Effects of galvanic mastoid stimulation in seated human subjects.

The vestibular responses evoked by transmastoid galvanic stimulation (GS) in the rectified soleus electromyogram (EMG) in freely standing human subjects disappear when seated. However, a GS-induced facilitation of the soleus monosynaptic (H and tendon jerk) reflex has been described in few experiments in subjects lying prone or seated. This study addresses the issue of whether this reflex facilitation while seated is of vestibulospinal origin. GS-induced responses in the soleus (modulation of the rectified ongoing EMG and of the monosynaptic reflexes) were compared in the same normal subjects while freely standing and sitting with back and head support. The polarity-dependent biphasic responses in the free-standing position were replaced by a non-polarity-dependent twofold facilitation while seated. The effects of GS were hardly detectable in the rectified ongoing voluntary EMG activity, weak for the H reflex, but large and constant for the tendon jerk. They were subject to habituation. Anesthesia of the skin beneath the GS electrodes markedly reduced the reflex facilitation, while a similar, although weaker, facilitation of the tendon jerk was observed when GS was replaced with purely cutaneous stimulation, a tap to the tendon of the sternomastoid muscle, or an auditory click. The stimulation polarity independence of the GS-induced reflex facilitation argues strongly against a vestibular response. However, the vestibular afferent volley, insufficient to produce a vestibular reflex response while seated, could summate with the GS-induced tactile or proprioceptive volley to produce a startle-like response responsible for the reflex facilitation.

MRI and FDG PET/CT findings in a case of probable Heidenhain variant Creutzfeldt-Jakob disease.

Creutzfeldt-Jakob disease (CJD) is a neurodegenerative disease caused by the accumulation of a pathogenic isoform of a prion protein in neurons that is responsible for subacute dementia. The Heidenhain variant is an atypical form of CJD in which visual signs are predominant. This is a report of the case of a 65-year-old man with probable CJD of the Heidenhain variant, with topographical concordance between findings on magnetic resonance imaging (MRI) and 18F-fluorodeoxyglucose (FDG) photopenic areas on positron emission tomography (PET)/computed tomography (CT) for cortical parietooccipital lesions.

Substantial improvement of nanotube processability by freeze-drying.

As-produced carbon nanotubes often contain a fraction of impurities such as metal catalysts, inorganic supports, and carbon by-products. These impurities can be partially removed by using acidic dissolution. The resulting nanotube materials have to be dried to form a powder. The processability of nanotubes subjected to regular (thermal vaporisation) drying is particularly difficult because capillary forces pack and stick the nanotubes irreversibly, which limits their dispersability in polymeric matrices or solvents. We show that this dramatic limitation can be circumvented by using freeze-drying instead of regular-drying during nanotube purification process. In this case, the nanotubes are trapped in frozen water which is then sublimated. As a result the final powder is significantly less compact and, more important, the nanotubes can be easily dispersed with no apparent aggregates, thereby greatly enhancing their processability, e.g., they can be used to make homogeneous composites and fibers. Results from coagulation spinning from water-based dispersions of regularly-dried and freeze-dried nanotubes are compared. We also show that freeze-dried materials, in contrast to regularly-dried materials, can be dissolved in organic polar solvents using alkali-doped nanotubes. High resolution TEM and XRD analysis demonstrate that the nanotube structure and quality are not affected at the nanoscale by freeze-drying treatments.

Raman studies of solutions of single-wall carbon nanotube salts.

Polyelectrolyte solutions of Na-doped single-wall carbon nanotube (SWNT) salts are studied by Raman spectroscopy. Their Raman signature is first compared to undoped SWNT suspensions and dry alkali-doped SWNT powders, and the results indicate that the nanotube solutions consist of heavily doped (charged) SWNT. Raman signature of doping is then used to monitor in situ the oxidation reaction of the nanotube salt solutions upon exposure to air and to an acceptor molecule (benzoquinone). The results indicate a direct charge-transfer reaction from the acceptor molecule to the SWNT, leading to their gradual charge neutralization and eventual precipitation in solution. The results are consistent with a simple redox titration process occurring at the thermodynamical equilibrium.

The disynaptic group I inhibition between wrist flexor and extensor muscles revisited in humans.

The present studies are designed to further characterise the interneuronal pathway mediating the disynaptic reciprocal group I inhibition between flexors and extensors at the wrist and the elbow levels in humans. In the first series of experiments, we compared the electrical threshold of the reciprocal group I inhibition at the wrist and the elbow level after a prolonged vibration aimed at raising the electrical threshold of the antagonistic activated Ia afferents. Prolonged vibration to the 'conditioning' tendon, which raised significantly the electrical threshold of the inhibition at the elbow level, did not alter it at the wrist level. These results suggest that the dominant input to the relevant interneurones is Ia in origin at the elbow level but Ib in origin at the wrist level. In the second series of experiments, using the spatial facilitation method, we compared the effects on the post-stimulus time histograms of single voluntarily activated motor units of two volleys delivered both separately and together to group I afferents in the nerves supplying the homonymous and antagonistic muscles. At the wrist, but not at the elbow level, the peak of homonymous monosynaptic group I excitation was reduced on combined stimulation, although the antagonistic IPSP was just at the threshold. Because the suppression did not involve the initial bins of the peak, it is argued that the suppression is not due to presynaptic inhibition of Ia terminals, but probably reflects convergence between the homonymous and antagonistic volleys onto the interneurones mediating the disynaptic inhibition. Taken together with the previously reported effects of recurrent inhibition on reciprocal inhibition, these results suggest that inhibition between flexors and extensors is differently organised at the elbow (reciprocal Ia inhibition) and the wrist (non-reciprocal group I inhibition) levels. It is argued that the particular connectivity at the wrist level might correspond to some functional requirements at this ball joint.

Post-activation depression in various group I spinal pathways in humans.

This investigation was designed to study the effects of post-activation depression in different spinal pathways fed by group I afferents available to investigation in human subjects. It was precipitated by a recent investigation in the cat showing that-contrary to the general assumption-post-activation depression is not a widespread phenomenon in the spinal cord. In 24 healthy subjects comparison was made between the effects of low and high-test stimulus rates on the monosynaptic Ia excitation, known to be subject to post-activation depression, and on oligosynaptic pathways fed by group I afferents. Both the amplitude of monosynaptic H reflexes and the amount of heteronymous monosynaptic Ia facilitation were significantly smaller at high than at low-test stimulus rates (1-2 s compared with 6-8 s between two consecutive stimuli). So was the amount of reciprocal Ia inhibition of tibialis anterior motoneurones. In contrast, the amount of other non-monosynaptic group I effects directed to the same motor nuclei (peroneal-induced excitation of quadriceps motoneurones, disynaptic non-reciprocal group I inhibition of flexor carpi radialis motoneurones, and D1 inhibition of flexor carpi radialis and soleus H reflexes) were enhanced at high stimulus rates. Results in humans confirm that post-activation depression depends on the type of group I afferents, and/or on the target neurones. The functional significance of the discrepancy between post-activation depression in pure Ia pathways and in other group I pathways is discussed with regard to the fusimotor-driven servo-assistance from Ia afferent discharges.

Evidence for recurrent inhibition of reciprocal inhibition from soleus to tibialis anterior in man.

Reciprocal inhibition between ankle flexors and extensors has been the subject of numerous studies in Man. They have demonstrated that this reciprocal inhibition is in all likelihood caused by a disynaptic pathway at least partly fed by Ia afferents. It is thus generally agreed that this reciprocally organized inhibition between ankle flexors and extensors in Man is similar to the reciprocal Ia inhibition described in the cat. This conclusion has, however, been challenged, when Jankowska and McCrea described in the cat a non-reciprocal group I inhibition involving interneurones co-excited by Ia and Ib afferents and mediating inhibition to both antagonistic and non-antagonistic motoneurones. The only way to distinguish between reciprocal Ia inhibition and non-reciprocal group I inhibition is to test if the inhibition is blocked by recurrent inhibition, since only Ia interneurones are inhibited by recurrent inhibition. In the present study, reciprocal inhibition from soleus to tibialis anterior was thus investigated following activation of soleus-coupled Renshaw cells in normal human subjects. It was found that reciprocal inhibition induced in tibialis anterior motoneurones by the activation of soleus group I afferents is deeply depressed by activation of soleus-coupled Renshaw cells. This finding provides the missing data to identify disynaptic inhibition between antagonistic ankle muscles as a reciprocal Ia inhibition.

Metal-insulator and structural phase transition observed by ESR spectroscopy and x-ray diffraction in KC60.

We have performed electron spin resonance (ESR) spectroscopy and x-ray diffraction experiments at low temperature on KC60 single crystals. ESR data reveal the occurrence of a metal-insulator phase transition at about 50 K. In the same temperature range, we observe the stabilization of a superstructure which doubles the volume of the unit cell. We suggest that displacements of the K atoms and a modulation of the C60 charge may be involved in the mechanism of this phase transition. These results shed new light on the subtle interplay of structure, dimensionality, and electronic properties in the AC60 fullerides.

Modulation of presynaptic inhibition of la afferents during voluntary wrist flexion and extension in man.

Changes in presynaptic inhibition of Ia terminals directed to flexor carpi radialis (FCR) motoneurones (MNs) were investigated in normal human subjects at rest and during voluntary wrist flexion and extension. To that end, two independent methods were used: (1) the radial-induced D1 inhibition of the FCR H reflex, which assesses the excitability of PAD (primary afferent depolarisation) interneurones controlling presynaptic inhibition of Ia terminals mediating the afferent volley of the FCR H reflex; and (2) the heteronymous monosynaptic Ia facilitation induced in the FCR H reflex by intrinsic muscle Ia afferent stimulation, which assesses the ongoing presynaptic inhibition of Ia terminals. With respect to results at rest, it was found that at the onset of (and during tonic) voluntary wrist flexion, D1 inhibition was reduced and heteronymous monosynaptic Ia facilitation was increased. This suggests that, as in the lower limb, presynaptic inhibition is decreased on Ia terminals projecting to MNs involved in the voluntary contraction. In contrast with results observed in the lower limb, presynaptic inhibition of Ia terminals to FCR MNs was also found to be reduced at the onset of a voluntary contraction involving the antagonistic wrist extensors, suggesting that presynaptic inhibition of Ia terminals projecting to wrist flexors and extensors might be mediated through the same subsets of PAD interneurones. This is in keeping with other features showing that the organisation of reflex pathways between wrist flexors and extensors differs from that observed at other (elbow, ankle) joints.

Macroscopic fibers and ribbons of oriented carbon nanotubes.

A simple method was used to assemble single-walled carbon nanotubes into indefinitely long ribbons and fibers. The processing consists of dispersing the nanotubes in surfactant solutions, recondensing the nanotubes in the flow of a polymer solution to form a nanotube mesh, and then collating this mesh to a nanotube fiber. Flow-induced alignment may lead to a preferential orientation of the nanotubes in the mesh that has the form of a ribbon. Unlike classical carbon fibers, the nanotube fibers can be strongly bent without breaking. Their obtained elastic modulus is 10 times higher than the modulus of high-quality bucky paper.

Presynaptic inhibition and homosynaptic depression: a comparison between lower and upper limbs in normal human subjects and patients with hemiplegia.

Presynaptic inhibition of Ia terminals and postactivation depression at the Ia fibre-motor neuron (MN) synapses were compared in the upper and lower limbs of both sides in subjects from different populations: 49 spastic patients with hemiplegia [mainly with a lesion in the middle cerebral artery (MCA) area], two tetraplegics and 35 healthy subjects. Presynaptic inhibition was assessed using D1 inhibition of the soleus and the flexor carpi radialis (FCR) H reflexes elicited by electrical stimuli applied to the nerve supplying antagonistic muscles, and postactivation depression was explored by varying the time interval between two consecutive H reflexes. In normal subjects no right-left asymmetry was found in the amount of presynaptic Ia inhibition, homosynaptic depression or the H(max)/M(max) ratio. In the hemiplegic side of patients with MCA area lesions, the H(max)/M(max) ratio was significantly increased in the soleus but not in the FCR. Presynaptic inhibition of Ia terminals, which was significantly reduced at the cervical level on the hemiplegic side (and also, but to a lesser extent, on the unaffected side), was unchanged at the lumbar level. Homosynaptic depression was similarly reduced at the cervical and lumbar levels on the hemiplegic side but not modified on the unaffected side. It is argued that the decrease in presynaptic inhibition of Ia terminals is more a correlate of spasticity than a mechanism underlying it. The decrease in postactivation depression, which very probably contributes to the exaggeration of the stretch reflex characterizing spasticity, might be a consequence of the changes in the pattern of activation of Ia afferents and MNs following the motor impairment.

Novel infinite three-dimensional network of neutral fullerene molecules in (C60)8(twin-TDAS)6

The crystal structure of (C60)8(twin-TDAS)6 [twin-TDAS = C4S6N4 = 3,3',4,4'-tetrathiobis(1,2,5-thiadiazole)] has been redetermined at low temperature in the correct space group [I23, a = 18.849 (1) A]. Structural analysis reveals a novel three-dimensional close-contact network of C60 molecules with tetrahedral holes similar to pristine C60.

Recurrent inhibition between motor nuclei innervating opposing wrist muscles in the human upper limb.

1. Effects of conditioning motor volleys of increasing amplitude on antagonistic motor nuclei were tested at the wrist and elbow level in man. 2. The modifications of biceps, triceps, flexor carpi radialis (FCR) and extensor carpi radialis (ECR) motoneurone excitability were tested both in post-stimulus time histogram (PSTH) and rectified averaged EMG experiments. 3. Conditioning motor volleys were evoked in biceps, triceps, FCR and ECR muscles by electrical stimuli applied to the corresponding nerves. The intensity of the conditioning stimuli was systematically varied from an intensity just below motor threshold to one giving rise to a motor response equal to half of the maximal direct motor response (Mmax). 4. The effect of the injection of a cholinergist agonist, L-acetylcarnitine (L-Ac), on antagonistic conditioning motor volleys was tested for each motor nucleus. 5. Results obtained at the wrist and elbow level were strikingly different. Antagonistic motor volleys resulted in an early and long-lasting inhibition enhanced by L-Ac injection in wrist motor nuclei, while no modification was observed in elbow motor nuclei. 6. The characteristics of the early and long-lasting inhibition evoked in wrist motor nuclei by antagonistic motor volleys suggest that they are due to the activation of Renshaw cells by the conditioning motor volleys. 7. It is therefore concluded that these experiments provide further evidence that the interconnections between motoneurones, interneurones mediating reciprocal inhibition and Renshaw cells at the wrist level are different from those described elsewhere. Indeed, it has already been shown that the interneurones mediating reciprocal inhibition between FCR and ECR are not inhibited by Renshaw cells, and the results presented here suggest that FCR-coupled Renshaw cells inhibit ECR motoneurones and vice versa.

Reciprocal inhibition between wrist flexors and extensors in man: a new set of interneurones?

1. Interneurones mediating reciprocal inhibition between wrist flexors and extensors in man are characterized using both Renshaw cells and transarticular group I afferent activation. 2. Renshaw cells were activated by reflex discharges evoked by a tendon tap. The tendon tap was applied to the tendon of the muscles from which the Ia fibres responsible for the reciprocal inhibition originated. Contrary to what was observed both in the cat hindlimb and in human elbow muscles, this Renshaw cell activation never resulted in a long depression of the reciprocal inhibition between wrist flexors and extensors. 3. Convergence from group I elbow muscle afferents and antagonistic group I afferents onto interneurones mediating reciprocal inhibition between wrist muscles was revealed in post-stimulus time histogram (PSTH) experiments using the technique of spatial facilitation. 4. The characteristics of the interneurones mediating reciprocal inhibition between wrist flexors and extensors could therefore be summarized as follows: (a) they are fed by antagonistic group I afferents and group I afferents originating from both flexor and extensor elbow muscles; (b) they are not inhibited by Renshaw cells; (c) they are not excited by low threshold cutaneous afferents; and (d) they are probably interposed in a disynaptic pathway. 5. It is therefore concluded that interneurones mediating reciprocal inhibition between wrist flexors and extensors in man differ both from Ia interneurones and from interneurones interposed in the Ib reflex pathways and these characteristics are related to the complex circumduction movements developed in the wrist.

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.

Distribution of recurrent inhibition in the human upper limb.

The distribution of homonymous and heteronymous recurrent inhibition among the motor nuclei innervating the main muscles of the human upper limb has been investigated in 25 healthy subjects. Homonymous recurrent inhibition was studied with a specially designed electrophysiological method with paired H-reflexes, previously described by Bussel & Pierrot-Deseilligny (1977), combined with a pharmacological study using a cholinergic agonist, the L-acetylcarnitine (Rossi & Mazzocchio 1991). These methods were used to investigate the Flexor Carpi Radialis (FCR), Extensor Carpi Radialis (ECR), Opponens Pollicis (OP) and Abductor Digiti Minimi (ADM) motor nuclei. In the Deltoid, Triceps, Flexor Carpi Ulnaris (FCU) and Extensor Carpi Ulnaris (ECU) motor nuclei in which it was impossible to evoke clearly distinguishable H-reflexes, homonymous recurrent inhibition was studied with the PSTH technique: homonymous recurrent inhibition was found in Deltoid, Triceps, FCR, FCU, ECR, ECU motor nuclei but not in OP and ADM motor nuclei. Heteronymous recurrent inhibition was studied with the PSTH technique in the Deltoid, Biceps, Triceps, FCR, ECR, FCU, ECU, Flexor Digitorum Communis (FDC), Extensor Digitorum Communis (EDC) motor nuclei as well as those innervating the intrinsic muscles of the hand. The following results were obtained: (1) motor neurones innervating muscles acting at digits do not receive any heteronymous recurrent inhibition; (2) motor neurones innervating muscles acting at the wrist give recurrent inhibition to motor nuclei of proximal muscles but do not receive any recurrent projections from the latter; and (3) motor neurones innervating proximal muscles (acting at shoulder or elbow) are interconnected by recurrent inhibition and receive heteronymous recurrent projections from some wrist muscles but not from intrinsic hand muscles.

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

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.

Pattern of projections of group I afferents from elbow muscles to motoneurones supplying wrist muscles in man.

The pattern of projections of low threshold afferents from triceps and biceps brachii muscles onto motoneurones innervating muscles acting at the wrist was assessed by a reflex and a poststimulus time histogram (psth) technique. Activation of low-threshold afferents originating from elbow flexors or extensors resulted in an early, short-lasting inhibition of wrist flexor motoneurones (flexor carpi radialis, flexor carpi ulnaris). An inhibition was also found in the extensor carpi radialis (ECR) motoneurones after stimulation of low-threshold afferents from triceps. Evidence is presented that Ia fibres contribute to these effects. The inhibitory effects were found in all subjects, but they were constant in only 57% of the reflex experimental sessions and in 25% of the explored motor units. Stimulation of biceps low-threshold afferents was always ineffective on ECR motoneurones. No early facilitation was ever seen in motor nuclei innervating wrist muscles following stimulation of low threshold afferents from biceps and triceps. The pattern of transjoint projections of group I afferents from proximal to distal muscles and from distal to proximal ones (Cavallari and Katz 1989) is discussed in relation to that described in the cat forelimb.

Reciprocal Ia inhibition between elbow flexors and extensors in the human.

1. Reciprocal inhibition between elbow flexor and extensor muscles (biceps and triceps brachii) has been investigated in nine healthy subjects. Two techniques were used to assess changes in motoneurone excitability after stimulation of antagonist muscle afferents: (1) monosynaptic reflexes elicited by a mechanical stimulation of the distal muscle tendon (tendon tap); (2) post-stimulus time histograms (PSTH) of voluntarily activated motor units. 2. Electrical stimulation of the antagonist muscle nerve produced a short-latency and short-lasting inhibition of the flexor and extensor motoneurones. The amount of this inhibition was found to be similar in both motor nuclei. 3. The inhibition could be evoked with conditioning electrical stimuli as low as 0.7 x motor threshold (MT) or by very weak tendon taps applied to the antagonist tendon. In the former case the threshold of this inhibition was found to be consistently increased after raising the threshold of Ia afferent fibres by a long-lasting muscle vibration. Since a contribution from cutaneous afferent fibres was ruled out, it is concluded that this inhibition was Ia in origin. 4. Post-stimulus time histograms of voluntarily activated triceps and biceps motor units were made following electrical stimulation of homonymous and antagonist muscle afferents. This enabled an estimate of the central synaptic delay of the inhibitory process. An average central delay of 0.94 ms in excess of that of monosynaptic facilitation was found, thus suggesting that the inhibitory process could be mediated by only one interneurone. 5. A conditioning reflex discharge elicited in the antagonist muscle by a tendon tap depressed or suppressed this inhibition. This depression was maximal when the reflex discharge was elicited 10-20 ms before the conditioning stimulus for the inhibition and never lasted more than 30 ms. It is argued that the only mechanism compatible with such a depression is the inhibitory activity of Renshaw cells acting on the pathway mediating reciprocal inhibition. 6. We conclude that group Ia afferent fibres from elbow extensor and flexor muscles project monosynaptically onto Ia inhibitory interneurones to mediate disynaptic reciprocal inhibition of antagonist motoneurones.

[Clinical and electrophysiologic study of the peripheral nerve in 28 cases of mitochondrial disease]. / Etude clinique et électrophysiologique du nerf périphérique dans 28 cas de maladie mitochondriale.

Twenty-eight patients with mitochondrial disease were systematically investigated on clinical and electrophysiological grounds for peripheral neuropathy (PN): 25 had predominant ophthalmoplegia (including 4 with Kearns-Sayre syndrome) and 3 had predominant central nervous system involvement. There were 11 men and 17 women, mean age 43 years. Nine of the 28 patients had signs of sensory polyneuropathy involving mainly the lower limbs. These 9 patients and another asymptomatic patient had electrophysiological abnormalities: in the lower limbs, sensory potentials were absent or decreased in amplitude in all cases. In peroneal nerves, motor conduction nerve velocities were decreased in 4/10 cases. These data were consistent with an axonopathy. No correlation was found between the presence of PN and the clinical features of the mitochondrial diseases or with the respiratory chain defect (studied in 14 cases).