initial MVC) and MEP area (% Mmax) did not differ significantly during the two tasks. At task failure, however, the increases in normalized aEMG and MEP area were significantly (P < 0.05) greater for the force task (36.4 and 219.9%) than for the position task (22.4 and 141.7%). Furthermore, the superimposed mechanical twitch (% initial MVC), evoked by TMS during a brief MVC of the elbow flexors immediately after task failure, was increased similarly in both tasks. Although the normalized H-reflex area (% Mmax) decreased during the two fatiguing contractions, the reduction was more rapid and greater during the position task (59.8%) compared with the force task (34.7%). Taken together, the results suggest that spinal mechanisms were a major determinant of the briefer time to failure for the position task.

Cortical and spinal modulation of antagonist coactivation during a submaximal fatiguing contraction in humans.

This study investigates the control mechanisms at the cortical and spinal levels of antagonist coactivation during a submaximal fatiguing contraction of the elbow flexors at 50% of maximal voluntary contraction (MVC). We recorded motor-evoked potentials in the biceps brachii and triceps brachii muscles in response to magnetic stimulation of the motor cortex (MEP) and corticospinal tract (cervicomedullary motor-evoked potentials--CMEPs), as well as the Hoffmann reflex (H-reflex) and maximal M-wave (Mmax) elicited by electrical stimulation of the brachial plexus, before, during, and after the fatigue task. The results showed that although the coactivation ratio did not change at task failure, the MVC torque produced by the elbow flexors declined by 48% (P < 0.01) with no change in MVC torque for the elbow extensors. While the MEP and CMEP areas (normalized to Mmax) of the biceps brachii increased ( approximately 50%) over the first 40% of the time to task failure and then plateaued, both responses in the triceps brachii increased ( approximately 150-180%) gradually throughout the fatigue task. In contrast to the monotonic increase in the MEP and CMEP of the antagonist muscles, the H-reflex of the triceps brachii exhibited a biphasic modulation, increasing during the first part of the contraction before declining subsequently to 65% of its initial value. Collectively, these results suggest that the level of coactivation during a fatiguing contraction is mediated by supraspinal rather than spinal mechanisms and involves differential control of agonist and antagonist muscles.

Changes in excitability of the cortical projections to the human tibialis anterior after paired associative stimulation.

Paired associative stimulation (PAS) based on Hebb's law of association can induce plastic changes in the intact human. The optimal interstimulus interval (ISI) between the peripheral nerve and transcranial magnetic stimulus is not known for muscles of the lower leg. The aims of this study were to investigate the effect of PAS for a variety of ISIs and to explore the efficacy of PAS when applied during dynamic activation of the target muscle. PAS was applied at 0.2 Hz for 30 min with the tibialis anterior (TA) at rest. The ISI was varied randomly in seven sessions (n = 5). Subsequently, PAS was applied (n = 14, ISI = 55 ms) with the TA relaxed or dorsi-flexing. Finally, an optimized ISI based on the subject somatosensory evoked potential (SEP) latency plus a central processing delay (6 ms) was used (n = 13). Motor-evoked potentials (MEPs) were elicited in the TA before and after the intervention, and the size of the TA MEP was extracted. ISIs of 45, 50, and 55 ms increased and 40 ms decreased TA MEP significantly (P = 0.01). PAS during dorsi-flexion increased TA MEP size by 92% (P = 0.001). PAS delivered at rest resulted in a nonsignificant increase; however, when the ISI was optimized from SEP latency recordings, all subjects showed significant increases (P = 0.002). No changes in MEP size occurred in the antagonist. Results confirm that the excitability of the corticospinal projections to the TA but not the antagonist can be increased after PAS. This is strongly dependent on the individualized ISI and on the activation state of the muscle.

Interhemispheric inhibition in distal and proximal arm representations in the primary motor cortex.

Interhemispheric inhibitory interactions (IHI) operate between homologous distal hand representations in primary motor cortex (M1). It is not known whether proximal arm representations exhibit comparable effects on their homologous counterparts. We studied IHI in different arm representations, targeting triceps brachii (TB, n = 13), first dorsal interosseous (FDI, n = 13), and biceps brachii (BB, n = 7) muscles in healthy volunteers. Transcranial magnetic stimulation test stimuli (TS) were delivered to M1 contralateral to the target muscle preceded 10 ms by a conditioning stimulus (CS) to the opposite M1 at 110-150% resting motor threshold (RMT). IHI was calculated as the ratio between motor-evoked potential (MEP) amplitudes in conditioned relative to unconditioned trials. Mean RMTs were 38.9, 46.9, and 46.0% of stimulator output in FDI, TB, and BB muscles, respectively. IHI was 0.45 +/- 0.41 (FDI), 0.78 +/- 0.38 (TB), and 0.52 +/- 0.32 (BB, P < 0.01) when test MEP amplitudes were matched and 0.28 +/- 0.17 (FDI) and 0.85 +/- 0.31 (TB, P < 0.05) when TS intensities expressed as percentage RMT were matched. Significant IHI (P < 0.05) was identified with minimal CS intensities (expressed as percentage stimulator output) in the 30 s for FDI, 60 s for TB, and 40 s for BB. Additionally, a CS of roughly 120% RMT suppressed the test MEP but not a test H-reflex in BB, suggesting IHI observed in BB is likely mediated by a supraspinal mechanism. We conclude that IHI differs between different arm muscle representations, comparable between BB and FDI but lesser for TB. This finding suggests the amount of IHI between different arm representations does not strictly follow a proximal-to-distal gradient, but may be related to the role of each muscle in functional movement synergies.

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.

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

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

Influence of posture and stimulus parameters on post-activation depression of the soleus H-reflex in individuals with chronic spinal cord injury.

In non-disabled (ND) individuals, reflexes are modulated by influences related to physiologic state (e.g., posture, joint position, load) and activation history. Repeated activation of the H-reflex results in post-activation depression (PAD) of the response amplitude. The modulation associated with physiologic state and activation history is suppressed or abolished in individuals with spinal cord injury (SCI). While posture is known to affect H-reflex amplitude and PAD in non-disabled individuals, the effect of posture on PAD in SCI individuals is not known. Further, while the amount of PAD is also known to be influenced by the stimulus rate and by the amplitude of the evoked reflex, the interaction of posture with stimulus parameters has not been previously investigated in either group. We investigated differences in PAD of the soleus H-reflex between SCI subjects and ND subjects during sitting versus supported standing. Subjects were tested using paired conditioning-test stimulus pulses of 2.5s and 5s interpulse intervals (ISI) and with stimulus intensity adjusted to evoke reflex responses of 20% and 40% of the maximum motor response. We found standing posture to be associated with significantly less PAD in SCI subjects compared to ND subjects. In both groups, shorter ISIs and smaller reflex amplitudes were associated with greater PAD of the H-reflex. These results indicate that postural influences on post-activation modulation, while present, are impaired in individuals with chronic incomplete SCI.

Neuronal function in chronic spinal cord injury: divergence between locomotor and flexion- and H-reflex activity.

OBJECTIVE: Knowledge about the long-term course of spinal neuronal function after spinal cord injury (SCI) is important if regeneration therapies become available in the future. The objective of this study was to examine the behavior of locomotor EMG activity and of spinal reflexes in patients with chronic motor-complete SCI. METHODS: EMG activity from rectus femoris (RF), biceps femoris (BF), medial gastrocnemius (GM) and tibialis anterior (TA) of both sides was investigated during locomotor movements assisted by a robotic device in 10 chronic (>1 year after accident) complete SCI and 5 healthy subjects. H-reflexes (recorded from GM) were induced during the onset and flexion reflexes (recorded from BF and TA) at the end of the stance phase. RESULTS: Only in the chronic SCI subjects an exhaustion of EMG activity--i.e. a decrease in amplitude--occurred within a few minutes in all leg muscles. The EMG exhaustion was not associated with a change in the H- or flexion reflex amplitude during a walking session. CONCLUSIONS: Exhaustion of neuronal function in chronic complete SCI might be restricted to unused motor tasks, i.e. locomotion. The fact that H- and flexion reflexes show a normal behavior might be due to the fact that they still become activated. SIGNIFICANCE: Training/pharmalogical approaches may be required to maintain neuronal function of unused tasks as a basis for future successful regeneration therapies.

Decreasing pain in electrical nerve stimulation.

OBJECTIVE: Do large stimulation electrodes reduce pain in nerve conduction testing? METHODS: Fourteen healthy subjects (6 men, 22-42 years) blinded for study design underwent 8 H-reflex studies, using a large (36 x 36 mm) or a small (6 x 6 mm) electrode, placed on either the patella or the popliteal fossa; either site could be used for the cathode or the anode. Stimulation intensity was adjusted to obtain M- and H-waves. Intensity and other characteristics of pain were noted, as were M- and H-wave parameters. RESULTS: Pain was felt strongest near the smaller electrode when two sizes were used. Large electrodes reduced pain without compromising H- or M-waves. CONCLUSIONS: Large stimulation electrodes reduce pain. SIGNIFICANCE: The results apply for tests relying on amplitude measurements.

Changes in corticomotor excitability of forearm muscles in relation to static shoulder positions.

We examined whether the recruitment properties of the corticospinal pathway to forearm muscles are influenced by variations of the shoulder joint angle. Flexor carpi radialis (FCR) and extensor carpi radialis (ECR) muscles motor evoked potentials (MEPs) in response to transcranial magnetic stimulation were examined during different static positions of shoulder joint in the horizontal plane: from 30 degrees abduction to 30 degrees adduction. We found that at 30 degrees shoulder adduction, maximum slope and plateau phase of the ECR and FCR input-output relationship (i.e., relation between MEP size and stimulus intensity) were significantly higher and lower than at 30 degrees abduction of the shoulder joint, respectively. Intracortical inhibition (ICI) and intracortical facilitation (ICF) of the FCR were assessed using a paired-magnetic pulse paradigm. A significant decrease in ICF was observed after changing shoulder position from 30 degrees abduction to 30 degrees adduction. On the contrary, no variation in the amount of ICI occurred in relation to the same changes in shoulder position. FCR H-reflex to electrical stimulation of median nerve at elbow did not differ significantly between the two shoulder positions. We conclude that shoulder position influences the recruitment efficiency (gain) of the corticospinal volleys to motoneurones of forearm muscles. It is proposed that activity of peripheral receptors signaling static shoulder position influences corticomotor excitability of forearm muscles mainly at cortical level, although C3-C4 propriospinal system could be also involved. It is proposed that the above changes in corticomotoneuronal excitability to forearm muscles as function of shoulder joint position are part of a global proximal-distal synergy operating throughout reaching movements.

Depression of involuntary activity in muscles paralyzed by spinal cord injury.

Involuntary muscle contractions are common after spinal cord injury (SCI). Increased sensitivity to Ia muscle afferent input may contribute to the development of these spasms. Since tendon vibration results in a period of postactivation depression of the Ia synapse, we sought to determine whether Achilles tendon vibration (80 HZ for 2 s) altered involuntary contractions evoked by superficial peroneal nerve (SPN) stimulation (5 pulses at 300 HZ) in paralyzed leg muscles of subjects with chronic (>1 year) SCI. Responses to SPN stimulation that were conditioned by vibration were reduced in 66% of trials (by 33+/-12% in tibialis anterior and 40+/-16% in soleus). These reductions in electromyographic activity are unlikely to be mediated by changes at the Ia synapse or motoneuron because vibration did not alter the magnitude of the soleus H reflex. The electromyographic reductions may involve long-lasting neuromodulatory effects on spinal inhibitory interneurons or synapses involved in the flexor reflex pathway. Vibration-evoked depression of electromyographic activity may be clinically useful in controlling involuntary muscle contractions after SCI.

Motor learning changes GABAergic terminals on spinal motoneurons in normal rats.

The role of spinal cord plasticity in motor learning is largely unknown. This study explored the effects of H-reflex operant conditioning, a simple model of motor learning, on GABAergic input to spinal motoneurons in rats. Soleus motoneurons were labeled by retrograde transport of a fluorescent tracer and GABAergic terminals on them were identified by glutamic acid decarboxylase (GAD)67 immunoreactivity. Three groups were studied: (i) rats in which down-conditioning had reduced the H-reflex (successful HRdown rats); (ii) rats in which down-conditioning had not reduced the H-reflex (unsuccessful HRdown rats) and (iii) unconditioned (naive) rats. The number, size and GAD density of GABAergic terminals, and their coverage of the motoneuron, were significantly greater in successful HRdown rats than in unsuccessful HRdown or naive rats. It is likely that these differences are due to modifications in terminals from spinal interneurons in lamina VI-VII and that the increased terminal number, size, GAD density and coverage in successful HRdown rats reflect and convey a corticospinal tract influence that changes motoneuron firing threshold and thereby decreases the H-reflex. GABAergic terminals in spinal cord change after spinal cord transection. The present results demonstrate that such spinal cord plasticity also occurs in intact rats in the course of motor learning and suggest that this plasticity contributes to skill acquisition.

Locomotor responses and neuron excitability in conditions of haloperidol blockade of dopamine in invertebrates and vertebrates.

Levels of movement activity were used to identify two groups of rats: those with high- and low-activity levels. Blockade of dopamine receptors with haloperidol led to suppression of locomotor activity in both groups of rats; in common snails, haloperidol decreased the rate of locomotion. The excitability of spinal centers in rats decreased 5 min after single i.v. injections, with gradual recovery seen by 30 min. Chronic administration of haloperidol suppressed post-tetanic potentiation of the H response in the gastrocnemius muscle of spinal rats. Prolonged use of haloperidol induced significant hyperpolarization of the membrane potential of command neurons in common snails and increased the action potential generation threshold. Selective pharmacological exclusion of the brain dopamine system was found to lead to decreases in the excitability of defined neurons in snails and the spinal motor centers in rats, also producing impairments in locomotor responses in these animals.

Inhibition from the plantar nerve to soleus muscle during the stance phase of walking.

The purpose of this study is to investigate the effect and the circuit from the branch of tibial (plantar) nerve to soleus muscle and its modulation during walking in humans. Stimulation of the plantar nerve produced short latency inhibition of soleus EMG activity and the H-reflex in humans. The threshold of afferent fibers was lower than that of motor fibers. This inhibition did not converge to disynaptic reciprocal Ia inhibition nor did inhibition from the cutaneous nerve of the big toe, but to Ib inhibition from the medial gastrocnemius nerve. The inhibitory pathway from the plantar nerve therefore is considered to include Ib inhibitory interneurones. Modulation of the inhibition was investigated during walking. Less EMG depression after plantar nerve stimulation occurred in the stance phase of walking than for tonic or dynamic plantar flexion at similar background EMG activity level. The inhibition of the soleus H-reflex after plantar nerve stimulation was also decreased during the stance phase. For investigating the influence of load on the inhibition from the plantar nerve, more EMG depression occurred in the stance phase with body unloading. Similar findings were observed in Ib inhibition from the medial gastrocnemius nerve, but not in disynaptic reciprocal Ia inhibition to soleus muscle. It is concluded that transmission of inhibition from the plantar nerve to soleus muscle is modulated during walking. It would minimize this inhibition during the stance phase of walking and might enhance soleus muscle activity via this reflex pathway for the support of weight.

Reliability of the FCR H-reflex.

This study examined the intraclass reliability of the latency and amplitude of the Hoffmann reflex (H-reflex) in the flexor carpi radialis (FCR). The stability and consistency of the latency and peak-to-peak amplitude of the H-reflex were assessed across four test sessions. The FCR H-reflex was evoked in 39 volunteers (20 males and 19 females) on four separate days. The maximum amplitude of the motor response (Mmax) was determined each day and 10 responses were recorded at that stimulus intensity. The H-reflex was then evoked at an intensity corresponding to 5% of Mmax (H5%) and 10 recordings were obtained. The latency of the H-reflex and the peak-to-peak amplitude of both Mmax and H5% were calculated for each trial. Determination of reliability involves the consideration of both the stability and consistency of the measures. The reliability of the measures in this investigation was assessed using an analysis of variance and corresponding Lindquist intraclass correlation coefficient (ICC) model. In contrast to previous investigations, the H-reflex was evoked without facilitation in 95% (37/39) of the subjects tested. Statistical evaluation revealed that the latency and amplitude of the H-reflex, as well as the amplitude of Mmax, were both stable and consistent across the four test days. The ICC for Mmax was 0.97. The ICC for H5% was 0.92, and for latency of the H-reflex was 0.89. It was shown that the H-reflex can be easily evoked in the FCR and that the latency and amplitude of these recordings are highly reliable. The demonstration that the H-reflex can be easily and consistently evoked in the FCR has important clinical implications. It provides a tool for clinicians to assess the C7 level of the spinal cord and median nerve function.

Facilitation of soleus H-reflex amplitude evoked by cutaneous nerve stimulation at the wrist is not suppressed by rhythmic arm movement.

Neural connections between the cervical and lumbosacral spinal cord may assist in arm and leg coordination during locomotion. Currently the extent to which arm activity can modulate reflex excitability of leg muscles is not fully understood. We showed recently that rhythmic arm movement significantly suppresses soleus H-reflex amplitude probably via modification of presynaptic inhibition of the IA afferent pathway. Further, during walking reflexes evoked in leg muscles by stimulation of a cutaneous nerve at the wrist (superficial radial nerve; SR) are phase and task dependent. However, during walking both the arms and legs are rhythmically active thus it is difficult to identify the locus of such modulation. Here we examined the influence of SR nerve stimulation on transmission through the soleus H-reflex pathway in the leg during static contractions and during rhythmic arm movements. Nerve stimulation was delivered with the right shoulder in flexion or extension. H-reflexes were evoked alone (unconditioned) or with cutaneous conditioning via stimulation of the SR nerve (also delivered alone without H-reflex in separate trials). SR nerve stimulation significantly facilitated H-reflex amplitude during static contractions with the arm extended and countered the suppression of reflex amplitude induced by arm cycling. The results demonstrate that cutaneous feedback from the hand on to the soleus H-reflex pathway in the legs is not suppressed during rhythmic arm movement. This contrasts with the observation that rhythmic arm movement suppresses facilitation of soleus H-reflex when cutaneous nerves innervating the leg are stimulated. In conjunction with other data taken during walking, this suggests that the modulation of transmission through pathways from the SR nerve to the lumbosacral spinal cord is partly determined by rhythmic activity of both the arms and legs.

Excitability changes in resting forearm muscles during voluntary foot movements depend on hand position: a neural substrate for hand-foot isodirectional coupling.

When associating hand and foot voluntary oscillations, isodirectional coupling is preferred irrespective of hand position (prone or supine). To investigate the neural correlates of this coupling modality, excitability of the motor projections innervating the resting forearm was tested during cyclic voluntary flexion-extensions of the ipsilateral foot. H-reflexes, in some experiments facilitated by subliminal Transcranial Magnetic Stimulation (TMS), and Compound Muscle Action Potentials (CMAPs), evoked by supraliminal TMS, were elicited in Flexor Carpi Radialis (FCR) and Extensor Carpi Radialis (ECR) muscles at five intervals during the foot movement cycle. With the hand prone, a sinusoidal excitability modulation was observed in wrist flexors and extensors, but reversed in phase: in FCR, excitability increased during plantar-flexion and decreased during dorsiflexion, while in ECR the opposite occurred. This reciprocal organisation was confirmed by the excitability modulation of CMAPs evoked simultaneously in the two antagonists. When the hand was supinated, the H-reflex modulation reversed in phase, i.e., FCR excitability increased during foot dorsiflexion and decreased during plantar-flexion. In both muscles and hand positions tested, when the muscle-to-movement phase-lag was increased by inertial loading of the foot, H-reflex excitability modulations remained phase linked to muscular contractions, not to movement. Together, these results suggest that the subliminal excitability modulation of hand movers has a common central origin with the parallel overt activation of foot movers, is reciprocally organised, and is direction- not muscle-dependent. It may therefore represent the neural substrate for isodirectional coupling of hand (prone or supine) with the foot.

Short-latency subliminal effects of transcranial magnetic stimulation on forearm motoneurones.

The H-reflex technique has been used to evaluate the time-course of the effects evoked by transcranial clockwise magnetic stimuli in flexor or extensor carpi radialis motoneurones. In six subjects, magnetic stimulation was applied over the scalp in the focus for the motor response of those muscles. At intensities below motor threshold, a facilitation of the H-reflex started at a conditioning-test interval of -4 ms (i.e. when the magnetic stimulus lagged the test stimulus by 4 ms), reached a peak at about -2 ms and rapidly decayed. At about -1 ms, the decay attained a local minimum, which in three subjects had values indicating the presence of an inhibition. Thereafter, a second facilitatory phase peaked at about +1 ms. By matching the time course with the latency of the cortical muscle action potential (CMAP) evoked by suprathreshold magnetic stimulation, it is inferred that the motoneuronal discharge coincides with the second peak of facilitation and is preceded by 3-4 ms of subliminal excitation. This early effect could be brought to threshold by convergence of a subliminal Ia EPSP, leading to a reduction of the CMAP latency. The early excitatory effects reported above are as fast as those described as following transcranial electrical stimulation, and should likewise be considered as monosynaptic.

The H-reflex of the flexor carpi radialis muscle; a study in controls and radiation-induced brachial plexus lesions.

H-reflexes of the flexor carpi radialis muscle were studied in 52 controls and 25 cancer patients with radiation-induced brachial plexopathy. It was found that H-reflex conduction velocity (H-RCV) decreased with increasing age. This was not true for H-reflex latency (H-RL) and inter-latency times. There were no H-RCV and latency differences between age-matched male and female subjects. In the affected arm the reflex was absent in nine patients and delayed in 16 patients in whom H-RCV was decreased in 13 patients. Three patients showed large H-RL differences which were also notable features in median nerve disease in the region of the brachial plexus.