Postsynaptic potentials of soleus motor neurons produced by transspinal stimulation: a human single-motor unit study.

Transspinal (or transcutaneous spinal cord) stimulation is a noninvasive, cost-effective, easily applied method with great potential as a therapeutic modality for recovering somatic and nonsomatic functions in upper motor neuron disorders. However, how transspinal stimulation affects motor neuron depolarization is poorly understood, limiting the development of effective transspinal stimulation protocols for rehabilitation. In this study, we characterized the responses of soleus α motor neurons to single-pulse transspinal stimulation using single-motor unit (SMU) discharges as a proxy given the 1:1 discharge activation between the motor neuron and the motor unit. Peristimulus time histogram, peristimulus frequencygram, and surface electromyography (sEMG) were used to characterize the postsynaptic potentials of soleus motor neurons. Transspinal stimulation produced short-latency excitatory postsynaptic potentials (EPSPs) followed by two distinct phases of inhibitory postsynaptic potentials (IPSPs) in most soleus motor neurons and only IPSPs in others. Transspinal stimulation generated double discharges at short interspike intervals in a few motor units. The short-latency EPSPs were likely mediated by muscle spindle group Ia and II afferents, and the IPSPs via excitation of group Ib afferents and recurrent collaterals of motor neurons leading to activation of diverse spinal inhibitory interneuronal circuits. Further studies are warranted to understand better how transspinal stimulation affects depolarization of α motor neurons over multiple spinal segments. This knowledge will be seminal for developing effective transspinal stimulation protocols in upper motor neuron lesions.NEW & NOTEWORTHY Transspinal stimulation produces distinct actions on soleus motor neurons: an early short-latency excitation followed by two inhibitions or only inhibition and doublets. These results show how transspinal stimulation affects depolarization of soleus α motor neurons in healthy humans.

Single motor unit estimation of the cutaneous silent period in ALS.

OBJECTIVE: Recent evidence indicated that amyotrophic lateral sclerosis (ALS) also impairs spinal circuits, including those mediating cutaneous silent period (CSP). However, most studies utilised surface electromyography (sEMG), which needs more resolution to pinpoint changes at the single motoneuron level. We aimed to investigate CSP properties using single motor unit discharges in ALS. METHODS: In mild and severe ALS patients and controls, CSP was recorded in the first dorsal interosseus and analysed using the discharge rate method, which accurately shows the inhibitory postsynaptic potentials (IPSPs) profile. RESULTS: Our findings confirmed that the CSP latency was prolonged only in severe ALS patients. Moreover, the CSP duration was similar in each group, but late-stage ALS patients tend to have a longer CSP duration. The discharge rate method revealed a significantly longer duration (up to 150 ms) than the duration detected using sEMG. Strikingly, the motoneuron discharge rate - IPSP duration inverse relationship is lost in ALS patients, indicating a possible impairment in the motoneuron integrative properties. CONCLUSIONS: Our data support previous findings of prolonged latency, presented input-output modifications of motoneurons, and revealed the entire course of the CSP, representing a much stronger inhibitory event than previously thought. SIGNIFICANCE: Motoneuron integrative property modification assessed by CSP could be a new biomarker for ALS.

Estimating and minimizing movement artifacts in surface electromyogram.

While recording surface electromyography [sEMG], it is possible to record the electrical activities coming from the muscles and transients in the half-cell potential at the electrode-electrolyte interface due to micromovements of the electrode-skin interface. Separating the two sources of electrical activity usually fails due to the overlapping frequency characteristics of the signals. This paper aims to develop a method that detects movement artifacts and suggests a minimization technique. Towards that aim, we first estimated the frequency characteristics of movement artifacts under various static and dynamic experimental conditions. We found that the extent of the movement artifact depended on the nature of the movement and varied from person to person. Our study's highest movement artifact frequency for the stand position was 10 Hz, tiptoe 22, walk 32, run 23, jump from box 41, and jump up and down 40 Hz. Secondly, using a 40 Hz highpass filter, we cut out most of the frequencies belonging to the movement artifacts. Finally, we checked whether the latencies and amplitudes of reflex and direct muscle responses were still observed in the highpass-filtered sEMG. We showed that the 40 Hz highpass filter did not significantly alter reflex and direct muscle variables. Therefore, we recommend that researchers who use sEMG under similar conditions employ the recommended level of highpass filtering to reduce movement artifacts from their records. However, suppose different movement conditions are used. In that case, it is best to estimate the frequency characteristics of the movement artifact before applying any highpass filtering to minimize movement artifacts and their harmonics from sEMG.

A reliability study on the cumulative averaging method for estimating effective stimulus time in vibration studies.

Finding the reflex circuitry responsible for high-frequency vibration-induced muscle contraction takes work. The main challenge is to determine the effective stimulus time (EST) point at which continuous (sinusoidal) stimulation (i.e., vibration) triggers the reflex response. A novel "cumulated averaging method" has been previously proposed for estimating the EST point. In the current study, we aimed to test the reliability of the cumulated average method. We used five different whole-body vibration (WBV) frequencies in two experiments. The consistency between the EST points estimated from the first and second experiments was analysed with the intraclass correlation (ICC) and technical error of measurement (TEM). The ICC coefficient with 95% CI for the EST point estimation was 0.988 (0.950-0.997). The relative TEM was 1.3%. We concluded that the cumulated average method is highly reliable in estimating the effective stimulus time point for high-frequency continuous sinusoidal signals.

Demonstration of chewing-related areas in the brain via functional magnetic resonance imaging.

Purpose: To localize and identify chewing-related areas and their connections with other centres in the human brain using functional magnetic resonance imaging (fMRI). Material and methods: The paradigm of the present study was block designed. Spontaneous and controlled chewing with sugar-free gum was used as the main task in a 3-Tesla fMRI unit with a 32-channel birdcage coil. Our study popu-lation comprised 32 healthy volunteers. To determine possible intersections, we also put the rosary pulling (silent tell one's beads) movement in the fMRI protocol. The data analyses were performed with the Statistical Parametric Mapping (SPM) toolbox integrated into the Matlab platform. Results: The superomedial part of the right cerebellum was activated during either pulling rosary beads or spontaneous chewing. This region, however, was not activated during controlled chewing. We did not find statistically significant activation or connection related to the brain stem. Conclusion: We have confirmed that the cerebellum plays an important role in chewing. However, we could not find a definite central pattern generator (CPG) in the brain stem, which has been hypothesized to underlie spontaneous chewing.

The reflex mechanism underlying the neuromuscular effects of whole-body vibration: Is it the tonic vibration reflex?

OBJECTIVES: Whole-body vibration (WBV) is applied to the sole of the foot, whereas local mechanical vibration (LMV) is applied directly to the muscle or tendon. The time required for the mechanical stimulus to reach the muscle belly is longer for WBV. Therefore, the WBV-induced muscular reflex (WBV-IMR) latency may be longer than the tonic vibration reflex (TVR) latency. The aim of this study was to determine whether the difference between WBV-IMR and TVR latencies is due to the distance between the vibration application point and the target muscle. METHODS: Eight volunteers participated in this study. The soleus reflex response was recorded during WBV, LMVs, and tendon tap. LMVs were applied to the Achilles tendon and sole of the foot. The latencies were calculated using the cumulative averaging technique. RESULTS: The latency (33.4±2.8 ms) of the soleus reflex induced by the local foot vibration was similar to the soleus TVR latency (30.9±3.2 ms) and T-reflex (32.0±2.4 ms) but significantly shorter than the latency of the soleus WBV-IMR (42.3±3.4 ms) (F(3,21)=27.46, p=0.0001, partial η2=0.797). CONCLUSIONS: The present study points out that the neuronal circuitries of TVR and WBV-IMR are different.

An opinion on the 'delayed spikes' in human motoneurons.

This is a note challenging the claim by Kudina and Andreeva's recent publication in Experimental Brain Research. In that publication, Kudina and Andreeva (Exp Brain Res 239:719-730, 2021) put forward a new idea about discovering two spiking modes in human motoneurons. We suggest that what they have shown in their publication maybe is the motor unit firing indicating the end of a net synaptic potential. We reason this challenge from our previous publication in the same journal. In that publication, we have shown that the "second spiking mode" after the H-reflex was a return to the regular prestimulus discharge rate.

A new method to determine stretch reflex latency.

INTRODUCTION/AIMS: Motion artifact signals (MASs) created by the relative movement of intramuscular wire electrodes are an indicator of the mechanical stimulus arrival time to the muscle belly. This study proposes a method that uses wire electrodes as an intramuscular mechanosensor to determine the stretch reflex (SR) latency without lag time. METHODS: Gastrocnemius SR was induced by tendon tap, heel tap, and forefoot tap. The MASs recorded by intramuscular wire electrodes were extracted from background electromyographic activity using the spike-triggered averaging technique. Simultaneous recordings were obtained from multiple sites to validate the MAS technique. RESULTS: Using intramuscular wire electrodes, the MASs were successfully determined and extracted for all stimulus sites. In the records from the rectus femoris, MASs were also successfully extracted; thus, the reflex latency could be calculated. DISCUSSION: Wire electrodes can be used as an intramuscular mechanosensor to determine the mechanical stimulus arrival time to the muscle belly.

Estimating Exercise-Induced Changes in Human Neuronal Networks.

Although several methods have been used to estimate exercise-induced changes in human neuronal networks, there are growing doubts about the methodologies used. This review describes a single motor unit-based method that minimizes the errors inherent in classical methods. With this method, it is now possible to identify human neuronal networks' changes due to exercise.

Amyotrophic lateral sclerosis weakens spinal recurrent inhibition and post-activation depression.

OBJECTIVES: Amyotrophic lateral sclerosis (ALS) disrupts motoneurons that control movement and some vital functions, however, exact details of the neuronal circuits involved in ALS have yet to be fully endorsed. To contribute to our understanding of the responsible neuronal circuits, we aimed to investigate the spinal recurrent inhibition (RI) and post-activation depression (P-AD) in ALS patients. METHODS: In two groups of ALS patients, i.e. lumbar-affected (clinical signs in leg muscles) and nonlumbar-affected (clinical signs in arms or bulbar region but not in the legs), RI and P-AD on the soleus muscle were investigated using single motor units and amplitude changes of H-reflex in surface electromyography, respectively. The data were compared with healthy subjects. RESULTS: Compared to controls, P-AD of H-reflex was reduced severely in lumbar-affected patients and reduced to a certain degree in nonlumbar-affected patients. Similarly, a significant reduction in the duration of RI on firing motoneurons was found in lumbar-affected patients (11.5 ± 2.6 ms) but not in nonlumbar-affected patients (29.7 ± 12.4 ms, P < 0.0001) compared to controls (30.8 ± 7.2 ms, P < 0.0001). CONCLUSION: The current study revealed that spinal inhibitory circuits are impaired in ALS. SIGNIFICANCE: These findings may provide insight for proposing new therapeutic approaches and following disease progression in humans.

A stimulus rate that is not influenced by homosynaptic post-activation depression in chronic stroke.

PURPOSE: To determine a stimulus rate that is not influenced by homosynaptic post-activation depression for H-reflex studies in patients with chronic spasticity. MATERIALS AND METHODS: A cohort of 15 chronic stroke patients with soleus spasticity who received inpatient treatment at our rehabilitation centre participated in this study. The effect of stimulus frequency related depression on H-reflex size was tested using four different stimulus rates (0.1, 0.2, 0.3 and 1 Hz). The affected sides stibial nerve was stimulated by a bipolar electrode. The H-reflex was recorded from the affected sideed sidee sidehe affected smine stimulus frequency related depression of H-reflex size, amplitude of the first H-reflex response (H1) was used as control and amplitude of the second H-reflex response (H2) as test. RESULTS: H2 amplitude for frequency of 1 Hz, 0.3 Hz, 0.2 Hz and 0.1 Hz were 74.3, 84.1, 85.5 and 92.7% of H1, respectively. Depression of H2 amplitude was statistically significant for 1 Hz, 0.3 Hz and 0.2 Hz (p < 0.001, p = 0.002, p = 0.024, respectively). CONCLUSIONS: Higher frequency stimulation of Ia afferents than 0.1 Hz induced a stimulus frequency-related depression of H-reflex size in patients with chronic spasticity. The optimal stimulus rate for H-reflex was found to be 0.1 Hz.

Cross-training effect of chronic whole-body vibration exercise: a randomized controlled study.

Purpose: To determine whether unilateral leg whole-body vibration (WBV) strength training induces strength gain in the untrained contralateral leg muscle. The secondary aim was to determine the potential role of spinal neurological mechanisms regarding the effect of WBV exercise on contralateral strength training.Materials and Methods: Forty-two young adult healthy volunteers were randomized into two groups: WBV exercise and Sham control. An isometric semi-squat exercise during WBV was applied regularly through 20 sessions. WBV training was applied to the right leg in the WBV group and the left leg was isolated from vibration. Sham WBV was applied to the right leg of participants in the Control group. Pre- and post-training isokinetic torque and reflex latency of both quadricepses were evaluated.Results: The increase in the strength of right (vibrated) knee extensors was 9.4 ± 10.7% in the WBV group (p = .001) and was 1.2 ± 6.6% in the Control group (p = .724). The left (non-vibrated) extensorsvibrated) knee extensors w4 ± 8.4% in the WBV group (p = .038), whereas it decreased by 1.4 ± 7.0% in the Control (p = .294). The strength gains were significant between the two groups. WBV induced the reflex response of the quadriceps muscle in the vibrated ipsilateral leg and also in the non-vibrated contralateral leg, though with a definite delay. The WBV-induced muscle reflex (WBV-IMR) latency was 22.5 ± 7.7 ms for the vibrated leg and 39.3 ± 14.6 ms for the non-vibrated leg.Conclusions: Chronic WBV training has an effect of the cross-transfer of strength to contralateral homologous muscles. The WBV-induced muscular reflex may have a role in the mechanism of cross-transfer strength.

Exploring the receptor origin of vibration-induced reflexes.

STUDY DESIGN: An experimental design. OBJECTIVES: The aim of this study was to determine the latencies of vibration-induced reflexes in individuals with and without spinal cord injury (SCI), and to compare these latencies to identify differences in reflex circuitries. SETTING: A tertiary rehabilitation center in Istanbul. METHODS: Seventeen individuals with chronic SCI (SCI group) and 23 participants without SCI (Control group) were included in this study. Latency of tonic vibration reflex (TVR) and whole-body vibration-induced muscular reflex (WBV-IMR) of the left soleus muscle was tested for estimating the reflex origins. The local tendon vibration was applied at six different vibration frequencies (50, 85, 140, 185, 235, and 265 Hz), each lasting for 15 s with 3-s rest intervals. The WBV was applied at six different vibration frequencies (35, 37, 39, 41, 43, and 45 Hz), each lasting for 15 s with 3-s rest intervals. RESULTS: Mean (SD) TVR latency was 39.7 (5.3) ms in the SCI group and 35.9 (2.7) ms in the Control group with a mean (95% CI) difference of -3.8 (-6.7 to -0.9) ms. Mean (SD) WBV-IMR latency was 45.8 (7.4) ms in the SCI group and 43.3 (3.0) ms in the Control group with a mean (95% CI) difference of -2.5 (-6.5 to 1.4) ms. There were significant differences between TVR latency and WBV-IMR latency in both the groups (mean (95% CI) difference; -6.2 (-9.3 to -3.0) ms, p = 0.0001 for the SCI group and -7.4 (-9.3 to -5.6) ms, p = 0.011 for Control group). CONCLUSIONS: The results suggest that the receptor of origin of TVR and WBV-IMR may be different.

Jendrassik maneuver effect on spinal and brainstem reflexes.

The effect of Jendrassik Maneuver (JM) has been extensively studied on monosynaptic reflexes in numerous muscles below the level at which the maneuver was performed. Here we hypothesize that the effect of JM could be observed also on other reflexes, indicating a widespread influence of performing a motor act such as the JM. We examined polysynaptic reflexes caudal (i.e., the withdrawal reflex of the lower extremities) and rostral (i.e., the blink reflex to supraorbital nerve stimulation) to the level of JM contraction. We have assessed soleus tendon (T) reflex; withdrawal reflex in tibialis anterior and soleus muscle; blink reflex (BR), blink reflex excitability recovery curve (BR-ER) and prepulse inhibition of the blink reflex. Our results showed that (1) T-reflex amplitude increased during JM and decreased just after and 15 min after JM; (2) no change in the withdrawal reflex; (3) R2 area of BR reduced significantly just after or 15 min after JM; (4) Prepulse inhibition in BR reduced significantly during JM; (5) no change in BR-ER. Our results indicate that JM leads to generalized effects on neural excitability at both caudal and rostral levels. Furthermore, JM has a selective effect on excitability of reflex circuitries.

Facial muscle activity contaminates EEG signal at rest: evidence from frontalis and temporalis motor units.

OBJECTIVE: In order to reach electroencephalography (EEG) electrodes on the scalp, synchronized activity of neurons needs to pass thorough several tissue layers, including the skull and muscles covering the scalp. The contamination of EEG signal by temporalis and frontalis muscles has been well documented for voluntary muscle contraction even at low contraction levels. The extent of myogenic contamination during postural and/or rest activity of the temporalis and frontalis remains an impediment for EEG research. APPROACH: In this study, we first aimed to observe involuntary, continuous motor unit activity of the frontalis muscle at rest and evaluate motor unit level frontalis interference on the EEG electrodes. Second, we compared motor unit interference from the frontalis before and after artefact pruning via an independent component analysis (ICA) algorithm. MAIN RESULTS: We demonstrated that motor unit activity of the frontalis muscle produces interference potentials on the frontal electrodes at rest and the interference was significantly reduced after ICA on the frontal electrodes, but not completely eliminated. Likewise, the temporalis interference at rest was significantly smaller after ICA on the fronto-temporal electrodes, but not completely removed. SIGNIFICANCE: We documented the existence of resting involuntary activity of the temporalis and frontalis muscles underneath EEG electrodes and the removal of the EEG signal from their contiguous interference is not possible even after the use of ICA technology. We recommend that EEG researchers readdress the definition of 'rest' for EEG recordings and the ICA experts should extend their electromyography removal strategies to motor unit level interference.

The effects of a single session of chiropractic care on strength, cortical drive, and spinal excitability in stroke patients.

The objective of this study was to investigate whether a single session of chiropractic care could increase strength in weak plantar flexor muscles in chronic stroke patients. Maximum voluntary contractions (strength) of the plantar flexors, soleus evoked V-waves (cortical drive), and H-reflexes were recorded in 12 chronic stroke patients, with plantar flexor muscle weakness, using a randomized controlled crossover design. Outcomes were assessed pre and post a chiropractic care intervention and a passive movement control. Repeated measures ANOVA was used to asses within and between group differences. Significance was set at p < 0.05. Following the chiropractic care intervention there was a significant increase in strength (F (1,11) = 14.49, p = 0.002; avg 64.2 ± 77.7%) and V-wave/Mmax ratio (F(1,11) = 9.67, p = 0.009; avg 54.0 ± 65.2%) compared to the control intervention. There was a significant strength decrease of 26.4 ± 15.5% (p = 0.001) after the control intervention. There were no other significant differences. Plantar flexor muscle strength increased in chronic stroke patients after a single session of chiropractic care. An increase in V-wave amplitude combined with no significant changes in H-reflex parameters suggests this increased strength is likely modulated at a supraspinal level. Further research is required to investigate the longer term and potential functional effects of chiropractic care in stroke recovery.

Using first bout effect to study the mechanisms underlying eccentric exercise induced force loss.

INTRODUCTION: The first bout of eccentric exercise is known to have a protective effect on the consequent bouts. This effect is still disputable as it is not known whether it protects muscle damage by reducing force production or by improving force recovery in the healing process. The underlying mechanisms of this protective effect have not been fully understood. OBJECTIVES: To determine the mechanisms of this protective effect, three different loads were used for the first eccentric bout. This was done to investigate whether the protective effect is related to the size of the load in the first bout. To determine the neural adaptations, voluntary activation was assessed and to determine the muscular adaptations, the resting twitch was measured. METHOD: Thirty healthy participants were selectively allocated into three groups (low-, moderate- and high-load group) to match for maximal voluntary contraction (MVC) (n = 10 per group). Participants in each group performed only one of the three sets of ten eccentric (ECC) exercises of the elbow flexors (10%, 20% and 40% of MVC) as their first eccentric bout. The second bout of eccentric exercise was performed two weeks later and was identical for all the three groups, i.e., 40% ECC. RESULTS: The results showed that for the first bout, MVC, voluntary activation and the resting twitch displayed significant (p < 0.0001) interaction (group x time). This was not the case however for the second bout as there was no significant (group x time) interaction in all outcome variables immediately after exercise. When the first and second bouts were compared, it was found that the high-load group had faster recovery in MVC at day 1 and 4 corresponding to voluntary activation and only at day 4 corresponding to the resting twitch. CONCLUSIONS: In this study, it was found that high-load exercise aids fast recovery either via neural or muscular adaptations.

Periodontal mechanoreceptors and bruxism at low bite forces.

OBJECTIVE: In this study, we examined if 6-9 Hz jaw tremor, an indirect indicator of Periodontal Mechanoreceptor (PMR) activity, is different in bruxists compared to healthy participants during production of a low-level constant bite force. METHODS: Bite force and surface EMG from the masseter muscle were recorded simultaneously as participants (13 patients, 15 controls) held a force transducer between the upper and lower incisors very gently. RESULTS: Tremor in 6-9 Hz band for bruxists was greater on average compared to controls, but the difference was not significant, both for force recordings and EMG activity. CONCLUSIONS: The low effect sizes measured with the current protocol contrast highly with those of our previous study, where larger, dynamic bite forces were used, and where jaw tremor was markedly different in bruxists compared with controls. SIGNIFICANCE: We have now gained important insight into the conditions under which abnormal jaw tremor can be elicited in bruxism. From a scientific standpoint, this is critical for understanding the 'abnormality' of PMR feedback in bruxism. From a clinical perspective, our results represent progress towards the development of an optimal protocol in which jaw tremor can serve as a biological marker of bruxism.

Standardization of the Jendrassik maneuver in Achilles tendon tap reflex.

OBJECTIVE: For many decades, the Jendrassik maneuver (JM) has been used as a reinforcement for stretch reflexes, although the underlying mechanism of this reinforcement is still not fully understood. Moreover, the term JM has been used for many different muscle contraction strategies as there is no fixed movement for the maneuver in the literature. In this study, we aimed to investigate the effects of clenched hand pull, teeth clenching, and their combined effects to reach standardization. METHODS: Achilles tendon tap reflex responses in the soleus were recorded during rest (R), hand pull (HP), teeth clench (TC), and HP + TC combined, hereafter referred to as the JM. RESULTS: Reflex response amplitudes significantly increased during JM, HP, and TC in the soleus. HP and JM significantly changed the background activity in the soleus, but TC alone did not. CONCLUSION: These results suggest that dominantly presynaptic disinhibitory mechanisms may be responsible for the increase in the tendon tap reflex during HP, TC, and JM. SIGNIFICANCE: Because the findings indicate that HP increases the background activity of the soleus, we suggest that researchers should use only TC during the Jendrassik maneuver to avoid any confounding background activity change.

Optimal location for eliciting the tibial H-reflex and motor response.

INTRODUCTION: Although there are numerous protocols to adjust the amplitude of the Hoffmann reflex (H-reflex) relative to the size of the direct motor response (M-response), the optimal stimulating location has not been described. We sought to determine the optimal positioning of the stimulating cathode when evoking the tibial nerve H-reflex and M-response. METHODS: A small cathode was placed on defined points in the popliteal fossa while an anode was fixed on the patella. The tibial nerve was stimulated electrically, and the response of the soleus muscle was recorded using intramuscular and surface electromyography. RESULTS: We found that more-lateral points along a line drawn across the popliteal fossa were the best locations to obtain only the M-response, whereas stimulating the midpoint was optimal for obtaining only the H-reflex. DISCUSSION: By using specified locations for electrical stimulation to evoke H-reflex and M-response, the functionality of the tibial nerve can be assessed. Muscle Nerve 58:828-833, 2018.