Pattern of projections of group I afferents from forearm muscles to motoneurones supplying biceps and triceps muscles in man

Summary (1) Two independent methods were used, in man, to assess the modifications of the excitability of biceps and triceps brachii motoneurone pools following the stimulation of group I afferents coming from muscles acting at the wrist: (a) the modifications of the excitability of a motoneuronal population were studied using a reflex technique, (b) the modifications of the excitability of an isolated motor unit were estimated using a post-stimulus time histogram (p.s.t.h.) method. (2) The activation of group I afferents contained in the median nerve, originating from wrist flexors and pronators, resulted in a strong, short-latency facilitation of the biceps brachii motoneurones. A similar effect was also evoked by stimulation of group I afferents in the radial nerve, distally to the branch supplying the brachio-radialis muscle. The latency of both median and radial-induced facilitations is compatible with a monosynaptic linkage. (3) The stimulation of group I afferents in the median or the radial nerves produced inhibition of triceps motoneurones, with a latency compatible with a disynaptic linkage. (4) The prolonged vibration of the tendon of the flexor carpi radialis (FCR) or of the extensor carpi radialis (ECR) raised the threshold for both the facilitation of biceps and the inhibition of triceps motoneurones. The same pattern of excitatory and inhibitory convergence could also be obtained when the electrical conditioning stimulus to the median or radial nerves was replaced by a tap applied to the tendons of FCR or ECR respectively. Both results suggest that the conditioning fibres were Ia fibres. (5) The pattern of distribution of Ia afferents from muscles acting at the wrist onto motoneurones of muscles acting at the elbow has been compared to that described in the cat and monkey. A comparison has also been made between Ia connections of muscles acting at different joints in the upper and lower limb in man. The differences are discussed in relation to the manipulating capacity of the hand.     

Distribution of heteronymous Ia facilitation and recurrent inhibition in the human deltoid motor nucleus

Summary Distribution of heteronymous Ia facilitation and of heteronymous recurrent inhibition in motoneurones innervating the anterior part of the deltoid muscle were investigated in normal human subjects following electrical stimulation of the nerves innervating the main muscles of the upper limb. Activation of group I afferents originating from deltoid, biceps, triceps and extensor carpi radialis (ECR) muscles resulted in an early increase in firing probability of voluntarily activated motor units belonging to the anterior part of the deltoid muscle whereas activation of motor axons supplying deltoid, triceps, ECR and flexor carpi radialis (FCR) muscles resulted in an early and long-lasting decrease in firing probability. No effect was seen following activation of group I afferents and motor axons contained in the ulnar nerve. The characteristics of the early facilitation suggest that it is at least partly due to heteronymous Ia monosynaptic connections while these of the long-lasting inhibition suggest that it is at least partly due to heteronymous recurrent inhibition. Their patterns of distribution are discussed with regards to the functional role of the human deltoid.     

Synaptic connections from large afferents of wrist flexor and extensor muscles to synergistic motoneurones in man

 Short-latency excitatory Ia reflex connections were determined between pairs of human wrist flexor and extensor muscles. Spindle Ia afferents were stimulated by either tendon tap or electrical stimulation. The activity of voluntarily activated single motor units was recorded intramuscularly from pairs of wrist flexor or extensor muscles. Cross-correlation between stimuli and the discharge of the motor units provided a measure of the homonymous or heteronymous excitatory input to a motoneurone. Homonymous motoneurone facilitation was generally stronger than that of the heteronymous motoneurones. The principal wrist flexors, flexor carpi radialis (FCR) and flexor carpi ulnaris (FCU), were tightly connected through a bidirectional short-latency reflex pathway. In contrast, the extensor carpi ulnaris (ECU) and the extensor carpi radialis (ECR) did not have similar connections. ECU motoneurones received no short-latency excitatory Ia input from the ECR. ECR motoneurones did receive excitatory Ia input from ECU Ia afferents; however, its latency was delayed by several milliseconds compared with other heteronymous Ia excitatory effects observed. The wrist and finger extensors were linked through heteronymous Ia excitatory reflexes. The reflex connections observed in humans are largely similar to those observed in the cat, with the exception of heteronymous effects from the ECU to the ECR and from the extensor digitorum communis (EDC) to the ECU, which are present only in humans. The differences in the reflex organization of the wrist flexors versus the extensors probably reflects the importance of grasping. Received: 19 August 1996 / Accepted: 6 March 1997     

Changes in reciprocal and transjoint inhibition induced by muscle fatigue in man

The effects of localised muscle fatigue on group I reflex pathways were studied in the human upper limb. Activation of group I afferents originating from biceps and extensor carpi radialis (ECR) resulted in an inhibition of flexor carpi radialis (FCR) motoneurones, probably through a disynaptic pathway. Reciprocal inhibition (from ECR to FCR) and transjoint inhibition (from biceps to FCR) were compared before and during localised fatigue induced in the muscle from which group I afferents originated. Fatigue of wrist extensors did not modify the reciprocal inhibition, while during fatigue of elbow flexors the transjoint inhibition was less pronounced. This striking difference between reciprocal and transjoint inhibition is discussed in relation to the pattern of diffusion of voluntary contractions during fatigue in the human upper limb.     

Integrated motor cortical control of task-related muscles during pointing in humans

A large body of compelling but indirect evidence suggests that the motor cortex controls the different forelimb segments as a whole rather than individually. The purpose of this study was to obtain physiological evidence in behaving human subjects on the mode of operation of the primary motor cortex during coordinated movements of the forelimb. We approached this problem by studying a pointing movement involving the shoulder, elbow, wrist, and index finger as follows. Focal transcranial magnetic stimulation (TMS) was used to measure the input-output (I/O) curves-a measure of the corticospinal pathway excitability-of proximal (anterior deltoid, AD, and triceps brachii, TB) and distal muscles (extensor carpi radialis, ECR, and first dorsal interosseus, 1DI) during isolated contraction of one of these muscles or during selective co-activation with other muscles involved in pointing. Compared to an isolated contraction of the ECR, the plateau-level of the ECR sigmoid I/O curve increased markedly during co-activation with the AD while pointing. In contrast, the I/O curve of AD was not influenced by activation of the more distal muscles involved in pointing. Moreover, the 1DI I/O curve was not influenced by activation of the more proximal muscles. Three arguments argue for a cortical site of facilitation of ECR motor potentials. First, ECR motor potentials evoked by a near threshold TMS stimulus were facilitated when the AD and ECR were co-activated during pointing but not those in response to a near threshold anodal electrical stimulus. Second, the ECR H reflex was not found to be task dependent, indicating that the recruitment gain of the ECR alpha-motoneuron pool did not differ between tasks. Finally, in comparison with an isolated ECR contraction, intracortical inhibition tested at the ECR cortical site was decreased during pointing. These results suggest that activation of shoulder, elbow, and wrist muscles involved in pointing appear to involve, at least in part, common motor cortical circuits. In contrast, at least in the pointing task, the motor cortical circuits involved in activation of the 1DI appear to act independently.     

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 × 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.     

Pattern of descending excitation of presumed propriospinal neurones at the onset of voluntary movement in humans

Mazevet , D. & Pierrot -Deseilligny , E. 1994. Pattern of descending excitation of presumed propriospinal neurones at the onset of voluntary movement in humans. Acta Physiol Scand 150, 27–38. Received 2 April 1993, accepted 30 July 1993. ISSN 0001–6772. Neurophysiologie Clinique, Rééducation, Hôpital de la Salpêtrière, 47 boulevard de ? Hôpital, 75651 Paris cedex 13, France. The pattern of activation of presumed ‘propriospinal’ neurones was investigated in human subjects during phasic voluntary contractions of one of the following muscles: biceps, triceps, flexor carpi radialis (FCR), flexor carpi ulnaris (FCU) and extensor carpi radialis (ECR). Changes in the amplitude of the H reflex (FCR, ECR), or the tendon jerk (biceps, triceps) were used to assess the excitability of the corresponding motorneurone pools after conditioning stimulation. Conditioning stimuli were applied to the musculo-cutaneous, triceps and ulnar nerves. In most cases reflex facilitation was not observed at rest and was only disclosed at the onset of contraction. The characteristics of this facilitation (3–4 ms central delay, short duration, low threshold, depression when the afferent input was increased) are consistent with those previously attributed to ‘propriospinal’ excitation. It is argued that the contraction-associated facilitation was descending in origin. The descending facilitation of the ‘propriospinal’ system had a characteristic pattern in that the pathways selected by higher centres were those receiving the afferent feedback from the contracting muscle. These results provide further insight into the organization of human ‘propriospinal’ pathways: (1) it is confirmed that afferents from each muscle activate a specific subset of neurones; and (2) it is suggested that the projections of each subset are divergent, implying that individual neurones project onto diverse motor nuclei, an organization that would favour the coordination of multi-joint movements. Such an organization is discussed in relation to the possible role of the propriospinal system in the control of normal human upper limb movements.     

Influence of load type on presynaptic modulation of Ia afferent input onto two synergist muscles

The present work was designed to investigate presynaptic modulation of Ia afferents in the extensor (ECR) and flexor carpi radialis (FCR) when the two muscles acted as synergists during radial deviation to either support an inertial load (position task) or exert an equivalent constant torque against a rigid restraint (force task). H reflexes were evoked in the ECR and FCR by stimulating at the elbow level (1-ms duration) the radial and median nerves, respectively. Conditioning stimulation was applied to the median and radial nerves at the elbow level to assess presynaptic inhibition of homonymous Ia afferent input (D1 inhibition) from the ECR and FCR, respectively. The ongoing presynaptic inhibition of heteronymous Ia afferents that converges onto ECR and FCR motor neuron pools (heteronymous Ia facilitation) was assessed by stimulating the median nerve at the wrist level (palmar branch) prior to the stimulus applied over the radial or median nerve. The heteronymous monosynaptic Ia facilitation was greater (P < 0.05) during the position task (ECR 121%; FCR 147%) compared with the force task (ECR 115%; FCR 132%), and was paralleled by the depression of D1 inhibition (P < 0.05) during the position task (ECR 75.4%; FCR 79.0%) compared with force task (ECR 58.7%; FCR 58.8%). These data indicate that Ia presynaptic inhibition is reduced during the position task relative to the force task. Such differential modulation of Ia afferent input onto the motor neuron pool likely reflects the requirement to heighten reflex responsiveness during the unstable task of maintaining limb position.     

Evidence for mutual inhibition of opposite Ia interneurones in the human upper limb

Summary The H-reflex technique was used to collect indirect evidence for changes in excitability of the interneurones mediating reciprocal Ia inhibition between wrist extensors and flexors. Stimulating the radial nerve results in an inhibition of the flexor carpi radialis (FCR) H-reflex and evidence has previously been presented that the early phase of inhibition is mediated by extensor-coupled Ia interneurones (Ext Ia INs), i.e. by inhibitory interneurones fed by muscle spindle Ia afferents from wrist extensors. Variations in the level of this inhibition were used to assess changes in excitability of Ext Ia INs. Stimulation of group I fibres from flexors was shown to depress the reference Ia inhibition, i.e. to inhibit the Ext Ia INs. The central latency of this interneuronal inhibition was compatible with a disynaptic linkage between flexor Ia afferents and Ext Ia INs. Its threshold and time course profile could almost exactly be superimposed on those of reciprocal Ia inhibition from flexors to extensor carpi radialis (ECR) motoneurones (MNs). This suggests that the Ia inhibitions to extensor MNs and extensor Ia INs are collateral effects mediated by the same flexor-coupled Ia interneurones. In two subjects, in whom it was possible to elicit an H-reflex in the ECR, inhibition of flexor-coupled Ia interneurones by activation of extensor Ia interneurones could similarly be demonstrated.     

A propriospinal-like contribution to electromyographic responses evoked in wrist extensor muscles by transcranial stimulation of the motor cortex in man

We tested the hypothesis that some of the electromyographic (EMG) responses elicited in preactivated forearm muscles by transcranial stimulation of the human motor cortex are produced by activity in a disynaptic corticospinal linkage involving propriospinal-like interneurones with cell bodies in the spinal C3–4 segments. The experimental design incorporated a previous observation that stimulation of afferents in the superficial radial nerve inhibits propriospinal-like neurones projecting to the extensor carpi radialis (ECR) muscle. Surface EMG responses were recorded from the active ECR muscle after transcranial electrical or magnetic stimulation over the motor cortex. In random trials, single conditioning stimuli at twice perceptual threshold were given to the superficial radial nerve at the wrist at different times before a cortical shock. When the cortex was stimulated electrically, the conditioning stimulus suppressed the EMG responses when the interval between the shocks was 11 ms or more. This was about 3.5 ms longer than the minimum time calculated for a possible direct cutaneous effect on spinal motoneurones. The time course of suppression began earlier and was more complex during magnetic stimulation of the cortex. It is argued that this difference is due to the repetitive I waves generated by the magnetic shock. Whether electrical or magnetic stimulation was used, the first 1–3 ms of the EMG response was relatively unaffected by superficial radial nerve stimulation at any interstimulus interval, whereas clear suppression was seen in the later portion of the response. In contrast, if the EMG response in ECR was suppressed by a conditioning stimulus to the median nerve at the elbow, then all portions of the EMG response were inhibited including the first 1–3 ms. The median nerve effect is thought to be due to direct reciprocal inhibition of the extensor motoneurones. Thus sparing of the initial part of the cortically evoked response with superficial radial stimulation suggests that the latter type of inhibition occurs at a premotoneuronal level. The timing of the effect is compatible with the explanation that corticospinal excitation is produced in ECR motoneurones through both monosynaptic and disynaptic (including propriospinal premotoneuronal) pathways, with superficial radial nerve inhibition being exerted at the propriospinal level.     

Medium-latency reflex response elicited from the flexor carpi radialis by radial nerve stimulation

The H reflex obtained from the flexor carpi radialis muscle by median nerve stimulation is a well-known monosynaptic reflex. However, the origin of the late responses is still contentious. Radial nerve stimulation was performed through the spiral groove, and EMG recording was obtained from the flexor carpi radialis (FCR) and extensor carpi radialis (ECR) muscles. An M response followed by an F response was achieved from the ECR by radial nerve stimulation; the antagonistic FCR muscle elicited a late response. A total of 25 cases were included in this study. In 22 of these cases, a response with a latency of 40.97 ± 5.35 ms was obtained from the FCR by radial nerve stimulation. When extension of the hand was restricted, the response disappeared in five of nine cases. Application of cold markedly suppressed the response and prolonged the latency of the FCR medium-latency reflex response (FCR-MLR). Oral tizanidine considerably suppressed the FCR-MLR response. Two out of eight cases did not exhibit any response. No response could be recorded from a patient with complete amputation of the right hand. The FCR-MLR is the reflex caused by stretching of the FCR muscle from radial nerve stimulation, and it is greatly influenced by group II afferents.     

Vibration-evoked reciprocal inhibition between human wrist muscles

Summary Reciprocal inhibition of the voluntarily contracting wrist extensor (extensor carpi radialis, ECR) evoked by proprioceptive afferent input from the flexor (flexor carpi radialis, FCR), was studied in healthy human subjects. Vibration of the FCR tendon was used to elicit Ia-dominated afferent discharge whilst inhibition of ECR was assessed as the reduction in asynchronous, on-going EMG. A small early phase of inhibition (I1) was evident in 25% of trials. The latency (ca. 25 ms) of this component suggested that it was mediated by an Ia oligosynaptic, possibly classical disynaptic, inhibitory pathway. A later and apparently separate phase of reduced activity (12, ca. 40 ms) was, however, far more consistently observed (96% of trials) and of greater magnitude. The 12 component was usually followed, some 20 ms later, by a phase of elevated activity (El, 72% trials). Reductions in simultaneously recorded net extensor torque commenced at about 60 ms following the onset of flexor tendon vibration, i.e. some 20 ms after the main I2 EMG component. These mechanical responses must have almost exclusively resulted from reciprocal inhibition of extensor EMG since vibration of the relaxed FCR evoked minimal excitatory flexor activity. The reflex pattern, in any individual subject, was relatively unaffected by altering the duration of the vibration train between one and nineteen cycles (125 Hz). This suggests that the entire response complex resulted largely from the initial afferent volley. The sizes of both the I1 and I2 reductions in ECR activity increased with increasing voluntary extensor contraction so that their depths remained constant proportions of background EMG. Very similar results were obtained when reciprocal inhibition of FCR was produced by vibration of the belly of ECR. Thus, reciprocal inhibition between wrist muscles is mainly expressed as a rather stereotyped, short duration reduction in EMG whose depth is determined by the pre-existing level of motor activity. Some functional implications of this form of reflex behaviour are discussed.     

Heteronymous reflex connections in human upper limb muscles in response to stretch of forearm muscles

Torque motor produced stretch of upper limb muscles results in two distinct reflex peaks in the electromyographic activity. Whereas the short-latency reflex (SLR) response is mediated largely by the spinal monosynaptic reflex pathway, the longer-latency reflex (LLR) is suggested to involve a transcortical loop. For the SLRs, patterns of heteronymous monosynaptic Ia connections have been well-studied for a large number of muscles in the cat and in humans. For LLRs, information is available for perturbations to proximal joints, although the protocols for most of these studies did not focus on heteronymous connections. The main objective of the present study was to elicit both SLRs and LLRs in wrist flexors and extensors and to examine heteronymous connections from these muscles to elbow flexors (biceps brachii; BiBr) and extensors (triceps brachii; TriBr) and to selected distal muscles, including abductor pollicis longus (APL), first dorsal interosseous (FDI), abductor digiti minimi (ADM), and Thenars. The stretch of wrist flexors produced SLR and LLR peaks in APL, FDI, ADM, Thenars, and BiBr while simultaneously inducing inhibition of wrist extensors and TriBr. When wrist extensors were stretched, SLR and LLR peaks were observed in TriBr, whereas the primary wrist flexors, APL and BiBr, were inhibited; response patterns of FDI, ADM, and Thenars were less consistent. The main conclusions from the observed data are that: 1) as in the cat, afferents from wrist flexors and extensors make heteronymous connections with proximal and distal upper limb muscles; and 2) the strength of heteronymous connections is greater for LLRs than SLRs in the distal muscles, whereas the opposite is true for the proximal muscles. In the majority of observations, SLR and LLR excitatory peaks were observed together. However, on occasion, LLRs were observed without the SLR response in hand muscles when wrist extensors were stretched.     

Ascending long spinal actions on forelimb motoneurons in the acute spinal cat

Ascending long spinal reflex system were investigated by means of monosynaptic reflex testing in the acutely spinalized unanesthetized cat. 1.Hindlimb nerve stimulation gave bilateral facilitatory effects on the motoneuron pools of pectoralis major and physiological flexors of the forelimb such as biceps brachii, extensor carpi radialis, extensor digitorum communis, but elicited depressive effects on the physiological extensors such as triceps brachii, flexor carpi radialis, flexor digitorum profundus. 2. In the latissimus dorsi, which is the antagonist of pectoralis major, a depressive effect was elicited by the stimulation of ipsilateral hindlimb nerves, and a facilitatory effect by contralateral stimulation. 3. These effects were evoked mainly from group II afferent fibers in muscle as well as cutaneous nerves. 4. Intracellular recordings from motoneurons of extensor carpi radialis revealed EPSPs following stimulation of hindlimb nerves with time courses corresponding to those of the facilitatory effects. We failed to detect any potential changes in the motoneurons of flexor carpi radialis following stimulation of hindlimb nerves.     

Induction of cortical plastic changes in wrist muscles by paired associative stimulation in healthy subjects and post-stroke patients

It has been shown on hand muscles in normal subjects that paired associative stimulation (PAS) combining peripheral nerve stimulation and transcranial magnetic stimulation (TMS) induces lasting changes in cortical motor excitability (Stefan et al., Brain 123 (Pt3):572–584, 2000). Because the motor recovery of distal upper limb and particularly wrist extension in post-stroke patients is one of the major rehabilitation challenge, we investigate here the effect of one session of paired associative stimulation on the excitability of the corticospinal projection to extensor carpi radialis (ECR) muscle (motor evoked potential size) before and after PAS in 17 healthy subjects and in two patients 5 months after stroke. The time course, the topographical specificity, changes in rest motor threshold (RMT), short intracortical inhibition and intracortical facilitation (SICI and ICF), the respective role of cutaneous and muscular afferents and the effect of a prolonged peripheral stimulation alone were also studied in normal subjects. Using a protocol derived from that of Ridding et al. J Physiol 537:623–631 (2001), PAS was able to induce lasting changes in the excitability of corticospinal projection to wrist muscles in healthy subjects and in the two post-stroke patients studied. Electrophysiological features of these plastic changes were similar to those previously observed in hand muscles: rapid evolution, 30–60 min duration, reversibility, relative topographical specificity and associative dependence suggesting an LTP-like mechanism. A contribution of cutaneous afferents in inducing PAS effects was also demonstrated. The decrease in ECR RMT after PAS observed in patients and in healthy subjects was an unexpected result because it has not been previously reported in the hand muscles of healthy subjects. However, it has been observed in dystonic patients (Quartarone et al., Brain 126:2586–2596, 2003). This suggests that other mechanisms like changes in membrane excitability could be involved in ECR facilitation after PAS. Further studies performed on patients using daily repeated PAS protocols and showing a functional improvement in hand motor function will be necessary to confirm that this technique could be relevant in motor rehabilitation, at least for some selected patients.     

Distribution of non-monosynaptic excitation to early and late recruited units in human forearm muscles

The distribution of monosynaptic and nonmonosynaptic excitation was investigated within flexor carpi radialis (FCR) and extensor carpi radialis (ECR) motoneurone (MN) pools. FCR H reflexes of different size were conditioned by various conditioning stimuli eliciting different effects: (1) musculocutaneous-induced non-monosynaptic excitation of FCR MNs at the onset of biceps contraction, (2) heteronymous monosynaptic Ia facilitation, (3) reciprocal Ia inhibition, and (4) presynaptic inhibition of Ia terminals. Musculocutaneous-induced non-monosynaptic excitation increased continuously with the size of the unconditioned reflex. In contrast, heteronymous monosynaptic Ia excitation first increased and then decreased, with increases in the unconditioned reflex size, reciprocal inhibition and presynaptic inhibition showing an approximately similar tendency. This suggests that the non-monosynaptic excitation is distributed more evenly to early and late recruited MNs than monosynaptic Ia excitation, reciprocal inhibition and presynaptic inhibition. A different pattern of homonymous radial-induced monosynaptic and non-monosynaptic excitation was also found for individual ECR MNs investigated with the poststimulus time histogram (PSTH) method. Whereas the monosynaptic Ia excitation tended to be most marked in lower threshold MUs, the nonmonosynaptic excitation was evenly distributed to lower and higher threshold MUs. We propose that the even distribution of the non-monosynaptic excitation in the motoneuronal pool may be of significance when it is necessary to activate a wide range of MNs more or less simultaneously.     

Task-related changes in propriospinal excitation from hand muscles to human flexor carpi radialis motoneurones

This study addresses whether there is excitation from human hand muscles to flexor carpi radialis (FCR) motoneurones mediated through propriospinal circuits and, if so, whether it is used in specific motor tasks. Electrical stimuli to the ulnar nerve at wrist level produced an excitation in FCR motoneurones with characteristics typical of a propriospinally mediated effect: low threshold (0.6 × motor threshold (MT)), a group I effect that was not reproduced by purely cutaneous stimuli, long central delay (4.1 ± 0.4 ms in single units), suppression when the stimulus intensity was increased, and facilitation of the corticospinal excitation at the premotoneuronal level. Ulnar-induced propriospinally mediated excitation was compared during selective voluntary contractions of the FCR and, at equivalent level of FCR EMG, during tasks in which the FCR was activated automatically in postural contractions rather than voluntarily (grip, pinching and pointing). The excitation was significantly greater during grip (and pinching) than during voluntary FCR contractions and pointing, whether measured in single motor units or tonic EMG activity, or whether the response to motor cortex stimulation was assessed as the compound motor-evoked potential or the corticospinal peak in single units. The discrepancy between the tasks appeared with ulnar intensities above 0.8 × MT and was then present across a wide range of stimulus intensities. This suggests a reduction in the corticospinal control of 'feedback inhibitory interneurones' mediating peripheral inhibition to propriospinal neurones during grip and pinching. The resulting more effective background excitation of propriospinal neurones by the peripheral input from hand muscles could contribute to stabilizing the wrist during grip.     

Inhibitory projection from brachioradialis to biceps brachii motoneurones in human

Neural projection from the brachioradialis to the biceps brachii motoneurones in human was studied using the method of post-stimulus time histogram. Electrical stimulation to the radial branch innervating the brachioradialis produced inhibition in 11 out of 21 biceps motor units. The central delays of the inhibition were 0.7–1.2 ms longer than those of the homonymous facilitation. The inhibition was evoked with the intensity below the motor threshold. Pure cutaneous stimulation provoked no effects on the motor-unit firing. These findings suggest that group I afferents from the brachioradials mediate an oligosynaptic inhibition of the biceps brachii motoneurones.     

Long-lasting modulation of human motor cortex following prolonged transcutaneous electrical nerve stimulation (TENS) of forearm muscles: evidence of reciprocal inhibition and facilitation

Several lines of evidence indicate that motor cortex excitability can be modulated by manipulation of afferent inputs, like peripheral electrical stimulation. Most studies in humans mainly dealt with the effects of prolonged low-frequency peripheral nerve stimulation on motor cortical excitability, despite its being known from animal studies that high-frequency stimulation can also result in changes of the cortical excitability. To investigate the possible effects of high-frequency peripheral stimulation on motor cortical excitability we recorded motor-evoked potentials (MEPs) to transcranial magnetic stimulation (TMS) of the left motor cortex from the right flexor carpi radialis (FCR), extensor carpi radialis (ECR), and first dorsal interosseous (FDI) in normal subjects, before and after transcutaneous electrical nerve stimulation (TENS) of 30 min duration applied over the FCR. The amplitude of MEPs from the FRC was significantly reduced from 10 to 35 min after TENS while the amplitude of MEPs from ECR was increased. No effects were observed in the FDI muscle. Indices of peripheral nerve (M-wave) and spinal cord excitability (H waves) did not change throughout the experiment. Electrical stimulation of the lateral antebrachial cutaneous nerve has no significant effect on motor cortex excitability. These findings suggest that TENS of forearm muscles can induce transient reciprocal inhibitory and facilitatory changes in corticomotoneuronal excitability of forearm flexor and extensor muscles lasting several minutes. These changes probably may occur at cortical site and seem to be mainly dependent on stimulation of muscle afferents. These findings might eventually lead to practical applications in rehabilitation, especially in those syndromes in which the excitatory and inhibitory balance between agonist and antagonist is severely impaired, such as spasticity and dystonia.M. Tinazzi and S. Zarattini contributed equally to the work     

Changes of reflex size in upper limbs using wrist splint in hemiplegic patients

We evaluated the effect of prolonged wrist extension on H reflex in the flexor carpi radialis (FCR) muscle and tendon jerk (T) reflex in the biceps brachii (BB) muscle of 17 chronic hemiplegic patients. H reflex of the FCR and T reflex of the BB were assessed every 5 minutes within 20 minutes during prolonged wrist extension and post-20 minutes after the extension. As a result, H reflex in the FCR was reduced by passive wrist stretch in 82% of the spastic limbs. The effect was larger in the higher spastic group. In 45% of the spastic limbs, T reflex in the BB also was reduced by passive wrist stretch. The inhibitory effects had a tendency to strengthen in accordance with the grade of muscle tone. We considered from these results, prolonged wrist extension generated inhibitory projections via probably group II afferents of the FCR in the homonym and in the transjoint in spastic limbs.