Effects of low-threshold afferents from the flexor digitorum superficialis (FDS) to the extensor carpi radialis (ECR) motoneurons were examined using a post-stimulus time-histogram (PSTH) and electromyogram-averaging (EMG-A) methods in eight healthy human subjects. In the PSTH study in five of the eight subjects, electrical conditioning stimuli (ES) to the median nerve branch innervating FDS with the intensity below the motor threshold induced excitatory effects (facilitation) in 39 out of 92 ECR motor units. In 11 ECR motor units, the central synaptic delay of the facilitation was -0.1 ± 0.3 ms longer than that of the homonymous facilitation of ECR. Mechanical conditioning stimuli (MS) to FDS with the intensity below the threshold of the tendon(T)-wave-induced facilitation in 51 out of 51 ECR motor units. With the EMG-A method, early and significant peaks were produced by ES and MS in all the eight subjects. The difference between latencies of the peaks by ES and MS was almost equivalent to that of the Hoffmann- and T-waves of FDS by ES and MS. The peak was diminished by tonic vibration stimuli to FDS. These findings suggest that a facilitation from FDS to ECR exists in humans and group Ia afferents mediate the facilitation through a monosynaptic path.

Repetitive peripheral magnetic stimulation (rPMS) may improve motor function following central nervous system lesions, but the optimal parameters of rPMS to induce neural plasticity and mechanisms underlying its action remain unclear. We examined the effects of rPMS over wrist extensor muscles on neural plasticity and motor performance in 26 healthy volunteers. In separate experiments, the effects of rPMS on motor evoked potentials (MEPs), short-interval intracortical inhibition (SICI), intracortical facilitation (ICF), direct motor response (M-wave), Hoffmann-reflex, and ballistic wrist extension movements were assessed before and after rPMS. First, to examine the effects of stimulus frequency, rPMS was applied at 50, 25, and 10 Hz by setting a fixed total number of stimuli. A significant increase in MEPs of wrist extensors was observed following 50 and 25 Hz rPMS, but not 10 Hz rPMS. Next, we examined the time required to induce plasticity by increasing the number of stimuli, and found that at least 15 min of 50 and 25 Hz rPMS was required. Based on these parameters, lasting effects were evaluated following 15 min of 50 or 25 Hz rPMS. A significant increase in MEP was observed up to 60 min following 50 and 25 Hz rPMS; similarly, an attenuation of SICI and enhancement of ICF were also observed. The maximal M-wave and Hoffmann-reflex did not change, suggesting that the increase in MEP was due to plastic changes at the motor cortex. This was accompanied by increasing force and electromyograms during wrist ballistic extension movements following 50 and 25 Hz rPMS. These findings suggest that 15 min of rPMS with 25 Hz or more induces an increase in cortical excitability of the relevant area rather than altering the excitability of spinal circuits, and has the potential to improve motor output.

Following vibration, Hoffmann reflex (H-reflex) amplitude is depressed; however, the mechanisms underlying these phenomena remain unknown. This study investigated the influence of frequency and duration of vibration on the H-reflex amplitude, heteronymous facilitation of the tendon jerk (T-wave) mediated by group Ia afferents, and cervicomedullary motor evoked potential (CMEP) amplitude in 18 healthy human subjects. The H-reflex of the flexor carpi radialis (FCR) was induced by median nerve stimulation at the elbow, and the conditioning FCR stimulation enhanced the T-wave of the biceps brachii (BB). After vibration was applied to the FCR muscle belly, the amplitudes of the H-reflex and heteronymous facilitation of T-wave were depressed; these influences persisted after the removal of vibration in all subjects. For the H-reflex, there was no difference in the amount of depression among the frequencies of vibration used (57, 77, and 100 Hz). Higher frequencies of vibration were associated with longer recovery times of post-vibration depression, and a longer duration of vibration was associated with a longer recovery time of the depression. Similar results were observed for the heteronymous facilitation of T-wave, suggesting that the depression is caused by a decrease in post-synaptic potentials evoked by Ia afferents in spinal motoneurons. It was probably due to reduction in the number of Ia afferents recruited by the median nerve stimulation. Moreover, because the FCR CMEP amplitude was depressed after vibration, vibration should affect the responsiveness of spinal motoneurons. These mechanisms are considered to contribute to the H-reflex depression after vibration.

Introduction: This study examines effects of lowthreshold afferents from the brachioradialis (BR) on excitability of triceps brachii (TB) motor neurons in humans. Methods: We evaluated the effects using a post stimulus time histogram (PSTH) and electromyogram averaging (EMG-A) methods in 13 healthy human participants. Electrical conditioning stimulation to the radial nerve branch innervating BR with the intensity below the motor threshold was delivered. Results: In the PSTH study, the stimulation produced a trough (inhibition) in 36/69 TB motor units for all the participants. A cutaneous stimulation never provoked such inhibition. The central latency of the inhibition was 1.5 6 0.5 ms longer than that of the homonymous facilitation. In the EMG-A study, the stimulation produced inhibition in EMG-A of TB in all participants. The inhibition diminished with a tonic vibration stimulation to BR. Discussion: These findings suggest that oligosynaptic inhibition mediated by group Ia afferents from BR to TB exists in humans. 57: 122-128, 2018 Spinal reflex arcs mediated by low-threshold afferents (group I afferents, Ia fibers from muscle spindles and Ib fibers from Golgi tendon organs) among muscles in the human upper limb have been extensively studied. 1-23 Among human elbow flexors and extensors, oligosynaptic inhibition mediated by group Ia afferents has been demonstrated between the biceps brachii (BB) and the triceps brachii (TB) muscles, which are elbow flexors and extensors, respectively, and between the BB and the brachioradialis (BR) muscles, both of which are elbow flexors. 4,9,11,16 However, no reports describe a reflex arc between BR and TB. Muscle NerveOn the other hand, Cavallari and Katz 3 demonstrated disynaptic inhibition mediated by group Ia afferents from the radial nerve branch innervating wrist extensor muscles to TB. However, their conditioning stimulation did not include the radial nerve branch innervating BR. Therefore, the reflex arc between BR and TB is still unknown. The current study explores inhibition from BR to TB motor neurons using a post stimulus time histogram (PSTH) and electromyogram averaging (EMG-A) methods. MATERIALS AND METHODS Participants.The experiments were performed with 13 healthy human participants, ages 20-42 years, all of whom gave their informed consent to the experimental procedures, which were approved by the ethics committee of Yamagata University School of Medicine. The PSTH study was carried out with 8 of the 13 participants (males, 25-42 years old) and the EMG-A study was carried out with 9 (eight males, 1 female, 20-38 years old) of the 13 participants. Each participant was seated comfortably in an armchair. The examined (right) arm lay on an armrest with the shoulder slightly flexed (about 308), the elbow semiflexed, and the forearm pronated.Conditioning Stimulation. In both the PSTH and the EMG-A studies, electrical rectangular pulses of 1.0-ms duration were delivered to the radial nerve branch innervating BR (BR nerve) with surface electrodes (1.0-c...

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