Oligosynaptic inhibition of group Ia afferents from brachioradialis to triceps brachii motor neurons in humans

Abstract

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...

Monosynaptic facilitation of flexor digitorum superficialis motoneurons mediated by Group Ia afferents from the extensor carpi radialis in humans

“…The effects of conditioning stimulation on the motoneuron pool were investigated using the EMG‐A method (Capaday et al, 1990; Meunier et al, 1996; Nito, Hashizume, Jimenji, et al, 2018; Nito, Hashizume, Suzuki, et al, 2018; Nito et al, 2016; Ogawa et al, 2005; Petersen et al, 1998, 1999; Sato et al, 2018; Suzuki et al, 2005, 2012). The EMG signals were recorded with the above‐mentioned pair of Ag/AgCl disk surface electrodes.…”

“…To estimate the central synaptic delay of the process, the latencies of the facilitation and the monosynaptic peak in the PSTH following stimulation of the respective ECR and homonymous FDS afferents were compared for motor units receiving facilitation (Cavallari & Katz, 1989;Cavallari et al, 1992;Katz et al, 1991;Kobayashi et al, 2016;Marchand-Pauvert et al, 2000;Miyasaka et al, 2007;Naito et al, 1996Naito et al, , 1998Nakano et al, 2014;Nito, Hashizume, Jimenji, et al, 2018;Nito et al, 2016;Sato et al, 2018;Shinozaki et al, 2017). The homonymous FDS afferents were stimulated at the proximal part of the medial intermuscular septum of the arm (axilla) with bipolar surface electrodes.…”

Monosynaptic facilitation of motoneurons innervating intrinsic hand muscles mediated by group Ia afferents from the extensor carpi radialis in humans

“…Next, we analyzed the facilitation of motor units induced by ECR afferents. To estimate the central synaptic delay of the process, the latencies of the facilitation and the monosynaptic peak in PSTH following stimuli of the respective ECR and homonymous afferents from intrinsic hand muscles (APB, FDI, ADM) were compared (Cavallari et al, 1992 ; Cavallari & Katz, 1989 ; Katz et al, 1991 ; Kobayashi et al, 2016 ; Marchand‐Pauvert et al, 2000 ; Miyasaka et al, 2007 ; Naito et al, 1996 , 1998 ; Nakano et al, 2014 ; Nito et al, 2016 ; Nito, Hashizume, Jimenji, et al, 2018 ; Sato et al, 2018 ; Shinozaki et al, 2017 ) (Figure 1 ). To investigate the effect on APB motor units, APB homonymous afferents were stimulated at the proximal part of the medial intermuscular septum of the arm (axilla) with bipolar surface electrodes (0.8 cm diameter, 1.5 cm interelectrode distance).…”

“…We investigated the effect of conditioning stimulation on the motoneuron pool using the EMG‐A method (Capaday et al, 1990 ; Meunier et al, 1996 ; Nito et al, 2016 ; Nito, Hashizume, Jimenji, et al, 2018 ; Nito, Hashizume, Suzuki, et al, 2018 ; Ogawa et al, 2005 ; Petersen et al, 1998 , 1999 ; Sato et al, 2018 ; Suzuki et al, 2005 , 2012 ). The EMG signals were recorded with the pair of Ag/AgCl disk surface electrodes mentioned above.…”

Corticospinal excitability to the biceps and triceps brachii during forward and backward arm cycling is direction- and phase-dependent

“…This suggests that there may be another mechanism aside from the stretch of the triceps brachii during the elbow flexion phase that is contributing to the increase in spinal excitability at the 6 o'clock position. For example, flexion of the shoulder at the 12 o'clock position may result in an increase in inhibitory synaptic input to the triceps brachii motoneurone pool from the biceps brachii, brachioradialis or other ascending or descending inputs, thus decreasing excitatory input to the triceps brachii motoneurones (Sato et al 2018) and resulting in decreased CMEP amplitudes.…”