Syusaku Sasada

Modulation of interhemispheric interactions across symmetric and asymmetric bimanual force regulations

The corpus callosum is essential for neural communication between the left and right hemispheres. Although spatiotemporal coordination of bimanual movements is mediated by the activity of the transcallosal circuit, it remains to be addressed how transcallosal neural activity is involved in the dynamic control of bimanual force execution in human. To address this issue, we investigated transcallosal inhibition (TCI) elicited by single‐pulse transcranial magnetic stimulation (TMS) in association with the coordination condition of bimanual force regulation. During a visually‐guided bimanual force tracking task, both thumbs were abducted either in‐phase (symmetric condition) or 180° out‐of‐phase (asymmetric condition). TMS was applied to the left primary motor cortex to elicit the disturbance of ipsilateral left force tracking due to TCI. The tracking accuracy was equivalent between the two conditions, but the synchrony of the left and right tracking trajectories was higher in the symmetric condition than in the asymmetric condition. The magnitude of force disturbance and TCI were larger during the symmetric condition than during the asymmetric condition. Right unimanual force tracking influenced neither the force disturbance nor TCI during tonic left thumb abduction. Additionally, these TMS‐induced ipsilateral motor disturbances only appeared when the TMS intensity was strong enough to excite the transcallosal circuit, irrespective of whether the crossed corticospinal tract was activated. These findings support the hypotheses that interhemispheric interactions between the motor cortices play an important role in modulating bimanual force coordination tasks, and that TCI is finely tuned depending on the coordination condition of bimanual force regulation.

Effects of Leg Pedaling on Early Latency Cutaneous Reflexes in Upper Limb Muscles

The functional coupling of neural circuits between the upper and lower limbs involving rhythmic movements is of interest to both motor control research and rehabilitation science. This coupling can be detected by examining the effect of remote rhythmic limb movement on the modulation of reflex amplitude in stationary limbs. The present study investigated the extent to which rhythmic leg pedaling modulates the amplitude of an early latency (peak 30-70 ms) cutaneous reflex (ELCR) in the upper limb muscles. Thirteen neurologically intact volunteers performed leg pedaling (60 or 90 rpm) while simultaneously contracting their arm muscles isometrically. Control experiments included isolated isometric contractions and discrete movements of the leg. ELCRs were evoked by stimulation of the superficial radial nerve with a train of rectangular pulses (three pulses at 333 Hz, intensity 2.0- to 2.5-fold perceptual threshold). Reflex amplitudes were significantly increased in the flexor carpi radialis and posterior deltoid and significantly decreased in the biceps brachii muscles during leg pedaling compared with that during stationary isometric contraction of the lower leg muscles. This effect was also sensitive to cadence. No significant modulation was seen during the isometric contractions or discrete movements of the leg. Additionally, there was no phase-dependent modulation of the ELCR. These findings suggest that activation of the rhythm generating system of the legs affects the excitability of the early latency cutaneous reflex pathways in the upper limbs.

Volitional Walking via Upper Limb Muscle-Controlled Stimulation of the Lumbar Locomotor Center in Man

Gait disturbance in individuals with spinal cord lesion is attributed to the interruption of descending pathways to the spinal locomotor center, whereas neural circuits below and above the lesion maintain their functional capability. An artificial neural connection (ANC), which bridges supraspinal centers and locomotor networks in the lumbar spinal cord beyond the lesion site, may restore the functional impairment. To achieve an ANC that sends descending voluntary commands to the lumbar locomotor center and bypasses the thoracic spinal cord, upper limb muscle activity was converted to magnetic stimuli delivered noninvasively over the lumbar vertebra. Healthy participants were able to initiate and terminate walking-like behavior and to control the step cycle through an ANC controlled by volitional upper limb muscle activity. The walking-like behavior stopped just after the ANC was disconnected from the participants even when the participant continued to swing arms. Furthermore, additional simultaneous peripheral electrical stimulation to the foot via the ANC enhanced this walking-like behavior. Kinematics of the induced behaviors were identical to those observed in voluntary walking. These results demonstrate that the ANC induces volitionally controlled, walking-like behavior of the legs. This paradigm may be able to compensate for the dysfunction of descending pathways by sending commands to the preserved locomotor center at the lumbar spinal cord and may enable individuals with paraplegia to regain volitionally controlled walking.

A common neural element receiving rhythmic arm and leg activity as assessed by reflex modulation in arm muscles

Neural interactions between regulatory systems for rhythmic arm and leg movements are an intriguing issue in locomotor neuroscience. Amplitudes of early latency cutaneous reflexes (ELCRs) in stationary arm muscles are modulated during rhythmic leg or arm cycling but not during limb positioning or voluntary contraction. This suggests that interneurons mediating ELCRs to arm muscles integrate outputs from neural systems controlling rhythmic limb movements. Alternatively, outputs could be integrated at the motoneuron and/or supraspinal levels. We examined whether a separate effect on the ELCR pathways and cortico-motoneuronal excitability during arm and leg cycling is integrated by neural elements common to the lumbo-sacral and cervical spinal cord. The subjects performed bilateral leg cycling (LEG), contralateral arm cycling (ARM), and simultaneous contralateral arm and bilateral leg cycling (A&L), while ELCRs in the wrist flexor and shoulder flexor muscles were evoked by superficial radial (SR) nerve stimulation. ELCR amplitudes were facilitated by cycling tasks and were larger during A&L than during ARM and LEG. A low stimulus intensity during ARM or LEG generated a larger ELCR during A&L than the sum of ELCRs during ARM and LEG. We confirmed this nonlinear increase in single motor unit firing probability following SR nerve stimulation during A&L. Furthermore, motor-evoked potentials following transcranial magnetic and electrical stimulation did not show nonlinear potentiation during A&L. These findings suggest the existence of a common neural element of the ELCR reflex pathway that is active only during rhythmic arm and leg movement and receives convergent input from contralateral arms and legs.

P13-10 Arm pedaling modulates short latency reflex from ankle dorsiflexor afferents to knee extensor muscles

techniques combined with independent component analysis and EMG signal decomposition. Triggering MU discharge times, each signal MU action potentials on parallel arrayed EMG signal channels were cut out. Then, conduction velocity could be estimated from the single MU action potentials on parallel arrayed channels with cross-correlation methods. The simulated EMG signals were generated from a simple dipole model. Results: As compared with the parameters of simulated data, identified MU discharge times and estimated muscle fiber conduction velocities were sufficiently coincided when S/N ratios of the simulated EMG signals are over 10 dB. Also, the MU discharge times and muscle fiber conduction velocities could be measured on lower S/N ratios of the simulated EMG signals. The accuracies of the identified or estimated parameters were decreased. Conclusion: Our results show that our developed system can simultaneously measure MU discharge properties for evaluating central nervous system activities and muscle fiber conduction velocity for evaluating peripheral nervous system activities or composition of muscle fibers.

P31-24 Follow-up study of changes in cutaneous reflex in the peroneus longus after acute ankle sprain

MEPs remained above 50% of baseline in all of 23 patients and no patients had postoperative vagus nerve deficits. Conclusions: Reliable facial and vagus nerve MEPs could be recorded throughout the procedure during skull base procedures. CB MEP monitoring can circumvent difficulties of standard EMG monitoring techniques, provide ongoing evaluation of facial and vagus nerve function and predict outcome with sufficiently useful accuracy.

P13-2 Modulation of cutaneous reflexes during preparation of the contralateral finger movement

Hereditary neuropathy with liability to pressure palsies is an autosomal dominant disorder due to a 1.5 Mb deletion in chromosome 17p11.2p12 resulting in reduced expression of the PMP22 gene, the same gene that is duplicated in CMT1A. HNPP generally presents with an increased susceptibility to pressure palsies although phenotype within one family is variable. Nerve conduction studies show prolonged distal latencies and histopathologically dysmyelination, with characteristic tomaculi, is seen. The underlying pathophysiology of the disorder however is still not well understood. Methods: Axonal excitability studies were performed on four patients with genetically and biopsy proven HNPP. All subjects had been symptomatic on at least one occasion. Excitability testing of motor axons was performed by stimulating the median nerve at the wrist and elbow and recording from the thenar muscles and forearm flexors respectively. Excitability studies were performed using the TROND protocol of QTRACs. Comparisons were made with seven age-matched control subjects. Results: Motor thresholds were significantly greater in all HNPP patients and strength duration time constants were longer for HNPP subjects. Studies stimulating the median nerve at the wrist showed fanning out of the threshold electrotonus graph with a notch in the depolarising direction. These changes in threshold electrotonus were not present with stimulation at the elbow. Conclusion: There are changes in axonal excitability in HNPP, some of which resemble those observed in CMT1A. The changes are not uniform along the nerve.

47. Effects of leg pedaling on cutaneous reflexes in the upper limb muscles

F response is known as the indicator for excitability of spinal motor neuron. To know how much influence double stimulation has on spinal motor neuron excitability, F response was investigated on median nerve in 8 normal subjects using paired supramaximal stimuli with the inter-stimulus intervals (ISI) ranging from 5 to 500 ms. A hundred pair stimuli at 1 Hz interval were delivered to median nerve at the wrist in each session. F-wave was recorded from thenar muscles. Persistence of F-wave and mean amplitude of Fwave were examined. F-wave persistence was markedly decreased at ISI from 20 to 50 ms, and was increased at ISI from 150 to 200 ms. Mean F-wave amplitude showed no obvious change. Supramaximal conditioning stimulus includes some kinds of sensory input through posterior rami and antidromic invasion to the motor neuron through motor fibers. All such input may make inhibitory and facilitatory effects. Father conditioning stimulus of different intensity should be needed to clear the mechanism happened in spinal cord.