Motor Unit Size Research Articles

Early Surgical Decompression Ameliorates Dysfunction of Spinal Motor Neuron in Patients With Acute Traumatic Central Cord Syndrome: An Ambispective Cohort Analysis.

An ambispective cohort analysis. The aim of this study was to investigate the impact of early (≤2 weeks) versus delayed (>2 weeks) surgical intervention on the spinal motor neurons at and distal to injury site in acute traumatic central cord syndrome (ATCCS). Accumulating evidence demonstrated degeneration in distal lower motor neurons (LMNs) following spinal cord injury, and this secondary degeneration may exacerbate motor impairments and limit spontaneous motor recovery. However, few studies involved this pathological process in ATCCS. Motor unit number estimation (MUNE) was performed on both abductor pollicis brevis (APB) and extensor digitorum brevis (EDB) in 69 ATCCS patients (early vs. delayed surgical-treatment: 29 vs. 35) and 42 healthy subjects. All patients were assessed by American spinal injury association and Medical Research Council scales. These examinations and disabilities of arm, shoulder, and hand (c) questionnaire were administered approximately 21 months after operation in 65 of these patients. Preoperatively, MUNE values were lower in cervical-innervated muscles of ATCCS patients than in those of controls, and reduced motor units were observed in lumbosacral-innervated muscles in ATCCS patients with preoperative duration over 6 months (P < 0.05). Increased motor unit size without modification of MUNE values was found in delayed-surgical patients, whereas early-surgical patients mainly showed increased MUNE values in tested muscles between two assessments (P < 0.05). The postoperative follow-up analysis identified larger motor unit size and relatively fewer motor units in tested muscles, as well as higher DASH scores, in delayed-surgical patients than in early-surgical patients (P < 0.05). ATCCS has adverse downstream effects on the LMNs distal to injury site. Surgical intervention within 2 weeks after injury in ATCCS patients may be beneficial in ameliorating dysfunction of spinal motor neurons at and distal to injury site, reducing secondary motor neuron loss, and eventually improving neurologic outcomes. 3.

Non-invasive imaging of single human motor units

ObjectiveTo determine the size, shape and distribution of single human motor units in-vivo in healthy controls of different ages. MethodsA novel diffusion-weighted magnetic resonance imaging (MRI) technique was used in combination with in-scanner electrical stimulation to quantify the shape, cross-sectional area, and dimensions of individual motor units in 10 healthy subjects. ResultsThirty-one discrete motor units were studied. The majority were elliptical or crescent shaped, but occasional split motor units were observed. The mean motor unit cross sectional area was 26.7 ± 11.2 mm2, the mean maximum dimension was 10.7 ± 3.3 mm, and the mean minimum dimension was 4.5 ± 1.2 mm. Subjects aged over 40 had significantly larger maximum dimensions than those below this age (p < 0.05). ConclusionsMotor unit MRI (MUMRI) is a novel technique capable of revealing the size, shape and position of multiple motor units in human muscles. It is reproducible, non-invasive, and sufficiently sensitive to detect physiologically relevant changes in motor unit morphology with age. SignificanceTo our knowledge, these results provide the first imaging assessment of human motor unit morphology. The technique shows promise both as a diagnostic tool and as a biomarker in longitudinal studies of disease progression.

Interrelation between Sarcopenia and the Number of Motor Neurons in Patients with Parkinsonian Syndromes

Introduction: Pathogenesis in a subgroup of sarcopenic patients seems to be based on a reduced number of motor neurons. This study aimed at investigating the overlap between sarcopenia and neurodegeneration, as reflected by a low number of motor neurons in patients with Parkinsonian syndromes (PS). Methods: The motor unit number index (MUNIX) of the hypothenar muscle was used to assess the number and size (MUSIX) of motor units (MUs) in patients with idiopathic Parkinson disease (iPD, n = 53), patients with atypical Parkinsonian syndrome (aPS, n = 21), and a control group (n = 30). Mean age of participants was 70.3 years and 54.1% were female. Skeletal muscle mass by bioelectrical impedance analysis, hand-grip strength and gait speed were measured. Based on these assessments, sarcopenia was diagnosed according to the criteria of the European Working Group on Sarcopenia in Older People. Results: Sarcopenia criteria were met by 10 patients with PS (13.5%). The study group had significantly lower MUNIX values than the control group (109 [SD ±39.1] vs. 129 [SD ±45.1]; p = 0.020) even after adjustment for age and sex. Three of the 5 sarcopenic iPD patients (75%) had pathological low MUNIX values (<80). Discussion/Conclusion: Sarcopenia is a frequent comorbidity in PS. The pathologically low MUNIX values found in 75% of our sarcopenic iPD patients provides further support for the existence of a neurodegenerative overlap syndrome with a reduced number of MUs potentially leading to sarcopenia. This finding warrants further evaluation.

Eight weeks of resistance training increases strength, muscle cross-sectional area and motor unit size, but does not alter firing rates in the vastus lateralis.

Previous investigations analyzing resistance training's influence on motor unit (MU) firing rates have yielded mixed results. These mixed results may be clarified by concurrently measuring changes in MU size. Thus, this study analyzed whether post-training strength gains were due to increases in MU firing rates and/or sizes as measured indirectly via action potential amplitudes. Sixteen males (age = 20.7 ± 1.9years) completed 8 weeks of resistance training, while eight males (age = 19.4 ± 2.5years) served as controls. Vastus lateralis surface electromyography signals collected during submaximal isometric knee extensions were decomposed to yield an action potential amplitude (MUAPAMP), mean firing rate (MFR), and recruitment threshold (RT) for each MU. Each contraction's average MFR and MUAPAMP, and coefficients of the linear (y-intercept and slope) MUAPAMP-RT, linear MFR-RT and exponential (A and B terms) MFR-MUAPAMP relationships were analyzed. Firing instances and action potentials were validated via reconstruct-and-test and spike-triggered averaging procedures. Vastus lateralis cross-sectional area (CSA) was analyzed with ultrasonography. Resistance training increased isometric strength from 204.6 ± 34.9 to 239.8 ± 36.3 Nm and vastus lateralis CSA from 28.7 ± 4.7 to 34.0 ± 5.0 cm2. Resistance training did not affect MFR-RT relationship parameters or average MFRs but did increase the slopes of the MUAPAMP-RT relationships (0.0067 ± 0.0041 to 0.0097 ± 0.0045mV/%MVC) and average MUAPAMPs from 0.189 ± 0.093 to 0.249 ± 0.127mV. MU hypertrophy altered the MFR-MUAPAMP relationships (B terms: -3.63 ± 1.40 to -2.66 ± 1.38 pps/mV). Resistance training induced MU and muscle hypertrophy, but did not alter firing rates. Greater MU twitch forces resulting from larger MUs firing at pre-training rates likely explain resistance training strength gains.

Implementation and experimental validation of surface electromyogram and force model of Tibialis Anterior muscle for examining muscular factors.

This study reports a surface electromyogram and force of contraction model. The objective was to investigate the effect of changes in the size, type and number of motor units in the Tibialis Anterior muscle to surface electromyogram and force of dorsiflexion. A computational model to simulate surface electromyogram and associated force of contraction by the Tibialis Anterior muscle was developed. This model was simulated for isometric dorsiflexion, and comparative experiments were conducted for validation. Repeated simulations were performed to investigate the different parameters and evaluate inter-experimental variability. An equivalence statistical test and the Bland-Altman method were used to observe the significance between the simulated and experimental data. Simulated and experimentally recorded data had high similarity for the three measures: maximal power of power spectral density (p < 0.0001), root mean square of surface electromyogram (p < 0.0001) and force recorded at the footplate (p < 0.03). Inter-subject variability in the experimental results was in-line with the variability in the repeated simulation results. This experimentally validated computational model for the surface electromyogram and force of the Tibialis Anterior muscle is significant as it allows the examination of three important muscular factors associated with ageing and disease: size, fibre type and number of motor units.

Application value of motor unit number index in patients with amyotrophic lateral sclerosis

Objective: To investigate the application value of motor unit number index (MUNIX) for diagnosis and assessment progress in patients with amyotrophic lateral sclerosis (ALS). Method: Sixty healthy controls and 60 ALS patients in the clinic were enrolled from May 2017 to December 2018. The bilateral deltoid, abductor digiti minimi, quadriceps femoris and tibialis anterior muscles of the subjects were detected by MUNIX method, and the negative peak amplitude of compound muscle action potential (CMAP) of ulnar nerve, femoral nerve, peroneal nerve, axillary nerve in bilateral was collected. MUNIX and motor unit size index (MUSIX) of muscles were compared between ALS group and control group. The difference between the MUNIX abnormal rate of muscles and abnormal rate of the corresponding CMAP negative peak amplitude in ALS patients was further compared. Meanwhile, the correlation between the disease course of ALS patients and MUNIX and MUSIX was analyzed. Results: Compared with the control group, the MUNIX values of the deltoid, abductor digiti minimi, quadriceps femoris and tibialis anterior decreased significantly (97±24 vs 183±38, 48±17 vs 191±39, 54±15 vs 159±22, 49±16 vs 147±25, all P<0.05). MUSIX values increased ((175±32) μV vs (47±15) μV, (189±34) μV vs (54±16) μV, (170±30) μV vs (49±13) μV, (190± 36) μV vs (48± 14) μV, all P<0.05)). In ALS patients, the abnormal rate of MUNIX was respectively 81%, 87%, 75% and 89%. The negative peak amplitude abnormal rate of corresponding neuralCMAP was 35%, 40%, 31% and 36%, respectively, with a significant difference (P<0.05). There was a negative correlation between MUNIX value and the course of disease in ALS patients (P<0.05), and a positive correlation between MUSIX value and the course of disease (P<0.05). Conclusion: The MUNIX technique exhibits the features of quantifying the proximal upper and lower limb muscles and assessing the loss of motor units in motor neuron degeneration.

Detection of early motor involvement in diabetic polyneuropathy using a novel MUNE method – MScanFit MUNE

ObjectiveDetection of motor involvement in diabetic polyneuropathy (DPN) by nerve conduction studies (NCS) does not occur until there is substantial loss of motor units, because collateral reinnervation maintains compound muscle action potential (CMAP) amplitude. Motor unit number estimation (MUNE) methods may therefore be more sensitive. This study was undertaken to test whether the novel method, MScanFit MUNE (MScan) can detect motor involvement in DPN despite normal NCS. MethodsFifty-two type-2 diabetic patients and 38 healthy controls were included. The median nerve was examined in all participants using standard NCS and a detailed CMAP scan, used for MScan. Additional lower extremity NCS in patients were used for DPN diagnosis. ResultsOf 52 diabetic patients, 21 had NCS-defined DPN while lower extremity NCS were normal in 31 patients. MScan motor unit number and size showed higher sensitivity and incidence of abnormality than motor NCS parameters, and a similar sensitivity to sensory NCS. ConclusionsMScan is able to detect motor axonal damage at times when collateral reinnervation limits NCS changes. SignificanceMScan is a sensitive method to detect motor involvement in DPN, which our data suggests is present as early as sensory.

Motor unit number index (MUNIX) in myopathic disorders: Clinical correlations and potential pitfalls

Our aim was to study motor unit number index (MUNIX) in myopathic disorders. We studied 11 patients with myopathy, and healthy controls. We obtained MUNIX, compound muscle action potential (CMAP), motor unit size index (MUSIX) and alpha (α, power exponent from MUNIX equation) measurements from three different muscles. MUNIX and CMAP were significantly lower in one muscle. This MUNIX decrease may not be related to motor neuron loss, but rather to muscle fiber atrophy. MUSIX and α did not change and may be useful in determining whether the MUNIX decrease is indeed due to motor unit loss.

Motor unit number index and compound muscle action potential amplitude

ObjectivesMUNIX (motor unit number index), derived from the compound muscle action potential (CMAP) and surface EMG interference pattern (SIP) has become popular as a substitute for motor unit number estimation (MUNE). This study was undertaken to determine why, in recent recordings from amyotrophic lateral sclerosis (ALS) patients and healthy controls, we found that MUNIX values resembled CMAP amplitudes more closely than MUNE values. MethodsThe relationship between MUNIX and CMAP and SIP amplitudes was investigated by a theoretical analysis and by reanalysing the data from the previous study. ResultsTheory indicates that when motor unit potentials overlap extensively, information about motor unit size and number is lost, and MUNIX depends only on CMAP area and power. Accordingly, MUNIX values were found to be sensitive to changes in CMAP amplitude but insensitive to changes in SIP amplitude. The reproducibility of MUNIX measurements in healthy controls was found to depend almost entirely on correlation with CMAP properties. ConclusionsMUNIX gives misleading information about motor unit numbers in healthy controls, and provides little information about loss of motor units in ALS patients beyond that given by simple CMAP amplitude measurements. SignificanceMUNIX should not be interpreted as a MUNE method.

O-25 MUNIX alteration in three consecutive muscles innervated by ulnar nerve in Amiyotrophic Lateral Sclerosis

Background The motor unit number index (MUNIX) method is used to estimate alterations attributed to changes in number and size (MUSIX) of motor units (MUs) using the compound muscle action potential (CMAP) and surface electromyographic interference pattern (SIP). Material and methods In sixteen patients with Amyotrophic Lateral Sclerosis (ALS) and 16 controls, compound muscle action potentials (CMAP), MUNIXs and MUSIXs were recorded from flexor carpi ulnaris (FCU), abductor digiti minimi (ADM), and first dorsal interosseous (FDI) muscles. Results In ALS patients, mean values of MUNIXs recorded form FCU (76.7 ± 24.01 vs 71.5 ± 32.4, P = 0.22), ADM (130.3 ± 1.4 vs. 66.4 ± 2.3, P = 0.008), and FDI (181.8 ± 1.3 vs. 63.6 ± 2.8, P = 0001) muscles were lower and MUSIXs of FCU (59.9 ± 11.6 vs. 54.4 ± 13.4, P = 0.6), ADM (85.6 ± 1.4 vs. 99.1 ± 1.6, P = 0.34), and FDI (72.9 ± 1.2 vs. 99.9 ± 1.6, P = 0.03) were higher than controls. CMAPs deteriorated in proximal (i.e., FCU; 6 ALS vs. 0 controls, P = 0.009) muscle more than distal (i.e., ADM; 5 ALS vs. 0 control, P = 0.02 and FDI; 3 ALS vs. 1 control, P = 0.3) while MUNIXs deteriorated in distal (i.e., FDI; 9 ALS vs. 0 control, P = 0.003) more than proximal (i.e., ADM; 7 ALS vs. 1 control, P = 0.02, and FCU; 1 ALS vs. 0 control, P = 0.5) muscles. Conclusions Unlike CMAPs, MUNIX was considered to define denervation process overwhelming the impact of re-innervation using SIPs. In addition to utility in long term assessment of ALS patients, MUNIX could be used to estimate quantitative alterations in motor units for diagnostic purpose.

Long-Term Endurance and Power Training May Facilitate Motor Unit Size Expansion to Compensate for Declining Motor Unit Numbers in Older Age.

The evidence concerning the effects of exercise in older age on motor unit (MU) numbers, muscle fiber denervation and reinnervation cycles is inconclusive and it remains unknown whether any effects are dependent on the type of exercise undertaken or are localized to highly used muscles. MU characteristics of the vastus lateralis (VL) were assessed using surface and intramuscular electromyography in eighty-five participants, divided into sub groups based on age (young, old) and athletic discipline (control, endurance, power). In a separate study of the biceps brachii (BB), the same characteristics were compared in the favored and non-favored arms in eleven masters tennis players. Muscle size was assessed using MRI and ultrasound. In the VL, the CSA was greater in young compared to old, and power athletes had the largest CSA within their age groups. Motor unit potential (MUP) size was larger in all old compared to young (p < 0.001), with interaction contrasts showing this age-related difference was greater for endurance and power athletes than controls, and MUP size was greater in old athletes compared to old controls. In the BB, thickness did not differ between favored and non-favored arms (p = 0.575), but MUP size was larger in the favored arm (p < 0.001). Long-term athletic training does not prevent age-related loss of muscle size in the VL or BB, regardless of athletic discipline, but may facilitate more successful axonal sprouting and reinnervation of denervated fibers. These effects may be localized to muscles most involved in the exercise.

Assessment of the reliability of the motor unit size index (MUSIX) in single subject “round-robin” and multi-centre settings

ObjectiveThe motor unit size index (MUSIX) is incorporated into the motor unit number index (MUNIX). Our objective was to assess the intra-/inter-rater reliability of MUSIX in healthy volunteers across single subject “round robin” and multi-centre settings. MethodsData were obtained from (i) a round-robin assessment in which 12 raters (6 with prior experience and 6 without) assessed six muscles (abductor pollicis brevis, abductor digiti minimi, biceps brachii, tibialis anterior, extensor digitorum brevis and abductor hallucis) and (ii) a multi-centre study with 6 centres studying the same muscles in 66 healthy volunteers. Intra/inter-rater data were provided by 5 centres, 1 centre provided only intra-rater data.Intra/inter-rater variability was assessed using the coefficient of variation (COV), Bland-Altman plots, bias and 95% limits of agreement. ResultsIn the round-robin assessment intra-rater COVs for MUSIX ranged from 7.8% to 28.4%. Inter-rater variability was between 7.8% and 16.2%. Prior experience did not impact on MUSIX values. In the multi-centre study MUSIX was more consistent than the MUNIX. Abductor hallucis was the least reliable muscle. ConclusionsThe MUSIX is a reliable neurophysiological biomarker of reinnervation. SignificanceMUSIX could provide insights into the pathophysiology of a range of neuromuscular disorders, providing a quantitative biomarker of reinnervation.

Muscular strength and power are correlated with motor unit action potential amplitudes, but not myosin heavy chain isoforms in sedentary males and females

It remains unclear if the sizes of higher-threshold motor units (MU) are associated with muscular strength and power. Therefore, the purpose of this study was to examine sex-related differences in muscle cross-sectional area (mCSA), percent myosin heavy chain (%MHC) isoform expression, and the MU action potential amplitudes (MUAPAMPS)-recruitment threshold (RT) relationships of the vastus lateralis and isometric peak torque, isokinetic peak torque and mean power at 1.05 rad·s−1 of the leg extensors. Surface electromyographic decomposition techniques were used to quantify MUAPAMPS recorded during isometric muscle actions at 70% of maximal voluntary contractions and regressed against RTs with the slopes calculated. Ultrasound images were used to measure mCSA. Males had greater slopes from the MUAPAMP-RT relationship than the females (P < 0.05). The greater slopes likely reflected larger higher-threshold MUs for the males. The mCSAs and slopes from the relationships were strongly correlated with isometric and isokinetic peak torque and isokinetic mean power (r = 0.78–0.82), however, type I %MHC isoform was only moderately correlated with isometric peak torque (r = −0.54). The results indicated that sex-related differences in muscular strength and power were associated more so with the sizes of the higher-threshold MUs (slopes) and mCSA than MHC isoforms. The amount of cross-bridge activity within muscle fibers that comprise higher-threshold MUs may be the primary contributor to muscular strength and power rather than the contractile properties of the muscle.

The motor unit number index (MUNIX) profile of patients with adult spinal muscular atrophy

ObjectiveObjective of this study is the comprehensive characterisation of motor unit (MU) loss in type III and IV Spinal Muscular Atrophy (SMA) using motor unit number index (MUNIX), and evaluation of compensatory mechanisms based on MU size indices (MUSIX). MethodsNineteen type III and IV SMA patients and 16 gender- and age-matched healthy controls were recruited. Neuromuscular performance was evaluated by muscle strength testing and functional scales. Compound motor action potential (CMAP), MUNIX and MUSIX were studied in the abductor pollicis brevis (APB), abductor digiti minimi (ADM), deltoid, tibialis anterior and trapezius muscles. A composite MUNIX score was also calculated. ResultsSMA patients exhibited significantly reduced MUNIX values (p < 0.05) in all muscles, while MUSIX was increased, suggesting active re-innervation. Significant correlations were identified between MUNIX/MUSIX and muscle strength. Similarly, composite MUNIX scores correlated with disability scores. Interestingly, in SMA patients MUNIX was much lower in the ADM than in the ABP, a pattern which is distinctly different from that observed in Amyotrophic Lateral Sclerosis. ConclusionsMUNIX is a sensitive measure of MU loss in adult forms of SMA and correlates with disability. SignificanceMUNIX evaluation is a promising candidate biomarker for longitudinal studies and pharmacological trials in adult SMA patients.

T137. Detection of early motor involvement in diabetic polyneuropathy using a novel MUNE method – MScanFit MUNE

Detection of motor involvement in diabetic polyneuropathy (DPN) with nerve conduction studies (NCS) is delayed by collateral sprouting, resulting in fewer and larger motor units. Our aim is to examine whether a novel Motor Unit Number Estimation (MUNE) method so called MScanFit MUNE (MScan) can detect early motor involvement in DPN. We prospectively included 45 patients with diabetes mellitus type II and 35 healthy subjects (HS), age 59.56 ± 2.05. MScan in abductor pollicis brevis (APB) muscle were done in all participants. Additionally, NCS of three motor (median, peroneal, tibial) and three sensory (bilateral sural and median) nerves was done in all patients. NCS results were compared to laboratory controls. All participants with carpal tunnel syndrome were excluded. DPN diagnosis was given using Dyck’s criteria requiring NCS abnormality in at least two nerves, one must be the sural nerve. Twelve patients (26.6%) had DPN (DPN+), age 61.64 ± 1.83 and 33 patients had normal NCS (DPN−), age 63.5 ± 2.76. There was no significant difference in age between the tree groups. DPN+ patients had a significantly lower number of motor units (78.58 ± 8.01) when comparing with both HS (119.9 ± 5.21) (p = 0.00022) and DPN− patients (113.4 ± 6.44) (p = 0.005). There was no difference between HS and DPN− patients (p = 0.43709). Motor unit size was significantly larger in DPN+ patients (6.343 ± 0.658) compared to HS (3.851 ± 0.241) (p = 0.00083) and DPN− patients (3.84 (3.25, 5.17), p = 0.01384). No difference between HS and DPN− patients (p = 0.12388). Compound muscle action potential amplitude was within normal ranges for all DPN+ patients. MScan abnormality with normal CMAP amplitude in APB muscle suggests that MScan may be a sensitive measure of early motor involvement in DPN. Further studies with larger patient groups and application of MScan in different muscles particularly in lower extremities are needed.

S14-3. F-wave and spinal excitability in neurological disease

Depending on the neurological condition, the excitability of lower motoneuron (LMN) changes. Shortest latency, conduction velocity, frequency and amplitude are F-wave parameters we are able to measure. Because F-wave travels the entire length of peripheral nerve, parameters of latency and conduction velocity are used to evaluate motor neuropathies. However, frequency and amplitude are considered to reflect the excitability of LMN. Normally, LMN receives presynaptic inhibition from upper motoneuron (UMN) or Renshaw inhibitory cell. In case of central nervous system disorders like stroke and myelopathy, LMN excitability becomes higher because of disinhibition from those inhibitory neuron system, and is leading cause of F-wave frequency and amplitude increase. In diseases that lower LMN excitability such as motor neuron disease and neuropathy, F-wave frequency diminishes. Evaluation of amplitude would be somewhat complicated. Even though F-wave frequency diminishes, F-wave amplitude increases in relation to motor unit size, which is affected by reinnervation process. In amyotrophic lateral sclerosis, where both UMN and LMN progressively disappear, the balance of both neurons decides the excitability of LMN. Therefore, the frequency and amplitude of F-wave varies. Repeater F-wave might be the key to analyze this complicated condition.

The reliability of methods to estimate the number and size of human motor units and their use with large limb muscles

PurposeCurrent methods for estimating muscle motor unit (MU) number provide values which are remarkably similar for muscles of widely differing size, probably because surface electrodes sample from similar and relatively small volumes in each muscle. We have evaluated an alternative means of estimating MU number that takes into account differences in muscle size.MethodsIntramuscular motor unit potentials (MUPs) were recorded and muscle cross-sectional area (CSA) was measured using MRI to provide a motor unit number estimate (iMUNE). This was compared to the traditional MUNE method, using compound muscle action potentials (CMAP) and surface motor unit potentials (sMUPs) recorded using surface electrodes. Data were collected from proximal and distal regions of the vastus lateralis (VL) in young and old men while test–retest reliability was evaluated with VL, tibialis anterior and biceps brachii.ResultsMUPs, sMUPs and CMAPs were highly reliable (r = 0.84–0.91). The traditional MUNE, based on surface recordings, did not differ between proximal and distal sites of the VL despite the proximal CSA being twice the distal CSA. iMUNE, however, gave values that differed between young and old and were proportional to the muscle size.ConclusionWhen evaluating the contribution that MU loss makes to muscle atrophy, such as in disease or ageing, it is important to have a method such as iMUNE, which takes into account any differences in total muscle size.

Modified motor unit number index (MUNIX) algorithm for assessing excitability of alpha motor neuron in spasticity

ObjectiveThe understanding of the spasticity mechanism is still a problem in the literature, as its definition can be made on the basis of more than one parameter. Therefore, we studied alpha motor neuron excitability, dynamic changes based on force production, and patellar tendon (T) reflex in spasticity and healthy control groups. MethodsAlpha motor neuron excitability, force production, and patellar T reflex were evaluated through three different test protocols. Motor Unit Number Index (MUNIX) measurement was applied for understanding motor neuron pool properties in the first protocol. Voluntary force production and patellar T reflex parameters were evaluated by voluntary force production and triggering patellar T reflex. Twenty spasticity and 20 healthy volunteers participated in the study. ResultsIn the spasticity group, both MUNIX numbers and Motor Unit Size Index (MUSIX) numbers were lower than those in the control group. The results for the Ideal Case Motor Unit Count (ICMUC) parameter show that there is no significant difference between spasticity and healthy individuals for low-level contractions, whereas there is a significant difference for high-level contractions (p < 0.05). In the spasticity group, an increase was observed in the ratio of maximal voluntary force to the T reflex triggered force production (Tf/Vf). ConclusionSpasticity and healthy subjects can be distinguished easily and clearly by evaluating the changes in both kinesiological and electrophysiological findings and the decreasing threshold in the alpha motor neuron pool. SignificanceThis study shows that such combined methods, which allow the evaluation of the alpha motor neuron pool, as well as kinesiological and electrophysiological parameters, are tools that cannot be overlooked in understanding spasticity.

Detection radius of EMG for fasciculations: Empiric study combining ultrasonography and electromyography

ObjectiveThe aims of this study were to investigate the detection radius and sensitivity of EMG for fasciculations. MethodsMuscle ultrasonography was performed simultaneously to EMG recordings in patients with fasciculations in the context of amyotrophic lateral sclerosis. Ultrasonography and EMG parameters were analyzed for selected fasciculations. ResultsA total of 381 fasciculations were detected by ultrasonography in 18 muscles of 10 patients. Out of these, 125 (33%) were EMG-negative. In contrast, none of the fasciculations detected by EMG were ultrasonography-negative. EMG detection probability decreased significantly with increasing distance from the center of the fasciculation. EMG detection rate was 98% when the EMG needle was located within the fasciculation and 50% at 7.75 mm distance from the fasciculation center. In addition, EMG detection depended significantly on cross-sectional area of the fasciculation and presence of neurogenic changes. ConclusionsFor detecting the same fasciculations, EMG is less sensitive than ultrasonography. EMG detection probability decreases sharply at a distance comparable to motor unit size. SignificanceThese results extend previous knowledge about superior sensitivity of ultrasonography for fasciculations. Moreover, our novel bimodal detection method provides first in vivo data about the EMG detection radius for fasciculations in a clinical setting.

Organization of the motor-unit pool for different directions of isometric contraction of the first dorsal interosseous muscle.

Muscle force generation involves recruitment and firing rate modulation of motor units (MUs). The control of MUs in producing multidirectional forces remains unclear. We studied MU recruitment and firing properties, recorded from the first dorsal interosseous muscle, for 3 different directions of contraction: abduction; abduction/flexion combination; and flexion. MUs were recruited systematically at higher threshold force during flexion. Larger MUs were recruited and firing rates of MUs were lower during abduction. There was an orderly recruitment of MUs according to MU size regardless of contraction direction, obeying the "size principle." Firing rates of earlier-recruited MUs were consistently higher than later-recruited MUs, affirming the "onion-skin" property. Our findings suggest that the size principle and onion-skin organization together provide a general description of MU recruitment patterns and firing properties. The directional alternations of MU control properties likely reflect changes in neural drive to the muscle. Muscle Nerve 57: E85-E93, 2018.