Nerve specific modulation of somatosensory inflow to cerebral cortex during submaximal sustained contraction in first dorsal interosseous muscle

Cutaneous information processing differs with load type during isometric finger abduction

During submaximal isometric contraction, there are two different load types: maintenance of a constant limb angle while supporting an inertial load (position task) and maintenance of a constant force by pushing against a rigid restraint (force task). Previous studies demonstrated that performing the position task requires more proprioceptive information. The purpose of this study was to investigate whether there would be a difference in cutaneous information processing between the position and force tasks by assessing the gating effect, which is reduction of amplitude of somatosensory evoked potentials (SEPs), and cutaneomuscular reflex (CMR). Eighteen healthy adults participated in this study. They contracted their right first dorsal interosseous muscle by abducting their index finger to produce a constant force against a rigid restraint that was 20% maximum voluntary contraction (force task), or to maintain a target position corresponding to 10° abduction of the metacarpophalangeal joint while supporting a load equivalent to 20% maximum voluntary contraction (position task). During each task, electrical stimulation was applied to the digital nerves of the right index finger, and SEPs and CMR were recorded from C3' of the International 10-20 system and the right first dorsal interosseous muscle, respectively. Reduction of the amplitude of N33 component of SEPs was significantly larger during the force than position task. In addition, the E2 amplitude of CMR was significantly greater for the force than position task. These findings suggest that cutaneous information processing differs with load type during static muscle contraction.

Subcortical Contribution of Corticospinal Transmission during Visually Guided Switching Movements of the Arm

In animal experiments, the indirect corticospinal tract (CST) system via cervical interneurons has been shown to mediate motor commands for online adjustment of visuomotor behaviors, such as target-reaching. However, it is still unclear whether the similar CST system functions to perform similar motor behaviors in humans. To clarify this, we investigated changes in motor-evoked potentials (MEPs) in the elbow muscles following transcranial magnetic stimulation, transcranial electrical stimulation, or cervicomedullary stimulation while participants executed target-reaching and switching movements. We found that the MEP, whether elicited cortically or subcortically, was modulated depending on the direction of the switching movements. MEP facilitation began around the onset of the switching activities in an agonist muscle. Furthermore, ulnar nerve-induced MEP facilitation, which could be mediated by presumed cervical interneuronal systems, also increased at the onset of MEP facilitation. In a patient with cortical hemianopsia who showed switching movements in the scotoma, the MEPs were facilitated just before the switching activities. Our findings suggested that CST excitation was flexibly tuned with the switching movement initiation, which could partly take place in the subcortical networks, including the presumed cervical interneuronal systems.

Attenuation of sensory processing in the primary somatosensory cortex during rubber hand illusion

The neural representation of the body is easily altered by the integration of multiple sensory signals in the brain. The “rubber hand illusion” (RHI) is one of the most popular experimental paradigms to investigate this phenomenon. During this illusion, a feeling of ownership of the rubber hand is created. Some studies have shown that somatosensory processing in the brain is attenuated when RHI occurs. However, it is unknown where attenuation of somatosensory processing occurs. Here, we show that somatosensory processing is attenuated in the primary somatosensory cortex. We found that the earliest response of somatosensory evoked potentials, which is thought to originate from the primary somatosensory cortex, was attenuated during RHI. Furthermore, this attenuation was observed before the occurrence of the illusion. Our results suggest that attenuation of sensory processing in the primary somatosensory cortex is one of the factors influencing the occurrence of the RHI.

Visual information increases the indirect corticospinal excitation via cervical interneurons in humans

Modulatory actions of inputs from the visual system to cervical interneurons (IN) for arm muscle control are poorly understood in humans. In the present study, we examined whether visual stimulation modulates the excitation of cervical IN systems mediating corticospinal tract (CST) inputs to biceps brachii (BB). Twenty-eight healthy volunteers were seated, and electromyogram recordings from the BB were performed across six experiments, each with discrete objectives. A flash stimulator for visual stimulation (50-μs duration) was placed 60 cm from the participant's eye. The CST was stimulated with transcranial magnetic/electrical stimulation (TMS/TES, respectively) contralateral to the recording site. Visual stimulation with TMS/TES was randomly delivered during weak tonic BB contractions. Single TMS/TES-induced motor-evoked potentials (MEPs) were markedly enhanced from 60-100 ms after visual stimulation compared with the control condition. The MEPs were significantly increased by combining the electrical stimulation of the ulnar nerve at the wrist [7.5-12 ms of nerve stimulation (NERVE)/TMS interval] with and without visual stimulation compared with the algebraic summation of responses obtained with either TMS or NERVE. Interestingly, the combined stimulation-induced MEP facilitation was significantly increased after visual stimulation compared with the control. Single motor unit (MU) recording also revealed the further enhancement of combined stimulation effects on the firing probabilities of MU during visual stimulation, which was observed in the peaks of the peristimulus time histogram, 1-2 ms later than the onset latency. The present findings suggest that visual stimulation facilitates the oligosynaptic CST excitation of arm motoneurons mediated by the cervical IN system.NEW & NOTEWORTHY To date, little is known about how visual information modulates the human cervical motor systems, including the presumed interneuron (IN) circuitry. This study demonstrates that photic visual stimulation influences presumed oligosynaptic corticospinal transmission to arm motoneurons, which are mediated by cervical INs. In animals, these systems are known to be crucial for visually guided switching movements, and similar visual input systems to INs may exist in humans.

Motor imagery enhances corticospinal transmission mediated by cervical premotoneurons in humans

Imaging movement has positive effects on the reacquisition of motor functions after damage to the central nervous system. This study shows that motor imagery facilitates oligosynaptic corticospinal excitation that is mediated via cervical premotoneurons, which may be important for motor recovery in monkeys and humans. Current findings highlight how this imagery might be a beneficial tool for movement disorders through effects on premotoneuron circuitry.

Vestibular stimulation-induced facilitation of cervical premotoneuronal systems in humans

It is unclear how descending inputs from the vestibular system affect the excitability of cervical interneurons in humans. To elucidate this, we investigated the effects of galvanic vestibular stimulation (GVS) on the spatial facilitation of motor-evoked potentials (MEPs) induced by combined pyramidal tract and peripheral nerve stimulation. To assess the spatial facilitation, electromyograms were recorded from the biceps brachii muscles (BB) of healthy subjects. Transcranial magnetic stimulation (TMS) over the contralateral primary motor cortex and electrical stimulation of the ipsilateral ulnar nerve at the wrist were delivered either separately or together, with interstimulus intervals of 10 ms (TMS behind). Anodal/cathodal GVS was randomly delivered with TMS and/or ulnar nerve stimulation. The combination of TMS and ulnar nerve stimulation facilitated BB MEPs significantly more than the algebraic summation of responses induced separately by TMS and ulnar nerve stimulation (i.e., spatial facilitation). MEP facilitation significantly increased when combined stimulation was delivered with GVS (p < 0.01). No significant differences were found between anodal and cathodal GVS. Furthermore, single motor unit recordings showed that the short-latency excitatory peak in peri-stimulus time histograms during combined stimulation increased significantly with GVS. The spatial facilitatory effects of combined stimulation with short interstimulus intervals (i.e., 10 ms) indicate that facilitation occurred at the premotoneuronal level in the cervical cord. The present findings therefore suggest that GVS facilitates the cervical interneuron system that integrates inputs from the pyramidal tract and peripheral nerves and excites motoneurons innervating the arm muscles.

Tuning of the excitability of transcortical cutaneous reflex pathways during mirror-like activity

Voluntary contraction of a muscle generates electromyographic (EMG) activity in the homologous muscle on the opposite side (mirror-like activity), not only in pathological states and in infants but also in healthy adults. Few studies have examined whether the cutaneous reflexes during the preparatory period of a reaction time task are affected by mirror-like activity. In the present study, we investigated the modulation of the cutaneous reflexes in the left first interosseous (FDI) muscle in 9 healthy subjects while they performed a quick abduction of the right index finger during a reaction time task. Cutaneous reflexes were elicited by applying non-noxious electrical stimulation to the left index finger. We found that mirror-like activity occurred in the left FDI at approximately the onset of EMG activity in the right FDI. The excitatory E2 component was selectively increased at ~75 ms after the "Go" signal, which corresponded to the onset of mirror-like activity. The inhibitory I2 (~90 ms) component was tuned consistently into excitation after the "Go" signal. These findings suggest that long latency reflexes, possibly transcortical cutaneous reflexes, are finely tuned in relation to mirror-like activity.

Hypohydration and muscular fatigue of the thumb alter median nerve somatosensory evoked potentials

The mechanisms by which dehydration impairs endurance performance remain unresolved but may involve alterations in afferent neural processing. The purpose of this study was to determine the effect of hypohydration on somatosensory evoked potentials (SEPs) at rest and during recovery from fatiguing exercise. Fourteen volunteers (12 men, 2 women) performed repetitive isometric thumb contractions (50% maximal voluntary contractions (MVC) and 100% MVC in a 5:1 ratio, each contraction separated by 5 s of rest) until exhaustion when euhydrated (EU) and when hypohydrated by 4% body mass (HY). SEPs were obtained from the median nerve. The results indicated that HY did not produce statistical differences in time to exhaustion (EU = 754 (SD 255); HY = 714 (SD 318) s; p = 0.66) or rate of muscle fatigue. However, HY was associated with greater subjective feelings of fatigue and loss of vigor after exhaustive exercise (p < 0.01). HY affected N20 latency with an interaction effect of hydration by fatigue state (EU-Rest: 18.5 (SD 1.6) ms; EU-Fatigue: 19.0 (SD 1.6) ms; HY-Rest: 18.3 (SD 1.3) ms; HY-Fatigue: 18.4 (SD 1.5) ms; p = 0.034), but N20 and N20-P22 amplitude responses were similar between HY and EU trials. We concluded that moderate water deficits appear to alter afferent signal processing within the cerebral cortex.

Location specificity of plantar cutaneous reflexes involving lower limb muscles in humans

It is known that cutaneous reflexes in human hand muscles show strong location-specificity dependent on the digit stimulated. We hypothesized that in lower leg muscles the cutaneous reflex following tactile sensation of the plantar surface of the foot is also organized in a location-specific manner. The purpose of the present study was to test this hypothesis. Middle latency reflexes (approximately 70-110 ms, MLR) following non-noxious electrical stimulation to different locations on the plantar foot were recorded from 16 neurologically intact volunteers (15 males, 1 female). Electrical stimulation was given to the fore-medial (f-M), fore-lateral (f-L) and heel (HL) regions of the plantar surface of the right foot while the subjects performed isometric dorsiflexion (tibialis anterior, TA), plantarflexion (soleus, Sol and medial gastrocnemius, MG), eversion (peroneus longus, PL) and knee extension (vastus lateralis, VL) while sitting and standing. In the Sol and MG, an excitatory response was observed following HL stimulation, which was switched to an inhibitory response following f-M or f-L stimulation (P < 0.001). A reciprocal pattern in contrast to Sol was observed in the TA. In the PL, MLR exhibited significant excitation following both f-L and HL stimulation, which, however, was switched to an inhibitory response following f-M stimulation (P < 0.001). Moderate inhibition of the MLR was seen in the VL for all stimulated positions. Systematic stimulation along the lateral side of the plantar foot demonstrated that the reflex reversal occurred around the middle of the plantar foot in the Sol and TA. In all muscles tested, the slope of the regression line between the magnitude of the MLR and background electromyographic activity significantly decreased during standing compared with sitting except for the PL following f-L simulation. These results suggest that reflex effects from cutaneous nerves in the plantar foot onto the motoneurons innervating the lower leg muscles are organized in a highly topographic manner in humans. The organization of these reflexes may play an important role in the alteration of limb loading and/or ground contact in response to tactile sensation of the plantar foot while sitting and standing.