Decrease in short-latency afferent inhibition during corticomotor postexercise depression following repetitive finger movement

Transcutaneous auricular vagus nerve stimulation enhances short-latency afferent inhibition via central cholinergic system activation

The present study examined the effects of transcutaneous auricular vagus nerve stimulation (taVNS) on short-latency afferent inhibition (SAI), as indirect biomarker of cholinergic system activation. 24 healthy adults underwent intermittent taVNS (30 s on/30 s off, 30 min) or continuous taVNS at a frequency of 25 Hz (15 min) along with earlobe temporary stimulation (15 min or 30 min) were performed in random order. The efficiency with which the motor evoked potential from the abductor pollicis brevis muscle by transcranial magnetic stimulation was attenuated by the preceding median nerve conditioning stimulus was compared before taVNS, immediately after taVNS, and 15 min after taVNS. Continuous taVNS significantly increased SAI at 15 min post-stimulation compared to baseline. A positive correlation (Pearson coefficient = 0.563, p = 0.004) was observed between baseline SAI and changes after continuous taVNS. These results suggest that 15 min of continuous taVNS increases the activity of the cholinergic nervous system, as evidenced by the increase in SAI. In particular, the increase after taVNS was more pronounced in those with lower initial SAI. This study provides fundamental insight into the clinical potential of taVNS for cholinergic dysfunction.

Short-latency afferent inhibition is reduced in people with multiple sclerosis during fatiguing muscle contractions

Understanding how inhibitory pathways influence motor cortical activity during fatiguing contractions may provide valuable insight into mechanisms associated with multiple sclerosis (MS) muscle activation. Short-latency afferent inhibition (SAI) reflects inhibitory interactions between the somatosensory cortex and the motor cortex, and although SAI is typically reduced with MS, it is unknown how SAI is regulated during exercise-induced fatigue. The current study examined how SAI modulates motor evoked potentials (MEPs) during fatiguing contractions. Fourteen people with relapsing-remitting MS (39 ± 6 years, nine female) and 10 healthy individuals (36 ± 6 years, six female) participated. SAI was induced by stimulation of the median nerve that was paired with TMS over the motor representation of the abductor pollicis brevis. A contraction protocol was employed that depressed force generating capacity using a sustained 3-min 15% MVC, immediately followed by a low-intensity (15% MVC) intermittent contraction protocol so that MEP and SAI could be measured during the rest phases of each duty cycle. Similar force, electromyography and MEP responses were observed between groups. However, the MS group had significantly reduced SAI during the contraction protocol compared to the healthy control group (p < .001). Despite the MS group reporting greater scores on the Fatigue Severity Scale and Modified Fatigue Impact Scale, these scales did not correlate with inhibitory measures. As there were no between-group differences in SSEPs, MS-related SAI differences during the fatiguing contractions were most likely associated with disease-related changes in central integration.

Association of short- and long-latency afferent inhibition with human behavior

Transcranial magnetic stimulation (TMS) paired with nerve stimulation evokes short-latency afferent inhibition (SAI) and long-latency afferent inhibition (LAI), which are non-invasive assessments of the excitability of the sensorimotor system. SAI and LAI are abnormally reduced in various special populations in comparison to healthy controls. However, the relationship between afferent inhibition and human behavior remains unclear. The purpose of this review is to survey the current literature and synthesize observations and patterns that affect the interpretation of SAI and LAI in the context of human behavior. We discuss human behaviour across the motor and cognitive domains, and in special and control populations. Further, we discuss future considerations for research in this field and the potential for clinical applications. By understanding how human behavior is mediated by changes in SAI and LAI, this can allow us to better understand the neurophysiological underpinnings of human motor control.

Intracortical GABAergic dysfunction in patients with fatigue and dysexecutive syndrome after COVID-19

Objective

A high proportion of patients experience fatigue and impairment of cognitive functions after coronavirus disease 2019 (COVID-19). Here we applied transcranial magnetic stimulation (TMS) to explore the activity of the main inhibitory intracortical circuits within the primary motor cortex (M1) in a sample of patients complaining of fatigue and presenting executive dysfunction after resolution of COVID-19 with neurological manifestations.

Methods

Twelve patients who recovered from typical COVID-19 pneumonia with neurological complications and complained of profound physical and mental fatigue underwent, 9 to 13 weeks from disease onset, a psychometric evaluation including a self-reported fatigue numeric-rating scale (FRS, Fatigue Rating Scale) and the Frontal Assessment Battery (FAB). Intracortical activity was evaluated by means of well-established TMS protocols including short-interval intracortical inhibition (SICI), reflecting GABA_A-mediated inhibition, long-interval intracortical inhibition (LICI), a marker of GABA_B receptor activity, and short-latency afferent inhibition (SAI) that indexes central cholinergic transmission. TMS data were compared to those obtained in a control group of ten healthy subjects (HS) matched by age, sex and education level.

Results

Post-COVID-19 patients reported marked fatigue according to FRS score (8.1 ± 1.7) and presented pathological scores at the FAB based on Italian normative data (12.2 ± 0.7). TMS revealed marked reduction of SICI, and disruption of LICI as compared to HS. SAI was also slightly diminished.

Conclusions

The present study documents for the first time reduced GABAergic inhibition in the M1 in patients who recovered from COVID-19 with neurological complications and manifested fatigue and dysexecutive syndrome.

Significance

TMS may serve as diagnostic tool in cognitive disturbances and fatigue in post-COVID-19 patients.

Effect of Repetitive Passive Movement Before Motor Skill Training on Corticospinal Excitability and Motor Learning Depend on BDNF Polymorphisms

A decrease in cortical excitability tends to be easily followed by an increase induced by external stimuli via a mechanism aimed at restoring it; this phenomenon is called “homeostatic plasticity.” In recent years, although intervention methods aimed at promoting motor learning using this phenomenon have been studied, an optimal intervention method has not been established. In the present study, we examined whether subsequent motor learning can be promoted further by a repetitive passive movement, which reduces the excitability of the primary motor cortex (M1) before motor learning tasks. We also examined the relationship between motor learning and the brain-derived neurotrophic factor. Forty healthy subjects (Val/Val genotype, 17 subjects; Met carrier genotype, 23 subjects) participated. Subjects were divided into two groups of 20 individuals each. The first group was assigned to perform the motor learning task after an intervention consisting in the passive adduction–abduction movement of the right index finger at 5 Hz for 10 min (RPM condition), while the second group was assigned to perform the task without the passive movement (control condition). The motor learning task consisted in the visual tracking of the right index finger. The results showed that the corticospinal excitability was transiently reduced after the passive movement in the RPM condition, whereas it was increased to the level detected in the control condition after the motor learning task. Furthermore, the motor learning ability was decreased immediately after the passive movement; however, the motor performance finally improved to the level observed in the control condition. In individuals carrying the Val/Val genotype, higher motor learning was also found to be related to the more remarkable changes in corticospinal excitability caused by the RPM condition. This study revealed that the implementation of a passive movement before a motor learning tasks did not affect M1 excitatory changes and motor learning efficiency; in contrast, in subjects carrying the Val/Val polymorphism, the more significant excitatory changes in the M1 induced by the passive movement and motor learning task led to the improvement of motor learning efficiency. Our results also suggest that homeostatic plasticity occurring in the M1 is involved in this improvement.

Corticospinal excitability of untrained side depends on the type of motor task and degree of improvement in motor function

Unimanual motor tasks change the corticospinal excitability of the trained and untrained side. However, whether the motor task type influences the modulation of the corticospinal excitability of the untrained side remains unclear. This study aimed to clarify the effects of motor tasks on the corticospinal excitability of the untrained side and the relationship between the excitability and motor function. In Experiment I, we measured the corticospinal excitability of the untrained side and motor function after 10 min of motor training in two conditions (gripping task and ball rotation task). The gripping task decreased the excitability. In contrast, excitability remained unchanged after the ball rotation task; further, the modulation of excitability and motor function showed a correlation. In Experiment II, we measured the corticospinal excitability of the untrained side and motor function after two sessions of the ball rotation task. The excitability increased, but motor function remained unchanged after the first session, whereas the excitability decreased to the level observed before training, and motor function improved after the second session. We suggest that the training condition modulates the corticospinal excitability of the untrained side and that this is related to the modulation of motor function.

Neuropsychological and neurophysiological correlates of fatigue in post-acute patients with neurological manifestations of COVID-19: Insights into a challenging symptom

More than half of patients who recover from COVID-19 experience fatigue. We studied fatigue using neuropsychological and neurophysiological investigations in post-COVID-19 patients and healthy subjects. Neuropsychological assessment included: Fatigue Severity Scale (FSS), Fatigue Rating Scale, Beck Depression Inventory, Apathy Evaluation Scale, cognitive tests, and computerized tasks. Neurophysiological examination was assessed before (PRE) and 2 min after (POST) a 1-min fatiguing isometric pinching task and included: maximum compound muscle action potential (CMAP) amplitude in first dorsal interosseous muscle (FDI) following ulnar nerve stimulation, resting motor threshold, motor evoked potential (MEP) amplitude and silent period (SP) duration in right FDI following transcranial magnetic stimulation of the left motor cortex. Maximum pinch strength was measured. Perceived exertion was assessed with the Borg-Category-Ratio scale. Patients manifested fatigue, apathy, executive deficits, impaired cognitive control, and reduction in global cognition. Perceived exertion was higher in patients. CMAP and MEP were smaller in patients both PRE and POST. CMAP did not change in either group from PRE to POST, while MEP amplitudes declined in controls POST. SP duration did not differ between groups PRE, increased in controls but decreased in patients POST. Patients' change of SP duration from PRE to POST was negatively correlated to FSS. Abnormal SP shortening and lack of MEP depression concur with a reduction in post-exhaustion corticomotor inhibition, suggesting a possible GABA_B-ergic dysfunction. This impairment might be related to the neuropsychological alterations. COVID-19-associated inflammation might lead to GABAergic impairment, possibly representing the basis of fatigue and explaining apathy and executive deficits.

Learning to Expect: Predicting Sounds During Movement Is Related to Sensorimotor Association During Listening

Sensory experiences, such as sound, often result from our motor actions. Over time, repeated sound-producing performance can generate sensorimotor associations. However, it is not clear how sensory and motor information are associated. Here, we explore if sensory prediction is associated with the formation of sensorimotor associations during a learning task. We recorded event-related potentials (ERPs) while participants produced index and little finger-swipes on a bespoke device, generating novel sounds. ERPs were also obtained as participants heard those sounds played back. Peak suppression was compared to assess sensory prediction. Additionally, transcranial magnetic stimulation (TMS) was used during listening to generate finger-motor evoked potentials (MEPs). MEPs were recorded before and after training upon hearing these sounds, and then compared to reveal sensorimotor associations. Finally, we explored the relationship between these components. Results demonstrated that an increased positive-going peak (e.g., P2) and a suppressed negative-going peak (e.g., N2) were recorded during action, revealing some sensory prediction outcomes (P2: p = 0.050, ηp2 = 0.208; N2: p = 0.001, ηp2 = 0.474). Increased MEPs were also observed upon hearing congruent sounds compared with incongruent sounds (i.e., associated to a finger), demonstrating precise sensorimotor associations that were not present before learning (Index finger: p < 0.001, ηp2 = 0.614; Little finger: p < 0.001, ηp2 = 0.529). Consistent with our broad hypotheses, a negative association between the MEPs in one finger during listening and ERPs during performance of the other was observed (Index finger MEPs and Fz N1 action ERPs; r = −0.655, p = 0.003). Overall, data suggest that predictive mechanisms are associated with the fine-tuning of sensorimotor associations.

Repetitive Passive Movement Modulates Corticospinal Excitability: Effect of Movement and Rest Cycles and Subject Attention

Repetitive passive movement (PM) affects corticospinal excitability; however, it is unknown whether a duty cycle which repeats movement and rest, or subjects’ conscious attention to movements, affects corticospinal excitability. We aimed to clarify the effect of the presence or absence of a duty cycle and subjects’ attention on corticospinal excitability. Three experiments were conducted. In Experiment 1, PM of the right index finger was performed for 10 min. Three conditions were used: (1) continuous PM (cPM) at a rate of 40°/s; (2) intermittent PM (iPM) with a duty cycle at 40°/s; and (3) iPM at 100°/s. In conditions 1 and 3, motor evoked potential (MEP) amplitude was significantly reduced. In Experiment 2, PM was performed for 30 min: condition 1 comprised cPM at a rate of 40°/s and Condition 2 comprised iPM at 40°/s. MEP amplitude significantly decreased in both conditions. In Experiment 3, PM was performed for 10 min: condition 1 comprised paying attention to the moving finger during iPM and Condition 2 was similar to Condition 1 but while counting images on a monitor without looking at the movement finger, and Condition 3 comprised counting images on a monitor without performing PM. MEP amplitude significantly increased only under Condition 1. Thus, afferent input from movements above a certain threshold may affect corticospinal excitability reduction. Furthermore, corticospinal excitability increases when paying attention to passive finger movement.

Cortical excitability following passive movement

In brain injury rehabilitation, passive movement exercises are frequently used to maintain or improve mobility and range of motion. They can also induce beneficial and sustained neuroplastic changes. Neuroimaging studies have revealed that passive movements without motor commands activate not only the primary somatosensory cortex but also the primary motor cortex, supplementary motor area, and posterior parietal cortex as well as the secondary somatosensory cortex (S2) in healthy subjects. Repetitive passive movement has also been reported to induce increases or decreases in cortical excitability. In this review, we focused on the following: cortical activity following passive movement; cortical excitability during passive movement; and changes in cortical excitability after repetitive passive movement.

Decrease in short-latency afferent inhibition during corticomotor postexercise depression following repetitive finger movement

INTRODUCTION

This study aimed to clarify cortical circuit mechanisms contributing to corticomotor excitability during postexercise depression (PED) following repetitive nonfatiguing movement. We investigated changes in short-latency afferent inhibition (SAI) and short-interval intracortical inhibition (SICI) by paired-pulse transcranial magnetic stimulation (TMS) during PED.

METHODS

A total of 16 healthy subjects performed repetitive abduction movements of the right index finger at 2.0 Hz for 6 min at 10% maximum voluntary contraction. We measured SAI evoked by pairing ulnar nerve stimulation with TMS (interstimulus interval, 22 ms) before and during PED (n = 10, experiment 1). We also measured SICI evoked by paired TMS (interstimulus interval, 2 ms) at 80% resting motor threshold (n = 10, experiment 2), and at 80% active motor threshold (n = 8, experiment 3) before and during PED.

RESULTS

Single motor evoked potential amplitude significantly decreased 1-2 min after the movement task in all experiments, indicating reliable PED induction. In experiment 1, SAI significantly decreased (disinhibited) 1-2 min during PED, whereas in experiments 2 and 3, SICI showed no significant change during PED.

CONCLUSION

This study suggests that cholinergic inhibitory circuit activity decreases during PED following repetitive nonfatiguing movement, whereas GABA_A circuit activity remains stable.