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

INTRODUCTION: This study examines effects of low-threshold 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 ± 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. Muscle Nerve 57: 122-128, 2018.

Oligosynaptic inhibition mediated by group Ia afferents from flexor digitorum superficialis to wrist flexors in humans.

Effects of low-threshold afferents from the flexor digitorum superficialis (FDS) to the flexor carpi radialis (FCR), flexor carpi ulnaris (FCU) and extensor carpi ulnaris (ECU) motoneurons were examined using a post-stimulus time-histogram (PSTH) and electromyogram-averaging (EMG-A) methods in seven healthy human subjects. Electrical conditioning stimulation to the median nerve branch innervating FDS with the intensity immediately below the motor threshold was delivered. In the PSTH study, the stimulation produced a trough (inhibition) in 19/44 (43%) of FCR and 17/41 (41%) of FCU motor units. Remaining motor units received no facilitatory and inhibitory effects. The central latency of the inhibition was 1.1 ± 0.6 ms (mean ± SD) and 0.6 ± 0.4 ms longer than that of the homonymous monosynaptic Ia facilitation of FCR and FCU, respectively. In the EMG-A study, the stimulation produced a trough (inhibition) in EMG-A of FCR and FCU in all the seven subjects. Amount of the inhibition was 14.5 ± 3.8% (FCR) and 17.9 ± 2.5% (FCU). Since the inhibition diminished after withdrawal of tonic vibration stimuli to the FDS muscle belly, group Ia afferents should be responsible for the inhibition. The stimulation did not produce facilitatory or inhibitory effect on ECU motoneurons in both the PSTH and EMG-A studies. These findings suggest that group Ia afferents from FDS inhibit excitability of motoneurons supplying FCR and FCU through an oligo (di- or tri-) synaptic path in the spinal cord. The reflex arcs would function to prevent wrist flexion during hand grasping movements.

Oligosynaptic inhibition of group I afferents between the brachioradialis and flexor carpi radialis in humans.

Spinal reflex arcs mediated by low threshold afferents between the brachioradialis (BR) and flexor carpi radialis (FCR) were studied in eleven healthy human subjects using a post-stimulus time-histogram method. Electrical conditioning stimuli (ES) to the radial nerve branch innervating BR with the intensity below the motor threshold (MT) induced an early and significant trough (inhibition) in 32/85 FCR motor units (MUs) in 9/9 subjects. Such inhibition was never provoked by cutaneous stimulation. The central synaptic delay (CSD) of the inhibition was approximately 1.1ms longer than that of the homonymous FCR facilitation. ES to the median nerve branch innervating FCR with the intensity below MT induced an inhibition in 27/71 BR-MUs in 10/10 subjects. CSD of the inhibition was about 1.1ms longer than that of the homonymous BR facilitation. These findings suggest that inhibition between BR and FCR exists in humans. Group I afferents seem to mediate the inhibition through an oligo(di or tri)-synaptic path.