Left Abductor Digiti Minimi Research Articles

Mirror movements – A simple algorithm for mirror activity signal processing and normative values

Mirror activity is an involuntary activation of a muscle when the respective contralateral muscle is contracting. This phenomenon has been described primarily in children and in disease states, and, more recently, also in healthy adults. Different ways of assessing mirror activity have been described.In this work we propose a simple protocol for quantifying the amount of mirror activity during a brief isolated full force isometric contraction of a given muscle. The signal was analyzed by a custom-built algorithm that detects the beginning and the end of muscle contraction. The amount of EMG signal on the mirror muscle in relation to the amount of EMG signal of the active muscle is then calculated.We studied 57 right-handed healthy subjects. Mirror activity was evaluated in the Abductor digiti minimi (ADM) and Tibialis anterior (TA) muscles during a 2–3 s full force isometric contraction. The intensity of mirror movement was represented as a percentage of the signal from maximal voluntary contraction.The performance of the algorithm for the detection of the beginning of muscle contraction was very good, when compared to 2 human operators. Intraclass correlation coefficient was excellent (0.998). The Bland-Altman plots showed similar performances of the algorithm and the human operators.We found a significant correlation of mirror activity with intensity and age. There was significantly more intense mirror activity in the left limbs (non-dominant) when compared to the right limbs.The upper limits of normality for mirror EMG signal was 27.4% for right ADM, 15.4% for left ADM, 10.4% for right TA and 2.1% for left TA.This simple protocol allows for an objective measurement of the amount of mirror activity. We propose this technique for investigation of neurological disorders.

Non-local muscle fatigue is mediated at spinal and supraspinal levels.

The objective was to measure the corticospinal excitability and motoneuron responsiveness of the right and left Biceps Brachii (BB), and left Abductor Digiti Minimi (ADM) muscles in response to submaximal isotonic fatiguing contractions performed by the right BB muscle. With the familiarization session, ten young moderately active male subjects came to the lab on seven occasions. Three sets of 3min seated elbow curls at 25% of one-repetition maximum (1RM) separated by a 1-min rest performed by the right BB muscle were used as the fatiguing protocol. The motor evoked potential (MEP), cervicomedullary motor evoked potential (CMEP), and compound muscle action potential (Mmax) of the right BB muscle (baseline and after each set of the fatiguing task), the left BB and ADM muscles (baseline, post-fatigue, post-10, and post-20min) were measured. MEP and CMEP were then normalized to Mmax for statistical analysis. The results showed that in the right BB muscle, there was a significant reduction in the MEP after performing the fatiguing task (p= 0.03), while no significant effect of time was seen in the CMEP (p= 0.07). In the left BB muscle, the MEP significantly decreased from pre-fatigue to post-fatigue (p= 0.01) and post-10 (p= 0.001), while there was a significant decline in the CMEP post-fatigue (p= 0.03). In the left ADM muscle, MEP significantly decreased post-fatigue (p= 0.03) and no changes were seen in the CMEP (p= 0.12). These results not only confirm the incidence of non-local muscle fatigue (NLMF) in response to performing submaximal isotonic fatiguing contractions but also as a new finding, imply that both spinal and supraspinal modulations account for the NLMF response.

Thenar Muscle Motor Imagery Increases Spinal Motor Neuron Excitability of the Abductor Digiti Minimi Muscle.

When a person attempts intended finger movements, unintended finger movement also occur, a phenomenon called “enslaving”. Given that motor imagery (MI) and motor execution (ME) share a common neural foundation, we hypothesized that the enslaving effect on the spinal motor neuron excitability occurs during MI. To investigate this hypothesis, electromyography (EMG) and F-wave analysis were conducted in 11 healthy male volunteers. Initially, the EMG activity of the left abductor digiti minimi (ADM) muscle during isometric opposition pinch movement by the left thumb and index finger at 50% maximal effort was compared with EMG activity during the Rest condition. Next, the F-wave and background EMG recordings were performed under the Rest condition, followed by the MI condition. Specifically, in the Rest condition, subjects maintained relaxation. In the MI condition, they imagined isometric left thenar muscle activity at 50% maximal voluntary contraction (MVC). During ME, ADM muscle activity was confirmed. During the MI condition, both F-wave persistence and the F-wave/M-wave amplitude ratio obtained from the ADM muscle were significantly increased compared with that obtained during the Rest condition. No difference was observed in the background EMG between the Rest and MI conditions. These results suggest that MI of isometric intended finger muscle activity at 50% MVC facilitates spinal motor neuron excitability corresponding to unintended finger muscle. Furthermore, MI may induce similar modulation of spinal motor neuron excitability as actual movement.

101 Amyotrophic Lateral Sclerosis (ALS) - Not Just a Motor Disease? Isolated Bitter and Sweet Taste Loss in ALS

Specific taste quality deficits in ALS has not heretofore been described. Case Study: A 71 year old right handed female presented with a two year course of progressive reduction in strength in her hands, arms and legs with difficulty tying shoe laces, opening jars, writing and walking. She described nocturnal muscle spasms involving all extremities. Gradually, over eight months prior to presentation, all food began to taste bad and horribly bitter. Associated with no appetite and a seven pounds weight loss. Abnormalities in Neurological examination: Cranial Nerve (CN) examination: CN IX and X: Gag absent bilaterally. Motor examination: Bulk: atrophy in thenar and hypothenar eminences and intrinsics in both upper extremities. Percussion induced fasciculation and myotonia in both shoulders and arms. Fasciculation of tongue with percussion myotonia of tongue. Strength: Intrinsic 4/5 in both upper extremities, 3/5 in abductor policis brevis bilaterally, 3/5 right gastrocnemius soleus, 4/5 bilateral anterior tibialis. Drift testing: left abductor digiti minimi sign. Gait: Heel and toe walking unstable with circumduction of left leg. Tandem gait unstable. Cerebellar: Holmes rebound phenomena positive in the left upper extremity. Deep tendon reflexes: 1+ left brachioradialis. 1+ left triceps. 3+ right ankle jerks. 0 left ankle jerk. Positive jaw jerk. Chemosensory Testing: Normosmia to: Alcohol Sniff Test (46), Pocket Smell Test (3/3) and Retronasal Smell Index (9). Taste Quadrant Testing: ageusia in the palate to sodium chloride and citric acid. Ageusia throughout the palate, tongue and whole mouth to sucrose and quinine hydrochloride. Fungiform papillae count: left 18, right 20 (normal). Lip biopsy (normal). MRI: T2 flair in bilateral corticospinal tracts, left greater than right in the spinal cord and the brain. EMG: fibrillation, positive waves with fasciculation in all four extremities. Voluntary contraction with polyphasic unstable motor unit action potentials. While Lang found no taste loss in ALS (Lang, 2011), Pelletier found reduction in intensity of taste to all modalities in different sectors of the tongue, but paradoxically demonstrated normogeusia in whole mouth taste perception (Pelletier, 2013). Pathological specimens of those with ALS revealed degeneration in the nucleus parabrachialis medialis and tractus trigeminothalamicus dorsalis (Oyanagi, 2015), suggesting that taste deficit may be due to central white matter abnormalities. Sweet taste is localized in the most posterior and rostral aspect of the right insular cortex, immediately adjacent to bitter (Prinster, 2017), suggesting a neighborhood effect phenomena. Weight loss in ALS may be due to sensory distortion and secondary impairment of appetite. It would be worthwhile to investigate those with ALS for evidence of otherwise overlooked gustatory deficits, correction of which may improve appetite and nutritional state.

T122. Does TMS primarily target premotor or motor cortex in precentral gyrus?

Introduction Transcranial magnetic stimulation (TMS) can be used to map the corticomotor representations of hand muscles in the precentral gyrus. The spatial peak of the corticomotor representations is often not located in the primary motor hand area (M1HAND), but shows an anterior shift towards the caudal part of dorsal premotor cortex (PMd). Here we used magnetic resonance imaging (MRI) to test the hypothesis that the “hand knob” of the precentral gyrus shows different structural properties in individuals with a clear “premotor” representation compared to individuals with a preponderant “primary-motor” representation of hand muscles. Methods Twenty-four volunteers (mean age: 24.3 ± 0.9 SE, 12 women) underwent whole-brain structural MRI and sulcus-shape based TMS mapping of the right precentral gyrus. High-resolution T1-weighted MRIs were used for neuronavigation of TMS and to calculate regional cortical thickness of the precentral gyrus. We also performed quantitative multiparameter mapping of the longitudinal relaxation rate (R1 = 1/T1) as an index of cortical myelination and simulated the electric field strength induced by TMS at motor threshold (—E—) in the hand-knob region (SimNIBS software). Sulcus-shape based TMS mapping was used to obtain mediolateral and posterior-anterior corticomotor excitability profiles of the left abductor digiti minimi and first dorsal interosseus muscles. Results In 14 out of 24 individuals (58%), TMS mapping disclosed a clear spatial peak in the PMd (referred to as “PMd” subjects”), whereas the remaining 10 subjects (42%) showed maximal motor responses more posteriorly in M1HAND (referred to as M1HAND subjects). Motor threshold was not different between the two groups (p = 0.265). Compared “PMd” subjects, “M1” subjects displayed a higher electric field strength in the very rostral part of the M1HAND in the transition zone from the wall to the crown of the precentral gyrus (p = 0.01). In addition, “PMd” subjects showed an overall increase of regional myelination in the precentral gyrus relative to the “M1” subjects (mixed model ANOVA: F(1,18) = 5.362, p = 0.033). No difference between two groups was evident for curvature and cortical thickness. Conclusion Differences in location of the “motor hot spot” are associated with differences in microstructure in the precentral gyrus and the magnitude of the induced electrical field. The results support the notion of two distinct functional and microstructural phenotypes of corticomotor hand representations in human precentral gyrus. They also underscore the usefulness of electrical field modelling to account for inter-individual differences in the biophysiological effects of TMS in the human brain.

Long-term progressive motor skill training enhances corticospinal excitability for the ipsilateral hemisphere and motor performance of the untrained hand.

It is well established that unilateral motor practice can lead to increased performance in the opposite non-trained hand. Here, we test the hypothesis that progressively increasing task difficulty during long-term skill training with the dominant right hand increase performance and corticomotor excitability of the left non-trained hand. Subjects practiced a visuomotor tracking task engaging right digit V for 6weeks with either progressively increasing task difficulty (PT) or no progression (NPT). Corticospinal excitability (CSE) was evaluated from the resting motor threshold (rMT) and recruitment curve parameters following application of transcranial magnetic stimulation (TMS) to the ipsilateral primary motor cortex (iM1) hotspot of the left abductor digiti minimi muscle (ADM). PT led to significant improvements in left-hand motor performance immediately after 6weeks of training (63±18%, P<0.001) and 8days later (76±14%, P<0.001). In addition, PT led to better task performance compared to NPT (19±15%, P=0.024 and 27±15%, P=0.016). Following the initial training session, CSE increased across all subjects. After 6weeks of training and 8days later, only PT was accompanied by increased CSE demonstrated by a left and upwards shift in the recruitment curves, e.g. indicated by increased MEPmax (P=0.012). Eight days after training similar effects were observed, but 14months later motor performance and CSE were similar between groups. We suggest that progressively adjusting demands for timing and accuracy to individual proficiency promotes motor skill learning and drives the iM1-CSE resulting in enhanced performance of the non-trained hand. The results underline the importance of increasing task difficulty progressively and individually in skill learning and rehabilitation training.

Long-term plasticity may be manifested as reduction or expansion of cortical representations of actively used muscles in motor skill specialists

Our aim was to study long-term plasticity in the organization of cortical muscle representations due to extensive motor training for different skills. We were especially interested in whether skill-specific demands on independent hand muscle movements and synchronous leg muscle movements are reflected differently in the reorganization of muscle representations. We used navigated transcranial magnetic stimulation to estimate the size of cortical representations of opponens pollicis, abductor digiti minimi, and tibialis anterior muscles in five string instrument players, five figure skaters, and five controls. The extent of the representation area was presented as an amplitude-area curve showing the spatial distribution of motor evoked potentials. The size of representation areas was compared between the dominant and nondominant hemispheres and between the groups. The representation area of the left abductor digiti minimi (critical for reaching right tones) in the right, nondominant hemisphere was smaller in string players and the representation area of the tibialis anterior in the dominant hemisphere (critical for jumps) was larger in figure skaters when compared with controls. Reorganization in the motor cortex may differ depending upon the skill and an individual muscle's role in the skill. A smaller representation area of the independently used hand muscle in masters of fine motor skills may reflect long-term plasticity toward more focused representation, which may be beneficial in accurate and discrete cortical control of the muscle. Larger cortical representations are related to skill demanding coactivation of proximal and distal lower limb muscles.

Unilateral contractions modulate interhemispheric inhibition most strongly and most adaptively in the homologous muscle of the contralateral limb

We investigated how volitional contractions affect interhemispheric inhibition (IHI) from the active to the passive hemisphere. Younger and older adults isometrically contracted their dominant thumb (abductor pollicis brevis, APB) to various force targets. In ballistic contraction trials, transcranial magnetic stimulation (TMS) was administered very shortly after the onset of APB activity. In tonic contraction trials, TMS was delivered while the target force was maintained. In control trials both thumbs remained quiescent. In all trials, a test stimulus (TS) was directed to the APB hotspot in the non-dominant hemisphere (130% left APB resting motor threshold, RMT). In half the trials, a conditioning stimulus (130% right APB RMT) was applied to the APB hotspot in the dominant hemisphere 10 ms prior to the TS. Targeted ballistic contractions of the right APB were found to modulate (increase) IHI measured in the left APB, as previously reported for tonic contractions. Furthermore, the extent of the IHI increase was found to scale with the strength of the contralateral ballistic or tonic contraction. Less pronounced, but statistically significant, IHI increases were also observed in the left abductor digiti minimi and extensor carpi radialis during right APB contraction. For these muscles, however, the extent of the IHI modulation was independent of APB contraction strength. The capacity to modulate inhibition during contractions was unaffected by advancing age. During volitional actions, the ability to modulate IHI most adaptively in the homologous muscle of the resting limb may contribute to the prevention of mirror movements.

Poster 80: Proximal Ulnar Neuropathy Around Mid-arm Combined With Martin-Gruber Anastomosis: A Case Report

Setting: Secondary care hospital. Patient: A 65-year-old man with ulnar neuropathy (UN) around mid-arm combined with Martin-Gruber anstomosis. Case Description: The patient presented with the left fourth and fifth finger tingling sensation, which had developed after falling down in a drunken state 4 months earlier. Touch and pin-prick sensations were decreased on the left ulnar and dorsal ulnar sensory nerve distribution. According to the Medical Research Council, the left abductor digiti minimi (ADM) and first dorsal interosseous (FDI) muscles were grade 2, and wrist flexion and distal interphalangeal joint flexion of fourth and fifth fingers were grade 3.

RTMS of Motor Cortex Diminishes Gains in Maximal Voluntary Force

Repetitive transcranial magnetic stimulation (rTMS) of the human primary motor cortex (M1) at 1 Hz interferes with the acute consolidation of motor memory without affecting motor performance (Muellbacher et al. Nature 415:640, 2002). It is however unclear if the human M1 plays a role in chronic adaptations associated with exercise intended to increase maximal voluntary force (MVC). PURPOSE: To determine if 1 Hz rTMS of the human M1 affects gains in maximal voluntary force following strength training. METHODS: Healthy volunteers performed 5 bouts of 10 repetitions of index finger abduction at an intensity of 70% MVC in each of the 10 sessions over 3 weeks (VOL, n = 13). Subjects in VOL+rTMS (n = 8) also exercised but in the 1-minute-long inter-bout rest interval received 1 Hz rTMS at 120% of the resting motor threshold (rMT) targeting the M1 area of the first dorsal interosseus (FDI) (60 pulses, 5 times in a session for 10 sessions, 3,000 pulses total). The rTMS group received only 5 bouts of rTMS in 10 sessions and did not exercise (n = 8). VOL+sham RTMs (n = 6) exercised as did VOL and received sham rTMS to M1 from a coil turned perpendicular relative to M1. RESULTS: The Group by Time interaction was significant (F = 7.2, p = 0.0001) and the Tukey post-hoc contrast revealed that VOL and VOL+sham rTMS, respectively, increased MVC 39% (±19 SD) and 33% (±16). These increases were more (p < 0.05) than the 19% (±15) in VOL+rTMS, the 2% (±10) in rTMS. MVC did not change in a control muscle, the left abductor digiti minimi. rMT remained stable. There were no side effects due to chronic rTMS in these healthy volunteers. CONCLUSION: The human motor cortex may play a role in mediating neural adaptations to strength training.

Corticospinal tract conduction block results in the prolongation of central motor conduction time in compressive cervical myelopathy

Objective The objective of this study was to analyze corticospinal function in patients with compressive cervical myelopathy and to elucidate the mechanism underlying its prolonged central motor conduction time (CMCT). Methods Motor evoked potentials following transcranial magnetic stimulation (TMS) and peripheral conduction time in the ulnar and tibial nerves following electrical stimulation were measured from the abductor digiti minimi (ADM) and abductor hallucis (AH) muscles in 24 patients with compressive cervical myelopathy and used to calculate CMCT. Spinal cord evoked potentials (SCEPs) following transcranial electric stimulation were recorded intraoperatively from the C2-3 to C6-7 intervertebral levels. Correlations between prolonged CMCT and SCEP values were then estimated. Results The shorter/longer CMCT between the patients' right and left ADM and AH were 8.5±2.9/11.5±3.3 and 16.2±3.1/18.4±3.3 ms, respectively (mean±SD). The SCEPs amplitude at C6-7, compared to C2-3, was 25.7±21.0%. The attenuation of SCEP amplitude, but not latency, at C6-7 correlated significantly with CMCT prolongation. Conclusions Our data support the view that CMCT prolongation is primarily due to corticospinal conduction block, rather than conduction delay. Significance Insight was provided into the mechanism of corticospinal dysfunction in compressive cervical myelopathy.

Time-related changes of excitability of the human motor system contingent upon immobilisation of the ring and little fingers

Objectives: To examine possible changes of excitability of the human motor system contingent upon immobilisation of two hand fingers. Methods: Two series of 5 transcranial magnetic stimulation (TMS) sessions were carried out on different days (1, 2, 3, 4, and 7). In one series (fingers immobilised, FI), subjects wore for 4 days a device that kept immobilised the left fourth and fifth finger. In the other series (fingers free, FF), no constraining device was used. Focal TMS was applied over the right motor cortex and motor evoked potentials (MEPs) were recorded from left abductor digiti minimi (immobilised) and first dorsal interosseus (non-immobilised) muscles. Intensities of 10, 30, and 50% above the resting motor threshold (rMT), were used. Results: In FI series, rMT for both muscles showed significant increase on days 3, 4, and 7 with respect to day 1. At high stimulation intensity a clear decrease of MEPs amplitude was observed on days 3 and 4 for both muscles. Since no time-related changes of peripheral (M-wave) and spinal (F-wave) excitability were noted, MEPs and rMT changes are likely to have a cortical origin. In FF series, no changes of excitability were detected. Conclusions: Sensorimotor restriction of two fingers induces an early decrease of excitability, possibly at cortical level, which involves not only the immobilised muscle but also muscles with purportedly overlapping neural representations.

Inhibition of motor system excitability at cortical and spinal level by tonic muscle pain

Objective: To assess whether the motor system excitability can be modified by experimental tonic pain induced either in muscles or in subcutis. Methods: Transcranial magnetic stimulation of the left primary motor cortex was used to record motor evoked potentials (MEPs) from the right abductor digiti minimi (ADM) muscle. Recordings were made before, during and after experimental pain induced by (1) injection of hypertonic (5%) saline into the right ADM, the right first dorsal interosseum (FDI) and the left ADM muscles, and (2) injection of hypertonic saline in the subcutaneous region of the right ADM. Both MEPs and H-reflex were recorded also from the right flexor carpi radialis (FCR) before, during and after muscle pain. Results: MEPs recorded from the ADM muscle were significantly reduced in amplitude during pain induced in the right ADM and right FDI muscles, but not during pain in the left ADM muscle or during subcutaneous pain. This inhibitory effect was observed during the peak-pain and persisted also after the disappearance of the pain sensation. In the FCR muscle, the MEP inhibition was observed during the peak-pain, while a significant reduction of the H-reflex's amplitude was observed starting 1 min after the peak-pain. Conclusions: Tonic muscle pain can inhibit the motor system. The motor cortex inhibition observed at an early phase is followed by a reduction of the excitability of both cortical and spinal motoneurones.

Effects of a competitive stressor on motor cortex excitability: a pilot study

Several intervening variables may influence motor evoked potentials (MEP) from transcranial magnetic stimulation (TMS). Among the most common are motor (e.g. executed or intended muscle contraction) and sensory factors. However, little is known about the effects of a well defined stressor on MEPs and thus on central motor control. We studied 11 healthy right-handed volunteers (five men and six women), aged 21-32 years (mean 25.2 years. SD = 3.5). All subjects underwent a 20 min rest and a 20 min stress period (sequence-controlled, cross-over design). The stressor employed a competitive videogame. MEPs from the left abductor digiti minimi muscle were obtained immediately after rest and stress. We found that six (54.5 per cent) subjects presented a significant increase (p < 0.05) of MEP amplitudes from rest to stress, three (27.3 per cent; p < 0.05) decreased, and two (18.2 per cent) did not change, F- and M-waves did not change significantly. Psychological testing (state and trait anxiety inventory, STAI) revealed that individuals who presented an increase in MEP latency scored lower in state and trait anxiety than others, probably mdicating a lower vulnerability to stress. We believe that the observed increase in MEP amplitudes, combined with a decrease in MEP latency, can be regarded as neurophysiological evidence of stress-induced facilitation related to higher excitability of the corticospinal system and/or presynaptic neurons.

Patterns of motor control reorganization in a patient with mirror movements

Objective: To explore motor control reorganization in a 40-year-old, left-handed patient with perinatally acquired mirror movements. Methods: We performed simultaneous bilateral recordings of motor evoked potentials (MEPs) following focal transcranial magnetic stimulation (fTMS) and of central silent period (cSP) during unilateral voluntary contraction in abductor pollicis brevis (APB) and abductor digiti minimi (ADM) muscles. Results: For both muscles the MEP study showed bilateral fast-conducting corticospinal projections from the right undamaged hemisphere, and residual contralateral projections from the left hemisphere. The cSP findings differed in the two muscles: the mirror phenomenon was bilateral in the ADM, but present only on the right side in the APB muscles; the mirror activity of right ADM and APB muscles was inhibited only by fTMS of the ipsilateral right motor cortex; the mirror phenomenon in the left ADM muscle was inhibited only by fTMS of the contralateral right motor cortex. Conclusions: Mirror movements of right APB and ADM muscles were sustained by the ipsilateral connections from the undamaged motor cortex, while the mirror phenomenon in the left ADM muscle could be explained by hypothesizing a bilateral activation of motor cortices. This previously unreported electrophysiological picture demonstrates that different patterns of motor control may realize after perinatal cerebral lesions, even in different distal muscles of the same patient.

A new method for reproducible coil positioning in transcranial magnetic stimulation mapping

A new method is presented for mapping the motor cortex by transcranial magnetic stimulation in which the position of the stimulation coil on the scalp is measured using a 3D digitizer. The reproducibility of the method was tested by mapping 3 times the left abductor digiti minimi of 6 right-handed subjects and calculating the position of the centre of gravity (CoG), the are and volume of the individual maps. For individual maps, the coordinates of the CoG were found to be reproducible within ±3mm and the map areas and normalized volumes to within ± 20%, when the induced current flows anteriorly. Six more subjects were mapped to estimate interindividual variability of the position of the CoG. The method's ability to differentiate the cortical representation of two close muscles was successfully tested by mapping the flexor carpi radialis and the biceps brachii in another subject. Coordinates are given in a Cartesian frame of reference defined by the two tragi and the nasion. This feature will facilitate the comparison of results and their superposition on MR images.

Central motor Pathway Evaluation using magnetic coil stimulation in hereditary motor and sensory neuropathy type I (HMSN type I, Charco-Marie-Tooth disease

Central Motor Conduction Time (CMCT) was investigated in 18 patients (5 m, 13 f; age range: 11–69 yrs) with clinical and electrophysiological features of HMSN type I, using Magnetic Coil (MC) stimulation. No one exhibited clinically pyramidal signs. Brain stimulation Motor Evoked Potential (MEPs), recorded monolaterally from the left abductor digiti minimi (ADM) and tibialis anterior (TA) muscles, were evoked in all patients from upper extremities and absent in 11.1% from lower limbs. Total Motor Conduction Time (TMCT), as well as Peripheral Motor Conduction Time estimated by either magnetic nerve root stimulation (mag-PMCT) or F-wave latency values (F-PMCT), were markedly delayed in all patients. Central Motor Conduction Time was calculated by subtracting both the latency of mag-PMCT (mag-CMCT) and F-PMCT (F-CMCT) from that one obtained by cortical stimulation. F-CMCT was abnormal in 22.2% from ADM muscles and in 33.3% from TA muscles. Furthermore, CMCT to both methods was not possible to evaluate in 5.6% from upper and lower extremities and following magnetic root stimulation in 11.1% from lower limbs. These findings prove lower motor neuron involvement, in agreement with electroneurographic data, and suggest a possible central motor pathways impairment, even in patients without any clinical evidence, but they cannot explain which is the underlying pathophysiological mechanism, a true motor neuron involvement or an abnormal spinal motor neuron excitability.

The changes in excitability of spinal motoneurons at the notion of movement

Objectives: To examine possible changes of excitability of the human motor system contingent upon immobilisation of two hand fingers.Methods: Two series of 5 transcranial magnetic stimulation (TMS) sessions were carried out on different days (1, 2, 3, 4, and 7). In one series (fingers immobilised, FI), subjects wore for 4 days a device that kept immobilised the left fourth and fifth finger. In the other series (fingers free, FF), no constraining device was used. Focal TMS was applied over the right motor cortex and motor evoked potentials (MEPs) were recorded from left abductor digiti minimi (immobilised) and first dorsal interosseus (non-immobilised) muscles. Intensities of 10, 30, and 50% above the resting motor threshold (rMT), were used.Results: In FI series, rMT for both muscles showed significant increase on days 3, 4, and 7 with respect to day 1. At high stimulation intensity a clear decrease of MEPs amplitude was observed on days 3 and 4 for both muscles. Since no time-related changes of peripheral (M-wave) and spinal (F-wave) excitability were noted, MEPs and rMT changes are likely to have a cortical origin. In FF series, no changes of excitability were detected.Conclusions: Sensorimotor restriction of two fingers induces an early decrease of excitability, possibly at cortical level, which involves not only the immobilised muscle but also muscles with purportedly overlapping neural representations.