Measures Of Motor Excitability Research Articles

When Corticospinal Inhibition Favors an Efficient Motor Response

Many daily activities involve responding to the actions of other people. However, the functional relationship between the motor preparation and execution phases still needs to be clarified. With the combination of different and complementary experimental techniques (i.e., motor excitability measures, reaction times, electromyography, and dyadic 3-D kinematics), we investigated the behavioral and neurophysiological signatures characterizing different stages of a motor response in contexts calling for an interactive action. Participants were requested to perform an action (i.e., stirring coffee or lifting a coffee cup) following a co-experimenter's request gesture. Another condition, in which a non-interactive gesture was used, was also included. Greater corticospinal inhibition was found when participants prepared their motor response after observing an interactive request, compared to a non-interactive gesture. This, in turn, was associated with faster and more efficient action execution in kinematic terms (i.e., a social motor priming effect). Our results provide new insights on the inhibitory and facilitatory drives guiding social motor response generation. Altogether, the integration of behavioral and neurophysiological indexes allowed us to demonstrate that a more efficient action execution followed a greater corticospinal inhibition. These indexes provide a full picture of motor activity at both planning and execution stages.

Motor excitability measurements in early stage familial amyloid polyneuropathy: The influence of tafamidis treatment

To test motor fiber excitability in early affected patients with transthyretin (TTR)-type familial amyloid polyneuropathy (TTR-FAP) before and during tafamidis treatment. We examined the left median nerve of 21 healthy-matched controls and 10 early affected TTR-FAP patients using the automated threshold-tracking program, QTRAC. TTR-FAP patients were tested one day before the initiation of tafamidis treatment, 3 and 6 months later. The drug was well-tolerated in all patients; there was no drop-out. No statistical difference was found between healthy controls and TTR-FAP patients at study entry. On treatment, both stimulus intensity for 50% of the maximal motor response and rheobase increased significantly from entry to the last evaluation at 6 months (P<0.05). Strength duration time constant decreased significantly from the 3rd to the 6th month of evaluation (P<0.05). There was also a "fanning-out" effect on the late depolarization phase (TEd 90-100ms) as well as a shortened relative refractory period from study entry to the 6th month of evaluation. Threshold-tracking of median nerve motor fibers is not a helpful technique for the early diagnosis of TTR-FAP patients. Tafamidis was well-tolerated. We observed possible membrane hyperpolarization during treatment. Threshold tracking can contribute to documenting the action of new drugs to treat neuropathies. Tafamidis may change nerve electrical properties by reducing the burden of amyloid fibrils.

Two-week repetitive transcranial magnetic stimulation of the dorsal lateral prefrontal cortex does not affect cortical excitability in chronic smokers

Background: High-frequency repetitive transcranial magnetic stimulation (rTMS) generally induces motor cortical facilitation. However, no study has evaluated the effect of multiple rTMS sessions over the prefrontal cortex with cortical motor excitability measures in an addiction population. Here we test the hypothesis that multiple rTMS sessions over the left dorsal lateral prefrontal cortex (DLPFC) will not change motor cortical excitability, as measured with TMS in chronic smokers.

F127. Abnormal axonal inward rectification revealed in ALS patients with a low number of functionally intact motor units

Introduction Amyotrophic Lateral Sclerosis (ALS) is characterized by progressive loss of motor units. Changes in axonal excitability parameters have already been shown in earlier studies differing between severely and moderately affected disease by staging the disease using compound muscle action potential (CMAP) amplitude. However, CMAP amplitude may be normal until 50% of motor units are lost due to collateral sprouting. We aimed in this study to examine the axonal excitability properties in ALS patients correlated to disease severity determined by the novel motor unit number estimation (MUNE) method MScanFit MUNE (MScan). Methods Twenty-five patients with ALS (mean age: 64.7 ± 3.2) were prospectively evaluated by multiple motor excitability measurements (strength duration time constant (SDTC), threshold electrotonus (TE), recovery cycle (RC), current–threshold relationship (I/V)) and MScan using Qtrac software in abductor pollicis brevis muscle. The patients were divided into two groups according to the number of motor units in MScan MUNE (MSFNUnits): (1) patients with MSFNUnits ⩾40 and (2) patients with MSFNUnits Results S2 accommodation decreased significantly in patients (20.67 ± 1.08) compared to healthy subjects (24.13 ± 0.838) (p = 0.03) whereas none of the other axonal excitability parameters differed between groups. S2 accommodation decreased in patients with MSFNUnits Conclusion Axonal excitability parameters differ in different stages of ALS. Using MScan MUNE in staging of the disease severity may provide a more objective measure. The results in this study may suggest an increase in axonal inward rectification in ALS patients with low number of motor units. Further studies with larger patient groups are warranted.

S54 Motor axon excitability changes during anti-epileptic voltage-gated NA+ channel blocker therapy

Objectives Anti-epileptic drugs targeting voltage-gated Na channels (VGSC) are not selective for the brain. The aim of this study was to investigate the excitability of myelinated median nerve motor axons in epileptic patients treated with carbamazepine and other anti-VGSC drugs. Methods Stimulation was carried out at wrist and evoked compound muscle action potential (CMAP) was recorded from the abductor pollicis brevis muscle. Multiple measures of motor excitability using the TRONDNF protocol were carried out using an automatic threshold-tracking setup. In addition, the current required to evoke threshold responses targeting 40–50% of maximal CMAP amplitude was also determined by manual trial-and-error, for cathodic pulses of 0.2, 0.5 and 1 ms duration to measure the strength-duration time constant (SDTC). Results Across the anti-VGSC treated group there was an impairment in motor axon excitability with a 30% reduction in SDTC as compared to age-matched normal controls. A similar reduction in SDTC could be detected by manual threshold tracking in 3 different laboratories, using conventional nerve conduction equipment. The time for manual SDTC measurements was comparable to the full automatic multiple excitability measures protocol. Discussion The excitability impairment by partial VGSC block did not affect conduction of normal motor axons. Conclusions Our data suggest that, at the therapeutic doses of anti-VGSC drugs, there is an impairment in motor axon excitability that can be measured with conventional neurophysiological equipment. Significance Nerve excitability measurements could prove useful for monitoring the functional consequences of the anti-VGSC therapy in epilepsy.

P042 Investigating the relationship between contagious yawning and motor cortex excitability

Automatic imitation of other people’s actions is termed echophenomena (EP). EP is a prominent feature in several neurological and psychiatric conditions where patients are not able to suppress imitative behaviour of others. EPs also occur in neurologically healthy individuals ( Platek et al., 2005 ). As an example, observing a yawn can produce echophenomena-like behaviour in humans and is viewed as an involuntary mirror movement ( Dijksterhuis, 2001 , Schurmann, 2005 ). Objective In this study we wanted to assess whether the propensity to yawn is related to motor excitability measures in neurologically healthy volunteers. Materials & methods Thirty-three participants were recruited for the study. Corticospinal pathway excitability was measured using single-pulse transcranial magnetic stimulation (sp-TMS). The TMS coil was placed over the left motor cortex and recruitment curves (I/O curves) were collected from the contralateral first dorsal interosseus (FDI) muscle. Post stimulation, participants sat in front of a 15” laptop computer screen and watched a 30-min movie of actors yawning. Whilst watching the movie, participants were asked to either suppress yawning or yawn freely. In order to measure the propensity to yawn, participants were recorded using a webcam. Two independent raters, blinded to condition, counted the number of yawns from the participant recordings. The inter-rater reliability was calculated using Chronbach’s alpha. Results & conclusion Participants yawned significantly more in the free condition compared to the suppress condition. Multiple linear regression showed that the propensity to yawn was not significantly related to I/O curves. Moreover, no linear relationship was observed between I/O curves and resting motor threshold. Results show that the propensity to yawn might not be related to individual I/O curves. Future studies could include further TMS parameters such as SICI.

Nerve excitability in the rat forelimb: a technique to improve translational utility

BackgroundNerve excitability testing by threshold-tracking is the only available method to study axonal ion channel function and membrane potential in the clinical setting. The measures are, however, indirect and the interpretation of neuropathic changes remains challenging. The same multiple measures of axonal excitability were adapted to further explore the pathophysiological changes in rodent disease models under pharmacologic and genetic manipulations. These studies are typically limited to the investigation of the “long nerves” such as the tail or the tibial nerves. New methodWe introduce a novel setup to explore the ulnar nerve excitability in rodents. We provide normative ulnar data in 11 adult female Long Evans rats under anaesthesia by comparison with tibial and caudal nerves. Additionally, these measures were repeated weekly on 3 occasions to determine the repeatability of these tests. ResultsNerve excitability assessment of ulnar nerve proved to be a longitudinally repeatable measure of axonal function mature in rats, as were measures in tibial and caudal nerves.Comparison with existing method: Ulnar nerve motor excitability measures were different from the caudal and tibial excitability measures. Most notably, ulnar nerve showed the largest threshold changes during both depolarizing and hyperpolarizing threshold electrotonus. ConclusionsUlnar nerves demonstrate a distinct nerve excitability profile than the caudal and tibial nerves which could have functional and pathological implications.

Effects of age on motor excitability measures from children and adolescents with Tourette syndrome

Tourette syndrome (TS) is a neurological disorder characterised by vocal and motor tics. It is associated with cortical–striatal–thalamic–cortical circuit [CSTC] dysfunction and hyper-excitability of cortical motor regions. TS follows a developmental time course, in which tics often become increasingly more controlled during adolescence. Importantly, however, a substantial minority of patients continue to have debilitating tics into adulthood. This indicates that there may be important differences between adult TS patients and children and adolescents with the disorder. We use TMS to examine cortical motor excitability in a sample of children, adolescents and young adults with TS. We demonstrate that, in contrast to studies of adult patients, resting motor threshold and the variability of MEP responses are increased in children with TS, while the gain of motor excitability in reduced. Importantly, we demonstrate that these differences normalise with age over adolescence. We conclude that these effects are likely due to a developmental delay in the maturation of key brain networks in TS, consistent with recent brain imaging studies of structural and functional brain connectivity. Importantly, these findings suggest that the alterations in brain network structure and function associated with TS may be quite different in children and adult patients with the condition.

P854: Motor excitability measurements in familial amyloid polyneuropathy: the influence of tafamidis treatment

P854: Motor excitability measurements in familial amyloid polyneuropathy: the influence of tafamidis treatment

Motor excitability measurements: The influence of gender, body mass index, age and temperature in healthy controls

The technique of threshold tracking to test axonal excitability gives information about nodal and internodal ion channel function. We aimed to investigate variability of the motor excitability measurements in healthy controls, taking into account age, gender, body mass index (BMI) and small changes in skin temperature. We examined the left median nerve of 47 healthy controls using the automated threshold-tacking program, QTRAC. Statistical multiple regression analysis was applied to test relationship between nerve excitability measurements and subject variables. Comparisons between genders did not find any significant difference (P>0.2 for all comparisons). Multiple regression analysis showed that motor amplitude decreases with age and temperature, stimulus-response slope decreases with age and BMI, and that accommodation half-time decrease with age and temperature. The changes related to demographic features on TRONDE protocol parameters are small and less important than in conventional nerve conduction studies. Nonetheless, our results underscore the relevance of careful temperature control, and indicate that interpretation of stimulus-response slope and accommodation half-time should take into account age and BMI. In contrast, gender is not of major relevance to axonal threshold findings in motor nerves.

Increased Use-Dependent Plasticity in Chronic Insomnia

During normal sleep several neuroplasticity changes occur, some of which are considered to be fundamental to strengthen memories. Given the evidence linking sleep to neuroplasticity, it is conceivable that individuals with chronic sleep disruption, such as patients with chronic insomnia (CI), would experience abnormalities in neuroplastic processes during daytime. Protocols testing use-dependent plasticity (UDP), one of the mechanisms underlying formation of motor memories traces, provide a sensitive measure to assess neuroplasticity in the context of motor training. A well-established transcranial magnetic stimulation (TMS) paradigm was used to evaluate the ability of patients with CI and age-matched good sleeper controls to undergo UDP. We also investigated the effect of insomnia on intracortical motor excitability measures reflecting GABAergic and glutamatergic mechanisms. Human Brain Physiology Laboratory, Johns Hopkins Medical Institutions. We found that patients with CI experienced increased UDP changes relative to controls. This effect was not due to differences in motor training. In addition, patients with CI showed enhanced intracortical facilitation relative to controls, in the absence of changes in intracortical inhibitory measures. This study provides the first evidence that patients with chronic insomnia have an increased plasticity response to physical exercise, possibly due to larger activation of glutamatergic mechanisms. This suggests a heightened state of neuroplasticity, which may reflect a form of maladaptive plasticity, similar to what has been described in dystonia patients and chronic phantom pain after amputation. These results could lead to development of novel treatments for chronic insomnia.

Dissociation of motor task-induced cortical excitability and pain perception changes in healthy volunteers.

BackgroundThere is evidence that interventions aiming at modulation of the motor cortex activity lead to pain reduction. In order to understand further the role of the motor cortex on pain modulation, we aimed to compare the behavioral (pressure pain threshold) and neurophysiological effects (transcranial magnetic stimulation (TMS) induced cortical excitability) across three different motor tasks.Methodology/Principal FindingsFifteen healthy male subjects were enrolled in this randomized, controlled, blinded, cross-over designed study. Three different tasks were tested including motor learning with and without visual feedback, and simple hand movements. Cortical excitability was assessed using single and paired-pulse TMS measures such as resting motor threshold (RMT), motor-evoked potential (MEP), intracortical facilitation (ICF), short intracortical inhibition (SICI), and cortical silent period (CSP). All tasks showed significant reduction in pain perception represented by an increase in pressure pain threshold compared to the control condition (untrained hand). ANOVA indicated a difference among the three tasks regarding motor cortex excitability change. There was a significant increase in motor cortex excitability (as indexed by MEP increase and CSP shortening) for the simple hand movements.Conclusions/SignificanceAlthough different motor tasks involving motor learning with and without visual feedback and simple hand movements appear to change pain perception similarly, it is likely that the neural mechanisms might not be the same as evidenced by differential effects in motor cortex excitability induced by these tasks. In addition, TMS-indexed motor excitability measures are not likely good markers to index the effects of motor-based tasks on pain perception in healthy subjects as other neural networks besides primary motor cortex might be involved with pain modulation during motor training.

Effects of botulinum toxin on motor system excitability in patients with writer's cramp.

To investigate botulinum toxin (BTX) effects on central and peripheral motor excitability in writer's cramp. Using transcranial magnetic stimulation over the motor cortex and brainstem and peripheral electrical stimulation, the authors investigated measures of motor cortical and peripheral motor excitability on the affected and unaffected side before and 2 to 4 weeks after BTX A injection into the affected muscles in six patients with writer's cramp. The following motor excitability measures were studied: resting and active motor threshold, cortical silent period, intracortical inhibition and facilitation using the double-pulse technique, recruitment curves, motor evoked potentials following magnetic brainstem stimulation, and maximum M-response amplitude following supramaximal peripheral nerve stimulation. BTX injection improved function and reduced the M response. No other measures of motor system excitability showed significant changes following BTX injection, and there was no difference in these measures between sides. BTX A effects on motor system excitability seem to be based mainly on its peripheral mechanisms of action. There was no difference in the motor system excitability between the clinically affected and unaffected sides in this group of patients.

Motor system excitability in patients with restless legs syndrome.

In 18 patients with idiopathic restless legs syndrome (RLS), intracortical inhibition by paired transcranial magnetic stimulation (TMS) was significantly reduced for both foot and hand muscles, suggesting that the entire motor cortex is disinhibited in RLS. Decreased intracortical facilitation in the foot muscle but not in the hand muscle may be due to subliminal activation of the symptomatic lower limbs. Motor excitability measurements of single TMS were not altered. These results support a subcortical origin of RLS.