Evidence for early activation of primary motor cortex and SMA after electrical lower limb stimulation using EEG source reconstruction

Mechanism of Action of Tibial Nerve Stimulation in the Treatment of Lower Urinary Tract Dysfunction

BACKGROUND AND OBJECTIVE

Tibial nerve stimulation (TNS) has long been used to effectively treat lower urinary tract dysfunction (LUTD). Although numerous studies have concentrated on TNS, its mechanism of action remains elusive. This review aimed to concentrate on the mechanism of action of TNS against LUTD.

MATERIALS AND METHODS

A literature search was performed in PubMed on October 31, 2022. In this study, we introduced the application of TNS for LUTD, summarized different methods used in exploring the mechanism of TNS, and discussed the next direction to investigate the mechanism of TNS.

RESULTS AND CONCLUSIONS

In this review, 97 studies, including clinical studies, animal experiments, and reviews, were used. TNS is an effective treatment for LUTD. The study of its mechanisms primarily concentrated on the central nervous system, tibial nerve pathway, receptors, and TNS frequency. More advanced equipment will be used in human experiments to investigate the central mechanism, and diverse animal experiments will be performed to explore the peripheral mechanism and parameters of TNS in the future.

Corticospinal excitability and somatosensory information processing of the lower limb muscle during upper limb voluntary or electrically induced muscle contractions

Neural interactions between upper and lower limbs underlie motor coordination in humans. Specifically, upper limb voluntary muscle contraction can facilitate spinal and corticospinal excitability of the lower limb muscles. However, little remains known on the involvement of somatosensory information in arm-leg neural interactions. Here, we investigated effects of voluntary and electrically induced wrist flexion on corticospinal excitability and somatosensory information processing of the lower limbs. In Experiment 1, we measured transcranial magnetic stimulation (TMS)-evoked motor evoked potentials (MEPs) of the resting soleus (SOL) muscle at rest or during voluntary or neuromuscular electrical stimulation (NMES)-induced wrist flexion. The wrist flexion force was matched to 10% of the maximum voluntary contraction (MVC). We found that SOL MEPs were significantly increased during voluntary, but not NMES-induced, wrist flexion, compared to the rest (P < 0.001). In Experiment 2, we examined somatosensory evoked potentials (SEPs) following tibial nerve stimulation under the same conditions. The results showed that SEPs were unchanged during both voluntary and NMES-induced wrist flexion. In Experiment 3, we examined the modulation of SEPs during 10%, 20%, and 30% MVC voluntary wrist flexion. During 30% MVC voluntary wrist flexion, P50-N70 SEP component was significantly attenuated compared to the rest (P = 0.003). Our results propose that the somatosensory information generated by NMES-induced upper limb muscle contractions may have a limited effect on corticospinal excitability and somatosensory information processing of the lower limbs. However, voluntary wrist flexion modulated corticospinal excitability and somatosensory information processing of the lower limbs via motor areas.

Identification of Lower-Limb Motor Tasks via Brain–Computer Interfaces: A Topical Overview

Recent engineering and neuroscience applications have led to the development of brain–computer interface (BCI) systems that improve the quality of life of people with motor disabilities. In the same area, a significant number of studies have been conducted in identifying or classifying upper-limb movement intentions. On the contrary, few works have been concerned with movement intention identification for lower limbs. Notwithstanding, lower-limb neurorehabilitation is a major topic in medical settings, as some people suffer from mobility problems in their lower limbs, such as those diagnosed with neurodegenerative disorders, such as multiple sclerosis, and people with hemiplegia or quadriplegia. Particularly, the conventional pattern recognition (PR) systems are one of the most suitable computational tools for electroencephalography (EEG) signal analysis as the explicit knowledge of the features involved in the PR process itself is crucial for both improving signal classification performance and providing more interpretability. In this regard, there is a real need for outline and comparative studies gathering benchmark and state-of-art PR techniques that allow for a deeper understanding thereof and a proper selection of a specific technique. This study conducted a topical overview of specialized papers covering lower-limb motor task identification through PR-based BCI/EEG signal analysis systems. To do so, we first established search terms and inclusion and exclusion criteria to find the most relevant papers on the subject. As a result, we identified the 22 most relevant papers. Next, we reviewed their experimental methodologies for recording EEG signals during the execution of lower limb tasks. In addition, we review the algorithms used in the preprocessing, feature extraction, and classification stages. Finally, we compared all the algorithms and determined which of them are the most suitable in terms of accuracy.

Effects of motor style on timing control and EEG waveforms in self-paced and synchronization tapping tasks

We investigated the effects of tapping style on motor performance and neural activity in self-paced and synchronization tapping tasks in three conditions (drum sticking [DS], one-finger tapping [1FT], and four-finger tapping [4FT]). In the synchronization task, participants tapped in synchrony with a metronomic sound. No significant differences were detected in the accuracy of timing control among the tapping styles, whereas larger potentials on EEG waveforms before tap onset were found in 4FT than in DS or 1FT; these may be readiness potentials for the motor commands required to control multiple fingers. As expected, tap intervals were more stable under the synchronization condition than under the selfpaced condition, but no difference was detected in the neural activity evoked before tap onset. Larger neural potentials observed in the early stage after tap onset in DS might be involved in the sensory feedback associated with tool use.

Dipole orientation of receptive fields in the somatosensory cortex after stimulation of the posterior tibial nerve in humans

The origins of the earliest evoked potentials and magnetic fields after tibial nerve electrical stimulation are still controversial. We focused on the initial activity from the gyrus area and analyzed the components for the coronal and sagittal planes. In 12 healthy right-handed subjects, electrical stimuli were delivered to the left posterior tibial nerve at the ankle. The cortical somatosensory evoked fields were recorded, and the equivalent current dipoles were calculated and separated into the sagittal plane (y-components) and coronal plane (x-components) components. In nine subjects, we observed two deflections (y1 and y2) in the y-component waveform and two deflections (x1 and x2) in the x-component waveform over 60 milliseconds; y1 was directed anteriorly, y2 posteriorly, x1 to the left, and x2 to the right. The y1 was originated in the anterior wall of the central sulcus. By focusing on the y-component, we elucidated the existence of the posteroanterior component, which may originate from the area 3b (gyrus) in tibial nerve somatosensory evoked fields and has the same quality as N20m for median nerve somatosensory evoked fields. This is the first report to suggest that the posteroanterior component in the tibial nerve is analogous to N20m in the median nerve using magnetoencephalography.

The Effect of tDCS on Cognition and Neurologic Recovery of Rats with Alzheimer's Disease

[Purpose] This study examined the effect of the application of transcranial direct current stimulation (tDCS) on neurologic recovery and cognitive function of rats with Alzheimer-like dementia induced by scopolamine injections. [Subjects] To create a cognition dysfunction model, intraperitoneal injection of scopolamine was given to Sprague-Dawley rats that subsequently received tDCS for 4 weeks. [Methods] Changes in motor behavior were evaluated by conducting an open field test. Acetylcholine content in the cerebral cortex and hippocampus was examined for a biochemical assessment. [Results] With respect to changes in motor behavior, group II showed the most meaningful difference after scopolamine injection, followed by group III. In the biochemical assessment, the results of the examination of acetylcholine content in the tissue of the cerebral cortex and the hippocampus on the 14th and 28th days, respectively, showed the most significant increase in group II, followed by group III. [Conclusion] The above findings confirm that tDCS application after the onset of cognitive dysfunction caused by Alzheimer's disease leads to a positive effect on motor behavior and biochemical changes, and this effect is maintained over a specific period of time.

The optimal interstimulus interval and repeatability of paired associative stimulation when the soleus muscle is targeted

Changes in the excitability of the cortical projections to muscles in the upper and lower limbs can be induced in the intact human by paired associative stimulation (PAS). An interstimulus interval (ISI) of 25 ms between peripheral nerve and transcranial magnetic stimuli has been found to be effective when targeting hand muscles. The optimal ISI to induce plasticity changes in the cortical projections to lower limbs is still not well established. The purpose of this study was twofold: first, to investigate the effect of PAS with four different ISIs based on the individual latency of the sensory evoked potential (SEP plus 6, 12, 18 and 24 ms) and second, to evaluate the repeatability of the established optimal ISI. Transcranial magnetic stimulation was used to measure changes in the motor evoked potentials (MEPs) of the soleus (SOL) muscle before and after the PAS interventions. Significant increases in the amplitude of SOL MEPs (88 %) were attained with an ISI of SEP latency plus 18 ms (P32 + 18 ms). The PAS effect was long-lasting, input-specific and supraspinal in origin. The intraclass correlation coefficient to test the repeatability of the PAS intervention with the optimal ISI was 0.85. The results show that the excitability of cortical projections to the soleus muscle can be repeatedly increased after PAS with an optimal ISI of SEP plus 18 ms.

Afferent Regulation of Leg Motor Cortex Excitability After Incomplete Spinal Cord Injury

An incomplete spinal cord injury (SCI) impairs neural conduction along spared ascending sensory pathways to disrupt the control of residual motor movements. To characterize how SCI affects the activation of the motor cortex by spared ascending sensory pathways, we examined how stimulation of leg afferents facilitates the excitability of the motor cortex in subjects with incomplete SCI. Homo- and heteronymous afferents to the tibialis anterior (TA) representation in the motor cortex were electrically stimulated, and the responses were compared with uninjured controls. In addition, we examined if cortical excitability could be transiently increased by repetitively pairing stimulation of spared ascending sensory pathways with transcranial magnetic stimulation (TMS), an intervention termed paired associative stimulation (PAS). In uninjured subjects, activating the tibial nerve at the ankle 45–50 ms before a TMS pulse in a conditioning-test paradigm facilitated the motor-evoked potential (MEP) in the heteronymous TA muscle by twofold on average. In contrast, prior tibial nerve stimulation did not facilitate the TA MEP in individuals with incomplete SCI ( n = 8 SCI subjects), even in subjects with less severe injuries. However, we provide evidence that ascending sensory inputs from the homonymous common peroneal nerve (CPN) can, unlike the heteronymous pathways, facilitate the motor cortex to modulate the TA MEP ( n = 16 SCI subjects) but only in subjects with less severe injuries. Finally, by repetitively coupling CPN stimulation with coincident TA motor cortex activation during PAS, we show that 7 of 13 SCI subjects produced appreciable (>20%) facilitation of the MEP following the intervention. The increase in corticospinal tract excitability by PAS was transient (<20 min) and tended to be more prevalent in SCI subjects with stronger functional ascending sensory pathways.

Peripheral sensory activation of cortical circuits in the leg motor cortex of man

Peripheral sensory afferents in the hand activate both excitatory and inhibitory intracortical circuits to potentially facilitate and prune descending motor commands. In this study, we characterized how afferent inputs modulate the excitability of cortical circuits in the leg area of the primary motor cortex by examining how stimulation of the tibial nerve (TN) at the ankle alters motor evoked potentials (MEPs) activated by transcranial magnetic stimulation (TMS). Resting MEPs in the tibialis anterior (TA) muscle were facilitated in response to heteronymous activation of the TN 45-50 ms earlier, whereas MEPs were inhibited at interstimulus intervals of 32.5-37.5 ms. Similar time-dependent modulation occurred in the soleus (SOL) muscle with stimulation of the homonymous posterior tibial nerve (PTN) at the knee. To determine the site of this afferent-evoked facilitation and inhibition (spinal or cortical), we compared the effects of afferent stimulation to responses evoked at subcortical sites. At interstimulus intervals where MEP facilitation was observed (near 50 ms), spinal H-reflexes and responses evoked from corticospinal tract stimulation at the brainstem were predominantly depressed by the sensory stimulus suggesting that the observed MEP facilitation was cortical in origin. At interstimulus intervals where MEP depression was observed (near 35 ms), brainstem evoked responses were depressed to a similar degree and, in contrast to the hand, this suggests that spinal rather than cortical circuits mediate short-latency afferent inhibition (SAI) of leg MEPs. When the MEP was facilitated by afferent inputs, short-interval intracortical inhibition (SICI) was reduced and intracortical facilitation (ICF) was increased, but long-interval intracortical inhibition (LICI) at a 100 ms interval was unchanged. In addition, sensory excitation increased the recruitment of early, middle and late descending corticospinal volleys as evidenced from increases in MEP facilitation at the corresponding I-wave periodicity. We propose that sensory activation from the leg has a diffuse and predominantly facilitatory effect on the leg primary motor cortex.