Takuji Miyasaka

Inhibitory projections from pronator teres to biceps brachii motoneurones in human

Abstract Neural projections from the pronator teres (PT) muscle to biceps brachii (BB) motoneurones were studied in three healthy human subjects using a post-stimulus time histogram method. In 25 BB motor units, electrical stimulation to the PT nerve with intramuscular needle electrodes induced inhibition in nine units (36%), whereas facilitation was produced in 18 units (72%) by stimulation to the median nerve trunk with surface electrodes at the distal end of the intermuscular septum of the arm or in the cubital fossa. Six motor units (24%) received both inhibition (PT nerve stimulation) and facilitation (median nerve trunk stimulation). In the six, the latency of the inhibition was, on average, 1.2 ms longer than that of the facilitation. The stimulation site for the inhibition was, on average, 4.8 cm distal to that for the facilitation. The inhibition was evoked with an intensity well below the motor threshold. These findings suggest that BB motoneurones receive oligosynaptic inhibition of group I afferents from PT in human.

Inhibitory projection from brachioradialis to biceps brachii motoneurones in human

Neural projection from the brachioradialis to the biceps brachii motoneurones in human was studied using the method of post-stimulus time histogram. Electrical stimulation to the radial branch innervating the brachioradialis produced inhibition in 11 out of 21 biceps motor units. The central delays of the inhibition were 0.7–1.2 ms longer than those of the homonymous facilitation. The inhibition was evoked with the intensity below the motor threshold. Pure cutaneous stimulation provoked no effects on the motor-unit firing. These findings suggest that group I afferents from the brachioradials mediate an oligosynaptic inhibition of the biceps brachii motoneurones.

Facilitation between extensor carpi radialis and pronator teres in humans: A study using a post-stimulus time histogram method

Abstract Group I muscle afferents modulate the excitability of motor neurons through excitatory and inhibitory spinal reflexes. Spinal reflex relationships between various muscle pairs are well described in experimental animals but not in the human upper limb, which exhibits a fine control of movement. In the present study, spinal reflexes between the extensor carpi radialis (ECR) and pronator teres (PT) muscles were examined in healthy human subjects using a post-stimulus time histogram method. Electrical stimulation of low-threshold afferents of ECR nerves increased the motor neuron excitability in 31 of 76 PT motor units (MUs) in all eight subjects tested, while stimulation of low-threshold afferents of PT nerves increased the motor neuron excitability in 36 of 102 ECR MUs in all 10 subjects. The estimated central synaptic delay was almost equivalent to that of homonymous facilitation. Mechanical stimulation (MS) of ECR facilitated 16 of 30 PT MUs in all five subjects tested, while MS of PT facilitated 17 of 30 ECR MUs in all six subjects. These results suggest excitatory reflex (facilitation) between PT and ECR. Group I afferents should mediate the facilitation through a monosynaptic path.

1639 Inhibitory projections from the biceps brachii to the pronator teres motoneurons in the human

Projections of low threshold afferents from the biceps brachii (BB) to the pronator teres (PT) motoneurons were studied in four healthy human volunteers using a “triggered” post-stimulus time histogram technique on voluntarily-activated motor units. For conditioning stimuli, electrical rectangular pulses with 0.4 ms duration were delivered to the musculocutaneous branch innervating the BB with surface electrodes. Needle electrodes were used for recording single motor unit discharges from the PT. In 26 out of 52 units (50%), early (14-35 ms of latency) and significant troughs @<0.05) were induced with the stimulus intensity of less than 1.0 x MT (motor threshold). The central latency of the trough was less than 1 ms longer than that of the monosynaptic peak induced by the homonymous nerve stimulation. No significant troughs or peaks were provoked by pure cutaneous stimulation, These findings suggest that motoneurons of the PT receive inhibitory projections of group I afferents from the BB through an oligo(di or tri)-synaptic linkage in the spinal cord.

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.

Monosynaptic facilitation of group I afferents between brachioradialis and extensor carpi radialis in humans.

Spinal reflex arcs mediated by low-threshold (group I) afferents from muscle spindles and Golgi tendon organs modulate motoneuron excitabilities to coordinate smooth movements. In this study, the reflex arcs between the brachioradialis (BR) and extensor carpi radialis muscles (ECR) were examined in nine healthy human subjects using a post-stimulus time-histogram method. Electrical conditioning stimuli (ES) to the radial nerve branches innervating BR (BR nerve) and ECR (ECR nerve) with the intensity just below the motor threshold were delivered and firings of the ECR and BR motor units were recorded in 6 and 7 of the nine subjects, respectively. ES to the BR and ECR nerves induced a peak (facilitation) in 27/59 ECR and 22/68 BR motor units, respectively, in every subject. Such facilitation was never provoked by pure cutaneous stimulation. The remaining motor units received no effects by ES. The central synaptic delay of the facilitation was almost equal to that of the homonymous facilitation. These findings suggest that facilitation between BR and ECR exists in humans. Group I afferents should mediate the facilitation through a monosynaptic path in the spinal cord.

1553 Inhibition from the pronator teres to biceps brachii in human: An electrophysiological study with a PSTH method

KOICHI FUJIMURA Neonatal ablation of unilateral sensorimotor cortex of rat leads to the appearance of aberrant ipsilateral projecting corticospinal neurons in the spared hemisphere. The unablated cortex is suggested electrophysilogically to lack any primary somatosensory input from ipsilateral distal forelimb. The present study investigated the terminal distribution of the primary somatosensory distal forelimb area neurons within the motor cortex in relation to the distribution of corticospinal neurons by fluorescent tracer injections. There appeared to be little difference in distribution of the terminal within the motor cortex between lesioned and normal rats.

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