Motor cortex stimulation does not reset primary orthostatic tremor.
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Electromyography↗
Biomedical subjects
K R Mills
Publications and source records attributed to K R Mills.
Motor cortex excitability during ballistic forearm and finger movements.
In a ballistic forearm flexion movement, a centrally programmed triphasic pattern of electromyogram (EMG) is seen with two bursts in biceps and a single burst in triceps. Rapid abduction of the index finger, in contrast, is achieved with a single agonist burst. Transcranial magnetic and electrical stimuli, triggered at the onset of the EMG burst, have been used to probe cortical and spinal cord excitability during and after self-paced ballistic finger and forearm movements. The first phase is coincident with the initial agonist burst. The second phase in biceps is associated with the second agonist burst, but in the finger movement, the raised motor cortical excitability is not associated with any EMG. It is argued that the motor program for the two movements may be similar, despite there being large differences in the EMG pattern generated.
Action Potentials↗
Cortical and spinal mechanisms of facilitation to brain stimulation.
Compound muscle action potentials (CMAPs) evoked by magnetic brain stimuli are larger if the subject provides a steady background voluntary contraction of the target muscle. This facilitation could be due either to cortical or spinal mechanisms, or both. Both magnetic and electrical stimuli given immediately after the onset of a ballistic contraction also evoke markedly facilitated CMAPs. By contrast, responses some 200 ms after the onset of such a contraction are facilitated if stimuli are magnetic but not if they are electrical. This second phase of facilitation is largely cortical in origin. By comparing the size of CMAPs evoked by magnetic stimuli at two different delays after electromyogram onset, the total facilitation could be dissected into its spinal and cortical components. The relationship between CMAP area in the first dorsal interosseous and stimulus intensity was different in the two phases of facilitation, suggesting a constant background level of spinal facilitation upon which an increasing descending volley operated. In experiments in which ballistic contractions at increasing force levels were performed it was found that at low force levels, spinal facilitation predominated, but at forces greater than 10% maximum there were roughly equal contributions from increased spinal cord and cortex excitability.
Action Potentials↗
Abnormal excitability of the corticospinal pathway in patients with amyotrophic lateral sclerosis: a single motor unit study using transcranial magnetic stimulation.
The pathophysiology of corticospinal tract degeneration in amyotrophic lateral sclerosis (ALS) was investigated by studying the effect of transcranial magnetic stimulation on discharge characteristics of single motor units during voluntary activation. The motor units were recorded from the first dorsal interosseus muscles of 12 patients with ALS, 14 healthy subjects, 12 patients with upper motor neuron lesions and 9 with pure lower motor neuron diseases. More than 100 magnetic stimuli were delivered over the scalp during minimal muscle contraction. The occurrence of motor unit discharges was plotted in a peristimulus time histogram. An increase in discharge probability at latencies of 20-30 msec, that represents monosynaptic activation (primary peak) was found in normal units. Motor units from ALS patients with short disease durations had significantly increased discharge probabilities in the primary peak (P < 0.001). Motor units from 4 ALS patients with upper motor neuron signs showed double primary peaks: an initial synchronized peak followed by a dispersed peak. The latter was ascribed to a slow corticospinal pathway, which remains undetected or is functionally insignificant in healthy subjects. We conclude that the excitabilities of the surviving corticospinal tract pathways are abnormally increased in ALS, especially in the early stage.
Adolescent↗
Human muscle fatigue investigated by transcranial magnetic stimulation.
Corticospinal motor output was studied during a 1.5 min maximum isometric voluntary contraction of the first dorsal interosseous muscle in five healthy subjects. Compound muscle action potentials (CMAPs) evoked by suprathreshold magnetic cortical stimuli and by supramaximal ulnar nerve stimuli were compared. On average, force fell to 25% of its initial value, CMAPs from nerve stimuli declined to 58% of initial, but CMAPs from cortical stimuli showed no significant change in amplitude after the first 5 s. It is concluded that there is no marked net change in corticospinal output during fatigue. It is postulated that cortical stimuli engage a subgroup of low threshold, fatigue-resistant motor units which remain active throughout the contraction.
Action Potentials↗
Short term synchronization of human motor units and their responses to transcranial magnetic stimulation.
1. The voluntary discharge characteristics, short term synchronization, coherence and responses to transcranial magnetic stimulation of a sample of twenty-two pairs of simultaneously recorded low threshold motor units in the right human first dorsal interosseus muscle have been determined. 2. Peristimulus time histograms and cumulative sums (cusums) of motor unit discharge showed either excitatory or inhibitory responses to cortical stimuli. Over the whole motor unit sample, the primary excitatory response had a mean onset of 24.6 ms and the inhibitory response a mean onset of 31.0 ms. Responses of pairs of motor units to magnetic stimulation could be dissimilar; at some stimulus intensities one of the motor unit pair could be excited by the stimulus whilst the other was inhibited. 3. Most pairs of motor units showed short term synchrony in their voluntary discharge, but the same motor units driven by magnetic cortical stimuli showed little tendency to discharge together more frequently than would be predicted from their independent behaviour. This held true for discharges in both the early primary excitatory peak and in the later secondary peak in peristimulus time histograms. 4. Series of magnetic stimuli, causing either excitation or inhibition of individual tonically active motor units, had no effect on the size of the central peak of the cross-correlogram of the motor unit pair. However, frequency analysis of pairs of motor unit spike trains showed an increase in coherence in the 16-32 Hz band during magnetic stimulation. 5. It is concluded that, whilst many corticospinal fibres branch extensively within motoneurone pools, there are also fast conducting corticomotoneuronal fibres allowing the independent monosynaptic activation of individual motoneurones. 6. It is also suggested that during sustained voluntary tonic activity, magnetic stimuli applied to the brain can increase the synchrony in common input fibres to pairs of motoneurones.
Adult↗
Motor neuron disease. Studies of the corticospinal excitation of single motor neurons by magnetic brain stimulation.
The responses to magnetic brain stimulation of 57 motor units (MUs) in the first dorsal interosseous muscle of 33 patients with sporadic motor neuron disease (MND), and 44 MUs of 15 healthy control subjects have been compared. Thresholds for MU excitation both in the resting state and during tonic activity have been measured and the modulation of MU discharge by series of magnetic stimuli has been determined from peristimulus time histograms (PSTHs). Abnormalities in the MND MUs have been correlated with the physical signs of impaired hand muscle function. Motor units from hands showing pure lower motor neuron (LMN) signs fired at increased frequency when recruited, had normal or reduced thresholds for activation by magnetic scalp stimuli and had normal or large primary excitatory peaks with normal inter-subpeak intervals on PSTHs. Occasionally muscles with pure LMN signs also had fasciculations that could be driven by cortical stimuli. In contrast, MUs from hands showing pure upper motor neuron (UMN) signs fired at reduced frequency when recruited, had raised thresholds for excitation by cortical stimuli, had prolonged primary peaks with wide inter-subpeak intervals and had large secondary peaks on PSTHs. The rise times and amplitudes of the underlying post-synaptic potentials released by cortical stimuli in spinal motor neurons have been deduced from measurements of the peaks in PSTHs. This analysis suggests that in some MUs from muscles showing pure LMN signs, stimuli evoke larger than normal excitatory post-synaptic potentials (EPSPs), probably resulting from abnormalities in central motor fibres. It is proposed that a major cause of the changes found in MND MUs from hands showing either pure UMN or pure LMN signs may be an abnormality in the motor cortex leading to faulty initiation of the descending corticospinal volley from magnetic stimuli.
Adult↗
Suppression and long latency excitation of single spinal motoneurons by transcranial magnetic stimulation in health, multiple sclerosis, and stroke.
Whether or not suppression at the level of the spinal motoneuron plays a role in motor deficits such as central paresis is unknown. In this study suppression in the firing of tonically active low threshold single motoneurons following low intensity transcranial magnetic stimulation is described in health and disease. Changes in firing probability in the absence of an early excitatory response were studied in a total of 14 motor units from 4 healthy subjects, 5 patients with multiple sclerosis, and 1 patient with stroke. Firing probability began to fall 18-59 ms after the stimulus and remained low for a period of 27-133 ms. There were no obvious differences between the three subject groups. The change in firing probability was not associated with specific physical signs. Late rises in firing probability were seen in 7 of the 14 motor units at latencies that were similar to the secondary peak which is known to occur with higher stimulus intensities. It is argued that the mechanism of partial suppression is not dependent on the full integrity of the pyramidal tract and is likely to involve a transient withdrawal of descending excitatory drive rather than an inhibitory postsynaptic potential at the spinal motoneuron.
Adult↗
A clinical, serological and neurophysiological study of restless legs syndrome in rheumatoid arthritis.
The restless legs syndrome (RLS), defined by Gibb and Lees criteria, was investigated in patients with RA. RLS symptoms were more frequent in RA patients (25%) than in non-RA controls with OA or seronegative arthropathy (4%). RLS was significantly more common in females. Judged by a variety of clinical and laboratory indices, RA disease severity and current disease activity were greater in patients with RLS than in RA patients unaffected by RLS. In five out of 14 RLS patients undergoing neurophysiological study, delays in the P40 component of posterior tibial somatosensory evoked potentials (SSEPs) were observed suggesting the existence of myelopathy, whilst in another four RLS patients evidence of peripheral neuropathy was found. Though the higher frequency of neurophysiological abnormalities in RA patients with RLS was not statistically significant, possibly because of the small numbers of patients studied, these data suggest that RLS symptoms in RA may reflect the presence of neurological disorder.
Aged↗
Pure motor demyelinating neuropathy: deterioration after steroid treatment and improvement with intravenous immunoglobulin.
Within one month of starting oral prednisolone treatment weakness unexpectedly increased in four patients aged 34 to 75 years with purely motor forms of acquired chronic demyelinating neuropathy. By contrast, steroids produced the expected improvement in 11 other patients with symmetric sensorimotor chronic inflammatory demyelinating polyneuropathy. Two of the patients with purely motor demyelinating neuropathy were subsequently treated with high dose IVIg (0.4 g/kg/day for five days) with prompt improvements in strength measurements and motor nerve conduction. Thus IVIg seems to be the treatment of choice and steroids should be used with extreme caution, if at all, in patients with purely motor forms of acquired demyelinating polyneuropathy.
Adolescent↗
The optimal current direction for excitation of human cervical motor roots with a double coil magnetic stimulator.
Upper limb muscles can easily be excited using magnetic stimulation with a circular coil over the cervical vertebrae. Double coil magnetic stimulators generate a more focal and directed form of stimulus and may offer an advantage in the more specific stimulation of individual motor roots. The aims of this study were to determine the optimal rostro-caudal locations and orientations of a double coil magnetic stimulator for excitation of the human 5th cervical to 1st thoracic motor roots. Compound muscle action potentials (CMAPs) evoked by magnetic stimuli were recorded from deltoid, brachioradialis, extensor digitorum communis and first dorsal interosseus muscles. Optimal rostro-caudal location was determined with the coil current flowing horizontally towards the midline. Optimal orientation was determined by rotating the coil in increments of 22.5 degrees about the optimal rostro-caudal location. In all subjects with all muscles examined, the best orientation was with the coil current flowing downwards towards the midline at a mean angle between 14.9 degrees and 27.6 degrees to the horizontal. Review of the anatomy of motor roots reveals a short upwardly directed segment within the intervertebral foramen and it is postulated that excitation occurs exclusively within this segment.
Action Potentials↗
Immunological associations of acquired neuromyotonia (Isaacs' syndrome). Report of five cases and literature review.
Neuromyotonia is a syndrome of spontaneously occurring muscle activity of peripheral nerve origin, which can be triggered by voluntary or induced muscle contraction. It is one among several causes of visible myokymia. Although neuromyotonia may sometimes accompany hereditary neuropathies, most cases are acquired. The abnormal activity is characterized electromyographically by doublet, triplet or multiplet single unit discharges that have a high intraburst frequency, the frequency of the bursts themselves being irregular. Fibrillation potentials and fasciculations are often also present. We report five patients meeting these criteria. Clinical evidence suggesting a possible autoimmune aetiology included the presence of oligoclonal bands in the spinal fluid of the three patients examined and improvement following plasma exchange in two of three patients treated. Other supporting evidence from an English language literature review of approximately 40 patients reported in the last 20 years includes an associated thymoma in five cases, myasthenia gravis (two cases), raised anti-acetylcholine receptor antibody titres (two cases) and induction by penicillamine (one case). Finally, two further patients have recently been reported to benefit from plasma exchange. These clinical data, taken together with the physiological changes observed in mice injected with patients' immunoglobulin G (reported separately), suggest that antibody-mediated autoimmune mechanisms, possibly directed to peripheral nerve K+ channels, may be important in the aetiology of acquired neuromyotonia.
Adult↗
Acute myopathy associated with large parenteral dose of corticosteroid in myasthenia gravis.
A 13 year old Greek girl with myasthenia gravis developed widespread muscle paralysis and atrophy after large parenteral doses of corticosteroids (5.48 g methylprednisolone). An electromyogram showed myopathy, creatine kinase concentration below normal, and a muscle biopsy showed severe myopathy with selective loss of the thick filaments (myosin). Previous reports of myopathy associated with large steroid doses have mostly been in patients who were also receiving non-depolarising neuromuscular blocking drugs. This patient is unique in that severe myopathy was associated with neuromuscular blockade caused by antibodies to acetylcholine receptors. The findings in this case suggest that high doses of parenteral corticosteroids in patients with myasthenia gravis may be dangerous and that blocking the neuromuscular junction with drugs or antibodies predisposes skeletal muscles to the injurious effects of corticosteroids.
Adolescent↗
Comparative morphology of sulfur mustard effects in the hairless guinea pig and a human skin equivalent.
A commercially available human skin equivalent (HSE) was used as an in vitro organotypic skin model to study temporal morphological effects of sulfur mustard gas (HD). Light and electron microscopic analyses of the HD-human skin equivalent model (HD-HSE) were compared to the HD-hairless guinea pig model (HD-HGP). HSE samples were exposed to 10 microliters HD vapor for 8 min and harvested at selected times up to 24 h. Skin sites of HGP were exposed to the same vapor dose or to 2.0 microliters liquid HD for 30 min and collected at 12 and 24 h. In both models, basal cells of the stratum germinativum were selectively affected. The HD-HSE study revealed that basal cell changes began 3 to 6 h following exposure. These early cellular changes included an acantholysis of some basal cells with widening of intercellular spaces, disruption of desmosomal attachments, nuclear pyknosis, perinuclear blebbing and repositioning of cytoplasmic tonofilaments to a perinuclear position. At 12 to 24 h, basal cell pathology progressed to diffuse swelling of endoplasmic reticula, cytoplasmic vacuolations and necrosis which now extended to supra basal cell layers. Comparing basement membrane zone effects, HD-HGP consistently developed characteristic microblisters at the dermal-epidermal junction; however, HD-HSE with its absence of a morphologically distinguishable basement membrane did not. Instead, cellular fragments, granules and debris accumulated early in this area to thicken regions usually assigned to the lamina lucida and lamina densa of a true basement membrane leading to complete separation of dermis from epidermis at later time periods.
Animals↗
Magnetic brain stimulation with a double coil: the importance of coil orientation.
The human motor cortex can be excited by currents induced by a transient magnetic field generated in a coil over the scalp. A 9 cm mean diameter circular coil centered at the vertex is optimally placed for exciting the hand area. Anticlockwise current flow in the coil preferentially excites the left hemisphere and vice versa. A double coil has been used to investigate the orientation of inducing currents at which activation of cortical neural elements is maximal. The inducing current flowed in the same direction in the central segment of the coil and followed a monophasic wave form. The coil was rotated through 360 degrees over the motor area in increments of 45 degrees and compound muscle action potentials from the first dorsal interosseous muscle were recorded. The largest responses were obtained with the coil at about 50 degrees to the parasagittal plane with a backward flowing inducing current. The optimal angle did not depend on stimulus intensity or background voluntary contraction. This orientation corresponds to an maximal induced current flowing forwards approximately at right angles to the central sulcus. It is postulated that horizontal neural elements are aligned in this direction and are preferentially excited by these monophasic magnetic stimuli. The results have important implications for mapping the motor areas with magnetic stimulators.
Action Potentials↗
Suppression of motor neuron firing by transcranial magnetic stimulation in a patient with multiple sclerosis.
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Biofeedback, Psychology↗
Type 2 fiber predominance in Lambert-Eaton myasthenic syndrome.
Serial muscle biopsies in a noncarcinomatous case of Lambert-Eaton myasthenic syndrome (LEMS) have shown progressive atrophy and loss of type 1 fibers, resulting in overwhelming type 2 predominance. A similar abnormality was found in a single biopsy from a second case of LEMS without associated carcinoma. Review of the literature suggests that type 2 fiber predominance has been observed in at least one other biopsied case. Interference with transmitter release caused by anti-voltage-gated calcium channel antibodies may deprive type 1 muscle fibers of the low frequency discharge necessary to maintain their metabolic properties.
Adolescent↗
Magnetic brain stimulation: a tool to explore the action of the motor cortex on single human spinal motoneurones.
The human brain can be stimulated by a single intense magnetic pulse over the scalp. Currents induced within the cranium excite the motor cortex and cause limb muscles to contract. The discharge of single motor units, the firing of which is maintained by voluntary effort, can be modulated by magnetic stimuli. Peri-stimulus time histograms suggest that after a cortical stimulus spinal motoneurones are induced to fire by a sequence of EPSPs arising from a train of impulses transmitted monosynaptically over fast-conducting corticospinal fibres. In multiple sclerosis both dispersion of this descending volley and partial transmission failure can impair motoneurone excitation and may explain motor symptoms in these patients.
Brain↗