Effect of motion imagery to counter rest-induced suppression of F-wave as a measure of anterior horn cell excitability

Changes in the excitability of anterior horn cells in a mental rotation task of body parts

INTRODUCTION/AIMS

Mental rotation (MR), a tool of implicit motor imagery, is the ability to rotate mental representations of two- or three-dimensional objects. Although many reports have described changes in brain activity during MR tasks, it is not clear whether the excitability of anterior horn cells in the spinal cord can be changed. In this study, we examined whether MR tasks of hand images affect the excitability of anterior horn cells using F-wave analysis.

METHODS

Right-handed, healthy participants were recruited for this study. F-waves of the right abductor pollicis brevis were recorded after stimulation of the right median nerve at rest, during a non-MR task, and during an MR task. The F-wave persistence and the F/M amplitude ratio were calculated and analyzed.

RESULTS

Twenty participants (11 men and 9 women; mean age, 29.2 ± 4.4 years) were initially recruited, and data from the 18 that met the inclusion criteria were analyzed. The F-wave persistence was significantly higher in the MR task than in the resting condition (p = .001) or the non-MR task (p = .012). The F/M amplitude ratio was significantly higher in the MR task than in the resting condition (p = .019).

DISCUSSION

The MR task increases the excitability of anterior horn cells corresponding to the same body part. MR tasks may have the potential for improving motor function in patients with reduced excitability of the anterior horn cells, although this methodology must be further verified in a clinical setting.

Exploring the peripheral mechanisms of lower limb immobilisation on muscle function using novel electrophysiological methods

OBJECTIVE

To explore the effects of short-term immobilisation and subsequent retraining on peripheral nervous system (PNS) measures using two novel electrophysiological methods, muscle velocity recovery cycles (MVRC) and MScanFit motor unit number estimation (MUNE) alongside lower limb muscle strength, muscle imaging and walking capacity.

METHODS

Twelve healthy participants underwent 1-week of ankle immobilisation and 2-weeks of retraining. Assessments before and after immobilisation, and after retraining, included MVRC [muscle membrane properties; muscle relative refractory period (MRRP), early and late supernormality], MScanFit, MRI-scans [muscle contractile cross-sectional area (cCSA)], isokinetic dynamometry [dorsal and plantar flexor muscle strength], and 2-minute maximal walk test [physical function].

RESULTS

After immobilisation, compound muscle action potential (CMAP) amplitude reduced (-1.35[-2.00;-0.69]mV); mean change [95%CI]) alongside reductions in plantar (but not dorsal) flexor muscle cCSA (-124[-246;3]mm2), dorsal flexor muscle strength (isometric -0.06[-0.10;-0.02]Nm/kg, dynamicslow -0.08[-0.11;-0.04]Nm/kg, dynamicfast no changes), plantar flexor muscle strength (isometric -0.20[-0.30;-0.10]Nm/kg, dynamicslow -0.19[-0.28;-0.09]Nm/kg, dynamicfast -0.12[-0.19;-0.05]Nm/kg) and walking capacity (-31[-39;-23]m). After retraining, all immobilisation-affected parameters returned to baseline levels. In contrast, neither MScanFit nor MVRC were affected apart from slightly prolonged MRRP in gastrocnemius.

CONCLUSIONS

PNS do not contribute to the changes observed in muscle strength and walking capacity.

SIGNIFICANCE

Further studies should include both corticospinal and peripheral mechanisms.

Lumbar trans-spinal direct current stimulation: A modeling-experimental approach to dorsal root ganglia stimulation

Introduction

The excitability of spinal motor neurons (MN) can be altered through subthreshold currents, such as transcutaneous spinal direct-current stimulation (tsDCS). Current evidence shows that tsDCS can interfere with ascending somatosensory pathways and lower motor neurons' (LMN) excitability, which points to its therapeutic potential for repairing altered spinal responses. We aim to define the best tsDCS montage for maximizing the electric field (E-field) in the lumbar spinal cord (L-SC) by computer modeling; and to apply this montage to measure the effect on LMN excitability and somatosensory evoked potentials (SSEP).

Methods

A human volume conductor model was obtained from an available database. The E-field distribution was calculated considering three different electrode settings aiming at maximizing the field at L-SC and right dorsal root ganglia (DRG). The best electrode setting was then selected and applied in a blind crossover pseudo-randomized study including 14 subjects. tsDCS was delivered for 15 min (cathodal vs. sham) over L2 vertebra level (4 mA, 144 mC/cm²), and its effect on F-waves, H-reflex (including homosynaptic depression, HD) and SSEPs was investigated in the lower limbs.

Results

All simulated montages showed higher current density and E-field magnitudes between the electrodes (>0.15 V/m), with a major longitudinal component and with rostral-caudal direction. The induced E-field involved the sensory ganglia and was maximum in the right T8-left L2 montage, which was the one selected for the experimental protocol. We disclosed a statistically significant increase of the H-reflex amplitude at 0.1 Hz, after cathodal tsDCS (c-tsDCS) on both sides. No other significant change was observed.

Discussion

Our results can suggest the c-tsDCS applied to the L-SC and DRG can modulate synaptic efficiency increasing lower motor neurons response to Ia fibers excitation. The possible implications of our findings for treating clinical conditions will be addressed in future studies.

Exercise following immobility increases lower motor neuron excitability: F-wave and H-reflex studies

OBJECTIVES

The excitability of lower motor neurons can be explored non-invasively by several neurophysiological techniques, e.g., F-wave and H-reflex studies after a period of immobility and then after subsequent exercise. The aim of this study is to investigate the impact of exercise and high frequency repetitive nerve stimulation (RNS) following changes induced by 75 min of immobility.

METHODS

We studied 10 healthy subjects following 75 min lower limb immobility, then randomized to RNS or cycling on different days. The neurophysiological studies of M-response, F-wave latency, F/M amplitude ratio and persistence; H-reflex threshold and latency, H/M amplitude ratio, and homosynaptic depression were performed at baseline, after immobility and immediately following the intervention, using stimulation of posterior tibial and peroneal nerves.

RESULTS

After immobility F-wave latencies were delayed and homosynaptic depression at 2 Hz was increased (p < 0.025). RNS had no effect, but cycling exercise reduced H-reflex latencies (p = 0.025) and decreased homosynaptic depression at 2 Hz.

DISCUSSION

Our findings suggest that both proprioceptive stimulation and supraspinal pathways modulate intraspinal physiological changes after immobility. These observations suggest that specific exercise protocols may be useful in managing patients recovering from periods of immobility.

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.

Promising Effect of Visually-Assisted Motor Imagery Against Arthrogenic Muscle Inhibition - A Human Experimental Pain Study

Purpose

Clinically, arthrogenic muscle inhibition (AMI) has a negative impact on functional recovery in musculoskeletal disorders. One possible technique to relieve AMI is motor imagery, which is widely used in neurological rehabilitation to enhance motor neuron excitability. The purpose of this study was to verify the efficacy of visually-assisted motor imagery against AMI using a human experimental pain model.

Methods

Ten healthy volunteers were included. Experimental ankle pain was induced by hypertonic saline infusion into unilateral Kager’s fat pad. Isotonic saline was used as control. Subjects were instructed to imagine while watching a movie in which repetitive motion of their own ankle or fingers was shown. H-reflex normalized by the motor response (H/M ratio) on soleus muscle, maximal voluntary contraction (MVC) force of ankle flexion, and contractile activities of the calf muscles during MVC were recorded at baseline, pre-intervention, post-intervention, and 10 minutes after the pain had subsided.

Results

Hypertonic saline produced continuous and constant peri-ankle pain (VAS peak [median]= 6.7 [2.1–8.4] cm) compared to isotonic saline (0 [0–0.8] cm). In response to pain, there were significant decreases in the H/M ratio, MVC and contractile activities (P<0.01), all of which were successfully reversed after the ankle motion imagery. In contrast, no significant changes were observed with the finger motion imagery.

Conclusion

Visually-assisted motor imagery improved the pain-induced AMI. Motor imagery of the painful joint itself would efficiently work for relieving AMI. This investigation possibly shows the potential of a novel and versatile approach against AMI for patients with musculoskeletal pain.

State-Dependent Gain Modulation of Spinal Motor Output

Afferent somatosensory information plays a crucial role in modulating efferent motor output. A better understanding of this sensorimotor interplay may inform the design of neurorehabilitation interfaces. Current neurotechnological approaches that address motor restoration after trauma or stroke combine motor imagery (MI) and contingent somatosensory feedback, e.g., via peripheral stimulation, to induce corticospinal reorganization. These interventions may, however, change the motor output already at the spinal level dependent on alterations of the afferent input. Neuromuscular electrical stimulation (NMES) was combined with measurements of wrist deflection using a kinematic glove during either MI or rest. We investigated 360 NMES bursts to the right forearm of 12 healthy subjects at two frequencies (30 and 100 Hz) in random order. For each frequency, stimulation was assessed at nine intensities. Measuring the induced wrist deflection across different intensities allowed us to estimate the input-output curve (IOC) of the spinal motor output. MI decreased the slope of the IOC independent of the stimulation frequency. NMES with 100 Hz vs. 30 Hz decreased the threshold of the IOC. Human-machine interfaces for neurorehabilitation that combine MI and NMES need to consider bidirectional communication and may utilize the gain modulation of spinal circuitries by applying low-intensity, high-frequency stimulation.

Emergence of F-waves after repetitive nerve stimulation

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Absence of F-waves in weak patients may be due to reduced F-wave excitability.

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Re-emergence of F-waves was elicited after a 1-second train of 20 Hz supramaximal RNS.

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“Re-awakening” of F-waves using RNS may help exclude AIDP as a cause of weakness.

Aim

Absence of the F-wave may represent the inability of spinal motor neurons to be excited after periods of inactivity. Repetitive stimulation in an otherwise immobile patient acts as a voluntary movement therefore allowing for the production of an F-wave in a patient with previously demonstrated absent F-waves. Through this case report, we attempt to highlight that the absence of the F-wave may result from inexcitability of spinal motor neurons after reduced mobilization.

Case

We present the case of a 48-year-old woman who had been hospitalized in an ICU setting for almost one month due to a subarachnoid hemorrhage, pancreatitis, and respiratory failure. An electromyogram and nerve conduction study (NCS) was performed for weakness in all four extremities. On routine NCS, her F-waves were absent, but after repetitive stimulation was performed, her F-waves appeared.

Discussion

This may be further evidence that the absence of the F-wave may result from inexcitability of spinal motor neurons after immobilization or reduced mobility rather than true pathology of the peripheral nerve. The ability to recover F-waves after an initial absence could assist in differentiating between inexcitability of the anterior horn cell and proximal nerve conduction block. This case presentation is an attempt to show that repetitive nerve stimulation may prove to be a useful technique to restore F-waves in patients who are unable to voluntarily contract their muscles, which may help exclude certain pathologic processes.

Age-related characteristics and normative values of F waves in healthy infants

OBJECTIVE

To establish age-related characteristics and normative values of F waves in healthy Chinese infants.

METHODS

We studied median, ulnar and tibial nerves on one side distally in 229 healthy Chinese infants (108 males) ranging from 1 to 12 months old.

RESULTS

Minimal F-wave latencies (Fmin) showed a strong negative correlation to the age for median, ulnar and tibial nerves (P < 0.01) but no correlation to the height. Statistical analyses revealed a significant (P < 0.01) decrease of Fmin during the second month of life and no change (P > 0.05) thereafter. Dividing the infants into 1 month old (Group 1) and 2-12 months old (Group 2), normal values (Mean ± SD ms) of Fmin for tibial, median and ulnar nerves consisted of 23.38 ± 1.68, 17.19 ± 0.95 and 16.47 ± 1.06 for Group 1 and 21.42 ± 1.25, 14.50 ± 1.15 and 14.52 ± 0.90 for Group 2.

CONCLUSION

F-wave latencies shorten in the 2nd month of life and change little thereafter when age-related maturation counters the concomitant growth of the nerve length.

SIGNIFICANCE

F waves can assess infantile neuropathies as a reliable measure, complementing the technically difficult conventional nerve conduction study in short limbs.

Immobility and F-waves: Impact on lower motor neuron excitability

INTRODUCTION

Immobility of the upper limbs has been associated with reduction of F-wave frequency. However, there are no similar studies on lower limb (LL) F-waves. We investigated the impact of LL rest on F-wave and H-reflex parameters.

METHODS

The LLs of 14 healthy participants were studied after 90 minutes rest. F-waves (frequency, latencies, chronodispersion, and mean amplitude) and H-reflexes (latency and recruitment curve) were investigated bilaterally. In seven participants the protocol was repeated, but the temperature of one limb was reduced.

RESULTS

Immobility only changed F-wave latencies, which increased significantly (mean value of 2 ms, P < .01). Limb cooling did not influence results.

DISCUSSION

Contrary to what occurred in cervical lower motor neurons (LMN), LL LMNs did not show a reduced F-wave response to immobility, but their latency increased significantly. This could have been due to reduced Renshaw inhibition of small LMNs, thus facilitating their response to antidromic stimulation and causing delayed late responses.

F wave, A wave, H reflex, and blink reflex

Late responses include F waves, A waves, H reflex, and the blink reflex. These responses help enhance routine nerve conduction studies. Despite the use of F waves in multiple clinical applications, their studies can technically challenge even the most experienced electromyographers. They vary in latency, amplitude, and configuration, whereas A waves show no change in latency or morphology. Electrical stimulation of the supraorbital branch of the trigeminal nerve on one side results in a reflexive activation of the facial nucleus causing contraction of the orbicularis oculi muscle, short latency R1 ipsilaterally, and long latency R2 bilaterally. F waves can help determine the presence of a polyneuropathy. A waves can reflect axonal damage. H reflexes provide nerve conduction measurements along the entire length of the nerve, demonstrating abnormalities in neuropathies and radiculopathies. Abnormalities in the blink reflex can suggest the presence of an acoustic neuroma or a demyelinating polyneuropathy, which can affect the cranial nerves. This reflex, which also needs appropriate technical expertise, helps to assess cranial nerves V and VII along with their connections in the pons and medulla. The blink reflex, the electrical version of the corneal reflex, represents a polysynaptic reflex.

Does the duration of motor imagery affect the excitability of spinal anterior horn cells?

PURPOSE

Motor imagery, the process of imagining a physical action, has been shown to facilitate the excitability of spinal anterior horn cells. In the acute phase after a stroke, the excitability of spinal anterior horn cells is significantly reduced, which leads to motor deficits. This loss of movement can be prevented by increasing the excitability of spinal anterior horn cells immediately following an injury. Motor imagery is an effective method for facilitating the excitability of spinal anterior horn cells in patients with impaired movement; however, the optimal duration for motor imagery is unclear.

MATERIALS AND METHODS

To investigate time-dependent changes in spinal anterior horn cell excitability during motor imagery, healthy adult participants were recruited to measure the F-wave, an indicator of anterior horn cell excitability. F-waves were measured from participants at baseline, during motor imagery, and post-motor imagery. During motor imagery, participants imagined isometric thenar muscle activity at 50% maximum voluntary contraction for 5 min. F-waves were measured at 1, 3, and 5 min after beginning motor imagery and analysed for persistence and F/M amplitude ratio.

RESULTS

Persistence and F/M amplitude ratios at 1- and 3-min after motor imagery initiation were significantly greater than at baseline. The persistence and F/M amplitude ratio at 5-min after motor imagery initiation, however, was comparable to baseline levels.

CONCLUSION

Therefore, 1 to 3 min of motor imagery is likely sufficient to facilitate the excitability of spinal anterior horn cells.

Unilateral repetitive tibial nerve stimulation improves neurogenic claudication and bilateral F-wave conduction in central lumbar spinal stenosis

BACKGROUND

Repetitive electrical nerve stimulation of the lower limb may improve neurogenic claudication in patients with lumbar spinal stenosis (LSS) as originally described by Tamaki et al. We tested if this neuromodulation technique affects the F-wave conduction on both sides to explore the underlying physiologic mechanisms.

METHODS

We studied a total of 26 LSS patients, assigning 16 to a study group receiving repetitive tibial nerve stimulation at the ankle (RTNS) on one leg, and 10 to a group without RTNS. RTNS conditioning consisted of a 0.3-ms duration square-wave pulse with an intensity 20% above the motor threshold, delivered at a rate of 5 Hz for 5 min. All patients underwent the walking test and the F-wave and M-wave studies for the tibial nerve on both sides twice; once as the baseline, and once after either the 5-min RTNS or 5-min rest.

RESULTS

Compared to the baselines, a 5-min RTNS increased claudication distance (176 ± 96 m vs 329 ± 133 m; p = 0.0004) and slightly but significantly shortened F-wave minimal onset latency (i.e., increased F-wave conduction velocity) not only on the side receiving RTNS (50.7 ± 4.0 ms vs 49.2 ± 4.2 ms; p = 0.00081) but also on the contralateral side (50.1 ± 4.6 ms vs 47.9 ± 4.2 ms; p = 0.011). A 5-min rest in the group not receiving RTNS neither had a significant change on claudication distance nor on any F-wave measurements. The M response remained unchanged in both groups.

CONCLUSIONS

The present study verified a beneficial effect of unilaterally applied RTNS of a mild intensity on neurogenic claudication and bilateral F-wave conduction. Our F-wave data suggest that this type of neuromodulation could be best explained by an RTNS-induced widespread sympathetic tone reduction with vasodilation, which partially counters a walking-induced further decline in nerve blood flow in LSS patients who already have ischemic cauda equina.

Correlation between Motor Cortex Excitability Changes and Cognitive Impairment in Vascular Depression: Pathophysiological Insights from a Longitudinal TMS Study

Background. Transcranial magnetic stimulation (TMS) highlighted functional changes in dementia, whereas there are few data in patients with vascular cognitive impairment-no dementia (VCI-ND). Similarly, little is known about the neurophysiological impact of vascular depression (VD) on deterioration of cognitive functions. We test whether depression might affect not only cognition but also specific cortical circuits in subcortical vascular disease. Methods. Sixteen VCI-ND and 11 VD patients, age-matched with 15 controls, underwent a clinical-cognitive, neuroimaging, and TMS assessment. After approximately two years, all participants were prospectively reevaluated. Results. At baseline, a significant more pronounced intracortical facilitation (ICF) was found in VCI-ND patients. Reevaluation revealed an increase of the global excitability in both VCI-ND and VD subjects. At follow-up, the ICF of VCI-ND becomes similar to the other groups. Only VD patients showed cognitive deterioration. Conclusions. Unlike VD, the hyperfacilitation found at baseline in VCI-ND patients suggests enhanced glutamatergic neurotransmission that might contribute to the preservation of cognitive functioning. The hyperexcitability observed at follow-up in both groups of patients also indicates functional changes in glutamatergic neurotransmission. The mechanisms enhancing the risk of dementia in VD might be related either to subcortical vascular lesions or to the lack of compensatory functional cortical changes.

Cholinergic circuitry functioning in patients with vascular cognitive impairment--no dementia

BACKGROUND

An impairment of central cholinergic activity, as evaluated non-invasively by the short-latency afferent inhibition (SAI) of motor responses evoked by transcranial magnetic stimulation (TMS), was observed in patients with Alzheimer's disease (AD) and amnestic Mild Cognitive Impairment. Conversely, the involvement of central cholinergic neurotransmission in vascular dementia (VaD) is still under debate and data on Vascular Cognitive Impairment--No Dementia (VCI-ND) at risk for future VaD are lacking.

OBJECTIVE

To test for the first time SAI in patients with VCI-ND.

METHODS

Single-pulse TMS measures of cortical excitability and SAI were evaluated in 25 VCI-ND patients with subcortical ischemic lesions and 20 age-matched healthy controls. Functional status, neuropsychological tests evaluating frontal lobe abilities, and white matter lesions (WMLs) load were assessed.

RESULTS

A significant difference was found between patients and controls for the mean SAI, although this result did not resist after the Bonferroni correction. In the whole group of patients and controls, SAI showed a correlation with worse scores at the Montreal Cognitive Assessment (r = 0.376, p < 0.01). SAI also positively correlated with the total vascular burden (r = 0.345, p < 0.05) but not with the WML severity.

CONCLUSIONS

Central cholinergic pathway does not seem to be involved in VCI-ND, and the current results differ from those reported in primary cholinergic forms of dementia, such as AD. SAI might represent a valuable additional tool in the differential diagnosis of the dementing processes and in identifying potential responders to cholinergic agents.

Correlation between Motor Cortex Excitability Changes and Cognitive Impairment in Vascular Depression: Pathophysiological Insights from a Longitudinal TMS Study

Background. Transcranial magnetic stimulation (TMS) highlighted functional changes in dementia, whereas there are few data in patients with vascular cognitive impairment-no dementia (VCI-ND). Similarly, little is known about the neurophysiological impact of vascular depression (VD) on deterioration of cognitive functions. We test whether depression might affect not only cognition but also specific cortical circuits in subcortical vascular disease. Methods. Sixteen VCI-ND and 11 VD patients, age-matched with 15 controls, underwent a clinical-cognitive, neuroimaging, and TMS assessment. After approximately two years, all participants were prospectively reevaluated. Results. At baseline, a significant more pronounced intracortical facilitation (ICF) was found in VCI-ND patients. Reevaluation revealed an increase of the global excitability in both VCI-ND and VD subjects. At follow-up, the ICF of VCI-ND becomes similar to the other groups. Only VD patients showed cognitive deterioration. Conclusions. Unlike VD, the hyperfacilitation found at baseline in VCI-ND patients suggests enhanced glutamatergic neurotransmission that might contribute to the preservation of cognitive functioning. The hyperexcitability observed at follow-up in both groups of patients also indicates functional changes in glutamatergic neurotransmission. The mechanisms enhancing the risk of dementia in VD might be related either to subcortical vascular lesions or to the lack of compensatory functional cortical changes.

Excitability of spinal motor neurons during motor imagery of thenar muscle activity under maximal voluntary contractions of 50% and 100

[Purpose] We often perform physical therapy using motor imagery of muscle contraction to improve motor function for healthy subjects and central nerve disorders. This study aimed to determine the differences in the excitability of spinal motor neurons during motor imagery of a muscle contraction at different contraction strengths. [Subjects] We recorded the F-wave in 15 healthy subjects. [Methods] In resting trial, the muscle was relaxed during F-wave recording. For motor imagery trial, subjects were instructed to imagine maximal voluntary contractions of 50% and 100% while holding the sensor of a pinch meter, and F-waves were recorded for each contraction. The F-wave was recorded immediately after motor imagery. [Results] Persistence and F/M amplitude ratio during motor imagery under maximal voluntary contractions of 50% and 100% were significantly higher than that at rest. In addition, the relative values of persistence, F/M amplitude ratio, and latency were similar during motor imagery under the two muscle contraction strengths. [Conclusion] Motor imagery under maximal voluntary contractions of 50% and 100% can increase the excitability of spinal motor neurons. Differences in the imagined muscle contraction strengths are not involved in changes in the excitability of spinal motor neurons.

Effect of Motor Imagery on the F-Wave Parameters in Hemiparetic Stroke Survivors

Objective

To assess the effect of motor imagery, as a rehabilitation method in stroke, on F-wave parameters that undergo changes during upper motor neuron involvement.

Methods

Twenty-one fully conscious hemiparetic stroke survivors with a completely plegic hand (power 0/5) and a minimum interval of 72 hours since stroke were recruited into this study. The mean F-wave latency, amplitude, and persistence in the median and ulnar nerves were measured in both the affected and non-affected sides at rest and in the paretic hand during a mental task. Comparison was made between data from the affected hand and the non-affected hand as well as between data from the affected hand at baseline and during motor imagery.

Results

Patients had significantly different F-wave persistence between the affected and non-affected sides (paired t-test, p<0.001). Motor imagery could improve F-wave persistence in both the investigated nerves (paired t-test, p=0.01 for ulnar nerve and p<0.001 for median nerve) and F-response amplitude in the median nerve (paired t-test, p=0.01) of the affected limb.

Conclusion

The amplitude and persistence of F-wave were improved during motor imagery, representing F-wave facilitation. This result suggests that motor imagery can restore motor neuron excitability, which is depressed after stroke.

Sensorimotor event-related desynchronization represents the excitability of human spinal motoneurons

Amplitudes of mu and beta (7-26Hz) oscillations measured by electroencephalography over the sensorimotor areas are suppressed during motor imagery as well as during voluntary movements. This phenomenon is referred to as event-related desynchronization (ERD) and is known to reflect motor cortical excitability. The increased motor cortical excitability associated with ERD during hand motor imagery would induce a descending cortical volley to spinal motoneurons, resulting in facilitation of spinal motoneuronal excitability. Therefore, in the present study, we tested the association of ERD during motor imagery with the excitability of spinal motoneurons in 15 healthy participants. Spinal excitability was tested using the F-wave recorded from the right abductor pollicis brevis muscle. The F-wave results from antidromic activation of spinal motoneurons and is induced by peripheral nerve stimulation. Participants performed 5s of motor imagery of right thumb abduction following 7s of rest. The right median nerve was stimulated at wrist level when the ERD magnitude of the contralateral hand sensorimotor area exceeded predetermined thresholds during motor imagery. The results showed ERD magnitude during hand motor imagery was associated with an increase in F-wave persistence, but not with the response average of F-wave amplitude or F-wave latency. These findings suggest that the ERD magnitude may be a biomarker representing increases in the excitability of both cortical and spinal levels.

F-wave and motor-evoked potentials during motor imagery and observation in apraxia of Parkinson disease

INTRODUCTION

The amplitudes of F-waves and motor-evoked potentials (MEPs) increase during imagination or active motor performance. The aim of this study was to investigate F-wave and MEP facilitation during assessment of apraxia.

METHODS

Eight Parkinson disease (PD) patients with apraxia, 11 patients without apraxia, and 8 healthy volunteers were enrolled. F-waves and MEPs were recorded during 4 states (resting, imagination, observation, and active movement).

RESULTS

The mean amplitude of the F-waves increased significantly during imagination and active movement as compared with at rest in healthy individuals (P = 0.028) and in the nonapraxia group (P = 0.005). PD patients with apraxia did not have similar facilitation. The mean amplitude of the MEPs also showed a similar loss of facilitation in PD with apraxia.

CONCLUSIONS

Loss of facilitation during the preparation for movement is closely related to the "gold standard" clinical praxis battery. This study provides additional support and a potential electrophysiological assessment method for apraxia in PD.

Influence of motor imagery of isometric opponens pollicis activity on the excitability of spinal motor neurons: a comparison using different muscle contraction strengths

[Purpose] This study aimed to determine the differences in the excitability of spinal motor neurons during motor imagery of a muscle contraction at different contraction strengths. [Methods] We recorded the F-wave in 15 healthy subjects. First, in a trial at rest, the muscle was relaxed during F-wave recording. Next, during motor imagery, subjects were instructed to imagine maximum voluntary contractions of 10%, 30%, and 50% while holding the sensor of a pinch meter, and F-waves were recorded for each contraction. F-waves were recorded immediately and at 5, 10, and 15 min after motor imagery. [Results] Both persistence and F/M amplitude ratios during motor imagery under maximum voluntary contractions of 10%, 30%, and 50% were significantly higher than that at rest. In addition, persistence, F/M amplitude ratio, and latency were similar during motor imagery under the three muscle contraction strengths. [Conclusion] Motor imagery under maximum voluntary contractions of 10%, 30%, and 50% can increase the excitability of spinal motor neurons. The results indicated that differences in muscle contraction strengths during motor imagery are not involved in changes in the excitability of spinal motor neurons.

F-wave suppression induced by suprathreshold high-frequency repetitive trascranial magnetic stimulation in poststroke patients with increased spasticity

OBJECTIVE

High-intensity and high-frequency repetitive transcranial magnetic stimulation (rTMS) of the primary motor cortex was carried out in poststroke patients with increased spasticity, and the changes in F-wave parameters in comparison with M-wave parameters induced by rTMS were examined.

METHODS

Ten-hertz rTMS pulses were delivered to the primary motor cortex of the lesion side at 110% intensity of the resting motor threshold, and F-waves were obtained from the first dorsal interosseous muscle. F-waves were recorded before (pre-stim) and immediately after the end of rTMS (post-stim) in poststroke patients.

RESULTS

F-wave persistence and F/M Amp.Ratio increased significantly in patients with lesions in upper motor tract as compared with healthy subjects (Wilcoxon rank sum test, p = 0.00023 and p = 0.0073, respectively). After the rTMS application, both F-wave persistence and F/M Amp.Ratio decreased significantly (paired t-test, p = 0.0095 and p = 0.037, respectively). However, the F-wave amplitude did not show a statistically significant variance in poststroke patients.

CONCLUSIONS

High-frequency suprathreshold rTMS may suppress the F-waves by enhancing the inhibitory effect on spinal excitability through the corticospinal tract, and F-wave persistence and F/M Amp.Ratio can be used to determine the effect of rTMS on patients with increased spasticity.

Imagining is Not Doing but Involves Specific Motor Commands: A Review of Experimental Data Related to Motor Inhibition

There is now compelling evidence that motor imagery (MI) and actual movement share common neural substrate. However, the question of how MI inhibits the transmission of motor commands into the efferent pathways in order to prevent any movement is largely unresolved. Similarly, little is known about the nature of the electromyographic activity that is apparent during MI. In addressing these gaps in the literature, the present paper argues that MI includes motor execution commands for muscle contractions which are blocked at some level of the motor system by inhibitory mechanisms. We first assemble data from neuroimaging studies that demonstrate that the neural networks mediating MI and motor performance are not totally overlapping, thereby highlighting potential differences between MI and actual motor execution. We then review MI data indicating the presence of subliminal muscular activity reflecting the intrinsic characteristics of the motor command as well as increased corticomotor excitability. The third section not only considers the inhibitory mechanisms involved during MI but also examines how the brain resolves the problem of issuing the motor command for action while supervising motor inhibition when people engage in voluntary movement during MI. The last part of the paper draws on imagery research in clinical contexts to suggest that some patients move while imagining an action, although they are not aware of such movements. In particular, experimental data from amputees as well as from patients with Parkinson’s disease are discussed. We also review recent studies based on comparing brain activity in tetraplegic patients with that from healthy matched controls that provide insights into inhibitory processes during MI. We conclude by arguing that based on available evidence, a multifactorial explanation of motor inhibition during MI is warranted.